PuTTY User Manual
=================
PuTTY is a free (MIT-licensed) Windows Telnet and SSH client. This manual
documents PuTTY, and its companion utilities PSCP, PSFTP, Plink, Pageant
and PuTTYgen.
_Note to Unix users:_ this manual currently primarily documents the Windows
versions of the PuTTY utilities. Some options are therefore mentioned
that are absent from the Unix version; the Unix version has features
not described here; and the pterm and command-line puttygen and pageant
utilities are not described at all. The only Unix-specific documentation
that currently exists is the man pages.
This manual is copyright 1997-2025 Simon Tatham. All rights reserved. You
may distribute this documentation under the MIT licence. See appendix D for
the licence text in full.
Chapter 1: Introduction to PuTTY
--------------------------------
PuTTY is a free SSH, Telnet, Rlogin, and SUPDUP client for Windows
systems.
1.1 What are SSH, Telnet, Rlogin, and SUPDUP?
If you already know what SSH, Telnet, Rlogin, and SUPDUP are, you
can safely skip on to the next section.
SSH, Telnet, Rlogin, and SUPDUP are four ways of doing the same
thing: logging in to a multi-user computer from another computer,
over a network.
Multi-user operating systems, typically of the Unix family (such
as Linux, MacOS, and the BSD family), usually present a command-
line interface to the user, much like the `Command Prompt' or `MS-
DOS Prompt' in Windows. The system prints a prompt, and you type
commands which the system will obey.
Using this type of interface, there is no need for you to be sitting
at the same machine you are typing commands to. The commands,
and responses, can be sent over a network, so you can sit at one
computer and give commands to another one, or even to more than one.
SSH, Telnet, Rlogin, and SUPDUP are _network protocols_ that allow
you to do this. On the computer you sit at, you run a _client_,
which makes a network connection to the other computer (the
_server_). The network connection carries your keystrokes and
commands from the client to the server, and carries the server's
responses back to you.
These protocols can also be used for other types of keyboard-based
interactive session. In particular, there are a lot of bulletin
boards, talker systems and MUDs (Multi-User Dungeons) which support
access using Telnet. There are even a few that support SSH.
You might want to use SSH, Telnet, Rlogin, or SUPDUP if:
- you have an account on a Unix system (or some other multi-user
OS such as VMS or ITS) which you want to be able to access from
somewhere else
- your Internet Service Provider provides you with a login account
on a web server. (This might also be known as a _shell account_.
A _shell_ is the program that runs on the server and interprets
your commands for you.)
- you want to use a bulletin board system, talker or MUD which can
be accessed using Telnet.
You probably do _not_ want to use SSH, Telnet, Rlogin, or SUPDUP if:
- you only use Windows. Windows computers have their own ways
of networking between themselves, and unless you are doing
something fairly unusual, you will not need to use any of these
remote login protocols.
1.2 How do SSH, Telnet, Rlogin, and SUPDUP differ?
This list summarises some of the differences between SSH, Telnet,
Rlogin, and SUPDUP.
- SSH (which stands for `secure shell') is a recently designed,
high-security protocol. It uses strong cryptography to protect
your connection against eavesdropping, hijacking and other
attacks. Telnet, Rlogin, and SUPDUP are all older protocols
offering minimal security.
- SSH and Rlogin both allow you to log in to the server without
having to type a password. (Rlogin's method of doing this is
insecure, and can allow an attacker to access your account on
the server. SSH's method is much more secure, and typically
breaking the security requires the attacker to have gained
access to your actual client machine.)
- SSH allows you to connect to the server and automatically send
a command, so that the server will run that command and then
disconnect. So you can use it in automated processing.
The Internet is a hostile environment and security is everybody's
responsibility. If you are connecting across the open Internet,
then we recommend you use SSH. If the server you want to connect
to doesn't support SSH, it might be worth trying to persuade the
administrator to install it.
If your client and server are both behind the same (good) firewall,
it is more likely to be safe to use Telnet, Rlogin, or SUPDUP, but
we still recommend you use SSH.
Chapter 2: Getting started with PuTTY
-------------------------------------
This chapter gives a quick guide to the simplest types of
interactive login session using PuTTY.
2.1 Starting a session
When you start PuTTY, you will see a dialog box. This dialog box
allows you to control everything PuTTY can do. See chapter 4 for
details of all the things you can control.
You don't usually need to change most of the configuration options.
To start the simplest kind of session, all you need to do is to
enter a few basic parameters.
In the `Host Name' box, enter the Internet host name of the server
you want to connect to. You should have been told this by the
provider of your login account.
Now select a login protocol to use, from the `Connection type'
controls. For a login session, you should select SSH, Telnet,
Rlogin, or SUPDUP. See section 1.2 for a description of the
differences between these protocols, and advice on which one to use.
The _Raw_ protocol is not used for interactive login sessions; you
would usually use this for debugging other Internet services (see
section 3.7). The _Serial_ option is used for connecting to a local
serial line, and works somewhat differently: see section 3.6 for
more information on this.
When you change the selected protocol, the number in the `Port'
box will change. This is normal: it happens because the various
login services are usually provided on different network ports
by the server machine. Most servers will use the standard port
numbers, so you will not need to change the port setting. If your
server provides login services on a non-standard port, your system
administrator should have told you which one. (For example, many
MUDs run Telnet service on a port other than 23.)
Once you have filled in the `Host Name', `Connection type', and
possibly `Port' settings, you are ready to connect. Press the `Open'
button at the bottom of the dialog box, and PuTTY will begin trying
to connect you to the server.
2.2 Verifying the host key (SSH only)
If you are not using the SSH protocol, you can skip this section.
If you are using SSH to connect to a server for the first time, you
will probably see a message looking something like this:
The host key is not cached for this server:
ssh.example.com (port 22)
You have no guarantee that the server is the computer you think it is.
The server's ssh-ed25519 key fingerprint is:
ssh-ed25519 255 SHA256:TddlQk20DVs4LRcAsIfDN9pInKpY06D+h4kSHwWAj4w
If you trust this host, press "Accept" to add the key to PuTTY's
cache and carry on connecting.
If you want to carry on connecting just once, without adding the key
to the cache, press "Connect Once".
If you do not trust this host, press "Cancel" to abandon the connection.
This is a feature of the SSH protocol. It is designed to protect you
against a network attack known as _spoofing_: secretly redirecting
your connection to a different computer, so that you send your
password to the wrong machine. Using this technique, an attacker
would be able to learn the password that guards your login account,
and could then log in as if they were you and use the account for
their own purposes.
To prevent this attack, each server has a unique identifying code,
called a _host key_. These keys are created in a way that prevents
one server from forging another server's key. So if you connect to a
server and it sends you a different host key from the one you were
expecting, PuTTY can warn you that the server may have been switched
and that a spoofing attack might be in progress.
PuTTY records the host key for each server you connect to, in the
Windows Registry. Every time you connect to a server, it checks that
the host key presented by the server is the same host key as it was
the last time you connected. If it is not, you will see a stronger
warning, and you will have the chance to abandon your connection
before you type any private information (such as a password) into
it. (See section 10.2 for what that looks like.)
However, when you connect to a server you have not connected to
before, PuTTY has no way of telling whether the host key is the
right one or not. So it gives the warning shown above, and asks you
whether you want to trust this host key or not.
Whether or not to trust the host key is your choice. If you are
connecting within a company network, you might feel that all
the network users are on the same side and spoofing attacks are
unlikely, so you might choose to trust the key without checking
it. If you are connecting across a hostile network (such as the
Internet), you should check with your system administrator, perhaps
by telephone or in person. (When verifying the fingerprint, be
careful with letters and numbers that can be confused with each
other: `0'/`O', `1'/`I'/`l', and so on.)
Many servers have more than one host key. If the system
administrator sends you more than one fingerprint, you should make
sure the one PuTTY shows you is on the list, but it doesn't matter
which one it is.
If you don't have any fingerprints that look like the example
(`SHA256:' followed by a long string of characters), but instead
have pairs of characters separated by colons like `a4:db:96:a7:...',
try pressing the `More info...' button and see if you have a
fingerprint matching the `MD5 fingerprint' there. This is an older
and less secure way to summarise the same underlying host key; it's
possible for an attacker to create their own host key with the same
fingerprint; so you should avoid relying on this fingerprint format
unless you have no choice. The `More info...' dialog box also shows
the full host public key, in case that is easier to compare than a
fingerprint.
See section 4.19 for advanced options for managing host keys.
2.3 Logging in
After you have connected, and perhaps verified the server's host
key, you will be asked to log in, probably using a username and a
password. Your system administrator should have provided you with
these. (If, instead, your system administrator has asked you to
provide, or provided you with, a `public key' or `key file', see
chapter 8.)
PuTTY will display a text window (the `terminal window' - it will
have a black background unless you've changed the defaults), and
prompt you to type your username and password into that window.
(These prompts will include the PuTTY icon, to distinguish them from
any text sent by the server in the same window.)
Enter the username and the password, and the server should grant you
access and begin your session. If you have mistyped your password,
most servers will give you several chances to get it right.
While you are typing your password, you will not usually see the
cursor moving in the window, but PuTTY _is_ registering what you
type, and will send it when you press Return. (It works this way to
avoid revealing the length of your password to anyone watching your
screen.)
If you are using SSH, be careful not to type your username wrongly,
because you will not have a chance to correct it after you press
Return; many SSH servers do not permit you to make two login
attempts using different usernames. If you type your username
wrongly, you must close PuTTY and start again.
If your password is refused but you are sure you have typed it
correctly, check that Caps Lock is not enabled. Many login servers,
particularly Unix computers, treat upper case and lower case as
different when checking your password; so if Caps Lock is on, your
password will probably be refused.
2.4 After logging in
After you log in to the server, what happens next is up to the
server! Most servers will print some sort of login message and then
present a prompt, at which you can type commands which the server
will carry out. Some servers will offer you on-line help; others
might not. If you are in doubt about what to do next, consult your
system administrator.
2.5 Logging out
When you have finished your session, you should log out by typing
the server's own logout command. This might vary between servers; if
in doubt, try `logout' or `exit', or consult a manual or your system
administrator. When the server processes your logout command, the
PuTTY window should close itself automatically.
You _can_ close a PuTTY session using the Close button in the window
border, but this might confuse the server - a bit like hanging up a
telephone unexpectedly in the middle of a conversation. We recommend
you do not do this unless the server has stopped responding to you
and you cannot close the window any other way.
Chapter 3: Using PuTTY
----------------------
This chapter provides a general introduction to some more advanced
features of PuTTY. For extreme detail and reference purposes,
chapter 4 is likely to contain more information.
3.1 During your session
A lot of PuTTY's complexity and features are in the configuration
panel. Once you have worked your way through that and started
a session, things should be reasonably simple after that.
Nevertheless, there are a few more useful features available.
3.1.1 Copying and pasting text
Often in a PuTTY session you will find text on your terminal screen
which you want to type in again. Like most other terminal emulators,
PuTTY allows you to copy and paste the text rather than having to
type it again. Also, copy and paste uses the Windows clipboard, so
that you can paste (for example) URLs into a web browser, or paste
from a word processor or spreadsheet into your terminal session.
By default, PuTTY's copy and paste works entirely with the mouse.
(This will be familiar to people who have used `xterm' on Unix.) In
order to copy text to the clipboard, you just click the left mouse
button in the terminal window, and drag to select text. When you
let go of the button, the text is _automatically_ copied to the
clipboard. You do not need to press Ctrl-C or Ctrl-Ins; in fact, if
you do press Ctrl-C, PuTTY will send a Ctrl-C character down your
session to the server where it will probably cause a process to be
interrupted.
Pasting into PuTTY is done using the right button (or the middle
mouse button, if you have a three-button mouse and have set it up;
see section 4.11.1). (Pressing Shift-Ins, or selecting `Paste' from
the Ctrl+right-click context menu, have the same effect.) When you
click the right mouse button, PuTTY will read whatever is in the
Windows clipboard and paste it into your session. By default, this
behaves _exactly_ as if the clipboard contents had been typed at
the keyboard; therefore, be careful of pasting formatted text into
an editor that does automatic indenting, as you may find that the
spaces pasted from the clipboard plus the spaces added by the editor
add up to too many spaces and ruin the formatting. (Some remote
applications can ask PuTTY to identify text that is being pasted, to
avoid this sort of problem; but if your application does not, there
is nothing PuTTY can do to avoid this.)
If you double-click the left mouse button, PuTTY will select a whole
word. If you double-click, hold down the second click, and drag the
mouse, PuTTY will select a sequence of whole words. (You can adjust
precisely what PuTTY considers to be part of a word; see section
4.12.1.) If you _triple_-click, or triple-click and drag, then PuTTY
will select a whole line or sequence of lines.
If you want to select a rectangular region instead of selecting to
the end of each line, you can do this by holding down Alt when you
make your selection. You can also configure rectangular selection to
be the default, and then holding down Alt gives the normal behaviour
instead: see section 4.11.3 for details.
(In some Unix environments, Alt+drag is intercepted by the window
manager. Shift+Alt+drag should work for rectangular selection as
well, so you could try that instead.)
If you have a middle mouse button, then you can use it to adjust an
existing selection if you selected something slightly wrong. (If you
have configured the middle mouse button to paste, then the right
mouse button does this instead.) Click the button on the screen,
and you can pick up the nearest end of the selection and drag it to
somewhere else.
If you are running PuTTY itself on Unix (not just using it to
connect to a Unix system from Windows), by default you will likely
have to use similar mouse actions in other applications to paste the
text you copied from PuTTY, and to copy text for pasting into PuTTY;
actions like Ctrl-C and Ctrl-V will likely not behave as you expect.
Section 4.11.4 explains why this is, and how you can change the
behaviour. (On Windows there is only a single selection shared with
other applications, so this confusion does not arise.)
It's possible for the server to ask to handle mouse clicks in the
PuTTY window itself. If this happens, the mouse pointer will turn
into an arrow, and using the mouse to copy and paste will only work
if you hold down Shift. See section 4.6.2 and section 4.11.2 for
details of this feature and how to configure it.
You can customise much of this behaviour, for instance to enable
copy and paste from the keyboard; see section 4.11.
3.1.2 Scrolling the screen back
PuTTY keeps track of text that has scrolled up off the top of the
terminal. So if something appears on the screen that you want to
read, but it scrolls too fast and it's gone by the time you try to
look for it, you can use the scrollbar on the right side of the
window to look back up the session history and find it again.
As well as using the scrollbar, you can also page the scrollback up
and down by pressing Shift-PgUp and Shift-PgDn. You can scroll a
line at a time using Ctrl-PgUp and Ctrl-PgDn, or to the top/bottom
of the scrollback with Ctrl-Shift-PgUp and Ctrl-Shift-PgDn. These
are still available if you configure the scrollbar to be invisible.
By default the last 2000 lines scrolled off the top are preserved
for you to look at. You can increase (or decrease) this value using
the configuration box; see section 4.7.3.
3.1.3 The System menu
If you click the left mouse button on the icon in the top left
corner of PuTTY's terminal window, or click the right mouse button
on the title bar, you will see the standard Windows system menu
containing items like Minimise, Move, Size and Close.
PuTTY's system menu contains extra program features in addition
to the Windows standard options. These extra menu commands are
described below.
(These options are also available in a context menu brought up by
holding Ctrl and clicking with the right mouse button anywhere in
the PuTTY window.)
3.1.3.1 The PuTTY Event Log
If you choose `Event Log' from the system menu, a small window will
pop up in which PuTTY logs significant events during the connection.
Most of the events in the log will probably take place during
session startup, but a few can occur at any point in the session,
and one or two occur right at the end.
You can use the mouse to select one or more lines of the Event Log,
and hit the Copy button to copy them to the clipboard. If you are
reporting a bug, it's often useful to paste the contents of the
Event Log into your bug report.
(The Event Log is not the same as the facility to create a log file
of your session; that's described in section 3.2.)
3.1.3.2 Special commands
Depending on the protocol used for the current session, there may
be a submenu of `special commands'. These are protocol-specific
tokens, such as a `break' signal, that can be sent down a connection
in addition to normal data. Their precise effect is usually up to
the server. Currently only Telnet, SSH, and serial connections have
special commands.
The `break' signal can also be invoked from the keyboard with Ctrl-
Break.
In an SSH connection, the following special commands are available:
- IGNORE message
Should have no effect.
- Repeat key exchange
Only available in SSH-2. Forces a repeat key exchange
immediately (and resets associated timers and counters). For
more information about repeat key exchanges, see section 4.18.2.
- Cache new host key type
Only available in SSH-2. This submenu appears only if the server
has host keys of a type that PuTTY doesn't already have cached,
and so won't consider. Selecting a key here will allow PuTTY
to use that key now and in future: PuTTY will do a fresh key-
exchange with the selected key, and immediately add that key to
its permanent cache (relying on the host key used at the start
of the connection to cross-certify the new key). That key will
be used for the rest of the current session; it may not actually
be used for future sessions, depending on your preferences (see
section 4.19.1).
Normally, PuTTY will carry on using a host key it already knows,
even if the server offers key formats that PuTTY would otherwise
prefer, to avoid host key prompts. As a result, if you've been
using a server for some years, you may still be using an older
key than a new user would use, due to server upgrades in the
meantime. The SSH protocol unfortunately does not have organised
facilities for host key migration and rollover, but this allows
you to manually upgrade.
- Break
Only available in SSH-2, and only during a session. Optional
extension; may not be supported by server. PuTTY requests the
server's default break length.
- Signals (SIGINT, SIGTERM etc)
Only available in SSH-2, and only during a session. Sends
various POSIX signals. Not honoured by all servers.
The following special commands are available in Telnet:
- Are You There
- Break
- Synch
- Erase Character
PuTTY can also be configured to send this when the Backspace key
is pressed; see section 4.30.3.
- Erase Line
- Go Ahead
- No Operation
Should have no effect.
- Abort Process
- Abort Output
- Interrupt Process
PuTTY can also be configured to send this when Ctrl-C is typed;
see section 4.30.3.
- Suspend Process
PuTTY can also be configured to send this when Ctrl-Z is typed;
see section 4.30.3.
- End Of Record
- End Of File
With a serial connection, the only available special command is
`Break'.
3.1.3.3 Starting new sessions
PuTTY's system menu provides some shortcut ways to start new
sessions:
- Selecting `New Session' will start a completely new instance of
PuTTY, and bring up the configuration box as normal.
- Selecting `Duplicate Session' will start a session in a new
window with precisely the same options as your current one -
connecting to the same host using the same protocol, with all
the same terminal settings and everything.
- In an inactive window, selecting `Restart Session' will do the
same as `Duplicate Session', but in the current window.
- The `Saved Sessions' submenu gives you quick access to any sets
of stored session details you have previously saved. See section
4.1.2 for details of how to create saved sessions.
3.1.3.4 Changing your session settings
If you select `Change Settings' from the system menu, PuTTY will
display a cut-down version of its initial configuration box. This
allows you to adjust most properties of your current session. You
can change the terminal size, the font, the actions of various
keypresses, the colours, and so on.
Some of the options that are available in the main configuration box
are not shown in the cut-down Change Settings box. These are usually
options which don't make sense to change in the middle of a session
(for example, you can't switch from SSH to Telnet in mid-session).
You can save the current settings to a saved session for future use
from this dialog box. See section 4.1.2 for more on saved sessions.
3.1.3.5 Copy All to Clipboard
This system menu option provides a convenient way to copy the whole
contents of the terminal screen (up to the last nonempty line) and
scrollback to the clipboard in one go.
3.1.3.6 Clearing and resetting the terminal
The `Clear Scrollback' option on the system menu tells PuTTY to
discard all the lines of text that have been kept after they
scrolled off the top of the screen. This might be useful, for
example, if you displayed sensitive information and wanted to make
sure nobody could look over your shoulder and see it. (Note that
this only prevents a casual user from using the scrollbar to view
the information; the text is not guaranteed not to still be in
PuTTY's memory.)
The `Reset Terminal' option causes a full reset of the terminal
emulation. A VT-series terminal is a complex piece of software and
can easily get into a state where all the text printed becomes
unreadable. (This can happen, for example, if you accidentally
output a binary file to your terminal.) If this happens, selecting
Reset Terminal should sort it out.
3.1.3.7 Full screen mode
If you find the title bar on a maximised window to be ugly or
distracting, you can select Full Screen mode to maximise PuTTY `even
more'. When you select this, PuTTY will expand to fill the whole
screen and its borders, title bar and scrollbar will disappear. (You
can configure the scrollbar not to disappear in full-screen mode if
you want to keep it; see section 4.7.3.)
When you are in full-screen mode, you can still access the system
menu if you click the left mouse button in the _extreme_ top left
corner of the screen.
3.2 Creating a log file of your session
For some purposes you may find you want to log everything that
appears on your screen. You can do this using the `Logging' panel in
the configuration box.
To begin a session log, select `Change Settings' from the system
menu and go to the Logging panel. Enter a log file name, and select
a logging mode. (You can log all session output including the
terminal control sequences, or you can just log the printable text.
It depends what you want the log for.) Click `Apply' and your log
will be started. Later on, you can go back to the Logging panel and
select `Logging turned off completely' to stop logging; then PuTTY
will close the log file and you can safely read it.
See section 4.2 for more details and options.
3.3 Altering your character set configuration
If you find that special characters (accented characters, for
example, or line-drawing characters) are not being displayed
correctly in your PuTTY session, it may be that PuTTY is
interpreting the characters sent by the server according to the
wrong _character set_. There are a lot of different character sets
available, and no good way for PuTTY to know which to use, so it's
entirely possible for this to happen.
If you click `Change Settings' and look at the `Translation' panel,
you should see a large number of character sets which you can
select, and other related options. Now all you need is to find out
which of them you want! (See section 4.10 for more information.)
3.4 Using X11 forwarding in SSH
The SSH protocol has the ability to securely forward X Window System
graphical applications over your encrypted SSH connection, so that
you can run an application on the SSH server machine and have it put
its windows up on your local machine without sending any X network
traffic in the clear.
In order to use this feature, you will need an X display server
for your Windows machine, such as Cygwin/X, X-Win32, or Exceed.
This will probably install itself as display number 0 on your local
machine; if it doesn't, the manual for the X server should tell you
what it does do.
You should then tick the `Enable X11 forwarding' box in the X11
panel (see section 4.25) before starting your SSH session. The `X
display location' box is blank by default, which means that PuTTY
will try to use a sensible default such as `:0', which is the usual
display location where your X server will be installed. If that
needs changing, then change it.
Now you should be able to log in to the SSH server as normal. To
check that X forwarding has been successfully negotiated during
connection startup, you can check the PuTTY Event Log (see section
3.1.3.1). It should say something like this:
2001-12-05 17:22:01 Requesting X11 forwarding
2001-12-05 17:22:02 X11 forwarding enabled
If the remote system is Unix or Unix-like, you should also be able
to see that the `DISPLAY' environment variable has been set to point
at display 10 or above on the SSH server machine itself:
fred@unixbox:~$ echo $DISPLAY
unixbox:10.0
If this works, you should then be able to run X applications in the
remote session and have them display their windows on your PC.
For more options relating to X11 forwarding, see section 4.25.
3.5 Using port forwarding in SSH
The SSH protocol has the ability to forward arbitrary network
(TCP) connections over your encrypted SSH connection, to avoid the
network traffic being sent in clear. For example, you could use
this to connect from your home computer to a POP-3 server on a
remote machine without your POP-3 password being visible to network
sniffers.
In order to use port forwarding to connect from your local machine
to a port on a remote server, you need to:
- Choose a port number on your local machine where PuTTY should
listen for incoming connections. There are likely to be plenty
of unused port numbers above 3000. (You can also use a local
loopback address here; see below for more details.)
- Now, before you start your SSH connection, go to the Tunnels
panel (see section 4.26). Make sure the `Local' radio button
is set. Enter the local port number into the `Source port'
box. Enter the destination host name and port number into
the `Destination' box, separated by a colon (for example,
`popserver.example.com:110' to connect to a POP-3 server).
- Now click the `Add' button. The details of your port forwarding
should appear in the list box.
Now start your session and log in. (Port forwarding will not be
enabled until after you have logged in; otherwise it would be easy
to perform completely anonymous network attacks, and gain access to
anyone's virtual private network.) To check that PuTTY has set up
the port forwarding correctly, you can look at the PuTTY Event Log
(see section 3.1.3.1). It should say something like this:
2001-12-05 17:22:10 Local port 3110 forwarding to
popserver.example.com:110
Now if you connect to the source port number on your local PC, you
should find that it answers you exactly as if it were the service
running on the destination machine. So in this example, you could
then configure an e-mail client to use `localhost:3110' as a POP-
3 server instead of `popserver.example.com:110'. (Of course, the
forwarding will stop happening when your PuTTY session closes down.)
You can also forward ports in the other direction: arrange for a
particular port number on the _server_ machine to be forwarded back
to your PC as a connection to a service on your PC or near it. To do
this, just select the `Remote' radio button instead of the `Local'
one. The `Source port' box will now specify a port number on the
_server_ (note that most servers will not allow you to use port
numbers under 1024 for this purpose).
An alternative way to forward local connections to remote hosts
is to use dynamic SOCKS proxying. In this mode, PuTTY acts as a
SOCKS server, which SOCKS-aware programs can connect to and open
forwarded connections to the destination of their choice, so this
can be an alternative to long lists of static forwardings. To use
this mode, you will need to select the `Dynamic' radio button
instead of `Local', and then you should not enter anything into the
`Destination' box (it will be ignored). PuTTY will then listen for
SOCKS connections on the port you have specified. Most web browsers
can be configured to connect to this SOCKS proxy service; also, you
can forward other PuTTY connections through it by setting up the
Proxy control panel (see section 4.16 for details).
The source port for a forwarded connection usually does not accept
connections from any machine except the SSH client or server machine
itself (for local and remote forwardings respectively). There are
controls in the Tunnels panel to change this:
- The `Local ports accept connections from other hosts' option
allows you to set up local-to-remote port forwardings (including
dynamic port forwardings) in such a way that machines other than
your client PC can connect to the forwarded port.
- The `Remote ports do the same' option does the same thing for
remote-to-local port forwardings (so that machines other than
the SSH server machine can connect to the forwarded port.) Note
that this feature is only available in the SSH-2 protocol, and
not all SSH-2 servers honour it (in OpenSSH, for example, it's
usually disabled by default).
You can also specify an IP address to listen on. Typically a
Windows machine can be asked to listen on any single IP address
in the 127.*.*.* range, and all of these are loopback addresses
available only to the local machine. So if you forward (for example)
`127.0.0.5:79' to a remote machine's finger port, then you should be
able to run commands such as `finger
[email protected]'. This can be
useful if the program connecting to the forwarded port doesn't allow
you to change the port number it uses. This feature is available
for local-to-remote forwarded ports; SSH-1 is unable to support it
for remote-to-local ports, while SSH-2 can support it in theory but
servers will not necessarily cooperate.
(Note that if you're using Windows XP Service Pack 2, you may need
to obtain a fix from Microsoft in order to use addresses like
127.0.0.5 - see question A.7.17.)
For more options relating to port forwarding, see section 4.26.
If the connection you are forwarding over SSH is itself a second
SSH connection made by another copy of PuTTY, you might find the
`logical host name' configuration option useful to warn PuTTY of
which host key it should be expecting. See section 4.14.5 for
details of this.
3.6 Connecting to a local serial line
PuTTY can connect directly to a local serial line as an alternative
to making a network connection. In this mode, text typed into the
PuTTY window will be sent straight out of your computer's serial
port, and data received through that port will be displayed in the
PuTTY window. You might use this mode, for example, if your serial
port is connected to another computer which has a serial connection.
To make a connection of this type, simply select `Serial' from the
`Connection type' radio buttons on the `Session' configuration panel
(see section 4.1.1). The `Host Name' and `Port' boxes will transform
into `Serial line' and `Speed', allowing you to specify which serial
line to use (if your computer has more than one) and what speed
(baud rate) to use when transferring data. For further configuration
options (data bits, stop bits, parity, flow control), you can use
the `Serial' configuration panel (see section 4.29).
After you start up PuTTY in serial mode, you might find that you
have to make the first move, by sending some data out of the serial
line in order to notify the device at the other end that someone is
there for it to talk to. This probably depends on the device. If you
start up a PuTTY serial session and nothing appears in the window,
try pressing Return a few times and see if that helps.
A serial line provides no well defined means for one end of the
connection to notify the other that the connection is finished.
Therefore, PuTTY in serial mode will remain connected until you
close the window using the close button.
3.7 Making raw TCP connections
A lot of Internet protocols are composed of commands and responses
in plain text. For example, SMTP (the protocol used to transfer e-
mail), NNTP (the protocol used to transfer Usenet news), and HTTP
(the protocol used to serve Web pages) all consist of commands in
readable plain text.
Sometimes it can be useful to connect directly to one of these
services and speak the protocol `by hand', by typing protocol
commands and watching the responses. On Unix machines, you can do
this using the system's `telnet' command to connect to the right
port number. For example, `telnet mailserver.example.com 25' might
enable you to talk directly to the SMTP service running on a mail
server.
Although the Unix `telnet' program provides this functionality, the
protocol being used is not really Telnet. Really there is no actual
protocol at all; the bytes sent down the connection are exactly the
ones you type, and the bytes shown on the screen are exactly the
ones sent by the server. Unix `telnet' will attempt to detect or
guess whether the service it is talking to is a real Telnet service
or not; PuTTY prefers to be told for certain.
In order to make a debugging connection to a service of this
type, you simply select the fourth protocol name, `Raw', from the
`Protocol' buttons in the `Session' configuration panel. (See
section 4.1.1.) You can then enter a host name and a port number,
and make the connection.
3.8 Connecting using the Telnet protocol
PuTTY can use the Telnet protocol to connect to a server.
Telnet was perhaps the most popular remote login protocol before SSH
was introduced. It was general enough to be used by multiple server
operating systems (Unix and VMS in particular), and supported many
optional protocol extensions providing extra support for particular
server features.
Unlike SSH, Telnet runs over an unsecured network connection, so it
is a very bad idea to use it over the hostile Internet (though it is
still used to some extent as of 2020).
3.9 Connecting using the Rlogin protocol
PuTTY can use the Rlogin protocol to connect to a server.
Rlogin was similar to Telnet in concept, but more focused on
connections between Unix machines. It supported a feature for
passwordless login, based on use of `privileged ports' (ports with
numbers below 1024, which Unix traditionally does not allow users
other than root to allocate). Ultimately, based on the server
trusting that the client's IP address was owned by the Unix machine
it claimed to be, and that that machine would guard its privileged
ports appropriately.
Like Telnet, Rlogin runs over an unsecured network connection.
3.10 Connecting using the SUPDUP protocol
PuTTY can use the SUPDUP protocol to connect to a server.
SUPDUP is a login protocol used mainly by PDP-10 and Lisp machines
during the period 1975-1990. Like Telnet and Rlogin, it is
unsecured, so modern systems almost never support it.
To make a connection of this type, select `SUPDUP' from the
`Connection type' radio buttons on the `Session' panel (see section
4.1.1). For further configuration options (character set, more
processing, scrolling), you can use the `SUPDUP' configuration panel
(see section 4.32).
In SUPDUP, terminal emulation is more integrated with the network
protocol than in other protocols such as SSH. The SUPDUP protocol
can thus only be used with PuTTY proper, not with the command-line
tool Plink.
The SUPDUP protocol does not support changing the terminal
dimensions, so this capability is disabled during a SUPDUP session.
SUPDUP provides no well defined means for one end of the connection
to notify the other that the connection is finished. Therefore,
PuTTY in SUPDUP mode will remain connected until you close the
window using the close button.
3.11 The PuTTY command line
PuTTY can be made to do various things without user intervention
by supplying command-line arguments (e.g., from a command prompt
window, or a Windows shortcut).
3.11.1 Starting a session from the command line
These options allow you to bypass the configuration window and
launch straight into a session.
To start a connection to a server called `host':
putty.exe [-ssh | -ssh-connection | -telnet | -rlogin | -supdup | -raw] [user@]host
If this syntax is used, settings are taken from the Default Settings
(see section 4.1.2); `user' overrides these settings if supplied.
Also, you can specify a protocol, which will override the default
protocol (see section 3.11.3.2).
For telnet sessions, the following alternative syntax is supported
(this makes PuTTY suitable for use as a URL handler for telnet URLs
in web browsers):
putty.exe telnet://host[:port]/
To start a connection to a serial port, e.g. COM1:
putty.exe -serial com1
In order to start an existing saved session called `sessionname',
use the `-load' option (described in section 3.11.3.1).
putty.exe -load "session name"
3.11.2 `-cleanup'
If invoked with the `-cleanup' option, rather than running as
normal, PuTTY will remove its registry entries and random seed file
from the local machine (after confirming with the user). It will
also attempt to remove information about recently launched sessions
stored in the `jump list' on Windows 7 and up.
Note that on multi-user systems, `-cleanup' only removes registry
entries and files associated with the currently logged-in user.
3.11.3 Standard command-line options
PuTTY and its associated tools support a range of command-line
options, most of which are consistent across all the tools. This
section lists the available options in all tools. Options which are
specific to a particular tool are covered in the chapter about that
tool.
3.11.3.1 `-load': load a saved session
The `-load' option causes PuTTY to load configuration details out
of a saved session. If these details include a host name, then this
option is all you need to make PuTTY start a session.
You need double quotes around the session name if it contains
spaces.
If you want to create a Windows shortcut to start a PuTTY saved
session, this is the option you should use: your shortcut should
call something like
d:\path\to\putty.exe -load "my session"
(Note that PuTTY itself supports an alternative form of this option,
for backwards compatibility. If you execute `putty @sessionname' it
will have the same effect as `putty -load "sessionname"'. With the
`@' form, no double quotes are required, and the `@' sign must be
the very first thing on the command line. This form of the option is
deprecated.)
3.11.3.2 Selecting a protocol: `-ssh', `-ssh-connection', `-telnet', `-
rlogin', `-supdup', `-raw', `-serial'
To choose which protocol you want to connect with, you can use one
of these options:
- `-ssh' selects the SSH protocol.
- `-ssh-connection' selects the bare ssh-connection protocol.
(This is only useful in specialised circumstances; see section
4.28 for more information.)
- `-telnet' selects the Telnet protocol.
- `-rlogin' selects the Rlogin protocol.
- `-supdup' selects the SUPDUP protocol.
- `-raw' selects the raw protocol.
- `-serial' selects a serial connection.
Most of these options are not available in the file transfer tools
PSCP and PSFTP (which only work with the SSH protocol and the bare
ssh-connection protocol).
These options are equivalent to the protocol selection buttons
in the Session panel of the PuTTY configuration box (see section
4.1.1).
3.11.3.3 `-v': increase verbosity
Most of the PuTTY tools can be made to tell you more about what they
are doing by supplying the `-v' option. If you are having trouble
when making a connection, or you're simply curious, you can turn
this switch on and hope to find out more about what is happening.
3.11.3.4 `-l': specify a login name
You can specify the user name to log in as on the remote server
using the `-l' option. For example, `plink login.example.com -
l fred'.
These options are equivalent to the username selection box in
the Connection panel of the PuTTY configuration box (see section
4.15.1).
3.11.3.5 `-L', `-R' and `-D': set up port forwardings
As well as setting up port forwardings in the PuTTY configuration
(see section 4.26), you can also set up forwardings on the command
line. The command-line options work just like the ones in Unix `ssh'
programs.
To forward a local port (say 5110) to a remote destination (say
popserver.example.com port 110), you can write something like one of
these:
putty -L 5110:popserver.example.com:110 -load mysession
plink mysession -L 5110:popserver.example.com:110
To forward a remote port to a local destination, just use the `-R'
option instead of `-L':
putty -R 5023:mytelnetserver.myhouse.org:23 -load mysession
plink mysession -R 5023:mytelnetserver.myhouse.org:23
To specify an IP address for the listening end of the tunnel,
prepend it to the argument:
plink -L 127.0.0.5:23:localhost:23 myhost
To set up SOCKS-based dynamic port forwarding on a local port, use
the `-D' option. For this one you only have to pass the port number:
putty -D 4096 -load mysession
For general information on port forwarding, see section 3.5.
These options are not available in the file transfer tools PSCP and
PSFTP.
3.11.3.6 `-m': read a remote command or script from a file
The `-m' option performs a similar function to the `Remote command'
box in the SSH panel of the PuTTY configuration box (see section
4.17.1). However, the `-m' option expects to be given a local file
name, and it will read a command from that file.
With some servers (particularly Unix systems), you can even put
multiple lines in this file and execute more than one command in
sequence, or a whole shell script; but this is arguably an abuse,
and cannot be expected to work on all servers. In particular, it is
known _not_ to work with certain `embedded' servers, such as Cisco
routers.
This option is not available in the file transfer tools PSCP and
PSFTP.
3.11.3.7 `-P': specify a port number
The `-P' option is used to specify the port number to connect to. If
you have a Telnet server running on port 9696 of a machine instead
of port 23, for example:
putty -telnet -P 9696 host.name
plink -telnet -P 9696 host.name
(Note that this option is more useful in Plink than in PuTTY,
because in PuTTY you can write `putty -telnet host.name 9696' in any
case.)
This option is equivalent to the port number control in the Session
panel of the PuTTY configuration box (see section 4.1.1).
3.11.3.8 `-pwfile' and `-pw': specify a password
A simple way to automate a remote login is to supply your password
on the command line.
The `-pwfile' option takes a file name as an argument. The first
line of text in that file will be used as your password.
The `-pw' option takes the password itself as an argument. This is
*NOT SECURE* if anybody else uses the same computer, because the
whole command line (including the password) is likely to show up
if another user lists the running processes. `-pw' is retained for
backwards compatibility only; you should use `-pwfile' instead.
Note that these options only work when you are using the SSH
protocol. Due to fundamental limitations of Telnet, Rlogin,
and SUPDUP, these protocols do not support automated password
authentication.
3.11.3.9 `-agent' and `-noagent': control use of Pageant for authentication
The `-agent' option turns on SSH authentication using Pageant, and
`-noagent' turns it off. These options are only meaningful if you
are using SSH.
See chapter 9 for general information on Pageant.
These options are equivalent to the agent authentication checkbox in
the Auth panel of the PuTTY configuration box (see section 4.21.4).
3.11.3.10 `-A' and `-a': control agent forwarding
The `-A' option turns on SSH agent forwarding, and `-a' turns it
off. These options are only meaningful if you are using SSH.
See chapter 9 for general information on Pageant, and section 9.4
for information on agent forwarding. Note that there is a security
risk involved with enabling this option; see section 9.6 for
details.
These options are equivalent to the agent forwarding checkbox in the
Auth panel of the PuTTY configuration box (see section 4.21.7).
These options are not available in the file transfer tools PSCP and
PSFTP.
3.11.3.11 `-X' and `-x': control X11 forwarding
The `-X' option turns on X11 forwarding in SSH, and `-x' turns it
off. These options are only meaningful if you are using SSH.
For information on X11 forwarding, see section 3.4.
These options are equivalent to the X11 forwarding checkbox in the
X11 panel of the PuTTY configuration box (see section 4.25).
These options are not available in the file transfer tools PSCP and
PSFTP.
3.11.3.12 `-t' and `-T': control pseudo-terminal allocation
The `-t' option ensures PuTTY attempts to allocate a pseudo-terminal
at the server, and `-T' stops it from allocating one. These options
are only meaningful if you are using SSH.
These options are equivalent to the `Don't allocate a pseudo-
terminal' checkbox in the SSH panel of the PuTTY configuration box
(see section 4.24.1).
These options are not available in the file transfer tools PSCP and
PSFTP.
3.11.3.13 `-N': suppress starting a shell or command
The `-N' option prevents PuTTY from attempting to start a shell or
command on the remote server. You might want to use this option if
you are only using the SSH connection for port forwarding, and your
user account on the server does not have the ability to run a shell.
This feature is only available in SSH protocol version 2 (since the
version 1 protocol assumes you will always want to run a shell).
This option is equivalent to the `Don't start a shell or command at
all' checkbox in the SSH panel of the PuTTY configuration box (see
section 4.17.2).
This option is not available in the file transfer tools PSCP and
PSFTP.
3.11.3.14 `-nc': make a remote network connection in place of a remote
shell or command
The `-nc' option prevents Plink (or PuTTY) from attempting to start
a shell or command on the remote server. Instead, it will instruct
the remote server to open a network connection to a host name and
port number specified by you, and treat that network connection as
if it were the main session.
You specify a host and port as an argument to the `-nc' option, with
a colon separating the host name from the port number, like this:
plink host1.example.com -nc host2.example.com:1234
This can be useful if you're trying to make a connection to a target
host which you can only reach by SSH forwarding through a proxy
host. One way to do this would be to have an existing SSH connection
to the proxy host, with a port forwarding, but if you prefer to have
the connection started on demand as needed, then this approach can
also work.
However, this does depend on the program _using_ the proxy being
able to run a subprocess in place of making a network connection.
PuTTY itself can do this using the `Local' proxy type, but there's a
built-in more flexible way using the `SSH' proxy type. (See section
4.16.1 for a description of both.) So this feature is probably most
useful with another client program as the end user.
This feature is only available in SSH protocol version 2 (since the
version 1 protocol assumes you will always want to run a shell). It
is not available in the file transfer tools PSCP and PSFTP. It is
available in PuTTY itself, although it is unlikely to be very useful
in any tool other than Plink. Also, `-nc' uses the same server
functionality as port forwarding, so it will not work if your server
administrator has disabled port forwarding.
(The option is named `-nc' after the Unix program `nc', short for
`netcat'. The command `plink host1 -nc host2:port' is very similar
in functionality to `plink host1 nc host2 port', which invokes `nc'
on the server and tells it to connect to the specified destination.
However, Plink's built-in `-nc' option does not depend on the `nc'
program being installed on the server.)
3.11.3.15 `-C': enable compression
The `-C' option enables compression of the data sent across the
network. This option is only meaningful if you are using SSH.
This option is equivalent to the `Enable compression' checkbox in
the SSH panel of the PuTTY configuration box (see section 4.17.3).
3.11.3.16 `-1' and `-2': specify an SSH protocol version
The `-1' and `-2' options force PuTTY to use version 1 or version 2
of the SSH protocol. These options are only meaningful if you are
using SSH.
These options are equivalent to selecting the SSH protocol version
in the SSH panel of the PuTTY configuration box (see section
4.17.4).
3.11.3.17 `-4' and `-6': specify an Internet protocol version
The `-4' and `-6' options force PuTTY to use the older Internet
protocol IPv4 or the newer IPv6 for most outgoing connections.
These options are equivalent to selecting your preferred Internet
protocol version as `IPv4' or `IPv6' in the Connection panel of the
PuTTY configuration box (see section 4.14.4).
3.11.3.18 `-i': specify an SSH private key
The `-i' option allows you to specify the name of a private key file
in `*.PPK' format which PuTTY will use to authenticate with the
server. This option is only meaningful if you are using SSH.
If you are using Pageant, you can also specify a _public_ key file
(in RFC 4716 or OpenSSH format) to identify a specific key file to
use. (This won't work if you're not running Pageant, of course.)
For general information on public-key authentication, see chapter 8.
This option is equivalent to the `Private key file for
authentication' box in the Auth panel of the PuTTY configuration box
(see section 4.22.1).
3.11.3.19 `-cert': specify an SSH certificate
The `-cert' option allows you to specify the name of a certificate
file containing a signed version of your public key. If you specify
this option, PuTTY will present that certificate in place of the
plain public key, whenever it tries to authenticate with a key that
matches. (This applies whether the key is stored in Pageant or
loaded directly from a file by PuTTY.)
This option is equivalent to the `Certificate to use with the
private key' box in the Auth panel of the PuTTY configuration box
(see section 4.22.2).
3.11.3.20 `-no-trivial-auth': disconnect if SSH authentication succeeds
trivially
This option causes PuTTY to abandon an SSH session if the server
accepts authentication without ever having asked for any kind of
password or signature or token.
See section 4.21.3 for why you might want this.
3.11.3.21 `-loghost': specify a logical host name
This option overrides PuTTY's normal SSH host key caching policy by
telling it the name of the host you expect your connection to end up
at (in cases where this differs from the location PuTTY thinks it's
connecting to). It can be a plain host name, or a host name followed
by a colon and a port number. See section 4.14.5 for more detail on
this.
3.11.3.22 `-hostkey': manually specify an expected host key
This option overrides PuTTY's normal SSH host key caching policy
by telling it exactly what host key to expect, which can be
useful if the normal automatic host key store in the Registry is
unavailable. The argument to this option should be either a host key
fingerprint, or an SSH-2 public key blob. See section 4.19.3 for
more information.
You can specify this option more than once if you want to configure
more than one key to be accepted.
3.11.3.23 `-pgpfp': display PGP key fingerprints
This option causes the PuTTY tools not to run as normal, but instead
to display the fingerprints of the PuTTY PGP Master Keys, in
order to aid with verifying new versions. See appendix F for more
information.
3.11.3.24 `-sercfg': specify serial port configuration
This option specifies the configuration parameters for the serial
port (baud rate, stop bits etc). Its argument is interpreted as
a comma-separated list of configuration options, which can be as
follows:
- Any single digit from 5 to 9 sets the number of data bits.
- `1', `1.5' or `2' sets the number of stop bits.
- Any other numeric string is interpreted as a baud rate.
- A single lower-case letter specifies the parity: `n' for none,
`o' for odd, `e' for even, `m' for mark and `s' for space.
- A single upper-case letter specifies the flow control: `N' for
none, `X' for XON/XOFF, `R' for RTS/CTS and `D' for DSR/DTR.
For example, `-sercfg 19200,8,n,1,N' denotes a baud rate of 19200, 8
data bits, no parity, 1 stop bit and no flow control.
3.11.3.25 `-sessionlog', `-sshlog', `-sshrawlog': enable session logging
These options cause the PuTTY network tools to write out a log
file. Each of them expects a file name as an argument, e.g. `-
sshlog putty.log' causes an SSH packet log to be written to a file
called `putty.log'. The three different options select different
logging modes, all available from the GUI too:
- `-sessionlog' selects `All session output' logging mode.
- `-sshlog' selects `SSH packets' logging mode.
- `-sshrawlog' selects `SSH packets and raw data' logging mode.
For more information on logging configuration, see section 4.2.
3.11.3.26 `-logoverwrite', `-logappend': control behaviour with existing
log file
If logging has been enabled (in the saved configuration, or by
another command-line option), and the specified log file already
exists, these options tell the PuTTY network tools what to do so
that they don't have to ask the user. See section 4.2.2 for details.
3.11.3.27 `-proxycmd': specify a local proxy command
This option enables PuTTY's mode for running a command on the local
machine and using it as a proxy for the network connection. It
expects a shell command string as an argument.
See section 4.16.1 for more information on this, and on other proxy
settings. In particular, note that since the special sequences
described there are understood in the argument string, literal
backslashes must be doubled (if you want `\' in your command, you
must put `\\' on the command line).
3.11.3.28 `-restrict-acl': restrict the Windows process ACL
This option (on Windows only) causes PuTTY (or another PuTTY tool)
to try to lock down the operating system's access control on its own
process. If this succeeds, it should present an extra obstacle to
malware that has managed to run under the same user id as the PuTTY
process, by preventing it from attaching to PuTTY using the same
interfaces debuggers use and either reading sensitive information
out of its memory or hijacking its network session.
This option is not enabled by default, because this form of
interaction between Windows programs has many legitimate uses,
including accessibility software such as screen readers. Also,
it cannot provide full security against this class of attack in
any case, because PuTTY can only lock down its own ACL _after_ it
has started up, and malware could still get in if it attacks the
process between startup and lockdown. So it trades away noticeable
convenience, and delivers less real security than you might want.
However, if you do want to make that tradeoff anyway, the option is
available.
A PuTTY process started with `-restrict-acl' will pass that on to
any processes started with Duplicate Session, New Session etc.
(However, if you're invoking PuTTY tools explicitly, for instance as
a proxy command, you'll need to arrange to pass them the `-restrict-
acl' option yourself, if that's what you want.)
If Pageant is started with the `-restrict-acl' option, and you use
it to launch a PuTTY session from its System Tray submenu, then
Pageant will _not_ default to starting the PuTTY subprocess with
a restricted ACL. This is because PuTTY is more likely to suffer
reduced functionality as a result of restricted ACLs (e.g. screen
reader software will have a greater need to interact with it),
whereas Pageant stores the more critical information (hence benefits
more from the extra protection), so it's reasonable to want to run
Pageant but not PuTTY with the ACL restrictions. You can force
Pageant to start subsidiary PuTTY processes with a restricted ACL if
you also pass the `-restrict-putty-acl' option.
3.11.3.29 `-host-ca': launch the host CA configuration
If you start PuTTY with the `-host-ca' option, it will not launch
a session at all. Instead, it will just display the configuration
dialog box for host certification authorities, as described in
section 4.19.4. When you dismiss that dialog box, PuTTY will
terminate.
3.11.3.30 `-legacy-stdio-prompts': handle Windows console prompts like
older versions of PuTTY
This option applies to all of PSCP, PSFTP and Plink on Windows: all
the tools in the PuTTY suite that run in a Windows console and make
SSH connections.
These tools use the Windows console to prompt for various
information: usernames, passwords, answers to questions about host
keys, and so on.
In current versions of PuTTY, these prompts work by direct access
to the Windows console. This means that even if you redirect the
standard input or output of the tool, prompts will _still_ be sent
to the console (and not where you've redirected your output), and
the user's responses will be read from the console (and not from
where you've redirected your input).
Another advantage of reading directly from the Windows console is
that the tools can read input as Unicode. So this also allows you
to enter usernames and passwords that contain characters not in the
Windows system's default character set.
In versions of the PuTTY tools up to and including 0.81, the prompts
used the tool's ordinary I/O handles, so prompt output and user
responses could be redirected.
We think the new behaviour is more likely to be useful. For example,
if you have a local command that generates output, and you want to
pipe that output into a command running remotely via Plink, you can
run a command line such as
local_command | plink hostname remote_command
and the data piped into the remote command will be the same whether
or not Plink has to stop to ask for a password. With the old
behaviour you would have had to include the password in Plink's
input, which is more awkward.
However, we recognise that people may have customised complicated
workflows around the old behaviour. So if you need to switch back
to it, you can do so by specifying `-legacy-stdio-prompts' on the
command-line.
To fully revert to the previous behaviour, you'd also need to
specify `-legacy-charset-handling' (see the next section). (Even
without that option, prompt handling with `-legacy-stdio-prompts'
may not be fully Unicode-clean.)
3.11.3.31 `-legacy-charset-handling': handle character set in prompts like
older versions of PuTTY
This option applies to PuTTY (on all platforms), and also to all of
PSCP, PSFTP and Plink on Windows.
In current versions of PuTTY, when you are prompted in the terminal
window for things like SSH usernames and passwords, the responses
you type are interpreted as Unicode, and transmitted to the server
as such, even if the terminal is otherwise configured to use a
different character encoding (see section 4.10.1). Similarly, the
same prompts from the Windows console tools will unconditionally
interpret their input as Unicode.
This behaviour is in line with the SSH standards; it allows things
like usernames to use the full character set of the user's native
language, and ensures that different keystrokes you type for your
password are actually treated distinctly.
However, if you are used to the behaviour of the PuTTY tools up to
version 0.81, this could cause a previously working username and/or
password not to work as you expected. For instance, if you had
set a password including some accented characters, this change in
behaviour could cause the same keystrokes you've always entered to
start sending a different sequence of bytes to the server, denying
you access (and you wouldn't even be able to see the difference,
since the password is not shown when you type it).
`-legacy-charset-handling' reverts the PuTTY tools' behaviour to
how it was previously: what you type at these prompts will be
interpreted according to the `Remote character set' (for PuTTY) or
Windows' default character set (for the Windows console tools).
(For example, this could allow you to log in to change your password
to make using this option unnecessary in future. But if you're doing
that, make sure the terminal is configured as UTF-8!)
Chapter 4: Configuring PuTTY
----------------------------
This chapter describes all the configuration options in PuTTY.
PuTTY is configured using the control panel that comes up before you
start a session. Some options can also be changed in the middle of a
session, by selecting `Change Settings' from the window menu.
4.1 The Session panel
The Session configuration panel contains the basic options you need
to specify in order to open a session at all, and also allows you to
save your settings to be reloaded later.
4.1.1 The host name section
The top box on the Session panel, labelled `Specify the destination
you want to connect to', contains the details that need to be filled
in before PuTTY can open a session at all.
- The `Host Name' box is where you type the name, or the IP
address, of the server you want to connect to.
- The `Connection type' controls let you choose what type of
connection you want to make: an SSH network connection, a
connection to a local serial line, or various other kinds of
network connection.
- See section 1.2 for a summary of the differences between
the network remote login protocols SSH, Telnet, Rlogin, and
SUPDUP.
- See section 3.6 for information about using a serial line.
- See section 3.7 for an explanation of `raw' connections.
- See section 3.8 for a little information about Telnet.
- See section 3.9 for information about using Rlogin.
- See section 3.10 for information about using SUPDUP.
- The `Bare ssh-connection' option in the `Connection type'
control is intended for specialist uses not involving
network connections. See section 4.28 for some information
about it.
- The `Port' box lets you specify which port number on the server
to connect to. If you select Telnet, Rlogin, SUPDUP, or SSH,
this box will be filled in automatically to the usual value, and
you will only need to change it if you have an unusual server.
If you select Raw mode, you will almost certainly need to fill
in the `Port' box yourself.
If you select `Serial' from the `Connection type' radio buttons,
the `Host Name' and `Port' boxes are replaced by `Serial line' and
`Speed'; see section 4.29 for more details of these.
4.1.2 Loading and storing saved sessions
The next part of the Session configuration panel allows you to save
your preferred PuTTY options so they will appear automatically the
next time you start PuTTY. It also allows you to create _saved
sessions_, which contain a full set of configuration options plus a
host name and protocol. A saved session contains all the information
PuTTY needs to start exactly the session you want.
- To save your default settings: first set up the settings the way
you want them saved. Then come back to the Session panel. Select
the `Default Settings' entry in the saved sessions list, with a
single click. Then press the `Save' button.
If there is a specific host you want to store the details of how
to connect to, you should create a saved session, which will be
separate from the Default Settings.
- To save a session: first go through the rest of the
configuration box setting up all the options you want. Then come
back to the Session panel. Enter a name for the saved session in
the `Saved Sessions' input box. (The server name is often a good
choice for a saved session name.) Then press the `Save' button.
Your saved session name should now appear in the list box.
You can also save settings in mid-session, from the `Change
Settings' dialog. Settings changed since the start of the
session will be saved with their current values; as well as
settings changed through the dialog, this includes changes in
window size, window title changes sent by the server, and so on.
- To reload a saved session: single-click to select the session
name in the list box, and then press the `Load' button. Your
saved settings should all appear in the configuration panel.
- To modify a saved session: first load it as described above.
Then make the changes you want. Come back to the Session panel,
and press the `Save' button. The new settings will be saved over
the top of the old ones.
To save the new settings under a different name, you can enter
the new name in the `Saved Sessions' box, or single-click to
select a session name in the list box to overwrite that session.
To save `Default Settings', you must single-click the name
before saving.
- To start a saved session immediately: double-click on the
session name in the list box.
- To delete a saved session: single-click to select the session
name in the list box, and then press the `Delete' button.
Each saved session is independent of the Default Settings
configuration. If you change your preferences and update Default
Settings, you must also update every saved session separately.
Saved sessions are stored in the Registry, at the location
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\Sessions
If you need to store them in a file, you could try the method
described in section 4.33.
4.1.3 `Close window on exit'
Finally in the Session panel, there is an option labelled `Close
window on exit'. This controls whether the PuTTY terminal window
disappears as soon as the session inside it terminates. If you are
likely to want to copy and paste text out of the session after it
has terminated, or restart the session, you should arrange for this
option to be off.
`Close window on exit' has three settings. `Always' means always
close the window on exit; `Never' means never close on exit (always
leave the window open, but inactive). The third setting, and the
default one, is `Only on clean exit'. In this mode, a session which
terminates normally will cause its window to close, but one which is
aborted unexpectedly by network trouble or a confusing message from
the server will leave the window up.
4.2 The Logging panel
The Logging configuration panel allows you to save log files of your
PuTTY sessions, for debugging, analysis or future reference.
The main option is a radio-button set that specifies whether PuTTY
will log anything at all. The options are:
- `None'. This is the default option; in this mode PuTTY will not
create a log file at all.
- `Printable output'. In this mode, a log file will be created
and written to, but only printable text will be saved into it.
The various terminal control codes that are typically sent down
an interactive session alongside the printable text will be
omitted. This might be a useful mode if you want to read a log
file in a text editor and hope to be able to make sense of it.
- `All session output'. In this mode, _everything_ sent by the
server into your terminal session is logged. If you view the log
file in a text editor, therefore, you may well find it full of
strange control characters. This is a particularly useful mode
if you are experiencing problems with PuTTY's terminal handling:
you can record everything that went to the terminal, so that
someone else can replay the session later in slow motion and
watch to see what went wrong.
- `SSH packets'. In this mode (which is only used by SSH
connections), the SSH message packets sent over the encrypted
connection are written to the log file (as well as Event Log
entries). You might need this to debug a network-level problem,
or more likely to send to the PuTTY authors as part of a bug
report. _BE WARNED_ that if you log in using a password, the
password can appear in the log file; see section 4.2.5 for
options that may help to remove sensitive material from the log
file before you send it to anyone else.
- `SSH packets and raw data'. In this mode, as well as the
decrypted packets (as in the previous mode), the _raw_
(encrypted, compressed, etc) packets are _also_ logged. This
could be useful to diagnose corruption in transit. (The same
caveats as the previous mode apply, of course.)
Note that the non-SSH logging options (`Printable output' and `All
session output') only work with PuTTY proper; in programs without
terminal emulation (such as Plink), they will have no effect, even
if enabled via saved settings.
4.2.1 `Log file name'
In this edit box you enter the name of the file you want to log the
session to. The `Browse' button will let you look around your file
system to find the right place to put the file; or if you already
know exactly where you want it to go, you can just type a pathname
into the edit box.
There are a few special features in this box. If you use the `&'
character in the file name box, PuTTY will insert details of the
current session in the name of the file it actually opens. The
precise replacements it will do are:
- `&Y' will be replaced by the current year, as four digits.
- `&M' will be replaced by the current month, as two digits.
- `&D' will be replaced by the current day of the month, as two
digits.
- `&T' will be replaced by the current time, as six digits
(HHMMSS) with no punctuation.
- `&H' will be replaced by the host name you are connecting to (or
the serial line, for a serial connection).
- `&P' will be replaced by the port number you are connecting to
on the target host.
(These are all case-insensitive.)
For example, if you enter the file name `c:\puttylogs\log-&h-&y&m&d-
&t.dat', you will end up with files looking like
log-server1.example.com-20010528-110859.dat
log-unixbox.somewhere.org-20010611-221001.dat
4.2.2 `What to do if the log file already exists'
This control allows you to specify what PuTTY should do if it tries
to start writing to a log file and it finds the file already exists.
You might want to automatically destroy the existing log file and
start a new one with the same name. Alternatively, you might want to
open the existing log file and add data to the _end_ of it. Finally
(the default option), you might not want to have any automatic
behaviour, but to ask the user every time the problem comes up.
4.2.3 `Flush log file frequently'
This option allows you to control how frequently logged data is
flushed to disc. By default, PuTTY will flush data as soon as it
is displayed, so that if you view the log file while a session is
still open, it will be up to date; and if the client system crashes,
there's a greater chance that the data will be preserved.
However, this can incur a performance penalty. If PuTTY is running
slowly with logging enabled, you could try unchecking this option.
Be warned that the log file may not always be up to date as a result
(although it will of course be flushed when it is closed, for
instance at the end of a session).
4.2.4 `Include header'
This option allows you to choose whether to include a header line
with the date and time when the log file is opened. It may be useful
to disable this if the log file is being used as realtime input to
other programs that don't expect the header line.
4.2.5 Options specific to SSH packet logging
These options only apply if SSH packet data is being logged.
The following options allow particularly sensitive portions of
unencrypted packets to be automatically left out of the log file.
They are only intended to deter casual nosiness; an attacker could
glean a lot of useful information from even these obfuscated logs
(e.g., length of password).
4.2.5.1 `Omit known password fields'
When checked, decrypted password fields are removed from the log of
transmitted packets. (This includes any user responses to challenge-
response authentication methods such as `keyboard-interactive'.)
This does not include X11 authentication data if using X11
forwarding.
Note that this will only omit data that PuTTY _knows_ to be a
password. However, if you start another login session within your
PuTTY session, for instance, any password used will appear in the
clear in the packet log. The next option may be of use to protect
against this.
This option is enabled by default.
4.2.5.2 `Omit session data'
When checked, all decrypted `session data' is omitted; this is
defined as data in terminal sessions and in forwarded channels (TCP,
X11, and authentication agent). This will usually substantially
reduce the size of the resulting log file.
This option is disabled by default.
4.3 The Terminal panel
The Terminal configuration panel allows you to control the behaviour
of PuTTY's terminal emulation.
4.3.1 `Auto wrap mode initially on'
Auto wrap mode controls what happens when text printed in a PuTTY
window reaches the right-hand edge of the window.
With auto wrap mode on, if a long line of text reaches the right-
hand edge, it will wrap over on to the next line so you can still
see all the text. With auto wrap mode off, the cursor will stay at
the right-hand edge of the screen, and all the characters in the
line will be printed on top of each other.
If you are running a full-screen application and you occasionally
find the screen scrolling up when it looks as if it shouldn't, you
could try turning this option off.
Auto wrap mode can be turned on and off by control sequences sent by
the server. This configuration option controls the _default_ state,
which will be restored when you reset the terminal (see section
3.1.3.6). However, if you modify this option in mid-session using
`Change Settings', it will take effect immediately.
4.3.2 `DEC Origin Mode initially on'
DEC Origin Mode is a minor option which controls how PuTTY
interprets cursor-position control sequences sent by the server.
The server can send a control sequence that restricts the scrolling
region of the display. For example, in an editor, the server might
reserve a line at the top of the screen and a line at the bottom,
and might send a control sequence that causes scrolling operations
to affect only the remaining lines.
With DEC Origin Mode on, cursor coordinates are counted from the top
of the scrolling region. With it turned off, cursor coordinates are
counted from the top of the whole screen regardless of the scrolling
region.
It is unlikely you would need to change this option, but if you find
a full-screen application is displaying pieces of text in what looks
like the wrong part of the screen, you could try turning DEC Origin
Mode on to see whether that helps.
DEC Origin Mode can be turned on and off by control sequences sent
by the server. This configuration option controls the _default_
state, which will be restored when you reset the terminal (see
section 3.1.3.6). However, if you modify this option in mid-session
using `Change Settings', it will take effect immediately.
4.3.3 `Implicit CR in every LF'
Most servers send two control characters, CR and LF, to start a new
line of the screen. The CR character makes the cursor return to the
left-hand side of the screen. The LF character makes the cursor move
one line down (and might make the screen scroll).
Some servers only send LF, and expect the terminal to move the
cursor over to the left automatically. If you come across a server
that does this, you will see a stepped effect on the screen, like
this:
First line of text
Second line
Third line
If this happens to you, try enabling the `Implicit CR in every LF'
option, and things might go back to normal:
First line of text
Second line
Third line
4.3.4 `Implicit LF in every CR'
Most servers send two control characters, CR and LF, to start a new
line of the screen. The CR character makes the cursor return to the
left-hand side of the screen. The LF character makes the cursor move
one line down (and might make the screen scroll).
Some servers only send CR, and so the newly written line is
overwritten by the following line. This option causes a line feed so
that all lines are displayed.
4.3.5 `Use background colour to erase screen'
Not all terminals agree on what colour to turn the screen when the
server sends a `clear screen' sequence. Some terminals believe the
screen should always be cleared to the _default_ background colour.
Others believe the screen should be cleared to whatever the server
has selected as a background colour.
There exist applications that expect both kinds of behaviour.
Therefore, PuTTY can be configured to do either.
With this option disabled, screen clearing is always done in the
default background colour. With this option enabled, it is done in
the _current_ background colour.
Background-colour erase can be turned on and off by control
sequences sent by the server. This configuration option controls the
_default_ state, which will be restored when you reset the terminal
(see section 3.1.3.6). However, if you modify this option in mid-
session using `Change Settings', it will take effect immediately.
4.3.6 `Enable blinking text'
The server can ask PuTTY to display text that blinks on and off.
This is very distracting, so PuTTY allows you to turn blinking text
off completely.
When blinking text is disabled and the server attempts to make
some text blink, PuTTY will instead display the text with a bolded
background colour.
Blinking text can be turned on and off by control sequences sent by
the server. This configuration option controls the _default_ state,
which will be restored when you reset the terminal (see section
3.1.3.6). However, if you modify this option in mid-session using
`Change Settings', it will take effect immediately.
4.3.7 `Answerback to ^E'
This option controls what PuTTY will send back to the server if the
server sends it the ^E enquiry character. Normally it just sends the
string `PuTTY'.
If you accidentally write the contents of a binary file to your
terminal, you will probably find that it contains more than one ^E
character, and as a result your next command line will probably
read `PuTTYPuTTYPuTTY...' as if you had typed the answerback string
multiple times at the keyboard. If you set the answerback string
to be empty, this problem should go away, but doing so might cause
other problems.
Note that this is _not_ the feature of PuTTY which the server will
typically use to determine your terminal type. That feature is the
`Terminal-type string' in the Connection panel; see section 4.15.3
for details.
You can include control characters in the answerback string using
`^C' notation. (Use `^~' to get a literal `^'.)
4.3.8 `Local echo'
With local echo disabled, characters you type into the PuTTY window
are not echoed in the window _by PuTTY_. They are simply sent to the
server. (The _server_ might choose to echo them back to you; this
can't be controlled from the PuTTY control panel.)
Some types of session need local echo, and many do not. In its
default mode, PuTTY will automatically attempt to deduce whether
or not local echo is appropriate for the session you are working
in. If you find it has made the wrong decision, you can use this
configuration option to override its choice: you can force local
echo to be turned on, or force it to be turned off, instead of
relying on the automatic detection.
4.3.9 `Local line editing'
Normally, every character you type into the PuTTY window is sent
immediately to the server the moment you type it.
If you enable local line editing, this changes. PuTTY will let you
edit a whole line at a time locally, and the line will only be sent
to the server when you press Return. If you make a mistake, you can
use the Backspace key to correct it before you press Return, and the
server will never see the mistake.
Since it is hard to edit a line locally without being able to see
it, local line editing is mostly used in conjunction with local echo
(section 4.3.8). This makes it ideal for use in raw mode or when
connecting to MUDs or talkers. (Although some more advanced MUDs do
occasionally turn local line editing on and turn local echo off, in
order to accept a password from the user.)
Some types of session need local line editing, and many do not. In
its default mode, PuTTY will automatically attempt to deduce whether
or not local line editing is appropriate for the session you are
working in. If you find it has made the wrong decision, you can use
this configuration option to override its choice: you can force
local line editing to be turned on, or force it to be turned off,
instead of relying on the automatic detection.
4.3.10 Remote-controlled printing
A lot of VT100-compatible terminals support printing under control
of the remote server (sometimes called `passthrough printing').
PuTTY supports this feature as well, but it is turned off by
default.
To enable remote-controlled printing, choose a printer from
the `Printer to send ANSI printer output to' drop-down list
box. This should allow you to select from all the printers you
have installed drivers for on your computer. Alternatively, you
can type the network name of a networked printer (for example,
`\\printserver\printer1') even if you haven't already installed a
driver for it on your own machine.
When the remote server attempts to print some data, PuTTY will send
that data to the printer _raw_ - without translating it, attempting
to format it, or doing anything else to it. It is up to you to
ensure your remote server knows what type of printer it is talking
to.
Since PuTTY sends data to the printer raw, it cannot offer options
such as portrait versus landscape, print quality, or paper tray
selection. All these things would be done by your PC printer driver
(which PuTTY bypasses); if you need them done, you will have to find
a way to configure your remote server to do them.
To disable remote printing again, choose `None (printing disabled)'
from the printer selection list. This is the default state.
4.4 The Keyboard panel
The Keyboard configuration panel allows you to control the behaviour
of the keyboard in PuTTY. The correct state for many of these
settings depends on what the server to which PuTTY is connecting
expects. With a Unix server, this is likely to depend on the
`termcap' or `terminfo' entry it uses, which in turn is likely to be
controlled by the `Terminal-type string' setting in the Connection
panel; see section 4.15.3 for details. If none of the settings here
seems to help, you may find question A.7.13 to be useful.
4.4.1 Changing the action of the Backspace key
Some terminals believe that the Backspace key should send the same
thing to the server as Control-H (ASCII code 8). Other terminals
believe that the Backspace key should send ASCII code 127 (usually
known as Control-?) so that it can be distinguished from Control-H.
This option allows you to choose which code PuTTY generates when you
press Backspace.
If you are connecting over SSH, PuTTY by default tells the server
the value of this option (see section 4.24.2), so you may find that
the Backspace key does the right thing either way. Similarly, if
you are connecting to a Unix system, you will probably find that
the Unix `stty' command lets you configure which the server expects
to see, so again you might not need to change which one PuTTY
generates. On other systems, the server's expectation might be fixed
and you might have no choice but to configure PuTTY.
If you do have the choice, we recommend configuring PuTTY to
generate Control-? and configuring the server to expect it, because
that allows applications such as `emacs' to use Control-H for help.
(Typing Shift-Backspace will cause PuTTY to send whichever code
isn't configured here as the default.)
4.4.2 Changing the action of the Home and End keys
The Unix terminal emulator `rxvt' disagrees with the rest of the
world about what character sequences should be sent to the server by
the Home and End keys.
`xterm', and other terminals, send `ESC [1~' for the Home key, and
`ESC [4~' for the End key. `rxvt' sends `ESC [H' for the Home key
and `ESC [Ow' for the End key.
If you find an application on which the Home and End keys aren't
working, you could try switching this option to see if it helps.
4.4.3 Changing the action of the function keys and keypad
This option affects the function keys (F1 to F12) and the top row of
the numeric keypad.
- In the default mode, labelled `ESC [n~', the function keys
generate sequences like `ESC [11~', `ESC [12~' and so on. This
matches the general behaviour of Digital's terminals.
- In Linux mode, F6 to F12 behave just like the default mode, but
F1 to F5 generate `ESC [[A' through to `ESC [[E'. This mimics
the Linux virtual console.
- In Xterm R6 mode, F5 to F12 behave like the default mode, but
F1 to F4 generate `ESC OP' through to `ESC OS', which are the
sequences produced by the top row of the _keypad_ on Digital's
terminals.
- In VT400 mode, all the function keys behave like the default
mode, but the actual top row of the numeric keypad generates
`ESC OP' through to `ESC OS'.
- In VT100+ mode, the function keys generate `ESC OP' through to
`ESC O['
- In SCO mode, the function keys F1 to F12 generate `ESC [M'
through to `ESC [X'. Together with shift, they generate `ESC [Y'
through to `ESC [j'. With control they generate `ESC [k' through
to `ESC [v', and with shift and control together they generate
`ESC [w' through to `ESC [{'.
- In Xterm 216 mode, the unshifted function keys behave the same
as Xterm R6 mode. But pressing a function key together with
Shift or Alt or Ctrl generates a different sequence containing
an extra numeric parameter of the form (1 for Shift) + (2 for
Alt) + (4 for Ctrl) + 1. For F1-F4, the basic sequences like
`ESC OP' become ESC [1;_bitmap_P and similar; for F5 and above,
ESC[_index_~ becomes ESC[_index_;_bitmap_~.
If you don't know what any of this means, you probably don't need to
fiddle with it.
4.4.4 Changing the action of the shifted arrow keys
This option affects the arrow keys, if you press one with any of the
modifier keys Shift, Ctrl or Alt held down.
- In the default mode, labelled `Ctrl toggles app mode', the
Ctrl key toggles between the default arrow-key sequences like
`ESC [A' and `ESC [B', and the sequences Digital's terminals
generate in `application cursor keys' mode, i.e. `ESC O A' and
so on. Shift and Alt have no effect.
- In the `xterm-style bitmap' mode, Shift, Ctrl and Alt all
generate different sequences, with a number indicating which set
of modifiers is active.
If you don't know what any of this means, you probably don't need to
fiddle with it.
4.4.5 Controlling Application Cursor Keys mode
Application Cursor Keys mode is a way for the server to change the
control sequences sent by the arrow keys. In normal mode, the arrow
keys send `ESC [A' through to `ESC [D'. In application mode, they
send `ESC OA' through to `ESC OD'.
Application Cursor Keys mode can be turned on and off by the server,
depending on the application. PuTTY allows you to configure the
initial state.
You can also disable application cursor keys mode completely, using
the `Features' configuration panel; see section 4.6.1.
4.4.6 Controlling Application Keypad mode
Application Keypad mode is a way for the server to change the
behaviour of the numeric keypad.
In normal mode, the keypad behaves like a normal Windows keypad:
with NumLock on, the number keys generate numbers, and with NumLock
off they act like the arrow keys and Home, End etc.
In application mode, all the keypad keys send special control
sequences, _including_ Num Lock. Num Lock stops behaving like Num
Lock and becomes another function key.
Depending on which version of Windows you run, you may find the Num
Lock light still flashes on and off every time you press Num Lock,
even when application mode is active and Num Lock is acting like a
function key. This is unavoidable.
Application keypad mode can be turned on and off by the server,
depending on the application. PuTTY allows you to configure the
initial state.
You can also disable application keypad mode completely, using the
`Features' configuration panel; see section 4.6.1.
4.4.7 Using NetHack keypad mode
PuTTY has a special mode for playing NetHack. You can enable it
by selecting `NetHack' in the `Initial state of numeric keypad'
control.
In this mode, the numeric keypad keys 1-9 generate the NetHack
movement commands (hjklyubn). The 5 key generates the `.' command
(do nothing).
In addition, pressing Shift or Ctrl with the keypad keys generate
the Shift- or Ctrl-keys you would expect (e.g. keypad-7 generates
`y', so Shift-keypad-7 generates `Y' and Ctrl-keypad-7 generates
Ctrl-Y); these commands tell NetHack to keep moving you in the same
direction until you encounter something interesting.
For some reason, this feature only works properly when Num Lock is
on. We don't know why.
4.4.8 Enabling a DEC-like Compose key
DEC terminals have a Compose key, which provides an easy-to-remember
way of typing accented characters. You press Compose and then type
two more characters. The two characters are `combined' to produce
an accented character. The choices of character are designed to be
easy to remember; for example, composing `e' and ``' produces the
`e-grave' character.
If your keyboard has a Windows Application key, it acts as a Compose
key in PuTTY. Alternatively, if you enable the `AltGr acts as
Compose key' option, the AltGr key will become a Compose key.
4.4.9 `Control-Alt is different from AltGr'
Some old keyboards do not have an AltGr key, which can make it
difficult to type some characters. PuTTY can be configured to treat
the key combination Ctrl + Left Alt the same way as the AltGr key.
By default, this checkbox is checked, and the key combination Ctrl +
Left Alt does something completely different. PuTTY's usual handling
of the left Alt key is to prefix the Escape (Control-[) character to
whatever character sequence the rest of the keypress would generate.
For example, Alt-A generates Escape followed by `a'. So Alt-Ctrl-A
would generate Escape, followed by Control-A.
If you uncheck this box, Ctrl-Alt will become a synonym for AltGr,
so you can use it to type extra graphic characters if your keyboard
has any.
(However, Ctrl-Alt will never act as a Compose key, regardless of
the setting of `AltGr acts as Compose key' described in section
4.4.8.)
4.5 The Bell panel
The Bell panel controls the terminal bell feature: the server's
ability to cause PuTTY to beep at you.
In the default configuration, when the server sends the character
with ASCII code 7 (Control-G), PuTTY will play the Windows Default
Beep sound. This is not always what you want the terminal bell
feature to do; the Bell panel allows you to configure alternative
actions.
4.5.1 `Set the style of bell'
This control allows you to select various different actions to occur
on a terminal bell:
- Selecting `None' disables the bell completely. In this mode, the
server can send as many Control-G characters as it likes and
nothing at all will happen.
- `Make default system alert sound' is the default setting.
It causes the Windows `Default Beep' sound to be played. To
change what this sound is, or to test it if nothing seems to
be happening, use the Sound configurer in the Windows Control
Panel.
- `Visual bell' is a silent alternative to a beeping computer. In
this mode, when the server sends a Control-G, the whole PuTTY
window will flash white for a fraction of a second.
- `Beep using the PC speaker' is self-explanatory.
- `Play a custom sound file' allows you to specify a particular
sound file to be used by PuTTY alone, or even by a particular
individual PuTTY session. This allows you to distinguish your
PuTTY beeps from any other beeps on the system. If you select
this option, you will also need to enter the name of your sound
file in the edit control `Custom sound file to play as a bell'.
4.5.2 `Taskbar/caption indication on bell'
This feature controls what happens to the PuTTY window's entry in
the Windows Taskbar if a bell occurs while the window does not have
the input focus.
In the default state (`Disabled') nothing unusual happens.
If you select `Steady', then when a bell occurs and the window is
not in focus, the window's Taskbar entry and its title bar will
change colour to let you know that PuTTY session is asking for your
attention. The change of colour will persist until you select the
window, so you can leave several PuTTY windows minimised in your
terminal, go away from your keyboard, and be sure not to have missed
any important beeps when you get back.
`Flashing' is even more eye-catching: the Taskbar entry will
continuously flash on and off until you select the window.
4.5.3 `Control the bell overload behaviour'
A common user error in a terminal session is to accidentally run the
Unix command `cat' (or equivalent) on an inappropriate file type,
such as an executable, image file, or ZIP file. This produces a huge
stream of non-text characters sent to the terminal, which typically
includes a lot of bell characters. As a result of this the terminal
often doesn't stop beeping for ten minutes, and everybody else in
the office gets annoyed.
To try to avoid this behaviour, or any other cause of excessive
beeping, PuTTY includes a bell overload management feature. In the
default configuration, receiving more than five bell characters in a
two-second period will cause the overload feature to activate. Once
the overload feature is active, further bells will have no effect
at all, so the rest of your binary file will be sent to the screen
in silence. After a period of five seconds during which no further
bells are received, the overload feature will turn itself off again
and bells will be re-enabled.
If you want this feature completely disabled, you can turn it off
using the checkbox `Bell is temporarily disabled when over-used'.
Alternatively, if you like the bell overload feature but don't
agree with the settings, you can configure the details: how many
bells constitute an overload, how short a time period they have to
arrive in to do so, and how much silent time is required before the
overload feature will deactivate itself.
Bell overload mode is always deactivated by any keypress in the
terminal. This means it can respond to large unexpected streams of
data, but does not interfere with ordinary command-line activities
that generate beeps (such as filename completion).
4.6 The Features panel
PuTTY's terminal emulation is very highly featured, and can do a
lot of things under remote server control. Some of these features
can cause problems due to buggy or strangely configured server
applications.
The Features configuration panel allows you to disable some of
PuTTY's more advanced terminal features, in case they cause trouble.
4.6.1 Disabling application keypad and cursor keys
Application keypad mode (see section 4.4.6) and application cursor
keys mode (see section 4.4.5) alter the behaviour of the keypad and
cursor keys. Some applications enable these modes but then do not
deal correctly with the modified keys. You can force these modes to
be permanently disabled no matter what the server tries to do.
4.6.2 Disabling xterm-style mouse reporting
PuTTY allows the server to send control codes that let it take
over the mouse and use it for purposes other than copy and paste.
Applications which use this feature include the text-mode web
browser `links', the Usenet newsreader `trn' version 4, and the file
manager `mc' (Midnight Commander).
If you find this feature inconvenient, you can disable it using the
`Disable xterm-style mouse reporting' control. With this box ticked,
the mouse will _always_ do copy and paste in the normal way.
Note that even if the application takes over the mouse, you can
still manage PuTTY's copy and paste by holding down the Shift key
while you select and paste, unless you have deliberately turned this
feature off (see section 4.11.2).
4.6.3 Disabling remote terminal resizing
PuTTY has the ability to change the terminal's size and position in
response to commands from the server. If you find PuTTY is doing
this unexpectedly or inconveniently, you can tell PuTTY not to
respond to those server commands.
4.6.4 Disabling switching to the alternate screen
Many terminals, including PuTTY, support an `alternate screen'. This
is the same size as the ordinary terminal screen, but separate.
Typically a screen-based program such as a text editor might switch
the terminal to the alternate screen before starting up. Then at the
end of the run, it switches back to the primary screen, and you see
the screen contents just as they were before starting the editor.
Some people prefer this not to happen. If you want your editor to
run in the same screen as the rest of your terminal activity, you
can disable the alternate screen feature completely.
4.6.5 Disabling remote window title changing
PuTTY has the ability to change the window title in response
to commands from the server. If you find PuTTY is doing this
unexpectedly or inconveniently, you can tell PuTTY not to respond to
those server commands.
4.6.6 Response to remote window title querying
PuTTY can optionally provide the xterm service of allowing server
applications to find out the local window title. This feature is
disabled by default, but you can turn it on if you really want it.
NOTE that this feature is a _potential security hazard_. If a
malicious application can write data to your terminal (for example,
if you merely `cat' a file owned by someone else on the server
machine), it can change your window title (unless you have disabled
this as mentioned in section 4.6.5) and then use this service to
have the new window title sent back to the server as if typed at the
keyboard. This allows an attacker to fake keypresses and potentially
cause your server-side applications to do things you didn't want.
Therefore this feature is disabled by default, and we recommend you
do not set it to `Window title' unless you _really_ know what you
are doing.
There are three settings for this option:
`None'
PuTTY makes no response whatsoever to the relevant escape
sequence. This may upset server-side software that is expecting
some sort of response.
`Empty string'
PuTTY makes a well-formed response, but leaves it blank. Thus,
server-side software that expects a response is kept happy,
but an attacker cannot influence the response string. This is
probably the setting you want if you have no better ideas.
`Window title'
PuTTY responds with the actual window title. This is dangerous
for the reasons described above.
4.6.7 Disabling remote scrollback clearing
PuTTY has the ability to clear the terminal's scrollback buffer in
response to a command from the server. If you find PuTTY is doing
this unexpectedly or inconveniently, you can tell PuTTY not to
respond to that server command.
4.6.8 Disabling destructive backspace
Normally, when PuTTY receives character 127 (^?) from the server,
it will perform a `destructive backspace': move the cursor one
space left and delete the character under it. This can apparently
cause problems in some applications, so PuTTY provides the ability
to configure character 127 to perform a normal backspace (without
deleting a character) instead.
4.6.9 Disabling remote character set configuration
PuTTY has the ability to change its character set configuration
in response to commands from the server. Some programs send these
commands unexpectedly or inconveniently. In particular, BitchX (an
IRC client) seems to have a habit of reconfiguring the character set
to something other than the user intended.
If you find that accented characters are not showing up the way you
expect them to, particularly if you're running BitchX, you could try
disabling the remote character set configuration commands.
4.6.10 Disabling Arabic text shaping
PuTTY supports shaping of Arabic text, which means that if your
server sends text written in the basic Unicode Arabic alphabet then
it will convert it to the correct display forms before printing it
on the screen.
If you are using full-screen software which was not expecting
this to happen (especially if you are not an Arabic speaker and
you unexpectedly find yourself dealing with Arabic text files in
applications which are not Arabic-aware), you might find that the
display becomes corrupted. By ticking this box, you can disable
Arabic text shaping so that PuTTY displays precisely the characters
it is told to display.
You may also find you need to disable bidirectional text display;
see section 4.6.11.
4.6.11 Disabling bidirectional text display
PuTTY supports bidirectional text display, which means that if your
server sends text written in a language which is usually displayed
from right to left (such as Arabic or Hebrew) then PuTTY will
automatically flip it round so that it is displayed in the right
direction on the screen.
If you are using full-screen software which was not expecting
this to happen (especially if you are not an Arabic speaker and
you unexpectedly find yourself dealing with Arabic text files in
applications which are not Arabic-aware), you might find that the
display becomes corrupted. By ticking this box, you can disable
bidirectional text display, so that PuTTY displays text from left to
right in all situations.
You may also find you need to disable Arabic text shaping; see
section 4.6.10.
4.6.12 Disabling bracketed paste mode
By default, when you paste text into the terminal window, it's sent
to the server as terminal input, exactly as if you'd typed the same
text into the terminal window using the keyboard (except that it's
all sent at once, much faster than you could type it).
However, a terminal application can change that, by asking the
terminal to enable `bracketed paste mode'. In this mode, pasted data
is marked in the input stream, by sending a special control sequence
before the paste, and another one at the end.
A terminal application can use this information to treat pasted data
differently from keyboard input. For example, a terminal-based text
editor can treat the input as literal data, even if some of its
characters would normally trigger special editor functions. A shell
can treat pasted input as less trusted, in case another application
somehow sneaked a malicious shell command into your clipboard:
modern versions of bash will highlight pasted data on the command
line, and not run it until you've confirmed it by pressing Return,
even if the pasted data contained a newline character.
In edge cases, it's possible that bracketed paste mode introduces
bigger problems than the ones it solves. So you can use this
checkbox to turn it off completely. If you do that, then PuTTY will
always send your paste data exactly as if it had been typed at the
keyboard, whether or not the server asked for bracketed paste mode.
4.7 The Window panel
The Window configuration panel allows you to control aspects of the
PuTTY window.
4.7.1 Setting the size of the PuTTY window
The `Columns' and `Rows' boxes let you set the PuTTY window to a
precise size. Of course you can also drag the window to a new size
while a session is running.
4.7.2 What to do when the window is resized
These options allow you to control what happens when the user tries
to resize the PuTTY window using its window furniture.
There are four options here:
- `Change the number of rows and columns': the font size will not
change. (This is the default.)
- `Change the size of the font': the number of rows and columns in
the terminal will stay the same, and the font size will change.
- `Change font size when maximised': when the window is resized,
the number of rows and columns will change, _except_ when the
window is maximised (or restored), when the font size will
change. (In this mode, holding down the Alt key while resizing
will also cause the font size to change.)
- `Forbid resizing completely': the terminal will refuse to be
resized at all.
4.7.3 Controlling scrollback
These options let you configure the way PuTTY keeps text after it
scrolls off the top of the screen (see section 3.1.2).
The `Lines of scrollback' box lets you configure how many lines of
text PuTTY keeps. The `Display scrollbar' options allow you to hide
the scrollbar (although you can still view the scrollback using
the keyboard as described in section 3.1.2). You can separately
configure whether the scrollbar is shown in full-screen mode and in
normal modes.
If you are viewing part of the scrollback when the server sends
more text to PuTTY, the screen will revert to showing the current
terminal contents. You can disable this behaviour by turning off
`Reset scrollback on display activity'. You can also make the screen
revert when you press a key, by turning on `Reset scrollback on
keypress'.
4.7.4 `Push erased text into scrollback'
When this option is enabled, the contents of the terminal screen
will be pushed into the scrollback when a server-side application
clears the screen, so that your scrollback will contain a better
record of what was on your screen in the past.
If the application switches to the alternate screen (see section
4.6.4 for more about this), then the contents of the primary screen
will be visible in the scrollback until the application switches
back again.
This option is enabled by default.
4.8 The Appearance panel
The Appearance configuration panel allows you to control aspects of
the appearance of PuTTY's window.
4.8.1 Controlling the appearance of the cursor
The `Cursor appearance' option lets you configure the cursor to be a
block, an underline, or a vertical line. A block cursor becomes an
empty box when the window loses focus; an underline or a vertical
line becomes dotted.
The `Cursor blinks' option makes the cursor blink on and off. This
works in any of the cursor modes.
4.8.2 Controlling the font used in the terminal window
This option allows you to choose what font, in what size, the PuTTY
terminal window uses to display the text in the session.
By default, you will be offered a choice from all the fixed-width
fonts installed on the system, since VT100-style terminal handling
expects a fixed-width font. If you tick the box marked `Allow
selection of variable-pitch fonts', however, PuTTY will offer
variable-width fonts as well: if you select one of these, the font
will be coerced into fixed-size character cells, which will probably
not look very good (but can work OK with some fonts).
4.8.3 `Hide mouse pointer when typing in window'
If you enable this option, the mouse pointer will disappear if the
PuTTY window is selected and you press a key. This way, it will
not obscure any of the text in the window while you work in your
session. As soon as you move the mouse, the pointer will reappear.
This option is disabled by default, so the mouse pointer remains
visible at all times.
4.8.4 Controlling the window border
PuTTY allows you to configure the appearance of the window border to
some extent.
The checkbox marked `Sunken-edge border' changes the appearance of
the window border to something more like a DOS box: the inside edge
of the border is highlighted as if it sank down to meet the surface
inside the window. This makes the border a little bit thicker as
well. It's hard to describe well. Try it and see if you like it.
You can also configure a completely blank gap between the text in
the window and the border, using the `Gap between text and window
edge' control. By default this is set at one pixel. You can reduce
it to zero, or increase it further.
4.9 The Behaviour panel
The Behaviour configuration panel allows you to control aspects of
the behaviour of PuTTY's window.
4.9.1 Controlling the window title
The `Window title' edit box allows you to set the title of the PuTTY
window. By default the window title will contain the host name
followed by `PuTTY', for example `server1.example.com - PuTTY'. If
you want a different window title, this is where to set it.
PuTTY allows the server to send `xterm' control sequences which
modify the title of the window in mid-session (unless this is
disabled - see section 4.6.5); the title string set here is
therefore only the _initial_ window title.
As well as the _window_ title, there is also an `xterm' sequence
to modify the title of the window's _icon_. This makes sense in a
windowing system where the window becomes an icon when minimised,
such as Windows 3.1 or most X Window System setups; but in the
Windows 95-like user interface it isn't as applicable.
By default, PuTTY only uses the server-supplied _window_ title,
and ignores the icon title entirely. If for some reason you want
to see both titles, check the box marked `Separate window and icon
titles'. If you do this, PuTTY's window title and Taskbar caption
will change into the server-supplied icon title if you minimise the
PuTTY window, and change back to the server-supplied window title if
you restore it. (If the server has not bothered to supply a window
or icon title, none of this will happen.)
4.9.2 `Warn before closing window'
If you press the Close button in a PuTTY window that contains a
running session, PuTTY will put up a warning window asking if you
really meant to close the window. A window whose session has already
terminated can always be closed without a warning.
If you want to be able to close a window quickly, you can disable
the `Warn before closing window' option.
4.9.3 `Window closes on ALT-F4'
By default, pressing ALT-F4 causes the window to close (or a warning
box to appear; see section 4.9.2). If you disable the `Window closes
on ALT-F4' option, then pressing ALT-F4 will simply send a key
sequence to the server.
4.9.4 `System menu appears on ALT-Space'
If this option is enabled, then pressing ALT-Space will bring up the
PuTTY window's menu, like clicking on the top left corner. If it is
disabled, then pressing ALT-Space will just send `ESC SPACE' to the
server.
Some accessibility programs for Windows may need this option
enabling to be able to control PuTTY's window successfully. For
instance, Dragon NaturallySpeaking requires it both to open the
system menu via voice, and to close, minimise, maximise and restore
the window.
4.9.5 `System menu appears on Alt alone'
If this option is enabled, then pressing and releasing ALT will
bring up the PuTTY window's menu, like clicking on the top left
corner. If it is disabled, then pressing and releasing ALT will have
no effect.
4.9.6 `Ensure window is always on top'
If this option is enabled, the PuTTY window will stay on top of all
other windows.
4.9.7 `Full screen on Alt-Enter'
If this option is enabled, then pressing Alt-Enter will cause the
PuTTY window to become full-screen. Pressing Alt-Enter again will
restore the previous window size.
The full-screen feature is also available from the System menu, even
when it is configured not to be available on the Alt-Enter key. See
section 3.1.3.7.
4.10 The Translation panel
The Translation configuration panel allows you to control the
translation between the character set understood by the server and
the character set understood by PuTTY.
4.10.1 Controlling character set translation
During an interactive session, PuTTY receives a stream of 8-bit
bytes from the server, and in order to display them on the screen it
needs to know what character set to interpret them in. Similarly,
PuTTY needs to know how to translate your keystrokes into the
encoding the server expects. Unfortunately, there is no satisfactory
mechanism for PuTTY and the server to communicate this information,
so it must usually be manually configured.
There are a lot of character sets to choose from. The `Remote
character set' option lets you select one.
By default PuTTY will use the UTF-8 encoding of Unicode, which can
represent pretty much any character; data coming from the server
is interpreted as UTF-8, and keystrokes are sent UTF-8 encoded.
This is what most modern distributions of Linux will expect by
default. However, if this is wrong for your server, you can select a
different character set using this control.
A few other notable character sets are:
- The ISO-8859 series are all standard character sets that include
various accented characters appropriate for different sets of
languages.
- The Win125x series are defined by Microsoft, for similar
purposes. In particular Win1252 is almost equivalent to ISO-
8859-1, but contains a few extra characters such as matched
quotes and the Euro symbol.
- If you want the old IBM PC character set with block graphics and
line-drawing characters, you can select `CP437'.
If you need support for a numeric code page which is not listed in
the drop-down list, such as code page 866, then you can try entering
its name manually (`CP866' for example) in the list box. If the
underlying version of Windows has the appropriate translation table
installed, PuTTY will use it.
4.10.2 `Treat CJK ambiguous characters as wide'
There are some Unicode characters whose width is not well-defined.
In most contexts, such characters should be treated as single-
width for the purposes of wrapping and so on; however, in some CJK
contexts, they are better treated as double-width for historical
reasons, and some server-side applications may expect them to be
displayed as such. Setting this option will cause PuTTY to take the
double-width interpretation.
If you use legacy CJK applications, and you find your lines are
wrapping in the wrong places, or you are having other display
problems, you might want to play with this setting.
This option only has any effect in UTF-8 mode (see section 4.10.1).
4.10.3 `Caps Lock acts as Cyrillic switch'
This feature allows you to switch between a US/UK keyboard layout
and a Cyrillic keyboard layout by using the Caps Lock key, if you
need to type (for example) Russian and English side by side in the
same document.
Currently this feature is not expected to work properly if your
native keyboard layout is not US or UK.
4.10.4 Controlling display of line-drawing characters
VT100-series terminals allow the server to send control sequences
that shift temporarily into a separate character set for drawing
simple lines and boxes. However, there are a variety of ways in
which PuTTY can attempt to find appropriate characters, and the
right one to use depends on the locally configured font. In general
you should probably try lots of options until you find one that your
particular font supports.
- `Use Unicode line drawing code points' tries to use the box
characters that are present in Unicode. For good Unicode-
supporting fonts this is probably the most reliable and
functional option.
- `Poor man's line drawing' assumes that the font _cannot_
generate the line and box characters at all, so it will use the
`+', `-' and `|' characters to draw approximations to boxes. You
should use this option if none of the other options works.
- `Font has XWindows encoding' is for use with fonts that have a
special encoding, where the lowest 32 character positions (below
the ASCII printable range) contain the line-drawing characters.
This is unlikely to be the case with any standard Windows font;
it will probably only apply to custom-built fonts or fonts that
have been automatically converted from the X Window System.
- `Use font in both ANSI and OEM modes' tries to use the same font
in two different character sets, to obtain a wider range of
characters. This doesn't always work; some fonts claim to be a
different size depending on which character set you try to use.
- `Use font in OEM mode only' is more reliable than that, but can
miss out other characters from the main character set.
4.10.5 Controlling copy and paste of line drawing characters
By default, when you copy and paste a piece of the PuTTY screen that
contains VT100 line and box drawing characters, PuTTY will paste
them in the form they appear on the screen: either Unicode line
drawing code points, or the `poor man's' line-drawing characters
`+', `-' and `|'. The checkbox `Copy and paste VT100 line drawing
chars as lqqqk' disables this feature, so line-drawing characters
will be pasted as the ASCII characters that were printed to produce
them. This will typically mean they come out mostly as `q' and `x',
with a scattering of `jklmntuvw' at the corners. This might be
useful if you were trying to recreate the same box layout in another
program, for example.
Note that this option only applies to line-drawing characters which
_were_ printed by using the VT100 mechanism. Line-drawing characters
that were received as Unicode code points will paste as Unicode
always.
4.10.6 Combining VT100 line-drawing with UTF-8
If PuTTY is configured to treat data from the server as encoded in
UTF-8, then by default it disables the older VT100-style system
of control sequences that cause the lower-case letters to be
temporarily replaced by line drawing characters.
The rationale is that in UTF-8 mode you don't need those control
sequences anyway, because all the line-drawing characters they
access are available as Unicode characters already, so there's no
need for applications to put the terminal into a special state to
get at them.
Also, it removes a risk of the terminal _accidentally_ getting into
that state: if you accidentally write uncontrolled binary data to a
non-UTF-8 terminal, it can be surprisingly common to find that your
next shell prompt appears as a sequence of line-drawing characters
and then you have to remember or look up how to get out of that
mode. So by default, UTF-8 mode simply doesn't _have_ a confusing
mode like that to get into, accidentally or on purpose.
However, not all applications will see it that way. Even UTF-8
terminal users will still sometimes have to run software that tries
to print line-drawing characters in the old-fashioned way. So the
configuration option `Enable VT100 line drawing even in UTF-8 mode'
puts PuTTY into a hybrid mode in which it understands the VT100-
style control sequences that change the meaning of the ASCII lower
case letters, _and_ understands UTF-8.
4.11 The Selection panel
The Selection panel allows you to control the way copy and paste
work in the PuTTY window.
4.11.1 Changing the actions of the mouse buttons
PuTTY's copy and paste mechanism is by default modelled on the Unix
`xterm' application. The X Window System uses a three-button mouse,
and the convention in that system is that the left button selects,
the right button extends an existing selection, and the middle
button pastes.
Windows often only has two mouse buttons, so when run on
Windows, PuTTY is configurable. In PuTTY's default configuration
(`Compromise'), the _right_ button pastes, and the _middle_ button
(if you have one) extends a selection.
If you have a three-button mouse and you are already used to the
`xterm' arrangement, you can select it using the `Action of mouse
buttons' control.
Alternatively, with the `Windows' option selected, the middle button
extends, and the right button brings up a context menu (on which one
of the options is `Paste'). (This context menu is always available
by holding down Ctrl and right-clicking, regardless of the setting
of this option.)
(When PuTTY itself is running on Unix, it follows the X Window
System convention.)
4.11.2 `Shift overrides application's use of mouse'
PuTTY allows the server to send control codes that let it take
over the mouse and use it for purposes other than copy and paste.
Applications which use this feature include the text-mode web
browser `links', the Usenet newsreader `trn' version 4, and the file
manager `mc' (Midnight Commander).
When running one of these applications, pressing the mouse buttons
no longer performs copy and paste. If you do need to copy and paste,
you can still do so if you hold down Shift while you do your mouse
clicks.
However, it is possible in theory for applications to even detect
and make use of Shift + mouse clicks. We don't know of any
applications that do this, but in case someone ever writes one,
unchecking the `Shift overrides application's use of mouse' checkbox
will cause Shift + mouse clicks to go to the server as well (so that
mouse-driven copy and paste will be completely disabled).
If you want to prevent the application from taking over the mouse at
all, you can do this using the Features control panel; see section
4.6.2.
4.11.3 Default selection mode
As described in section 3.1.1, PuTTY has two modes of selecting
text to be copied to the clipboard. In the default mode (`Normal'),
dragging the mouse from point A to point B selects to the end of
the line containing A, all the lines in between, and from the very
beginning of the line containing B. In the other mode (`Rectangular
block'), dragging the mouse between two points defines a rectangle,
and everything within that rectangle is copied.
Normally, you have to hold down Alt while dragging the mouse to
select a rectangular block. Using the `Default selection mode'
control, you can set rectangular selection as the default, and then
you have to hold down Alt to get the _normal_ behaviour.
4.11.4 Assigning copy and paste actions to clipboards
Here you can configure which clipboard(s) are written or read by
PuTTY's various copy and paste actions.
Most platforms, including Windows, have a single system clipboard.
On these platforms, PuTTY provides a second clipboard-like
facility by permitting you to paste the text you last selected in
_this window_, whether or not it is currently also in the system
clipboard. This is not enabled by default.
The X Window System (which underlies most Unix graphical
interfaces) provides multiple clipboards (or `selections'), and
many applications support more than one of them by a different user
interface mechanism. When PuTTY itself is running on Unix, it has
more configurability relating to these selections.
The two most commonly used selections are called `PRIMARY' and
`CLIPBOARD'; in applications supporting both, the usual behaviour
is that PRIMARY is used by mouse-only actions (selecting text
automatically copies it to PRIMARY, and middle-clicking pastes from
PRIMARY), whereas CLIPBOARD is used by explicit Copy and Paste menu
items or keypresses such as Ctrl-C and Ctrl-V.
4.11.4.1 `Auto-copy selected text'
The checkbox `Auto-copy selected text to system clipboard' controls
whether or not selecting text in the PuTTY terminal window
automatically has the side effect of copying it to the system
clipboard, without requiring a separate user interface action.
On X, the wording of this option is changed slightly so that
`CLIPBOARD' is mentioned in place of the `system clipboard'. Text
selected in the terminal window will _always_ be automatically
placed in the PRIMARY selection, as is conventional, but if you tick
this box, it will _also_ be placed in `CLIPBOARD' at the same time.
4.11.4.2 Choosing a clipboard for UI actions
PuTTY has three user-interface actions which can be configured to
paste into the terminal (not counting menu items). You can click
whichever mouse button (if any) is configured to paste (see section
4.11.1); you can press Shift-Ins; or you can press Ctrl-Shift-V,
although that action is not enabled by default.
You can configure which of the available clipboards each of these
actions pastes from (including turning the paste action off
completely). On platforms with a single system clipboard (such as
Windows), the available options are to paste from that clipboard
or to paste from PuTTY's internal memory of the last selected text
within that window. On X, the standard options are CLIPBOARD or
PRIMARY.
(PRIMARY is conceptually similar in that it _also_ refers to the
last selected text - just across all applications instead of just
this window.)
The two keyboard options each come with a corresponding key to copy
_to_ the same clipboard. Whatever you configure Shift-Ins to paste
from, Ctrl-Ins will copy to the same location; similarly, Ctrl-
Shift-C will copy to whatever Ctrl-Shift-V pastes from.
On X, you can also enter a selection name of your choice. For
example, there is a rarely-used standard selection called
`SECONDARY', which Emacs (for example) can work with if you hold
down the Meta key while dragging to select or clicking to paste; if
you configure a PuTTY keyboard action to access this clipboard, then
you can interoperate with other applications' use of it. Another
thing you could do would be to invent a clipboard name yourself, to
create a special clipboard shared _only_ between instances of PuTTY,
or between just instances configured in that particular way.
4.11.5 `Permit control characters in pasted text'
It is possible for the clipboard to contain not just text (with
newlines and tabs) but also control characters such as ESC which
could have surprising effects if pasted into a terminal session,
depending on what program is running on the server side. Copying
text from a mischievous web page could put such characters onto the
clipboard.
By default, PuTTY filters out the more unusual control characters,
only letting through the more obvious text-formatting characters
(newlines, tab, backspace, and DEL).
Setting this option stops this filtering; on paste, any character
on the clipboard is sent to the session uncensored. This might be
useful if you are deliberately using control character pasting as a
simple form of scripting, for instance.
4.12 The Copy panel
The Copy configuration panel controls behaviour specifically related
to copying from the terminal window to the clipboard.
4.12.1 Character classes
PuTTY will select a word at a time in the terminal window if you
double-click to begin the drag. This section allows you to control
precisely what is considered to be a word.
Each character is given a _class_, which is a small number
(typically 0, 1 or 2). PuTTY considers a single word to be any
number of adjacent characters in the same class. So by modifying the
assignment of characters to classes, you can modify the word-by-word
selection behaviour.
In the default configuration, the character classes are:
- Class 0 contains white space and control characters.
- Class 1 contains most punctuation.
- Class 2 contains letters, numbers and a few pieces of
punctuation (the double quote, minus sign, period, forward slash
and underscore).
So, for example, if you assign the `@' symbol into character class
2, you will be able to select an e-mail address with just a double
click.
In order to adjust these assignments, you start by selecting a group
of characters in the list box. Then enter a class number in the edit
box below, and press the `Set' button.
This mechanism currently only covers ASCII characters, because it
isn't feasible to expand the list to cover the whole of Unicode.
Character class definitions can be modified by control sequences
sent by the server. This configuration option controls the _default_
state, which will be restored when you reset the terminal (see
section 3.1.3.6). However, if you modify this option in mid-session
using `Change Settings', it will take effect immediately.
4.12.2 Copying in Rich Text Format
If you enable `Copy to clipboard in RTF as well as plain text',
PuTTY will write formatting information to the clipboard as well
as the actual text you copy. The effect of this is that if you
paste into (say) a word processor, the text will appear in the word
processor in the same font, colour, and style (e.g. bold, underline)
PuTTY was using to display it.
This option can easily be inconvenient, so by default it is
disabled.
4.13 The Colours panel
The Colours panel allows you to control PuTTY's use of colour.
4.13.1 `Allow terminal to specify ANSI colours'
This option is enabled by default. If it is disabled, PuTTY will
ignore any control sequences sent by the server to request coloured
text.
If you have a particularly garish application, you might want to
turn this option off and make PuTTY only use the default foreground
and background colours.
4.13.2 `Allow terminal to use xterm 256-colour mode'
This option is enabled by default. If it is disabled, PuTTY will
ignore any control sequences sent by the server which use the
extended 256-colour mode supported by recent versions of xterm.
If you have an application which is supposed to use 256-colour mode
and it isn't working, you may find you need to tell your server that
your terminal supports 256 colours. On Unix, you do this by ensuring
that the setting of TERM describes a 256-colour-capable terminal.
You can check this using a command such as `infocmp':
$ infocmp | grep colors
colors#256, cols#80, it#8, lines#24, pairs#256,
If you do not see `colors#256' in the output, you may need to change
your terminal setting. On modern Linux machines, you could try
`xterm-256color'.
4.13.3 `Allow terminal to use 24-bit colour'
This option is enabled by default. If it is disabled, PuTTY will
ignore any control sequences sent by the server which use the
control sequences supported by modern terminals to specify arbitrary
24-bit RGB colour value.
4.13.4 `Indicate bolded text by changing...'
When the server sends a control sequence indicating that some text
should be displayed in bold, PuTTY can handle this in several ways.
It can either change the font for a bold version, or use the same
font in a brighter colour, or it can do both (brighten the colour
_and_ embolden the font). This control lets you choose which.
By default bold is indicated by colour, so non-bold text is
displayed in light grey and bold text is displayed in bright white
(and similarly in other colours). If you change the setting to `The
font' box, bold and non-bold text will be displayed in the same
colour, and instead the font will change to indicate the difference.
If you select `Both', the font and the colour will both change.
Some applications rely on `bold black' being distinguishable from a
black background; if you choose `The font', their text may become
invisible.
4.13.5 `Attempt to use logical palettes'
Logical palettes are a mechanism by which a Windows application
running on an 8-bit colour display can select precisely the colours
it wants instead of going with the Windows standard defaults.
If you are not getting the colours you ask for on an 8-bit display,
you can try enabling this option. However, be warned that it's never
worked very well.
4.13.6 `Use system colours'
Enabling this option will cause PuTTY to ignore the configured
colours for `Default Background/Foreground' and `Cursor Colour/Text'
(see section 4.13.7), instead going with the system-wide defaults.
Note that non-bold and bold text will be the same colour if this
option is enabled. You might want to change to indicating bold text
by font changes (see section 4.13.4).
4.13.7 Adjusting the colours in the terminal window
The main colour control allows you to specify exactly what colours
things should be displayed in. To modify one of the PuTTY colours,
use the list box to select which colour you want to modify. The
RGB values for that colour will appear on the right-hand side of
the list box. Now, if you press the `Modify' button, you will be
presented with a colour selector, in which you can choose a new
colour to go in place of the old one. (You may also edit the RGB
values directly in the edit boxes, if you wish; each value is an
integer from 0 to 255.)
PuTTY allows you to set the cursor colour, the default foreground
and background, and the precise shades of all the ANSI configurable
colours (black, red, green, yellow, blue, magenta, cyan, and white).
You can also modify the precise shades used for the bold versions
of these colours; these are used to display bold text if you have
chosen to indicate that by colour (see section 4.13.4), and can also
be used if the server asks specifically to use them. (Note that
`Default Bold Background' is _not_ the background colour used for
bold text; it is only used if the server specifically asks for a
bold background.)
4.14 The Connection panel
The Connection panel allows you to configure options that apply to
more than one type of connection.
4.14.1 Using keepalives to prevent disconnection
If you find your sessions are closing unexpectedly (most often with
`Connection reset by peer') after they have been idle for a while,
you might want to try using this option.
Some network routers and firewalls need to keep track of all
connections through them. Usually, these firewalls will assume a
connection is dead if no data is transferred in either direction
after a certain time interval. This can cause PuTTY sessions to be
unexpectedly closed by the firewall if no traffic is seen in the
session for some time.
The keepalive option (`Seconds between keepalives') allows you
to configure PuTTY to send data through the session at regular
intervals, in a way that does not disrupt the actual terminal
session. If you find your firewall is cutting idle connections off,
you can try entering a non-zero value in this field. The value
is measured in seconds; so, for example, if your firewall cuts
connections off after ten minutes then you might want to enter 300
seconds (5 minutes) in the box.
Note that keepalives are not always helpful. They help if you have
a firewall which drops your connection after an idle period; but
if the network between you and the server suffers from breaks in
connectivity then keepalives can actually make things worse. If a
session is idle, and connectivity is temporarily lost between the
endpoints, but the connectivity is restored before either side
tries to send anything, then there will be no problem - neither
endpoint will notice that anything was wrong. However, if one side
does send something during the break, it will repeatedly try to
re-send, and eventually give up and abandon the connection. Then
when connectivity is restored, the other side will find that the
first side doesn't believe there is an open connection any more.
Keepalives can make this sort of problem worse, because they
increase the probability that PuTTY will attempt to send data during
a break in connectivity. (Other types of periodic network activity
can cause this behaviour; in particular, SSH-2 re-keys can have this
effect. See section 4.18.2.)
Therefore, you might find that keepalives help connection loss,
or you might find they make it worse, depending on what _kind_ of
network problems you have between you and the server.
Keepalives are only supported in Telnet and SSH; the Rlogin, SUPDUP,
and Raw protocols offer no way of implementing them. (For an
alternative, see section 4.14.3.)
Note that if you are using SSH-1 and the server has a bug that makes
it unable to deal with SSH-1 ignore messages (see section 4.27.14),
enabling keepalives will have no effect.
4.14.2 `Disable Nagle's algorithm'
Nagle's algorithm is a detail of TCP/IP implementations that tries
to minimise the number of small data packets sent down a network
connection. With Nagle's algorithm enabled, PuTTY's bandwidth usage
will be slightly more efficient; with it disabled, you may find you
get a faster response to your keystrokes when connecting to some
types of server.
The Nagle algorithm is disabled by default for interactive
connections.
4.14.3 `Enable TCP keepalives'
_NOTE:_ TCP keepalives should not be confused with the application-
level keepalives described in section 4.14.1. If in doubt, you
probably want application-level keepalives; TCP keepalives are
provided for completeness.
The idea of TCP keepalives is similar to application-level
keepalives, and the same caveats apply. The main differences are:
- TCP keepalives are available on _all_ network connection types,
including Raw, Rlogin, and SUPDUP.
- The interval between TCP keepalives is usually much longer,
typically two hours; this is set by the operating system, and
cannot be configured within PuTTY.
- If the operating system does not receive a response to a
keepalive, it may send out more in quick succession and
terminate the connection if no response is received.
TCP keepalives may be more useful for ensuring that half-open
connections are terminated than for keeping a connection alive.
TCP keepalives are disabled by default.
4.14.4 `Internet protocol version'
This option allows the user to select between the old and new
Internet protocols and addressing schemes (IPv4 and IPv6). The
selected protocol will be used for most outgoing network connections
(including connections to proxies); however, tunnels have their own
configuration, for which see section 4.26.2.
The default setting is `Auto', which means PuTTY will do something
sensible and try to guess which protocol you wanted. (If you specify
a literal Internet address, it will use whichever protocol that
address implies. If you provide a hostname, it will see what kinds
of address exist for that hostname; it will use IPv6 if there is an
IPv6 address available, and fall back to IPv4 if not.)
If you need to force PuTTY to use a particular protocol, you can
explicitly set this to `IPv4' or `IPv6'.
4.14.5 `Logical name of remote host'
This allows you to tell PuTTY that the host it will really end up
connecting to is different from where it thinks it is making a
network connection.
You might use this, for instance, if you had set up an SSH port
forwarding in one PuTTY session so that connections to some
arbitrary port (say, localhost port 10022) were forwarded to a
second machine's SSH port (say, foovax port 22), and then started a
second PuTTY connecting to the forwarded port.
In normal usage, the second PuTTY will access the host key cache
under the host name and port it actually connected to (i.e.
localhost port 10022 in this example). Using the logical host name
option, however, you can configure the second PuTTY to cache the
host key under the name of the host _you_ know that it's _really_
going to end up talking to (here `foovax').
This can be useful if you expect to connect to the same actual
server through many different channels (perhaps because your port
forwarding arrangements keep changing): by consistently setting the
logical host name, you can arrange that PuTTY will not keep asking
you to reconfirm its host key. Conversely, if you expect to use the
same local port number for port forwardings to lots of different
servers, you probably didn't want any particular server's host
key cached under that local port number. (For this latter case,
you could instead explicitly configure host keys in the relevant
sessions; see section 4.19.3.)
If you just enter a host name for this option, PuTTY will cache the
SSH host key under the default SSH port for that host, irrespective
of the port you really connected to (since the typical scenario is
like the above example: you connect to a silly real port number and
your connection ends up forwarded to the normal port-22 SSH server
of some other machine). To override this, you can append a port
number to the logical host name, separated by a colon. E.g. entering
`foovax:2200' as the logical host name will cause the host key to be
cached as if you had connected to port 2200 of `foovax'.
If you provide a host name using this option, it is also displayed
in other locations which contain the remote host name, such as the
default window title and the default SSH password prompt. This
reflects the fact that this is the host you're _really_ connecting
to, which is more important than the mere means you happen to be
using to contact that host. (This applies even if you're using a
protocol other than SSH.)
4.15 The Data panel
The Data panel allows you to configure various pieces of data which
can be sent to the server to affect your connection at the far end.
Each option on this panel applies to more than one protocol.
Options which apply to only one protocol appear on that protocol's
configuration panels.
4.15.1 `Auto-login username'
All three of the SSH, Telnet, and Rlogin protocols allow you to
specify what user name you want to log in as, without having to type
it explicitly every time. (Some Telnet servers don't support this.)
In this box you can type that user name.
4.15.2 Use of system username
When the previous box (section 4.15.1) is left blank, by default,
PuTTY will prompt for a username at the time you make a connection.
In some environments, such as the networks of large organisations
implementing single sign-on, a more sensible default may be to use
the name of the user logged in to the local operating system (if
any); this is particularly likely to be useful with GSSAPI key
exchange and user authentication (see section 4.23 and section
4.18.1.1). This control allows you to change the default behaviour.
The current system username is displayed in the dialog as a
convenience. It is not saved in the configuration; if a saved
session is later used by a different user, that user's name will be
used.
4.15.3 `Terminal-type string'
Most servers you might connect to with PuTTY are designed to be
connected to from lots of different types of terminal. In order to
send the right control sequences to each one, the server will need
to know what type of terminal it is dealing with. Therefore, each
of the SSH, Telnet, and Rlogin protocols allow a text string to be
sent down the connection describing the terminal. On a Unix server,
this selects an entry from the `termcap' or `terminfo' database that
tells applications what control sequences to send to the terminal,
and what character sequences to expect the keyboard to generate.
PuTTY attempts to emulate the Unix `xterm' program, and by default
it reflects this by sending `xterm' as a terminal-type string. If
you find this is not doing what you want - perhaps the remote system
reports `Unknown terminal type' - you could try setting this to
something different, such as `vt220'.
If you're not sure whether a problem is due to the terminal type
setting or not, you probably need to consult the manual for your
application or your server.
4.15.4 `Terminal speeds'
The Telnet, Rlogin, and SSH protocols allow the client to specify
terminal speeds to the server.
This parameter does _not_ affect the actual speed of the connection,
which is always `as fast as possible'; it is just a hint that is
sometimes used by server software to modify its behaviour. For
instance, if a slow speed is indicated, the server may switch to a
less bandwidth-hungry display mode.
The value is usually meaningless in a network environment, but PuTTY
lets you configure it, in case you find the server is reacting badly
to the default value.
The format is a pair of numbers separated by a comma, for instance,
`38400,38400'. The first number represents the output speed (_from_
the server) in bits per second, and the second is the input speed
(_to_ the server). (Only the first is used in the Rlogin protocol.)
This option has no effect on Raw connections.
4.15.5 Setting environment variables on the server
The Telnet protocol provides a means for the client to pass
environment variables to the server. Many Telnet servers have
stopped supporting this feature due to security flaws, but PuTTY
still supports it for the benefit of any servers which have found
other ways around the security problems than just disabling the
whole mechanism.
Version 2 of the SSH protocol also provides a similar mechanism,
which is easier to implement without security flaws. Newer SSH-2
servers are more likely to support it than older ones.
This configuration data is not used in the SSH-1, rlogin or raw
protocols.
To add an environment variable to the list transmitted down the
connection, you enter the variable name in the `Variable' box, enter
its value in the `Value' box, and press the `Add' button. To remove
one from the list, select it in the list box and press `Remove'.
4.16 The Proxy panel
The Proxy panel allows you to configure PuTTY to use various types
of proxy in order to make its network connections. The settings in
this panel affect the primary network connection forming your PuTTY
session, and also any extra connections made as a result of SSH port
forwarding (see section 3.5).
Note that unlike some software (such as web browsers), PuTTY does
not attempt to automatically determine whether to use a proxy and
(if so) which one to use for a given destination. If you need to use
a proxy, it must always be explicitly configured.
4.16.1 Setting the proxy type
The `Proxy type' drop-down allows you to configure what type of
proxy you want PuTTY to use for its network connections. The default
setting is `None'; in this mode no proxy is used for any connection.
- Selecting `HTTP CONNECT' allows you to proxy your connections
through a web server supporting the HTTP CONNECT command, as
documented in RFC 2817.
- Selecting `SOCKS 4' or `SOCKS 5' allows you to proxy your
connections through a SOCKS server.
- Many firewalls implement a less formal type of proxy in
which a user can make a Telnet or TCP connection directly
to the firewall machine and enter a command such as
`connect myhost.com 22' to connect through to an external host.
Selecting `Telnet' allows you to tell PuTTY to use this type
of proxy, with the precise command specified as described in
section 4.16.5.
- There are several ways to use a SSH server as a proxy. All of
these cause PuTTY to make a secondary SSH connection to the
proxy host (sometimes called a `jump host' in this context).
The `Proxy hostname' field will be interpreted as the name of a
PuTTY saved session if one exists, or a hostname if not. This
allows multi-hop jump paths, if the referenced saved session is
itself configured to use an SSH proxy; and it allows combining
SSH and non-SSH proxying.
- `SSH to proxy and use port forwarding' causes PuTTY to use
the secondary SSH connection to open a port-forwarding
channel to the final destination host (similar to OpenSSH's
-J option).
- `SSH to proxy and execute a command' causes PuTTY to run an
arbitrary remote command on the proxy SSH server and use
that command's standard input and output streams to run
the primary connection over. The remote command line is
specified as described in section 4.16.5.
- `SSH to proxy and invoke a subsystem' is similar but causes
PuTTY to start an SSH `subsystem' rather than an ordinary
command line. This might be useful with a specially set up
SSH proxy server.
- Selecting `Local' allows you to specify an arbitrary command
on the local machine to act as a proxy. When the session is
started, instead of creating a TCP connection, PuTTY runs the
command (specified in section 4.16.5), and uses its standard
input and output streams.
This could be used, for instance, to talk to some kind of
network proxy that PuTTY does not natively support; or you could
tunnel a connection over something other than TCP/IP entirely.
You can also enable this mode on the command line; see section
3.11.3.27.
4.16.2 Excluding parts of the network from proxying
Typically you will only need to use a proxy to connect to non-local
parts of your network; for example, your proxy might be required for
connections outside your company's internal network. In the `Exclude
Hosts/IPs' box you can enter ranges of IP addresses, or ranges of
DNS names, for which PuTTY will avoid using the proxy and make a
direct connection instead.
The `Exclude Hosts/IPs' box may contain more than one exclusion
range, separated by commas. Each range can be an IP address or a DNS
name, with a `*' character allowing wildcards. For example:
*.example.com
This excludes any host with a name ending in `.example.com' from
proxying.
192.168.88.*
This excludes any host with an IP address starting with 192.168.88
from proxying.
192.168.88.*,*.example.com
This excludes both of the above ranges at once.
Connections to the local host (the host name `localhost', and any
loopback IP address) are never proxied, even if the proxy exclude
list does not explicitly contain them. It is very unlikely that this
behaviour would ever cause problems, but if it does you can change
it by enabling `Consider proxying local host connections'.
Note that if you are doing DNS at the proxy (see section 4.16.3),
you should make sure that your proxy exclusion settings do not
depend on knowing the IP address of a host. If the name is passed on
to the proxy without PuTTY looking it up, it will never know the IP
address and cannot check it against your list.
4.16.3 Name resolution when using a proxy
If you are using a proxy to access a private network, it can make a
difference whether DNS name resolution is performed by PuTTY itself
(on the client machine) or performed by the proxy.
The `Do DNS name lookup at proxy end' configuration option allows
you to control this. If you set it to `No', PuTTY will always do its
own DNS, and will always pass an IP address to the proxy. If you set
it to `Yes', PuTTY will always pass host names straight to the proxy
without trying to look them up first.
If you set this option to `Auto' (the default), PuTTY will do
something it considers appropriate for each type of proxy. Most
types of proxy (HTTP, SOCK5, SSH, Telnet, and local) will have host
names passed straight to them; SOCKS4 proxies will not.
Note that if you are doing DNS at the proxy, you should make sure
that your proxy exclusion settings (see section 4.16.2) do not
depend on knowing the IP address of a host. If the name is passed on
to the proxy without PuTTY looking it up, it will never know the IP
address and cannot check it against your list.
The original SOCKS 4 protocol does not support proxy-side DNS. There
is a protocol extension (SOCKS 4A) which does support it, but not
all SOCKS 4 servers provide this extension. If you enable proxy DNS
and your SOCKS 4 server cannot deal with it, this might be why.
If you want to avoid PuTTY making _any_ DNS query related to your
destination host name (for example, because your local DNS resolver
is very slow to return a negative response in that situation), then
as well as setting this control to `Yes', you may also need to
turn off GSSAPI authentication and GSSAPI key exchange in SSH (see
section 4.23 and section 4.18.1.1 respectively). This is because
GSSAPI setup also involves a DNS query for the destination host
name, and that query is performed by the separate GSSAPI library, so
PuTTY can't override or reconfigure it.
4.16.4 Username and password
You can enter a username and a password in the `Username' and
`Password' boxes, which will be used if your proxy requires
authentication.
Note that if you save your session, the proxy password will be saved
in plain text, so anyone who can access your PuTTY configuration
data will be able to discover it.
If PuTTY discovers that it needs a proxy username or password
and you have not specified one here, PuTTY will prompt for it
interactively in the terminal window.
Authentication is not fully supported for all forms of proxy:
- Username and password authentication is supported for HTTP
proxies and SOCKS 5 proxies.
- With SOCKS 5, authentication is via CHAP if the proxy
supports it (this is not supported in PuTTYtel); otherwise
the password is sent to the proxy in plain text.
- With HTTP proxying, authentication is via `HTTP Digest'
if possible (again, not supported in PuTTYtel), or `HTTP
Basic'. In the latter case, the password is sent to the
proxy in plain text.
- SOCKS 4 can use the `Username' field, but does not support
passwords.
- SSH proxying can use all the same forms of SSH authentication
supported by PuTTY for its main connection. If the SSH server
requests password authentication, any configured proxy password
will be used, but other authentication methods such as public
keys and GSSAPI will be tried first, just as for a primary
SSH connection, and if they require credentials such as a key
passphrase, PuTTY will interactively prompt for these.
- You can specify a way to include a username and password in the
Telnet/Local proxy command (see section 4.16.5). If you do so,
and don't also specify the actual username and/or password in
the configuration, PuTTY will interactively prompt for them.
4.16.5 Specifying the Telnet, SSH, or Local proxy command
If you are using the Telnet proxy type, the usual command required
by the firewall's Telnet server is `connect', followed by a host
name and a port number. If your proxy needs a different command, you
can enter an alternative in the `Command to send to proxy' box.
If you are using the Local proxy type, the local command to run is
specified here.
If you are using the `SSH to proxy and execute a command' type,
the command to run on the SSH proxy server is specified here.
Similarly, if you are using `SSH to proxy and invoke a subsystem',
the subsystem name is constructed as specified here.
In this string, you can use `\n' to represent a new-line, `\r' to
represent a carriage return, `\t' to represent a tab character, and
`\x' followed by two hex digits to represent any other character.
`\\' is used to encode the `\' character itself.
Also, the special strings `%host' and `%port' will be replaced by
the host name and port number you want to connect to. For Telnet and
Local proxy types, the strings `%user' and `%pass' will be replaced
by the proxy username and password (which, if not specified in the
configuration, will be prompted for) - this does not happen with SSH
proxy types (because the proxy username/password are used for SSH
authentication). The strings `%proxyhost' and `%proxyport' will be
replaced by the host details specified on the _Proxy_ panel, if any
(this is most likely to be useful for proxy types using a local or
remote command). To get a literal `%' sign, enter `%%'.
If a Telnet proxy server prompts for a username and password before
commands can be sent, you can use a command such as:
%user\n%pass\nconnect %host %port\n
This will send your username and password as the first two lines to
the proxy, followed by a command to connect to the desired host and
port. Note that if you do not include the `%user' or `%pass' tokens
in the Telnet command, then anything specified in `Username' and
`Password' configuration fields will be ignored.
4.16.6 Controlling proxy logging
Often the proxy interaction has its own diagnostic output; this is
particularly the case for local proxy commands.
The setting `Print proxy diagnostics in the terminal window' lets
you control how much of the proxy's diagnostics are printed to the
main terminal window, along with output from your main session.
By default (`No'), proxy diagnostics are only sent to the Event Log;
with `Yes' they are also printed to the terminal, where they may
get mixed up with your main session. `Only until session starts' is
a compromise; proxy messages will go to the terminal window until
the main session is deemed to have started (in a protocol-dependent
way), which is when they're most likely to be interesting; any
further proxy-related messages during the session will only go to
the Event Log.
4.17 The SSH panel
The SSH panel allows you to configure options that only apply to SSH
sessions.
4.17.1 Executing a specific command on the server
In SSH, you don't have to run a general shell session on the server.
Instead, you can choose to run a single specific command (such as
a mail user agent, for example). If you want to do this, enter the
command in the `Remote command' box.
Note that most servers will close the session after executing the
command.
4.17.2 `Don't start a shell or command at all'
If you tick this box, PuTTY will not attempt to run a shell or
command after connecting to the remote server. You might want to
use this option if you are only using the SSH connection for port
forwarding, and your user account on the server does not have the
ability to run a shell.
This feature is only available in SSH protocol version 2 (since the
version 1 protocol assumes you will always want to run a shell).
This feature can also be enabled using the `-N' command-line option;
see section 3.11.3.13.
If you use this feature in Plink, you will not be able to terminate
the Plink process by any graceful means; the only way to kill it
will be by pressing Control-C or sending a kill signal from another
program.
4.17.3 `Enable compression'
This enables data compression in the SSH connection: data sent by
the server is compressed before sending, and decompressed at the
client end. Likewise, data sent by PuTTY to the server is compressed
first and the server decompresses it at the other end. This can help
make the most of a low-bandwidth connection.
4.17.4 `SSH protocol version'
This allows you to select whether to use SSH protocol version 2 or
the older version 1.
You should normally leave this at the default of `2'. As well
as having fewer features, the older SSH-1 protocol is no longer
developed, has many known cryptographic weaknesses, and is generally
not considered to be secure. PuTTY's protocol 1 implementation is
provided mainly for compatibility, and is no longer being enhanced.
If a server offers both versions, prefer `2'. If you have some
server or piece of equipment that only talks SSH-1, select `1' here,
and do not treat the resulting connection as secure.
PuTTY will not automatically fall back to the other version of the
protocol if the server turns out not to match your selection here;
instead, it will put up an error message and abort the connection.
This prevents an active attacker downgrading an intended SSH-2
connection to SSH-1.
4.17.5 Sharing an SSH connection between PuTTY tools
The controls in this box allow you to configure PuTTY to reuse an
existing SSH connection, where possible.
The SSH-2 protocol permits you to run multiple data channels over
the same SSH connection, so that you can log in just once (and do
the expensive encryption setup just once) and then have more than
one terminal window open.
Each instance of PuTTY can still run at most one terminal session,
but using the controls in this box, you can configure PuTTY to check
if another instance of itself has already connected to the target
host, and if so, share that instance's SSH connection instead of
starting a separate new one.
To enable this feature, just tick the box `Share SSH connections if
possible'. Then, whenever you start up a PuTTY session connecting to
a particular host, it will try to reuse an existing SSH connection
if one is available. For example, selecting `Duplicate Session' from
the system menu will launch another session on the same host, and if
sharing is enabled then it will reuse the existing SSH connection.
When this mode is in use, the first PuTTY that connected to a given
server becomes the `upstream', which means that it is the one
managing the real SSH connection. All subsequent PuTTYs which reuse
the connection are referred to as `downstreams': they do not connect
to the real server at all, but instead connect to the upstream PuTTY
via local inter-process communication methods.
For this system to be activated, _both_ the upstream and downstream
instances of PuTTY must have the sharing option enabled.
The upstream PuTTY can therefore not terminate until all its
downstreams have closed. This is similar to the effect you get with
port forwarding or X11 forwarding, in which a PuTTY whose terminal
session has already finished will still remain open so as to keep
serving forwarded connections.
In case you need to configure this system in more detail, there
are two additional checkboxes which allow you to specify whether a
particular PuTTY can act as an upstream or a downstream or both.
(These boxes only take effect if the main `Share SSH connections if
possible' box is also ticked.) By default both of these boxes are
ticked, so that multiple PuTTYs started from the same configuration
will designate one of themselves as the upstream and share a single
connection; but if for some reason you need a particular PuTTY
configuration _not_ to be an upstream (e.g. because you definitely
need it to close promptly) or not to be a downstream (e.g. because
it needs to do its own authentication using a special private key)
then you can untick one or the other of these boxes.
I have referred to `PuTTY' throughout the above discussion, but
all the other PuTTY tools which make SSH connections can use this
mechanism too. For example, if PSCP or PSFTP loads a configuration
with sharing enabled, then it can act as a downstream and use an
existing SSH connection set up by an instance of GUI PuTTY. The one
special case is that PSCP and PSFTP will _never_ act as upstreams.
It is possible to test programmatically for the existence of a live
upstream using Plink. See section 7.2.3.4.
4.18 The Kex panel
The Kex panel (short for `key exchange') allows you to configure
options related to SSH-2 key exchange.
Key exchange occurs at the start of an SSH connection (and
occasionally thereafter); it establishes a shared secret that
is used as the basis for all of SSH's security features. It is
therefore very important for the security of the connection that the
key exchange is secure.
Key exchange is a cryptographically intensive process; if either
the client or the server is a relatively slow machine, the slower
methods may take several tens of seconds to complete.
If connection startup is too slow, or the connection hangs
periodically, you may want to try changing these settings.
If you don't understand what any of this means, it's safe to leave
these settings alone.
This entire panel is only relevant to SSH protocol version 2; none
of these settings affect SSH-1 at all.
4.18.1 Key exchange algorithm selection
PuTTY supports a variety of SSH-2 key exchange methods, and allows
you to choose which one you prefer to use; configuration is similar
to cipher selection (see section 4.20).
PuTTY currently supports the following key exchange methods:
- `NTRU Prime / Curve25519 hybrid': `Streamlined NTRU Prime' is
a lattice-based algorithm intended to resist quantum attacks.
In this key exchange method, it is run in parallel with a
conventional Curve25519-based method (one of those included in
`ECDH'), in such a way that it should be no _less_ secure than
that commonly-used method, and hopefully also resistant to a new
class of attacks.
- `ML-KEM / Curve25519 hybrid' and `ML-KEM NIST ECDH hybrid':
similar hybrid constructs of ML-KEM, another lattice-based key
exchange method intended to be quantum-resistant. In the former,
ML-KEM is hybridised with Curve25519; in the latter, with NIST
P384 or P256.
- `ECDH': elliptic curve Diffie-Hellman key exchange, with a
variety of standard curves and hash algorithms.
- The original form of Diffie-Hellman key exchange, with a variety
of well-known groups and hashes:
- `Group 18', a well-known 8192-bit group, used with the SHA-
512 hash function.
- `Group 17', a well-known 6144-bit group, used with the SHA-
512 hash function.
- `Group 16', a well-known 4096-bit group, used with the SHA-
512 hash function.
- `Group 15', a well-known 3072-bit group, used with the SHA-
512 hash function.
- `Group 14': a well-known 2048-bit group, used with the SHA-
256 hash function or, if the server doesn't support that,
SHA-1.
- `Group 1': a well-known 1024-bit group, used with the SHA-
1 hash function. Neither we nor current SSH standards
recommend using this method any longer, and it's not used by
default in new installations; however, it may be the only
method supported by very old server software.
- `Diffie-Hellman group exchange': with this method, instead of
using a fixed group, PuTTY requests that the server suggest a
group to use for a subsequent Diffie-Hellman key exchange; the
server can avoid groups known to be weak, and possibly invent
new ones over time, without any changes required to PuTTY's
configuration. This key exchange method uses the SHA-256 hash
or, if the server doesn't support that, SHA-1.
- `RSA-based key exchange': this requires much less computational
effort on the part of the client, and somewhat less on the part
of the server, than Diffie-Hellman key exchange.
- `GSSAPI key exchange': see section 4.18.1.1.
If the first algorithm PuTTY finds is below the `warn below here'
line, you will see a warning box when you make the connection,
similar to that for cipher selection (see section 4.20).
4.18.1.1 GSSAPI-based key exchange
PuTTY supports a set of key exchange methods that also incorporates
GSSAPI-based authentication. They are enabled with the `Attempt
GSSAPI key exchange' checkbox (which also appears on the `GSSAPI'
panel).
PuTTY can only perform the GSSAPI-authenticated key exchange methods
when using Kerberos V5, and not other GSSAPI mechanisms. If the user
running PuTTY has current Kerberos V5 credentials, then PuTTY will
select the GSSAPI key exchange methods in preference to any of the
ordinary SSH key exchange methods configured in the preference list.
There's a GSSAPI-based equivalent to most of the ordinary methods
listed in section 4.18.1; server support determines which one will
be used. (PuTTY's preference order for GSSAPI-authenticated key
exchange methods is fixed, not controlled by the preference list.)
The advantage of doing GSSAPI authentication as part of the SSH
key exchange is apparent when you are using credential delegation
(see section 4.23.1). The SSH key exchange can be repeated later in
the session, and this allows your Kerberos V5 credentials (which
are typically short-lived) to be automatically re-delegated to the
server when they are refreshed on the client. (This feature is
commonly referred to as `cascading credentials'.)
If your server doesn't support GSSAPI key exchange, it may still
support GSSAPI in the SSH user authentication phase. This will still
let you log in using your Kerberos credentials, but will only allow
you to delegate the credentials that are active at the beginning of
the session; they can't be refreshed automatically later, in a long-
running session. See section 4.23 for how to control GSSAPI user
authentication in PuTTY.
Another effect of GSSAPI key exchange is that it replaces the usual
SSH mechanism of permanent host keys described in section 2.2. So
if you use this method, then you won't be asked any interactive
questions about whether to accept the server's host key. Instead,
the Kerberos exchange will verify the identity of the host you
connect to, at the same time as verifying your identity to it.
4.18.2 Repeat key exchange
If the session key negotiated at connection startup is used too much
or for too long, it may become feasible to mount attacks against the
SSH connection. Therefore, the SSH-2 protocol specifies that a new
key exchange should take place every so often; this can be initiated
by either the client or the server.
While this renegotiation is taking place, no data can pass through
the SSH connection, so it may appear to `freeze'. (The occurrence
of repeat key exchange is noted in the Event Log; see section
3.1.3.1.) Usually the same algorithm is used as at the start of the
connection, with a similar overhead.
These options control how often PuTTY will initiate a repeat key
exchange (`rekey'). You can also force a key exchange at any time
from the Special Commands menu (see section 3.1.3.2).
- `Max minutes before rekey' specifies the amount of time that
is allowed to elapse before a rekey is initiated. If this is
set to zero, PuTTY will not rekey due to elapsed time. The SSH-
2 protocol specification recommends a timeout of at most 60
minutes.
You might have a need to disable time-based rekeys completely
for the same reasons that keepalives aren't always helpful. If
you anticipate suffering a network dropout of several hours in
the middle of an SSH connection, but were not actually planning
to send _data_ down that connection during those hours, then an
attempted rekey in the middle of the dropout will probably cause
the connection to be abandoned, whereas if rekeys are disabled
then the connection should in principle survive (in the absence
of interfering firewalls). See section 4.14.1 for more discussion
of these issues; for these purposes, rekeys have much the same
properties as keepalives. (Except that rekeys have cryptographic
value in themselves, so you should bear that in mind when deciding
whether to turn them off.) Note, however, the the SSH _server_ can
still initiate rekeys.
- `Minutes between GSSAPI checks', if you're using GSSAPI key
exchange, specifies how often the GSSAPI credential cache is
checked to see whether new tickets are available for delegation,
or current ones are near expiration. If forwarding of GSSAPI
credentials is enabled, PuTTY will try to rekey as necessary to
keep the delegated credentials from expiring. Frequent checks
are recommended; rekeying only happens when needed.
- `Max data before rekey' specifies the amount of data (in bytes)
that is permitted to flow in either direction before a rekey is
initiated. If this is set to zero, PuTTY will not rekey due to
transferred data. The SSH-2 protocol specification recommends a
limit of at most 1 gigabyte.
As well as specifying a value in bytes, the following shorthand
can be used:
- `1k' specifies 1 kilobyte (1024 bytes).
- `1M' specifies 1 megabyte (1024 kilobytes).
- `1G' specifies 1 gigabyte (1024 megabytes).
Disabling data-based rekeys entirely is a bad idea. The integrity,
and to a lesser extent, confidentiality of the SSH-2 protocol depend
in part on rekeys occurring before a 32-bit packet sequence number
wraps around. Unlike time-based rekeys, data-based rekeys won't
occur when the SSH connection is idle, so they shouldn't cause the
same problems. The SSH-1 protocol, incidentally, has even weaker
integrity protection than SSH-2 without rekeys.
4.19 The Host Keys panel
The Host Keys panel allows you to configure options related to host
key management.
Host keys are used to prove the server's identity, and assure you
that the server is not being spoofed (either by a man-in-the-middle
attack or by completely replacing it on the network). See section
2.2 for a basic introduction to host keys.
Much of this panel is only relevant to SSH protocol version 2; SSH-1
only supports one type of host key.
4.19.1 Host key type selection
PuTTY supports a variety of SSH-2 host key types, and allows you
to choose which one you prefer to use to identify the server.
Configuration is similar to cipher selection (see section 4.20).
PuTTY currently supports the following host key types:
- `Ed25519': Edwards-curve DSA using a twisted Edwards curve with
modulus 2^255-19.
- `Ed448': another Edwards-curve DSA type, using a larger elliptic
curve with a 448-bit instead of 255-bit modulus (so it has a
higher security level than Ed25519).
- `ECDSA': elliptic curve DSA using one of the NIST-standardised
elliptic curves.
- `DSA': straightforward DSA using modular exponentiation.
- `RSA': the ordinary RSA algorithm.
If PuTTY already has one or more host keys stored for the server, it
will by default prefer to use one of those, even if the server has
a key type that is higher in the preference order. You can add such
a key to PuTTY's cache from within an existing session using the
`Special Commands' menu; see section 3.1.3.2.
Otherwise, PuTTY will choose a key type based purely on the
preference order you specify in the configuration.
If the first key type PuTTY finds is below the `warn below here'
line, you will see a warning box when you make the connection,
similar to that for cipher selection (see section 4.20).
4.19.2 Preferring known host keys
By default, PuTTY will adjust the preference order for SSH-2 host
key algorithms so that any host keys it already knows are moved to
the top of the list.
This prevents you from having to check and confirm a new host key
for a server you already had one for (e.g. because the server has
generated an alternative key of a type higher in PuTTY's preference
order, or because you changed the preference order itself).
However, on the other hand, it can leak information to a listener
in the network about _whether_ you already know a host key for this
server.
For this reason, this policy is configurable. By turning this
checkbox off, you can reset PuTTY to always use the exact order of
host key algorithms configured in the preference list described in
section 4.19.1, so that a listener will find out nothing about what
keys you had stored.
4.19.3 Manually configuring host keys
In some situations, if PuTTY's automated host key management is not
doing what you need, you might need to manually configure PuTTY to
accept a specific host key, or one of a specific set of host keys.
One reason why you might want to do this is because the host name
PuTTY is connecting to is using round-robin DNS to return one of
multiple actual servers, and they all have different host keys. In
that situation, you might need to configure PuTTY to accept any of
a list of host keys for the possible servers, while still rejecting
any key not in that list.
Another reason is if PuTTY's automated host key management is
completely unavailable, e.g. because PuTTY (or Plink or PSFTP, etc)
is running in a Windows environment without access to the Registry.
In that situation, you will probably want to use the -hostkey
command-line option to configure the expected host key(s); see
section 3.11.3.22.
For situations where PuTTY's automated host key management simply
picks the wrong host name to store a key under, you may want to
consider setting a `logical host name' instead; see section 4.14.5.
To configure manual host keys via the GUI, enter some text
describing the host key into the edit box in the `Manually configure
host keys for this connection' container, and press the `Add'
button. The text will appear in the `Host keys or fingerprints
to accept' list box. You can remove keys again with the `Remove'
button.
The text describing a host key can be in one of the following
formats:
- An SHA-256-based host key fingerprint of the form displayed in
PuTTY's Event Log and host key dialog boxes, i.e. `SHA256:'
followed by 43 case-sensitive characters.
- An MD5-based host key fingerprint, i.e. sixteen 2-digit hex
numbers separated by colons, optionally preceded by the prefix
`MD5:'. (The case of the characters does not matter.)
- A base64-encoded blob describing an SSH-2 public key in
OpenSSH's one-line public key format. How you acquire
a public key in this format is server-dependent; on an
OpenSSH server it can typically be found in a location like
`/etc/ssh/ssh_host_rsa_key.pub'.
If this box contains at least one host key or fingerprint when PuTTY
makes an SSH connection, then PuTTY's automated host key management
is completely bypassed: the connection will be permitted if and only
if the host key presented by the server is one of the keys listed
in this box, and the host key store in the Registry will be neither
read _nor written_, unless you explicitly do so.
If the box is empty (as it usually is), then PuTTY's automated host
key management will work as normal.
4.19.4 Configuring PuTTY to accept host certificates
In some environments, the SSH host keys for a lot of servers will
all be signed in turn by a central `certification authority' (`CA'
for short). This simplifies host key configuration for users,
because if they configure their SSH client to accept host keys
certified by that CA, then they don't need to individually confirm
each host key the first time they connect to that server.
In order to do this, press the `Configure host CAs' button in the
`Host keys' configuration panel. This will launch a secondary
configuration dialog box where you can configure what CAs PuTTY will
accept signatures from.
*Note that this configuration is common to all saved sessions*.
Everything in the main PuTTY configuration is specific to one
saved session, and you can prepare a separate session with all the
configuration different. But there's only one copy of the host CA
configuration, and it applies to all sessions PuTTY runs, whether
saved or not.
(Otherwise, it would be useless - configuring a CA by hand for each
new host wouldn't be any more convenient than pressing the `confirm'
button for each new host's host key.)
To set up a new CA using this config box:
First, load the CA's public key from a file, or paste it directly
into the `Public key of certification authority' edit box. If your
organisation signs its host keys in this way, they will publish the
public key of their CA so that SSH users can include it in their
configuration.
Next, in the `Valid hosts this key is trusted to certify' box,
configure at least one hostname wildcard to say what servers PuTTY
should trust this CA to speak for. For example, suppose you work for
Example Corporation (example.com), and the Example Corporation IT
department has advertised a CA that signs all the Example internal
machines' host keys. Then probably you want to trust that CA to
sign host keys for machines in the domain example.com, but not for
anything else. So you might enter `*.example.com' into the `Valid
hosts' box.
*It's important to limit what the CA key is allowed to sign*. Don't
just enter `*' in that box! If you do that, you're saying that
Example Corporation IT department is authorised to sign a host key
for _anything at all_ you might decide to connect to - even if
you're connecting out of the company network to a machine somewhere
else, such as your own personal server. So that configuration would
enable the Example IT department to act as a `man-in-the-middle'
between your PuTTY process and your server, and listen in to your
communications - exactly the thing SSH is supposed to avoid.
So, if the CA was provided to you by the sysadmins responsible for
example.com (or whatever), make sure PuTTY will _only_ trust it for
machines in the example.com domain.
For the full syntax of the `Valid hosts' expression, see section
4.19.4.1.
Finally, choose an identifying name for this CA; enter that name
in the `Name for this CA' edit box at the top of the window, and
press `Save' to record the CA in your configuration. The name you
chose will appear in the list of saved CAs to the left of the `Save'
button.
The identifying name can be anything you like. It's there so that if
you store multiple certificates you can tell which is which later
when you want to edit or delete them. It also appears in the PuTTY
Event Log when a server presents a certificate signed by that CA.
To reload an existing CA configuration, select it in the list box
and press `Load'. Then you can make changes, and save it again.
To remove a CA from your configuration completely, select it in the
list and press `Delete'.
4.19.4.1 Expressions you can enter in `Valid hosts'
The simplest thing you can enter in the `Valid hosts this key is
trusted to certify' edit box is just a hostname wildcard such as
`*.example.com'. This matches any host in any subdomain, so both
`ssh.example.com' and `login.dept.example.com' would match, but
`prod.example.net' would not.
But you can also enter multiple host name wildcards, and port number
ranges, and make complicated Boolean expressions out of them using
the operators `&&' for `and', `||' for `or', `!' for `not', and
parentheses.
For example, here are some other things you could enter.
- `*.foo.example.com || *.bar.example.com'. This means the CA is
trusted to sign the host key for a connection if the host name
matches `*.foo.example.com' _or_ it matches `*.bar.example.com'.
In other words, the CA has authority over those two particular
subdomains of example.com, but not for anything else, like
www.example.com.
- `*.example.com && ! *.extrasecure.example.com'. This means
the CA is trusted to sign the host key for a connection if
the host name matches `*.example.com' _but does not_ match
`*.extrasecure.example.com'. (Imagine if there was one top-
secret set of servers in your company that the main IT
department didn't have security clearance to administer.)
- `*.example.com && port:22'. This means the CA is trusted to
sign the host key for a connection if the host name matches
`*.example.com' _and_ the port number is 22. SSH servers running
on other ports would not be covered.
- `(*.foo.example.com || *.bar.example.com) && port:0-1023'. This
matches two subdomains of example.com, as before, but _also_
restricts the port number to the range 0-1023.
A certificate configuration expression consists of one or more
individual requirements which can each be a hostname wildcard, a
single port number, or a port number range, combined together with
these Boolean operators.
Unlike other languages such as C, there is no implied priority
between `&&' and `||'. If you write `A && B || C' (where A, B
and C are some particular requirements), then PuTTY will report
a syntax error, because you haven't said which of the `&&' and
`||' takes priority tightly. You will have to write either
`(A && B) || C', meaning `both of A and B, or alternatively just C',
or `A && (B || C)' (`A, and also at least one of B and C'), to make
it clear.
4.19.4.2 RSA signature types in certificates
RSA keys can be used to generate signatures with a choice of
secure hash function. Typically, any version of OpenSSH new enough
to support certificates at all will also be new enough to avoid
using SHA-1, so the default settings of accepting the more modern
SHA-256 and SHA-512 should be suitable for nearly all cases.
For completeness, however, you can configure which types of RSA
signature PuTTY will accept in a certificate from a CA using an RSA
key.
4.20 The Cipher panel
PuTTY supports a variety of different encryption algorithms, and
allows you to choose which one you prefer to use. You can do this by
dragging the algorithms up and down in the list box (or moving them
using the Up and Down buttons) to specify a preference order. When
you make an SSH connection, PuTTY will search down the list from the
top until it finds an algorithm supported by the server, and then
use that.
PuTTY currently supports the following algorithms:
- ChaCha20-Poly1305, a combined cipher and MAC (SSH-2 only)
- AES (Rijndael) - 256, 192, or 128-bit SDCTR or CBC, or 256 or
128-bit GCM (SSH-2 only)
- Arcfour (RC4) - 256 or 128-bit stream cipher (SSH-2 only)
- Blowfish - 256-bit SDCTR (SSH-2 only) or 128-bit CBC
- Triple-DES - 168-bit SDCTR (SSH-2 only) or CBC
- Single-DES - 56-bit CBC (see below for SSH-2)
If the algorithm PuTTY finds is below the `warn below here' line,
you will see a warning box when you make the connection:
The first cipher supported by the server
is single-DES, which is below the configured
warning threshold.
Do you want to continue with this connection?
This warns you that the first available encryption is not a very
secure one. Typically you would put the `warn below here' line
between the encryptions you consider secure and the ones you
consider substandard. By default, PuTTY supplies a preference order
intended to reflect a reasonable preference in terms of security and
speed.
In SSH-2, the encryption algorithm is negotiated independently for
each direction of the connection, although PuTTY does not support
separate configuration of the preference orders. As a result you may
get two warnings similar to the one above, possibly with different
encryptions.
Single-DES is not recommended in the SSH-2 protocol standards, but
one or two server implementations do support it. PuTTY can use
single-DES to interoperate with these servers if you enable the
`Enable legacy use of single-DES in SSH-2' option; by default this
is disabled and PuTTY will stick to recommended ciphers.
4.21 The Auth panel
The Auth panel allows you to configure authentication options for
SSH sessions.
4.21.1 `Display pre-authentication banner'
SSH-2 servers can provide a message for clients to display to the
prospective user before the user logs in; this is sometimes known
as a pre-authentication `banner'. Typically this is used to provide
information about the server and legal notices.
By default, PuTTY displays this message before prompting for a
password or similar credentials (although, unfortunately, not before
prompting for a login name, due to the nature of the protocol
design). By unchecking this option, display of the banner can be
suppressed entirely.
4.21.2 `Bypass authentication entirely'
In SSH-2, it is in principle possible to establish a connection
without using SSH's mechanisms to identify or prove who you are to
the server. An SSH server could prefer to handle authentication
in the data channel, for instance, or simply require no user
authentication whatsoever.
By default, PuTTY assumes the server requires authentication (we've
never heard of one that doesn't), and thus must start this process
with a username. If you find you are getting username prompts that
you cannot answer, you could try enabling this option. However, most
SSH servers will reject this.
This is not the option you want if you have a username and just
want PuTTY to remember it; for that see section 4.15.1. It's
also probably not what if you're trying to set up passwordless
login to a mainstream SSH server; depending on the server, you
probably wanted public-key authentication (chapter 8) or perhaps
GSSAPI authentication (section 4.23). (These are still forms of
authentication, even if you don't have to interact with them.)
This option only affects SSH-2 connections. SSH-1 connections always
require an authentication step.
4.21.3 `Disconnect if authentication succeeds trivially'
This option causes PuTTY to abandon an SSH session and disconnect
from the server, if the server accepted authentication without ever
having asked for any kind of password or signature or token.
This might be used as a security measure. There are some forms
of attack against an SSH client user which work by terminating
the SSH authentication stage early, and then doing something in
the main part of the SSH session which _looks_ like part of the
authentication, but isn't really.
For example, instead of demanding a signature from your public key,
for which PuTTY would ask for your key's passphrase, a compromised
or malicious server might allow you to log in with no signature or
password at all, and then print a message that _imitates_ PuTTY's
request for your passphrase, in the hope that you would type it in.
(In fact, the passphrase for your public key should not be sent to
any server.)
PuTTY's main defence against attacks of this type is the `trust
sigil' system: messages in the PuTTY window that are truly
originated by PuTTY itself are shown next to a small copy of the
PuTTY icon, which the server cannot fake when it tries to imitate
the same message using terminal output.
However, if you think you might be at risk of this kind of thing
anyway (if you don't watch closely for the trust sigils, or if you
think you're at extra risk of one of your servers being malicious),
then you could enable this option as an extra defence. Then, if the
server tries any of these attacks involving letting you through the
authentication stage, PuTTY will disconnect from the server before
it can send a follow-up fake prompt or other type of attack.
On the other hand, some servers _legitimately_ let you through
the SSH authentication phase trivially, either because they are
genuinely public, or because the important authentication step
happens during the terminal session. (An example might be an SSH
server that connects you directly to the terminal login prompt of a
legacy mainframe.) So enabling this option might cause some kinds of
session to stop working. It's up to you.
4.21.4 `Attempt authentication using Pageant'
If this option is enabled, then PuTTY will look for Pageant (the
SSH private-key storage agent) and attempt to authenticate with any
suitable public keys Pageant currently holds.
This behaviour is almost always desirable, and is therefore enabled
by default. In rare cases you might need to turn it off in order
to force authentication by some non-public-key method such as
passwords.
This option can also be controlled using the `-noagent' command-line
option. See section 3.11.3.9.
See chapter 9 for more information about Pageant in general.
4.21.5 `Attempt TIS or CryptoCard authentication'
TIS and CryptoCard authentication are (despite their names) generic
forms of simple challenge/response authentication available in SSH
protocol version 1 only. You might use them if you were using S/Key
one-time passwords, for example, or if you had a physical security
token that generated responses to authentication challenges. They
can even be used to prompt for simple passwords.
With this switch enabled, PuTTY will attempt these forms of
authentication if the server is willing to try them. You will be
presented with a challenge string (which may be different every
time) and must supply the correct response in order to log in.
If your server supports this, you should talk to your system
administrator about precisely what form these challenges and
responses take.
4.21.6 `Attempt keyboard-interactive authentication'
The SSH-2 equivalent of TIS authentication is called `keyboard-
interactive'. It is a flexible authentication method using an
arbitrary sequence of requests and responses; so it is not only
useful for challenge/response mechanisms such as S/Key, but it can
also be used for (for example) asking the user for a new password
when the old one has expired.
PuTTY leaves this option enabled by default, but supplies a switch
to turn it off in case you should have trouble with it.
4.21.7 `Allow agent forwarding'
This option allows the SSH server to open forwarded connections back
to your local copy of Pageant. If you are not running Pageant, this
option will do nothing.
See chapter 9 for general information on Pageant, and section 9.4
for information on agent forwarding. Note that there is a security
risk involved with enabling this option; see section 9.6 for
details.
4.21.8 `Allow attempted changes of username in SSH-2'
In the SSH-1 protocol, it is impossible to change username after
failing to authenticate. So if you mis-type your username at the
PuTTY `login as:' prompt, you will not be able to change it except
by restarting PuTTY.
The SSH-2 protocol _does_ allow changes of username, in principle,
but does not make it mandatory for SSH-2 servers to accept them.
In particular, OpenSSH does not accept a change of username; once
you have sent one username, it will reject attempts to try to
authenticate as another user. (Depending on the version of OpenSSH,
it may quietly return failure for all login attempts, or it may send
an error message.)
For this reason, PuTTY will by default not prompt you for your
username more than once, in case the server complains. If you know
your server can cope with it, you can enable the `Allow attempted
changes of username' option to modify PuTTY's behaviour.
4.22 The Credentials panel
This subpane of the Auth panel contains configuration options that
specify actual _credentials_ to present to the server: key files and
certificates.
4.22.1 `Private key file for authentication'
This box is where you enter the name of your private key file if you
are using public key authentication. See chapter 8 for information
about public key authentication in SSH.
This key must be in PuTTY's native format (`*.PPK'). If you have a
private key in another format that you want to use with PuTTY, see
section 8.2.15.
You can use the authentication agent Pageant so that you do not need
to explicitly configure a key here; see chapter 9.
If a private key file is specified here with Pageant running, PuTTY
will first try asking Pageant to authenticate with that key, and
ignore any other keys Pageant may have. If that fails, PuTTY will
ask for a passphrase as normal. You can also specify a _public_
key file in this case (in RFC 4716 or OpenSSH format), as that's
sufficient to identify the key to Pageant, but of course if Pageant
isn't present PuTTY can't fall back to using this file itself.
4.22.2 `Certificate to use with the private key'
(This is optional. If you don't know you need it, you can leave this
blank.)
In some environments, user authentication keys can be signed in turn
by a `certifying authority' (`CA' for short), and user accounts
on an SSH server can be configured to automatically trust any key
that's certified by the right signature.
This can be a convenient setup if you have a very large number of
servers. When you change your key pair, you might otherwise have to
edit the authorized_keys file on every server individually, to make
them all accept the new key. But if instead you configure all those
servers _once_ to accept keys signed as yours by a CA, then when you
change your public key, all you have to do is to get the new key
certified by the same CA as before, and then all your servers will
automatically accept it without needing individual reconfiguration.
One way to use a certificate is to incorporate it into your private
key file. Section 8.2.9 explains how to do that using PuTTYgen. But
another approach is to tell PuTTY itself where to find the public
certificate file, and then it will automatically present that
certificate when authenticating with the corresponding private key.
To do this, enter the pathname of the certificate file into the
`Certificate to use with the private key' file selector.
When this setting is configured, PuTTY will honour it no matter
whether the private key is found in a file, or loaded into Pageant.
4.22.3 `Plugin to provide authentication responses'
An SSH server can use the `keyboard-interactive' protocol to present
a series of arbitrary questions and answers. Sometimes this is
used for ordinary passwords, but sometimes the server will use the
same mechanism for something more complicated, such as a one-time
password system.
Some of these systems can be automated. For this purpose, PuTTY
allows you to provide a separate program to act as a `plugin' which
will take over the authentication and send answers to the questions
on your behalf.
If you have been provided with a plugin of this type, you can
configure it here, by entering a full command line in the `Plugin
command to run' box.
(If you want to _write_ a plugin of this type, see appendix H for
the full specification of how the plugin is expected to behave.)
4.23 The GSSAPI panel
The `GSSAPI' subpanel of the `Auth' panel controls the use of
GSSAPI authentication. This is a mechanism which delegates the
authentication exchange to a library elsewhere on the client
machine, which in principle can authenticate in many different ways
but in practice is usually used with the Kerberos single sign-on
protocol to implement passwordless login.
GSSAPI authentication is only available in the SSH-2 protocol.
PuTTY supports two forms of GSSAPI-based authentication. In one of
them, the SSH key exchange happens in the normal way, and GSSAPI
is only involved in authenticating the user. The checkbox labelled
`Attempt GSSAPI authentication' controls this form.
In the other method, GSSAPI-based authentication is combined
with the SSH key exchange phase. If this succeeds, then the SSH
authentication step has nothing left to do. See section 4.18.1.1 for
more information about this method. The checkbox labelled `Attempt
GSSAPI key exchange' controls this form. (The same checkbox appears
on the `Kex' panel.)
If one or both of these controls is enabled, then GSSAPI
authentication will be attempted in one form or the other, and
(typically) if your client machine has valid Kerberos credentials
loaded, then PuTTY should be able to authenticate automatically to
servers that support Kerberos logins.
If both of those checkboxes are disabled, PuTTY will not try any
form of GSSAPI at all, and the rest of this panel will be unused.
4.23.1 `Allow GSSAPI credential delegation'
GSSAPI credential delegation is a mechanism for passing on your
Kerberos (or other) identity to the session on the SSH server. If
you enable this option, then not only will PuTTY be able to log in
automatically to a server that accepts your Kerberos credentials,
but also you will be able to connect out from that server to other
Kerberos-supporting services and use the same credentials just as
automatically.
(This option is the Kerberos analogue of SSH agent forwarding; see
section 9.4 for some information on that.)
Note that, like SSH agent forwarding, there is a security
implication in the use of this option: the administrator of
the server you connect to, or anyone else who has cracked the
administrator account on that server, could fake your identity
when connecting to further Kerberos-supporting services. However,
Kerberos sites are typically run by a central authority, so the
administrator of one server is likely to already have access to the
other services too; so this would typically be less of a risk than
SSH agent forwarding.
If your connection is not using GSSAPI key exchange, it is possible
for the delegation to expire during your session. See section
4.18.1.1 for more information.
4.23.2 Preference order for GSSAPI libraries
GSSAPI is a mechanism which allows more than one authentication
method to be accessed through the same interface. Therefore, more
than one authentication library may exist on your system which can
be accessed using GSSAPI.
PuTTY contains native support for a few well-known such libraries
(including Windows' SSPI), and will look for all of them on your
system and use whichever it finds. If more than one exists on your
system and you need to use a specific one, you can adjust the order
in which it will search using this preference list control.
One of the options in the preference list is to use a user-specified
GSSAPI library. If the library you want to use is not mentioned by
name in PuTTY's list of options, you can enter its full pathname in
the `User-supplied GSSAPI library path' field, and move the `User-
supplied GSSAPI library' option in the preference list to make sure
it is selected before anything else.
On Windows, such libraries are files with a .dll extension, and
must have been built in the same way as the PuTTY executable you're
running; if you have a 32-bit DLL, you must run a 32-bit version
of PuTTY, and the same with 64-bit (see question A.6.10). On Unix,
shared libraries generally have a .so extension.
4.24 The TTY panel
The TTY panel lets you configure the remote pseudo-terminal.
4.24.1 `Don't allocate a pseudo-terminal'
When connecting to a Unix system, most interactive shell sessions
are run in a _pseudo-terminal_, which allows the Unix system to
pretend it's talking to a real physical terminal device but allows
the SSH server to catch all the data coming from that fake device
and send it back to the client.
Occasionally you might find you have a need to run a session _not_
in a pseudo-terminal. In PuTTY, this is generally only useful for
very specialist purposes; although in Plink (see chapter 7) it is
the usual way of working.
4.24.2 Sending terminal modes
The SSH protocol allows the client to send `terminal modes' for
the remote pseudo-terminal. These usually control the server's
expectation of the local terminal's behaviour.
If your server does not have sensible defaults for these modes, you
may find that changing them here helps, although the server is at
liberty to ignore your changes. If you don't understand any of this,
it's safe to leave these settings alone.
(None of these settings will have any effect if no pseudo-terminal
is requested or allocated.)
You can change what happens for a particular mode by selecting it in
the list, choosing one of the options and specifying the exact value
if necessary, and hitting `Set'. The effect of the options is as
follows:
- If the `Auto' option is selected, the PuTTY tools will decide
whether to specify that mode to the server, and if so, will send
a sensible value.
PuTTY proper will send modes that it has an opinion on
(currently only the code for the Backspace key, ERASE, and
whether the character set is UTF-8, IUTF8). Plink on Unix will
propagate appropriate modes from the local terminal, if any.
- If `Nothing' is selected, no value for the mode will be
specified to the server under any circumstances.
- If a value is specified, it will be sent to the server under all
circumstances. The precise syntax of the value box depends on
the mode.
By default, all of the available modes are listed as `Auto', which
should do the right thing in most circumstances.
The precise effect of each setting, if any, is up to the server.
Their names come from POSIX and other Unix systems, and they are
most likely to have a useful effect on such systems. (These are the
same settings that can usually be changed using the `stty' command
once logged in to such servers.)
Some notable modes are described below; for fuller explanations, see
your server documentation.
- ERASE is the character that when typed by the user will delete
one space to the left. When set to `Auto' (the default setting),
this follows the setting of the local Backspace key in PuTTY
(see section 4.4.1).
This and other special characters are specified using `^C'
notation for Ctrl-C, and so on. Use `^<27>' or `^<0x1B>' to
specify a character numerically, and `^~' to get a literal `^'.
Other non-control characters are denoted by themselves. Leaving
the box entirely blank indicates that _no_ character should be
assigned to the specified function, although this may not be
supported by all servers.
- QUIT is a special character that usually forcefully ends the
current process on the server (SIGQUIT). On many servers its
default setting is Ctrl-backslash (`^\'), which is easy to
accidentally invoke on many keyboards. If this is getting in
your way, you may want to change it to another character or turn
it off entirely.
- Boolean modes such as ECHO and ICANON can be specified in PuTTY
in a variety of ways, such as true/false, yes/no, and 0/1.
(Explicitly specifying a value of no is different from not
sending the mode at all.)
- The boolean mode IUTF8 signals to the server whether the
terminal character set is UTF-8 or not, for purposes such as
basic line editing; if this is set incorrectly, the backspace
key may erase the wrong amount of text, for instance. However,
simply setting this is not usually sufficient for the server to
use UTF-8; POSIX servers will generally also require the locale
to be set (by some server-dependent means), although many newer
installations default to UTF-8. Also, since this mode was added
to the SSH protocol much later than the others, many servers
(particularly older servers) do not honour this mode sent over
SSH; indeed, a few poorly-written servers object to its mere
presence, so you may find you need to set it to not be sent
at all. When set to `Auto', this follows the local configured
character set (see section 4.10.1).
- Terminal speeds are configured elsewhere; see section 4.15.4.
4.25 The X11 panel
The X11 panel allows you to configure forwarding of X11 over an SSH
connection.
If your server lets you run X Window System graphical applications,
X11 forwarding allows you to securely give those applications access
to a local X display on your PC.
To enable X11 forwarding, check the `Enable X11 forwarding' box.
If your X display is somewhere unusual, you will need to enter its
location in the `X display location' box; if this is left blank,
PuTTY will try to find a sensible default in the environment, or use
the primary local display (`:0') if that fails.
See section 3.4 for more information about X11 forwarding.
4.25.1 Remote X11 authentication
If you are using X11 forwarding, the virtual X server created on the
SSH server machine will be protected by authorisation data. This
data is invented, and checked, by PuTTY.
The usual authorisation method used for this is called MIT-MAGIC-
COOKIE-1. This is a simple password-style protocol: the X client
sends some cookie data to the server, and the server checks that it
matches the real cookie. The cookie data is sent over an unencrypted
X11 connection; so if you allow a client on a third machine to
access the virtual X server, then the cookie will be sent in the
clear.
PuTTY offers the alternative protocol XDM-AUTHORIZATION-1. This is
a cryptographically authenticated protocol: the data sent by the X
client is different every time, and it depends on the IP address
and port of the client's end of the connection and is also stamped
with the current time. So an eavesdropper who captures an XDM-
AUTHORIZATION-1 string cannot immediately re-use it for their own X
connection.
PuTTY's support for XDM-AUTHORIZATION-1 is a somewhat experimental
feature, and may encounter several problems:
- Some X clients probably do not even support XDM-AUTHORIZATION-
1, so they will not know what to do with the data PuTTY has
provided.
- This authentication mechanism will only work in SSH-2. In SSH-
1, the SSH server does not tell the client the source address
of a forwarded connection in a machine-readable format, so it's
impossible to verify the XDM-AUTHORIZATION-1 data.
- You may find this feature causes problems with some SSH servers,
which will not clean up XDM-AUTHORIZATION-1 data after a
session, so that if you then connect to the same server using
a client which only does MIT-MAGIC-COOKIE-1 and are allocated
the same remote display number, you might find that out-of-date
authentication data is still present on your server and your X
connections fail.
PuTTY's default is MIT-MAGIC-COOKIE-1. If you change it, you should
be sure you know what you're doing.
4.25.2 X authority file for local display
If you are using X11 forwarding, the local X server to which your
forwarded connections are eventually directed may itself require
authorisation.
Some Windows X servers do not require this: they do authorisation
by simpler means, such as accepting any connection from the local
machine but not from anywhere else. However, if your X server does
require authorisation, then PuTTY needs to know what authorisation
is required.
One way in which this data might be made available is for the X
server to store it somewhere in a file which has the same format
as the Unix `.Xauthority' file. If this is how your Windows X
server works, then you can tell PuTTY where to find this file by
configuring this option. By default, PuTTY will not attempt to find
any authorisation for your local display.
4.26 The Tunnels panel
The Tunnels panel allows you to configure tunnelling of arbitrary
connection types through an SSH connection.
Port forwarding allows you to tunnel other types of network
connection down an SSH session. See section 3.5 for a general
discussion of port forwarding and how it works.
The port forwarding section in the Tunnels panel shows a list of all
the port forwardings that PuTTY will try to set up when it connects
to the server. By default no port forwardings are set up, so this
list is empty.
To add a port forwarding:
- Set one of the `Local' or `Remote' radio buttons, depending on
whether you want to forward a local port to a remote destination
(`Local') or forward a remote port to a local destination
(`Remote'). Alternatively, select `Dynamic' if you want PuTTY to
provide a local SOCKS 4/4A/5 proxy on a local port (note that
this proxy only supports TCP connections; the SSH protocol does
not support forwarding UDP).
- Enter a source port number into the `Source port' box. For local
forwardings, PuTTY will listen on this port of your PC. For
remote forwardings, your SSH server will listen on this port of
the remote machine. Note that most servers will not allow you to
listen on port numbers less than 1024.
- If you have selected `Local' or `Remote' (this step is not
needed with `Dynamic'), enter a hostname and port number
separated by a colon, in the `Destination' box. Connections
received on the source port will be directed to this
destination. For example, to connect to a POP-3 server, you
might enter `popserver.example.com:110'. (If you need to enter
a literal IPv6 address, enclose it in square brackets, for
instance `[::1]:2200'.)
- Click the `Add' button. Your forwarding details should appear in
the list box.
To remove a port forwarding, simply select its details in the list
box, and click the `Remove' button.
In the `Source port' box, you can also optionally enter an IP
address to listen on, by specifying (for instance) `127.0.0.5:79'.
See section 3.5 for more information on how this works and its
restrictions.
In place of port numbers, you can enter service names, if they are
known to the local system. For instance, in the `Destination' box,
you could enter `popserver.example.com:pop3'.
You can modify the currently active set of port forwardings in mid-
session using `Change Settings' (see section 3.1.3.4). If you delete
a local or dynamic port forwarding in mid-session, PuTTY will stop
listening for connections on that port, so it can be re-used by
another program. If you delete a remote port forwarding, note that:
- The SSH-1 protocol contains no mechanism for asking the server
to stop listening on a remote port.
- The SSH-2 protocol does contain such a mechanism, but not all
SSH servers support it. (In particular, OpenSSH does not support
it in any version earlier than 3.9.)
If you ask to delete a remote port forwarding and PuTTY cannot make
the server actually stop listening on the port, it will instead
just start refusing incoming connections on that port. Therefore,
although the port cannot be reused by another program, you can at
least be reasonably sure that server-side programs can no longer
access the service at your end of the port forwarding.
If you delete a forwarding, any existing connections established
using that forwarding remain open. Similarly, changes to global
settings such as `Local ports accept connections from other hosts'
only take effect on new forwardings.
If the connection you are forwarding over SSH is itself a second
SSH connection made by another copy of PuTTY, you might find the
`logical host name' configuration option useful to warn PuTTY of
which host key it should be expecting. See section 4.14.5 for
details of this.
4.26.1 Controlling the visibility of forwarded ports
The source port for a forwarded connection usually does not accept
connections from any machine except the SSH client or server machine
itself (for local and remote forwardings respectively). There are
controls in the Tunnels panel to change this:
- The `Local ports accept connections from other hosts' option
allows you to set up local-to-remote port forwardings in such a
way that machines other than your client PC can connect to the
forwarded port. (This also applies to dynamic SOCKS forwarding.)
- The `Remote ports do the same' option does the same thing for
remote-to-local port forwardings (so that machines other than
the SSH server machine can connect to the forwarded port.) Note
that this feature is only available in the SSH-2 protocol, and
not all SSH-2 servers support it (OpenSSH 3.0 does not, for
example).
4.26.2 Selecting Internet protocol version for forwarded ports
This switch allows you to select a specific Internet protocol (IPv4
or IPv6) for the local end of a forwarded port. By default, it is
set on `Auto', which means that:
- for a local-to-remote port forwarding, PuTTY will listen for
incoming connections in both IPv4 and (if available) IPv6
- for a remote-to-local port forwarding, PuTTY will choose a
sensible protocol for the outgoing connection.
This overrides the general Internet protocol version preference on
the Connection panel (see section 4.14.4).
Note that some operating systems may listen for incoming connections
in IPv4 even if you specifically asked for IPv6, because their IPv4
and IPv6 protocol stacks are linked together. Apparently Linux does
this, and Windows does not. So if you're running PuTTY on Windows
and you tick `IPv6' for a local or dynamic port forwarding, it will
_only_ be usable by connecting to it using IPv6; whereas if you do
the same on Linux, you can also use it with IPv4. However, ticking
`Auto' should always give you a port which you can connect to using
either protocol.
4.27 The Bugs and More Bugs panels
Not all SSH servers work properly. Various existing servers have
bugs in them, which can make it impossible for a client to talk to
them unless it knows about the bug and works around it.
Since most servers announce their software version number at the
beginning of the SSH connection, PuTTY will attempt to detect which
bugs it can expect to see in the server and automatically enable
workarounds. However, sometimes it will make mistakes; if the server
has been deliberately configured to conceal its version number, or
if the server is a version which PuTTY's bug database does not know
about, then PuTTY will not know what bugs to expect.
The Bugs and More Bugs panels (there are two because we have so many
bug compatibility modes) allow you to manually configure the bugs
PuTTY expects to see in the server. Each bug can be configured in
three states:
- `Off': PuTTY will assume the server does not have the bug.
- `On': PuTTY will assume the server _does_ have the bug.
- `Auto': PuTTY will use the server's version number announcement
to try to guess whether or not the server has the bug. (This
option is not available for bugs that _cannot_ be detected from
the server version, e.g. because they must be acted on before
the server version is known.)
(The PuTTY project has a defined policy about when we're prepared to
add auto-detection for a bug workaround. See section B.6.)
4.27.1 `Chokes on SSH-2 ignore messages'
An ignore message (SSH_MSG_IGNORE) is a message in the SSH protocol
which can be sent from the client to the server, or from the server
to the client, at any time. Either side is required to ignore the
message whenever it receives it. PuTTY uses ignore messages in
SSH-2 to confuse the encrypted data stream and make it harder to
cryptanalyse. It also uses ignore messages for connection keepalives
(see section 4.14.1).
If it believes the server to have this bug, PuTTY will stop using
ignore messages. If this bug is enabled when talking to a correct
server, the session will succeed, but keepalives will not work and
the session might be less cryptographically secure than it could be.
4.27.2 `Handles SSH-2 key re-exchange badly'
Some SSH servers cannot cope with repeat key exchange at all, and
will ignore attempts by the client to start one. Since PuTTY pauses
the session while performing a repeat key exchange, the effect of
this would be to cause the session to hang after an hour (unless
you have your rekey timeout set differently; see section 4.18.2 for
more about rekeys). Other, very old, SSH servers handle repeat key
exchange even more badly, and disconnect upon receiving a repeat key
exchange request.
If this bug is detected, PuTTY will never initiate a repeat key
exchange. If this bug is enabled when talking to a correct server,
the session should still function, but may be less secure than you
would expect.
This is an SSH-2-specific bug.
4.27.3 `Chokes on PuTTY's SSH-2 `winadj' requests'
PuTTY sometimes sends a special request to SSH servers in the middle
of channel data, with the name
[email protected]
(see section G.1). The purpose of this request is to measure the
round-trip time to the server, which PuTTY uses to tune its flow
control. The server does not actually have to _understand_ the
message; it is expected to send back a SSH_MSG_CHANNEL_FAILURE
message indicating that it didn't understand it. (All PuTTY needs
for its timing calculations is _some_ kind of response.)
It has been known for some SSH servers to get confused by this
message in one way or another - because it has a long name, or
because they can't cope with unrecognised request names even to the
extent of sending back the correct failure response, or because they
handle it sensibly but fill up the server's log file with pointless
spam, or whatever. PuTTY therefore supports this bug-compatibility
flag: if it believes the server has this bug, it will never send
its `
[email protected]' request, and will make do
without its timing data.
4.27.4 `Replies to requests on closed channels'
The SSH protocol as published in RFC 4254 has an ambiguity which
arises if one side of a connection tries to close a channel, while
the other side simultaneously sends a request within the channel and
asks for a reply. RFC 4254 leaves it unclear whether the closing
side should reply to the channel request after having announced its
intention to close the channel.
Discussion on the ietf-ssh mailing list in April 2014 formed a clear
consensus that the right answer is no. However, because of the
ambiguity in the specification, some SSH servers have implemented
the other policy; for example, OpenSSH used to until it was fixed.
Because PuTTY sends channel requests with the `want reply'
flag throughout channels' lifetime (see section 4.27.3), it's
possible that when connecting to such a server it might receive
a reply to a request after it thinks the channel has entirely
closed, and terminate with an error along the lines of `Received
SSH2_MSG_CHANNEL_FAILURE for nonexistent channel 256'.
4.27.5 `Ignores SSH-2 maximum packet size'
When an SSH-2 channel is set up, each end announces the maximum size
of data packet that it is willing to receive for that channel. Some
servers ignore PuTTY's announcement and send packets larger than
PuTTY is willing to accept, causing it to report `Incoming packet
was garbled on decryption'.
If this bug is detected, PuTTY never allows the channel's flow-
control window to grow large enough to allow the server to send an
over-sized packet. If this bug is enabled when talking to a correct
server, the session will work correctly, but download performance
will be less than it could be.
4.27.6 `Discards data sent before its greeting'
Just occasionally, an SSH connection can be established over some
channel that will accidentally discard outgoing data very early in
the connection.
This is not typically seen as a bug in an actual SSH server, but
it can sometimes occur in situations involving a complicated proxy
process. An example is Debian bug #991958, in which a connection
going over the console of a User Mode Linux kernel can lose outgoing
data before the kernel has fully booted.
You can work around this problem by manually enabling this bug flag,
which will cause PuTTY to wait to send its initial SSH greeting
until after it sees the greeting from the server.
Note that this bug flag can never be automatically detected,
since auto-detection relies on the version string in the server's
greeting, and PuTTY has to decide whether to expect this bug
_before_ it sees the server's greeting. So this is a manual
workaround only.
4.27.7 `Chokes on PuTTY's full KEXINIT'
At the start of an SSH connection, the client and server exchange
long messages of type SSH_MSG_KEXINIT, containing lists of all the
cryptographic algorithms they're prepared to use. This is used to
negotiate a set of algorithms that both ends can speak.
Occasionally, a badly written server might have a length limit on
the list it's prepared to receive, and refuse to make a connection
simply because PuTTY is giving it too many choices.
A workaround is to enable this flag, which will make PuTTY wait to
send KEXINIT until after it receives the one from the server, and
then filter its own KEXINIT to leave out any algorithm the server
doesn't also announce support for. This will generally make PuTTY's
KEXINIT at most the size of the server's, and will otherwise make no
difference to the algorithm negotiation.
This flag is a minor violation of the SSH protocol, because both
sides are supposed to send KEXINIT proactively. It still works
provided _one_ side sends its KEXINIT without waiting, but if both
client and server waited for the other one to speak first, the
connection would deadlock. We don't know of any servers that do
this, but if there is one, then this flag will make PuTTY unable to
speak to them at all.
4.27.8 `Old RSA/SHA2 cert algorithm naming'
If PuTTY is trying to do SSH-2 user authentication using an RSA
key, and the server is using one of the newer SHA-2 based versions
of the SSH RSA protocol, and the user's key is also a certificate,
then earlier versions of OpenSSH (up to 7.7) disagree with later
versions about the right key algorithm string to send in the
SSH2_MSG_USERAUTH_REQUEST packet. Modern versions send a string
that indicates both the SHA-2 nature and the certificate nature
of the key, such as `
[email protected]'. Earlier
versions would reject that, and insist on seeing `ssh-rsa-cert-
[email protected]' followed by a SHA-2 based signature.
PuTTY should auto-detect the presence of this bug in earlier OpenSSH
and adjust to send the right string.
4.27.9 `Requires padding on SSH-2 RSA signatures'
Versions below 3.3 of OpenSSH require SSH-2 RSA signatures to be
padded with zero bytes to the same length as the RSA key modulus.
The SSH-2 specification says that an unpadded signature MUST be
accepted, so this is a bug. A typical symptom of this problem is
that PuTTY mysteriously fails RSA authentication once in every few
hundred attempts, and falls back to passwords.
If this bug is detected, PuTTY will pad its signatures in the way
OpenSSH expects. If this bug is enabled when talking to a correct
server, it is likely that no damage will be done, since correct
servers usually still accept padded signatures because they're used
to talking to OpenSSH.
This is an SSH-2-specific bug.
4.27.10 `Only supports pre-RFC4419 SSH-2 DH GEX'
The SSH key exchange method that uses Diffie-Hellman group exchange
was redesigned after its original release, to use a slightly more
sophisticated setup message. Almost all SSH implementations switched
over to the new version. (PuTTY was one of the last.) A few old
servers still only support the old one.
If this bug is detected, and the client and server negotiate Diffie-
Hellman group exchange, then PuTTY will send the old message now
known as SSH2_MSG_KEX_DH_GEX_REQUEST_OLD in place of the new
SSH2_MSG_KEX_DH_GEX_REQUEST.
This is an SSH-2-specific bug.
4.27.11 `Miscomputes SSH-2 HMAC keys'
Versions 2.3.0 and below of the SSH server software from ssh.com
compute the keys for their HMAC message authentication codes
incorrectly. A typical symptom of this problem is that PuTTY dies
unexpectedly at the beginning of the session, saying `Incorrect MAC
received on packet'.
If this bug is detected, PuTTY will compute its HMAC keys in the
same way as the buggy server, so that communication will still be
possible. If this bug is enabled when talking to a correct server,
communication will fail.
This is an SSH-2-specific bug.
4.27.12 `Misuses the session ID in SSH-2 PK auth'
Versions below 2.3 of OpenSSH require SSH-2 public-key
authentication to be done slightly differently: the data to be
signed by the client contains the session ID formatted in a
different way. If public-key authentication mysteriously does
not work but the Event Log (see section 3.1.3.1) thinks it has
successfully sent a signature, it might be worth enabling the
workaround for this bug to see if it helps.
If this bug is detected, PuTTY will sign data in the way OpenSSH
expects. If this bug is enabled when talking to a correct server,
SSH-2 public-key authentication will fail.
This is an SSH-2-specific bug.
4.27.13 `Miscomputes SSH-2 encryption keys'
Versions below 2.0.11 of the SSH server software from ssh.com
compute the keys for the session encryption incorrectly. This
problem can cause various error messages, such as `Incoming packet
was garbled on decryption', or possibly even `Out of memory'.
If this bug is detected, PuTTY will compute its encryption keys
in the same way as the buggy server, so that communication will
still be possible. If this bug is enabled when talking to a correct
server, communication will fail.
This is an SSH-2-specific bug.
4.27.14 `Chokes on SSH-1 ignore messages'
An ignore message (SSH_MSG_IGNORE) is a message in the SSH protocol
which can be sent from the client to the server, or from the server
to the client, at any time. Either side is required to ignore the
message whenever it receives it. PuTTY uses ignore messages to hide
the password packet in SSH-1, so that a listener cannot tell the
length of the user's password; it also uses ignore messages for
connection keepalives (see section 4.14.1).
If this bug is detected, PuTTY will stop using ignore messages.
This means that keepalives will stop working, and PuTTY will
have to fall back to a secondary defence against SSH-1 password-
length eavesdropping. See section 4.27.15. If this bug is enabled
when talking to a correct server, the session will succeed, but
keepalives will not work and the session might be more vulnerable to
eavesdroppers than it could be.
4.27.15 `Refuses all SSH-1 password camouflage'
When talking to an SSH-1 server which cannot deal with ignore
messages (see section 4.27.14), PuTTY will attempt to disguise the
length of the user's password by sending additional padding _within_
the password packet. This is technically a violation of the SSH-
1 specification, and so PuTTY will only do it when it cannot use
standards-compliant ignore messages as camouflage. In this sense,
for a server to refuse to accept a padded password packet is not
really a bug, but it does make life inconvenient if the server can
also not handle ignore messages.
If this `bug' is detected, PuTTY will assume that neither ignore
messages nor padding are acceptable, and that it thus has no choice
but to send the user's password with no form of camouflage, so
that an eavesdropping user will be easily able to find out the
exact length of the password. If this bug is enabled when talking
to a correct server, the session will succeed, but will be more
vulnerable to eavesdroppers than it could be.
This is an SSH-1-specific bug. SSH-2 is secure against this type of
attack.
4.27.16 `Chokes on SSH-1 RSA authentication'
Some SSH-1 servers cannot deal with RSA authentication messages at
all. If Pageant is running and contains any SSH-1 keys, PuTTY will
normally automatically try RSA authentication before falling back
to passwords, so these servers will crash when they see the RSA
attempt.
If this bug is detected, PuTTY will go straight to password
authentication. If this bug is enabled when talking to a correct
server, the session will succeed, but of course RSA authentication
will be impossible.
This is an SSH-1-specific bug.
4.28 The `Bare ssh-connection' protocol
In addition to SSH itself, PuTTY also supports a second protocol
that is derived from SSH. It's listed in the PuTTY GUI under the
name `Bare ssh-connection'.
This protocol consists of just the innermost of SSH-2's three
layers: it leaves out the cryptography layer providing network
security, and it leaves out the authentication layer where you
provide a username and prove you're allowed to log in as that user.
It is therefore *completely unsuited to any network connection*.
Don't try to use it over a network!
The purpose of this protocol is for various specialist circumstances
in which the `connection' is not over a real network, but is a pipe
or IPC channel between different processes running on the _same_
computer. In these contexts, the operating system will already have
guaranteed that each of the two communicating processes is owned by
the expected user (so that no authentication is necessary), and that
the communications channel cannot be tapped by a hostile user on the
same machine (so that no cryptography is necessary either). Examples
of possible uses involve communicating with a strongly separated
context such as the inside of a container, or a VM, or a different
network namespace.
Explicit support for this protocol is new in PuTTY 0.75. As of 2021-
04, the only known server for the bare ssh-connection protocol is
the Unix program `psusan' that is also part of the PuTTY tool suite.
(However, this protocol is also the same one used between instances
of PuTTY to implement connection sharing: see section 4.17.5. In
fact, in the Unix version of PuTTY, when a sharing upstream records
`Sharing this connection at [pathname]' in the Event Log, it's
possible to connect another instance of PuTTY directly to that Unix
socket, by entering its pathname in the host name box and selecting
`Bare ssh-connection' as the protocol!)
Many of the options under the SSH panel also affect this protocol,
although options to do with cryptography and authentication do not,
for obvious reasons.
I repeat, *DON'T TRY TO USE THIS PROTOCOL FOR NETWORK CONNECTIONS!*
That's not what it's for, and it's not at all safe to do it.
4.29 The Serial panel
The Serial panel allows you to configure options that only apply
when PuTTY is connecting to a local serial line.
4.29.1 Selecting a serial line to connect to
The `Serial line to connect to' box allows you to choose which
serial line you want PuTTY to talk to, if your computer has more
than one serial port.
On Windows, the first serial line is called COM1, and if there is a
second it is called COM2, and so on.
This configuration setting is also visible on the Session panel,
where it replaces the `Host Name' box (see section 4.1.1) if the
connection type is set to `Serial'.
4.29.2 Selecting the speed of your serial line
The `Speed' box allows you to choose the speed (or `baud rate') at
which to talk to the serial line. Typical values might be 9600,
19200, 38400 or 57600. Which one you need will depend on the device
at the other end of the serial cable; consult the manual for that
device if you are in doubt.
This configuration setting is also visible on the Session panel,
where it replaces the `Port' box (see section 4.1.1) if the
connection type is set to `Serial'.
4.29.3 Selecting the number of data bits
The `Data bits' box allows you to choose how many data bits are
transmitted in each byte sent or received through the serial line.
Typical values are 7 or 8.
4.29.4 Selecting the number of stop bits
The `Stop bits' box allows you to choose how many stop bits are used
in the serial line protocol. Typical values are 1, 1.5 or 2.
4.29.5 Selecting the serial parity checking scheme
The `Parity' box allows you to choose what type of parity checking
is used on the serial line. The settings are:
- `None': no parity bit is sent at all.
- `Odd': an extra parity bit is sent alongside each byte, and
arranged so that the total number of 1 bits is odd.
- `Even': an extra parity bit is sent alongside each byte, and
arranged so that the total number of 1 bits is even.
- `Mark': an extra parity bit is sent alongside each byte, and
always set to 1.
- `Space': an extra parity bit is sent alongside each byte, and
always set to 0.
4.29.6 Selecting the serial flow control scheme
The `Flow control' box allows you to choose what type of flow
control checking is used on the serial line. The settings are:
- `None': no flow control is done. Data may be lost if either side
attempts to send faster than the serial line permits.
- `XON/XOFF': flow control is done by sending XON and XOFF
characters within the data stream.
- `RTS/CTS': flow control is done using the RTS and CTS wires on
the serial line.
- `DSR/DTR': flow control is done using the DSR and DTR wires on
the serial line.
4.30 The Telnet panel
The Telnet panel allows you to configure options that only apply to
Telnet sessions.
4.30.1 `Handling of OLD_ENVIRON ambiguity'
The original Telnet mechanism for passing environment variables was
badly specified. At the time the standard (RFC 1408) was written,
BSD telnet implementations were already supporting the feature, and
the intention of the standard was to describe the behaviour the BSD
implementations were already using.
Sadly there was a typing error in the standard when it was issued,
and two vital function codes were specified the wrong way round. BSD
implementations did not change, and the standard was not corrected.
Therefore, it's possible you might find either BSD or RFC-compliant
implementations out there. This switch allows you to choose which
one PuTTY claims to be.
The problem was solved by issuing a second standard, defining a
new Telnet mechanism called NEW_ENVIRON, which behaved exactly
like the original OLD_ENVIRON but was not encumbered by existing
implementations. Most Telnet servers now support this, and it's
unambiguous. This feature should only be needed if you have trouble
passing environment variables to quite an old server.
4.30.2 Passive and active Telnet negotiation modes
In a Telnet connection, there are two types of data passed between
the client and the server: actual text, and _negotiations_ about
which Telnet extra features to use.
PuTTY can use two different strategies for negotiation:
- In _active_ mode, PuTTY starts to send negotiations as soon as
the connection is opened.
- In _passive_ mode, PuTTY will wait to negotiate until it sees a
negotiation from the server.
The obvious disadvantage of passive mode is that if the server is
also operating in a passive mode, then negotiation will never begin
at all. For this reason PuTTY defaults to active mode.
However, sometimes passive mode is required in order to successfully
get through certain types of firewall and Telnet proxy server. If
you have confusing trouble with a firewall, you could try enabling
passive mode to see if it helps.
4.30.3 `Keyboard sends Telnet special commands'
If this box is checked, several key sequences will have their normal
actions modified:
- the Backspace key on the keyboard will send the Telnet special
backspace code;
- Control-C will send the Telnet special Interrupt Process code;
- Control-Z will send the Telnet special Suspend Process code.
You probably shouldn't enable this unless you know what you're
doing.
4.30.4 `Return key sends Telnet New Line instead of ^M'
Unlike most other remote login protocols, the Telnet protocol has
a special `new line' code that is not the same as the usual line
endings of Control-M or Control-J. By default, PuTTY sends the
Telnet New Line code when you press Return, instead of sending
Control-M as it does in most other protocols.
Most Unix-style Telnet servers don't mind whether they receive
Telnet New Line or Control-M; some servers do expect New Line,
and some servers prefer to see ^M. If you are seeing surprising
behaviour when you press Return in a Telnet session, you might try
turning this option off to see if it helps.
4.31 The Rlogin panel
The Rlogin panel allows you to configure options that only apply to
Rlogin sessions.
4.31.1 `Local username'
Rlogin allows an automated (password-free) form of login by means
of a file called `.rhosts' on the server. You put a line in your
`.rhosts' file saying something like `
[email protected]', and
then when you make an Rlogin connection the client transmits the
username of the user running the Rlogin client. The server checks
the username and hostname against `.rhosts', and if they match it
does not ask for a password.
This only works because Unix systems contain a safeguard to stop a
user from pretending to be another user in an Rlogin connection.
Rlogin connections have to come from port numbers below 1024, and
Unix systems prohibit this to unprivileged processes; so when the
server sees a connection from a low-numbered port, it assumes the
client end of the connection is held by a privileged (and therefore
trusted) process, so it believes the claim of who the user is.
Windows does not have this restriction: _any_ user can initiate
an outgoing connection from a low-numbered port. Hence, the
Rlogin `.rhosts' mechanism is completely useless for securely
distinguishing several different users on a Windows machine. If you
have a `.rhosts' entry pointing at a Windows PC, you should assume
that _anyone_ using that PC can spoof your username in an Rlogin
connection and access your account on the server.
The `Local username' control allows you to specify what user name
PuTTY should claim you have, in case it doesn't match your Windows
user name (or in case you didn't bother to set up a Windows user
name).
4.32 The SUPDUP panel
The SUPDUP panel allows you to configure options that only apply
to SUPDUP sessions. See section 3.10 for more about the SUPDUP
protocol.
4.32.1 `Location string'
In SUPDUP, the client sends a piece of text of its choice to the
server giving the user's location. This is typically displayed in
lists of logged-in users.
By default, PuTTY just defaults this to "The Internet". If you
want your location to show up as something more specific, you can
configure it here.
4.32.2 `Extended ASCII Character set'
This declares what kind of character set extension your terminal
supports. If the server supports it, it will send text using
that character set. `None' means the standard 95 printable ASCII
characters. `ITS' means ASCII extended with printable characters in
the control character range. This character set is documented in the
SUPDUP protocol definition. `WAITS' is similar to `ITS' but uses
some alternative characters in the extended set: most prominently,
it will display arrows instead of `^' and `_', and `}' instead of
`~'. `ITS' extended ASCII is used by ITS and Lisp machines, whilst
`WAITS' is only used by the WAITS operating system from the Stanford
AI Laboratory.
4.32.3 `**MORE** processing'
When **MORE** processing is enabled, the server causes output to
pause at the bottom of the screen, until a space is typed.
4.32.4 `Terminal scrolling'
This controls whether the terminal will perform scrolling when the
cursor goes below the last line, or if the cursor will return to the
first line.
4.33 Storing configuration in a file
PuTTY does not currently support storing its configuration in a file
instead of the Registry. However, you can work around this with a
couple of batch files.
You will need a file called (say) `PUTTY.BAT' which imports the
contents of a file into the Registry, then runs PuTTY, exports
the contents of the Registry back into the file, and deletes the
Registry entries. This can all be done using the Regedit command
line options, so it's all automatic. Here is what you need in
`PUTTY.BAT':
@ECHO OFF
regedit /s putty.reg
regedit /s puttyrnd.reg
start /w putty.exe
regedit /ea new.reg HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
copy new.reg putty.reg
del new.reg
regedit /s puttydel.reg
This batch file needs two auxiliary files: `PUTTYRND.REG' which sets
up an initial safe location for the `PUTTY.RND' random seed file,
and `PUTTYDEL.REG' which destroys everything in the Registry once
it's been successfully saved back to the file.
Here is `PUTTYDEL.REG':
REGEDIT4
[-HKEY_CURRENT_USER\Software\SimonTatham\PuTTY]
Here is an example `PUTTYRND.REG' file:
REGEDIT4
[HKEY_CURRENT_USER\Software\SimonTatham\PuTTY]
"RandSeedFile"="a:\\putty.rnd"
You should replace `a:\putty.rnd' with the location where you want
to store your random number data. If the aim is to carry around
PuTTY and its settings on one USB stick, you probably want to store
it on the USB stick.
Chapter 5: Using PSCP to transfer files securely
------------------------------------------------
PSCP, the PuTTY Secure Copy client, is a tool for transferring files
securely between computers using an SSH connection.
If you have an SSH-2 server, you might prefer PSFTP (see chapter
6) for interactive use. PSFTP does not in general work with SSH-1
servers, however.
5.1 Starting PSCP
PSCP is a command line application. This means that you cannot just
double-click on its icon to run it and instead you have to bring up
a console window. With Windows 95, 98, and ME, this is called an
`MS-DOS Prompt' and with Windows NT, 2000, and XP, it is called a
`Command Prompt'. It should be available from the Programs section
of your Start Menu.
To start PSCP it will need either to be on your `PATH' or in your
current directory. To add the directory containing PSCP to your
`PATH' environment variable, type into the console window:
set PATH=C:\path\to\putty\directory;%PATH%
This will only work for the lifetime of that particular console
window. To set your `PATH' more permanently on Windows NT, 2000, and
XP, use the Environment tab of the System Control Panel. On Windows
95, 98, and ME, you will need to edit your `AUTOEXEC.BAT' to include
a `set' command like the one above.
5.2 PSCP Usage
Once you've got a console window to type into, you can type `pscp -
h' to bring up a usage message. This tells you the version of PSCP
you're using, and gives you a brief summary of how to use PSCP:
C:\>pscp -h
PuTTY Secure Copy client
Release 0.83
Usage: pscp [options] [user@]host:source target
pscp [options] source [source...] [user@]host:target
pscp [options] -ls [user@]host:filespec
Options:
-V print version information and exit
-pgpfp print PGP key fingerprints and exit
-p preserve file attributes
-q quiet, don't show statistics
-r copy directories recursively
-v show verbose messages
-load sessname Load settings from saved session
-P port connect to specified port
-l user connect with specified username
-pwfile file login with password read from specified file
-1 -2 force use of particular SSH protocol version
-ssh -ssh-connection
force use of particular SSH protocol variant
-4 -6 force use of IPv4 or IPv6
-C enable compression
-i key private key file for user authentication
-noagent disable use of Pageant
-agent enable use of Pageant
-no-trivial-auth
disconnect if SSH authentication succeeds trivially
-hostkey keyid
manually specify a host key (may be repeated)
-batch disable all interactive prompts
-no-sanitise-stderr don't strip control chars from standard error
-proxycmd command
use 'command' as local proxy
-unsafe allow server-side wildcards (DANGEROUS)
-sftp force use of SFTP protocol
-scp force use of SCP protocol
-sshlog file
-sshrawlog file
log protocol details to a file
-logoverwrite
-logappend
control what happens when a log file already exists
(PSCP's interface is much like the Unix `scp' command, if you're
familiar with that.)
5.2.1 The basics
To receive (a) file(s) from a remote server:
pscp [options] [user@]host:source target
So to copy the file `/etc/hosts' from the server `example.com' as
user `fred' to the file `c:\temp\example-hosts.txt', you would type:
pscp
[email protected]:/etc/hosts c:\temp\example-hosts.txt
To send (a) file(s) to a remote server:
pscp [options] source [source...] [user@]host:target
So to copy the local file `c:\documents\foo.txt' to the server
`example.com' as user `fred' to the file `/tmp/foo' you would type:
pscp c:\documents\foo.txt
[email protected]:/tmp/foo
You can use wildcards to transfer multiple files in either
direction, like this:
pscp c:\documents\*.doc
[email protected]:docfiles
pscp
[email protected]:source/*.c c:\source
However, in the second case (using a wildcard for multiple remote
files) you may see a warning saying something like `warning: remote
host tried to write to a file called `terminal.c' when we requested
a file called `*.c'. If this is a wildcard, consider upgrading to
SSH-2 or using the `-unsafe' option. Renaming of this file has been
disallowed'.
This is due to a fundamental insecurity in the old-style SCP
protocol: the client sends the wildcard string (`*.c') to the
server, and the server sends back a sequence of file names that
match the wildcard pattern. However, there is nothing to stop the
server sending back a _different_ pattern and writing over one of
your other files: if you request `*.c', the server might send back
the file name `AUTOEXEC.BAT' and install a virus for you. Since the
wildcard matching rules are decided by the server, the client cannot
reliably verify that the filenames sent back match the pattern.
PSCP will attempt to use the newer SFTP protocol (part of SSH-2)
where possible, which does not suffer from this security flaw. If
you are talking to an SSH-2 server which supports SFTP, you will
never see this warning. (You can force use of the SFTP protocol, if
available, with `-sftp' - see section 5.2.2.6.)
If you really need to use a server-side wildcard with an SSH-1
server, you can use the `-unsafe' command line option with PSCP:
pscp -unsafe
[email protected]:source/*.c c:\source
This will suppress the warning message and the file transfer will
happen. However, you should be aware that by using this option you
are giving the server the ability to write to _any_ file in the
target directory, so you should only use this option if you trust
the server administrator not to be malicious (and not to let the
server machine be cracked by malicious people). Alternatively, do
any such download in a newly created empty directory. (Even in
`unsafe' mode, PSCP will still protect you against the server trying
to get out of that directory using pathnames including `..'.)
5.2.1.1 `user'
The login name on the remote server. If this is omitted, and `host'
is a PuTTY saved session, PSCP will use any username specified by
that saved session. Otherwise, PSCP will attempt to use the local
Windows username.
5.2.1.2 `host'
The name of the remote server, or the name of an existing PuTTY
saved session. In the latter case, the session's settings for
hostname, port number, cipher type and username will be used.
5.2.1.3 `source'
One or more source files. Wildcards are allowed. The syntax of
wildcards depends on the system to which they apply, so if you are
copying _from_ a Windows system _to_ a UNIX system, you should use
Windows wildcard syntax (e.g. `*.*'), but if you are copying _from_
a UNIX system _to_ a Windows system, you would use the wildcard
syntax allowed by your UNIX shell (e.g. `*').
If the source is a remote server and you do not specify a full
pathname (in UNIX, a pathname beginning with a `/' (slash)
character), what you specify as a source will be interpreted
relative to your home directory on the remote server.
5.2.1.4 `target'
The filename or directory to put the file(s). When copying from a
remote server to a local host, you may wish simply to place the
file(s) in the current directory. To do this, you should specify a
target of `.'. For example:
pscp
[email protected]:/home/tom/.emacs .
...would copy `/home/tom/.emacs' on the remote server to the current
directory.
As with the `source' parameter, if the target is on a remote server
and is not a full path name, it is interpreted relative to your home
directory on the remote server.
5.2.2 Options
PSCP accepts all the general command line options supported by the
PuTTY tools, except the ones which make no sense in a file transfer
utility. See section 3.11.3 for a description of these options. (The
ones not supported by PSCP are clearly marked.)
PSCP also supports some of its own options. The following sections
describe PSCP's specific command-line options.
5.2.2.1 `-ls' list remote files
If the `-ls' option is given, no files are transferred; instead,
remote files are listed. Only a hostname specification and optional
remote file specification need be given. For example:
pscp -ls
[email protected]:dir1
The SCP protocol does not contain within itself a means of listing
files. If SCP is in use, this option therefore assumes that the
server responds appropriately to the command `ls -la'; this may not
work with all servers.
If SFTP is in use, this option should work with all servers.
5.2.2.2 `-p' preserve file attributes
By default, files copied with PSCP are timestamped with the date
and time they were copied. The `-p' option preserves the original
timestamp on copied files.
5.2.2.3 `-q' quiet, don't show statistics
By default, PSCP displays a meter displaying the progress of the
current transfer:
mibs.tar | 168 kB | 84.0 kB/s | ETA: 00:00:13 | 13%
The fields in this display are (from left to right), filename, size
(in kilobytes) of file transferred so far, estimate of how fast the
file is being transferred (in kilobytes per second), estimated time
that the transfer will be complete, and percentage of the file so
far transferred. The `-q' option to PSCP suppresses the printing of
these statistics.
5.2.2.4 `-r' copies directories recursively
By default, PSCP will only copy files. Any directories you specify
to copy will be skipped, as will their contents. The `-r' option
tells PSCP to descend into any directories you specify, and to copy
them and their contents. This allows you to use PSCP to transfer
whole directory structures between machines.
5.2.2.5 `-batch' avoid interactive prompts
If you use the `-batch' option, PSCP will never give an interactive
prompt while establishing the connection. If the server's host key
is invalid, for example (see section 2.2), then the connection will
simply be abandoned instead of asking you what to do next.
This may help PSCP's behaviour when it is used in automated scripts:
using `-batch', if something goes wrong at connection time, the
batch job will fail rather than hang.
5.2.2.6 `-sftp', `-scp' force use of particular file transfer protocol
As mentioned in section 5.2.1, there are two different file transfer
protocols in use with SSH. Despite its name, PSCP (like many other
ostensible scp clients) can use either of these protocols.
The older SCP protocol does not have a written specification and
leaves a lot of detail to the server platform. Wildcards are
expanded on the server. The simple design means that any wildcard
specification supported by the server platform (such as brace
expansion) can be used, but also leads to interoperability issues
such as with filename quoting (for instance, where filenames contain
spaces), and also the security issue described in section 5.2.1.
The newer SFTP protocol, which is usually associated with SSH-
2 servers, is specified in a more platform independent way, and
leaves issues such as wildcard syntax up to the client. (PuTTY's
SFTP wildcard syntax is described in section 6.2.2.) This makes it
more consistent across platforms, more suitable for scripting and
automation, and avoids security issues with wildcard matching.
Normally PSCP will attempt to use the SFTP protocol, and only fall
back to the SCP protocol if SFTP is not available on the server.
The `-scp' option forces PSCP to use the SCP protocol or quit.
The `-sftp' option forces PSCP to use the SFTP protocol or quit.
When this option is specified, PSCP looks harder for an SFTP server,
which may allow use of SFTP with SSH-1 depending on server setup.
5.2.2.7 `-no-sanitise-stderr': control error message sanitisation
The `-no-sanitise-stderr' option will cause PSCP to pass through the
server's standard-error stream literally, without stripping control
characters from it first. This might be useful if the server were
sending coloured error messages, but it also gives the server the
ability to have unexpected effects on your terminal display. For
more discussion, see section 7.2.3.5.
5.2.3 Return value
PSCP returns an ERRORLEVEL of zero (success) only if the files were
correctly transferred. You can test for this in a batch file, using
code such as this:
pscp file*.* user@hostname:
if errorlevel 1 echo There was an error
5.2.4 Using public key authentication with PSCP
Like PuTTY, PSCP can authenticate using a public key instead of a
password. There are three ways you can do this.
Firstly, PSCP can use PuTTY saved sessions in place of hostnames
(see section 5.2.1.2). So you would do this:
- Run PuTTY, and create a PuTTY saved session (see section 4.1.2)
which specifies your private key file (see section 4.22.1). You
will probably also want to specify a username to log in as (see
section 4.15.1).
- In PSCP, you can now use the name of the session instead of
a hostname: type `pscp sessionname:file localfile', where
`sessionname' is replaced by the name of your saved session.
Secondly, you can supply the name of a private key file on the
command line, with the `-i' option. See section 3.11.3.18 for more
information.
Thirdly, PSCP will attempt to authenticate using Pageant if Pageant
is running (see chapter 9). So you would do this:
- Ensure Pageant is running, and has your private key stored in
it.
- Specify a user and host name to PSCP as normal. PSCP will
automatically detect Pageant and try to use the keys within it.
For more general information on public-key authentication, see
chapter 8.
Chapter 6: Using PSFTP to transfer files securely
-------------------------------------------------
PSFTP, the PuTTY SFTP client, is a tool for transferring files
securely between computers using an SSH connection.
PSFTP differs from PSCP in the following ways:
- PSCP should work on virtually every SSH server. PSFTP uses the
new SFTP protocol, which is a feature of SSH-2 only. (PSCP
will also use this protocol if it can, but there is an SSH-1
equivalent it can fall back to if it cannot.)
- PSFTP allows you to run an interactive file transfer session,
much like the Windows `ftp' program. You can list the contents
of directories, browse around the file system, issue multiple
`get' and `put' commands, and eventually log out. By contrast,
PSCP is designed to do a single file transfer operation and
immediately terminate.
6.1 Starting PSFTP
The usual way to start PSFTP is from a command prompt, much like
PSCP. To do this, it will need either to be on your `PATH' or in
your current directory. To add the directory containing PSFTP to
your `PATH' environment variable, type into the console window:
set PATH=C:\path\to\putty\directory;%PATH%
Unlike PSCP, however, PSFTP has no complex command-line syntax; you
just specify a host name and perhaps a user name:
psftp server.example.com
or perhaps
psftp
[email protected]
Alternatively, if you just type `psftp' on its own (or double-click
the PSFTP icon in the Windows GUI), you will see the PSFTP prompt,
and a message telling you PSFTP has not connected to any server:
C:\>psftp
psftp: no hostname specified; use "open host.name" to connect
psftp>
At this point you can type `open server.example.com' or
`open
[email protected]' to start a session.
PSFTP accepts all the general command line options supported by the
PuTTY tools, except the ones which make no sense in a file transfer
utility. See section 3.11.3 for a description of these options. (The
ones not supported by PSFTP are clearly marked.)
PSFTP also supports some of its own options. The following sections
describe PSFTP's specific command-line options.
6.1.1 `-b': specify a file containing batch commands
In normal operation, PSFTP is an interactive program which displays
a command line and accepts commands from the keyboard.
If you need to do automated tasks with PSFTP, you would probably
prefer to specify a set of commands in advance and have them
executed automatically. The `-b' option allows you to do this. You
use it with a file name containing batch commands. For example, you
might create a file called `myscript.scr' containing lines like
this:
cd /home/ftp/users/jeff
del jam-old.tar.gz
ren jam.tar.gz jam-old.tar.gz
put jam.tar.gz
chmod a+r jam.tar.gz
and then you could run the script by typing
psftp user@hostname -b myscript.scr
When you run a batch script in this way, PSFTP will abort the script
if any command fails to complete successfully. To change this
behaviour, you can add the `-be' option (section 6.1.3).
PSFTP will terminate after it finishes executing the batch script.
6.1.2 `-bc': display batch commands as they are run
The `-bc' option alters what PSFTP displays while processing a
batch script specified with `-b'. With the `-bc' option, PSFTP will
display prompts and commands just as if the commands had been typed
at the keyboard. So instead of seeing this:
C:\>psftp fred@hostname -b batchfile
Sent username "fred"
Remote working directory is /home/fred
Listing directory /home/fred/lib
drwxrwsr-x 4 fred fred 1024 Sep 6 10:42 .
drwxr-sr-x 25 fred fred 2048 Dec 14 09:36 ..
drwxrwsr-x 3 fred fred 1024 Apr 17 2000 jed
lrwxrwxrwx 1 fred fred 24 Apr 17 2000 timber
drwxrwsr-x 2 fred fred 1024 Mar 13 2000 trn
you might see this:
C:\>psftp fred@hostname -bc -b batchfile
Sent username "fred"
Remote working directory is /home/fred
psftp> dir lib
Listing directory /home/fred/lib
drwxrwsr-x 4 fred fred 1024 Sep 6 10:42 .
drwxr-sr-x 25 fred fred 2048 Dec 14 09:36 ..
drwxrwsr-x 3 fred fred 1024 Apr 17 2000 jed
lrwxrwxrwx 1 fred fred 24 Apr 17 2000 timber
drwxrwsr-x 2 fred fred 1024 Mar 13 2000 trn
psftp> quit
6.1.3 `-be': continue batch processing on errors
When running a batch file, this additional option causes PSFTP
to continue processing even if a command fails to complete
successfully.
You might want this to happen if you wanted to delete a file and
didn't care if it was already not present, for example.
6.1.4 `-batch': avoid interactive prompts
If you use the `-batch' option, PSFTP will never give an interactive
prompt while establishing the connection. If the server's host key
is invalid, for example (see section 2.2), then the connection will
simply be abandoned instead of asking you what to do next.
This may help PSFTP's behaviour when it is used in automated
scripts: using `-batch', if something goes wrong at connection time,
the batch job will fail rather than hang.
6.1.4.1 `-no-sanitise-stderr': control error message sanitisation
The `-no-sanitise-stderr' option will cause PSFTP to pass through
the server's standard-error stream literally, without stripping
control characters from it first. This might be useful if the server
were sending coloured error messages, but it also gives the server
the ability to have unexpected effects on your terminal display. For
more discussion, see section 7.2.3.5.
6.2 Running PSFTP
Once you have started your PSFTP session, you will see a `psftp>'
prompt. You can now type commands to perform file-transfer
functions. This section lists all the available commands.
Any line starting with a # will be treated as a comment and ignored.
6.2.1 General quoting rules for PSFTP commands
Most PSFTP commands are considered by the PSFTP command interpreter
as a sequence of words, separated by spaces. For example, the
command `ren oldfilename newfilename' splits up into three words:
`ren' (the command name), `oldfilename' (the name of the file to be
renamed), and `newfilename' (the new name to give the file).
Sometimes you will need to specify file names that _contain_ spaces.
In order to do this, you can surround the file name with double
quotes. This works equally well for local file names and remote file
names:
psftp> get "spacey file name.txt" "save it under this name.txt"
The double quotes themselves will not appear as part of the file
names; they are removed by PSFTP and their only effect is to stop
the spaces inside them from acting as word separators.
If you need to _use_ a double quote (on some types of remote system,
such as Unix, you are allowed to use double quotes in file names),
you can do this by doubling it. This works both inside and outside
double quotes. For example, this command
psftp> ren ""this"" "a file with ""quotes"" in it"
will take a file whose current name is `"this"' (with a double quote
character at the beginning and the end) and rename it to a file
whose name is `a file with "quotes" in it'.
(The one exception to the PSFTP quoting rules is the `!' command,
which passes its command line straight to Windows without splitting
it up into words at all. See section 6.2.19.)
6.2.2 Wildcards in PSFTP
Several commands in PSFTP support `wildcards' to select multiple
files.
For _local_ file specifications (such as the first argument to
`put'), wildcard rules for the local operating system are used. For
instance, PSFTP running on Windows might require the use of `*.*'
where PSFTP on Unix would need `*'.
For _remote_ file specifications (such as the first argument to
`get'), PSFTP uses a standard wildcard syntax (similar to POSIX
wildcards):
- `*' matches any sequence of characters (including a zero-length
sequence).
- `?' matches exactly one character.
- `[abc]' matches exactly one character which can be a, b, or c.
`[a-z]' matches any character in the range a to z.
`[^abc]' matches a single character that is _not_ a, b, or c.
Special cases: `[-a]' matches a literal hyphen (-) or a; `[^-a]'
matches all other characters. `[a^]' matches a literal caret (^)
or a.
- `\' (backslash) before any of the above characters (or itself)
removes that character's special meaning.
A leading period (.) on a filename is not treated specially, unlike
in some Unix contexts; `get *' will fetch all files, whether or not
they start with a leading period.
6.2.3 The `open' command: start a session
If you started PSFTP by double-clicking in the GUI, or just by
typing `psftp' at the command line, you will need to open a
connection to an SFTP server before you can issue any other commands
(except `help' and `quit').
To create a connection, type `open host.name', or if you need to
specify a user name as well you can type `open
[email protected]'. You
can optionally specify a port as well: `open
[email protected] 22'.
Once you have issued this command, you will not be able to issue it
again, _even_ if the command fails (for example, if you mistype the
host name or the connection times out). So if the connection is not
opened successfully, PSFTP will terminate immediately.
6.2.4 The `quit' command: end your session
When you have finished your session, type the command `quit' to
close the connection, terminate PSFTP and return to the command line
(or just close the PSFTP console window if you started it from the
GUI).
You can also use the `bye' and `exit' commands, which have exactly
the same effect.
6.2.5 The `close' command: close your connection
If you just want to close the network connection but keep PSFTP
running, you can use the `close' command. You can then use the
`open' command to open a new connection.
6.2.6 The `help' command: get quick online help
If you type `help', PSFTP will give a short list of the available
commands.
If you type `help' with a command name - for example, `help get'
- then PSFTP will give a short piece of help on that particular
command.
6.2.7 The `cd' and `pwd' commands: changing the remote working directory
PSFTP maintains a notion of your `working directory' on the server.
This is the default directory that other commands will operate on.
For example, if you type `get filename.dat' then PSFTP will look for
`filename.dat' in your remote working directory on the server.
To change your remote working directory, use the `cd' command. If
you don't provide an argument, `cd' will return you to your home
directory on the server (more precisely, the remote directory you
were in at the start of the connection).
To display your current remote working directory, type `pwd'.
6.2.8 The `lcd' and `lpwd' commands: changing the local working directory
As well as having a working directory on the remote server, PSFTP
also has a working directory on your local machine (just like
any other Windows process). This is the default local directory
that other commands will operate on. For example, if you type
`get filename.dat' then PSFTP will save the resulting file as
`filename.dat' in your local working directory.
To change your local working directory, use the `lcd' command. To
display your current local working directory, type `lpwd'.
6.2.9 The `get' command: fetch a file from the server
To download a file from the server and store it on your local PC,
you use the `get' command.
In its simplest form, you just use this with a file name:
get myfile.dat
If you want to store the file locally under a different name,
specify the local file name after the remote one:
get myfile.dat newname.dat
This will fetch the file on the server called `myfile.dat', but will
save it to your local machine under the name `newname.dat'.
To fetch an entire directory recursively, you can use the `-r'
option:
get -r mydir
get -r mydir newname
(If you want to fetch a file whose name starts with a hyphen, you
may have to use the `--' special argument, which stops `get' from
interpreting anything as a switch after it. For example, `get -- -
silly-name-'.)
6.2.10 The `put' command: send a file to the server
To upload a file to the server from your local PC, you use the `put'
command.
In its simplest form, you just use this with a file name:
put myfile.dat
If you want to store the file remotely under a different name,
specify the remote file name after the local one:
put myfile.dat newname.dat
This will send the local file called `myfile.dat', but will store it
on the server under the name `newname.dat'.
To send an entire directory recursively, you can use the `-r'
option:
put -r mydir
put -r mydir newname
(If you want to send a file whose name starts with a hyphen, you
may have to use the `--' special argument, which stops `put' from
interpreting anything as a switch after it. For example, `put -- -
silly-name-'.)
6.2.11 The `mget' and `mput' commands: fetch or send multiple files
`mget' works almost exactly like `get', except that it allows you to
specify more than one file to fetch at once. You can do this in two
ways:
- by giving two or more explicit file names
(`mget file1.txt file2.txt')
- by using a wildcard (`mget *.txt').
Every argument to `mget' is treated as the name of a file to fetch
(unlike `get', which will interpret at most one argument like
that, and a second argument will be treated as an alternative name
under which to store the retrieved file), or a wildcard expression
matching more than one file.
The `-r' and `--' options from `get' are also available with `mget'.
`mput' is similar to `put', with the same differences.
6.2.12 The `reget' and `reput' commands: resuming file transfers
If a file transfer fails half way through, and you end up with half
the file stored on your disk, you can resume the file transfer using
the `reget' and `reput' commands. These work exactly like the `get'
and `put' commands, but they check for the presence of the half-
written destination file and start transferring from where the last
attempt left off.
The syntax of `reget' and `reput' is exactly the same as the syntax
of `get' and `put':
reget myfile.dat
reget myfile.dat newname.dat
reget -r mydir
These commands are intended mainly for resuming interrupted
transfers. They assume that the remote file or directory structure
has not changed in any way; if there have been changes, you may end
up with corrupted files. In particular, the `-r' option will not
pick up changes to files or directories already transferred in full.
6.2.13 The `dir' command: list remote files
To list the files in your remote working directory, just type `dir'.
You can also list the contents of a different directory by typing
`dir' followed by the directory name:
dir /home/fred
dir sources
And you can list a subset of the contents of a directory by
providing a wildcard:
dir /home/fred/*.txt
dir sources/*.c
The `ls' command works exactly the same way as `dir'.
6.2.14 The `chmod' command: change permissions on remote files
PSFTP allows you to modify the file permissions on files and
directories on the server. You do this using the `chmod' command,
which works very much like the Unix `chmod' command.
The basic syntax is `chmod modes file', where `modes' represents a
modification to the file permissions, and `file' is the filename to
modify. You can specify multiple files or wildcards. For example:
chmod go-rwx,u+w privatefile
chmod a+r public*
chmod 640 groupfile1 groupfile2
The `modes' parameter can be a set of octal digits in the Unix
style. (If you don't know what this means, you probably don't want
to be using it!) Alternatively, it can be a list of permission
modifications, separated by commas. Each modification consists of:
- The people affected by the modification. This can be `u'
(the owning user), `g' (members of the owning group), or `o'
(everybody else - `others'), or some combination of those. It
can also be `a' (`all') to affect everybody at once.
- A `+' or `-' sign, indicating whether permissions are to be
added or removed.
- The actual permissions being added or removed. These can be
`r' (permission to read the file), `w' (permission to write
to the file), and `x' (permission to execute the file, or in
the case of a directory, permission to access files within the
directory).
So the above examples would do:
- The first example: `go-rwx' removes read, write and execute
permissions for members of the owning group and everybody else
(so the only permissions left are the ones for the file owner).
`u+w' adds write permission for the file owner.
- The second example: `a+r' adds read permission for everybody to
all files and directories starting with `public'.
In addition to all this, there are a few extra special cases for
Unix systems. On non-Unix systems these are unlikely to be useful:
- You can specify `u+s' and `u-s' to add or remove the Unix set-
user-ID bit. This is typically only useful for special purposes;
refer to your Unix documentation if you're not sure about it.
- You can specify `g+s' and `g-s' to add or remove the Unix set-
group-ID bit. On a file, this works similarly to the set-user-
ID bit (see your Unix documentation again); on a directory it
ensures that files created in the directory are accessible by
members of the group that owns the directory.
- You can specify `+t' and `-t' to add or remove the Unix `sticky
bit'. When applied to a directory, this means that the owner of
a file in that directory can delete the file (whereas normally
only the owner of the _directory_ would be allowed to).
6.2.15 The `del' command: delete remote files
To delete a file on the server, type `del' and then the filename or
filenames:
del oldfile.dat
del file1.txt file2.txt
del *.o
Files will be deleted without further prompting, even if multiple
files are specified.
`del' will only delete files. You cannot use it to delete
directories; use `rmdir' for that.
The `rm' command works exactly the same way as `del'.
6.2.16 The `mkdir' command: create remote directories
To create a directory on the server, type `mkdir' and then the
directory name:
mkdir newstuff
You can specify multiple directories to create at once:
mkdir dir1 dir2 dir3
6.2.17 The `rmdir' command: remove remote directories
To remove a directory on the server, type `rmdir' and then the
directory name or names:
rmdir oldstuff
rmdir *.old ancient
Directories will be deleted without further prompting, even if
multiple directories are specified.
Most SFTP servers will probably refuse to remove a directory if
the directory has anything in it, so you will need to delete the
contents first.
6.2.18 The `mv' command: move and rename remote files
To rename a single file on the server, type `mv', then the current
file name, and then the new file name:
mv oldfile newname
You can also move the file into a different directory and change the
name:
mv oldfile dir/newname
To move one or more files into an existing subdirectory, specify
the files (using wildcards if desired), and then the destination
directory:
mv file dir
mv file1 dir1/file2 dir2
mv *.c *.h ..
The `rename' and `ren' commands work exactly the same way as `mv'.
6.2.19 The `!' command: run a local Windows command
You can run local Windows commands using the `!' command. This is
the only PSFTP command that is not subject to the command quoting
rules given in section 6.2.1. If any command line begins with the
`!' character, then the rest of the line will be passed straight to
Windows without further translation.
For example, if you want to move an existing copy of a file out of
the way before downloading an updated version, you might type:
psftp> !ren myfile.dat myfile.bak
psftp> get myfile.dat
using the Windows `ren' command to rename files on your local PC.
6.3 Using public key authentication with PSFTP
Like PuTTY, PSFTP can authenticate using a public key instead of a
password. There are three ways you can do this.
Firstly, PSFTP can use PuTTY saved sessions in place of hostnames.
So you might do this:
- Run PuTTY, and create a PuTTY saved session (see section 4.1.2)
which specifies your private key file (see section 4.22.1). You
will probably also want to specify a username to log in as (see
section 4.15.1).
- In PSFTP, you can now use the name of the session instead of
a hostname: type `psftp sessionname', where `sessionname' is
replaced by the name of your saved session.
Secondly, you can supply the name of a private key file on the
command line, with the `-i' option. See section 3.11.3.18 for more
information.
Thirdly, PSFTP will attempt to authenticate using Pageant if Pageant
is running (see chapter 9). So you would do this:
- Ensure Pageant is running, and has your private key stored in
it.
- Specify a user and host name to PSFTP as normal. PSFTP will
automatically detect Pageant and try to use the keys within it.
For more general information on public-key authentication, see
chapter 8.
Chapter 7: Using the command-line connection tool Plink
-------------------------------------------------------
Plink is a command-line connection tool similar to UNIX `ssh'. It is
mostly used for automated operations, such as making CVS access a
repository on a remote server.
Plink is probably not what you want if you want to run an
interactive session in a console window.
7.1 Starting Plink
Plink is a command line application. This means that you cannot just
double-click on its icon to run it and instead you have to bring
up a console window. In Windows 95, 98, and ME, this is called an
`MS-DOS Prompt', and in Windows NT, 2000, and XP, it is called a
`Command Prompt'. It should be available from the Programs section
of your Start Menu.
In order to use Plink, the file `plink.exe' will need either to be
on your `PATH' or in your current directory. To add the directory
containing Plink to your `PATH' environment variable, type into the
console window:
set PATH=C:\path\to\putty\directory;%PATH%
This will only work for the lifetime of that particular console
window. To set your `PATH' more permanently on Windows NT, 2000, and
XP, use the Environment tab of the System Control Panel. On Windows
95, 98, and ME, you will need to edit your `AUTOEXEC.BAT' to include
a `set' command like the one above.
7.2 Using Plink
This section describes the basics of how to use Plink for
interactive logins and for automated processes.
Once you've got a console window to type into, you can type `plink -
-help' to bring up a usage message. This tells you the version of
Plink you're using, and gives you a brief summary of how to use
Plink:
C:\>plink --help
Plink: command-line connection utility
Release 0.83
Usage: plink [options] [user@]host [command]
("host" can also be a PuTTY saved session name)
Options:
-V print version information and exit
-pgpfp print PGP key fingerprints and exit
-v show verbose messages
-load sessname Load settings from saved session
-ssh -telnet -rlogin -raw -serial
force use of a particular protocol
-ssh-connection
force use of the bare ssh-connection protocol
-P port connect to specified port
-l user connect with specified username
-batch disable all interactive prompts
-proxycmd command
use 'command' as local proxy
-sercfg configuration-string (e.g. 19200,8,n,1,X)
Specify the serial configuration (serial only)
The following options only apply to SSH connections:
-pwfile file login with password read from specified file
-D [listen-IP:]listen-port
Dynamic SOCKS-based port forwarding
-L [listen-IP:]listen-port:host:port
Forward local port to remote address
-R [listen-IP:]listen-port:host:port
Forward remote port to local address
-X -x enable / disable X11 forwarding
-A -a enable / disable agent forwarding
-t -T enable / disable pty allocation
-1 -2 force use of particular SSH protocol version
-4 -6 force use of IPv4 or IPv6
-C enable compression
-i key private key file for user authentication
-noagent disable use of Pageant
-agent enable use of Pageant
-no-trivial-auth
disconnect if SSH authentication succeeds trivially
-noshare disable use of connection sharing
-share enable use of connection sharing
-hostkey keyid
manually specify a host key (may be repeated)
-sanitise-stderr, -sanitise-stdout, -no-sanitise-stderr, -no-sanitise-stdout
do/don't strip control chars from standard output/error
-no-antispoof omit anti-spoofing prompt after authentication
-m file read remote command(s) from file
-s remote command is an SSH subsystem (SSH-2 only)
-N don't start a shell/command (SSH-2 only)
-nc host:port
open tunnel in place of session (SSH-2 only)
-sshlog file
-sshrawlog file
log protocol details to a file
-logoverwrite
-logappend
control what happens when a log file already exists
-shareexists
test whether a connection-sharing upstream exists
Once this works, you are ready to use Plink.
7.2.1 Using Plink for interactive logins
To make a simple interactive connection to a remote server, just
type `plink' and then the host name:
C:\>plink login.example.com
Debian GNU/Linux 2.2 flunky.example.com
flunky login:
You should then be able to log in as normal and run a session. The
output sent by the server will be written straight to your command
prompt window, which will most likely not interpret terminal control
codes in the way the server expects it to. So if you run any full-
screen applications, for example, you can expect to see strange
characters appearing in your window. Interactive connections like
this are not the main point of Plink.
In order to connect with a different protocol, you can give the
command line options `-ssh', `-ssh-connection', `-telnet', `-
rlogin', or `-raw'. To make an SSH connection, for example:
C:\>plink -ssh login.example.com
login as:
If you have already set up a PuTTY saved session, then instead of
supplying a host name, you can give the saved session name. This
allows you to use public-key authentication, specify a user name,
and use most of the other features of PuTTY:
C:\>plink my-ssh-session
Sent username "fred"
Authenticating with public key "fred@winbox"
Last login: Thu Dec 6 19:25:33 2001 from :0.0
fred@flunky:~$
(You can also use the `-load' command-line option to load a saved
session; see section 3.11.3.1. If you use `-load', the saved session
exists, and it specifies a hostname, you cannot also specify a
`host' or `user@host' argument - it will be treated as part of the
remote command.)
7.2.2 Using Plink for automated connections
More typically Plink is used with the SSH protocol, to enable you
to talk directly to a program running on the server. To do this you
have to ensure Plink is _using_ the SSH protocol. You can do this in
several ways:
- Use the `-ssh' option as described in section 7.2.1.
- Set up a PuTTY saved session that describes the server you are
connecting to, and that also specifies the protocol as SSH.
- Set the Windows environment variable `PLINK_PROTOCOL' to the
word `ssh'.
Usually Plink is not invoked directly by a user, but run
automatically by another process. Therefore you typically do not
want Plink to prompt you for a user name or a password.
Next, you are likely to need to avoid the various interactive
prompts Plink can produce. You might be prompted to verify the host
key of the server you're connecting to, to enter a user name, or to
enter a password.
To avoid being prompted for the server host key when using Plink for
an automated connection, you can first make a _manual_ connection
(using either of PuTTY or Plink) to the same server, verify the host
key (see section 2.2 for more information), and select `Accept'
to add the host key to the Registry. After that, Plink commands
connecting to that server should not give a host key prompt unless
the host key changes. Alternatively, you can specify the appropriate
host key(s) on Plink's command line every time you use it; see
section 3.11.3.22.
To avoid being prompted for a user name, you can:
- Use the `-l' option to specify a user name on the command line.
For example, `plink login.example.com -l fred'.
- Set up a PuTTY saved session that describes the server you are
connecting to, and that also specifies the username to log in as
(see section 4.15.1).
To avoid being prompted for a password, you should almost certainly
set up public-key authentication. (See chapter 8 for a general
introduction to public-key authentication.) Again, you can do this
in two ways:
- Set up a PuTTY saved session that describes the server you
are connecting to, and that also specifies a private key file
(see section 4.22.1). For this to work without prompting, your
private key will need to have no passphrase.
- Store the private key in Pageant. See chapter 9 for further
information.
Once you have done all this, you should be able to run a remote
command on the SSH server machine and have it execute automatically
with no prompting:
C:\>plink login.example.com -l fred echo hello, world
hello, world
C:\>
Or, if you have set up a saved session with all the connection
details:
C:\>plink mysession echo hello, world
hello, world
C:\>
Then you can set up other programs to run this Plink command and
talk to it as if it were a process on the server machine.
7.2.3 Plink command line options
Plink accepts all the general command line options supported by the
PuTTY tools. See section 3.11.3 for a description of these options.
Plink also supports some of its own options. The following sections
describe Plink's specific command-line options.
7.2.3.1 `-batch': disable all interactive prompts
If you use the `-batch' option, Plink will never give an interactive
prompt while establishing the connection. If the server's host key
is invalid, for example (see section 2.2), then the connection will
simply be abandoned instead of asking you what to do next.
This may help Plink's behaviour when it is used in automated
scripts: using `-batch', if something goes wrong at connection time,
the batch job will fail rather than hang.
If another program is invoking Plink on your behalf, then you might
need to arrange that that program passes `-batch' to Plink. See
section 7.4 for an example involving Git.
7.2.3.2 `-s': remote command is SSH subsystem
If you specify the `-s' option, Plink passes the specified command
as the name of an SSH `subsystem' rather than an ordinary command
line.
(This option is only meaningful with the SSH-2 protocol.)
7.2.3.3 `-share': Test and try to share an existing connection.
This option tries to detect if an existing connection can be shared
(See section 4.17.5 for more information about SSH connection
sharing.) and reuses that connection.
A Plink invocation of the form:
plink -share <session>
will test whether there is currently a viable `upstream' for the
session in question, which can be specified using any syntax you'd
normally use with Plink to make an actual connection (a host/port
number, a bare saved session name, `-load', etc). If no `upstream'
viable session is found and `-share' is specified, this connection
will be become the `upstream' connection for subsequent connection
sharing tries.
(This option is only meaningful with the SSH-2 protocol.)
7.2.3.4 `-shareexists': test for connection-sharing upstream
This option does not make a new connection; instead it allows
testing for the presence of an existing connection that can
be shared. (See section 4.17.5 for more information about SSH
connection sharing.)
A Plink invocation of the form:
plink -shareexists <session>
will test whether there is currently a viable `upstream' for the
session in question, which can be specified using any syntax you'd
normally use with Plink to make an actual connection (a host/port
number, a bare saved session name, `-load', etc). It returns a zero
exit status if a usable `upstream' exists, nonzero otherwise.
(This option is only meaningful with the SSH-2 protocol.)
7.2.3.5 `-sanitise-'_stream_: control output sanitisation
In some situations, Plink applies a sanitisation pass to the output
received from the server, to strip out control characters such as
backspace and the escape character.
The idea of this is to prevent remote processes from sending
confusing escape sequences through the standard error channel when
Plink is being used as a transport for something like git or CVS. If
the server actually wants to send an error message, it will probably
be plain text; if the server abuses that channel to try to write
over unexpected parts of your terminal display, Plink will try to
stop it.
By default, this only happens for output channels which are sent
to a Windows console device, or a Unix terminal device. (Any
output stream going somewhere else is likely to be needed by an 8-
bit protocol and must not be tampered with at all.) It also stops
happening if you tell Plink to allocate a remote pseudo-terminal
(see section 3.11.3.12 and section 4.24.1), on the basis that in
that situation you often _want_ escape sequences from the server to
go to your terminal.
But in case Plink guesses wrong about whether you want this
sanitisation, you can override it in either direction, using one of
these options:
`-sanitise-stderr'
Sanitise server data written to Plink's standard error channel,
regardless of terminals and consoles and remote ptys.
`-no-sanitise-stderr'
Do not sanitise server data written to Plink's standard error
channel.
`-sanitise-stdout'
Sanitise server data written to Plink's standard output channel.
`-no-sanitise-stdout'
Do not sanitise server data written to Plink's standard output
channel.
7.2.3.6 -no-antispoof: turn off authentication spoofing protection prompt
In SSH, some possible server authentication methods require user
input (for example, password authentication, or entering a private
key passphrase), and others do not (e.g. a private key held in
Pageant).
If you use Plink to run an interactive login session, and if Plink
authenticates without needing any user interaction, and if the
server is malicious or compromised, it could try to trick you into
giving it authentication data that should not go to the server (such
as your private key passphrase), by sending what _looks_ like one of
Plink's local prompts, as if Plink had not already authenticated.
To protect against this, Plink's default policy is to finish the
authentication phase with a final trivial prompt looking like this:
Access granted. Press Return to begin session.
so that if you saw anything that looked like an authentication
prompt _after_ that line, you would know it was not from Plink.
That extra interactive step is inconvenient. So Plink will turn it
off in as many situations as it can:
- If Plink's standard input is not pointing at a console or
terminal device - for example, if you're using Plink as a
transport for some automated application like version control
- then you _can't_ type passphrases into the server anyway. In
that situation, Plink won't try to protect you from the server
trying to fool you into doing so.
- If Plink is in batch mode (see section 7.2.2), then it _never_
does any interactive authentication. So anything looking like an
interactive authentication prompt is automatically suspect, and
so Plink omits the anti-spoofing prompt.
But if you still find the protective prompt inconvenient, and you
trust the server not to try a trick like this, you can turn it off
using the `-no-antispoof' option.
7.3 Using Plink in batch files and scripts
Once you have set up Plink to be able to log in to a remote server
without any interactive prompting (see section 7.2.2), you can use
it for lots of scripting and batch purposes. For example, to start a
backup on a remote machine, you might use a command like:
plink root@myserver /etc/backups/do-backup.sh
Or perhaps you want to fetch all system log lines relating to a
particular web area:
plink mysession grep /~fred/ /var/log/httpd/access.log > fredlog
Any non-interactive command you could usefully run on the server
command line, you can run in a batch file using Plink in this way.
7.4 Using Plink with Git
To use Plink for Git operations performed over SSH, you can set the
environment variable `GIT_SSH_COMMAND' to point to Plink.
For example, if you've run PuTTY's full Windows installer and it has
installed Plink in the default location, you might do this:
set GIT_SSH_COMMAND="C:\Program Files\PuTTY\plink.exe"
or if you've put Plink somewhere else then you can do a similar
thing with a different path.
This environment variable accepts a whole command line, not just an
executable file name. So you can add Plink options to the end of
it if you like. For example, if you're using Git in a batch-mode
context, where your Git jobs are running unattended and nobody is
available to answer interactive prompts, you might also append the
`-batch' option (section 7.2.3.1):
set GIT_SSH_COMMAND="C:\Program Files\PuTTY\plink.exe" -batch
and then if Plink unexpectedly prints a prompt of some kind (for
example, because the SSH server's host key has changed), your batch
job will terminate with an error message, instead of stopping and
waiting for user input that will never arrive.
(However, you don't _always_ want to do this with Git. If you're
using Git interactively, you might _want_ Plink to stop for
interactive prompts - for example, to let you enter a password for
the SSH server.)
7.5 Using Plink with CVS
To use Plink with CVS, you need to set the environment variable
`CVS_RSH' to point to Plink:
set CVS_RSH=\path\to\plink.exe
You also need to arrange to be able to connect to a remote host
without any interactive prompts, as described in section 7.2.2.
You should then be able to run CVS as follows:
cvs -d :ext:user@sessionname:/path/to/repository co module
If you specified a username in your saved session, you don't even
need to specify the `user' part of this, and you can just say:
cvs -d :ext:sessionname:/path/to/repository co module
7.6 Using Plink with WinCVS
Plink can also be used with WinCVS. Firstly, arrange for Plink to be
able to connect to a remote host non-interactively, as described in
section 7.2.2.
Then, in WinCVS, bring up the `Preferences' dialogue box from the
_Admin_ menu, and switch to the `Ports' tab. Tick the box there
labelled `Check for an alternate rsh name' and in the text entry
field to the right enter the full path to `plink.exe'. Select `OK'
on the `Preferences' dialogue box.
Next, select `Command Line' from the WinCVS `Admin' menu, and type a
CVS command as in section 7.5, for example:
cvs -d :ext:user@hostname:/path/to/repository co module
or (if you're using a saved session):
cvs -d :ext:user@sessionname:/path/to/repository co module
Select the folder you want to check out to with the `Change Folder'
button, and click `OK' to check out your module. Once you've got
modules checked out, WinCVS will happily invoke plink from the GUI
for CVS operations.
Chapter 8: Using public keys for SSH authentication
---------------------------------------------------
8.1 Public key authentication - an introduction
Public key authentication is an alternative means of identifying
yourself to a login server, instead of typing a password. It is more
secure and more flexible, but more difficult to set up.
In conventional password authentication, you prove you are who you
claim to be by proving that you know the correct password. The
only way to prove you know the password is to tell the server what
you think the password is. This means that if the server has been
hacked, or _spoofed_ (see section 2.2), an attacker can learn your
password.
Public key authentication solves this problem. You generate a _key
pair_, consisting of a public key (which everybody is allowed to
know) and a private key (which you keep secret and do not give
to anybody). The private key is able to generate _signatures_. A
signature created using your private key cannot be forged by anybody
who does not have that key; but anybody who has your public key can
verify that a particular signature is genuine.
So you generate a key pair on your own computer, and you copy the
public key to the server. Then, when the server asks you to prove
who you are, PuTTY can generate a signature using your private key.
The server can verify that signature (since it has your public key)
and allow you to log in. Now if the server is hacked or spoofed, the
attacker does not gain your private key or password; they only gain
one signature. And signatures cannot be re-used, so they have gained
nothing.
There is a problem with this: if your private key is stored
unprotected on your own computer, then anybody who gains access to
_that_ will be able to generate signatures as if they were you. So
they will be able to log in to your server under your account. For
this reason, your private key is usually _encrypted_ when it is
stored on your local machine, using a passphrase of your choice. In
order to generate a signature, PuTTY must decrypt the key, so you
have to type your passphrase.
This can make public-key authentication less convenient than
password authentication: every time you log in to the server,
instead of typing a short password, you have to type a longer
passphrase. One solution to this is to use an _authentication
agent_, a separate program which holds decrypted private keys and
generates signatures on request. PuTTY's authentication agent is
called Pageant. When you begin a Windows session, you start Pageant
and load your private key into it (typing your passphrase once).
For the rest of your session, you can start PuTTY any number of
times and Pageant will automatically generate signatures without you
having to do anything. When you close your Windows session, Pageant
shuts down, without ever having stored your decrypted private key on
disk. Many people feel this is a good compromise between security
and convenience. See chapter 9 for further details.
There is more than one public-key algorithm available. The most
common are RSA and ECDSA, but others exist, notably DSA (otherwise
known as DSS), the USA's federal Digital Signature Standard. The key
types supported by PuTTY are described in section 8.2.2.
8.2 Using PuTTYgen, the PuTTY key generator
PuTTYgen is a key generator. It generates pairs of public and
private keys to be used with PuTTY, PSCP, PSFTP, and Plink, as well
as the PuTTY authentication agent, Pageant (see chapter 9). PuTTYgen
generates RSA, DSA, ECDSA, and EdDSA keys.
When you run PuTTYgen you will see a window where you have two main
choices: `Generate', to generate a new public/private key pair, or
`Load' to load in an existing private key.
8.2.1 Generating a new key
This is a general outline of the procedure for generating a new key
pair. The following sections describe the process in more detail.
- First, you need to select which type of key you want to
generate, and also select the strength of the key. This is
described in more detail in section 8.2.2 and section 8.2.3.
- Then press the `Generate' button, to actually generate the key.
Section 8.2.5 describes this step.
- Once you have generated the key, select a comment field (section
8.2.7) and a passphrase (section 8.2.8).
- Now you're ready to save the private key to disk; press the
`Save private key' button. (See section 8.2.10).
Your key pair is now ready for use. You may also want to copy the
public key to your server, either by copying it out of the `Public
key for pasting into OpenSSH authorized_keys file' box (see section
8.2.12), or by using the `Save public key' button (section 8.2.11).
However, you don't need to do this immediately; if you want, you can
load the private key back into PuTTYgen later (see section 8.2.14)
and the public key will be available for copying and pasting again.
Section 8.3 describes the typical process of configuring PuTTY to
attempt public-key authentication, and configuring your SSH server
to accept it.
8.2.2 Selecting the type of key
Before generating a key pair using PuTTYgen, you need to select
which type of key you need.
The current version of the SSH protocol, SSH-2, supports several
different key types, although specific servers may not support all
of them. PuTTYgen can generate:
- An RSA key for use with the SSH-2 protocol.
- A DSA key for use with the SSH-2 protocol.
- An ECDSA (elliptic curve DSA) key for use with the SSH-2
protocol.
- An EdDSA key (Edwards-curve DSA, another elliptic curve
algorithm) for use with the SSH-2 protocol.
PuTTYgen can also generate an RSA key suitable for use with the
old SSH-1 protocol (which only supports RSA); for this, you need
to select the `SSH-1 (RSA)' option. Since the SSH-1 protocol is no
longer considered secure, it's rare to need this option.
8.2.3 Selecting the size (strength) of the key
The `Number of bits' input box allows you to choose the strength of
the key PuTTYgen will generate.
- For RSA and DSA, 2048 bits should currently be sufficient for
most purposes. (Smaller keys of these types are no longer
considered secure, and PuTTYgen will warn if you try to generate
them.)
- For ECDSA, only 256, 384, and 521 bits are supported,
corresponding to NIST-standardised elliptic curves. (Elliptic-
curve keys do not need as many bits as RSA keys for equivalent
security, so these numbers are smaller than the RSA
recommendations.)
- For EdDSA, the only valid sizes are 255 bits (these keys are
also known as `Ed25519' and are commonly used) and 448 bits
(`Ed448', which is much less common at the time of writing).
(256 is also accepted for backward compatibility, but the effect
is the same as 255.)
8.2.4 Selecting the prime generation method
(This is entirely optional. Unless you know better, it's entirely
sensible to skip this and use the default settings.)
On the `Key' menu, you can also optionally change the method for
generating the prime numbers used in the generated key. This is
used for RSA and DSA keys only. (The other key types don't require
generating prime numbers at all.)
The prime-generation method does not affect compatibility: a key
generated with any of these methods will still work with all the
same SSH servers.
The available methods are:
- Use probable primes (fast)
- Use proven primes (slower)
- Use proven primes with even distribution (slowest)
The `probable primes' method sounds unsafe, but it's the most
commonly used prime-generation strategy. There is in theory a
possibility that it might accidentally generate a number that
isn't prime, but the software does enough checking to make that
probability vanishingly small (less than 1 in 2^80, or 1 in 10^24).
So, in practice, nobody worries about it very much.
The other methods cause PuTTYgen to use numbers that it is _sure_
are prime, because it generates the output number together with a
proof of its primality. This takes more effort, but it eliminates
that theoretical risk in the probabilistic method.
There in one way in which PuTTYgen's `proven primes' method is
not strictly better than its `probable primes' method. If you use
PuTTYgen to generate an RSA key on a computer that is potentially
susceptible to timing- or cache-based side-channel attacks, such
as a shared computer, the `probable primes' method is designed to
resist such attacks, whereas the `proven primes' methods are not.
(This is only a concern for RSA keys; for other key types, primes
are either not secret or not involved.)
You might choose to switch from probable to proven primes if you
have a local security standard that demands it, or if you don't
trust the probabilistic argument for the safety of the usual method.
For RSA keys, there's also an option on the `Key' menu to use
`strong' primes as the prime factors of the public key. A `strong'
prime is a prime number chosen to have a particular structure that
makes certain factoring algorithms more difficult to apply, so some
security standards recommend their use. However, the most modern
factoring algorithms are unaffected, so this option is probably not
worth turning on _unless_ you have a local standard that recommends
it.
8.2.5 The `Generate' button
Once you have chosen the type of key you want, and the strength of
the key, press the `Generate' button and PuTTYgen will begin the
process of actually generating the key.
First, a progress bar will appear and PuTTYgen will ask you to move
the mouse around to generate randomness. Wave the mouse in circles
over the blank area in the PuTTYgen window, and the progress bar
will gradually fill up as PuTTYgen collects enough randomness. You
don't need to wave the mouse in particularly imaginative patterns
(although it can't hurt); PuTTYgen will collect enough randomness
just from the fine detail of _exactly_ how far the mouse has moved
each time Windows samples its position.
When the progress bar reaches the end, PuTTYgen will begin creating
the key. The progress bar will reset to the start, and gradually
move up again to track the progress of the key generation. It will
not move evenly, and may occasionally slow down to a stop; this
is unfortunately unavoidable, because key generation is a random
process and it is impossible to reliably predict how long it will
take.
When the key generation is complete, a new set of controls will
appear in the window to indicate this.
8.2.6 The `Key fingerprint' box
The `Key fingerprint' box shows you a fingerprint value for the
generated key. This is derived cryptographically from the _public_
key value, so it doesn't need to be kept secret; it is supposed to
be more manageable for human beings than the public key itself.
The fingerprint value is intended to be cryptographically secure,
in the sense that it is computationally infeasible for someone to
invent a second key with the same fingerprint, or to find a key with
a particular fingerprint. So some utilities, such as the Pageant key
list box (see section 9.2.1) and the Unix `ssh-add' utility, will
list key fingerprints rather than the whole public key.
By default, PuTTYgen will display SSH-2 key fingerprints in the
`SHA256' format. If you need to see the fingerprint in the older
`MD5' format (which looks like `aa:bb:cc:...'), you can choose `Show
fingerprint as MD5' from the `Key' menu, but bear in mind that this
is less cryptographically secure; it may be feasible for an attacker
to create a key with the same fingerprint as yours.
8.2.7 Setting a comment for your key
If you have more than one key and use them for different purposes,
you don't need to memorise the key fingerprints in order to tell
them apart. PuTTYgen allows you to enter a _comment_ for your key,
which will be displayed whenever PuTTY or Pageant asks you for the
passphrase.
The default comment format, if you don't specify one, contains the
key type and the date of generation, such as `rsa-key-20011212'.
Another commonly used approach is to use your name and the name of
the computer the key will be used on, such as `simon@simons-pc'.
To alter the key comment, just type your comment text into the `Key
comment' box before saving the private key. If you want to change
the comment later, you can load the private key back into PuTTYgen,
change the comment, and save it again.
8.2.8 Setting a passphrase for your key
The `Key passphrase' and `Confirm passphrase' boxes allow you to
choose a passphrase for your key. The passphrase will be used to
encrypt the key on disk, so you will not be able to use the key
without first entering the passphrase.
When you save the key, PuTTYgen will check that the `Key passphrase'
and `Confirm passphrase' boxes both contain exactly the same
passphrase, and will refuse to save the key otherwise.
If you leave the passphrase fields blank, the key will be saved
unencrypted. You should _not_ do this without good reason; if you
do, your private key file on disk will be all an attacker needs to
gain access to any machine configured to accept that key. If you
want to be able to log in without having to type a passphrase every
time, you should consider using Pageant (chapter 9) so that your
decrypted key is only held in memory rather than on disk.
Under special circumstances you may genuinely _need_ to use a key
with no passphrase; for example, if you need to run an automated
batch script that needs to make an SSH connection, you can't be
there to type the passphrase. In this case we recommend you generate
a special key for each specific batch script (or whatever) that
needs one, and on the server side you should arrange that each
key is _restricted_ so that it can only be used for that specific
purpose. The documentation for your SSH server should explain how to
do this (it will probably vary between servers).
Choosing a good passphrase is difficult. Just as you shouldn't use
a dictionary word as a password because it's easy for an attacker
to run through a whole dictionary, you should not use a song
lyric, quotation or other well-known sentence as a passphrase.
DiceWare (www.diceware.com) recommends using at least five words
each generated randomly by rolling five dice, which gives over
2^64 possible passphrases and is probably not a bad scheme. If you
want your passphrase to make grammatical sense, this cuts down the
possibilities a lot and you should use a longer one as a result.
_Do not forget your passphrase_. There is no way to recover it.
8.2.9 Adding a certificate to your key
In some environments, user authentication keys can be signed in turn
by a `certifying authority' (`CA' for short), and user accounts
on an SSH server can be configured to automatically trust any key
that's certified by the right signature.
This can be a convenient setup if you have a very large number of
servers. When you change your key pair, you might otherwise have to
edit the authorized_keys file on every server individually, to make
them all accept the new key. But if instead you configure all those
servers _once_ to accept keys signed as yours by a CA, then when you
change your public key, all you have to do is to get the new key
certified by the same CA as before, and then all your servers will
automatically accept it without needing individual reconfiguration.
To get your key signed by a CA, you'll probably send the CA the new
_public_ key (not the private half), and get back a modified version
of the public key with the certificate included.
If you want to incorporate the certificate into your PPK file for
convenience, you can use the `Add certificate to key' menu option in
PuTTYgen's `Key' menu. This will give you a single file containing
your private key and the certificate, which is everything you need
to authenticate to a server prepared to accept that certificate.
To remove the certificate again and restore the uncertified PPK
file, there's also a `Remove certificate from key' option.
(However, you don't _have_ to incorporate the certificate into
your PPK file. You can equally well use it separately, via the
`Certificate to use with the private key' option in PuTTY itself.
See section 4.22.2. It's up to you which you find more convenient.)
When the currently loaded key in PuTTYgen contains a certificate,
the large `Public key for pasting' edit box (see section 8.2.12)
is replaced by a button that brings up an information box telling
you about the certificate, such as who it certifies your key as
belonging to, when it expires (if ever), and the fingerprint of the
CA key that signed it in turn.
8.2.10 Saving your private key to a disk file
Once you have generated a key, set a comment field and set a
passphrase, you are ready to save your private key to disk.
Press the `Save private key' button. PuTTYgen will put up a dialog
box asking you where to save the file. Select a directory, type in a
file name, and press `Save'.
This file is in PuTTY's native format (`*.PPK'); it is the one you
will need to tell PuTTY to use for authentication (see section
4.22.1) or tell Pageant to load (see section 9.2.2).
(You can optionally change some details of the PPK format for your
saved key files; see section 8.2.13. But the defaults should be fine
for most purposes.)
8.2.11 Saving your public key to a disk file
RFC 4716 specifies a standard format for storing SSH-2 public keys
on disk. Some SSH servers (such as ssh.com's) require a public
key in this format in order to accept authentication with the
corresponding private key. (Others, such as OpenSSH, use a different
format; see section 8.2.12.)
To save your public key in the SSH-2 standard format, press the
`Save public key' button in PuTTYgen. PuTTYgen will put up a dialog
box asking you where to save the file. Select a directory, type in a
file name, and press `Save'.
You will then probably want to copy the public key file to your
SSH server machine. See section 8.3 for general instructions on
configuring public-key authentication once you have generated a key.
If you use this option with an SSH-1 key, the file PuTTYgen saves
will contain exactly the same text that appears in the `Public key
for pasting' box. This is the only existing standard for SSH-1
public keys.
8.2.12 `Public key for pasting into OpenSSH authorized_keys file'
The OpenSSH server, among others, requires your public key
to be given to it in a one-line format before it will accept
authentication with your private key. (SSH-1 servers also used this
method.)
The `Public key for pasting into OpenSSH authorized_keys file' gives
the public-key data in the correct one-line format. Typically you
will want to select the entire contents of the box using the mouse,
press Ctrl+C to copy it to the clipboard, and then paste the data
into a PuTTY session which is already connected to the server.
See section 8.3 for general instructions on configuring public-key
authentication once you have generated a key.
8.2.13 Parameters for saving key files
Selecting `Parameters for saving key files...' from the `Key'
menu lets you adjust some aspects of PPK-format private key files
stored on disk. None of these options affect compatibility with SSH
servers.
In most cases, it's entirely sensible to leave all of these at their
default settings.
8.2.13.1 PPK file version
This defaults to version 3, which is fine for most uses.
You might need to select PPK version 2 if you need your private key
file to be loadable in older versions of PuTTY (0.74 and older), or
in other tools which do not yet support the version 3 format (which
was introduced in 2021).
The version 2 format is less resistant to brute-force decryption,
and doesn't support any of the following options to control that.
8.2.13.2 Options affecting passphrase hashing
All of the following options only affect keys saved with
passphrases. They control how much work is required to decrypt the
key (which happens every time you type its passphrase). This allows
you to trade off the cost of legitimate use of the key against the
resistance of the encrypted key to password-guessing attacks.
These options only affect PPK version 3.
Key derivation function
The variant of the Argon2 key derivation function to use. You
might change this if you consider your exposure to side-channel
attacks to be different to the norm.
Memory to use for passphrase hash
The amount of memory needed to decrypt the key, in Kbyte.
Time to use for passphrase hash
Controls how much time is required to attempt decrypting the
key. You can either specify an approximate time in milliseconds
(on this machine), or explicitly specify a number of hash passes
(which is what the time is turned into during encryption).
Parallelism for passphrase hash
Number of parallelisable threads that can be used to decrypt the
key. The default, 1, forces the process to run single-threaded,
even on machines with multiple cores.
8.2.14 Reloading a private key
PuTTYgen allows you to load an existing private key file into
memory. If you do this, you can then change the passphrase and
comment before saving it again; you can also make extra copies of
the public key.
To load an existing key, press the `Load' button. PuTTYgen will put
up a dialog box where you can browse around the file system and find
your key file. Once you select the file, PuTTYgen will ask you for a
passphrase (if necessary) and will then display the key details in
the same way as if it had just generated the key.
If you use the Load command to load a foreign key format, it will
work, but you will see a message box warning you that the key you
have loaded is not a PuTTY native key. See section 8.2.15 for
information about importing foreign key formats.
8.2.15 Dealing with private keys in other formats
SSH-2 private keys have no standard format. OpenSSH and ssh.com
have different formats, and PuTTY's is different again. So a key
generated with one client cannot immediately be used with another.
Using the `Import' command from the `Conversions' menu, PuTTYgen can
load SSH-2 private keys in OpenSSH's format and ssh.com's format.
Once you have loaded one of these key types, you can then save it
back out as a PuTTY-format key (`*.PPK') so that you can use it with
the PuTTY suite. The passphrase will be unchanged by this process
(unless you deliberately change it). You may want to change the key
comment before you save the key, since some OpenSSH key formats
contained no space for a comment, and ssh.com's default comment
format is long and verbose.
PuTTYgen can also export private keys in OpenSSH format and in
ssh.com format. To do so, select one of the `Export' options from
the `Conversions' menu. Exporting a key works exactly like saving
it (see section 8.2.10) - you need to have typed your passphrase in
beforehand, and you will be warned if you are about to save a key
without a passphrase.
For OpenSSH there are two options. Modern OpenSSH actually has two
formats it uses for storing private keys: an older (`PEM-style')
format, and a newer `native' format with better resistance to
passphrase guessing and support for comments. `Export OpenSSH key'
will automatically choose the oldest format supported for the key
type, for maximum backward compatibility with older versions of
OpenSSH; for newer key types like Ed25519, it will use the newer
format as that is the only legal option. If you have some specific
reason for wanting to use OpenSSH's newer format even for RSA,
DSA, or ECDSA keys - for instance, you know your file will only
be used by OpenSSH 6.5 or newer (released in 2014), and want the
extra security - you can choose `Export OpenSSH key (force new file
format)'.
Most clients for the older SSH-1 protocol use a standard format for
storing private keys on disk. PuTTY uses this format as well; so if
you have generated an SSH-1 private key using OpenSSH or ssh.com's
client, you can use it with PuTTY, and vice versa. Hence, the export
options are not available if you have generated an SSH-1 key.
8.2.16 PuTTYgen command-line configuration
PuTTYgen supports a set of command-line options to configure many
of the same settings you can select in the GUI. This allows you to
start it up with your own preferences ready-selected, which might be
useful if you generate a lot of keys. (For example, you could make a
Windows shortcut that runs PuTTYgen with some command line options,
or a batch file or Powershell script that you could distribute to a
whole organisation containing your local standards.)
The options supported on the command line are:
-t _keytype_
Type of key to generate. You can select `rsa', `dsa', `ecdsa',
`eddsa', `ed25519', `ed448', or `rsa1'. See section 8.2.2.
-b _bits_
Size of the key to generate, in bits. See section 8.2.3.
--primes _method_
Method for generating prime numbers. You can select `probable',
`proven', and `proven-even'. See section 8.2.4.
--strong-rsa
When generating an RSA key, make sure the prime factors of the
key modulus are `strong primes'. See section 8.2.4.
--ppk-param _key_=_value_,...
Allows setting all the same details of the PPK save file format
described in section 8.2.13.
Aspects to change are specified as a series of _key_=_value_
pairs separated by commas. The _key_s are:
version
The PPK format version: either 3 or 2.
kdf
The variant of Argon2 to use: argon2id, argon2i, and
argon2d.
memory
The amount of memory needed to decrypt the key, in Kbyte.
time
Specifies how much time is required to attempt decrypting
the key, in milliseconds.
passes
Alternative to time: specifies the number of hash passes
required to attempt decrypting the key.
parallelism
Number of parallelisable threads that can be used to decrypt
the key.
-E _fptype_
Algorithm to use when displaying key fingerprints. You can
select `sha256' or `md5'. See section 8.2.6.
8.3 Getting ready for public key authentication
Connect to your SSH server using PuTTY with the SSH protocol. When
the connection succeeds you will be prompted for your user name and
password to login. Once logged in, you must configure the server to
accept your public key for authentication:
- If your server is OpenSSH, you should change into the `.ssh'
directory under your home directory, and open the file
`authorized_keys' with your favourite editor. (You may have
to create this file, if this is the first key you have put in
it.) Then switch to the PuTTYgen window, select all of the text
in the `Public key for pasting into OpenSSH authorized_keys
file' box (see section 8.2.12), and copy it to the clipboard
(`Ctrl+C'). Then, switch back to the PuTTY window and insert the
data into the open file, making sure it ends up all on one line.
Save the file.
(In very old versions of OpenSSH, SSH-2 keys had to be put
in a separate file called `authorized_keys2'. In all current
versions, the same `authorized_keys' file is used for both SSH-1
and SSH-2 keys.)
- If your server is ssh.com's product and is using SSH-2, you need
to save a _public_ key file from PuTTYgen (see section 8.2.11),
and copy that into the `.ssh2' directory on the server. Then you
should go into that `.ssh2' directory, and edit (or create) a
file called `authorization'. In this file you should put a line
like `Key mykey.pub', with `mykey.pub' replaced by the name of
your key file.
- For other SSH server software, you should refer to the manual
for that server.
You may also need to ensure that your home directory, your `.ssh'
directory, and any other files involved (such as `authorized_keys',
`authorized_keys2' or `authorization') are not group-writable or
world-writable; servers will typically ignore the keys unless this
is done. You can typically do this by using a command such as
chmod go-w $HOME $HOME/.ssh $HOME/.ssh/authorized_keys
Your server should now be configured to accept authentication using
your private key. Now you need to configure PuTTY to _attempt_
authentication using your private key. You can do this in any of
three ways:
- Select the private key in PuTTY's configuration. See section
4.22.1 for details.
- Specify the key file on the command line with the `-i' option.
See section 3.11.3.18 for details.
- Load the private key into Pageant (see chapter 9). In this case
PuTTY will automatically try to use it for authentication if it
can.
Chapter 9: Using Pageant for authentication
-------------------------------------------
Pageant is an SSH authentication agent. It holds your private keys
in memory, already decoded, so that you can use them often without
needing to type a passphrase.
9.1 Getting started with Pageant
Before you run Pageant, you need to have a private key in `*.PPK'
format. See chapter 8 to find out how to generate and use one.
When you run Pageant, it will put an icon of a computer wearing a
hat into the System tray. It will then sit and do nothing, until
you load a private key into it. (You may need to use Windows' `Show
hidden icons' arrow to see the Pageant icon.)
If you click the Pageant icon with the right mouse button, you will
see a menu. Select `View Keys' from this menu. The Pageant main
window will appear. (You can also bring this window up by double-
clicking on the Pageant icon.)
The Pageant window contains a list box. This shows the private keys
Pageant is holding. When you start Pageant, it has no keys, so the
list box will be empty. After you add one or more keys, they will
show up in the list box.
To add a key to Pageant, press the `Add Key' button. Pageant will
bring up a file dialog, labelled `Select Private Key File'. Find
your private key file in this dialog, and press `Open'.
Pageant will now load the private key. If the key is protected by
a passphrase, Pageant will ask you to type the passphrase. When
the key has been loaded, it will appear in the list in the Pageant
window.
Now start PuTTY and open an SSH session to a site that accepts your
key. PuTTY will notice that Pageant is running, retrieve the key
automatically from Pageant, and use it to authenticate. You can now
open as many PuTTY sessions as you like without having to type your
passphrase again.
(PuTTY can be configured not to try to use Pageant, but it will
try by default. See section 4.21.4 and section 3.11.3.9 for more
information.)
When you want to shut down Pageant, click the right button on the
Pageant icon in the System tray, and select `Exit' from the menu.
Closing the Pageant main window does _not_ shut down Pageant.
If you want Pageant to stay running but forget all the keys it has
acquired, select `Remove All Keys' from the System tray menu.
9.2 The Pageant main window
The Pageant main window appears when you left-click on the Pageant
system tray icon, or alternatively right-click and select `View
Keys' from the menu. You can use it to keep track of what keys are
currently loaded into Pageant, and to add new ones or remove the
existing keys.
9.2.1 The key list box
The large list box in the Pageant main window lists the private
keys that are currently loaded into Pageant. The list might look
something like this:
Ed25519 SHA256:TddlQk20DVs4LRcAsIfDN9pInKpY06D+h4kSHwWAj4w
RSA 2048 SHA256:8DFtyHm3kQihgy52nzX96qMcEVOq7/yJmmwQQhBWYFg
For each key, the list box will tell you:
- The type of the key. Currently, this can be `RSA' (an RSA key
for use with the SSH-2 protocol), `DSA' (a DSA key for use with
the SSH-2 protocol), `NIST' (an ECDSA key for use with the
SSH-2 protocol), `Ed25519' (an Ed25519 key for use with the
SSH-2 protocol), `Ed448' (an Ed448 key for use with the SSH-
2 protocol), or `SSH-1' (an RSA key for use with the old SSH-
1 protocol). (If the key has an associated certificate, this is
shown here with a `cert' suffix.)
- The size (in bits) of the key, for key types that come in
different sizes. (For ECDSA `NIST' keys, this is indicated as
`p256' or `p384' or `p521'.)
- The fingerprint for the public key. This should be the same
fingerprint given by PuTTYgen, and (hopefully) also the same
fingerprint shown by remote utilities such as `ssh-keygen' when
applied to your `authorized_keys' file.
For SSH-2 keys, by default this is shown in the `SHA256' format.
You can change to the older `MD5' format (which looks like
`aa:bb:cc:...') with the `Fingerprint type' drop-down, but bear
in mind that this format is less secure and should be avoided
for comparison purposes where possible.
If some of the keys loaded into Pageant have certificates
attached, then Pageant will default to showing the fingerprint
of the underlying key. This way, a certified and uncertified
version of the same key will have the same fingerprint, so you
can see that they match. You can instead use the `Fingerprint
type' drop-down to ask for a different fingerprint to be shown
for certified keys, which includes the certificate as part of
the fingerprinted data. That way you can tell two certificates
apart.
- The comment attached to the key.
- The state of deferred decryption, if enabled for this key. See
section 9.5.
9.2.2 The `Add Key' button
To add a key to Pageant by reading it out of a local disk file,
press the `Add Key' button in the Pageant main window, or
alternatively right-click on the Pageant icon in the system tray and
select `Add Key' from there.
Pageant will bring up a file dialog, labelled `Select Private Key
File'. Find your private key file in this dialog, and press `Open'.
If you want to add more than one key at once, you can select
multiple files using Shift-click (to select several adjacent files)
or Ctrl-click (to select non-adjacent files).
Pageant will now load the private key(s). If a key is protected by a
passphrase, Pageant will ask you to type the passphrase.
(This is not the only way to add a private key to Pageant. You can
also add one from a remote system by using agent forwarding; see
section 9.4 for details.)
9.2.3 The `Remove Key' button
If you need to remove a key from Pageant, select that key in the
list box, and press the `Remove Key' button. Pageant will remove the
key from its memory.
You can apply this to keys you added using the `Add Key' button, or
to keys you added remotely using agent forwarding (see section 9.4);
it makes no difference.
9.3 The Pageant command line
Pageant can be made to do things automatically when it starts up,
by specifying instructions on its command line. If you're starting
Pageant from the Windows GUI, you can arrange this by editing the
properties of the Windows shortcut that it was started from.
If Pageant is already running, invoking it again with the options
below causes actions to be performed with the existing instance, not
a new one.
9.3.1 Making Pageant automatically load keys on startup
Pageant can automatically load one or more private keys when it
starts up, if you provide them on the Pageant command line. Your
command line might then look like:
C:\PuTTY\pageant.exe d:\main.ppk d:\secondary.ppk
If the keys are stored encrypted, Pageant will request the
passphrases on startup.
If Pageant is already running, this syntax loads keys into the
existing Pageant.
You can specify the `--encrypted' option to defer decryption of
these keys; see section 9.5.
9.3.2 Making Pageant run another program
You can arrange for Pageant to start another program once it has
initialised itself and loaded any keys specified on its command
line. This program (perhaps a PuTTY, or a WinCVS making use of
Plink, or whatever) will then be able to use the keys Pageant has
loaded.
You do this by specifying the `-c' option followed by the command,
like this:
C:\PuTTY\pageant.exe d:\main.ppk -c C:\PuTTY\putty.exe
9.3.3 Integrating with Windows OpenSSH
Windows's own port of OpenSSH uses the same mechanism as Pageant to
talk to its SSH agent (Windows named pipes). This means that Windows
OpenSSH can talk directly to Pageant, if it knows where to find
Pageant's named pipe.
When Pageant starts up, it can optionally write out a file
containing an OpenSSH configuration directive that tells the Windows
`ssh.exe' where to find Pageant. If you include this file from your
Windows SSH configuration, then `ssh.exe' should automatically use
Pageant as its agent, so that you can keep your keys in one place
and have both SSH clients able to use them.
The option is `--openssh-config', and you follow it with a filename.
To refer to this file from your main OpenSSH configuration, you can
use the `Include' directive. For example, you might run Pageant like
this (with your own username substituted, of course):
pageant --openssh-config C:\Users\Simon\.ssh\pageant.conf
and then add a directive like this to your main `.ssh\config'
file (assuming that lives in the same directory that you just put
pageant.conf):
Include pageant.conf
*Note*: this technique only works with _Windows's_ port of OpenSSH,
which lives at C:\Windows\System32\OpenSSH\ssh.exe if you have
it installed. (If not, it can be installed as a Windows optional
feature, e.g., via Settings > Apps & features > Optional features >
Add a feature > OpenSSH Client.)
There are other versions of OpenSSH for Windows, notably the one
that comes with Windows git. Those will likely not work with the
same configuration, because they tend to depend on Unix emulation
layers like MinGW or MSys, so they won't speak Windows native
pathname syntax or understand named pipes. The above instructions
will only work with Windows's own version of OpenSSH.
So, if you want to use Windows git with an SSH key held in
Pageant, you'll have to set the environment variable GIT_SSH,
to point at a different program. You could point it at
c:\Windows\System32\OpenSSH\ssh.exe once you've done this setup -
but it's just as easy to point it at Plink!
9.3.4 Unix-domain sockets: integrating with WSL 1
Pageant can listen on the WinSock implementation of `Unix-domain
sockets'. These interoperate with the Unix-domain sockets found in
the original Windows Subsystem for Linux (now known as WSL 1). So if
you ask Pageant to listen on one of these, then your WSL 1 processes
can talk directly to Pageant.
To configure this, run Pageant with the option `--unix', followed
with a pathname. Then, in WSL 1, set the environment variable
SSH_AUTH_SOCK to point at the WSL translation of that pathname.
For example, you might run
pageant --unix C:\Users\Simon\.ssh\agent.sock
and in WSL 1, set the environment variable
SSH_AUTH_SOCK=/mnt/c/Users/Simon/.ssh/agent.sock
Alternatively, you can add a line to your .ssh/config file inside
WSL that says
IdentityAgent /mnt/c/Users/Simon/.ssh/agent.sock
although doing it like that may mean that ssh-add commands won't
find the agent, even though ssh itself will.
*Security note*: Unix-domain sockets are protected against access by
other users by the file protections on their containing directory.
So if your Windows machine is multiuser, make sure you create the
socket inside a directory that other users can't access at all. (In
fact, that's a good idea on general principles.)
*Compatibility note*: WSL 2 processes cannot talk to Pageant by this
mechanism, because WSL 2's Unix-domain sockets are managed by a
separate Linux kernel, and not by the same kernel that WinSock talks
to.
9.3.5 Starting with the key list visible
Start Pageant with the `--keylist' option to show the main window as
soon as it starts up.
9.3.6 Restricting the Windows process ACL
Pageant supports the same `-restrict-acl' option as the other PuTTY
utilities to lock down the Pageant process's access control; see
section 3.11.3.28 for why you might want to do this.
By default, if Pageant is started with `-restrict-acl', it won't
pass this to any PuTTY sessions started from its System Tray
submenu. Use `-restrict-putty-acl' to change this. (Again, see
section 3.11.3.28 for details.)
9.4 Using agent forwarding
Agent forwarding is a mechanism that allows applications on your SSH
server machine to talk to the agent on your client machine.
Note that at present, whether agent forwarding in SSH-2 is available
depends on your server. Pageant's protocol is compatible with the
OpenSSH server, but the ssh.com server uses a different agent
protocol, which PuTTY does not yet support.
To enable agent forwarding, first start Pageant. Then set up a
PuTTY SSH session in which `Allow agent forwarding' is enabled (see
section 4.21.7). Open the session as normal. (Alternatively, you
can use the `-A' command line option; see section 3.11.3.10 for
details.)
If this has worked, your applications on the server should now have
access to a Unix domain socket which the SSH server will forward
back to PuTTY, and PuTTY will forward on to the agent. To check that
this has actually happened, you can try this command on Unix server
machines:
unixbox:~$ echo $SSH_AUTH_SOCK
/tmp/ssh-XXNP18Jz/agent.28794
unixbox:~$
If the result line comes up blank, agent forwarding has not been
enabled at all.
Now if you run `ssh' on the server and use it to connect through to
another server that accepts one of the keys in Pageant, you should
be able to log in without a password:
unixbox:~$ ssh -v otherunixbox
[...]
debug: next auth method to try is publickey
debug: userauth_pubkey_agent: trying agent key my-putty-key
debug: ssh-userauth2 successful: method publickey
[...]
If you enable agent forwarding on _that_ SSH connection as well (see
the manual for your server-side SSH client to find out how to do
this), your authentication keys will still be available on the next
machine you connect to - two SSH connections away from where they're
actually stored.
In addition, if you have a private key on one of the SSH servers,
you can send it all the way back to Pageant using the local `ssh-
add' command:
unixbox:~$ ssh-add ~/.ssh/id_rsa
Need passphrase for /home/fred/.ssh/id_rsa
Enter passphrase for /home/fred/.ssh/id_rsa:
Identity added: /home/fred/.ssh/id_rsa (/home/simon/.ssh/id_rsa)
unixbox:~$
and then it's available to every machine that has agent forwarding
available (not just the ones downstream of the place you added it).
9.5 Loading keys without decrypting them
You can add keys to Pageant _without_ decrypting them. The key file
will be held in Pageant's memory still encrypted, and when a client
program first tries to use the key, Pageant will display a dialog
box prompting for the passphrase so that the key can be decrypted.
This works the same way whether the key is used by an instance of
PuTTY running locally, or a remote client connecting to Pageant
through agent forwarding.
To add a key to Pageant in this encrypted form, press the `Add Key
(encrypted)' button in the Pageant main window, or alternatively
right-click on the Pageant icon in the system tray and select `Add
Key (encrypted)' from there. Pageant will bring up a file dialog, in
just the same way as it would for the plain `Add Key' button. But it
won't ask for a passphrase. Instead, the key will be listed in the
main window with `(encrypted)' after it.
To start Pageant up in the first place with encrypted keys loaded
into it, you can use the `--encrypted' option on the command line.
For example:
C:\PuTTY\pageant.exe --encrypted d:\main.ppk
After a key has been decrypted for the first use, it remains
decrypted, so that it can be used again. The main window will list
the key with `(re-encryptable)' after it. You can revert it to the
previous state, where a passphrase is required, using the `Re-
encrypt' button in the Pageant main window.
You can also `re-encrypt' all keys that were added encrypted by
choosing `Re-encrypt All Keys' from the System tray menu. (Note that
this does _not_ discard cleartext keys that were not previously
added encrypted!)
*CAUTION*: When Pageant displays a prompt to decrypt an already-
loaded key, it cannot give keyboard focus to the prompt dialog box.
As far as I know this is a deliberate defensive measure by Windows,
against malicious software. So make sure you click in the prompt
window before typing your passphrase, or else the passphrase might
be sent to somewhere you didn't want to trust with it!
9.6 Security considerations
Using Pageant for public-key authentication gives you the
convenience of being able to open multiple SSH sessions without
having to type a passphrase every time, but also gives you the
security benefit of never storing a decrypted private key on disk.
Many people feel this is a good compromise between security and
convenience.
It _is_ a compromise, however. Holding your decrypted private keys
in Pageant is better than storing them in easy-to-find disk files,
but still less secure than not storing them anywhere at all. This is
for two reasons:
- Windows unfortunately provides no way to protect pieces of
memory from being written to the system swap file. So if Pageant
is holding your private keys for a long period of time, it's
possible that decrypted private key data may be written to the
system swap file, and an attacker who gained access to your hard
disk later on might be able to recover that data. (However,
if you stored an unencrypted key in a disk file they would
_certainly_ be able to recover it.)
- Although, like most modern operating systems, Windows prevents
programs from accidentally accessing one another's memory space,
it does allow programs to access one another's memory space
deliberately, for special purposes such as debugging. This means
that if you allow a virus, trojan, or other malicious program
on to your Windows system while Pageant is running, it could
access the memory of the Pageant process, extract your decrypted
authentication keys, and send them back to its master.
Similarly, use of agent _forwarding_ is a security improvement on
other methods of one-touch authentication, but not perfect. Holding
your keys in Pageant on your Windows box has a security advantage
over holding them on the remote server machine itself (either in an
agent or just unencrypted on disk), because if the server machine
ever sees your unencrypted private key then the sysadmin or anyone
who cracks the machine can steal the keys and pretend to be you for
as long as they want.
However, the sysadmin of the server machine can always pretend to
be you _on that machine_. So if you forward your agent to a server
machine, then the sysadmin of that machine can access the forwarded
agent connection and request signatures from any of your private
keys, and can therefore log in to other machines as you. They
can only do this to a limited extent - when the agent forwarding
disappears they lose the ability - but using Pageant doesn't
actually _prevent_ the sysadmin (or hackers) on the server from
doing this.
Therefore, if you don't trust the sysadmin of a server machine, you
should _never_ use agent forwarding to that machine. (Of course you
also shouldn't store private keys on that machine, type passphrases
into it, or log into other machines from it in any way at all;
Pageant is hardly unique in this respect.)
Chapter 10: Common error messages
---------------------------------
This chapter lists a number of common error messages which PuTTY and
its associated tools can produce, and explains what they mean in
more detail.
We do not attempt to list _all_ error messages here: there are many
which should never occur, and some which should be self-explanatory.
If you get an error message which is not listed in this chapter and
which you don't understand, report it to us as a bug (see appendix
B) and we will add documentation for it.
10.1 `The host key is not cached for this server'
This error message occurs when PuTTY connects to a new SSH server.
Every server identifies itself by means of a host key; once PuTTY
knows the host key for a server, it will be able to detect if a
malicious attacker redirects your connection to another machine.
If you see this message, it means that PuTTY has not seen this host
key before, and has no way of knowing whether it is correct or not.
You should attempt to verify the host key by other means, such as
asking the machine's administrator.
If you see this message and you know that your installation of PuTTY
_has_ connected to the same server before, it may have been recently
upgraded to SSH protocol version 2. SSH protocols 1 and 2 use
separate host keys, so when you first use SSH-2 with a server you
have only used SSH-1 with before, you will see this message again.
You should verify the correctness of the key as before.
See section 2.2 for more information on host keys.
10.2 `WARNING - POTENTIAL SECURITY BREACH!'
This message, followed by `The server's host key does not match
the one PuTTY has cached for this server', means that PuTTY has
connected to the SSH server before, knows what its host key _should_
be, but has found a different one.
(If the message instead talks about a `certified host key', see
instead section 10.3.)
This may mean that a malicious attacker has replaced your server
with a different one, or has redirected your network connection
to their own machine. On the other hand, it may simply mean that
the administrator of your server has accidentally changed the key
while upgrading the SSH software; this _shouldn't_ happen but it is
unfortunately possible.
You should contact your server's administrator and see whether they
expect the host key to have changed. If so, verify the new host key
in the same way as you would if it was new.
See section 2.2 for more information on host keys.
10.3 `This server presented a certified host key which was signed by a
different certification authority ...'
If you've configured PuTTY to trust at least one certification
authority for signing host keys (see section 4.19.4), then it will
ask the SSH server to send it any available certified host keys.
If the server sends back a certified key signed by a _different_
certification authority, PuTTY will present this variant of the host
key prompt, preceded by `WARNING - POTENTIAL SECURITY BREACH!'
One reason why this can happen is a deliberate attack. Just like an
ordinary man-in-the-middle attack which substitutes a wrong host
key, a particularly ambitious attacker might substitute an entire
wrong certification authority, and hope that you connect anyway.
But it's also possible in some situations that this error might
arise legitimately. For example, if your organisation's IT
department has just rolled out a new CA key which you haven't yet
entered in PuTTY's configuration, or if your CA configuration
involves two overlapping domains, or something similar.
So, unfortunately, you'll have to work out what to do about it
yourself: make an exception for this specific case, or abandon this
connection and install a new CA key before trying again (if you're
really sure you trust the CA), or edit your configuration in some
other way, or just stop trying to use this server.
If you're convinced that this particular server is legitimate even
though the CA is not one you trust, PuTTY will let you cache the
certified host key, treating it in the same way as an uncertified
one. Then that particular certificate will be accepted for future
connections to this specific server, even though other certificates
signed by the same CA will still be rejected.
10.4 `SSH protocol version 2 required by our configuration but remote
only provides (old, insecure) SSH-1'
By default, PuTTY only supports connecting to SSH servers that
implement SSH protocol version 2. If you see this message, the
server you're trying to connect to only supports the older SSH-1
protocol.
If the server genuinely only supports SSH-1, then you need to either
change the `SSH protocol version' setting (see section 4.17.4), or
use the `-1' command-line option; in any case, you should not treat
the resulting connection as secure.
You might start seeing this message with new versions of PuTTY
(from 0.68 onwards) where you didn't before, because it used to be
possible to configure PuTTY to automatically fall back from SSH-2 to
SSH-1. This is no longer supported, to prevent the possibility of a
downgrade attack.
10.5 `The first cipher supported by the server is ... below the
configured warning threshold'
This occurs when the SSH server does not offer any ciphers which you
have configured PuTTY to consider strong enough. By default, PuTTY
puts up this warning only for Blowfish, single-DES, and Arcfour
encryption.
See section 4.20 for more information on this message.
(There are similar messages for other cryptographic primitives, such
as host key algorithms.)
10.6 `Remote side sent disconnect message type 2 (protocol error): "Too
many authentication failures for root"'
This message is produced by an OpenSSH (or Sun SSH) server if it
receives more failed authentication attempts than it is willing to
tolerate.
This can easily happen if you are using Pageant and have a large
number of keys loaded into it, since these servers count each offer
of a public key as an authentication attempt. This can be worked
around by specifying the key that's required for the authentication
in the PuTTY configuration (see section 4.22.1); PuTTY will ignore
any other keys Pageant may have, but will ask Pageant to do the
authentication, so that you don't have to type your passphrase.
On the server, this can be worked around by disabling public-key
authentication or (for Sun SSH only) by increasing `MaxAuthTries' in
`sshd_config'.
10.7 `Out of memory'
This occurs when PuTTY tries to allocate more memory than the system
can give it. This _may_ happen for genuine reasons: if the computer
really has run out of memory, or if you have configured an extremely
large number of lines of scrollback in your terminal. PuTTY is
not able to recover from running out of memory; it will terminate
immediately after giving this error.
However, this error can also occur when memory is not running out at
all, because PuTTY receives data in the wrong format. In SSH-2 and
also in SFTP, the server sends the length of each message before the
message itself; so PuTTY will receive the length, try to allocate
space for the message, and then receive the rest of the message.
If the length PuTTY receives is garbage, it will try to allocate
a ridiculous amount of memory, and will terminate with an `Out of
memory' error.
This can happen in SSH-2, if PuTTY and the server have not enabled
encryption in the same way (see question A.7.3 in the FAQ).
This can also happen in PSCP or PSFTP, if your login scripts on the
server generate output: the client program will be expecting an
SFTP message starting with a length, and if it receives some text
from your login scripts instead it will try to interpret them as a
message length. See question A.7.4 for details of this.
10.8 `Internal error', `Internal fault', `Assertion failed'
Any error beginning with the word `Internal' should _never_ occur.
If it does, there is a bug in PuTTY by definition; please see
appendix B and report it to us.
Similarly, any error message starting with `Assertion failed' is a
bug in PuTTY. Please report it to us, and include the exact text
from the error message box.
10.9 `Unable to use key file', `Couldn't load private key', `Couldn't
load this key'
Various forms of this error are printed in the PuTTY window, or
written to the PuTTY Event Log (see section 3.1.3.1) when trying
public-key authentication, or given by Pageant when trying to load a
private key.
If you see one of these messages, it often indicates that you've
tried to load a key of an inappropriate type into PuTTY, Plink,
PSCP, PSFTP, or Pageant.
You may have tried to load an SSH-2 key in a `foreign' format
(OpenSSH or ssh.com) directly into one of the PuTTY tools, in which
case you need to import it into PuTTY's native format (`*.PPK')
using PuTTYgen - see section 8.2.15.
Alternatively, you may have specified a key that's inappropriate for
the connection you're making. The SSH-2 and the old SSH-1 protocols
require different private key formats, and a SSH-1 key can't be used
for a SSH-2 connection (or vice versa).
10.10 `Server refused our key', `Server refused our public key', `Key
refused'
Various forms of this error are printed in the PuTTY window, or
written to the PuTTY Event Log (see section 3.1.3.1) when trying
public-key authentication.
If you see one of these messages, it means that PuTTY has sent a
public key to the server and offered to authenticate with it, and
the server has refused to accept authentication. This usually means
that the server is not configured to accept this key to authenticate
this user.
This is almost certainly not a problem with PuTTY. If you see
this type of message, the first thing you should do is check your
_server_ configuration carefully. Common errors include having the
wrong permissions or ownership set on the public key or the user's
home directory on the server. Also, read the PuTTY Event Log; the
server may have sent diagnostic messages explaining exactly what
problem it had with your setup.
Section 8.3 has some hints on server-side public key setup.
10.11 `Access denied', `Authentication refused'
Various forms of this error are printed in the PuTTY window,
or written to the PuTTY Event Log (see section 3.1.3.1) during
authentication.
If you see one of these messages, it means that the server has
refused all the forms of authentication PuTTY has tried and it has
no further ideas.
It may be worth checking the Event Log for diagnostic messages from
the server giving more detail.
This error can be caused by buggy SSH-1 servers that fail to cope
with the various strategies we use for camouflaging passwords in
transit. Upgrade your server, or use the workarounds described in
section 4.27.14 and possibly section 4.27.15.
10.12 `No supported authentication methods available'
This error indicates that PuTTY has run out of ways to authenticate
you to an SSH server. This may be because PuTTY has TIS or keyboard-
interactive authentication disabled, in which case see section
4.21.5 and section 4.21.6.
10.13 `Incorrect MAC received on packet' or `Incorrect CRC received on
packet'
This error occurs when PuTTY decrypts an SSH packet and its checksum
is not correct. This probably means something has gone wrong in the
encryption or decryption process. It's difficult to tell from this
error message whether the problem is in the client, in the server,
or in between.
In particular, if the network is corrupting data at the TCP level,
it may only be obvious with cryptographic protocols such as SSH,
which explicitly check the integrity of the transferred data and
complain loudly if the checks fail. Corruption of protocols without
integrity protection (such as HTTP) will manifest in more subtle
failures (such as misdisplayed text or images in a web browser)
which may not be noticed.
Occasionally this has been caused by server bugs. An example is the
bug described at section 4.27.11, although you're very unlikely to
encounter that one these days.
In this context MAC stands for Message Authentication Code. It's a
cryptographic term, and it has nothing at all to do with Ethernet
MAC (Media Access Control) addresses, or with the Apple computer.
10.14 `Incoming packet was garbled on decryption'
This error occurs when PuTTY decrypts an SSH packet and the
decrypted data makes no sense. This probably means something has
gone wrong in the encryption or decryption process. It's difficult
to tell from this error message whether the problem is in the
client, in the server, or in between.
If you get this error, one thing you could try would be to fiddle
with the setting of `Miscomputes SSH-2 encryption keys' (see section
4.27.13) or `Ignores SSH-2 maximum packet size' (see section 4.27.5)
on the Bugs panel.
10.15 `PuTTY X11 proxy: _various errors_'
This family of errors are reported when PuTTY is doing X forwarding.
They are sent back to the X application running on the SSH server,
which will usually report the error to the user.
When PuTTY enables X forwarding (see section 3.4) it creates a
virtual X display running on the SSH server. This display requires
authentication to connect to it (this is how PuTTY prevents other
users on your server machine from connecting through the PuTTY proxy
to your real X display). PuTTY also sends the server the details it
needs to enable clients to connect, and the server should put this
mechanism in place automatically, so your X applications should just
work.
A common reason why people see one of these messages is because they
used SSH to log in as one user (let's say `fred'), and then used
the Unix `su' command to become another user (typically `root').
The original user, `fred', has access to the X authentication data
provided by the SSH server, and can run X applications which are
forwarded over the SSH connection. However, the second user (`root')
does not automatically have the authentication data passed on to it,
so attempting to run an X application as that user often fails with
this error.
If this happens, _it is not a problem with PuTTY_. You need to
arrange for your X authentication data to be passed from the user
you logged in as to the user you used `su' to become. How you do
this depends on your particular system; in fact many modern versions
of `su' do it automatically.
10.16 `Network error: Software caused connection abort'
This is a generic error produced by the Windows network code when
it kills an established connection for some reason. For example, it
might happen if you pull the network cable out of the back of an
Ethernet-connected computer, or if Windows has any other similar
reason to believe the entire network has become unreachable.
Windows also generates this error if it has given up on the machine
at the other end of the connection ever responding to it. If the
network between your client and server goes down and your client
then tries to send some data, Windows will make several attempts
to send the data and will then give up and kill the connection. In
particular, this can occur even if you didn't type anything, if you
are using SSH-2 and PuTTY attempts a key re-exchange. (See section
4.18.2 for more about key re-exchange.)
(It can also occur if you are using keepalives in your connection.
Other people have reported that keepalives _fix_ this error for
them. See section 4.14.1 for a discussion of the pros and cons of
keepalives.)
We are not aware of any reason why this error might occur that would
represent a bug in PuTTY. The problem is between you, your Windows
system, your network and the remote system.
10.17 `Network error: Connection reset by peer'
This error occurs when the machines at each end of a network
connection lose track of the state of the connection between them.
For example, you might see it if your SSH server crashes, and
manages to reboot fully before you next attempt to send data to it.
However, the most common reason to see this message is if you are
connecting through a firewall or a NAT router which has timed the
connection out. See question A.7.8 in the FAQ for more details.
You may be able to improve the situation by using keepalives; see
section 4.14.1 for details on this.
Note that Windows can produce this error in some circumstances
without seeing a connection reset from the server, for instance if
the connection to the network is lost.
10.18 `Network error: Connection refused'
This error means that the network connection PuTTY tried to make to
your server was rejected by the server. Usually this happens because
the server does not provide the service which PuTTY is trying to
access.
Check that you are connecting with the correct protocol (SSH,
Telnet, etc), and check that the port number is correct. If that
fails, consult the administrator of your server.
This error can also be caused by a firewall in between you and the
server, which rejects the connection and sends back the same type of
error packet as the real server would have sent.
10.19 `Network error: Connection timed out'
This error means that the network connection PuTTY tried to make to
your server received no response at all from the server. Usually
this happens because the server machine is completely isolated from
the network, or because it is turned off.
Check that you have correctly entered the host name or IP address
of your server machine. If that fails, consult the administrator of
your server.
Unix also generates this error when it tries to send data down a
connection and contact with the server has been completely lost
during a connection. (There is a delay of minutes before Unix gives
up on receiving a reply from the server.) This can occur if you type
things into PuTTY while the network is down, but it can also occur
if PuTTY decides of its own accord to send data: due to a repeat key
exchange in SSH-2 (see section 4.18.2) or due to keepalives (section
4.14.1).
10.20 `Network error: Cannot assign requested address'
This means that the operating system rejected the parameters of the
network connection PuTTY tried to make, usually without actually
trying to connect to anything, because they were simply invalid.
A common way to provoke this error is to accidentally try to connect
to port 0, which is not a valid port number.
Appendix A: PuTTY FAQ
---------------------
This FAQ is published on the PuTTY web site, and also provided as an
appendix in the manual.
A.1 Introduction
A.1.1 What is PuTTY?
PuTTY is a client program for the SSH, Telnet, Rlogin, and SUPDUP
network protocols.
These protocols are all used to run a remote session on a computer,
over a network. PuTTY implements the client end of that session: the
end at which the session is displayed, rather than the end at which
it runs.
In really simple terms: you run PuTTY on a Windows machine, and
tell it to connect to (for example) a Unix machine. PuTTY opens a
window. Then, anything you type into that window is sent straight
to the Unix machine, and everything the Unix machine sends back is
displayed in the window. So you can work on the Unix machine as if
you were sitting at its console, while actually sitting somewhere
else.
A.2 Features supported in PuTTY
In general, if you want to know if PuTTY supports a particular
feature, you should look for it on the PuTTY web site. In
particular:
- try the changes page, and see if you can find the feature on
there. If a feature is listed there, it's been implemented. If
it's listed as a change made _since_ the latest version, it
should be available in the development snapshots, in which case
testing will be very welcome.
- try the Wishlist page, and see if you can find the feature
there. If it's on there, and not in the `Recently fixed'
section, it probably _hasn't_ been implemented.
A.2.1 Does PuTTY support SSH-2?
Yes. SSH-2 support has been available in PuTTY since version 0.50 in
2000.
Public key authentication (both RSA and DSA) in SSH-2 was new in
version 0.52 in 2002.
A.2.2 Does PuTTY support reading OpenSSH or ssh.com SSH-2 private key
files?
PuTTY doesn't support this natively (see the wishlist entry for
reasons why not), but as of 0.53 PuTTYgen can convert both OpenSSH
and ssh.com private key files into PuTTY's format.
A.2.3 Does PuTTY support SSH-1?
Yes. SSH-1 support has always been available in PuTTY.
However, the SSH-1 protocol has many weaknesses and is no longer
considered secure; you should use SSH-2 instead if at all possible.
As of 0.68, PuTTY will no longer fall back to SSH-1 if the server
doesn't appear to support SSH-2; you must explicitly ask for SSH-1.
A.2.4 Does PuTTY support local echo?
Yes. Version 0.52 has proper support for local echo.
In version 0.51 and before, local echo could not be separated from
local line editing (where you type a line of text locally, and it
is not sent to the server until you press Return, so you have the
chance to edit it and correct mistakes _before_ the server sees it).
New in version 0.52, local echo and local line editing are separate
options, and by default PuTTY will try to determine automatically
whether to enable them or not, based on which protocol you have
selected and also based on hints from the server. If you have a
problem with PuTTY's default choice, you can force each option to be
enabled or disabled as you choose. The controls are in the Terminal
panel, in the section marked `Line discipline options'.
A.2.5 Does PuTTY support storing settings, so I don't have to change them
every time?
Yes, all of PuTTY's settings can be saved in named session profiles.
You can also change the default settings that are used for new
sessions. See section 4.1.2 in the documentation for how to do this.
A.2.6 Does PuTTY support storing its settings in a disk file?
Not at present, although section 4.33 in the documentation gives a
method of achieving the same effect.
A.2.7 Does PuTTY support full-screen mode, like a DOS box?
Yes; this was added in version 0.52, in 2002.
A.2.8 Does PuTTY have the ability to remember my password so I don't have
to type it every time?
No, it doesn't.
Remembering your password is a bad plan for obvious security
reasons: anyone who gains access to your machine while you're away
from your desk can find out the remembered password, and use it,
abuse it or change it.
In addition, it's not even _possible_ for PuTTY to automatically
send your password in a Telnet session, because Telnet doesn't
give the client software any indication of which part of the login
process is the password prompt. PuTTY would have to guess, by
looking for words like `password' in the session data; and if your
login program is written in something other than English, this won't
work.
In SSH, remembering your password would be possible in theory,
but there doesn't seem to be much point since SSH supports public
key authentication, which is more flexible and more secure. See
chapter 8 in the documentation for a full discussion of public key
authentication.
A.2.9 Is there an option to turn off the annoying host key prompts?
No, there isn't. And there won't be. Even if you write it yourself
and send us the patch, we won't accept it.
Those annoying host key prompts are the _whole point_ of SSH.
Without them, all the cryptographic technology SSH uses to secure
your session is doing nothing more than making an attacker's job
slightly harder; instead of sitting between you and the server with
a packet sniffer, the attacker must actually subvert a router and
start modifying the packets going back and forth. But that's not all
that much harder than just sniffing; and without host key checking,
it will go completely undetected by client or server.
Host key checking is your guarantee that the encryption you put
on your data at the client end is the _same_ encryption taken off
the data at the server end; it's your guarantee that it hasn't
been removed and replaced somewhere on the way. Host key checking
makes the attacker's job _astronomically_ hard, compared to packet
sniffing, and even compared to subverting a router. Instead of
applying a little intelligence and keeping an eye on oss-security,
the attacker must now perform a brute-force attack against at least
one military-strength cipher. That insignificant host key prompt
really does make _that_ much difference.
If you're having a specific problem with host key checking - perhaps
you want an automated batch job to make use of PSCP or Plink, and
the interactive host key prompt is hanging the batch process -
then the right way to fix it is to add the correct host key to the
Registry in advance, or if the Registry is not available, to use the
-hostkey command-line option. That way, you retain the _important_
feature of host key checking: the right key will be accepted and the
wrong ones will not. Adding an option to turn host key checking off
completely is the wrong solution and we will not do it.
If you have host keys available in the common `known_hosts' format,
we have a script called `kh2reg.py' to convert them to a Windows
.REG file, which can be installed ahead of time by double-clicking
or using `REGEDIT'.
A.2.10 Will you write an SSH server for the PuTTY suite, to go with the
client?
Not one that you'd want to use.
While much of the protocol and networking code can be made common
between a client and server, to make a _useful_ general-purpose
server requires all sorts of fiddly new code like interacting with
OS authentication databases and the like.
A special-purpose SSH server (called Uppity) can now be built from
the PuTTY source code, and indeed it is not usable as a general-
purpose server; it exists mainly as a test harness.
If someone else wants to use this as a basis for writing a general-
purpose SSH server, they'd be perfectly welcome to of course; but we
don't have time, and we don't have motivation. The code is available
if anyone else wants to try it.
A.2.11 Can PSCP or PSFTP transfer files in ASCII mode?
Unfortunately not.
This was a limitation of the file transfer protocols as originally
specified: the SCP and SFTP protocols had no notion of transferring
a file in anything other than binary mode. (This is still true of
SCP.)
The current draft protocol spec of SFTP proposes a means of
implementing ASCII transfer. At some point PSCP/PSFTP may implement
this proposal.
A.3 Ports to other operating systems
The eventual goal is for PuTTY to be a multi-platform program, able
to run on at least Windows, Mac OS and Unix.
PuTTY has been gaining a generalised porting layer, drawing a clear
line between platform-dependent and platform-independent code. The
general intention was for this porting layer to evolve naturally as
part of the process of doing the first port; a Unix port has now
been released and the plan seems to be working so far.
A.3.1 What ports of PuTTY exist?
Currently, release versions of PuTTY tools only run on Windows
systems and Unix.
As of 0.68, the supplied PuTTY executables run on versions of
Windows from XP onwards, up to and including Windows 11; and
we know of no reason why PuTTY should not continue to work on
future versions of Windows. We provide 32-bit and 64-bit Windows
executables for the common x86 processor family; see question A.6.10
for discussion of the compatibility issues around that. The 32-bit
executables require a Pentium 4 or newer processor. We also provide
executables for Windows on Arm processors.
(We used to also provide executables for Windows for the Alpha
processor, but stopped after 0.58 due to lack of interest.)
In the development code, a partial port to Mac OS exists (see
question A.3.6).
Currently PuTTY does _not_ run on Windows CE (see question A.3.4).
We do not have release-quality ports for any other systems at the
present time. If anyone told you we had an Android port, or an iOS
port, or any other port of PuTTY, they were mistaken. We don't.
There are some third-party ports to various platforms, mentioned on
the Links page of our website.
A.3.2 Is there a port to Unix?
There are Unix ports of most of the traditional PuTTY tools, and
also one entirely new application.
If you look at the source release, you should find a `unix'
subdirectory. You need `cmake' to build it; see the file `README' in
the source distribution. This should build you:
- Unix ports of PuTTY, Plink, PSCP, and PSFTP, which work pretty
much the same as their Windows counterparts;
- Command-line versions of PuTTYgen and Pageant, whose user
interface is quite different to the Windows GUI versions;
- `pterm' - an xterm-type program which supports the same terminal
emulation as PuTTY.
If you don't have Gtk, you should still be able to build the
command-line tools.
A.3.3 What's the point of the Unix port? Unix has OpenSSH.
All sorts of little things. `pterm' is directly useful to anyone who
prefers PuTTY's terminal emulation to `xterm''s, which at least some
people do. Unix Plink has apparently found a niche among people who
find the complexity of OpenSSL makes OpenSSH hard to install (and
who don't mind Plink not having as many features). Some users want
to generate a large number of SSH keys on Unix and then copy them
all into PuTTY, and the Unix PuTTYgen should allow them to automate
that conversion process.
There were development advantages as well; porting PuTTY to Unix
was a valuable path-finding effort for other future ports, and also
allowed us to use the excellent Linux tool Valgrind to help with
debugging, which has already improved PuTTY's stability on _all_
platforms.
However, if you're a Unix user and you can see no reason to switch
from OpenSSH to PuTTY/Plink, then you're probably right. We don't
expect our Unix port to be the right thing for everybody.
A.3.4 Will there be a port to Windows CE or PocketPC?
We once did some work on such a port, but it only reached an early
stage, and certainly not a useful one. It's no longer being actively
worked on.
A.3.5 Is there a port to Windows 3.1?
PuTTY is a 32-bit application from the ground up, so it won't run on
Windows 3.1 as a native 16-bit program; and it would be _very_ hard
to port it to do so, because of Windows 3.1's vile memory allocation
mechanisms.
However, it is possible in theory to compile the existing PuTTY
source in such a way that it will run under Win32s (an extension to
Windows 3.1 to let you run 32-bit programs). In order to do this
you'll need the right kind of C compiler - modern versions of Visual
C at least have stopped being backwards compatible to Win32s. Also,
the last time we tried this it didn't work very well.
A.3.6 Will there be a port to the Mac?
We hope so!
We attempted one around 2005, written as a native Cocoa application,
but it turned out to be very slow to redraw its window for some
reason we never got to the bottom of.
In 2015, after porting the GTK front end to work with GTK 3, we
began another attempt based on making small changes to the GTK
code and building it against the OS X Quartz version of GTK 3.
This doesn't seem to have the window redrawing problem any more,
so it's already got further than the last effort, but it is still
substantially unfinished.
If any OS X and/or GTK programming experts are keen to have a
finished version of this, we urge them to help out with some of
the remaining problems! See the TODO list in `unix/main-gtk-
application.c' in the source code.
A.3.7 Will there be a port to EPOC?
I hope so, but given that ports aren't really progressing very fast
even on systems the developers _do_ already know how to program for,
it might be a long time before any of us get round to learning a new
system and doing the port for that.
However, some of the work has been done by other people; see the
Links page of our website for various third-party ports.
A.3.8 Will there be a port to the iPhone?
We have no plans to write such a port ourselves; none of us has an
iPhone, and developing and publishing applications for it looks
awkward and expensive.
However, there is a third-party SSH client for the iPhone and
iPod Touch called pTerm, which is apparently based on PuTTY. (This
is nothing to do with our similarly-named `pterm', which is a
standalone terminal emulator for Unix systems; see question A.3.2.)
A.4 Embedding PuTTY in other programs
A.4.1 Is the SSH or Telnet code available as a DLL?
No, it isn't. It would take a reasonable amount of rewriting for
this to be possible, and since the PuTTY project itself doesn't
believe in DLLs (they make installation more error-prone) none of us
has taken the time to do it.
Most of the code cleanup work would be a good thing to happen in
general, so if anyone feels like helping, we wouldn't say no.
See also the wishlist entry.
A.4.2 Is the SSH or Telnet code available as a Visual Basic component?
No, it isn't. None of the PuTTY team uses Visual Basic, and none of
us has any particular need to make SSH connections from a Visual
Basic application. In addition, all the preliminary work to turn it
into a DLL would be necessary first; and furthermore, we don't even
know how to write VB components.
If someone offers to do some of this work for us, we might consider
it, but unless that happens I can't see VB integration being
anywhere other than the very bottom of our priority list.
A.4.3 How can I use PuTTY to make an SSH connection from within another
program?
Probably your best bet is to use Plink, the command-line connection
tool. If you can start Plink as a second Windows process, and
arrange for your primary process to be able to send data to the
Plink process, and receive data from it, through pipes, then you
should be able to make SSH connections from your program.
This is what CVS for Windows does, for example.
A.5 Details of PuTTY's operation
A.5.1 What terminal type does PuTTY use?
For most purposes, PuTTY can be considered to be an xterm terminal.
PuTTY also supports some terminal control sequences not supported by
the real xterm: notably the Linux console sequences that reconfigure
the colour palette, and the title bar control sequences used by
DECterm (which are different from the xterm ones; PuTTY supports
both).
By default, PuTTY announces its terminal type to the server as
`xterm'. If you have a problem with this, you can reconfigure it to
say something else; `vt220' might help if you have trouble.
A.5.2 Where does PuTTY store its data?
On Windows, PuTTY stores most of its data (saved sessions, SSH host
keys) in the Registry. The precise location is
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
and within that area, saved sessions are stored under `Sessions'
while host keys are stored under `SshHostKeys'.
PuTTY also requires a random number seed file, to improve the
unpredictability of randomly chosen data needed as part of the
SSH cryptography. This is stored by default in a file called
`PUTTY.RND'; this is stored by default in the `Application Data'
directory, or failing that, one of a number of fallback locations.
If you want to change the location of the random number seed file,
you can put your chosen pathname in the Registry, at
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\RandSeedFile
You can ask PuTTY to delete all this data; see question A.8.2.
On Unix, PuTTY stores all of this data in a directory ~/.putty by
default.
A.5.3 Why do small PuTTY icons appear next to the login prompts?
As of PuTTY 0.71, some lines of text in the terminal window are
marked with a small copy of the PuTTY icon (as far as pixels allow).
This is to show trustworthiness. When the PuTTY icon appears next to
a line of text, it indicates that that line of text was generated by
PuTTY itself, and not generated by the server and sent to PuTTY.
Text that comes from the server does not have this icon, and we've
arranged that the server should not be able to fake it. (There's
no control sequence the server can send which will make PuTTY draw
its own icon, and if the server tries to move the cursor back up to
a line that _already_ has an icon and overwrite the text, the icon
will disappear.)
This lets you tell the difference between (for example) a legitimate
prompt in which PuTTY itself asks you for your private key
passphrase, and a fake prompt in which the server tries to send
the identical text to trick you into telling _it_ your private key
passphrase.
A.5.4 Why has Plink started saying `Press Return to begin session'?
As of PuTTY 0.71, if you use Plink for an interactive SSH session,
then after the login phase has finished, it will present a final
interactive prompt saying `Access granted. Press Return to begin
session'.
This is another defence against servers trying to mimic the real
authentication prompts after the session has started. When you pass
through that prompt, you know that everything after it is generated
by the server and not by Plink itself, so any request for your
private key passphrase should be treated with suspicion.
In Plink, we can't use the defence described in section A.5.3: Plink
is running _in_ the terminal, so anything it can write into the
terminal, the server could write in the same way after the session
starts. And we can't just print a separator line without a pause,
because then the server could simply move the cursor back up to
it and overwrite it (probably with a brief flicker, but you might
easily miss that). The only robust defence anyone has come up with
involves this pause.
If you trust your server not to be abusive, you can turn this
off. It will also not appear in various other circumstances where
Plink can be confident it isn't necessary. See section 7.2.3.6 for
details.
A.6 HOWTO questions
A.6.1 What login name / password should I use?
This is not a question you should be asking _us_.
PuTTY is a communications tool, for making connections to other
computers. We maintain the tool; we _don't_ administer any computers
that you're likely to be able to use, in the same way that the
people who make web browsers aren't responsible for most of the
content you can view in them. We cannot help with questions of this
sort.
If you know the name of the computer you want to connect to, but
don't know what login name or password to use, you should talk to
whoever administers that computer. If you don't know who that is,
see the next question for some possible ways to find out.
A.6.2 What commands can I type into my PuTTY terminal window?
Again, this is not a question you should be asking _us_. You need
to read the manuals, or ask the administrator, of _the computer you
have connected to_.
PuTTY does not process the commands you type into it. It's only a
communications tool. It makes a connection to another computer; it
passes the commands you type to that other computer; and it passes
the other computer's responses back to you. Therefore, the precise
range of commands you can use will not depend on PuTTY, but on what
kind of computer you have connected to and what software is running
on it. The PuTTY team cannot help you with that.
(Think of PuTTY as being a bit like a telephone. If you phone
somebody up and you don't know what language to speak to make them
understand you, it isn't _the telephone company_'s job to find that
out for you. We just provide the means for you to get in touch;
making yourself understood is somebody else's problem.)
If you are unsure of where to start looking for the administrator
of your server, a good place to start might be to remember how you
found out the host name in the PuTTY configuration. If you were
given that host name by e-mail, for example, you could try asking
the person who sent you that e-mail. If your company's IT department
provided you with ready-made PuTTY saved sessions, then that IT
department can probably also tell you something about what commands
you can type during those sessions. But the PuTTY maintainer team
does not administer any server you are likely to be connecting to,
and cannot help you with questions of this type.
A.6.3 How can I make PuTTY start up maximised?
Create a Windows shortcut to start PuTTY from, and set it as `Run
Maximized'.
A.6.4 How can I create a Windows shortcut to start a particular saved
session directly?
To run a PuTTY session saved under the name `mysession', create a
Windows shortcut that invokes PuTTY with a command line like
\path\name\to\putty.exe -load "mysession"
(Note: prior to 0.53, the syntax was `@session'. This is now
deprecated and may be removed at some point.)
A.6.5 How can I start an SSH session straight from the command line?
Use the command line `putty -ssh host.name'. Alternatively, create a
saved session that specifies the SSH protocol, and start the saved
session as shown in question A.6.4.
A.6.6 How do I copy and paste between PuTTY and other Windows
applications?
Copy and paste works similarly to the X Window System. You use the
left mouse button to select text in the PuTTY window. The act of
selection _automatically_ copies the text to the clipboard: there
is no need to press Ctrl-Ins or Ctrl-C or anything else. In fact,
pressing Ctrl-C will send a Ctrl-C character to the other end of
your connection (just like it does the rest of the time), which may
have unpleasant effects. The _only_ thing you need to do, to copy
text to the clipboard, is to select it.
To paste the clipboard contents into a PuTTY window, by default you
click the right mouse button. If you have a three-button mouse and
are used to X applications, you can configure pasting to be done by
the middle button instead, but this is not the default because most
Windows users don't have a middle button at all.
You can also paste by pressing Shift-Ins.
A.6.7 How do I use all PuTTY's features (public keys, proxying, cipher
selection, etc.) in PSCP, PSFTP and Plink?
Most major features (e.g., public keys, port forwarding) are
available through command line options. See section 3.11.3.
Not all features are accessible from the command line yet, although
we'd like to fix this. In the meantime, you can use most of PuTTY's
features if you create a PuTTY saved session, and then use the name
of the saved session on the command line in place of a hostname.
This works for PSCP, PSFTP and Plink (but don't expect port
forwarding in the file transfer applications!).
A.6.8 How do I use PSCP.EXE? When I double-click it gives me a command
prompt window which then closes instantly.
PSCP is a command-line application, not a GUI application. If you
run it without arguments, it will simply print a help message and
terminate.
To use PSCP properly, run it from a Command Prompt window. See
chapter 5 in the documentation for more details.
A.6.9 How do I use PSCP to copy a file whose name has spaces in?
If PSCP is using the newer SFTP protocol (which is usual with most
modern servers), this is straightforward; all filenames with spaces
in are specified using a single pair of quotes in the obvious way:
pscp "local file" user@host:
pscp user@host:"remote file" .
However, if PSCP is using the older SCP protocol for some reason,
things are more confusing. If you're specifying a file at the local
end, you just use one set of quotes as you would normally do:
pscp "local filename with spaces" user@host:
pscp user@host:myfile "local filename with spaces"
But if the filename you're specifying is on the _remote_ side, you
have to use backslashes and two sets of quotes:
pscp user@host:"\"remote filename with spaces\"" local_filename
pscp local_filename user@host:"\"remote filename with spaces\""
Worse still, in a remote-to-local copy you have to specify the local
file name explicitly, otherwise PSCP will complain that they don't
match (unless you specified the `-unsafe' option). The following
command will give an error message:
c:\>pscp user@host:"\"oo er\"" .
warning: remote host tried to write to a file called 'oo er'
when we requested a file called '"oo er"'.
Instead, you need to specify the local file name in full:
c:\>pscp user@host:"\"oo er\"" "oo er"
A.6.10 Should I run the 32-bit or the 64-bit version?
If you're not sure, the 32-bit version is generally the safe option.
It will run perfectly well on all processors and on all versions of
Windows that PuTTY supports. PuTTY doesn't require to run as a 64-
bit application to work well, and having a 32-bit PuTTY on a 64-bit
system isn't likely to cause you any trouble.
The 64-bit version (first released in 0.68) will only run if you
have a 64-bit processor _and_ a 64-bit edition of Windows (both of
these things are likely to be true of any recent Windows PC). It
will run somewhat faster (in particular, the cryptography will be
faster, especially during link setup), but it will consume slightly
more memory.
If you need to use an external DLL for GSSAPI authentication, that
DLL may only be available in a 32-bit or 64-bit form, and that will
dictate the version of PuTTY you need to use. (You will probably
know if you're doing this; see section 4.23.2 in the documentation.)
A.7 Troubleshooting
A.7.1 Why do I see `Fatal: Protocol error: Expected control record' in
PSCP?
This happens because PSCP was expecting to see data from the server
that was part of the PSCP protocol exchange, and instead it saw data
that it couldn't make any sense of at all.
This almost always happens because the startup scripts in your
account on the server machine are generating output. This is
impossible for PSCP, or any other SCP client, to work around. You
should never use startup files (`.bashrc', `.cshrc' and so on) which
generate output in non-interactive sessions.
This is not actually a PuTTY problem. If PSCP fails in this way,
then all other SCP clients are likely to fail in exactly the same
way. The problem is at the server end.
A.7.2 I clicked on a colour in the Colours panel, and the colour didn't
change in my terminal.
That isn't how you're supposed to use the Colours panel.
During the course of a session, PuTTY potentially uses _all_ the
colours listed in the Colours panel. It's not a question of using
only one of them and you choosing which one; PuTTY will use them
_all_. The purpose of the Colours panel is to let you adjust the
appearance of all the colours. So to change the colour of the
cursor, for example, you would select `Cursor Colour', press the
`Modify' button, and select a new colour from the dialog box that
appeared. Similarly, if you want your session to appear in green,
you should select `Default Foreground' and press `Modify'. Clicking
on `ANSI Green' won't turn your session green; it will only allow
you to adjust the _shade_ of green used when PuTTY is instructed by
the server to display green text.
A.7.3 After trying to establish an SSH-2 connection, PuTTY says `Out of
memory' and dies.
If this happens just while the connection is starting up, this often
indicates that for some reason the client and server have failed to
establish a session encryption key. Somehow, they have performed
calculations that should have given each of them the same key, but
have ended up with different keys; so data encrypted by one and
decrypted by the other looks like random garbage.
This causes an `out of memory' error because the first encrypted
data PuTTY expects to see is the length of an SSH message. Normally
this will be something well under 100 bytes. If the decryption has
failed, PuTTY will see a completely random length in the region of
two _gigabytes_, and will try to allocate enough memory to store
this non-existent message. This will immediately lead to it thinking
it doesn't have enough memory, and panicking.
If this happens to you, it is quite likely to still be a PuTTY bug
and you should report it (although it might be a bug in your SSH
server instead); but it doesn't necessarily mean you've actually run
out of memory.
A.7.4 When attempting a file transfer, either PSCP or PSFTP says `Out of
memory' and dies.
This is almost always caused by your login scripts on the server
generating output. PSCP or PSFTP will receive that output when they
were expecting to see the start of a file transfer protocol, and
they will attempt to interpret the output as file-transfer protocol.
This will usually lead to an `out of memory' error for much the same
reasons as given in question A.7.3.
This is a setup problem in your account on your server, _not_ a
PSCP/PSFTP bug. Your login scripts should _never_ generate output
during non-interactive sessions; secure file transfer is not the
only form of remote access that will break if they do.
On Unix, a simple fix is to ensure that all the parts of your login
script that might generate output are in `.profile' (if you use a
Bourne shell derivative) or `.login' (if you use a C shell). Putting
them in more general files such as `.bashrc' or `.cshrc' is liable
to lead to problems.
A.7.5 PSFTP transfers files much slower than PSCP.
The throughput of PSFTP 0.54 should be much better than 0.53b and
prior; we've added code to the SFTP backend to queue several blocks
of data rather than waiting for an acknowledgement for each. (The
SCP backend did not suffer from this performance issue because SCP
is a much simpler protocol.)
A.7.6 When I run full-colour applications, I see areas of black space
where colour ought to be, or vice versa.
You almost certainly need to change the `Use background colour to
erase screen' setting in the Terminal panel. If there is too much
black space (the commoner situation), you should enable it, while
if there is too much colour, you should disable it. (See section
4.3.5.)
In old versions of PuTTY, this was disabled by default, and would
not take effect until you reset the terminal (see question A.7.7).
Since 0.54, it is enabled by default, and changes take effect
immediately.
A.7.7 When I change some terminal settings, nothing happens.
Some of the terminal options (notably Auto Wrap and background-
colour screen erase) actually represent the _default_ setting,
rather than the currently active setting. The server can send
sequences that modify these options in mid-session, but when the
terminal is reset (by server action, or by you choosing `Reset
Terminal' from the System menu) the defaults are restored.
In versions 0.53b and prior, if you change one of these options in
the middle of a session, you will find that the change does not
immediately take effect. It will only take effect once you reset the
terminal.
In version 0.54, the behaviour has changed - changes to these
settings take effect immediately.
A.7.8 My PuTTY sessions unexpectedly close after they are idle for a
while.
Some types of firewall, and almost any router doing Network Address
Translation (NAT, also known as IP masquerading), will forget about
a connection through them if the connection does nothing for too
long. This will cause the connection to be rudely cut off when
contact is resumed.
You can try to combat this by telling PuTTY to send _keepalives_:
packets of data which have no effect on the actual session, but
which reassure the router or firewall that the network connection is
still active and worth remembering about.
Keepalives don't solve everything, unfortunately; although they
cause greater robustness against this sort of router, they can also
cause a _loss_ of robustness against network dropouts. See section
4.14.1 in the documentation for more discussion of this.
A.7.9 PuTTY's network connections time out too quickly when network
connectivity is temporarily lost.
This is a Windows problem, not a PuTTY problem. The timeout value
can't be set on per application or per session basis. To increase
the TCP timeout globally, you need to tinker with the Registry.
On Windows 95, 98 or ME, the registry key you need to create or
change is
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\
MSTCP\MaxDataRetries
(it must be of type DWORD in Win95, or String in Win98/ME). (See MS
Knowledge Base article 158474 for more information.)
On Windows NT, 2000, or XP, the registry key to create or change is
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
Parameters\TcpMaxDataRetransmissions
and it must be of type DWORD. (See MS Knowledge Base articles 120642
and 314053 for more information.)
Set the key's value to something like 10. This will cause Windows to
try harder to keep connections alive instead of abandoning them.
A.7.10 When I cat a binary file, I get `PuTTYPuTTYPuTTY' on my command
line.
Don't do that, then.
This is designed behaviour; when PuTTY receives the character
Control-E from the remote server, it interprets it as a request
to identify itself, and so it sends back the string `PuTTY' as if
that string had been entered at the keyboard. Control-E should only
be sent by programs that are prepared to deal with the response.
Writing a binary file to your terminal is likely to output many
Control-E characters, and cause this behaviour. Don't do it. It's a
bad plan.
To mitigate the effects, you could configure the answerback string
to be empty (see section 4.3.7); but writing binary files to your
terminal is likely to cause various other unpleasant behaviour, so
this is only a small remedy.
A.7.11 When I cat a binary file, my window title changes to a nonsense
string.
Don't do that, then.
It is designed behaviour that PuTTY should have the ability
to adjust the window title on instructions from the server.
Normally the control sequence that does this should only be sent
deliberately, by programs that know what they are doing and intend
to put meaningful text in the window title. Writing a binary file to
your terminal runs the risk of sending the same control sequence by
accident, and cause unexpected changes in the window title. Don't do
it.
A.7.12 My keyboard stops working once PuTTY displays the password prompt.
No, it doesn't. PuTTY just doesn't display the password you type, so
that someone looking at your screen can't see what it is.
Unlike the Windows login prompts, PuTTY doesn't display the password
as a row of asterisks either. This is so that someone looking at
your screen can't even tell how _long_ your password is, which might
be valuable information.
A.7.13 One or more function keys don't do what I expected in a server-side
application.
If you've already tried all the relevant options in the PuTTY
Keyboard panel, you may need to mail the PuTTY maintainers and ask.
It is _not_ usually helpful just to tell us which application,
which server operating system, and which key isn't working; in
order to replicate the problem we would need to have a copy of
every operating system, and every application, that anyone has ever
complained about.
PuTTY responds to function key presses by sending a sequence of
control characters to the server. If a function key isn't doing what
you expect, it's likely that the character sequence your application
is expecting to receive is not the same as the one PuTTY is sending.
Therefore what we really need to know is _what_ sequence the
application is expecting.
The simplest way to investigate this is to find some other terminal
environment, in which that function key _does_ work; and then
investigate what sequence the function key is sending in that
situation. One reasonably easy way to do this on a Unix system is
to type the command `cat', and then press the function key. This is
likely to produce output of the form `^[[11~'. You can also do this
in PuTTY, to find out what sequence the function key is producing in
that. Then you can mail the PuTTY maintainers and tell us `I wanted
the F1 key to send `^[[11~', but instead it's sending `^[OP', can
this be done?', or something similar.
You should still read the Feedback page on the PuTTY website (also
provided as appendix B in the manual), and follow the guidelines
contained in that.
A.7.14 Why do I see `Couldn't load private key from ...'? Why can PuTTYgen
load my key but not PuTTY?
It's likely that you've generated an SSH protocol 2 key with
PuTTYgen, but you're trying to use it in an SSH-1 connection. SSH-
1 and SSH-2 keys have different formats, and (at least in 0.52)
PuTTY's reporting of a key in the wrong format isn't optimal.
To connect using SSH-2 to a server that supports both versions, you
need to change the configuration from the default (see question
A.2.1).
A.7.15 When I'm connected to a Red Hat Linux 8.0 system, some characters
don't display properly.
A common complaint is that hyphens in man pages show up as a-acute.
With release 8.0, Red Hat appear to have made UTF-8 the default
character set. There appears to be no way for terminal emulators
such as PuTTY to know this (as far as we know, the appropriate
escape sequence to switch into UTF-8 mode isn't sent).
A fix is to configure sessions to RH8 systems to use UTF-8
translation - see section 4.10.1 in the documentation. (Note that if
you use `Change Settings', changes may not take place immediately -
see question A.7.7.)
If you really want to change the character set used by the server,
the right place is `/etc/sysconfig/i18n', but this shouldn't be
necessary.
A.7.16 Since I upgraded to PuTTY 0.54, the scrollback has stopped working
when I run `screen'.
PuTTY's terminal emulator has always had the policy that when the
`alternate screen' is in use, nothing is added to the scrollback.
This is because the usual sorts of programs which use the alternate
screen are things like text editors, which tend to scroll back and
forth in the same document a lot; so (a) they would fill up the
scrollback with a large amount of unhelpfully disordered text, and
(b) they contain their _own_ method for the user to scroll back to
the bit they were interested in. We have generally found this policy
to do the Right Thing in almost all situations.
Unfortunately, `screen' is one exception: it uses the alternate
screen, but it's still usually helpful to have PuTTY's scrollback
continue working. The simplest solution is to go to the Features
control panel and tick `Disable switching to alternate terminal
screen'. (See section 4.6.4 for more details.) Alternatively, you
can tell `screen' itself not to use the alternate screen: the
`screen' FAQ suggests adding the line `termcapinfo xterm ti@:te@' to
your .screenrc file.
The reason why this only started to be a problem in 0.54 is because
`screen' typically uses an unusual control sequence to switch to the
alternate screen, and previous versions of PuTTY did not support
this sequence.
A.7.17 Since I upgraded Windows XP to Service Pack 2, I can't use addresses
like 127.0.0.2.
Some people who ask PuTTY to listen on localhost addresses other
than 127.0.0.1 to forward services such as SMB and Windows Terminal
Services have found that doing so no longer works since they
upgraded to WinXP SP2.
This is apparently an issue with SP2 that is acknowledged by
Microsoft in MS Knowledge Base article 884020. The article links to
a fix you can download.
(_However_, we've been told that SP2 _also_ fixes the bug that means
you need to use non-127.0.0.1 addresses to forward Terminal Services
in the first place.)
A.7.18 PSFTP commands seem to be missing a directory separator (slash).
Some people have reported the following incorrect behaviour with
PSFTP:
psftp> pwd
Remote directory is /dir1/dir2
psftp> get filename.ext
/dir1/dir2filename.ext: no such file or directory
This is not a bug in PSFTP. There is a known bug in some versions of
portable OpenSSH (bug 697) that causes these symptoms; it appears
to have been introduced around 3.7.x. It manifests only on certain
platforms (AIX is what has been reported to us).
There is a patch for OpenSSH attached to that bug; it's also fixed
in recent versions of portable OpenSSH (from around 3.8).
A.7.19 Do you want to hear about `Software caused connection abort'?
In the documentation for PuTTY 0.53 and 0.53b, we mentioned that
we'd like to hear about any occurrences of this error. Since the
release of PuTTY 0.54, however, we've been convinced that this error
doesn't indicate that PuTTY's doing anything wrong, and we don't
need to hear about further occurrences. See section 10.16 for our
current documentation of this error.
A.7.20 My SSH-2 session locks up for a few seconds every so often.
Recent versions of PuTTY automatically initiate repeat key exchange
once per hour, to improve session security. If your client or server
machine is slow, you may experience this as a delay of anything up
to thirty seconds or so.
These delays are inconvenient, but they are there for your
protection. If they really cause you a problem, you can choose to
turn off periodic rekeying using the `Kex' configuration panel (see
section 4.18), but be aware that you will be sacrificing security
for this. (Falling back to SSH-1 would also remove the delays, but
would lose a _lot_ more security still. We do not recommend it.)
A.7.21 PuTTY fails to start up. Windows claims that `the application
configuration is incorrect'.
This is caused by a bug in certain versions of Windows XP which is
triggered by PuTTY 0.58. This was fixed in 0.59. The `xp-wont-run'
entry in PuTTY's wishlist has more details.
A.7.22 When I put 32-bit PuTTY in C:\WINDOWS\SYSTEM32 on my 64-bit Windows
system, `Duplicate Session' doesn't work.
The short answer is not to put the PuTTY executables in that
location.
On 64-bit systems, C:\WINDOWS\SYSTEM32 is intended to contain
only 64-bit binaries; Windows' 32-bit binaries live in
C:\WINDOWS\SYSWOW64. When a 32-bit PuTTY executable runs on a 64-
bit system, it cannot by default see the `real' C:\WINDOWS\SYSTEM32
at all, because the File System Redirector arranges that the running
program sees the appropriate kind of binaries in SYSTEM32. Thus,
operations in the PuTTY suite that involve it accessing its own
executables, such as `New Session' and `Duplicate Session', will not
work.
A.7.23 After I upgraded PuTTY to 0.68, I can no longer connect to my
embedded device or appliance.
If your SSH server has started unexpectedly closing SSH connections
after you enter your password, and it worked before 0.68, you may
have a buggy server that objects to certain SSH protocol extensions.
The SSH protocol recently gained a new `terminal mode', IUTF8, which
PuTTY sends by default; see section 4.24.2. This is the first new
terminal mode since the SSH-2 protocol was defined. While servers
are supposed to ignore modes they don't know about, some buggy
servers will unceremoniously close the connection if they see
anything they don't recognise. SSH servers in embedded devices,
network appliances, and the like seem to disproportionately have
this bug.
If you think you have such a server, from 0.69 onwards you can
disable sending of the IUTF8 mode: on the SSH / TTY panel, select
IUTF8 on the list, select `Nothing', and press `Set'. (It's not
possible to disable sending this mode in 0.68.)
A.7.24 Since 0.78, I can't find where to configure my SSH private key.
In PuTTY 0.78, the `Private key file for authentication' control,
where you specify a `.PPK' file for SSH public key authentication,
moved to a new `Credentials' panel in the configuration dialog. You
can find this by opening the `SSH' category in the tree view on the
left, then opening the `Auth' subcategory under that, then clicking
on `Credentials'. On this page you'll find the `Browse...' button
you need to select a `.PPK' file for authentication, as described in
section 4.22.1.
(This control had previously been on the `Auth' panel since public
key authentication was first released in 2002, so many online how-
to guides still describe it there. The configuration controls were
reorganised to make room for features added in 0.78, such as OpenSSH
certificates.)
A.8 Security questions
A.8.1 Is it safe for me to download PuTTY and use it on a public PC?
It depends on whether you trust that PC. If you don't trust the
public PC, don't use PuTTY on it, and don't use any other software
you plan to type passwords into either. It might be watching your
keystrokes, or it might tamper with the PuTTY binary you download.
There is _no_ program safe enough that you can run it on an actively
malicious PC and get away with typing passwords into it.
If you do trust the PC, then it's probably OK to use PuTTY on it
(but if you don't trust the network, then the PuTTY download might
be tampered with, so it would be better to carry PuTTY with you on a
USB stick).
A.8.2 What does PuTTY leave on a system? How can I clean up after it?
PuTTY will leave some Registry entries, and a random seed file, on
the PC (see question A.5.2). Windows 7 and up also remember some
information about recently launched sessions for the `jump list'
feature.
If you are using PuTTY on a public PC, or somebody else's PC, you
might want to clean this information up when you leave. You can do
that automatically, by running the command `putty -cleanup'. See
section 3.11.2 in the documentation for more detail. (Note that this
only removes settings for the currently logged-in user on multi-user
systems.)
If PuTTY was installed from the installer package, it will also
appear in `Add/Remove Programs'. Current versions of the installer
do not offer to remove the above-mentioned items, so if you want
them removed you should run `putty -cleanup' before uninstalling.
A.8.3 How come PuTTY now supports DSA, when the website used to say how
insecure it was?
DSA has a major weakness _if badly implemented_: it relies on a
random number generator to far too great an extent. If the random
number generator produces a number an attacker can predict, the DSA
private key is exposed - meaning that the attacker can log in as you
on all systems that accept that key.
The PuTTY policy changed because the developers were informed of
ways to implement DSA which do not suffer nearly as badly from this
weakness, and indeed which don't need to rely on random numbers at
all. For this reason we now believe PuTTY's DSA implementation is
probably OK.
The recently added elliptic-curve signature methods are also DSA-
style algorithms, so they have this same weakness in principle. Our
ECDSA implementation uses the same defence as DSA, while our Ed25519
implementation uses the similar system (but different in details)
that the Ed25519 spec mandates.
A.8.4 Couldn't Pageant use VirtualLock() to stop private keys being
written to disk?
Unfortunately not. The VirtualLock() function in the Windows API
doesn't do a proper job: it may prevent small pieces of a process's
memory from being paged to disk while the process is running, but
it doesn't stop the process's memory as a whole from being swapped
completely out to disk when the process is long-term inactive. And
Pageant spends most of its time inactive.
A.8.5 Is the version of PuTTY in the Microsoft Store legit?
The free-of-charge `PuTTY' application at this link is published
and maintained by us. The copy there is the latest release, usually
updated within a few days of us publishing it on our own website.
There have been other copies of PuTTY on the store, some looking
quite similar, and some charging money. Those were uploaded by other
people, and we can't guarantee anything about them.
The first version we published to the Microsoft Store was 0.76 (some
time after its initial release on our website).
A.9 Administrative questions
A.9.1 Is putty.org your website?
No, it isn't. putty.org is run by an opportunist who uses it to
advertise their own commercial SSH implementation to people looking
for our free one. We don't own that site, we can't control it, and
we don't advise anyone to use it in preference to our own site.
The real PuTTY web site, run by the PuTTY team, has always been at
https://www.chiark.greenend.org.uk/~sgtatham/putty/.
A.9.2 Why do the download links point to the.earth.li and not chiark? Has
your website been hacked?
We haven't been hacked: links to the.earth.li are legit. The files
for released versions of PuTTY are hosted on a different server from
the web pages, for bandwidth reasons.
The download site the.earth.li is hosted by Mythic Beasts, and we're
very grateful to them!
A.9.3 Would you like me to register you a nicer domain name?
No, thank you. Even if you can find one (most of them seem to
have been registered already, by people who didn't ask whether we
actually wanted it before they applied), we're happy with the PuTTY
web site being exactly where it is. It's not hard to find (just type
`putty' into google.com and we're the first link returned), and we
don't believe the administrative hassle of moving the site would be
worth the benefit.
In addition, if we _did_ want a custom domain name, we would want to
run it ourselves, so we knew for certain that it would continue to
point where we wanted it, and wouldn't suddenly change or do strange
things. Having it registered for us by a third party who we don't
even know is not the best way to achieve this.
A.9.4 Would you like free web hosting for the PuTTY web site?
We already have some, thanks.
A.9.5 Would you link to my web site from the PuTTY web site?
Only if the content of your web page is of definite direct interest
to PuTTY users. If your content is unrelated, or only tangentially
related, to PuTTY, then the link would simply be advertising for
you.
One very nice effect of the Google ranking mechanism is that by and
large, the most popular web sites get the highest rankings. This
means that when an ordinary person does a search, the top item in
the search is very likely to be a high-quality site or the site they
actually wanted, rather than the site which paid the most money for
its ranking.
The PuTTY web site is held in high esteem by Google, for precisely
this reason: lots of people have linked to it simply because they
like PuTTY, without us ever having to ask anyone to link to us. We
feel that it would be an abuse of this esteem to use it to boost the
ranking of random advertisers' web sites. If you want your web site
to have a high Google ranking, we'd prefer that you achieve this the
way we did - by being good enough at what you do that people will
link to you simply because they like you.
In particular, we aren't interested in trading links for money
(see above), and we _certainly_ aren't interested in trading links
for other links (since we have no advertising on our web site, our
Google ranking is not even directly worth anything to us). If we
don't want to link to you for free, then we probably won't want to
link to you at all.
If you have software based on PuTTY, or specifically designed to
interoperate with PuTTY, or in some other way of genuine interest to
PuTTY users, then we will probably be happy to add a link to you on
our Links page. And if you're running a particularly valuable mirror
of the PuTTY web site, we might be interested in linking to you from
our Mirrors page.
A.9.6 Why don't you move PuTTY to SourceForge?
Partly, because we don't want to move the web site location (see
question A.9.3).
Also, security reasons. PuTTY is a security product, and as such it
is particularly important to guard the code and the web site against
unauthorised modifications which might introduce subtle security
flaws. Therefore, we prefer that the Git repository, web site and
FTP site remain where they are, under the direct control of system
administrators we know and trust personally, rather than being run
by a large organisation full of people we've never met and which is
known to have had breakins in the past.
No offence to SourceForge; I think they do a wonderful job. But
they're not ideal for everyone, and in particular they're not ideal
for us.
A.9.7 Why can't I subscribe to the putty-bugs mailing list?
Because you're not a member of the PuTTY core development team. The
putty-bugs mailing list is not a general newsgroup-like discussion
forum; it's a contact address for the core developers, and an
_internal_ mailing list for us to discuss things among ourselves.
If we opened it up for everybody to subscribe to, it would turn
into something more like a newsgroup and we would be completely
overwhelmed by the volume of traffic. It's hard enough to keep up
with the list as it is.
A.9.8 If putty-bugs isn't a general-subscription mailing list, what is?
There isn't one, that we know of.
If someone else wants to set up a mailing list or other forum for
PuTTY users to help each other with common problems, that would be
fine with us, though the PuTTY team would almost certainly not have
the time to read it.
A.9.9 How can I donate to PuTTY development?
Please, _please_ don't feel you have to. PuTTY is completely free
software, and not shareware. We think it's very important that
_everybody_ who wants to use PuTTY should be able to, whether they
have any money or not; so the last thing we would want is for a
PuTTY user to feel guilty because they haven't paid us any money. If
you want to keep your money, please do keep it. We wouldn't dream of
asking for any.
Having said all that, if you still really _want_ to give us money,
we won't argue :-) The easiest way for us to accept donations is if
you send money to <
[email protected]> using PayPal (www.paypal.com).
If you don't like PayPal, talk to us; we can probably arrange some
alternative means.
Small donations (tens of dollars or tens of euros) will probably be
spent on beer or curry, which helps motivate our volunteer team to
continue doing this for the world. Larger donations will be spent on
something that actually helps development, if we can find anything
(perhaps new hardware, or a new version of Windows), but if we
can't find anything then we'll just distribute the money among the
developers. If you want to be sure your donation is going towards
something worthwhile, ask us first. If you don't like these terms,
feel perfectly free not to donate. We don't mind.
A.9.10 Can I have permission to put PuTTY on a cover disk / distribute it
with other software / etc?
Yes. For most things, you need not bother asking us explicitly for
permission; our licence already grants you permission.
See section B.9 for more details.
A.9.11 Can you sign an agreement indemnifying us against security problems
in PuTTY?
No!
A vendor of physical security products (e.g. locks) might plausibly
be willing to accept financial liability for a product that failed
to perform as advertised and resulted in damage (e.g. valuables
being stolen). The reason they can afford to do this is because they
sell a _lot_ of units, and only a small proportion of them will
fail; so they can meet their financial liability out of the income
from all the rest of their sales, and still have enough left over
to make a profit. Financial liability is intrinsically linked to
selling your product for money.
There are two reasons why PuTTY is not analogous to a physical
lock in this context. One is that software products don't exhibit
random variation: _if_ PuTTY has a security hole (which does happen,
although we do our utmost to prevent it and to respond quickly
when it does), every copy of PuTTY will have the same hole, so
it's likely to affect all the users at the same time. So even if
our users were all paying us to use PuTTY, we wouldn't be able to
_simultaneously_ pay every affected user compensation in excess of
the amount they had paid us in the first place. It just wouldn't
work.
The second, much more important, reason is that PuTTY users _don't_
pay us. The PuTTY team does not have an income; it's a volunteer
effort composed of people spending their spare time to try to write
useful software. We aren't even a company or any kind of legally
recognised organisation. We're just a bunch of people who happen to
do some stuff in our spare time.
Therefore, to ask us to assume financial liability is to ask us to
assume a risk of having to pay it out of our own _personal_ pockets:
out of the same budget from which we buy food and clothes and pay
our rent. That's more than we're willing to give. We're already
giving a lot of our spare _time_ to developing software for free; if
we had to pay our own _money_ to do it as well, we'd start to wonder
why we were bothering.
Free software fundamentally does not work on the basis of financial
guarantees. Your guarantee of the software functioning correctly is
simply that you have the source code and can check it before you
use it. If you want to be sure there aren't any security holes, do
a security audit of the PuTTY code, or hire a security engineer if
you don't have the necessary skills yourself: instead of trying to
ensure you can get compensation in the event of a disaster, try to
ensure there isn't a disaster in the first place.
If you _really_ want financial security, see if you can find a
security engineer who will take financial responsibility for the
correctness of their review. (This might be less likely to suffer
from the everything-failing-at-once problem mentioned above, because
such an engineer would probably be reviewing a lot of _different_
products which would tend to fail independently.) Failing that,
see if you can persuade an insurance company to insure you against
security incidents, and if the insurer demands it as a condition
then get our code reviewed by a security engineer they're happy
with.
A.9.12 Can you sign this form granting us permission to use/distribute
PuTTY?
If your form contains any clause along the lines of `the undersigned
represents and warrants', we're not going to sign it. This is
particularly true if it asks us to warrant that PuTTY is secure;
see question A.9.11 for more discussion of this. But it doesn't
really matter what we're supposed to be warranting: even if it's
something we already believe is true, such as that we don't infringe
any third-party copyright, we will not sign a document accepting
any legal or financial liability. This is simply because the PuTTY
development project has no income out of which to satisfy that
liability, or pay legal costs, should it become necessary. We cannot
afford to be sued. We are assuring you that _we have done our best_;
if that isn't good enough for you, tough.
The existing PuTTY licence document already gives you permission
to use or distribute PuTTY in pretty much any way which does not
involve pretending you wrote it or suing us if it goes wrong. We
think that really ought to be enough for anybody.
See also question A.9.14 for another reason why we don't want to do
this sort of thing.
A.9.13 Can you write us a formal notice of permission to use PuTTY?
We could, in principle, but it isn't clear what use it would be. If
you think there's a serious chance of one of the PuTTY copyright
holders suing you (which we don't!), you would presumably want a
signed notice from _all_ of them; and we couldn't provide that even
if we wanted to, because many of the copyright holders are people
who contributed some code in the past and with whom we subsequently
lost contact. Therefore the best we would be able to do _even
in theory_ would be to have the core development team sign the
document, which wouldn't guarantee you that some other copyright
holder might not sue.
See also question A.9.14 for another reason why we don't want to do
this sort of thing.
A.9.14 Can you sign _anything_ for us?
Not unless there's an incredibly good reason.
We are generally unwilling to set a precedent that involves us
having to enter into individual agreements with PuTTY users.
We estimate that we have literally _millions_ of users, and we
absolutely would not have time to go round signing specific
agreements with every one of them. So if you want us to sign
something specific for you, you might usefully stop to consider
whether there's anything special that distinguishes you from 999,999
other users, and therefore any reason we should be willing to sign
something for you without it setting such a precedent.
If your company policy requires you to have an individual agreement
with the supplier of any software you use, then your company policy
is simply not well suited to using popular free software, and we
urge you to consider this as a flaw in your policy.
A.9.15 If you won't sign anything, can you give us some sort of assurance
that you won't make PuTTY closed-source in future?
Yes and no.
If what you want is an assurance that some _current version_ of
PuTTY which you've already downloaded will remain free, then you
already have that assurance: it's called the PuTTY Licence. It
grants you permission to use, distribute and copy the software to
which it applies; once we've granted that permission (which we
have), we can't just revoke it.
On the other hand, if you want an assurance that _future_ versions
of PuTTY won't be closed-source, that's more difficult. We could
in principle sign a document stating that we would never release a
closed-source PuTTY, but that wouldn't assure you that we _would_
keep releasing _open_-source PuTTYs: we would still have the option
of ceasing to develop PuTTY at all, which would surely be even worse
for you than making it closed-source! (And we almost certainly
wouldn't _want_ to sign a document guaranteeing that we would
actually continue to do development work on PuTTY; we certainly
wouldn't sign it for free. Documents like that are called contracts
of employment, and are generally not signed except in return for a
sizeable salary.)
If we _were_ to stop developing PuTTY, or to decide to make all
future releases closed-source, then you would still be free to copy
the last open release in accordance with the current licence, and in
particular you could start your own fork of the project from that
release. If this happened, I confidently predict that _somebody_
would do that, and that some kind of a free PuTTY would continue
to be developed. There's already precedent for that sort of thing
happening in free software. We can't guarantee that somebody _other
than you_ would do it, of course; you might have to do it yourself.
But we can assure you that there would be nothing _preventing_
anyone from continuing free development if we stopped.
(Finally, we can also confidently predict that if we made PuTTY
closed-source and someone made an open-source fork, most people
would switch to the latter. Therefore, it would be pretty stupid of
us to try it.)
A.9.16 Can you provide us with export control information / FIPS
certification for PuTTY?
Some people have asked us for an Export Control Classification
Number (ECCN) for PuTTY. We don't know whether we have one, and as a
team of free software developers based in the UK we don't have the
time, money, or effort to deal with US bureaucracy to investigate
any further. We believe that PuTTY falls under 5D002 on the US
Commerce Control List, but that shouldn't be taken as definitive.
If you need to know more you should seek professional legal advice.
The same applies to any other country's legal requirements and
restrictions.
Similarly, some people have asked us for FIPS certification of the
PuTTY tools. Unless someone else is prepared to do the necessary
work and pay any costs, we can't provide this.
A.9.17 As one of our existing software vendors, can you just fill in this
questionnaire for us?
We periodically receive requests like this, from organisations
which have apparently sent out a form letter to everyone listed in
their big spreadsheet of `software vendors' requiring them all to
answer some long list of questions about supported OS versions, paid
support arrangements, compliance with assorted local regulations
we haven't heard of, contact phone numbers, and other such
administrivia. Many of the questions are obviously meaningless when
applied to PuTTY (we don't provide any paid support in the first
place!), most of the rest could have been answered with only a very
quick look at our website, and some we are actively unwilling to
answer (we are private individuals, why would we want to give out
our home phone numbers to large corporations?).
We don't make a habit of responding in full to these questionnaires,
because _we are not a software vendor_.
A software _vendor_ is a company to which you are paying lots of
money in return for some software. They know who you are, and they
know you're paying them money; so they have an incentive to fill in
your forms and questionnaires, to research any local regulations you
cite if they don't already know about them, and generally to provide
every scrap of information you might possibly need in the most
convenient manner for you, because they want to keep being paid.
But we are a team of free software developers, and that means your
relationship with us is nothing like that at all. If you once
downloaded our software from our website, that's great and we hope
you found it useful, but it doesn't mean we have the least idea who
you are, or any incentive to do lots of unpaid work to support our
`relationship' with you.
It's not that we are unwilling to _provide information_. We put as
much of it as we can on our website for your convenience, and if you
actually need to know some fact about PuTTY which you haven't been
able to find on the website (and which is not obviously inapplicable
to free software in the first place) then please do ask us, and
we'll try to answer as best we can. But we put up the website and
this FAQ precisely so that we _don't_ have to keep answering the
same questions over and over again, so we aren't prepared to fill in
completely generic form-letter questionnaires for people who haven't
done their best to find the answers here first.
If you work for an organisation which you think might be at risk of
making this mistake, we urge you to reorganise your list of software
suppliers so that it clearly distinguishes paid vendors who know
about you from free software developers who don't have any idea who
you are. Then, only send out these mass mailings to the former.
A.9.18 The `sha1sums' / `sha256sums' / etc files on your download page
don't match the binaries.
People report this every so often, and usually the reason turns out
to be that they've matched up the wrong checksums file with the
wrong binaries.
The PuTTY download page contains more than one version of the
software. There's a _latest release_ version; there are the
_development snapshots_; and when we're in the run-up to making a
release, there are also _pre-release_ builds of the upcoming new
version. Each one has its own collection of binaries, and its own
collection of checksums files to go with them.
So if you've downloaded the release version of the actual program,
you need the release version of the checksums too, otherwise you
will see a mismatch. Similarly, the development snapshot binaries go
with the development snapshot checksums, and so on. (We've colour-
coded the download page in an effort to reduce this confusion a
bit.)
Another thing to watch out for: as of 0.71, executables like
`putty.exe' come in two flavours for each platform: the standalone
versions on the website, each of which contains embedded help, and
the versions installed by the installer, which use a separate help
file also in the installer. We provide checksums for both; the
latter are indicated with `(installer version)' after the filename.
If you have double-checked all that, and you still think there's
a real mismatch, then please send us a report carefully quoting
everything relevant:
- the exact URL you got your binary from
- the checksum of the binary after you downloaded
- the exact URL you got your checksums file from
- the checksum that file says the binary should have.
A.10 Miscellaneous questions
A.10.1 Is PuTTY a port of OpenSSH, or based on OpenSSH or OpenSSL?
No, it isn't. PuTTY is almost completely composed of code written
from scratch for PuTTY. The only code we share with OpenSSH is the
detector for SSH-1 CRC compensation attacks, written by CORE SDI
S.A; we share no code at all with OpenSSL.
A.10.2 Where can I buy silly putty?
You're looking at the wrong web site; the only PuTTY we know about
here is the name of a computer program.
If you want the kind of putty you can buy as an executive toy, the
PuTTY team can personally recommend Thinking Putty, which you can
buy from Crazy Aaron's Putty World, at www.puttyworld.com.
A.10.3 What does `PuTTY' mean?
It's the name of a popular SSH and Telnet client. Any other meaning
is in the eye of the beholder. It's been rumoured that `PuTTY' is
the antonym of `getty', or that it's the stuff that makes your
Windows useful, or that it's a kind of plutonium Teletype. We
couldn't possibly comment on such allegations.
A.10.4 How do I pronounce `PuTTY'?
Exactly like the English word `putty', which we pronounce /'pVti/.
Appendix B: Feedback and bug reporting
--------------------------------------
This is a guide to providing feedback to the PuTTY development team.
It is provided as both a web page on the PuTTY site, and an appendix
in the PuTTY manual.
Section B.1 gives some general guidelines for sending any kind of e-
mail to the development team. Following sections give more specific
guidelines for particular types of e-mail, such as bug reports and
feature requests.
B.1 General guidelines
The PuTTY development team gets a _lot_ of mail. If you can possibly
solve your own problem by reading the manual, reading the FAQ,
reading the web site, asking a fellow user, or some other means,
then it would make our lives much easier.
We get so much e-mail that we literally do not have time to answer
it all. We regret this, but there's nothing we can do about it.
So if you can _possibly_ avoid sending mail to the PuTTY team, we
recommend you do so. In particular, support requests (section B.7)
are probably better sent to some public forum, or passed to a local
expert if possible.
The PuTTY contact email address is a private mailing list containing
four or five core developers. Don't be put off by it being a mailing
list: if you need to send confidential data as part of a bug report,
you can trust the people on the list to respect that confidence.
Also, the archives aren't publicly available, so you shouldn't be
letting yourself in for any spam by sending us mail.
Please use a meaningful subject line on your message. We get a lot
of mail, and it's hard to find the message we're looking for if they
all have subject lines like `PuTTY bug'.
B.1.1 Sending large attachments
Since the PuTTY contact address is a mailing list, e-mails larger
than 40Kb will be held for inspection by the list administrator, and
will not be allowed through unless they really appear to be worth
their large size.
If you are considering sending any kind of large data file to the
PuTTY team, it's almost always a bad idea, or at the very least it
would be better to ask us first whether we actually need the file.
Alternatively, you could put the file on a web site and just send us
the URL; that way, we don't have to download it unless we decide we
actually need it, and only one of us needs to download it instead of
it being automatically copied to all the developers.
(If the file contains confidential information, then you could
encrypt it with our Secure Contact Key; see section F.1 for
details. Please _only_ use this for information that _needs_ to be
confidential.)
Some people like to send mail in MS Word format. Please _don't_
send us bug reports, or any other mail, as a Word document. Word
documents are roughly fifty times larger than writing the same
report in plain text. In addition, most of the PuTTY team read their
e-mail on Unix machines, so copying the file to a Windows box to run
Word is very inconvenient. Not only that, but several of us don't
even _have_ a copy of Word!
Some people like to send us screen shots when demonstrating a
problem. Please don't do this without checking with us first
- we almost never actually need the information in the screen
shot. Sending a screen shot of an error box is almost certainly
unnecessary when you could just tell us in plain text what the
error was. (On some versions of Windows, pressing Ctrl-C when the
error box is displayed will copy the text of the message to the
clipboard.) Sending a full-screen shot is _occasionally_ useful, but
it's probably still wise to check whether we need it before sending
it.
If you _must_ mail a screen shot, don't send it as a .BMP file. BMPs
have no compression and they are _much_ larger than other image
formats such as PNG, TIFF and GIF. Convert the file to a properly
compressed image format before sending it.
Please don't mail us executables, at all. Our mail server blocks all
incoming e-mail containing executables, as a defence against the
vast numbers of e-mail viruses we receive every day. If you mail us
an executable, it will just bounce.
If you have made a tiny modification to the PuTTY code, please
send us a _patch_ to the source code if possible, rather than
sending us a huge .ZIP file containing the complete sources plus
your modification. If you've only changed 10 lines, we'd prefer to
receive a mail that's 30 lines long than one containing multiple
megabytes of data we already have.
B.2 Reporting bugs
If you think you have found a bug in PuTTY, your first steps should
be:
- Check the Wishlist page on the PuTTY website, and see if we
already know about the problem. If we do, it is almost certainly
not necessary to mail us about it, unless you think you have
extra information that might be helpful to us in fixing it. (Of
course, if we actually _need_ specific extra information about a
particular bug, the Wishlist page will say so.)
- Check the Change Log on the PuTTY website, and see if we have
already fixed the bug in the development snapshots.
- Check the FAQ on the PuTTY website (also provided as appendix
A in the manual), and see if it answers your question. The FAQ
lists the most common things which people think are bugs, but
which aren't bugs.
- Download the latest development snapshot and see if the problem
still happens with that. This really is worth doing. As a
general rule we aren't very interested in bugs that appear in
the release version but not in the development version, because
that usually means they are bugs we have _already fixed_. On the
other hand, if you can find a bug in the development version
that doesn't appear in the release, that's likely to be a new
bug we've introduced since the release and we're definitely
interested in it.
If none of those options solved your problem, and you still need
to report a bug to us, it is useful if you include some general
information:
- Tell us what version of PuTTY you are running. To find this
out, use the `About PuTTY' option from the System menu. Please
_do not_ just tell us `I'm running the latest version'; e-mail
can be delayed and it may not be obvious which version was the
latest at the time you sent the message.
- PuTTY is a multi-platform application; tell us what version of
what OS you are running PuTTY on. (If you're running on Unix,
or Windows for Arm, tell us, or we'll assume you're running on
Windows for Intel as this is overwhelmingly the case.)
- Tell us what protocol you are connecting with: SSH, Telnet,
Rlogin, SUPDUP, or Raw mode, or a serial connection.
- Tell us what kind of server you are connecting to; what OS, and
if possible what SSH server (if you're using SSH). You can get
some of this information from the PuTTY Event Log (see section
3.1.3.1 in the manual).
- Send us the contents of the PuTTY Event Log, unless you have a
specific reason not to (for example, if it contains confidential
information that you think we should be able to solve your
problem without needing to know).
- Try to give us as much information as you can to help us see
the problem for ourselves. If possible, give us a step-by-step
sequence of _precise_ instructions for reproducing the fault.
- Don't just tell us that PuTTY `does the wrong thing'; tell us
exactly and precisely what it did, and also tell us exactly
and precisely what you think it should have done instead. Some
people tell us PuTTY does the wrong thing, and it turns out that
it was doing the right thing and their expectations were wrong.
Help to avoid this problem by telling us exactly what you think
it should have done, and exactly what it did do.
- If you think you can, you're welcome to try to fix the
problem yourself. A patch to the code which fixes a bug is an
excellent addition to a bug report. However, a patch is never a
_substitute_ for a good bug report; if your patch is wrong or
inappropriate, and you haven't supplied us with full information
about the actual bug, then we won't be able to find a better
solution.
-
https://www.chiark.greenend.org.uk/~sgtatham/bugs.html is an
article on how to report bugs effectively in general. If your
bug report is _particularly_ unclear, we may ask you to go away,
read this article, and then report the bug again.
It is reasonable to report bugs in PuTTY's documentation, if you
think the documentation is unclear or unhelpful. But we do need
to be given exact details of _what_ you think the documentation
has failed to tell you, or _how_ you think it could be made
clearer. If your problem is simply that you don't _understand_
the documentation, we suggest asking around and seeing if someone
will explain what you need to know. _Then_, if you think the
documentation could usefully have told you that, send us a bug
report and explain how you think we should change it.
B.3 Reporting security vulnerabilities
If you've found a security vulnerability in PuTTY, you might well
want to notify us using an encrypted communications channel, to
avoid disclosing information about the vulnerability before a fixed
release is available.
For this purpose, we provide a GPG key suitable for encryption: the
Secure Contact Key. See section F.1 for details of this.
(Of course, vulnerabilities are also bugs, so please do include as
much information as possible about them, the same way you would with
any other bug report.)
B.4 Requesting extra features
If you want to request a new feature in PuTTY, the very first things
you should do are:
- Check the Wishlist page on the PuTTY website, and see if your
feature is already on the list. If it is, it probably won't
achieve very much to repeat the request. (But see section B.5 if
you want to persuade us to give your particular feature higher
priority.)
- Check the Wishlist and Change Log on the PuTTY website, and
see if we have already added your feature in the development
snapshots. If it isn't clear, download the latest development
snapshot and see if the feature is present. If it is, then it
will also be in the next release and there is no need to mail us
at all.
If you can't find your feature in either the development snapshots
_or_ the Wishlist, then you probably do need to submit a feature
request. Since the PuTTY authors are very busy, it helps if you try
to do some of the work for us:
- Do as much of the design as you can. Think about `corner cases';
think about how your feature interacts with other existing
features. Think about the user interface; if you can't come
up with a simple and intuitive interface to your feature, you
shouldn't be surprised if we can't either. Always imagine
whether it's possible for there to be more than one, or less
than one, of something you'd assumed there would be one of.
(For example, if you were to want PuTTY to put an icon in the
System tray rather than the Taskbar, you should think about what
happens if there's more than one PuTTY active; how would the
user tell which was which?)
- If you can program, it may be worth offering to write the
feature yourself and send us a patch. However, it is likely to
be helpful if you confer with us first; there may be design
issues you haven't thought of, or we may be about to make big
changes to the code which your patch would clash with, or
something. If you check with the maintainers first, there is a
better chance of your code actually being usable. Also, read the
design principles listed in appendix E: if you do not conform to
them, we will probably not be able to accept your patch.
B.5 Requesting features that have already been requested
If a feature is already listed on the Wishlist, then it usually
means we would like to add it to PuTTY at some point. However, this
may not be in the near future. If there's a feature on the Wishlist
which you would like to see in the _near_ future, there are several
things you can do to try to increase its priority level:
- Mail us and vote for it. (Be sure to mention that you've seen
it on the Wishlist, or we might think you haven't even _read_
the Wishlist). This probably won't have very _much_ effect; if
a huge number of people vote for something then it may make a
difference, but one or two extra votes for a particular feature
are unlikely to change our priority list immediately. Offering a
new and compelling justification might help. Also, don't expect
a reply.
- Offer us money if we do the work sooner rather than later. This
sometimes works, but not always. The PuTTY team all have full-
time jobs and we're doing all of this work in our free time; we
may sometimes be willing to give up some more of our free time
in exchange for some money, but if you try to bribe us for a
_big_ feature it's entirely possible that we simply won't have
the time to spare - whether you pay us or not. (Also, we don't
accept bribes to add _bad_ features to the Wishlist, because
our desire to provide high-quality software to the users comes
first.)
- Offer to help us write the code. This is probably the _only_ way
to get a feature implemented quickly, if it's a big one that we
don't have time to do ourselves.
B.6 Workarounds for SSH server bugs
It's normal for SSH implementations to automatically enable
workarounds for each other's bugs, using the software version
strings that are exchanged at the start of the connection. Typically
an SSH client will have a list of server version strings that it
believes to have particular bugs, and auto-enable the appropriate
set of workarounds when it sees one of those strings. (And servers
will have a similar list of workarounds for _client_ software they
believe to be buggy.)
If you've found a bug in an SSH server, and you'd like us to add
an auto-detected workaround for it, our policy is that *the server
implementor should fix it first*.
If the server implementor has fixed it in the latest version, and
can give us a complete description of the version strings that go
with the bug, then we're happy to use those version strings as a
trigger to automatically enable our workaround (assuming one is
possible). We _won't_ accept requests to auto-enable workarounds
for an open-ended set of version strings, such as `any version of
FooServer, including future ones not yet released'.
The aim of this policy is to encourage implementors to gradually
converge on the actual standardised SSH protocol. If we enable
people to continue violating the spec, by installing open-ended
workarounds in PuTTY for bugs they're never going to fix, then we're
contributing to an ecosystem in which everyone carries on having
bugs and everyone else carries on having to work around them.
An exception: if an SSH server is no longer maintained _at all_
(e.g. the company that produced it has gone out of business), and
every version of it that was ever released has a bug, then that's
one situation in which we may be prepared to add a workaround rule
that matches all versions of that software. (The aim is to stop
implementors from continuing to release software with the bug - and
if they're not releasing it _at all_ any more, then that's already
done!)
We do recognise that sometimes it will be difficult to get the
server maintainer to fix a bug, or even to answer support requests
at all. Or it might take them a very long time to get round to doing
anything about it. We're not completely unwilling to compromise:
we're prepared to add _manually enabled_ workarounds to PuTTY even
for bugs that an implementation hasn't fixed yet. We just won't
_automatically_ enable the workaround unless the server maintainer
has also done their part.
B.7 Support requests
If you're trying to make PuTTY do something for you and it isn't
working, but you're not sure whether it's a bug or not, then
_please_ consider looking for help somewhere else. This is one of
the most common types of mail the PuTTY team receives, and we simply
don't have time to answer all the questions. Questions of this type
include:
- If you want to do something with PuTTY but have no idea where to
start, and reading the manual hasn't helped, try posting to a
public forum and see if someone can explain it to you.
- If you have tried to do something with PuTTY but it hasn't
worked, and you aren't sure whether it's a bug in PuTTY or a bug
in your SSH server or simply that you're not doing it right,
then try posting to some public forum and see if someone can
solve your problem. Or try doing the same thing with a different
SSH client and see if it works with that. Please do not report
it as a PuTTY bug unless you are really sure it _is_ a bug in
PuTTY.
- If someone else installed PuTTY for you, or you're using PuTTY
on someone else's computer, try asking them for help first.
They're more likely to understand how they installed it and what
they expected you to use it for than we are.
- If you have successfully made a connection to your server and
now need to know what to type at the server's command prompt,
or other details of how to use the server-end software, talk to
your server's system administrator. This is not the PuTTY team's
problem. PuTTY is only a communications tool, like a telephone;
if you can't speak the same language as the person at the other
end of the phone, it isn't the telephone company's job to teach
it to you.
If you absolutely cannot get a support question answered any other
way, you can try mailing it to us, but we can't guarantee to have
time to answer it.
B.8 Web server administration
If the PuTTY web site is down (Connection Timed Out), please don't
bother mailing us to tell us about it. Most of us read our e-mail on
the same machines that host the web site, so if those machines are
down then we will notice _before_ we read our e-mail. So there's no
point telling us our servers are down.
Of course, if the web site has some other error (Connection Refused,
404 Not Found, 403 Forbidden, or something else) then we might _not_
have noticed and it might still be worth telling us about it.
If you want to report a problem with our web site, check that you're
looking at our _real_ web site and not a mirror. The real web site
is at `
https://www.chiark.greenend.org.uk/~sgtatham/putty/'; if
that's not where you're reading this, then don't report the problem
to us until you've checked that it's really a problem with the
main site. If it's only a problem with the mirror, you should try
to contact the administrator of that mirror site first, and only
contact us if that doesn't solve the problem (in case we need to
remove the mirror from our list).
B.9 Asking permission for things
PuTTY is distributed under the MIT Licence (see appendix D for
details). This means you can do almost _anything_ you like with our
software, our source code, and our documentation. The only things
you aren't allowed to do are to remove our copyright notices or the
licence text itself, or to hold us legally responsible if something
goes wrong.
So if you want permission to include PuTTY on a magazine cover disk,
or as part of a collection of useful software on a CD or a web site,
then _permission is already granted_. You don't have to mail us and
ask. Just go ahead and do it. We don't mind.
(If you want to distribute PuTTY alongside your own application for
use with that application, or if you want to distribute PuTTY within
your own organisation, then we recommend, but do not insist, that
you offer your own first-line technical support, to answer questions
about the interaction of PuTTY with your environment. If your users
mail us directly, we won't be able to tell them anything useful
about your specific setup.)
If you want to use parts of the PuTTY source code in another
program, then it might be worth mailing us to talk about technical
details, but if all you want is to ask permission then you don't
need to bother. You already have permission.
If you just want to link to our web site, just go ahead. (It's not
clear that we _could_ stop you doing this, even if we wanted to!)
B.10 Mirroring the PuTTY web site
If you want to set up a mirror of the PuTTY website, go ahead and
set one up. Please don't bother asking us for permission before
setting up a mirror. You already have permission.
If the mirror is in a country where we don't already have plenty
of mirrors, we may be willing to add it to the list on our mirrors
page. Read the guidelines on that page, make sure your mirror works,
and email us the information listed at the bottom of the page.
Note that we do not _promise_ to list your mirror: we get a lot of
mirror notifications and yours may not happen to find its way to the
top of the list.
Also note that we link to all our mirror sites using the
`rel="nofollow"' attribute. Running a PuTTY mirror is not intended
to be a cheap way to gain search rankings.
If you have technical questions about the process of mirroring, then
you might want to mail us before setting up the mirror (see also the
guidelines on the Mirrors page); but if you just want to ask for
permission, you don't need to. You already have permission.
B.11 Praise and compliments
One of the most rewarding things about maintaining free software
is getting e-mails that just say `thanks'. We are always happy to
receive e-mails of this type.
Regrettably we don't have time to answer them all in person. If you
mail us a compliment and don't receive a reply, _please_ don't think
we've ignored you. We did receive it and we were happy about it; we
just didn't have time to tell you so personally.
To everyone who's ever sent us praise and compliments, in the past
and the future: _you're welcome_!
B.12 E-mail address
The actual address to mail is <
[email protected]>.
Appendix C: PPK file format
---------------------------
This appendix documents the file format used by PuTTY to store
private keys.
In this appendix, binary data structures are described using data
type representations such as `uint32', `string' and `mpint' as
used in the SSH protocol standards themselves. These are defined
authoritatively by RFC 4251 section 5, `Data Type Representations
Used in the SSH Protocols'.
C.1 Overview
A PPK file stores a private key, and the corresponding public key.
Both are contained in the same file.
The file format can be completely unencrypted, or it can encrypt
the private key. The _public_ key is stored in cleartext in both
cases. (This enables PuTTY to send the public key to an SSH server
to see whether it will accept it, and not bother prompting for the
passphrase unless the server says yes.)
When the key file is encrypted, the encryption key is derived from a
passphrase. An encrypted PPK file is also tamper-proofed using a MAC
(authentication code), also derived from the same passphrase. The
MAC protects the encrypted private key data, but it also covers the
cleartext parts of the file. So you can't edit the public half of
the key without invalidating the MAC and causing the key file as a
whole to become useless.
This MAC protects the key file against active cryptographic attacks
in which the public half of a key pair is modified in a controlled
way that allows an attacker to deduce information about the private
half from the resulting corrupted signatures. Any attempt to do
that to a PPK file should be reliably caught by the MAC failing to
validate.
(Such an attack would only be useful if the key file was stored in a
location where the attacker could modify it without also having full
access to the process that you type passphrases into. But that's not
impossible; for example, if your home directory was on a network
file server, then the file server's administrator could access the
key file but not processes on the client machine.)
The MAC also covers the _comment_ on the key. This stops an attacker
from swapping keys with each other and editing the comments to
disguise the fact. As a consequence, PuTTYgen cannot edit the
comment on a key unless you decrypt the key with your passphrase
first.
(The circumstances in which _that_ attack would be useful are even
more restricted. One example might be that the different keys
trigger specific actions on the server you're connecting to and one
of those actions is more useful to the attacker than the other. But
once you have a MAC at all, it's no extra effort to make it cover as
much as possible, and usually sensible.)
C.2 Outer layer
The outer layer of a PPK file is text-based. The PuTTY tools will
always use LF line termination when writing PPK files, but will
tolerate CR+LF and CR-only on input.
The first few lines identify it as a PPK, and give some initial data
about what's stored in it and how. They look like this:
PuTTY-User-Key-File-version: algorithm-name
Encryption: encryption-type
Comment: key-comment-string
*version* is a decimal number giving the version number of the file
format itself. The current file format version is 3.
*algorithm-name* is the SSH protocol identifier for the public key
algorithm that this key is used for (such as `ssh-dss' or `ecdsa-
sha2-nistp384').
*encryption-type* indicates whether this key is stored encrypted,
and if so, by what method. Currently the only supported encryption
types are `aes256-cbc' and `none'.
*key-comment-string* is a free text field giving the comment. This
can contain any byte values other than 13 and 10 (CR and LF).
The next part of the file gives the public key. This is stored
unencrypted but base64-encoded (RFC 4648), and is preceded by a
header line saying how many lines of base64 data are shown, looking
like this:
Public-Lines: number-of-lines
that many lines of base64 data
The base64-encoded data in this blob is formatted in exactly the
same way as an SSH public key sent over the wire in the SSH protocol
itself. That is also the same format as the base64 data stored in
OpenSSH's `authorized_keys' file, except that in a PPK file the
base64 data is split across multiple lines. But if you remove the
newlines from the middle of this section, the resulting base64 blob
is in the right format to go in an `authorized_keys' line.
If the key is encrypted (i.e. *encryption-type* is not `none'),
then the next thing that appears is a sequence of lines specifying
how the keys for encrypting the file are to be derived from the
passphrase:
Key-Derivation: argon2-flavour
Argon2-Memory: decimal-integer
Argon2-Passes: decimal-integer
Argon2-Parallelism: decimal-integer
Argon2-Salt: hex-string
*argon2-flavour* is one of the identifiers `Argon2d', `Argon2i' or
`Argon2id', all describing variants of the Argon2 password-hashing
function.
The three integer values are used as parameters for Argon2, which
allows you to configure the amount of memory used (in Kbyte), the
number of passes of the algorithm to run (to tune its running time),
and the degree of parallelism required by the hash function. The
salt is decoded into a sequence of binary bytes and used as an
additional input to Argon2. (It is chosen randomly when the key file
is written, so that a guessing attack can't be mounted in parallel
against multiple key files.)
The next part of the file gives the private key. This is base64-
encoded in the same way:
Private-Lines: number-of-lines
that many lines of base64 data
The binary data represented in this base64 blob may be encrypted,
depending on the _encryption-type_ field in the key file header
shown above:
- If *encryption-type* is `none', then this data is stored in
plain text.
- If *encryption-type* is `aes256-cbc', then this data is
encrypted using AES, with a 256-bit key length, in the CBC
cipher mode. The key and initialisation vector are derived from
the passphrase: see section C.4.
In order to encrypt the private key data with AES, it must be
a multiple of 16 bytes (the AES cipher block length). This
is achieved by appending random padding to the data before
encrypting it. When decoding it after decryption, the random
data can be ignored: the internal structure of the data is
enough to tell you when you've reached the end of the meaningful
part.
Unlike public keys, the binary encoding of private keys is not
specified at all in the SSH standards. See section C.3 for details
of the private key format for each key type supported by PuTTY.
The final thing in the key file is the MAC:
Private-MAC: hex-mac-data
*hex-mac-data* is a hexadecimal-encoded value, 64 digits long (i.e.
32 bytes), generated using the HMAC-SHA-256 algorithm with the
following binary data as input:
- string: the *algorithm-name* header field.
- string: the *encryption-type* header field.
- string: the *key-comment-string* header field.
- string: the binary public key data, as decoded from the base64
lines after the `Public-Lines' header.
- string: the plaintext of the binary private key data, as decoded
from the base64 lines after the `Private-Lines' header. If that
data was stored encrypted, then the decrypted version of it
is used in this MAC preimage, _including_ the random padding
mentioned above.
The MAC key is derived from the passphrase: see section C.4.
C.3 Private key encodings
This section describes the private key format for each key type
supported by PuTTY.
Because the PPK format also contains the public key (and both public
and private key are protected by the same MAC to ensure they can't
be made inconsistent), there is no need for the private key section
of the file to repeat data from the public section. So some of these
formats are very short.
In all cases, a decoding application can begin reading from the
start of the decrypted private key data, and know when it has read
all that it needs. This allows random padding after the meaningful
data to be safely ignored.
C.3.1 RSA
RSA keys are stored using an *algorithm-name* of `ssh-rsa'. (Keys
stored like this are also used by the updated RSA signature schemes
that use hashes other than SHA-1.)
The public key data has already provided the key modulus and the
public encoding exponent. The private data stores:
- mpint: the private decoding exponent of the key.
- mpint: one prime factor _p_ of the key.
- mpint: the other prime factor _q_ of the key. (RSA keys stored
in this format are expected to have exactly two prime factors.)
- mpint: the multiplicative inverse of _q_ modulo _p_.
C.3.2 DSA
DSA keys are stored using an *algorithm-name* of `ssh-dss'.
The public key data has already provided the key parameters (the
large prime _p_, the small prime _q_ and the group generator _g_),
and the public key _y_. The private key stores:
- mpint: the private key _x_, which is the discrete logarithm of
_y_ in the group generated by _g_ mod _p_.
C.3.3 NIST elliptic-curve keys
NIST elliptic-curve keys are stored using one of the following
*algorithm-name* values, each corresponding to a different elliptic
curve and key size:
- `ecdsa-sha2-nistp256'
- `ecdsa-sha2-nistp384'
- `ecdsa-sha2-nistp521'
The public key data has already provided the public elliptic curve
point. The private key stores:
- mpint: the private exponent, which is the discrete log of the
public point.
C.3.4 EdDSA elliptic-curve keys (Ed25519 and Ed448)
EdDSA elliptic-curve keys are stored using one of the following
*algorithm-name* values, each corresponding to a different elliptic
curve and key size:
- `ssh-ed25519'
- `ssh-ed448'
The public key data has already provided the public elliptic curve
point. The private key stores:
- mpint: the private exponent, which is the discrete log of the
public point.
C.4 Key derivation
When a key file is encrypted, there are three pieces of key material
that need to be computed from the passphrase:
- the key for the symmetric cipher used to encrypt the private key
- the initialisation vector for that cipher encryption
- the key for the MAC.
If *encryption-type* is `aes256-cbc', then the symmetric cipher key
is 32 bytes long, and the initialisation vector is 16 bytes (one
cipher block). The length of the MAC key is also chosen to be 32
bytes.
If *encryption-type* is `none', then all three of these pieces of
data have zero length. (The MAC is still generated and checked in
the key file format, but it has a zero-length key.)
If the amount of key material required is not zero, then the
passphrase is fed to the Argon2 key derivation function, in
whichever mode is described in the `Key-Derivation' header in
the key file, with parameters derived from the various `Argon2-
_Parameter_:' headers.
(If the key is unencrypted, then all those headers are omitted, and
Argon2 is not run at all.)
Argon2 takes two extra string inputs in addition to the passphrase
and the salt: a secret key, and some `associated data'. In PPK's use
of Argon2, these are both set to the empty string.
The `tag length' parameter to Argon2 (i.e. the amount of data it
is asked to output) is set to the sum of the lengths of all of the
data items required, i.e. (cipher key length + IV length + MAC key
length). The output data is interpreted as the concatenation of the
cipher key, the IV and the MAC key, in that order.
So, for `aes256-cbc', the tag length will be 32+16+32 = 80 bytes; of
the 80 bytes of output data, the first 32 bytes are used as the 256-
bit AES key, the next 16 as the CBC IV, and the final 32 bytes as
the HMAC-SHA-256 key.
C.5 Older versions of the PPK format
C.5.1 Version 2
PPK version 2 was used by PuTTY 0.52 to 0.74 inclusive.
In PPK version 2, the MAC algorithm used was HMAC-SHA-1 (so the
Private-MAC line contained only 40 hex digits).
The `Key-Derivation:' header and all the `Argon2-_Parameter_:'
headers were absent. Instead of using Argon2, the key material for
encrypting the private blob was derived from the passphrase in a
totally different way, as follows.
The cipher key for `aes256-cbc' was constructed by generating two
SHA-1 hashes, concatenating them, and taking the first 32 bytes of
the result. (So you'd get all 20 bytes of the first hash output, and
the first 12 of the second). Each hash preimage was as follows:
- uint32: a sequence number. This is 0 in the first hash, and 1 in
the second. (The idea was to extend this mechanism to further
hashes by continuing to increment the sequence number, if future
changes required even longer keys.)
- the passphrase, without any prefix length field.
In PPK v2, the CBC initialisation vector was all zeroes.
The MAC key was 20 bytes long, and was a single SHA-1 hash of the
following data:
- the fixed string `putty-private-key-file-mac-key', without any
prefix length field.
- the passphrase, without any prefix length field. (If the key is
stored unencrypted, the passphrase was taken to be the empty
string for these purposes.)
C.5.2 Version 1
PPK version 1 was a badly designed format, only used during initial
development, and not recommended for production use.
PPK version 1 was never used by a released version of PuTTY. It was
only emitted by some early development snapshots between version
0.51 (which did not support SSH-2 public keys at all) and 0.52
(which already used version 2 of this file format). I _hope_ there
are no PPK v1 files in use anywhere. But just in case, the old badly
designed format is documented here anyway.
In PPK version 1, the input to the MAC does not include any of the
header fields or the public key. It is simply the private key data
(still in plaintext and including random padding), all by itself
(without a wrapping string).
PPK version 1 keys must therefore be rigorously validated after
loading, to ensure that the public and private parts of the key were
consistent with each other.
PPK version 1 only supported the RSA and DSA key types. For RSA,
this validation can be done using only the provided data (since the
private key blob contains enough information to reconstruct the
public values anyway). But for DSA, that isn't quite enough.
Hence, PPK version 1 DSA keys extended the private data so that
immediately after _x_ was stored an extra value:
- string: a SHA-1 hash of the public key data, whose preimage
consists of
- string: the large prime _p_
- string: the small prime _q_
- string: the group generator _g_
The idea was that checking this hash would verify that the key
parameters had not been tampered with, and then the loading
application could directly verify that _g_^_x_ = _y_.
In an _unencrypted_ version 1 key file, the MAC is replaced by a
plain SHA-1 hash of the private key data. This is indicated by the
`Private-MAC:' header being replaced with `Private-Hash:' instead.
Appendix D: PuTTY Licence
-------------------------
PuTTY is copyright 1997-2025 Simon Tatham.
Portions copyright Robert de Bath, Joris van Rantwijk, Delian
Delchev, Andreas Schultz, Jeroen Massar, Wez Furlong, Nicolas Barry,
Justin Bradford, Ben Harris, Malcolm Smith, Ahmad Khalifa, Markus
Kuhn, Colin Watson, Christopher Staite, Lorenz Diener, Christian
Brabandt, Jeff Smith, Pavel Kryukov, Maxim Kuznetsov, Svyatoslav
Kuzmich, Nico Williams, Viktor Dukhovni, Josh Dersch, Lars
Brinkhoff, and CORE SDI S.A.
Permission is hereby granted, free of charge, to any person
obtaining a copy of this software and associated documentation files
(the `Software'), to deal in the Software without restriction,
including without limitation the rights to use, copy, modify, merge,
publish, distribute, sublicense, and/or sell copies of the Software,
and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED `AS IS', WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE
FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Appendix E: PuTTY hacking guide
-------------------------------
This appendix lists a selection of the design principles applying
to the PuTTY source code. If you are planning to send code
contributions, you should read this first.
E.1 Cross-OS portability
Despite Windows being its main area of fame, PuTTY is no longer a
Windows-only application suite. It has a working Unix port; a Mac
port is in progress; more ports may or may not happen at a later
date.
Therefore, embedding Windows-specific code in core modules such as
ssh.c is not acceptable. We went to great lengths to _remove_ all
the Windows-specific stuff from our core modules, and to shift it
out into Windows-specific modules. Adding large amounts of Windows-
specific stuff in parts of the code that should be portable is
almost guaranteed to make us reject a contribution.
The PuTTY source base is divided into platform-specific modules
and platform-generic modules. The Unix-specific modules are all in
the `unix' subdirectory; the Windows-specific modules are in the
`windows' subdirectory.
All the modules in the main source directory and other
subdirectories - notably _all_ of the code for the various back ends
- are platform-generic. We want to keep them that way.
This also means you should stick to the C semantics guaranteed by
the C standard: try not to make assumptions about the precise size
of basic types such as `int' and `long int'; don't use pointer casts
to do endianness-dependent operations, and so on.
(Even _within_ a platform front end you should still be careful of
some of these portability issues. The Windows front end compiles on
both 32- and 64-bit x86 and also Arm.)
Our current choice of C standards version is _mostly_ C99. With a
couple of exceptions, you can assume that C99 features are available
(in particular <stdint.h>, <stdbool.h> and `inline'), but you
shouldn't use things that are new in C11 (such as <uchar.h> or
_Generic).
The exceptions to that rule are due to the need for Visual Studio
compatibility:
- Don't use variable-length arrays. Visual Studio doesn't support
them even now that it's adopted the rest of C99. We use -Wvla
when building with gcc and clang, to make it easier to avoid
accidentally breaking that rule.
- For historical reasons, we still build with one older VS version
which lacks <inttypes.h>. So that file is included centrally
in `defs.h', and has a set of workaround definitions for the
PRIx64-type macros we use. If you need to use another one
of those macros, you need to add a workaround definition in
`defs.h', and don't casually re-include <inttypes.h> anywhere
else in the source file.
Here are a few portability assumptions that we _do_ currently allow
(because we'd already have to thoroughly vet the existing code if
they ever needed to change, and it doesn't seem worth doing that
unless we really have to):
- You can assume `int' is _at least_ 32 bits wide. (We've never
tried to port PuTTY to a platform with 16-bit int, and it
doesn't look likely to be necessary in future.)
- Similarly, you can assume `char' is exactly 8 bits. (Exceptions
to that are even less likely to be relevant to us than short
int.)
- You can assume that using `memset' to write zero bytes over a
whole structure will have the effect of setting all its pointer
fields to NULL. (The standard itself guarantees this for
_integer_ fields, but not for pointers.)
- You can assume that `time_t' has POSIX semantics, i.e. that
it represents an integer number of non-leap seconds since
1970-01-01 00:00:00 UTC. (Times in this format are used in
X authorisation, but we could work around that by carefully
distinguishing local `time_t' from time values used in the wire
protocol; but these semantics of `time_t' are also baked into
the shared library API used by the GSSAPI authentication code,
which would be much harder to change.)
- You can assume that the execution character encoding is a
superset of the printable characters of ASCII. (In particular,
it's fine to do arithmetic on a `char' value representing a
Latin alphabetic character, without bothering to allow for
EBCDIC or other non-consecutive encodings of the alphabet.)
On the other hand, here are some particular things _not_ to assume:
- Don't assume anything about the _signedness_ of `char'. In
particular, you _must_ cast `char' values to `unsigned char'
before passing them to any <ctype.h> function (because those
expect a non-negative character value, or EOF). If you need
a particular signedness, explicitly specify `signed char' or
`unsigned char', or use C99 int8_t or uint8_t.
- From past experience with MacOS, we're still a bit nervous about
'\n' and '\r' potentially having unusual meanings on a given
platform. So it's fine to say `\n' in a string you're passing to
`printf', but in any context where those characters appear in a
standardised wire protocol or a binary file format, they should
be spelled '\012' and '\015' respectively.
E.2 Multiple backends treated equally
PuTTY is not an SSH client with some other stuff tacked on the side.
PuTTY is a generic, multiple-backend, remote VT-terminal client
which happens to support one backend which is larger, more popular
and more useful than the rest. Any extra feature which can possibly
be general across all backends should be so: localising features
unnecessarily into the SSH back end is a design error. (For example,
we had several code submissions for proxy support which worked by
hacking ssh.c. Clearly this is completely wrong: the network.h
abstraction is the place to put it, so that it will apply to all
back ends equally, and indeed we eventually put it there after
another contributor sent a better patch.)
The rest of PuTTY should try to avoid knowing anything about
specific back ends if at all possible. To support a feature which is
only available in one network protocol, for example, the back end
interface should be extended in a general manner such that _any_
back end which is able to provide that feature can do so. If it so
happens that only one back end actually does, that's just the way it
is, but it shouldn't be relied upon by any code.
E.3 Multiple sessions per process on some platforms
Some ports of PuTTY - notably the in-progress Mac port - are
constrained by the operating system to run as a single process
potentially managing multiple sessions.
Therefore, the platform-independent parts of PuTTY never use global
variables to store per-session data. The global variables that do
exist are tolerated because they are not specific to a particular
login session. The random number state in sshrand.c, the timer list
in timing.c and the queue of top-level callbacks in callback.c serve
all sessions equally. But most data is specific to a particular
network session, and is therefore stored in dynamically allocated
data structures, and pointers to these structures are passed around
between functions.
Platform-specific code can reverse this decision if it likes. The
Windows code, for historical reasons, stores most of its data as
global variables. That's OK, because _on Windows_ we know there is
only one session per PuTTY process, so it's safe to do that. But
changes to the platform-independent code should avoid introducing
global variables, unless they are genuinely cross-session.
E.4 C, not C++
PuTTY is written entirely in C, not in C++.
We have made _some_ effort to make it easy to compile our code using
a C++ compiler: notably, our `snew', `snewn' and `sresize' macros
explicitly cast the return values of malloc and realloc to the
target type. (This has type checking advantages even in C: it means
you never accidentally allocate the wrong size piece of memory for
the pointer type you're assigning it to. C++ friendliness is really
a side benefit.)
We want PuTTY to continue being pure C, at least in the platform-
independent parts and the currently existing ports. Patches which
switch the Makefiles to compile it as C++ and start using classes
will not be accepted.
The one exception: a port to a new platform may use languages other
than C if they are necessary to code on that platform. If your
favourite PDA has a GUI with a C++ API, then there's no way you can
do a port of PuTTY without using C++, so go ahead and use it. But
keep the C++ restricted to that platform's subdirectory; if your
changes force the Unix or Windows ports to be compiled as C++, they
will be unacceptable to us.
E.5 Security-conscious coding
PuTTY is a network application and a security application. Assume
your code will end up being fed deliberately malicious data by
attackers, and try to code in a way that makes it unlikely to be a
security risk.
In particular, try not to use fixed-size buffers for variable-size
data such as strings received from the network (or even the user).
We provide functions such as dupcat and dupprintf, which dynamically
allocate buffers of the right size for the string they construct.
Use these wherever possible.
E.6 Independence of specific compiler
Windows PuTTY can currently be compiled with any of three Windows
compilers: MS Visual C, the Cygwin / mingw32 GNU tools, and clang
(in MS compatibility mode).
This is a really useful property of PuTTY, because it means people
who want to contribute to the coding don't depend on having a
specific compiler; so they don't have to fork out money for MSVC if
they don't already have it, but on the other hand if they _do_ have
it they also don't have to spend effort installing gcc alongside it.
They can use whichever compiler they happen to have available, or
install whichever is cheapest and easiest if they don't have one.
Therefore, we don't want PuTTY to start depending on which compiler
you're using. Using GNU extensions to the C language, for example,
would ruin this useful property (not that anyone's ever tried
it!); and more realistically, depending on an MS-specific library
function supplied by the MSVC C library (_snprintf, for example)
is a mistake, because that function won't be available under the
other compilers. Any function supplied in an official Windows DLL
as part of the Windows API is fine, and anything defined in the C
library standard is also fine, because those should be available
irrespective of compilation environment. But things in between,
available as non-standard library and language extensions in only
one compiler, are disallowed.
(_snprintf in particular should be unnecessary, since we provide
dupprintf; see section E.5.)
Compiler independence should apply on all platforms, of course, not
just on Windows.
E.7 Small code size
PuTTY is tiny, compared to many other Windows applications. And
it's easy to install: it depends on no DLLs, no other applications,
no service packs or system upgrades. It's just one executable. You
install that executable wherever you want to, and run it.
We want to keep both these properties - the small size, and the ease
of installation - if at all possible. So code contributions that
depend critically on external DLLs, or that add a huge amount to the
code size for a feature which is only useful to a small minority of
users, are likely to be thrown out immediately.
We do vaguely intend to introduce a DLL plugin interface for PuTTY,
whereby seriously large extra features can be implemented in plugin
modules. The important thing, though, is that those DLLs will be
_optional_; if PuTTY can't find them on startup, it should run
perfectly happily and just won't provide those particular features.
A full installation of PuTTY might one day contain ten or twenty
little DLL plugins, which would cut down a little on the ease of
installation - but if you really needed ease of installation you
_could_ still just install the one PuTTY binary, or just the DLLs
you really needed, and it would still work fine.
Depending on _external_ DLLs is something we'd like to avoid if
at all possible (though for some purposes, such as complex SSH
authentication mechanisms, it may be unavoidable). If it can't be
avoided, the important thing is to follow the same principle of
graceful degradation: if a DLL can't be found, then PuTTY should run
happily and just not supply the feature that depended on it.
E.8 Single-threaded code
PuTTY and its supporting tools, or at least the vast majority of
them, run in only one OS thread.
This means that if you're devising some piece of internal mechanism,
there's no need to use locks to make sure it doesn't get called by
two threads at once. The only way code can be called re-entrantly is
by recursion.
That said, most of Windows PuTTY's network handling is triggered
off Windows messages requested by WSAAsyncSelect(), so if you call
MessageBox() deep within some network event handling code you should
be aware that you might be re-entered if a network event comes in
and is passed on to our window procedure by the MessageBox() message
loop.
Also, the front ends can use multiple threads if they like. For
example, the Windows front-end code spawns subthreads to deal with
bidirectional blocking I/O on non-network streams such as Windows
pipes. However, it keeps tight control of its auxiliary threads, and
uses them only for that one purpose, as a form of select(). Pretty
much all the code outside windows/handle-io.c is _only_ ever called
from the one primary thread; the others just loop round blocking
on file handles, and signal the main thread (via Windows event
objects) when some real work needs doing. This is not considered a
portability hazard because that code is already Windows-specific and
needs rewriting on other platforms.
One important consequence of this: PuTTY has only one thread in
which to do everything. That `everything' may include managing
more than one login session (section E.3), managing multiple data
channels within an SSH session, responding to GUI events even when
nothing is happening on the network, and responding to network
requests from the server (such as repeat key exchange) even when the
program is dealing with complex user interaction such as the re-
configuration dialog box. This means that _almost none_ of the PuTTY
code can safely block.
E.9 Keystrokes sent to the server wherever possible
In almost all cases, PuTTY sends keystrokes to the server. Even
weird keystrokes that you think should be hot keys controlling
PuTTY. Even Alt-F4 or Alt-Space, for example. If a keystroke has a
well-defined escape sequence that it could usefully be sending to
the server, then it should do so, or at the very least it should be
configurably able to do so.
To unconditionally turn a key combination into a hot key to control
PuTTY is almost always a design error. If a hot key is really truly
required, then try to find a key combination for it which _isn't_
already used in existing PuTTYs (either it sends nothing to the
server, or it sends the same thing as some other combination).
Even then, be prepared for the possibility that one day that key
combination might end up being needed to send something to the
server - so make sure that there's an alternative way to invoke
whatever PuTTY feature it controls.
E.10 640x480 friendliness in configuration panels
There's a reason we have lots of tiny configuration panels instead
of a few huge ones, and that reason is that not everyone has a
1600x1200 desktop. 640x480 is still a viable resolution for running
Windows (and indeed it's still the default if you start up in safe
mode), so it's still a resolution we care about.
Accordingly, the PuTTY configuration box, and the PuTTYgen control
window, are deliberately kept just small enough to fit comfortably
on a 640x480 display. If you're adding controls to either of these
boxes and you find yourself wanting to increase the size of the
whole box, _don't_. Split it into more panels instead.
E.11 Coroutines in protocol code
Large parts of the code in modules implementing wire protocols
(mainly SSH) are structured using a set of macros that implement
(something close to) Donald Knuth's `coroutines' concept in C.
Essentially, the purpose of these macros are to arrange that a
function can call crReturn() to return to its caller, and the next
time it is called control will resume from just after that crReturn
statement.
This means that any local (automatic) variables declared in such a
function will be corrupted every time you call crReturn. If you need
a variable to persist for longer than that, you _must_ make it a
field in some appropriate structure containing the persistent state
of the coroutine - typically the main state structure for a protocol
layer.
See `
https://www.chiark.greenend.org.uk/~sgtatham/coroutines.html'
for a more in-depth discussion of what these macros are for and how
they work.
Another caveat: most of these coroutines are not _guaranteed_ to run
to completion, because the SSH connection (or whatever) that they're
part of might be interrupted at any time by an unexpected network
event or user action. So whenever a coroutine-managed variable
refers to a resource that needs releasing, you should also ensure
that the cleanup function for its containing state structure can
reliably release it even if the coroutine is aborted at an arbitrary
point.
For example, if an SSH packet protocol layer has to have a field
that sometimes points to a piece of allocated memory, then you
should ensure that when you free that memory you reset the pointer
field to NULL. Then, no matter when the protocol layer's cleanup
function is called, it can reliably free the memory if there is any,
and not crash if there isn't.
E.12 Explicit vtable structures to implement traits
A lot of PuTTY's code is written in a style that looks structurally
rather like an object-oriented language, in spite of PuTTY being a
pure C program.
For example, there's a single data type called ssh_hash, which is
an abstraction of a secure hash function, and a bunch of functions
called things like ssh_hash__foo_ that do things with those data
types. But in fact, PuTTY supports many different hash functions,
and each one has to provide its own implementation of those
functions.
In C++ terms, this is rather like having a single abstract base
class, and multiple concrete subclasses of it, each of which fills
in all the pure virtual methods in a way that's compatible with the
data fields of the subclass. The implementation is more or less the
same, as well: in C, we do explicitly in the source code what the
C++ compiler will be doing behind the scenes at compile time.
But perhaps a closer analogy in functional terms is the Rust concept
of a `trait', or the Java idea of an `interface'. C++ supports a
multi-level hierarchy of inheritance, whereas PuTTY's system - like
traits or interfaces - has only two levels, one describing a generic
object of a type (e.g. a hash function) and another describing a
specific implementation of that type (e.g. SHA-256).
The PuTTY code base has a standard idiom for doing this in C, as
follows.
Firstly, we define two struct types for our trait. One of them
describes a particular _kind_ of implementation of that trait,
and it's full of (mostly) function pointers. The other describes
a specific _instance_ of an implementation of that trait, and it
will contain a pointer to a const instance of the first type. For
example:
typedef struct MyAbstraction MyAbstraction;
typedef struct MyAbstractionVtable MyAbstractionVtable;
struct MyAbstractionVtable {
MyAbstraction *(*new)(const MyAbstractionVtable *vt);
void (*free)(MyAbstraction *);
void (*modify)(MyAbstraction *, unsigned some_parameter);
unsigned (*query)(MyAbstraction *, unsigned some_parameter);
};
struct MyAbstraction {
const MyAbstractionVtable *vt;
};
Here, we imagine that MyAbstraction might be some kind of object
that contains mutable state. The associated vtable structure shows
what operations you can perform on a MyAbstraction: you can create
one (dynamically allocated), free one you already have, or call the
example methods `modify' (to change the state of the object in some
way) and `query' (to return some value derived from the object's
current state).
(In most cases, the vtable structure has a name ending in `vtable'.
But for historical reasons a lot of the crypto primitives that
use this scheme - ciphers, hash functions, public key methods and
so on - instead have names ending in `alg', on the basis that the
primitives they implement are often referred to as `encryption
algorithms', `hash algorithms' and so forth.)
Now, to define a concrete instance of this trait, you'd define a
struct that contains a MyAbstraction field, plus any other data it
might need:
struct MyImplementation {
unsigned internal_data[16];
SomeOtherType *dynamic_subthing;
MyAbstraction myabs;
};
Next, you'd implement all the necessary methods for that
implementation of the trait, in this kind of style:
static MyAbstraction *myimpl_new(const MyAbstractionVtable *vt)
{
MyImplementation *impl = snew(MyImplementation);
memset(impl, 0, sizeof(*impl));
impl->dynamic_subthing = allocate_some_other_type();
impl->myabs.vt = vt;
return &impl->myabs;
}
static void myimpl_free(MyAbstraction *myabs)
{
MyImplementation *impl = container_of(myabs, MyImplementation, myabs);
free_other_type(impl->dynamic_subthing);
sfree(impl);
}
static void myimpl_modify(MyAbstraction *myabs, unsigned param)
{
MyImplementation *impl = container_of(myabs, MyImplementation, myabs);
impl->internal_data[param] += do_something_with(impl->dynamic_subthing);
}
static unsigned myimpl_query(MyAbstraction *myabs, unsigned param)
{
MyImplementation *impl = container_of(myabs, MyImplementation, myabs);
return impl->internal_data[param];
}
Having defined those methods, now we can define a const instance of
the vtable structure containing pointers to them:
const MyAbstractionVtable MyImplementation_vt = {
.new = myimpl_new,
.free = myimpl_free,
.modify = myimpl_modify,
.query = myimpl_query,
};
_In principle_, this is all you need. Client code can construct a
new instance of a particular implementation of MyAbstraction by
digging out the new method from the vtable and calling it (with
the vtable itself as a parameter), which returns a MyAbstraction *
pointer that identifies a newly created instance, in which the vt
field will contain a pointer to the same vtable structure you passed
in. And once you have an instance object, say MyAbstraction *myabs,
you can dig out one of the other method pointers from the vtable it
points to, and call that, passing the object itself as a parameter.
But in fact, we don't do that, because it looks pretty ugly at all
the call sites. Instead, what we generally do in this code base is
to write a set of static inline wrapper functions in the same header
file that defined the MyAbstraction structure types, like this:
static inline MyAbstraction *myabs_new(const MyAbstractionVtable *vt)
{ return vt->new(vt); }
static inline void myabs_free(MyAbstraction *myabs)
{ myabs->vt->free(myabs); }
static inline void myimpl_modify(MyAbstraction *myabs, unsigned param)
{ myabs->vt->modify(myabs, param); }
static inline unsigned myimpl_query(MyAbstraction *myabs, unsigned param)
{ return myabs->vt->query(myabs, param); }
And now call sites can use those reasonably clean-looking wrapper
functions, and shouldn't ever have to directly refer to the vt field
inside any myabs object they're holding. For example, you might
write something like this:
MyAbstraction *myabs = myabs_new(&MyImplementation_vtable);
myabs_update(myabs, 10);
unsigned output = myabs_query(myabs, 2);
myabs_free(myabs);
and then all this code can use a different implementation of the
same abstraction by just changing which vtable pointer it passed in
in the first line.
Some things to note about this system:
- The implementation instance type (here `MyImplementation'
contains the abstraction type (`MyAbstraction') as one of its
fields. But that field is not necessarily at the start of the
structure. So you can't just _cast_ pointers back and forth
between the two types. Instead:
- You `up-cast' from implementation to abstraction by taking
the address of the MyAbstraction field. You can see the
example new method above doing this, returning &impl->myabs.
All new methods do this on return.
- Going in the other direction, each method that was
passed a generic MyAbstraction *myabs parameter has to
recover a pointer to the specific implementation type
MyImplementation *impl. The idiom for doing that is to use
the `container_of' macro, also seen in the Linux kernel
code. Generally, container_of(p, Type, field) says: `I'm
confident that the pointer value `p' is pointing to the
field called `field' within a larger struct of type Type.
Please return me the pointer to the containing structure.'
So in this case, we take the `myabs' pointer passed to the
function, and `down-cast' it into a pointer to the larger
and more specific structure type MyImplementation, by
adjusting the pointer value based on the offset within that
structure of the field called `myabs'.
This system is flexible enough to permit `multiple inheritance',
or rather, multiple _implementation_: having one object type
implement more than one trait. For example, the ProxySocket
type implements both the Socket trait and the Plug trait that
connects to it, because it has to act as an adapter between
another instance of each of those types.
It's also perfectly possible to have the same object implement
the _same_ trait in two different ways. At the time of writing
this I can't think of any case where we actually do this, but a
theoretical example might be if you needed to support a trait
like Comparable in two ways that sorted by different criteria.
There would be no difficulty doing this in the PuTTY system:
simply have your implementation struct contain two (or more)
fields of the same abstraction type. The fields will have
different names, which makes it easy to explicitly specify which
one you're returning a pointer to during up-casting, or which
one you're down-casting from using container_of. And then both
sets of implementation methods can recover a pointer to the same
containing structure.
- Unlike in C++, all objects in PuTTY that use this system are
dynamically allocated. The `constructor' functions (whether
they're virtualised across the whole abstraction or specific to
each implementation) always allocate memory and return a pointer
to it. The `free' method (our analogue of a destructor) always
expects the input pointer to be dynamically allocated, and frees
it. As a result, client code doesn't need to know how large
the implementing object type is, because it will never need to
allocate it (on the stack or anywhere else).
- Unlike in C++, the abstraction's `vtable' structure does not
only hold methods that you can call on an instance object. It
can also hold several other kinds of thing:
- Methods that you can call _without_ an instance object,
given only the vtable structure identifying a particular
implementation of the trait. You might think of these as
`static methods', as in C++, except that they're _virtual_
- the same code can call the static method of a different
`class' given a different vtable pointer. So they're more
like `virtual static methods', which is a concept C++
doesn't have. An example is the pubkey_bits method in
ssh_keyalg.
- The most important case of a `virtual static method' is the
new method that allocates and returns a new object. You can
think of it as a `virtual constructor' - another concept C++
doesn't have. (However, not all types need one of these: see
below.)
- The vtable can also contain constant data relevant to the
class as a whole - `virtual constant data'. For example, a
cryptographic hash function will contain an integer field
giving the length of the output hash, and most crypto
primitives will contain a string field giving the identifier
used in the SSH protocol that describes that primitive.
The effect of all of this is that you can make other pieces of
code able to use any instance of one of these types, by passing
it an actual vtable as a parameter. For example, the hash_simple
function takes an ssh_hashalg vtable pointer specifying any
hash algorithm you like, and internally, it creates an object
of that type, uses it, and frees it. In C++, you'd probably
do this using a template, which would mean you had multiple
specialisations of hash_simple - and then it would be much more
difficult to decide _at run time_ which one you needed to use.
Here, hash_simple is still just one function, and you can decide
as late as you like which vtable to pass to it.
- The abstract _instance_ structure can also contain publicly
visible data fields (this time, usually treated as mutable)
which are common to all implementations of the trait. For
example, BinaryPacketProtocol has lots of these.
- Not all abstractions of this kind want virtual constructors. It
depends on how different the implementations are.
With a crypto primitive like a hash algorithm, the constructor
call looks the same for every implementing type, so it makes
sense to have a standardised virtual constructor in the
vtable and a ssh_hash_new wrapper function which can make an
instance of whatever vtable you pass it. And then you make
all the vtable objects themselves globally visible throughout
the source code, so that any module can call (for example)
ssh_hash_new(&ssh_sha256).
But with other kinds of object, the constructor for each
implementing type has to take a different set of parameters.
For example, implementations of Socket are not generally
interchangeable at construction time, because constructing
different kinds of socket require totally different kinds of
address parameter. In that situation, it makes more sense to
keep the vtable structure itself private to the implementing
source file, and instead, publish an ordinary constructing
function that allocates and returns an instance of that
particular subtype, taking whatever parameters are appropriate
to that subtype.
- If you do have virtual constructors, you can choose whether
they take a vtable pointer as a parameter (as shown above), or
an _existing_ instance object. In the latter case, they can
refer to the object itself as well as the vtable. For example,
you could have a trait come with a virtual constructor called
`clone', meaning `Make a copy of this object, no matter which
implementation it is.'
- Sometimes, a single vtable structure type can be shared
between two completely different object types, and contain
all the methods for both. For example, ssh_compression_alg
contains methods to create, use and free ssh_compressor and
ssh_decompressor objects, which are not interchangeable - but
putting their methods in the same vtable means that it's easy to
create a matching pair of objects that are compatible with each
other.
- Passing the vtable itself as an argument to the new method is
not compulsory: if a given new implementation is only used by
a single vtable, then that function can simply hard-code the
vtable pointer that it writes into the object it constructs.
But passing the vtable is more flexible, because it allows
a single constructor function to be shared between multiple
slightly different object types. For example, SHA-384 and SHA-
512 share the same new method and the same implementation data
type, because they're very nearly the same hash algorithm - but
a couple of the other methods in their vtables are different,
because the `reset' function has to set up the initial algorithm
state differently, and the `digest' method has to write out a
different amount of data.
One practical advantage of having the myabs__foo_ family of
inline wrapper functions in the header file is that if you
change your mind later about whether the vtable needs to be
passed to new, you only have to update the myabs_new wrapper,
and then the existing call sites won't need changing.
- Another piece of `stunt object orientation' made possible by
this scheme is that you can write two vtables that both use the
same structure layout for the implementation object, and have
an object _transform from one to the other_ part way through
its lifetime, by overwriting its own vtable pointer field. For
example, the sesschan type that handles the server side of an
SSH terminal session will sometimes transform in mid-lifetime
into an SCP or SFTP file-transfer channel in this way, at the
point where the client sends an `exec' or `subsystem' request
that indicates that that's what it wants to do with the channel.
This concept would be difficult to arrange in C++. In Rust,
it wouldn't even _make sense_, because in Rust, objects
implementing a trait don't even contain a vtable pointer at
all - instead, the `trait object' type (identifying a specific
instance of some implementation of a given trait) consists of
a pair of pointers, one to the object itself and one to the
vtable. In that model, the only way you could make an existing
object turn into a different trait would be to know where all
the pointers to it were stored elsewhere in the program, and
persuade all their owners to rewrite them.
- Another stunt you can do is to have a vtable that doesn't have
a corresponding implementation structure at all, because the
only methods implemented in it are the constructors, and they
always end up returning an implementation of some other vtable.
For example, some of PuTTY's crypto primitives have a hardware-
accelerated version and a pure software version, and decide at
run time which one to use (based on whether the CPU they're
running on supports the necessary acceleration instructions).
So, for example, there are vtables for ssh_sha256_sw and
ssh_sha256_hw, each of which has its own data layout and its own
implementations of all the methods; and then there's a top-level
vtable ssh_sha256, which only provides the `new' method, and
implements it by calling the `new' method on one or other of the
subtypes depending on what it finds out about the machine it's
running on. That top-level selector vtable is nearly always the
one used by client code. (Except for the test suite, which has
to instantiate both of the subtypes in order to make sure they
both pass the tests.)
As a result, the top-level selector vtable ssh_sha256 doesn't
need to implement any method that takes an ssh_cipher *
parameter, because no ssh_cipher object is ever constructed
whose vt field points to &ssh_sha256: they all point to one of
the other two full implementation vtables.
E.13 Do as we say, not as we do
The current PuTTY code probably does not conform strictly to _all_
of the principles listed above. There may be the occasional SSH-
specific piece of code in what should be a backend-independent
module, or the occasional dependence on a non-standard X library
function under Unix.
This should not be taken as a licence to go ahead and violate the
rules. Where we violate them ourselves, we're not happy about it,
and we would welcome patches that fix any existing problems. Please
try to help us make our code better, not worse!
Appendix F: PuTTY download keys and signatures
----------------------------------------------
We create GPG signatures for all the PuTTY files distributed from
our web site, so that users can be confident that the files have not
been tampered with. Here we identify our public keys, and explain
our signature policy so you can have an accurate idea of what each
signature guarantees. This description is provided as both a web
page on the PuTTY site, and an appendix in the PuTTY manual.
As of release 0.58, all of the PuTTY executables contain fingerprint
material (usually accessed via the `-pgpfp' command-line option),
such that if you have an executable you trust, you can use it to
establish a trust path, for instance to a newer version downloaded
from the Internet.
As of release 0.67, the Windows executables and installer also
contain built-in signatures that are automatically verified by
Windows' own mechanism (`Authenticode'). The keys used for that are
different, and are not covered here.
See question A.9.18 in the FAQ for some gotchas when verifying
checksums and signatures.
(Note that none of the keys, signatures, etc mentioned here have
anything to do with keys used with SSH - they are purely for
verifying the origin of files distributed by the PuTTY team.)
F.1 Public keys
We maintain multiple keys, stored with different levels of security
due to being used in different ways. See section F.2 below for
details.
The keys we provide are:
Snapshot Key
Used to sign routine development builds of PuTTY: nightly
snapshots, pre-releases, and sometimes also custom diagnostic
builds we send to particular users.
Release Key
Used to sign manually released versions of PuTTY.
Secure Contact Key
An encryption-capable key suitable for people to send
confidential messages to the PuTTY team, e.g. reports of
vulnerabilities.
Master Key
Used to tie all the above keys into the GPG web of trust. The
Master Key signs all the other keys, and other GPG users have
signed it in turn.
The current issue of those keys are available for download from the
PuTTY website, and are also available on PGP keyservers using the
key IDs listed below.
*Master Key* (2023)
RSA, 4096-bit. Key ID: B15D9EFC216B06A1. Fingerprint:
28D4 7C46 55E7 65A6 D827 AC66 B15D 9EFC 216B 06A1
*Release Key* (2023)
RSA, 3072-bit. Key ID: 1993D21BCAD1AA77. Fingerprint:
F412 BA3A A30F DC0E 77B4 E387 1993 D21B CAD1 AA77
*Snapshot Key* (2023)
RSA, 3072-bit. Key ID: 10625E553F53FAAD. Fingerprint:
74CC 6DD9 ABA7 31D4 C5A0 C2D0 1062 5E55 3F53 FAAD
*Secure Contact Key* (2023)
RSA, 3072-bit. Key ID: 1559F6A8929F5EFC. Fingerprint:
01F5 A2B1 1388 D64B 707F 897F 1559 F6A8 929F 5EFC
F.2 Security details
The various keys have various different security levels. This
section explains what those security levels are, and how far you can
expect to trust each key.
F.2.1 The Development Snapshots key
The Development Snapshots private key is stored _without a
passphrase_. This is necessary, because the snapshots are generated
every night without human intervention, so nobody would be able to
type a passphrase.
The snapshots are built and signed on a team member's home
computers, before being uploaded to the web server from which you
download them.
Therefore, a signature from the Development Snapshots key _DOES_
protect you against:
- People tampering with the PuTTY binaries between the PuTTY web
site and you.
- The maintainers of our web server attempting to abuse their root
privilege to tamper with the binaries.
But it _DOES NOT_ protect you against:
- People tampering with the binaries before they are uploaded to
our download servers.
- People tampering with the build machines so that the next set of
binaries they build will be malicious in some way.
- People stealing the unencrypted private key from the build
machine it lives on.
Of course, we take all reasonable precautions to guard the build
machines. But when you see a signature, you should always be certain
of precisely what it guarantees and precisely what it does not.
F.2.2 The Releases key
The Releases key is more secure: because it is only used at release
time, to sign each release by hand, we can store it encrypted.
The Releases private key is kept encrypted on the developers' own
local machines. So an attacker wanting to steal it would have to
also steal the passphrase.
F.2.3 The Secure Contact Key
The Secure Contact Key is stored with a similar level of security to
the Release Key: it is stored with a passphrase, and no automated
script has access to it.
F.2.4 The Master Keys
The Master Key signs almost nothing. Its purpose is to bind the
other keys together and certify that they are all owned by the same
people and part of the same integrated setup. The only signatures
produced by the Master Key, _ever_, should be the signatures on the
other keys.
The Master Key is especially long, and its private key and
passphrase are stored with special care.
We have collected some third-party signatures on the Master Key, in
order to increase the chances that you can find a suitable trust
path to them.
We have uploaded our various keys to public keyservers, so that even
if you don't know any of the people who have signed our keys, you
can still be reasonably confident that an attacker would find it
hard to substitute fake keys on all the public keyservers at once.
F.3 Key rollover
Our current keys were generated in July 2023.
Each new Master Key is signed with the old one, to show that it
really is owned by the same people and not substituted by an
attacker.
Each new Master Key also signs the previous Release Keys, in case
you're trying to verify the signatures on a release prior to the
rollover and can find a chain of trust to those keys from any of the
people who have signed our new Master Key.
Each release is signed with the Release Key that was current at the
time of release. We don't go back and re-sign old releases with
newly generated keys.
The details of all previous keys are given here.
*Keys generated in the 2021 rollover*
*Master Key* (2021)
RSA, 3072-bit. Key ID: DD4355EAAC1119DE. Fingerprint:
A872 D42F 1660 890F 0E05 223E DD43 55EA AC11 19DE
*Release Key* (2021)
RSA, 3072-bit. Key ID: E4F83EA2AA4915EC. Fingerprint:
2CF6 134B D3F7 7A65 88EB D668 E4F8 3EA2 AA49 15EC
*Snapshot Key* (2021)
RSA, 3072-bit. Key ID: B43979F89F446CFD. Fingerprint:
1FD3 BCAC E532 FBE0 6A8C 09E2 B439 79F8 9F44 6CFD
*Secure Contact Key* (2021)
RSA, 3072-bit. Key ID: 012C59D4211BD62A. Fingerprint:
E30F 1354 2A04 BE0E 56F0 5801 012C 59D4 211B D62A
*Keys generated in the 2018 rollover*
*Master Key* (2018)
RSA, 4096-bit. Key ID: 76BC7FE4EBFD2D9E. Fingerprint:
24E1 B1C5 75EA 3C9F F752 A922 76BC 7FE4 EBFD 2D9E
*Release Key* (2018)
RSA, 3072-bit. Key ID: 6289A25F4AE8DA82. Fingerprint:
E273 94AC A3F9 D904 9522 E054 6289 A25F 4AE8 DA82
*Snapshot Key* (2018)
RSA, 3072-bit. Key ID: 38BA7229B7588FD1. Fingerprint:
C92B 52E9 9AB6 1DDA 33DB 2B7A 38BA 7229 B758 8FD1
*Secure Contact Key* (2018)
RSA, 3072-bit. Key ID: 657D487977F95C98. Fingerprint:
A680 0082 2998 6E46 22CA 0E43 657D 4879 77F9 5C98
*Key generated in 2016* (when we first introduced the Secure Contact
Key)
*Secure Contact Key* (2016)
RSA, 2048-bit. Main key ID: 2048R/8A0AF00B (long version:
2048R/C4FCAAD08A0AF00B). Encryption subkey ID: 2048R/50C2CF5C
(long version: 2048R/9EB39CC150C2CF5C). Fingerprint:
8A26 250E 763F E359 75F3 118F C4FC AAD0 8A0A F00B
*Keys generated in the 2015 rollover*
*Master Key* (2015)
RSA, 4096-bit. Key ID: 4096R/04676F7C (long
version: 4096R/AB585DC604676F7C). Fingerprint:
440D E3B5 B7A1 CA85 B3CC 1718 AB58 5DC6 0467 6F7C
*Release Key* (2015)
RSA, 2048-bit. Key ID: 2048R/B43434E4 (long
version: 2048R/9DFE2648B43434E4). Fingerprint:
0054 DDAA 8ADA 15D2 768A 6DE7 9DFE 2648 B434 34E4
*Snapshot Key* (2015)
RSA, 2048-bit. Key ID: 2048R/D15F7E8A (long
version: 2048R/EEF20295D15F7E8A). Fingerprint:
0A3B 0048 FE49 9B67 A234 FEB6 EEF2 0295 D15F 7E8A
*Original keys generated in 2000* (two sets, RSA and DSA)
*Master Key* (original RSA)
RSA, 1024-bit. Key ID: 1024R/1E34AC41 (long
version: 1024R/9D5877BF1E34AC41). Fingerprint:
8F 15 97 DA 25 30 AB 0D 88 D1 92 54 11 CF 0C 4C
*Master Key* (original DSA)
DSA, 1024-bit. Key ID: 1024D/6A93B34E (long
version: 1024D/4F5E6DF56A93B34E). Fingerprint:
313C 3E76 4B74 C2C5 F2AE 83A8 4F5E 6DF5 6A93 B34E
*Release Key* (original RSA)
RSA, 1024-bit. Key ID: 1024R/B41CAE29 (long
version: 1024R/EF39CCC0B41CAE29). Fingerprint:
AE 65 D3 F7 85 D3 18 E0 3B 0C 9B 02 FF 3A 81 FE
*Release Key* (original DSA)
DSA, 1024-bit. Key ID: 1024D/08B0A90B (long
version: 1024D/FECD6F3F08B0A90B). Fingerprint:
00B1 1009 38E6 9800 6518 F0AB FECD 6F3F 08B0 A90B
*Snapshot Key* (original RSA)
RSA, 1024-bit. Key ID: 1024R/32B903A9 (long
version: 1024R/FAAED21532B903A9). Fingerprint:
86 8B 1F 79 9C F4 7F BD 8B 1B D7 8E C6 4E 4C 03
*Snapshot Key* (original DSA)
DSA, 1024-bit. Key ID: 1024D/7D3E4A00 (long
version: 1024D/165E56F77D3E4A00). Fingerprint:
63DD 8EF8 32F5 D777 9FF0 2947 165E 56F7 7D3E 4A00
Appendix G: SSH-2 names specified for PuTTY
-------------------------------------------
There are various parts of the SSH-2 protocol where things
are specified using a textual name. Names ending in
@putty.projects.tartarus.org are reserved for allocation by the
PuTTY team. Allocated names are documented here.
G.1 Connection protocol channel request names
These names can be sent in a SSH_MSG_CHANNEL_REQUEST message.
[email protected]
This is sent by a client to announce that it will not have more
than one channel open at a time in the current connection (that
one being the one the request is sent on). The intention is that
the server, knowing this, can set the window on that one channel
to something very large, and leave flow control to TCP. There is
no message-specific data.
[email protected]
PuTTY sends this request along with some
SSH_MSG_CHANNEL_WINDOW_ADJUST messages as part of its window-
size tuning. It can be sent on any type of channel. There is no
message-specific data. Servers MUST treat it as an unrecognised
request and respond with SSH_MSG_CHANNEL_FAILURE.
(Some SSH servers get confused by this message, so there is a
bug-compatibility mode for disabling it. See section 4.27.3.)
G.2 Key exchange method names
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
These appeared in various drafts of what eventually became
RFC 4432. They have been superseded by rsa1024-sha1 and rsa2048-
sha256.
G.3 Encryption algorithm names
[email protected]
[email protected]
These were used in drafts of what eventually became RFC 4345.
They have been superseded by arcfour128 and arcfour256.
G.4 Agent extension request names
The SSH agent protocol, which is only specified in an Internet-
Draft at the time of writing (draft-miller-ssh-agent),
defines an extension mechanism. These names can be sent in an
SSH_AGENTC_EXTENSION message.
[email protected]
The payload is a single SSH-2 string containing a keypair in
the PPK format defined in appendix C. Compared to the standard
SSH_AGENTC_ADD_IDENTITY, this extension allows adding keys
in encrypted form, with the agent requesting a decryption
passphrase from the user on demand, and able to revert the key
to encrypted form.
[email protected]
The payload is a single SSH-2 string specifying a public key
blob, as in SSH_AGENTC_REMOVE_IDENTITY. Requests that the agent
forget any cleartext form of a specific key.
Returns SSH_AGENT_SUCCESS if the agent ended up holding the
key only in encrypted form (even if it was already encrypted);
returns SSH_AGENT_EXTENSION_FAILURE if not (if it wasn't held by
the agent at all, or only in cleartext form).
[email protected]
No payload. Requests that the agent forget the cleartext form of
any keys for which it holds an encrypted form.
If the agent holds any keys with an encrypted form (or no keys
at all), returns SSH_AGENT_SUCCESS to indicate that no such keys
are now held in cleartext form, followed by a uint32 specifying
how many keys remain in cleartext form (because the agent didn't
hold an encrypted form for them). If the agent holds nothing but
keys in cleartext form, returns SSH_AGENT_EXTENSION_FAILURE.
[email protected]
No payload. Returns SSH_AGENT_SUCCESS followed by a list of
identities similar to SSH_AGENT_IDENTITIES_ANSWER, except that
each key has an extra SSH-2 string at the end. Currently that
string contains a single uint32 flags word, with the following
bits defined:
Bit 0
If set, key is held with an encrypted form (so that the
`reencrypt' extension can do something useful with it).
Bit 1
If set, key's cleartext form is not currently held (so the
user will have to supply a passphrase before the key can be
used).
Appendix H: PuTTY authentication plugin protocol
------------------------------------------------
This appendix contains the specification for the protocol spoken
over local IPC between PuTTY and an authentication helper plugin.
If you already have an authentication plugin and want to configure
PuTTY to use it, see section 4.22.3 for how to do that. This
appendix is for people writing new authentication plugins.
H.1 Requirements
The following requirements informed the specification of this
protocol.
*Automate keyboard-interactive authentication.* We're motivated in
the first place by the observation that the general SSH userauth
method `keyboard-interactive' (defined in [RFC4256]) can be used
for many kinds of challenge/response or one-time-password styles
of authentication, and in more than one of those, the necessary
responses might be obtained from an auxiliary network connection,
such as an HTTPS transaction. So it's useful if a user doesn't have
to manually copy-type or copy-paste from their web browser into
their SSH client, but instead, the process can be automated.
*Be able to pass prompts on to the user.* On the other hand, some
userauth methods can be only _partially_ automated; some of the
server's prompts might still require human input. Also, the plugin
automating the authentication might need to ask its own questions
that are not provided by the SSH server. (For example, `please
enter the master key that the real response will be generated by
hashing'.) So after the plugin intercepts the server's questions, it
needs to be able to ask its own questions of the user, which may or
may not be the same questions sent by the server.
*Allow automatic generation of the username.* Sometimes, the
authentication method comes with a mechanism for discovering the
username to be used in the SSH login. So the plugin has to start up
early enough that the client hasn't committed to a username yet.
*Future expansion route to other SSH userauth flavours.* The initial
motivation for this protocol is specific to keyboard-interactive.
But other SSH authentication methods exist, and they may also
benefit from automation in future. We're making no attempt here to
predict what those methods might be or how they might be automated,
but we do need to leave a space where they can be slotted in later
if necessary.
*Minimal information loss.* Keyboard-interactive prompts and replies
should be passed to and from the plugin in a form as close as
possible to the way they look on the wire in SSH itself. Therefore,
the protocol resembles SSH in its data formats and marshalling
(instead of, for example, translating from SSH binary packet style
to another well-known format such as JSON, which would introduce
edge cases in character encoding).
*Half-duplex.* Simultaneously trying to read one I/O stream and
write another adds a lot of complexity to software. It becomes
necessary to have an organised event loop containing select or
WaitForMultipleObjects or similar, which can invoke the handler
for whichever event happens soonest. There's no need to add that
complexity in an application like this, which isn't transferring
large amounts of bulk data or multiplexing unrelated activities. So,
to keep life simple for plugin authors, we set the ground rule that
it must always be 100% clear which side is supposed to be sending a
message next. That way, the plugin can be written as sequential code
progressing through the protocol, making simple read and write calls
to receive or send each message.
*Communicate success/failure, to facilitate caching in the plugin.*
A plugin might want to cache recently used data for next time, but
only in the case where authentication using that data was actually
successful. So the client has to tell the plugin what the outcome
was, if it's known. (But this is best-effort only. Obviously the
plugin cannot _depend_ on hearing the answer, because any IPC
protocol at all carries the risk that the other end might crash or
be killed by things outside its control.)
H.2 Transport and configuration
Plugins are executable programs on the client platform.
The SSH client must be manually configured to use a plugin for
a particular connection. The configuration takes the form of a
command line, including the location of the plugin executable,
and optionally command-line arguments that are meaningful to the
particular plugin.
The client invokes the plugin as a subprocess, passing it a pair of
8-bit-clean pipes as its standard input and output. On those pipes,
the client and plugin will communicate via the protocol specified
below.
H.3 Data formats and marshalling
This protocol borrows the low-level data formatting from SSH itself,
in particular the following wire encodings from [RFC4251] section 5:
*byte*
An integer between 0 and 0xFF inclusive, transmitted as a single
byte of binary data.
*boolean*
The values `true' or `false', transmitted as the bytes 1 and 0
respectively.
*uint32*
An integer between 0 and 0xFFFFFFFF inclusive, transmitted as 4
bytes of binary data, in big-endian (`network') byte order.
*string*
A sequence of bytes, preceded by a *uint32* giving the number of
bytes in the sequence. The length field does not include itself.
For example, the empty string is represented by four zero bytes
(the *uint32* encoding of 0); the string "AB" is represented by
the six bytes 0,0,0,2,'A','B'.
Unlike SSH itself, the protocol spoken between the client and the
plugin is unencrypted, because local inter-process pipes are assumed
to be secured by the OS kernel. So the binary packet protocol is
much simpler than SSH proper, and is similar to SFTP and the OpenSSH
agent protocol.
The data sent in each direction of the conversation consists of a
sequence of *messages* exchanged between the SSH client and the
plugin. Each message is encoded as a *string*. The contents of the
string begin with a *byte* giving the message type, which determines
the format of the rest of the message.
H.4 Protocol versioning
This protocol itself is versioned. At connection setup, the client
states the highest version number it knows how to speak, and then
the plugin responds by choosing the version number that will
actually be spoken (which may not be higher than the client's
value).
Including a version number makes it possible to make breaking
changes to the protocol later.
Even version numbers represent released versions of this spec.
Odd numbers represent drafts or development versions in between
releases. A client and plugin negotiating an odd version number
are not guaranteed to interoperate; the developer testing the
combination is responsible for ensuring the two are compatible.
This document describes version 2 of the protocol, the first
released version. (The initial drafts had version 1.)
H.5 Overview and sequence of events
At the very beginning of the user authentication phase of SSH, the
client launches the plugin subprocess, if one is configured. It
immediately sends the PLUGIN_INIT message, telling the plugin some
initial information about where the SSH connection is to.
The plugin responds with PLUGIN_INIT_RESPONSE, which may optionally
tell the SSH client what username to use.
The client begins trying to authenticate with the SSH server in the
usual way, using the username provided by the plugin (if any) or
alternatively one obtained via its normal (non-plugin) policy.
The client follows its normal policy for selecting authentication
methods to attempt. If it chooses a method that this protocol does
not cover, then the client will perform that method in its own way
without consulting the plugin.
However, if the client and server decide to attempt a method that
this protocol _does_ cover, then the client sends PLUGIN_PROTOCOL
specifying the SSH protocol id for the authentication method being
used. The plugin responds with PLUGIN_PROTOCOL_ACCEPT if it's
willing to assist with this auth method, or PLUGIN_PROTOCOL_REJECT
if it isn't.
If the plugin sends PLUGIN_PROTOCOL_REJECT, then the client will
proceed as if the plugin were not present. Later, if another auth
method is negotiated (either because this one failed, or because it
succeeded but the server wants multiple auth methods), the client
may send a further PLUGIN_PROTOCOL and try again.
If the plugin sends PLUGIN_PROTOCOL_ACCEPT, then a protocol segment
begins that is specific to that auth method, terminating in either
PLUGIN_AUTH_SUCCESS or PLUGIN_AUTH_FAILURE. After that, again, the
client may send a further PLUGIN_PROTOCOL.
Currently the only supported method is `keyboard-interactive',
defined in [RFC4256]. Once the client has announced this to the
server, the followup protocol is as follows:
Each time the server sends an SSH_MSG_USERAUTH_INFO_REQUEST message
requesting authentication responses from the user, the SSH client
translates the message into PLUGIN_KI_SERVER_REQUEST and passes it
on to the plugin.
At this point, the plugin may optionally send back
PLUGIN_KI_USER_REQUEST containing prompts to be presented
to the actual user. The client will reply with a matching
PLUGIN_KI_USER_RESPONSE after asking the user to reply to the
question(s) in the request message. The plugin can repeat this cycle
multiple times.
Once the plugin has all the information it needs to
respond to the server's authentication prompts, it sends
PLUGIN_KI_SERVER_RESPONSE back to the client, which translates it
into SSH_MSG_USERAUTH_INFO_RESPONSE to send on to the server.
After that, as described in [RFC4256], the server is free
to accept authentication, reject it, or send another
SSH_MSG_USERAUTH_INFO_REQUEST. Each SSH_MSG_USERAUTH_INFO_REQUEST is
dealt with in the same way as above.
If the server terminates keyboard-interactive authentication
with SSH_MSG_USERAUTH_SUCCESS or SSH_MSG_USERAUTH_FAILURE, the
client informs the plugin by sending either PLUGIN_AUTH_SUCCESS
or PLUGIN_AUTH_FAILURE. PLUGIN_AUTH_SUCCESS is sent when _that
particular authentication method_ was successful, regardless of
whether the SSH server chooses to request further authentication
afterwards: in particular, SSH_MSG_USERAUTH_FAILURE with the
`partial success' flag (see [RFC4252] section 5.1) translates into
PLUGIN_AUTH_SUCCESS.
The plugin's standard input will close when the client no longer
requires the plugin's services, for any reason. This could be
because authentication is complete (with overall success or overall
failure), or because the user has manually aborted the session in
mid-authentication, or because the client crashed.
H.6 Message formats
This section describes the format of every message in the protocol.
As described in section H.3, every message starts with the same two
fields:
- *uint32*: overall length of the message
- *byte*: message type.
The length field does not include itself, but does include the type
code.
The following subsections each give the format of the remainder of
the message, after the type code.
The type codes themselves are defined here:
#define PLUGIN_INIT 1
#define PLUGIN_INIT_RESPONSE 2
#define PLUGIN_PROTOCOL 3
#define PLUGIN_PROTOCOL_ACCEPT 4
#define PLUGIN_PROTOCOL_REJECT 5
#define PLUGIN_AUTH_SUCCESS 6
#define PLUGIN_AUTH_FAILURE 7
#define PLUGIN_INIT_FAILURE 8
#define PLUGIN_KI_SERVER_REQUEST 20
#define PLUGIN_KI_SERVER_RESPONSE 21
#define PLUGIN_KI_USER_REQUEST 22
#define PLUGIN_KI_USER_RESPONSE 23
If this protocol is extended to be able to assist with further
auth methods, their message type codes will also begin from 20,
overlapping the codes for keyboard-interactive.
H.6.1 PLUGIN_INIT
*Direction*: client to plugin
*When*: the first message sent at connection startup
*What happens next*: the plugin will send PLUGIN_INIT_RESPONSE or
PLUGIN_INIT_FAILURE
*Message contents after the type code*:
- *uint32*: the highest version number of this protocol that the
client knows how to speak.
- *string*: the hostname of the server. This will be the
_logical_ hostname, in cases where it differs from the physical
destination of the network connection. Whatever name would be
used by the SSH client to cache the server's host key, that's
the same name passed in this message.
- *uint32*: the port number on the server. (Together with the host
name, this forms a primary key identifying a particular server.
Port numbers may be vital because a single host can run two
unrelated SSH servers with completely different authentication
requirements, e.g. system sshd on port 22 and Gerrit on port
29418.)
- *string*: the username that the client will use to log in, if
the plugin chooses not to override it. An empty string means
that the client has no opinion about this (and might, for
example, prompt the user).
H.6.2 PLUGIN_INIT_RESPONSE
*Direction*: plugin to client
*When*: response to PLUGIN_INIT
*What happens next*: the client will send PLUGIN_PROTOCOL, or
perhaps terminate the session (if no auth method is ever negotiated
that the plugin can help with)
*Message contents after the type code*:
- *uint32*: the version number of this protocol that the
connection will use. Must be no greater than the max version
number sent by the client in PLUGIN_INIT.
- *string*: the username that the plugin suggests the client use.
An empty string means that the plugin has no opinion and the
client should stick with the username it already had (or prompt
the user, if it had none).
H.6.3 PLUGIN_INIT_FAILURE
*Direction*: plugin to client
*When*: response to PLUGIN_INIT
*What happens next*: the session is over
*Message contents after the type code*:
- *string*: an error message to present to the user indicating why
the plugin was unable to start up.
H.6.4 PLUGIN_PROTOCOL
*Direction*: client to plugin
*When*: sent after PLUGIN_INIT_RESPONSE, or after a previous auth
phase terminates with PLUGIN_AUTH_SUCCESS or PLUGIN_AUTH_FAILURE
*What happens next*: the plugin will send PLUGIN_PROTOCOL_ACCEPT or
PLUGIN_PROTOCOL_REJECT
*Message contents after the type code*:
- *string*: the SSH protocol id of the auth method the client
intends to attempt. Currently the only method specified for use
in this protocol is `keyboard-interactive'.
H.6.5 PLUGIN_PROTOCOL_REJECT
*Direction*: plugin to client
*When*: sent after PLUGIN_PROTOCOL
*What happens next*: the client will either send another
PLUGIN_PROTOCOL or terminate the session
*Message contents after the type code*:
- *string*: an error message to present to the user, explaining
why the plugin cannot help with this authentication protocol.
An example might be `unable to open <config file>: <OS error
message>', if the plugin depends on some configuration that the
user has not set up.
If the plugin does not support this this particular
authentication protocol at all, this string should be left
blank, so that no message will be presented to the user at all.
H.6.6 PLUGIN_PROTOCOL_ACCEPT
*Direction*: plugin to client
*When*: sent after PLUGIN_PROTOCOL
*What happens next*: depends on the auth protocol agreed on. For
keyboard-interactive, the client will send PLUGIN_KI_SERVER_REQUEST
or PLUGIN_AUTH_SUCCESS or PLUGIN_AUTH_FAILURE. No other method is
specified.
*Message contents after the type code*: none.
H.6.7 PLUGIN_KI_SERVER_REQUEST
*Direction*: client to plugin
*When*: sent after PLUGIN_PROTOCOL, or after a previous
PLUGIN_KI_SERVER_RESPONSE, when the SSH server has sent
SSH_MSG_USERAUTH_INFO_REQUEST
*What happens next*: the plugin will send either
PLUGIN_KI_USER_REQUEST or PLUGIN_KI_SERVER_RESPONSE
*Message contents after the type code*: the exact contents of the
SSH_MSG_USERAUTH_INFO_REQUEST just sent by the server. See [RFC4256]
section 3.2 for details. The summary:
- *string*: name of this prompt collection (e.g. to use as a
dialog-box title)
- *string*: instructions to be displayed before this prompt
collection
- *string*: language tag (deprecated)
- *uint32*: number of prompts in this collection
- That many copies of:
- *string*: prompt (in UTF-8)
- *boolean*: whether the response to this prompt is safe to
echo to the screen
H.6.8 PLUGIN_KI_SERVER_RESPONSE
*Direction*: plugin to client
*When*: response to PLUGIN_KI_SERVER_REQUEST, perhaps after
one or more intervening pairs of PLUGIN_KI_USER_REQUEST and
PLUGIN_KI_USER_RESPONSE
*What happens next*: the client will send a further
PLUGIN_KI_SERVER_REQUEST, or PLUGIN_AUTH_SUCCESS or
PLUGIN_AUTH_FAILURE
*Message contents after the type code*: the exact contents of the
SSH_MSG_USERAUTH_INFO_RESPONSE that the client should send back to
the server. See [RFC4256] section 3.4 for details. The summary:
- *uint32*: number of responses (must match the `number of
prompts' field from the corresponding server request)
- That many copies of:
- *string*: response to the _n_th prompt (in UTF-8)
H.6.9 PLUGIN_KI_USER_REQUEST
*Direction*: plugin to client
*When*: response to PLUGIN_KI_SERVER_REQUEST, if the plugin cannot
answer the server's auth prompts without presenting prompts of its
own to the user
*What happens next*: the client will send PLUGIN_KI_USER_RESPONSE
*Message contents after the type code*: exactly the same as in
PLUGIN_KI_SERVER_REQUEST (see section H.6.7).
H.6.10 PLUGIN_KI_USER_RESPONSE
*Direction*: client to plugin
*When*: response to PLUGIN_KI_USER_REQUEST
*What happens next*: the plugin will send PLUGIN_KI_SERVER_RESPONSE,
or another PLUGIN_KI_USER_REQUEST
*Message contents after the type code*: exactly the same as in
PLUGIN_KI_SERVER_RESPONSE (see section H.6.8).
H.6.11 PLUGIN_AUTH_SUCCESS
*Direction*: client to plugin
*When*: sent after PLUGIN_KI_SERVER_RESPONSE, or (in unusual cases)
after PLUGIN_PROTOCOL_ACCEPT
*What happens next*: the client will either send another
PLUGIN_PROTOCOL or terminate the session
*Message contents after the type code*: none
H.6.12 PLUGIN_AUTH_FAILURE
*Direction*: client to plugin
*When*: sent after PLUGIN_KI_SERVER_RESPONSE, or (in unusual cases)
after PLUGIN_PROTOCOL_ACCEPT
*What happens next*: the client will either send another
PLUGIN_PROTOCOL or terminate the session
*Message contents after the type code*: none
H.7 References
[RFC4251] RFC 4251, `The Secure Shell (SSH) Protocol Architecture'.
[RFC4252] RFC 4252, `The Secure Shell (SSH) Authentication
Protocol'.
[RFC4256] RFC 4256, `Generic Message Exchange Authentication for the
Secure Shell Protocol (SSH)' (better known by its wire id `keyboard-
interactive').
Appendix I: PuTTY privacy considerations
----------------------------------------
This appendix lists the implications of using PuTTY for your privacy
and personal data.
The short summary: PuTTY never `phones home' to us, the developers.
It does store data on your own computer, and it does transmit data
over the network, but in both cases, only as necessary to do its
job. In particular, data is only transmitted over the network to the
server you told PuTTY to connect to.
But if you're concerned about exactly _what_ information is stored
or transmitted, then here's a more detailed description.
I.1 Information that PuTTY stores locally
When you use PuTTY, it stores a small amount of information on
your computer, necessary for doing its own job. This information
is stored in the user account of the user who runs PuTTY, so it is
under your control: you can view it, change it, or delete it.
If you need to delete all of this data, you can use the `-cleanup'
command-line option, as described in section 3.11.2.
PuTTY does not transmit your saved session data to any other site.
However, you may need to be aware of the fact that it is stored on
_your_ computer. (For example, somebody else accessing your computer
might be able to find a list of sites you have connected to, if you
have saved details of them.)
I.1.1 Host key cache
If you use the SSH protocol, then PuTTY stores a list of the SSH
servers you have connected to, together with their host keys.
This is known as the `host key cache'. It is used to detect network
attacks, by notifying you if a server you've connected to before
doesn't look like the same one you thought it was. (See section 2.2
for a basic introduction to host keys.)
The host key cache is optional. An entry is only saved in the host
key cache if you select the `Accept' action at one of the PuTTY
suite's host key verification prompts. So if you want to make an SSH
connection without PuTTY saving any trace of where you connected to,
you can press `Connect Once' instead of `Accept', which does not
store the host key in the cache.
However, if you do this, PuTTY can't automatically detect the host
key changing in the future, so you should check the key fingerprint
yourself every time you connect. *This is vitally important.* If you
don't let PuTTY cache host keys _and_ don't check them yourself,
then it becomes easy for an attacker to interpose a listener between
you and the server you're connecting to. The entire cryptographic
system of SSH depends on making sure the host key is right.
The host key cache is only used by SSH. No other protocol supported
by PuTTY has any analogue of it.
I.1.2 Saved sessions
After you set up PuTTY's configuration for a particular network
connection, you can choose to save it as a `saved session', so that
you can make the same connection again later without having to re-
enter all the details.
PuTTY will not do this unless you use the `Save' button in
its configuration box. It never saves session configuration
automatically.
So if you want to make an SSH connection without leaving any trace
of where you connected to, you should not make a saved session for
that connection. Instead, re-enter the details by hand every time
you do it.
I.1.3 Jump list
On Windows, the operating system provides a feature called a `jump
list'. This is a menu that pops up from an application's icon in
the Windows taskbar, and the application can configure entries that
appear in it. Applications typically include menu items to re-launch
recently used documents or configurations.
PuTTY updates its jump list whenever a saved session is loaded,
either to launch it immediately or to load it within the
configuration dialog box. So if you have a collection of saved
sessions, the jump list will contain a record of which ones you have
recently used.
An exception is that saved sessions are not included in the jump
list if they are not `launchable', meaning that they actually
specify a host name or serial port to connect to. A non-launchable
session can specify all the other configuration details (such as
fonts, window size, keyboard setup, SSH features, etc), but leave
out the hostname.
If you want to avoid leaving any evidence of having made a
particular connection, then make the connection without creating
a launchable saved session for it: either make no saved session
at all, or create a non-launchable one which sets up every detail
_except_ the destination host name. Then it won't appear in the jump
list.
(The saved session itself would also be evidence, of course, as
discussed in the previous section.)
I.1.4 Log files
PuTTY can be configured to save a log file of your entire session
to the computer you run it on. By default it does not do so: the
content of your session is not saved.
See section 4.2 for details of the logging features. Some logging
modes store only output sent by the server and printed in PuTTY's
terminal window. Other more thorough modes also store your input
that PuTTY sends _to_ the server.
If the logging feature is enabled, then by default, PuTTY will avoid
saving data in the log file that it knows to be sensitive, such as
passwords. However, it cannot reliably identify _all_ passwords. If
you use a password for your initial login to an SSH server, PuTTY
knows that is a password, and will omit it from the log file. But if
after login you type a password into an application on the server,
then PuTTY will not know that _that_ is a password, so it will
appear in the log file, if PuTTY is writing a type that includes
keyboard input.
PuTTY can also be configured to include all passwords in its log
files, even the ones it would normally leave out. This is intended
for debugging purposes, for example if a server is refusing your
password and you need to check whether the password is being sent
correctly. We do not recommend enabling this option routinely.
I.1.5 Random seed file
PuTTY stores a small file of random bytes under the name
`putty.rnd', which is reloaded the next time it is run and used to
seed its random number generator. These bytes are meaningless and
random, and do not contain an encrypted version of anything.
I.2 Sending information over the network
PuTTY is a communications tool. Its _purpose_ is to connect to
another computer, over a network or a serial port, and send
information. However it only makes the network connections that its
configuration instructs it to.
I.2.1 PuTTY only connects to the specified destination host
No PuTTY tool will `phone home' to any site under the control of
us (the development team), or to any other site apart from the
destination host or proxy host in its configuration, and any DNS
server that is needed to look up the IP addresses corresponding to
those host names.
No information about your network sessions, and no information from
the computer you run PuTTY on, is collected or recorded by the PuTTY
developers.
Information you provide to PuTTY (via keyboard input, the command
line, or files loaded by the file transfer tools) is sent to the
server that PuTTY's configuration tells it to connect to. It is not
sent anywhere else.
I.2.2 What data is sent to the destination host
When you log in to a server, PuTTY will send your username. If you
use a password to authenticate to the server, PuTTY will send it
that password as well.
(Therefore, the server is told what your password is during login.
This means that if you use the same password on two servers, the
administrator of one could find out your password and log in to your
account on the other.)
If you use an SSH private key to authenticate, PuTTY will send the
_public_ key, but not the private key. If you typed a passphrase to
decrypt the private key, PuTTY will not send the passphrase either.
(Therefore, it is safer to use the same _public key_ to authenticate
to two SSH servers. Neither server gains the ability to impersonate
you to the other server. However, if the server maintainers talked
to each other, they would at least be able to find out that your
accounts on the two machines were owned by the same person, if they
didn't already know.)
When PuTTY prompts for a private key passphrase, a small copy of the
PuTTY icon appears to the left of the prompt, to indicate that the
prompt was genuinely from PuTTY. (We call this a `trust sigil'.)
That icon never appears next to text sent from the server. So if a
server tries to mimic that prompt to trick you into telling it your
private key passphrase, it won't be able to fake that trust sigil,
and you can tell the difference.
If you're running Pageant, and you haven't configured a specific
public key to authenticate to this server, then PuTTY will try
all the keys in Pageant one after the other, sending each public
key to the server to see if it's acceptable. This can lead to the
server finding out about other public keys you own. However, if you
configure PuTTY to use a specific public key, then it will ignore
all the other keys in Pageant.
Once you have logged in, keystrokes you type in the PuTTY terminal
window, and data you paste in with the mouse, are sent to the
destination host. That is PuTTY's primary job.
The server can request PuTTY to send details of mouse movements in
the terminal window, in order to implement mouse-controlled user
interfaces on the server. If you consider this to be a privacy
intrusion, you can turn off that terminal feature in the Features
configuration panel (`Disable xterm-style mouse reporting', as
described in section 4.6.2).
I.3 Configuration
The operation of a PuTTY network tool is controlled by its
configuration. This configuration is obtained from:
- the command line used to run the tool
- settings configured in the GUI before opening a network session
- optionally, the contents of a saved session, if the command line
or a GUI action instructed PuTTY to load one
- the special saved session called `Default Settings', which
applies if no other saved session is loaded
- defaults built in to PuTTY itself.
The defaults built in to PuTTY do not tell it to save log files, or
specify the name of any network site to connect to.
However, if PuTTY has been installed for you by somebody else,
such as an organisation, then that organisation may have provided
their own default configuration. In that situation you may wish
to check that the defaults they have set are compatible with your
privacy needs. For example, an organisation providing your PuTTY
configuration might configure PuTTY to save log files of your
sessions, even though PuTTY's own default is not to do so.
I.4 Modified versions of PuTTY
PuTTY is free software. Its source code is available, so anyone
can make a modified version of it. The modified version can behave
differently from the original in any way it likes.
This list of privacy considerations only applies to the original
version of PuTTY, as distributed by its development team. We
cannot make any promises about the behaviour of modified versions
distributed by other people.
[PuTTY release 0.83]
