The VPN HOWTO
Arpad Magosanyi <mag@bunuel.tii.matav.hu> v0.1, 3 May 1997
1. Blurb
This is the Linux VPN howto, a collection of information on how to set
up a Virtual Protected Network in Linux (and other unices in general).
1.1. Copyright
This document is part of the Linux HOWTO project. The copyright notice
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1.2. Disclaimer
As usual: the author not responsible for any damage. For the correct
wording, see the relevant part of the GNU GPL 0.1.1
1.3. Disclaimer
We are dealing with security: you are not safe if you haven't got good
security policy, and other rather boring things.
1.4. State of this document
This is very preliminary. You should have thorough knowledge of
administrating IP, at least some knowledge of firewalls, ppp and ssh.
You should know them anyway if you want to set up a VPN. I just
decided to write down my experiences not to forget them. There are
possibly some security holes indeed. To be fair I've tried it on hosts
configured as routers not firewalls, saying: It's simple from that
point.
1.5. Related documentations
o The Linux Firewall-HOWTO /usr/doc/HOWTO/Firewall-HOWTO
o The Linux PPP-HOWTO /usr/doc/HOWTO/PPP-HOWTO.gz
o The ssh documentations /usr/doc/ssh/*
o The Linux Network Admins' Guide
o NIST Computer Security Special Publications
http://csrc.ncsl.nist.gov/nistpubs/
o Firewall list (majordomo@greatcircle.com)
2. Introduction
As firewalls are in more and more widely use in internet and intranet
security, the ability to do nice VPNs is important. Here are my
experiences. Comments are welcome.
2.1. Naming conventions
I will use the terms "master firewall" and "slave firewall", though
making a VPN has nothing to do with client-server architecture. I
simply refer to them as the active and passive participants of the
connection's setup. The host which is starts the setup will be
referred as the master, and the passive participant will be the slave.
3. Doing it
3.1. Planning
Before you start to set up your system, you should know the networking
details. I assume you have two firewalls protecting one intranet per
firewall, and they are both connected to the internet. So now you
should have two network interfaces (at least) per firewall. Take a
sheet of paper, write down their IP addresses and network mask. You
will need one more IP adresses per firewall for the VPN you want to do
now. Those addresses should be outside of your existing subnets. I
suggest using addresses from the "private" address ranges. They are
the followings:
o 10.0.0.0 - 10.255.255.255
o 172.16.0.0 - 172.31.255.255
o 192.168.0.0 - 192.168.255.255
For the sake of example, here's a sample configuration: The two
bastions are called fellini and polanski. They have one interface for
the internet (-out), one for the intranet (-in), and one for the vpn
(-vpn). The addresses and netmasks:
o fellini-out: 193.6.34.12 255.255.255.0
o fellini-in: 193.6.35.12 255.255.255.0
o fellini-vpn: 192.168.0.1 point-to-point
o polanski-out: 193.6.36.12 255.255.255.0
o polanski-in: 193.6.37.12 255.255.255.0
o polanski-vpn: 192.168.0.2 point-to-point
So we have the plan.
3.2. Gathering the tools
You will need a
o Linux firewall
o kernel
o very minimal configuration
o ipfwadm
o fwtk
o Tools for the VPN
o ssh
o pppd
o sudo
o pty-redir
Current versions:
o kernel: 2.0.30. Use a stable kernel, and it must be newer than
2.0.20, because the ping'o'death bug.
o base system: I prefer Debian. YMMV. You absolutely don't want to
use any big packages, and you never even tought of using sendmail,
of course. You also definitely don't want to enable telnet, ftp,
and the 'r' commands (as usual in case of any other unix hosts).
o ipfwadm: I've used 2.3.0 fwtk: I've used 1.3
o ssh: >= 1.2.20. There are problems with the underlying protocol in
the older versions.
o pppd: I've used 2.2.0f for the tests, but I'm not sure if is it
secure, this is why I turned the setuid bit off, and used sudo to
launch it.
o sudo: 1.5.2 the newest I am aware of
o pty-redir: It is written by me. Try
ftp://ftp.vein.hu/ssa/contrib/mag/pty-redir-0.1.tar.gz. Its version
number is 0.1 now. Tell me it there is any problem with it.
3.3. Compile and install
Compile or otherwise install the gathered tools. Look at every one's
documentation (and the firewall-howto) for details. Now we have the
tools.
3.4. Configure the other subsystems
Configure your firewall rules, etc. You need to enable ssh traffic
between the two firewll hosts. It means a connection to port 22 on the
slave from the master. Start sshd on the slave and verify if you can
login. This step is untested, please tell me your results.
3.5. Set up the accounts for the VPN
Create an account on the slave firewall use your favourite tool (e.g.
vi, mkdir, chown, chmod) you might create an account on the master
also, but I think you want to set up the connection at boot time, so
your ordinary root account will do. Can anyone point out risks on
using the root account on the master?
3.6. Generate an ssh key for your master account
Use the ssh-keygen program. Set empty password for the private key if
you want to do automatic setup of the VPN.
3.7. Set up automatic ssh login for the slave account
Copy the newly generated public key in the slave account under
.ssh/authorized_keys, and set up file permissions like the following:
drwx------ 2 slave slave 1024 Apr 7 23:49 ./
drwx------ 4 slave slave 1024 Apr 24 14:05 ../
-rwx------ 1 slave slave 328 Apr 7 03:04 authorized_keys
-rw------- 1 slave slave 660 Apr 14 15:23 known_hosts
-rw------- 1 slave slave 512 Apr 21 10:03 random_seed
The first row being ~slave/.ssh, and the second is ~slave.
3.8. Tighten ssh security on the bastions.
It means the followings on my setup in sshd_conf:
PermitRootLogin no
IgnoreRhosts yes
StrictModes yes
QuietMode no
FascistLogging yes
KeepAlive yes
RhostsAuthentication no
RhostsRSAAuthentication no
RSAAuthentication yes
PasswordAuthentication no
PermitEmptyPasswords no
Password authentication is turned off, so login is only possible with
authorized keys. (You've turned off telnet and the 'r' commands of
course).
3.9. Enable execution of ppp and route for both accounts.
As the master account is the root in my case, it has nothing to do.
For the slave account, the following lines appear in /etc/sudoers:
Cmnd_Alias VPN=/usr/sbin/pppd,/usr/local/vpn/route
slave ALL=NOPASSWD: VPN
As you can see, I am using some scripts to set up ppp and the routing
tables on the slave host.
3.10. Do the scripting
On the master host there is a full-blown init script I am using:
#! /bin/sh
# skeleton example file to build /etc/init.d/ scripts.
# This file should be used to construct scripts for /etc/init.d.
#
# Written by Miquel van Smoorenburg <miquels@cistron.nl>.
# Modified for Debian GNU/Linux
# by Ian Murdock <imurdock@gnu.ai.mit.edu>.
#
# Version: @(#)skeleton 1.6 11-Nov-1996 miquels@cistron.nl
#
PATH=/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/bin/X11/:
PPPAPP=/home/slave/ppp
ROUTEAPP=/home/slave/route
PPPD=/usr/sbin/pppd
NAME=VPN
REDIR=/usr/local/bin/pty-redir
SSH=/usr/bin/ssh
MYPPPIP=192.168.0.1
TARGETIP=192.168.0.2
TARGETNET=193.6.37.0
MYNET=193.6.35.0
SLAVEWALL=polanski-out
SLAVEACC=slave
test -f $PPPD || exit 0
set -e
case "$1" in
start)
echo setting up vpn
$REDIR $SSH -t -l $SLAVEACC $SLAVEWALL sudo $PPPAPP >/tmp/device
TTYNAME=`cat /tmp/device`
echo tty is $TTYNAME
sleep 10s
if [ ! -z $TTYNAME ]
then
$PPPD $TTYNAME ${MYPPPIP}:${TARGETIP}
else
echo FAILED!
logger "vpn setup failed"
fi
sleep 5s
route add -net $TARGETNET gw $TARGETIP
ssh -l $SLAVEACC $SLAVEWALL sudo $ROUTEAPP
;;
stop)
ps -ax | grep "ssh -t -l $SLAVEACC " | grep -v grep | awk '{print $1}' | xargs kill
;;
*)
# echo "Usage: /etc/init.d/$NAME {start|stop|reload}"
echo "Usage: /etc/init.d/$NAME {start|stop}"
exit 1
;;
esac
exit 0
The slave uses one script for routing setup (/usr/local/vpn/route):
#!/bin/bash
/sbin/route add -net 193.6.35.0 gw 192.168.0.1
and its .ppprc consists of the following:
passive
4. Look at what's happening:
The master logs in into the slave, starts pppd, and redirects this all
thing into a local pty. It consists of the following steps:
o allocating a new pty
o sshing into the slave
o running pppd on the slave
o the master runs pppd in this local pty
o and sets up the routing table on the client.
There are (not very tight) timing considerations involved, this is why
that 'sleep 10s'.
5. Doing it by hand.
5.1. Logging in
You've already tried if ssh works well, aren't you? If the slave
refuses to log you in, read the logs. Perhaps there are problems with
file permissions or the sshd setup.
5.2. Firing up ppp
Log in into slave, and issue:
sudo /usr/sbin/pppd passive
You should see garbage coming at this point. If it works good, if not,
there is some problem either with sudo, either with pppd. Look what
the commands had said, and at the logs and at the /etc/ppp/options,
and the .ppprc file. If it works, write this 'passive' word into
.ppprc, and try again. To get rid off the garbage and continue
working, press enter,'~' and '^Z'. You should have the master's
prompt now, and kill %1. See the section about tuning if you want to
know more of the escape character.
5.3. Together the two
Well, then
ssh -l slave polanski sudo /usr/sbin/pppd
should work also, and deliver the garbage right into your face.
5.4. Pty redirecting
Try to redirect this whole thing this time:
/usr/local/bin/pty-redir /usr/bin/ssh -l slave polanski sudo /usr/sbin/pppd
Nice long sentence isn't it? You should use the full path into the
ssh executable, as the pty-redir program allows only this form for
security reasons. Now you've got a device name from the program.
Let's say, you've got /dev/ttyp0 You can use the ps command to look
what has happened. Look for 'p0'
5.5. Is anything on the device?
Try
/usr/sbin/pppd /dev/ttyp0 local 192.168.0.1:192.168.0.2
to establish the connection. Look at the output of the ifconfig
command to see if the device has established, and use ping to check
your virtual net.
5.6. Setting up the routes
Set up the routes on the master host, and on the slave also. Now you
should be able to ping one host in one intranet from other host in the
other intranet. Set up the additional firewalling rules. Now as you
have the VPN, you can set up the rules concerning the connectivity of
the two intranets.
6. Tuning
6.1. Configuration tuning
As I said this HOWTO is mainly a quick memo on how I had set up a VPN.
There are things in the configuration I didn't experiment yet. These
things will go into their place when I try them, or anyone tells me
"it works in the following way" The most important thing is that the
connection ppp uses is not 8-bit yet. I believe it has something to do
either with ssh configuration or the pty setup. In this configuration
ssh uses the tilde (~) character as an escape character. It might stop
or slow down the communication, as any newline-tilde sequence causes
ssh to give a prompt. Ssh documentation said: <On most systems,
setting the escape character to ``none'' will also make the session
transparent even if a tty is used.> The corresponding flag to ssh is
'-e', and you can also set it in the configuration file.
6.2. Bandwith vs. cicles
Creating anything virtual comes with utilization of real-world
resources. A VPN eats up bandwidth and computing resources. The goal
would be to get balance between the two. You can tune it with the '-C'
switch or the 'CompressionLevel' option. You might try using another
cipher, but I don't recommend it. Also note that the round-trip-time
can be longer if you use better compression. Any experiments on it are
welcome.
7. Vulnerability analisis
I try to cover here the vulnerability issues arising from this
particular setup and VPNs in general. Any comments are warmly welcome.
o sudo: Well, I'm excessively using sudo. I believe it's still safer
than using setuid bits. It's still a backdraw of Linux that it
hasn't got more fine-grained access control. Waiting for POSIX.6
compatibility <http://www.xarius.demon.co.uk/software/posix6/>.
What is worse, there are shell scripts which are getting called
through sudo. Bad enough. Any idea out there?
o pppd: It runs suid root also. It can be configured by user's
.ppprc. There might be some nice buffer overruns in it. The bottom
line: secure your slave account as tightly as you can.
o ssh: Beware that ssh older than 1.2.20 has security holes. What is
worse, we made a configuration such when the master account had
been compromised, the slave account is also compromised, and wide
open to attacks using the two sudoed programs. It is because I've
choosen not to have password on the master's secret key to enable
automatic setup of the VPN.
o firewall: With inproperly set firewall rules on one bastion, you
open both of the intranets. I recommend using IP masquerading (as
setting up incorrect routes is a bit less trivial), and doing hard
control on the VPN interfaces.