"Pocket" ISP based on RedHat Linux HOWTO
Anton Chuvakin,
[email protected]
v2.0.0 10 January 2001
This document outlines the setup of a single RedHat box for dialins,
virtual web hosting, virtual email, POP3 and ftp servers. Why anybody
might need this in one box is beyond the scope of this document. The
idea is a complete ISP solution based on RedHat Linux. Any part of
this setup can be implemented separately though. I will try to empha�
size all the commands so one can just paste them to configure his own
box. The list of documents that I borrowed from and some further
reading is provided below (see References section). I will keep secu�
rity in mind on all stages of the setup and will make clear all the
security limitations of this setup, that I am aware of. I should add
that assets that are to be protected in this case are considered not
very valuable (e.g. personal pages etc) thus efforts spent on securing
the setup are allowed to be limited.
______________________________________________________________________
Table of Contents
1. Introduction
2. Changes
3. TODO Tue Jan 9 13:14:15 EST 2001
3.1 New versions of this document
3.2 Feedback
3.3 Standard disclaimer
3.4 Support
3.5 Copyright information
4. Step by step guide
4.1 Get RH
4.2 Install RH
4.3 Some install tips
4.4 Some preliminary security configuration
4.5 Remove unnecessary services
4.6 Enable multiple IP addresses
4.7 Configure DNS
4.8 Configure httpd
4.9 Configure sendmail
4.9.1 Setup
4.9.2 Troubleshooting
4.10 Configure POP3
4.10.1 Setup
4.10.2 Troubleshooting
4.11 Configure FTP server
4.11.1 Anonymous FTP setup
4.11.2 Guest FTP setup
4.12 Configure dialin
4.12.1 Linux setup
4.12.2 Windows setup
4.13 Open access
5. Conclusion
6. References
______________________________________________________________________
1. Introduction
The guide assumes some familiarity with Linux functionality and
general Linux/UNIX setup procedure (although not very detailed). Fully
functional brain is also required for some stages of the procedure.
All setup would be done manually (without the use of linuxconf, Webmin
or other tools). Not that those are bad or that there is anything
wrong with them. The reasons for that are: 1) it is comparatively hard
to give step by step directions that produce predictable results as
these tools pretend they are intelligent and "know better" (also known
as "Windows syndrome") 2) layout of tools changes with time and is
different in some distributions 3) manual setup gives better
understanding of system works (not that it is always required though)
4)some tools allow only limited configuration of Linux system or do
not keep up with updated features of services they try to configure.
I should add, that another solution seems to be very promising. It is
virtfs developed by Afra Ahmad. Its main part is a perl script so it
does not suffer from being a "black box". It will automatically
configure all virtual services in a highly customizable fashion.
It is based on taking advantage of the chroot environment. A separate
and smaller filesystem is created for each virtual server, and when a
service is requested, the main server will chroot to the desired
virtual server.
This method may take up more disk space, but it is much more flexible,
especially when dealing with the services. For example, it is possible
to have two different email accounts
[email protected] and
[email protected] (as you are dealing with two different passwd files).
It might be essential for a bigger hosting site.
For more information please visit the Virtfs page at
http://www.prongs.org/virtfs.
While many improvements are possible to the setup described in this
HOWTO they might be described in later editions of this document - I
just outline one possible way (accidentally, the one I used). The
writeup is aimed at RedHat Linux, but with trivial changes can be used
on any modern Linux distribution. The resulting configuration loosely
follows the setup of some particular machines built by the author.
2. Changes
from 1.1.1 to 2.0.0
� References updated (single IP hosting)
� RedHat 7.0 based
�
from 1.1.0 to 1.1.1
� Partitioning scheme described
� Some comments to dialin server setup added (based on user feedback)
� Some comments to ftp server setup added (based on user feedback)
from 1.0.3 to 1.1.0
� Description of virtfs added
� Qpopper and WUftpd bugs described
� Double connections issue mentioned as requested by one of the
readers
� POP-only accounts described
� References added
from 1.0.2 to 1.0.3
� Some spelling errors corrected (thanks to Eugene Shishkin for that)
from 1.0.1 to 1.0.2
� Some errors corrected (spelling)
� Method to chroot non-anonymous ftp users ("guest" users; those with
password and usernames, but with access only to their home
directories; used for easy web updates)
� References section updated
� Troubleshooting subsections added to two sections
� Qpopper update
from 1.0.0 to 1.0.1
� Many errors corrected (both spelling and factual)
� References section updated
� Minor changes in wording and syntax to improve clarity
� More security info added to several sections
� Windows configuration for dialup added
Next update planned at: upon request or when new program versions are
released
3. TODO Tue Jan 9 13:14:15 EST 2001
� How to test each service section added! (including maybe strace:
(mkdir /tmp/strace; mv .../in.ftpd .../in.ftpd.binary, create a two
line shell script named in.ftpd with: #!/bin/sh and I've described
strace wrappers and reading strace output in past LG articles;
search for some hints))
� More on security of all the services we install (clear text
password, DoS by overflowing partition in mail and ftp, http access
configs etc), including maybe the very basic ipchains setup
(ipchains as safer alternative to tcp wrappers)
� Add info on POP3 and ftp tunneling via ssh (just for fun) or refer
to other HOWTOs
� Add troubleshooting subsections to various sections
� Add SSL-enabled Apache install and basic configuration
� Add news server setup - who needs it?
� User accounting setup (acc, acua)
� Description of mail-only users (playing with proxyarp and
restricting their access only to the local machine)
3.1. New versions of this document
New versions of this document can be found at
http://www.chuvakin.org/ispdoc
3.2. Feedback
All comments, error reports, additional information (very much
appreciated!!!) and criticism of all sorts should be directed to:
[email protected]
http://www.chuvakin.org/
My PGP key is located at
http://www.chuvakin.org/pgpkey
Please direct spelling error comments to your friendly local spell
checker.
If you plan to ask for help, see support section first.
3.3. Standard disclaimer
No liability for the contents of this document can be accepted. Use
the concepts, examples and other content at your own risk.
Additionally, this is an early version, with many possibilities for
inaccuracies and errors.
One of many possible setups will be described. In the Linux world,
there is usually a number of ways in which to accomplish things.
As far as I know, only programs that under certain terms may be used
or evaluated for personal purposes will be described. Most of the
programs will be available complete with source under GNU-like terms.
3.4. Support
This is ridiculous, right? Who may ask for support after seeing such a
comprehensive doc ;-) ?
Anyhow, if you are curious about some particular aspect of this setup
or some of my writing is unclear, just drop me an email and I might
answer it (at least, be assured that I will read it).
Now, if you require a phone, hand-holding style support or my work on
your system, I might be able to provide it on certain terms (if I have
time and your case seems interesting enough ;-) )
3.5. Copyright information
This document is copyrighted (c) 2000,2001 Anton Chuvakin and
distributed under the following terms:
� Linux HOWTO documents may be reproduced and distributed in whole or
in part, in any medium physical or electronic, as long as this
copyright notice is retained on all copies. Commercial
redistribution is allowed and encouraged; however, the author would
like to be notified of any such distributions.
� All translations, derivative works, or aggregate works
incorporating any Linux HOWTO documents must be covered under this
copyright notice. That is, you may not produce a derivative work
from a HOWTO and impose additional restrictions on its
distribution. Exceptions to these rules may be granted under
certain conditions; please contact the Linux HOWTO coordinator at
the address given below.
� If you have questions, please contact Greg Hankins, the Linux HOWTO
coordinator, at
[email protected]
4. Step by step guide
Ingredients needed:
� RedHat Linux distribution (the instructions are exactly applicable
to RedHat 6.x or 7.x and, I think, with some minor changes to
5.x))
� compatible hardware (also known as a PC), that includes network
card and modem (at least one)
� 3-256 IP addresses (as the machine will give out some IP addresses
for modem callers and use others for virtual hosting more than 1 is
needed, the upper number is the maximum number of IP-based virtual
hosts allowed without recompiling the stock RedHat kernel, lower is
one real IP, one modem and one virtual IP - see reference for
sinlge IP virtual hosting below).
� some sort of permanent network connection (using some modems for
dialin while providing the Internet access via another modem is
considered totally weird and not recommended)
Here follows the procedure:
4.1. Get RH
Purchase or otherwise procure the RedHat 7.0 (further referred as RH,
latest version number is 7.0 at the time of updating) distribution and
compatible hardware. One can get a full RH CDROM for about $3.00
including shipping and handling at
http://www.cheapbytes.com. This
version will not contain such luxuries as secure web server and extra
software. For those you should turn to RedHat website. Or probably
buying the PC with Linux RH pre-installed is an option for some.
4.2. Install RH
Install the RH following the *instructions on the package* (might be
added here later). CDROM install is very easy to perform. I suggest
using text-mode setup, in my case their graphical one failed
miserably. When asked about the installation type
(Server/Workstation/Custom) choose Server or Custom (if you know what
you are doing)-you can always add software later. Some other important
installation decisions are outlined further. For RH 6.0 and 6.1 you
might be able to add packages to Workstation setup as well, but in RH
6.2 and later (7.0) all the server services are disabled and
significant amount of tweaking is required-so only Server or Custom is
strongly recommended.
4.3. Some install tips
If your hardware really is compatible the installation process will
detect and configure it correctly. Otherwise, refer to corresponding
documentation for troubleshooting network card, modem, video card, etc
problems (mostly HOWTOs and mini-HOWTOs, some are in References
section below).
Here are some ideas on disk space partitioning. Read Linux Partitions
HOWTO (a bit outdated) to get some general hints on functions of
partitions and their sizes for different kinds of server setups.
Lest assume we are setting up a server for under one hundred users. We
will need separate /tmp, /var and /home partititons (and swap, of
course). If you hard drive is around 4 GB than roughly 300 MB is
/tmp, 100MB swap, 1 GB /var (you want ample logging) and 1GB /home.
The remaining 1.6GB will be root partition (no separate /usr). The
split between /home and / might depend upon the amount of web pages
you plan to host - the more pages the more space goes to /home. To
enhance security it is nice to put some restricions (in /etc/fstab) to
/tmp, /var and /home partitions (similar to those described in my
Public Browser Station HOWTO .
If your network card is detected properly you will be asked for an IP
address of your machine, gateway address and network mask and the
address of the DNS server (might be your own machine if you plan to
set it up this way). Have all this info handy. Also you will be asked
for a machine name and domain name. We will use a sample domain name
you.com and the machine will be named ns (that gives us a fully
qualified domain name (FQDN) ns.you.com). You should use whatever
domain you registered (see Setting Up Your New Domain Mini-HOWTO, link
in References section below) and intend to use as your primary domain
(not a virtual). For the gateway address we will use a sample
111.222.333.111 address. Gateway is likely the router that connects
your machine (or your LAN) to the outside world.
Enable shadow and MD5 passwords for greater security. First of those
makes the file that contains encrypted passwords readable only to root
user and the second allows longer and harder to crack passwords. As
it will be a standalone machine do not enable NIS/NFS.
After installation finishes and machine reboots you will see the login
prompt. Enter login and password (for the root account) and start
configuring you new Linux station.
4.4. Some preliminary security configuration
First (and fast), add a line: ALL:ALL to your /etc/hosts.deny file.
That would (to some known extent) prevent other people from accessing
your machine while you are doing the configuration. That will also
prevent you from doing the same. For further configuration efforts
(that can be done remotely, by the way) secure shell is recommended.
Download the RPM package for RH from one of the many sites and install
it (as root) using: rpm -U ssh*rpm or similar command (depends upon
the version). You will have to get both client and server packages (if
you want to ssh from this machines as well as to this machine). Upon
installation all necessary post-installation commands (like server key
generation) are run automatically by the RPM package. You will have to
start server manually using command /etc/rc.d/init.d/sshd start.Some
early versions of ssh1 and also all versions of ssh1 compiled with
RSAREF library contain a buffer-overflow bug. Use ssh2 or the latest
version of ssh1 without RSAREF. If you do this you will have to allow
access using ssh from some trusted machine (described later) in
/etc/hosts.allow file. RedHat 7.0 now includes OpenSSH clone that
supports both ssh1 and ssh2 protocols. Its configuration is almost the
same as ssh. It has some minor configuration advantages over ssh (for
instance, no X11 forwarding by default) and is otherwise the same.
Sshd (when run as daemon) will also refer to /etc/hosts.deny and
/etc/hosts.allow for access control.
If you want to be really rigorous in you configuration pursuits go to
single use mode by giving the command init 1, in this case all work is
to be done locally and you would not be able to test you network-
related configuration as network is not available in this mode.
To further enhance your security ipchains software (that is usually
part of your Linux distribution) can be used (for that refer to
IPCHAINS HOWTO, link in References). It takes quite a bit more
efforts to configure it than TCP wrappers, although some automated
tools are available for that too.
4.5. Remove unnecessary services
Now lets deal with unnecessary services. Please note that my idea of
"unnecessary" might not be 100% same as yours. Also, telnet is now
considered by many to be not only unnecessary, but really utterly
undesirable. Use ssh, and forget telnet once and for all!
1. Services started from /etc/inetd.conf (RedHat 7.0 introduced the
more advanced /etc/xinetd.conf which uses somewhat different
syntax, see below):
comment out all the lines, but those
ftp stream tcp nowait root /usr/sbin/tcpd in.ftpd -L -l -i -a
telnet stream tcp nowait root /usr/sbin/tcpd in.telnetd
Check this by using the command: grep -v '\#' /etc/inetd.conf
If you will be using the secure shell (ssh), telnet is also not
necessary and can be removed. Secure shell can either be started as a
daemon on system startup or as a service from /etc/inetd.conf. Default
configuration (used by the RPM package) is to start is as a daemon.
Sshd can be compiled to refer to /etc/hosts.allow file for access
control. In this case, while you will not have it in your
/etc/inetd.conf, it will still use the settings from /etc/hosts.allow
and /etc/hosts.deny. The advantages of this method is faster
connection as the sshd will not have to regenerate server key every
time somebody connects. On the other hand, if you start it from
/etc/inetd.conf it will be more isolated from the outside world. More
lines will be added to /etc/inetd.conf as necessary (POP3 is one of
those).
Here goes the note for RedHat 7.0 users. Inetd daemon (while still
present in the distribution) is now replaced with xinetd. Its
configuration file format is as follows:
#
# Simple configuration file for xinetd
#
# Some defaults, and include /etc/xinetd.d/
defaults
{
instances = 60
log_type = SYSLOG authpriv
log_on_success = HOST PID
log_on_failure = HOST RECORD
}
includedir /etc/xinetd.d
where /etc/xinetd.d directory looks like (with probably more file in
your case):
-rw-r--r-- 1 root root 498 Aug 23 00:17 tftp
-rw-r--r-- 1 root root 414 Jul 21 08:43 rsh
-rw-r--r-- 1 root root 362 Jul 21 08:43 rexec
-rw-r--r-- 1 root root 361 Jul 21 08:43 rlogin
-rw-r--r-- 1 root root 347 Aug 9 05:55 wu-ftpd
Files in the directory configure individual services like finger,
telnet or ftp. There format is (this service, ftp, defaults to on on
stock RedHat 7.0)
# default: on
# description: The wu-ftpd FTP server serves FTP connections. It uses \
# normal, unencrypted usernames and passwords for authentication.
service ftp
{
socket_type = stream
wait = no
user = root
server = /usr/sbin/in.ftpd
server_args = -l -a
log_on_success += DURATION USERID
log_on_failure += USERID
nice = 10
}
Or (this service, tftp, defaults to off on stock RedHat 7.0)
# default: off
# description: The tftp server serves files using the trivial file transfer \
# protocol. The tftp protocol is often used to boot diskless \
# workstations, download configuration files to network-aware printers, \
# and to start the installation process for some operating systems.
service tftp
{
socket_type = dgram
wait = yes
user = nobody
log_on_success += USERID
log_on_failure += USERID
server = /usr/sbin/in.tftpd
server_args = /tftpboot
disable = yes
}
So, to disable services add "disable= yes" to the end of correspondent
file or just remove the file.
2. Services started on system startup from /etc/rc.d directory:
Check what services are running by using: ps ax. You will get
something similar to the sample output below:
PID TTY STAT TIME COMMAND
1 ? S 0:04 init
2 ? SW 0:30 [kflushd]
3 ? SW 0:32 [kupdate]
4 ? SW 0:00 [kpiod]
5 ? SW 0:03 [kswapd]
6 ? SW< 0:00 [mdrecoveryd]
296 ? SW 0:00 [apmd]
349 ? S 0:00 syslogd -m 0
360 ? S 0:00 klogd
376 ? S 0:00 /usr/sbin/atd
392 ? S 0:00 crond
412 ? S 0:00 inetd
454 ttyS0 S 0:00 gpm -t ms
533 tty2 SW 0:00 [mingetty]
534 tty3 SW 0:00 [mingetty]
535 tty4 SW 0:00 [mingetty]
536 tty5 SW 0:00 [mingetty]
537 tty6 SW 0:00 [mingetty]
667 tty1 SW 0:00 [mingetty]
4540 ? S 0:00 httpd
5176 ? S 0:00 httpd
5177 ? S 0:00 httpd
5178 ? S 0:00 httpd
5179 ? S 0:00 httpd
5180 ? S 0:00 httpd
5181 ? S 0:00 httpd
5182 ? S 0:00 httpd
5183 ? S 0:00 httpd
7321 ? S 0:00 /usr/sbin/sshd <<< only after you installed sshd to run on startup
7323 pts/0 S 0:00 -bash
7336 pts/0 R 0:00 ps ax
Lets concentrate on processes that listen to network, such as lpd.
Since we do not plan to use our server for printing (we sure might, I
just don't describe it here), I suggest we remove the printer daemon
by: rpm -e lpd . If rpm complains about any dependencies (like, in my
case, printfilter and rhprinttool), add them to your rpm -e command
and repeat it. Other services that should be removed are NFS, NIS,
samba etc, if they got installed by mistake. Make sure you remove
NFS/NIS (if you are not using them) as bugs are often found in them.
Again, these are useful things, I am just following the *golden rule*
"remove the software you don't currently use". And, with RH RPM it is
really easy to add it any time in the future.
Some more basic security settings can be obtained from Armoring Linux
paper. As suggested there, lets make a wheel group with trusted users
(in our case, only user youwill be able to do /bin/su and to run cron
jobs (together with root).
� wheel group for sensitive commands:
1. vi /etc/group, add a line (if it doesn't exist):
wheel:x:10:root,you
as shown. You don't have to use vi (and somehow I understand it
very well ;-)), just use your favorite editor (for a nice reason�
ably user-friendly non-X editor try pico, distributed together with
mail program pine, the latter is part of most Linux distributions)
2.
/bin/chgrp wheel /bin/su
wheel group on /bin/su
3.
/bin/chmod 4750 /bin/su
� restrict cron:
To only allow root and you to submit cron jobs create a file
called /etc/cron.allow that contains usernames that you want to be
able to run cron jobs. This file might look like this:
root
you
Why should one restrict cron jobs? Local exploits to elevate privi�
leges to root from, say, nobody, exist for some versions of cron.
I suggest you do not install X Windows as it will bring new concern
that you might not be prepared to deal with.
4.6. Enable multiple IP addresses
Now we are ready to enable our machine to handle multiple IP addresses
for virtual hosting. At that point, the IP Aliasing HOWTO might come
handy (see link in References). For several reasons, IP-based virtual
hosting is better (if you have enough IP addresses, that is). For
instance, reverse lookups would succeed, if done from the browser
side. It might also be needed for hosting cryptographically enabled
websites (commonly known as "secure websites"). Older browsers (not
supporting HTTP 1.1) will get unhappy too.
The changes would be concentrated in /etc/rc.d/ directory. To enable
multiple IP addresses your kernel should support this. On a freshly
installed RH Linux it does. To verify it one should look into the
config file that was used to compile the kernel. In my case, it was
/usr/src/linux/configs/kernel-2.2.17-i686.config since the machine has
Pentium III processor. This file exists, if the kernel-source RPM
package was installed. If line CONFIG_IP_ALIAS=y is present in the
file than you are OK. While we are here, we can also confirm the
ability to forward IP packets (needed for dialup users PPP). This
ability is present, but not turned on by default (to turn it on do
execute the following command echo 1 > /proc/sys/net/ipv4/ip_forward
or add a line into /etc/sysctl.conf). Also needed is the support for
PPP protocol (line CONFIG_PPP=m, this means PPP support is compiled as
a kernel loadable module, CONFIG_PPP=y is also OK)
The examples will use the ridiculous IP addresses
111.222.333.444-111.222.333.777 from C block 111.222.333.0.
111.222.333.444 is a real host IP (that is configured during RH
installation), 111.222.333.555-777 are virtual addresses and
111.222.333.888 is a dialin user address (can be more of those).
Lets assume we want to configure 3 virtual hosts.
Two sets of commands will be used:
1.
/sbin/ifconfig eth0:0 111.222.333.555
/sbin/ifconfig eth0:1 111.222.333.666
/sbin/ifconfig eth0:2 111.222.333.777
These will bind the IP addresses to (virtual) interfaces
eth0:0-eth0:2.
2.
/sbin/route add -host 111.222.333.555 dev eth0
/sbin/route add -host 111.222.333.666 dev eth0
/sbin/route add -host 111.222.333.777 dev eth0
These commands will add routes for those addresses and connect those
to real interface eth0 (ethernet card).
After doing them the ifconfig command output (ifconfig) will look like
this:
eth0 Link encap:Ethernet HWaddr 02:60:8C:4D:24:CE
inet addr:111.222.333.444 Bcast:255.255.255.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:901597 errors:33 dropped:0 overruns:0 frame:823
TX packets:433589 errors:0 dropped:0 overruns:0 carrier:0
collisions:128327 txqueuelen:100
Interrupt:5 Base address:0x280
eth0:0 Link encap:Ethernet HWaddr 02:60:8C:4D:24:CE
inet addr:111.222.333.555 Bcast:111.222.333.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:5 Base address:0x280
eth0:1 Link encap:Ethernet HWaddr 02:60:8C:4D:24:CE
inet addr:111.222.333.666 Bcast:111.222.333.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:5 Base address:0x280
eth0:2 Link encap:Ethernet HWaddr 02:60:8C:4D:24:CE
inet addr:111.222.333.777 Bcast:111.222.333.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:5 Base address:0x280
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:3924 Metric:1
RX packets:26232 errors:0 dropped:0 overruns:0 frame:0
TX packets:26232 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
All commands can be added to the bottom of /etc/rc.d/rc.local so that
the changes are saved after reboot. Strictly speaking, rebooting
machine is not required for adding new IP addresses. Please, do docu�
ment all changes you do to your machines. Many a good sysadmin (or,
should I say not-so-good?) were burned on that at some point in their
careers.
TO TEST THE CHANGES
Do: ping virtual addresses as
ping 111.222.333.555
ping 111.222.333.666
ping 111.222.333.777
Should get: interfaces should be up
4.7. Configure DNS
Now we are ready to configure DNS. The easiest way would be to add
the hostnames (real and all the virtual) that we want to be seen by
the world to the configuration of some machine that already has bind
(DNS daemon) running. But, since we are setting up ISP-in-a-box we
might not be able to avoid "DNS fun".
Now, let me also try to defend the (well, questionable) choice of
"outdated" version of bind 4.9.7 (last of the pre-8 series). I know
that my arguments can be beaten, now that even bind 9 is out, but I
consider bind 4.9.7 much more time-tested and stable. The arguments
for upgrading to 8.x (provided
http://www.acmebw.com/askmrdns/00444.htm and
http://www.dns.net/dnsrd/servers.html and, I guess, at many other
places. Here is a message from Theo de Raadt himself (the head of
OpenBSD development) where he justifies the choice of bind 4 as part
of OpenBSD-the most secure UNIX OS around. He also shudders at the
amount of bugs the OpenBSD auditing team saw in BIND 8 source code)
still didn't seem to convince many people. And, lets not forget the
"exploit of 1999" - ADMROCKS, that gives remote root access to almost
any Linux machine running bind prior to 8.1.2 patch 3. Judging by the
INCIDENTS mailing list, this is still a very popular way to attack RH
versions 5.0-6.1 if no recommended upgrades are installed. It is
claimed that named (whatever version) should always be run in a chroot
jail.
Here are the instructions, loosely following the DNS book from
O'Reilly (a good one, highly recommended to all, but very casual DNS
user).
1. Find and install bind 4.9.7 either from RPM package (RH 4.2, if I
am not mistaken - for that you can use RPMFIND.net, personally I
didn't try this and so I am somewhat skeptical about installing RH
4.2 package on RH 6.1 system, but it might work) or from source
(bind 4.9.7, compiling it is a bit troublesome, but reading all the
README files in the archive will definitely help).
2. Create files and directories needed for bind:
� /etc/named.boot
� /etc/namedb
� /etc/namedb/db.you
� /etc/namedb/db.111.222.333
� /etc/namedb/db.127.0.0
� /etc/namedb/db.yoursite1
� /etc/namedb/db.yoursite2
� /etc/namedb/db.yoursite3
This will be used for 3 virtual domains:
yoursite1.com,yoursite2.com and yoursite3.com. One more important
comment refers to secondary DNS issue. As all your domains and all
their services will be hosted on the same machine, DNS backup in
the form of secondary server doesn't make much sense: if your
primary DNS is down everything else (mail, www, ftp, pop, etc) is
down as well. But you do have to have a secondary DNS to register a
domain. Try to convince somebody to put you in as a secondary or
use a free DNS service (link is in Setting Up Your New Domain Mini-
HOWTO).
3.
That is how they look like (if you are unfamiliar with bind 4.x
configuration file format, please, do read either the O'Reilly DNS
book or any of the HOWTOs or documents at bind pages, or, better,
all of the above. You also have an option of using them without
understanding, but this is a bad idea in general):
/etc/named.boot
This is the main config file for bind 4.9.x.
directory /etc/namedb
;cache-obtained from internic, usually
cache . db.cache
;main config files
primary you.com db.you
;reverse lookups
primary 333.222.111.in-addr.arpa db.111.222.333
;localhost.localnet configs
primary 0.0.127.in-addr.arpa db.127.0.0
;virtual Domains
primary yoursite1.net db.yoursite1
primary yoursite2.net db.yoursite2
primary yoursite3.net db.yoursite3
4.
/etc/namedb/db.you
; defines our local hosts at you.com, just one in our case, and its aliases
@ IN SOA ns.you.com. root.ns.you.com. (
2000012190 7200 1800 3600000 7200 )
;name servers and mail servers
IN NS ns.you.com.
IN MX 10 ns.you.com.
IN A 111.222.333.444
ns IN A 111.222.333.444
;address of the canonical names
localhost IN A 127.0.0.1
gateway IN A 111.222.333.111
;aliases (to use in ftp: ftp ftp.you.com etc, for clarity)
www CNAME ns
mail CNAME ns
ftp CNAME ns
pop3 CNAME ns
5.
/etc/namedb/db.111.222.333
;reverse mapping of our IP addresses
.
;origin is 333.222.111.in-addr.arpa
333.222.111.in-addr.arpa. IN SOA ns.you.com. root.ns.you.com. (
1999121501 7200 1800 3600000 7200 )
;name Servers
IN NS ns.you.com.
;addresses point to canonical name
444.333.222.111.in-addr.arpa. IN PTR ns.you.com.
;dialins
888 IN PTR dialup.you.com.
;virtual hosts
555 IN PTR yoursite1.com.
666 IN PTR yoursite2.com.
777 IN PTR yoursite3.com.
6. /etc/namedb/db.127.0.0
;local loop config file
0.0.127.in-addr.arpa. IN SOA ns.you.com. root.ns.you.com. (
1997072200 7200 1800 3600000 7200 )
IN NS ns.you.com.
1 IN PTR localhost.
7. /etc/namedb/db.yoursite1
; yoursite1.com
@ IN SOA virtual root.virtual (
1999092201 ; Serial: update each time the file is changed
7200 ; refresh, sec
1800 ; retry, sec
3600000 ; expire, sec
7200 ) ; minimum TTL
;name servers
IN NS ns.you.com.
IN MX 10 virtual
IN A 111.222.333.555
;address of the canonical names
localhost IN A 127.0.0.1
gateway IN A 111.222.333.111
virtual IN A 111.222.333.555
IN MX 10 virtual
;aliases
www CNAME virtual
mail CNAME virtual
ftp CNAME virtual
pop3 CNAME virtual
8. /etc/namedb/db.yoursite2
; yoursite2.com
@ IN SOA virtual root.virtual (
1999092201 ; Serial: update each time the file is changed
7200 ; refresh, sec
1800 ; retry, sec
3600000 ; expire, sec
7200 ) ; minimum TTL
;name servers
IN NS ns.you.com.
IN MX 10 virtual
IN A 111.222.333.666
;address of the canonical names
localhost IN A 127.0.0.1
gateway IN A 111.222.333.111
virtual IN A 111.222.333.666
IN MX 10 virtual
;aliases
www CNAME virtual
mail CNAME virtual
ftp CNAME virtual
pop3 CNAME virtual
9. /etc/namedb/db.yoursite3
; yoursite3.com
@ IN SOA virtual root.virtual (
1999092201 ; Serial: update each time the file is changed
7200 ; refresh, sec
1800 ; retry, sec
3600000 ; expire, sec
7200 ) ; minimum TTL
;name servers
IN NS ns.you.com.
IN MX 10 virtual
IN A 111.222.333.777
;address of the canonical names
localhost IN A 127.0.0.1
gateway IN A 111.222.333.111
virtual IN A 111.222.333.777
IN MX 10 virtual
;aliases
www CNAME virtual
mail CNAME virtual
ftp CNAME virtual
pop3 CNAME virtual
These configuration files will allow you to host these three virtual
domains and your real domain you.com.
TO TEST THE CHANGES
Do: check address resolution
nslookup www.you.com
nslookup www.yoursite1.com
nslookup www.yoursite2.com
nslookup www.yoursite3.com
Should get: nslookup returns the correct IP addresses for all
hostnames
4.8. Configure httpd
To server html pages httpd daemon is used. RH 7.0 comes with Apache
1.3.12 (latest version is currently 1.3.14 and the alpha of the
upcoming 2.0 is released). At that point it is wise to check RH site
or its mirrors (RH Mirrors) for updates.
Most changes that we are about to make concentrate in
/etc/httpd/httpd.conf (RH standard location for Apache configuration).
Default location for html pages (shown when you go to www.you.com) is
/home/httpd/html. You can allocate directories for virtual hosts
within the same /home/httpd, shown below are the following locations
for them: /home/httpd/yoursite1, /home/httpd/yoursite2 and
/home/httpd/yoursite3.
Below I provide the minimum necessary changes for your
/etc/httpd/httpd.conf file:
<VirtualHost 111.222.333.555>
ServerAdmin
[email protected]
DocumentRoot /home/httpd/yoursite1
ServerName www.yoursite1.com
ErrorLog yoursite1-error_log
TransferLog yoursite1-access_log
</VirtualHost>
<VirtualHost 111.222.333.666>
ServerAdmin
[email protected]
DocumentRoot /home/httpd/yoursite2
ServerName www.yoursite2.com
ErrorLog yoursite2-error_log
TransferLog yoursite2-access_log
</VirtualHost>
<VirtualHost 111.222.333.777>
ServerAdmin
[email protected]
DocumentRoot /home/httpd/yoursite3
ServerName www.yoursite3.com
ErrorLog yoursite3-error_log
TransferLog yoursite3-access_log
</VirtualHost>
That configuration will cause all logs to be stored in one directory
(whatever is specified as such) for all sites. If that is not desired
the ErrorLog and TransferLog directives can be changed to point to the
proper location separately for each virtual host. The pages for the
"real" www.you.com will be stored in default location
/home/httpd/html.
For more information, look at
http://www.apache.org, Apache http
server homepage. They have a lot of support pages, including those for
virtual hosting setup (both IP-based and name-based [uses just 1 IP
address]). Also useful is Linux WWW HOWTO (link in References
section), section on virtual hosting.
TO TEST THE CHANGES
Do: access the test pages via Lynx browser or telnet to port 80
lynx
http://www.you.com
lynx
http://www.yoursite1.com
lynx
http://www.yoursite2.com
lynx
http://www.yoursite3.com
Should get: Test pages will be returned (if you put them in the proper
directories)
4.9. Configure sendmail
4.9.1. Setup
Now we will deal with sendmail. Again, proposed are the minimum
necessary changes to the stock RH /etc/sendmail.cf and
/etc/sendmail.cw.
1. look for the lines that starts from Dj$w.foo.com and change it to
point to your main ("real", not virtual) server name (you.com, so
it will looks like this Dj$w.you.com).
2. locate file /etc/sendmail.cw and make it look like this
# sendmail.cw - include all aliases for your machine here.
you.com
ns.you.com
mail.you.com
yoursite1.com
mail.yoursite1.com
yoursite2.com
mail.yoursite2.com
yoursite3.com
mail.yoursite3.com
These are necessary so that sendmail accepts mail for these domains.
This does not address the issue of
[email protected] and
[email protected] mail getting to different mailboxes. For that look
into /etc/mail/virtusertable functionality (appropriate line in
/etc/sendmail.cw is Kvirtuser hash -o /etc/mail/virtusertable,
detailed info may be added here later). Excellent documentation on
that is on
http://www.sendmail.org/virtual, sendmail reference on
virtual hosting.
It is worthwhile to add that linuxconf proposes a somewhat different
scheme for virtual email with separate spool directories for all
domains (that cleanly solves the above "name-conflict" issue"), but
that requires a special virtual-aware POP/IMAP server (included with
RH) and is somewhat more complicated. It is recommended for bigger
email volume sites with many users within each domain.
A few words about sendmail, it is a good idea (good from the security
standpoint) to have sendmail run from inetd.conf and not as a
standalone daemon. For that we need to add it to /etc/inetd.conf,
remove it from /etc/rc.d/init.d, add the sendmail queue processing to
cron. Here is what you have to do:
1. Add the following line to /etc/inetd.conf:
smtp stream tcp nowait root /usr/sbin/tcpd /usr/sbin/sendmail -bs
Or, if using xinetd create a file sendmail in /etc/xinetd.d/ similar
to
# default: on
service sendmail
{
socket_type = stream
wait = no
user = root
server = /usr/bin/sendmail -bs
}
2. Edit /etc/rc.d/init.d/sendmail to have exit 0 somewhere in the
very beginning (might not be the best way, be sure to document the
changes you do to these files) so that this file does nothing
instead of starting sendmail
3. By editing your (root's) crontab (to edit do crontab -e) add a
line like this
*/20 * * * * /usr/sbin/sendmail -q
That would process sendmail queue every 20 min (if it exists). The
described steps will simplify sendmail access control and will let you
regulate who can talk to your 25 port, not just who can send email
through you. The lines in /etc/hosts.allow that let all machines from
.com and .org domains send you email are as follows
sendmail: .com .org
Please, note, that the daemon name, not protocol name is used here
(sendmail, NOT smtp).
That would allow your system to handle email for all those domains.
4.9.2. Troubleshooting
PROBLEM: mail that you are trying to send is denied with a message
Relaying denied
SOLUTION:Look into your /etc/sendmail.cw. Are you sure all possible
variations of your hostname and of your virtual hostnames are here?
Look in the message headers and see from what machine it was rejected
from: does it look like another name of yours that you missed?
TO TEST THE CHANGES
Do: access the SMTP port 25 via telnet
telnet www.you.com 25
telnet www.yoursite1.com 25
telnet www.yoursite2.com 25
telnet www.yoursite3.com 25
Should get: Sendmail should respond with prompt and version number!
Type QUIT to get out of the prompt.
4.10. Configure POP3
4.10.1. Setup
POP3 configuration is easy (no "virtualization" is required for this
setup). RH comes equipped with imapd IMAP server. If you do not want
to use IMAP functionality or do not like this particular
implementation (buffer overflow bugs were discovered in it at some
point) the good idea is to use qpopper, free POP3 daemon from Eudora
http://www.eudora.com/freeware/qpop.html. At the time of writing the
released version is qpopper 3.0.2. It is important to note that
versions earlier than 2.5 contain a buffer overflow error that allows
remote root exploit to be executed. Same problem plagues "public
betas" up to 3.0 release 21. Use either 2.53 or the latest 3.0 (the
former is better audited and the latter is better suited for RH -
seamlessly works with PAM authentication). I suggest using 3.0, so the
instructions below apply to that case. As of April 13, Qpopper 3.0 is
no longer beta, but a regular software. As of recently, the bug was
discovered even in Qpopper 2.53 that allows the attacker to obtain a
shell with group-id 'mail', potentially allowing read/write access to
all mail.
1. wget
ftp://ftp.qualcomm.com/eudora/servers/unix/popper/qpopper3.0.tar.Z
Retrieve the archive from Eudora site.
2. tar zxvf qpopper3.0.tar.Z
Uncompress and untar the contents.
3. cd qpopper
If you need explanation for this step, please, discontinue reading
the document.
4. ./configure --enable-specialauth --with-pam --enable-log-login
--enable-shy
The options here are:
--enable-specialauth : allows MD5 and shadow passwords
--with-pam: allows the use of RH Pluggable Authentication Modules
(PAM) technology
--enable-log-login: log successful logins, not only failures (not
really that useful as it will use tcpd wrappers logging anyway)
--enable-shy: conceal version number (yeah, a little pesky
manifestation of "security through obscurity")
5. make
That compiles the popper
6.
/bin/cp popper/popper /usr/local/bin
Copies the binary to /usr/local/bin
7. Now set the mode to
-rwx------ 1 root root 297008 Feb 16 15:41 /usr/local/bin/popper
by using the command:
chmod 700 /usr/local/bin/popper
8. Add a line to /etc/inetd.conf
pop3 stream tcp nowait root /usr/sbin/tcpd /usr/local/bin/popper -s
That would cause the tcpd wrapper to control access to popper.
The lines to add in /etc/hosts.allow are
popper: .good.com .nice.org
That will allow people from domains good.com and nice.org to read
email via POP3 client from your machine.
To cause qpopper to use PAM authentication one must create a file for
POP3 service in /etc/pam.d/ directory. File should be named "pop3"
(same as line in /etc/services and qpopper compile-time option). The
file looks like this:
auth required /lib/security/pam_pwdb.so shadow
account required /lib/security/pam_pwdb.so
password required /lib/security/pam_cracklib.so
password required /lib/security/pam_pwdb.so nullok use_authtok md5 shadow
session required /lib/security/pam_pwdb.so
9. For whatever reason stock RH lists line in /etc/services file for
POP3 protocol as "pop-3". And since qpopper prefers to see "pop3",
it should be edited to be:
pop3 110/tcp # pop3 service
That would allow all user to get their email via any reasonable mail
client.
4.10.2. Troubleshooting
PROBLEM: you are connecting to your POP server with valid password and
username and they are rejected with a message Password incorrect.
SOLUTION: PAM doesn't like your setup. This message is common for
qpopper 2.53, use 3.0 and it should disappear. Otherwise, look into
/etc/pam.d/pop3 that you created. Is it OK?
TO TEST THE CHANGES
Do: access the POP3 port 110 via telnet
telnet www.you.com 110
Should get: Qpopper should respond with prompt and version number!
Type QUIT to get out of the prompt.
4.11. Configure FTP server
4.11.1. Anonymous FTP setup
We will use only anonymous ftp and will not allow any non-anonymous
user any access. Here we describe the anonymous ftp server setup that
allows anonymous uploads. Any self-respecting guide on the subject
will tell you that "this is a bad thing". But how is it worse than
allowing users to ftp from untrusted location and transfer their
passwords in clear text? Not everybody (especially, using Windows) can
easily setup an ftp tunnel via ssh. But you definitely should restrict
access via tcp wrappers and watch for "warez puppies" (people who will
try to exchange stolen software via your ftp site if you allow
unlimited downloads!).
I suggest using the stock RH wu-ftpd (version 2.6.1 at the time of
writing). While it is rumored that there are "more secure" ftp daemons
(Pro-ftpd), wu-ftp appears to be one most commonly used. Recently a
series of bugs was again discovered in wu-ftp (even in 2.6.x versions)
and its reputation as the most popular ftp daemon seem to be
dwindling. CERT has issued an advisory concerning WU-FTPD and all ftp
daemons derived from BSD's final release.
RH installs the wu-ftpd (package wu-ftpd-2.6.1-1) by default in server
configuration. You are encouraged to check for updates as running ftp
is an important security concern. There is also a separate rpm package
that creates a separate directory structure for anonymous ftp home
(anonftp-2.8-1). As anonymous ftp always does a chroot() system call
(puts the user in the restricted file system) all necessary binaries
and libraries are required. The typical directory looks like this
(output of ls -lRa in /home/ftp):
.:
total 20
d--x--x--x 2 root root 4096 Feb 15 06:22 bin
d--x--x--x 2 root root 4096 Feb 15 06:22 etc
drwxrws-wt 2 root wheel 4096 Feb 18 19:51 incoming
drwxr-xr-x 2 root root 4096 Feb 15 06:22 lib
drwxr-sr-x 3 root ftp 4096 Feb 15 23:34 pub
bin:
total 344
---x--x--x 1 root root 15204 Mar 21 1999 compress
---x--x--x 1 root root 52388 Mar 21 1999 cpio
---x--x--x 1 root root 50384 Mar 21 1999 gzip
---x--x--x 1 root root 29308 Mar 21 1999 ls
---------- 1 root root 62660 Mar 21 1999 sh
---x--x--x 1 root root 110668 Mar 21 1999 tar
lrwxrwxrwx 1 root root 4 Feb 15 06:22 zcat -> gzip
etc:
total 40
-r--r--r-- 1 root root 53 Mar 21 1999 group
-rw-r--r-- 1 root root 31940 Mar 21 1999 ld.so.cache
-r--r--r-- 1 root root 79 Mar 21 1999 passwd
incoming:
total 0
lib:
total 1212
-rwxr-xr-x 1 root root 77968 Mar 21 1999 ld-2.1.1.so
lrwxrwxrwx 1 root root 11 Feb 15 06:22 ld-linux.so.2 -> ld-2.1.1.so
-rwxr-xr-x 1 root root 1031004 Mar 21 1999 libc-2.1.1.so
lrwxrwxrwx 1 root root 13 Feb 15 06:22 libc.so.6 -> libc-2.1.1.so
-rwxr-xr-x 1 root root 77196 Mar 21 1999 libnsl-2.1.1.so
lrwxrwxrwx 1 root root 15 Feb 15 06:22 libnsl.so.1 -> libnsl-2.1.1.so
-rwxr-xr-x 1 root root 33596 Mar 21 1999 libnss_files-2.1.1.so
lrwxrwxrwx 1 root root 21 Feb 15 06:22 libnss_files.so.2 -> libnss_fi
les-2.1.1.so
pub:
total 0
Notice though, that for whatever reason, RH puts a copy of /bin/sh in
/home/ftp/bin. I do not feel good about having it there, so it is
chmoded to 0 by chmod 0 sh (can also be removed completely, but RPM
might be slightly unhappy if you attempt to remove the package
afterwards).
Permissions on /home/ftp directories and files should be carefully
considered. In the above example, all of the system files are owned by
root and are only readable (executable where necessary) by all. Files
in bin are only executable (as is the directory itself to prevent
listing of its contents).
The interesting part is permissions on pub and incoming.
Below follows the configuration file for ftp daemon (/etc/ftpaccess).
It is well commented to the degree of being self-explanatory:
#ideas from <htmlurl url="
ftp://ftp.wu-ftpd.org/pub/wu-ftpd/upload.configuration.HOWTO" name="
ftp://ftp.wu-ftpd.org/pub/wu-ftpd/upload.configuration.HOWTO">
#only allow anonymous users-no other classes defined
class anonftp anonymous *
#number of users restriction with message shown when too many
limit remote 10 Any /toomany.msg
#prevent uploads everywhere (for now)
upload /home/ftp * no
#display the contents of some files upon login/cd
readme README* login
readme README* cwd=*
message /welcome.msg login
message .message cwd=*
#log all file transfers DISABLED
#log transfers anonymous
#prevent these file operations for anon users
delete no anonymous
overwrite no anonymous
#fast cd and aliasing for the same reason (not really necessary, but convenient)
alias inc: /incoming
cdpath /incoming
cdpath /pub
cdpath /
#what is allowed in paths
path-filter anonymous /etc/pathmsg ^[-A-Za-z0-9_\.]*$ ^\. ^-
#prevent the retrieval of some file
noretrieve .notar
#allow upload with NO subdirectory creation by anon users
upload /home/ftp /incoming yes root wheel 0400 nodirs
#allow upload with subdirectory creation by anon users DISABLED
#upload /home/ftp /incoming yes root wheel 0400 dirs
#prevent anon users to GET files from incoming (you might not like it, but it
#is a good idea-to prevent some people from using your ftp server to store
#their own stuff, pics, warez etc)
noretrieve /home/ftp/incoming
That would allow only anonymous users to do downloads and uploads in
somewhat (!) controlled manner. Make sure you update the permissions
on files that you changed after you upgrade the RPM packages next
time.
4.11.2. Guest FTP setup
Guest FTP users are those that have valid usernames and passwords
(unlike anonymous), but do not have access to the whole directory
structure (unlike real ones). So they are chrooted after
authentication. Guest users can do uploads in this configuration.
Easy 21-step directions for that are provided below ;-)
Software used: wu-ftpd-2.6.1
Sample username will be created: ftpguy, user ID=505.
Her group will be: lusers, group ID=701.
If you want more users of the same sort, they should be the members of
the same group. For that it might be good to change the directory
structure somewhat so that all of them use the same passwd file and
the same static ls. But, for better separation you can give each of
them their own files.
1. adduser ftpguy
creates an entry in /etc/passwd
2. passwd ftpguy change password to whatever
3. Edit file /etc/passwd, last line (that contains our new user)
should look like this
ftpguy:x:505:701::/home/ftpguy/./:/etc/ftponly
yes, that is "slash"-"dot"-"slash" after his home directory.
4. Edit file /etc/shells, add line, below
/etc/ftponly
This file has to exist in some newer Linux distributions (contrary to
what is claimed at Guest FTP HOWTO). Sometimes one can put /bin/true
in its place.
5. Edit file /etc/group, add line, below
lusers:x:701:ftpguy
6. cd /home
7.
chown ftpguy.lusers ftpguy
this directory is created by adduser command
8.
cd ftpguy; mkdir etc bin ; chown root.daemon etc bin
this creates a directory tree for chroot
9.
chmod 111 etc bin
this sets very conservative permissions on directories within the
chrooted tree
10.
cp ~/static_ls /home/ftpguy/bin/ls
obtaining static (not calling any libraries) version of /bin/ls: this
directory (
http://www.stanford.edu/group/itss-ccs/security/bina�
ries/linux/redhat/) contains static version of many RH 6.x/7.x-compat�
ible utilities, including ls (local copy is
http://www.chu�
vakin.org/ispdoc/ls.gz here, gunzip ls.gz to run)
11.
cd bin ; chown root.bin ls
12.
chmod 111 ls
this sets very conservative permissions on binaries within chroot
13.
cd ../etc
14.
Create file /home/ftpguy/etc/passwd as follows
root:*:0:0::/:/etc/ftponly
ftpguy:*:505:701::/home/ftpguy/./:/etc/ftponly
15.
Create file /home/ftpguy/etc/group, contents follow
root::0:root
lusers::701:ftpguy
16.
chown root.daemon passwd group
this sets proper ownership of these files
17.
chmod 444 passwd group
this sets minimum necessary permission on that file
18.
cd ~ftpguy; touch .forward
this creates .forward file
19.
chown root.root .forward ; chmod 400 .forward
and locks it for security reasons
20.
cd /etc
21.
Add the facilities for handling guest users into /etc/ftpaccess
#=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
class anonftp guest,anonymous *
delete no anonymous,guest # delete permission?
overwrite no anonymous,guest # overwrite permission?
rename no anonymous,guest # rename permission?
chmod no anonymous,guest # chmod permission?
umask no anonymous,guest # umask permission?
guestgroup lusers
limit remote 10 Any /toomany.msg
upload /home/ftp * no
readme README* login
readme README* cwd=*
message /welcome.msg login
message .message cwd=*
alias inc: /incoming
cdpath /incoming
cdpath /pub
cdpath /
path-filter anonymous /etc/pathmsg ^[-A-Za-z0-9_\.]*$ ^\. ^-
noretrieve .notar
upload /home/ftp /incoming yes root wheel 0400 nodirs
noretrieve /home/ftp/incoming
Lets test this beast:
localhost[anton]#1008: ftp localhost
Connected to anton.
220 anton FTP server (Version wu-2.6.1(1) Mon Feb 28 10:30:36 EST 2000) ready.
Name (localhost:anton): ftpguy
331 Password required for ftpguy.
Password:
230 User ftpguy logged in. Access restrictions apply.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls -la
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 4
drwx------ 4 505 701 1024 Apr 8 02:16 .
drwx------ 4 505 701 1024 Apr 8 02:16 ..
-r-------- 1 0 0 0 Apr 8 02:16 .forward
d--x--x--x 2 0 2 1024 Apr 8 02:09 bin
d--x--x--x 2 0 2 1024 Apr 8 02:15 etc
226 Transfer complete.
ftp> mkdir TEST
257 "/TEST" new directory created.
ftp> ls -l
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 3
-r-------- 1 0 0 0 Apr 8 02:16 .forward
drwxr-xr-x 2 505 701 1024 Apr 8 02:32 TEST
d--x--x--x 2 0 2 1024 Apr 8 02:09 bin
d--x--x--x 2 0 2 1024 Apr 8 02:15 etc
226 Transfer complete.
ftp>
and so on.
TO TEST THE CHANGES
Do: access the ftp server using ftp client
ftp www.you.com
Should get: ftp daemon should respond with prompt and version number!
4.12. Configure dialin
Now the fun part starts. We want the machine to allow dial-in access
via attached (inserted?) modem or modems. It will provide either
regular shell or restricted shell (that only executes pppd daemon).
Windows 95/98 users should be able to effortlessly dial in using all
default settings of their computers.
4.12.1. Linux setup
To handle login via serial line some version of getty program is
needed. This program monitors the serial line (/dev/ttyS1 will be
used throughout the document, see serial HOWTO for details) and upon
connection shown the login prompt or starts a program.
I suggest using the mgetty program (as it has more features and is
easier to setup than some of the competitors).
RH comes with mgetty-1.1.21-2, that also has extensions to receive
faxes and voice mail (if the modem supports this). Check whether
mgetty is installed by doing: rpm -qa | grep mgetty.
After installing mgetty some reconfiguration is necessary. The files
that should be changed and the details follow:
1. /etc/inittab
That enables mgetty to start when system is booted and be respawned
accordingly. These lines should be added in the end.
#for dialins use mgetty
#note this S1 in the beginning of the line and ttyS1 in the end
S1:2345:respawn:/sbin/mgetty ttyS1
2. /etc/ppp/options
This file controls the pppd daemon whenever it is started. Some of
the options here are optional (hey, that why they are called
options, right?).
auth -chap +pap login modem crtscts debug proxyarp lock
ms-dns 111.222.333.444
Here is their brief meaning:
� auth : use some sort of authentication for dialin clients
� -chap: not CHAP
� +pap: use PAP
� login : use the system password file for authenticating the
client using PAP and record the user in the system wtmp file,
/etc/ppp/pap-secrets should still be present (see below)
� modem : use the modem control lines (for carrier detection and
other stuff)
� crtscts : use hardware flow control
� debug : log extra info (might be removed after everything is fine)
� proxyarp : this is needed to connect from the client to the
Internet, not just to the LAN you dialed into
� lock: pppd should create a lock file for the serial device
� ms-dns 111.222.333.444: this info is provided to Windows box as a
default DNS server
Look at pppd man page for all the juicy details (parts of the above
info is adapted from there)
Another note is appropriate here. Some people reported that they
had more success with +chap -pap in authenticating both Windows and
Linux dial-up clients. If you are having problems, try changing
/etc/ppp/options to have +chap -pap. In this case the new file
/etc/ppp/chap-secrets should be created (same contents as
recommended /etc/ppp/pap-secrets).
Some other people reported that having default line from
/etc/mgetty+sendfax/login.config works fine. I am very happy to
hear that, and I never claimed that my way to set things up is the
only true way.
3. /etc/ppp/options.ttyS1
This file serves purpose similar to the previous one, but only
applies to particular modem line. It specifies the IP address given
to the remote machine (dynamic, in some sense, if you have more
than one line) and the local IP as well.
111.222.333.444:111.222.333.888
4. /etc/mgetty+sendfax/login.config
This file is the main mgetty control file. Mgetty is Windows-PPP-
aware, so it has provisions to start pppd automatically upon
receiving connect from the Windows machine.
These lines should be present:
/AutoPPP/ - - /usr/sbin/pppd
Before adding them, check that some other version of similar command
is absent there (commented out by default).
5. /etc/ppp/pap-secrets
This is similar to /etc/password file, but only used for dialins
and contains plain text passwords (apparently, only visible to
root). All users that you want to be able to dialin must have their
usernames and password listed in this file. They should enter the
same username and password into Windows Dial Up Networking
configuration.
# Secrets for authentication using PAP
# these two users below can use dialin
# client server secret pword remote IP addresses
dialinuser1 * b1ab1a!? 111.222.333.888
dialinuser2 * p8sSw0rD 111.222.333.888
Check that mgetty is running by looking for similar line in the output
of ps ax command.
4625 ? S 0:00 /sbin/mgetty ttyS1
Now this machine will allow modem calls from any Windows 95/98 box.
As was noted by one of the readers some steps are to be taken to
prevent users from sharing their dialin password with others. A simple
perl/shell script will do the job by killing and logging connections
that use the same username.
Also, if it is desirable to prevent users from using dialing in their
usernames should not be put into /etc/ppp/pap-secrets .
4.12.2. Windows setup
This is really straightforward.
1. Click on My Computer
2. Click on Dial Up networking
3. Click on Make New Connection
4. Proceed according to directions, enter the phone number etc
5. After a new connection is created click on it and enter the
username and password (same as mentioned in /etc/passwd and
/etc/ppp/pap-secrets)
6. Click Connect and it should work (it did in my case ;-) )
TO TEST THE CHANGES
Do: try to dial in using terminal program (UNIX:minicom
/Windows:terminal or other)
Should get: Mgetty should respond with prompt and you Linux
distribution version!
4.13. Open access
Now, after testing all the services, we are ready to open the access
to this machine. The main access control facility in our case is TCP
wrappers (tcpd). In case of RH 7 the xinetd will check the same access
control files itself without any need to wrap services with
/usr/sbin/tcpd. These facilities are controlled by 2 files
/etc/hosts.allow and /etc/hosts.deny, as was mentioned in the sections
devoted to various network services. TCP wrappers configuration can be
done in 2 distinct ways and we will employ the simplest.
Let our /etc/hosts.deny contain ALL:ALL clause, thus denying the
access to all services (started from /etc/inetd.conf ) for all hosts
and all users on them. Now we can allow what we need explicitly in
/etc/hosts.allow, thus following the philosophy "what is not expressly
allowed is denied".
Lets assume we want to allow people to read and send email, we want
some trusted hosts to update contents of the web pages and we want
admin workstation to have full access. So we arrive at the following
/etc/hosts.allow:
#
# hosts.allow This file describes the names of the hosts which are
# allowed to use the local INET services, as decided
# by the '/usr/sbin/tcpd' server.
#
ALL: 127.0.0.1 adminbox.some.net
#we rely on anti-relaying features of sendmail 8.9+ to fight spam
#and also restrict some sites that we don't want to see email from
sendmail: ALL EXCEPT .kr .cn
popper: .com .edu .gov .mil
#these people can upload/download stuff, make it restrictive to avoid warez!
in.ftpd: .this.net .that.net
5. Conclusion
There must be the conclusion, right?
6. References
Useful LDP HOWTOs (well, actually, all others are useful too)
1. Setting Up Your New Domain Mini-HOWTO., really good guide of DNS
setup and general network setup (recommended reading)
2. Linux WWW HOWTO, provides more details on Apache setup, including
virtual hosting
3. Red Hat Linux 6.X as an Internet Gateway for a Home Network, some
hints on network setup
4. IP Aliasing On A Linux Machine, used for multiple IP on the same
interface
5. Ethernet HOWTO, look here in case of network card trouble
6. IPCHAINS HOWTO, turn to this if more security is desired
7. Serial HOWTO, serial ports, lines, modems and related stuff
8. PPP HOWTO,some notes on PPP server setup
Software (used or mentioned) websites
1. Eudora POP3 server
2. WU-FPTD ftp server
3. Sendmail MTA
4. Mgetty pages
5. Apache httpd server
Other documents
1. Armoring Linux
2. Setting Up POP/PPP server
3. Mgetty and Windows dialin info
4. Using RedHat 5.1 to Start an ISP, the short article on how to start
an ISP if all you have is a Linux RH ;-)
5. Guest FTP server setup
6. Linux Dialin Server Setup Guide Yet Another Guide about that
7. virtfs a nice automatic tool for configuring virtual services based
on Perl script
8. Linux Public Access HOWTOan old and not updated for 5 years
document describing Linux-based ISP, some nice hints on equipment
(serial boards) and performance
9. Sinlge IP virtual hosting, nice doc describing how to host
everything on a single IP.
Resources, not related to the topic of the document ;-)
1. I also maintain a list of computer/network security related books
with (where available) reviews and online availability. It is
posted at
http://www.chuvakin.org/books. If you have a book that I
don't list please use the form on the page and I will add it to the
list and maybe review it later.
2. Public Browser Station HOWTO, my mini-HOWTO on web-access terminal
based on RedHat Linux
3. Access the Web Anywhere, my article in Linux Journal about Internet
Kiosks
