Firewall Piercing mini-HOWTO
Fran�ois-Ren� Rideau,
[email protected]
v0.7, 4 November 2000
Directions for using ppp over ssh or telnet so as to do network stuff
transparently through an Internet firewall. Also applies to VPN con�
struction.
______________________________________________________________________
Table of Contents
1. Stuff
1.1 DISCLAIMER
1.2 Legal Blurp
1.3 Looking for a maintainer
1.4 Credits
2. Introduction
2.1 Foreword
2.2 Security issues
2.3 Other requirements
2.4 Downloading software
3. Understanding the problem
3.1 Giving names to things
3.2 The main problem
3.3 The secondary problem
4. Secure solution: piercing using ssh
4.1 Principle
4.2 A sample session
5. Unsecure solution: piercing using telnet
5.1 Principle
5.2 fwprc
5.3 .fwprcrc
6. Reverse piercing
6.1 Rationale
6.2 Getting the triggering mail
7. Final notes
7.1 Other settings
7.2 HOWTO maintenance
7.3 Related Documents
7.4 Extra copy of IMPORTANT DISCLAIMER --- BELIEVE IT!!!
______________________________________________________________________
1. Stuff
1.1. DISCLAIMER
READ THIS IMPORTANT SECTION !!!
I hereby disclaim all responsibility for your use of this hack. If it
backfires on you in any way whatsoever, that's the breaks. Not my
fault. If you don't understand the risks inherent in doing this,
don't do it. If you use this hack and it allows vicious vandals to
break into your company's computers and costs you your job and your
company millions of dollars, well that's just tough nuggies. Don't
come crying to me.
1.2. Legal Blurp
Copyright � 1998-2000 by Fran�ois-Ren� Rideau.
This document is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at
your option) any later version.
1.3. Looking for a maintainer
I haven't been actively developing this mini-HOWTO recently; in
particular, the french version is lagging behind. I'm looking for a
maintainer to take over this document, and maybe develop software to
make it easier to pierce firewalls. I also have a lot of ideas for
active maintainers to expand this HOWTO, if anyone is interested.
1.4. Credits
Even though the only thing left is the disclaimers, this document owes
a lot to the Term-Firewall mini-HOWTO by Barak Pearlmutter
<mailto:
[email protected]>. Barak's mini-HOWTO relies on an ancient and
no-more-supported program named Term (a great program in its time, and
maybe still useful in some unhappy circumstances), as well as on
peculiarities of a not-so-standard telnet implementation, that is,
many obsolete and non-portable facts. Nevertheless, there was a
necessity for a mini-HOWTO about piercing firewalls, and despite its
shortcomings, his mini-HOWTO was a model and an encouragement.
I'd also like to congratulate Lars Brinkhoff <mailto:
[email protected]>
and Magnus Lundstr�m <mailto:
[email protected]> for their fine
http, mail and icmp tunnels.
2. Introduction
2.1. Foreword
This is document with a moral. And the moral is: a firewall cannot
protect a network against its own internal users, and should not even
try to.
When an internal user asks you system administrator to open an
outbound port to an external machine, or an inbound port to an
internal machine, then you should do it for him. Of course you should
help the user to make sure that his transactions are secure, and that
his software is robust. But a flat out denial of service is plain
incompetence. For unless he is so firewalled as be completely cut
from the outside world, with no ssh, no telnet, no web browsing, no
email, no ping, no phone line, no radio, no nothing, then the user can
and will use firewall piercing techniques to access the machines he
wants nonetheless, and the net result for security will be an
unaudited connection with the outside world. So either you trust your
users, after proper training and selection, or you shouldn't grant
them access to the network at all. You can and you shall protect them
from the outside world, but you can't protect them from themselves.
Because there exists such things as system administrators who are
either unresponsive, absent, plain incompetent, or more generally
managed by incompetent people, it so happens that a user may find
himself behind a firewall that he may cross, but only in awkward ways.
This mini-HOWTO explains a generic and portable way to pierce tunnels
into firewalls, by turning any tiny small crack into a full-fledged
information superhighway, so the user can seamlessly use standard
tools to access computers on the other side of the firewall. The very
same technique can be used by competent system administrators to build
virtual private networks (VPN).
2.2. Security issues
Of course, if your sysadm has setup a firewall s/he might have a good
reason, and you may have signed an agreement to not circumvent it. On
the other hand, the fact that you can use telnet, the web, e-mail, or
whatever other bidirectional information flux with the outside of the
firewall (which is a prerequisite for the presented hacks to work)
means that you are allowed to access external systems, and the fact
that you can log into a particular external system somehow means
you're allowed to do it, too.
So this is all a matter of conveniently using legal holes in a
firewall, and allow generic programs to work from there with generic
protocols, as opposed to requiring special or modified (and
recompiled) programs going through lots of special-purpose proxies
that be misconfigured by an uncaring or incompetent sysadm, or to
installing lots of special-purpose converters to access each of your
usual services (like e-mail) through ways supported by the firewall
(like the web).
Moreover, the use of a user-level IP emulator such as SLiRP should
still prevent external attackers from piercing the firewall back in
the other way, unless explicitly permitted by you (or they are clever
and wicked, and root or otherwise able to spy you on the remote host).
All in all, the presented hack should be relatively safe. However, it
all depends on the particular circumstances in which you set things
up, and I can give no guarantee about this hack. Lots of things are
intrinsically unsafe about any Internet connection, be it with this
hack or not, so don't you assume anything is safe unless you have good
reasons, and/or use some kind of encryption all the way.
Let's repeat the basics of networking security: you cannot trust
anything about a connection more than you trust the hosts that can
handle the unencrypted data, including hosts on both ends of the
connection, and all hosts that can intercept the communication, unless
the communication is properly encrypted with secret keys. If you
misplace your trust, your passwords may be stolen and used against
you, your credit card number may be stolen and used against you, and
you may be fired from your work for endangering the whole company.
Tough nuggies.
To sum it up, don't use this hack unless you know what you're doing.
Re-read the disclaimer above.
2.3. Other requirements
It is assumed that you know what you're doing, that you know about
setting up a network connection, that in case of doubt, you will have
read all relevant documentation (HOWTOs, manual pages, web pages,
mailing-list archives, RFCs, courses, tutorials).
It is assumed that you have shell accounts on both sides of the
firewall, that you can somehow transmit packets of information both
ways across the firewall (with telnet, ssh, e-mail, and the web being
the ways currently known to work), and that you can let a daemon run
as a background task on the remote site (or benefit from and existing
daemon, sshd, telnetd, or sendmail/procmail).
It is assumed that you'll know how to configure an IP emulator (pppd,
slirp) or an Internet access daemon and its associated library (SOCKS,
Term) on each side, according to your needs in terms of connectivity
and to your access rights, with your recompiling some software if
needed.
2.4. Downloading software
Most software named in this HOWTO should be available from your
standard Linux distribution, possibly among contrib's. At least, the
four first below are available in as .rpm and .deb packages. In case
you want to fetch the latest sources (after all, one of the ends of
the connection may not be running under Linux), use the addresses
below:
� SLiRP can be found at <
http://blitzen.canberra.edu.au/slirp> and/or
<
ftp://www.ibc.wustl.edu/pub/slirp_bin/>.
� zsh can be found at <
http://www.zsh.org/>.
� ppp can be found at <
ftp://cs.anu.edu.au/pub/software/ppp/>.
� ssh can be found at <
http://www.openssh.com/>.
� fwprc and cotty can be found at
<
http://fare.tunes.org/files/fwprc/>.
� httptunnel can be found at
<
http://www.nocrew.org/software/httptunnel/>.
� mailtunnel can be found at <
http://www.detached.net/mailtunnel/>.
� icmptunnel can be found at <
http://www.detached.net/icmptunnel/>.
3. Understanding the problem
Understanding a problem is the first half of the path to solving it.
3.1. Giving names to things
If you want this hack to work for you, you'll have to get an idea of
how it works, so that in case anything breaks, you know where to look
for.
The first step toward understanding the problem is to give a name to
relevant concepts.
As usual, we'll herein call "local" the client machine that decides to
initiate the connection, as well as programs and files on that
machine; conversely, we'll call "remote" what's on the other side of
the connection, where a server runs that waits for connections.
3.2. The main problem
The main problem with firewall piercing is to create a tunnel: a
continuous connection from the local machine to a remote machine on
the other side of the firewall, that allows for bidirectional exchange
of information. Optionally, this connection should be a secure one.
The secondary problem is to transform this connection into a full IP
access for normal programs to use transparently.
For the main problem, we'll assume that either (1) you can establish
normal TCP/IP connections from the local side of the firewall to some
port on a remote machine where a sshd runs or can be set to run, or
(2) you can somehow establish a telnet connection through a telnet
proxy. In case you cannot, we give you pointers to other software
that allows you to pierce a tunnel accross a firewall. Although we
only give a secure solution in the first case, you can hack your own
secure solution in the other cases, if you understand the principle
(if you don't, someone, e.g. I, can do it for you in exchange for
money).
3.3. The secondary problem
For the secondary problem, IP emulators (pppd or SLiRP) are run on
each side of the tunnel.
On the side that wants full IP access to the other side, you'll want
to run pppd. On the other side, you want to run pppd if you also want
full IP access to the first side, or SLiRP if you want to prevent any
access. Go to your usual pppd or SLiRP documentation for more
information, if you have specific needs not covered by the examples
given below.
Although this is conceptually trivial, it nonetheless requires a few
silly tricks, so as to work, since (a) in case you're using some kind
of programmed interactive shell session to start the remote IP
emulator on either side, you need to correctly synchronize the start
of the IP emulator on the other side, so as not to send garbage into
the shell session, and (b) IP emulators are designed to be run on a
"tty" interface so you have to convert your tunnel's interface into a
tty one.
Issue (a) is just your usual synchronization problem, and doesn't even
exist if you use ssh, that transparently handles remote command
launching.
Issue (b) requires the use of a simple external utility. We wrote
one, cotty just for that purpose.
<FLAME ON>
Among the silly problems caused by pppd maintainers' shortmindedness,
you can only run it through either a device in /dev or the current
tty. You cannot run it through a pair of pipe (which would be the
obvious design). This is fine for the remote pppd if any, as it can
use the telnet or ssh session's tty; but for the local pppd, this
conflicts with the possible use of telnet as a way to establish a
connection.
Indeed, telnet, too wants to be on a tty; it behaves almost correctly
with a pair of pipe, except that it will still insist on doing ioctl's
to the current tty, with which it will interfere; using telnet without
a tty also causes race conditions, so that the whole connection will
fail on "slow" computers (fwprc 0.1 worked perfectly on a P/MMX 233,
one time out of 6 on a 6x86-P200+, and never on a 486dx2/66). All in
all, when using telnet, you need cotty to run as a daemon to copy
output from one tty on which runs pppd into another tty on which runs
telnet, and conversely.
If I find the sucker (probably a MULTICS guy, though there must have
been UNIX people stupid enough to copy the idea) who invented the
principle of "tty" devices by which you read and write from a "same"
pseudo-file, instead of having clean pairs of pipes, I strangle him!
</FLAME>
4. Secure solution: piercing using ssh
4.1. Principle
Let's assume that your site administrator allows transparent TCP
connections to some port on some remote machine, (be it the standard
SSH port 22, or an alternate destination port, like the HTTP port 80
or whatever), or that you somehow managed to get some port in one side
of the firewall to get redirected to a port on the other side (using
httptunnel, mailtunnel, icmptunnel, some tunnel over telnet, or
whatelse).
Then, you can run an sshd on the remote port, and connect to it with
an ssh on the local port. On both sides of the ssh connection, you
run IP emulators (pppd), and there you have your VPN, Virtual Public
Network, that circumvents the stupid firewall limitations, with the
added bonus of being encrypted for privacy (beware: the firewall
administrator still knows the other end of the tunnel, and whatever
authentication information you might have sent before to run ssh).
The exact same technology can be used to build a VPN, Virtual Private
Network, whereby you securely join physical sites into a one logical
network without sacrificing security with respect to the transport
network between the sites.
4.2. A sample session
Below is a sample session to integrate in a shell script (it assumes
sh/bash syntax; YMMV).
Be sure to edit this into a script with the right values for your
needs. Use option -p for ssh to try another port than port 22 (but
then, be sure to run sshd on same port). You can use slirp on the
remote end, if you are not root there, or simply want to screen your
local network from outbound connections.
Automatic reconnection is left as an exercise to the reader.
[email protected]
REMOTE_PPPD="pppd ipcp-accept-local ipcp-accept-remote"
LOCAL_PPPD="pppd silent 192.168.0.1:192.168.0.2"
cotty -d -- $LOCAL_PPPD -- ssh -t $REMOTE_ACCOUNT $REMOTE_PPPD
(Note: this command requires cotty 0.4 or later.)
5. Unsecure solution: piercing using telnet
5.1. Principle
If all you can do is telnet (because of a telnet proxy), then this
solution might be fit for you.
The firewall-piercing program, fwprc, will use a "tty proxy", cotty,
that opens two pseudo-tty devices, launches some command on each of
those devices' slaves, and stubbornly copies every character that one
outputs to the tty that serves as input of the other command. One
command will be telnet connection to remote site, and the other will
be the local pppd. pppd can then open and control the telnet session
with a chat script as usual.
Actually, if your telnet proxy allows connection to an arbitrary port,
and if you can reliably run a daemon on the remote host (with a cron
job to relaunch it in case of breakage), then you'd better write some
program that will just connect a local port to the remote one through
the proxy, so you can use the above secure solution, possibly using
some variant of ssh -t -o "ProxyCommand ..." (if you submit it to me,
I'll gladly integrate such a solution to the fwprc distribution).
Note: if you must use the unsecure telnet-based solution, be sure that
nothing lies in your target account that you want to keep secret or
untampered, since the password will be sent in clear text accross the
Internet.
5.2. fwprc
I wrote a very well self-documented script to pierce firewalls, fwprc,
available from my site <
http://fare.tunes.org/files/fwprc/>, together
with cotty (which is required by fwprc 0.2 and later). At the time of
my writing these lines, latest versions are fwprc 0.3e and cotty 0.4.
The name "fwprc" is voluntarily made unreadable and unpronounceable,
so that it will confuse the incompetent paranoid sysadm who might be
the cause of the firewall that annoys you (of course, there can be
legitimate firewalls, too, and even indispensable ones; security is
all a matter of correct configuration). If you must read it aloud,
choose the worst way you can imagine.
CONTEST! CONTEST! Send me a .au audio file with a digital audio
recording of how you pronounce "fwprc". The worst entry will win a
free upgrade and his name on the fwprc 1.0 page!
I tested the program in several settings, by configuring it through
resource files. But of course, by Murphy's law, it will break for
you. Feel free to contribute enhancements that will make life easier
to other people who'll configure it after you.
5.3. .fwprcrc
fwprc can be customized through a file .fwprcrc meant to be the same
on both sides of the firewall. Having several alternate
configurations to choose from is sure possible (for instance, I do
it), and is left as an exercise to the reader.
To begin with, copy the appropriate section of fwprc (the previous to
last) into a file named .fwprcrc in your home directory. Then replace
variable values with stuff that fits your configuration. Finally,
copy to the other host, and test.
Default behavior is to use pppd locally, and slirp remotely. To
modify that, you can redefine the appropriate function in your
.fwprcrc with such a line as:
remote_IP_emu () { remote_pppd }
Note that SLiRP is safer than pppd, and easier to have access to,
since it does not require being root on the remote machine, and
needn't additional firewall configuration to prevent connections from
the outside world into the firewalled network. The basic
functionality in SLiRP works quite well, but I haven't managed to get
some advertised pluses to work (like run-time controllability). Of
course, since it is free software, feel free to hack the source so as
to actually implement or fix whichever feature you need.
6. Reverse piercing
6.1. Rationale
Sometimes, only one side of the firewall can launch telnet sessions
into the other side; however, some means of communication is possible
(typically, through e-mail). Piercing the firewall is still possible,
by triggering with whatever messaging capability is available a telnet
connection from the ``right'' side of the firewall to the other.
fwprc includes code to trigger such connections from a PGP-
authentified e-mail message; all you need is add fwprc as a procmail
filter to messages using the protocol, (instructions included in fwprc
itself). Note however, that if you are to launch pppd with
appropriate privileges, you might need create your own suid wrapper to
become root. Instructions enclosed in fwprc.
Also, authentified trigger does not remotely mean secure connection.
You should really use ssh (perhaps over telnet) for secure
connections. And then, beware of what happens between the triggering
of a telnet connection, and ssh taking over that connection.
Contribution in that direction welcome.
6.2. Getting the triggering mail
If you are firewalled, your mail may as well be in a central server
that doesn't do procmail filtering or allow telnet sessions. No
problem! You can use fetchmail to run in daemon mode to poll and get
mail to your client linux system, and/or add a cron job to
automatically poll for mail every 1-5 minutes. fetchmail will forward
mail to a local address through sendmail, which itself will have been
configured to use procmail for delivery. Note that if you run
fetchmail as a background daemon, it will lock away any other
fetchmail that you'd like to run only at other times, like when you
open a fwprc; of course, if you can also run a fetchmail daemon as a
fake user. Too frequent a poll won't be nice to either the server or
your host. Too infrequent a poll means you'll have to wait before the
message gets read and the reverse connection gets established. I use
two-minute poll frequency.
7. Final notes
7.1. Other settings
There are other kinds of firewalls than those that allow for direct
ssh or telnet connections. As long as a continuous flow of packets
may transmit information through a firewall in both directions, it is
possible to pierce it; only the price of writing the piercer may be
higher or lower.
In a very easy case, we saw that you can just launch ssh over a pty
master and do some pppd in the slave tty. You may even want to do it
without an adverse firewall, just so as to build a secure ``VPN''
(Virtual Private Network). The VPN mini-HOWTO gives all the details
you need about this. We invite you, as an exercise, to modify fwprc
so as to use this technique, or perhaps even so as to use it inside a
previous non-secure fwprc session.
Now, if the only way through the firewall is a WWW proxy (usually, a
minimum for an Internet-connected network), you might want to use Lars
Brinkoff <
http://lars.nocrew.org/>'s httptunnel
<
http://www.nocrew.org/software/httptunnel/>, a http daemon and client
combination that achieves a TCP/IP tunnel connection through the
proxy-friendly HTTP protocol. You should then be able to run fwprc
(preferably over ssh) over that connection, although I haven't tried
it yet. Could anyone test and report? Note that httptunnel is still
under development, so you may help implement the features it currently
lacks, like, having multiple connections, and/or serving fake pages so
as to mislead suspicious adverse firewall administrators.
Whatever goes through your firewall, be it telnet, HTTP or other
TCP/IP connections, or something real weird like DNS queries, ICMP
packets, e-mail (see mailtunnel <
http://www.detached.net/mailtunnel/>,
icmptunnel <
http://www.detached.net/icmptunnel/>), or whatelse, you
can always write a tunnel client/daemon combination, and run a ssh
and/or PPP connection through it. The performance mightn't be high,
depending on the effective information communication rate after paying
the overhead for coding around filters and proxies; but such a tunnel
is still interesting as long as it's good enough to use fetchmail,
suck, and other non-interactive programs.
If you need cross a 7-bit line, you'll want to use SLIP instead of
PPP. I never tried, because lines are more or less 8-bit clean these
days, but it shouldn't be difficult. If necessary, fall back to using
the Term-Firewall mini-HOWTO.
If you have an 8-bit clean connection and you're root on linux both
sides of the firewall, you might want to use ethertap for better
performance, encapsulating raw ethernet communications on top of your
connection. David Madore has written ethertap-over-TCP and ethertap-
over-UDP tunneling <
ftp://quatramaran.ens.fr/pub/madore/misc/>. There
remains to write some ethertap-over-tty to combine with fwprc-like
tools.
If you really need more performance than you can get while paying for
a user-space sequential communication tunnel through which to run PPP,
then you're in the very hard case where you might have to re-hack a
weird IP stack, using (for instance) the Fox project's packet-protocol
functors. You'll then achieve some direct IP-over-HTTP, IP-over-DNS,
IP-over-ICMP, or such, which requires not only an elaborate protocol,
but also an interface to an OS kernel, both of which are costly to
implement.
7.2. HOWTO maintenance
I felt it was necessary to write it, but I don't have that much time
for that, so this mini-HOWTO is very rough. Thus will it stay, until
I get enough feedback so as to know what sections to enhance, or
better, until someone comes and takes over maintenance for the mini-
HOWTO. Feedback welcome. Help welcome. mini-HOWTO maintenance take-
over welcome.
In any case, the above sections have shown many problems whose
solution is just a matter of someone (you?) spending some time (or
money, by hiring someone else) to sit down and write it: nothing
conceptually complicated, though the details might be burdensome or
tricky.
Do not hesitate to contribute more problems, and hopefully more
solutions, to this mini-HOWTO.
For instance, there is some need for a section on setting up routes
correctly with fwprc, including example use of getroute.pl from
/etc/ppp/ip-up.
7.3. Related Documents
The LDP <
http://www.linuxdoc.org/> publishes many documents related to
this mini-HOWTO, most notably the Linux Security Knowledge Base, the
VPN HOWTO, the VPN mini-HOWTO.
Then again, when facing a problem with some program, one reflex for
any Linux user should be to RTFM: Read The Fscking Manual pages for
the considered programs.
7.4. Extra copy of IMPORTANT DISCLAIMER --- BELIEVE IT!!!
I hereby disclaim all responsibility for your use of this
hack. If it backfires on you in any way whatsoever, that's
the breaks. Not my fault. If you don't understand the risks
inherent in doing this, don't do it. If you use this hack
and it allows vicious vandals to break into your company's
computers and costs you your job and your company millions
of dollars, well that's just tough nuggies. Don't come cry�
ing to me.
