Chip 1998 February/CHIP_2_98.iso/doc/HOWTO/mini/VPN

Text File | 1997-07-14 | 15KB | 506 lines

  The VPN HOWTO
  Arpad Magosanyi <[e-mail]> 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
  is the following: Unless otherwise stated, Linux HOWTO documents are
  copyrighted by their respective authors. 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. In short, we wish to
  promote dissemination of this information through as many channels as
  possible. However, we do wish to retain copyright on the HOWTO
  documents, and would like to be notified of any plans to redistribute
  the HOWTOs. If you have questions, please contact Greg Hankins, the
  Linux HOWTO coordinator, at [e-mail] via email.

  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 ([e-mail])

  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 <[e-mail]>.

  #        Modified for Debian GNU/Linux

  #        by Ian Murdock <[e-mail]>.

  #

  # Version:    @(#)skeleton    1.6  11-Nov-1996  [e-mail]

  #

  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.