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4. Client

Now we examine the client end. In practice, when used to allow access to a remote network, this box can easily serve as a Samba (Windows Networking) server, DHCP server, and even an internal web server. The important thing to remember is that this box should be as secure as possible, as it runs your whole remote network.

4.1 The Kernel

First things first, you must have ppp available in your kernel. If you are going to allow multiple machines to use the tunnel, then you need to have firewalling and forwarding available too. If the client is going to be a single machine, ppp is enough.

4.2 Bring up the link

The link is created by running pppd through a pseudo terminal that is created by pty-redir and connected to ssh. This is done with something similar to the following sequence of commands: 

# /usr/sbin/pty-redir /usr/bin/ssh -t -e none -o 'Batchmode yes' -c blowfish -i /root/.ssh/identity.vpn -l joe > /tmp/vpn-device
# sleep 10

# /usr/sbin/pppd `cat /tmp/vpn-device`
# sleep 15

# /sbin/route add -net 172.16.0.0 gw vpn-internal.mycompany.com netmask 255.240.0.0
# /sbin/route add -net 192.168.0.0 gw vpn-internal.mycompany.com netmask 255.255.0.0

Simply, what this does is run ssh, redirecting it's input and output to pppd. The options passed to ssh configure it to run without escape characters (-e), using the blowfish crypto algorithm (-c), using the identity file specified (-i), in terminal mode (-t), with the options 'Batchmode yes' (-o). The sleep commands are used to space out the executions of the commands so that each can complete their startup before the next is run.

4.3 scripting

Of course you don't want to have to type those commands in every time that you want to get the tunnel running. I've written a set of bash scripts that keep the tunnel up and running. You can download the package from here. Just download that an uncompress it into /usr/local/vpn. Inside you'll find three files:

You'll need to edit the vpnd script to set things like the client's username and the server's names. You may also need to modify the starttunnel section of the script to specify which networks you are using. Below is a copy of the script for your reading enjoyment. You'll note that you could put the script in a different directory, you just need to change the VPN_DIR variable. 

#! /bin/bash
#
# vpnd: Monitor the tunnel, bring it up and down as necessary
#

USERNAME=vpn-username
IDENTITY=/root/.ssh/identity.vpn

VPN_DIR=/usr/local/vpn
LOCK_DIR=/var/run
VPN_EXTERNAL=vpn.mycompany.com
VPN_INTERNAL=vpn-internal.mycompany.com
PTY_REDIR=${VPN_DIR}/pty-redir
SSH=${VPN_DIR}/${VPN_EXTERNAL}
PPPD=/usr/sbin/pppd
ROUTE=/sbin/route
CRYPTO=blowfish
PPP_OPTIONS="noipdefault ipcp-accept-local ipcp-accept-remote local noauth nocrtscts lock nodefaultroute"
ORIG_SSH=/usr/bin/ssh


starttunnel () {
   $PTY_REDIR $SSH -t -e none -o 'Batchmode yes' -c $CRYPTO -i $IDENTITY -l $USERNAME > /tmp/vpn-device
   sleep 15

   $PPPD `cat /tmp/vpn-device` $PPP_OPTIONS
   sleep 15

   # Add routes (modify these lines as necessary)
   /sbin/route add -net 10.0.0.0 gw $VPN_INTERNAL netmask 255.0.0.0
   /sbin/route add -net 172.16.0.0 gw $VPN_INTERNAL netmask 255.240.0.0
   /sbin/route add -net 192.168.0.0 gw $VPN_INTERNAL netmask 255.255.0.0
}

stoptunnel () {
   kill `ps ax | grep $SSH | grep -v grep | awk '{print $1}'`
}

resettunnel () {
   echo "reseting tunnel."
   date >> ${VPN_DIR}/restart.log
   eval stoptunnel
   sleep 5
   eval starttunnel
}

checktunnel () {
   ping -c 4 $VPN_EXTERNAL 2>/dev/null 1>/dev/null

   if [ $? -eq 0 ]; then
      ping -c 4 $VPN_INTERNAL 2>/dev/null 1>/dev/null
      if [ $? -ne 0 ]; then
         eval resettunnel
      fi
   fi
}

settraps () {
   trap "eval stoptunnel; exit 0" INT TERM
   trap "eval resettunnel" HUP
   trap "eval checktunnel" USR1
}

runchecks () {
   if [ -f ${LOCK_DIR}/tunnel.pid ]; then
      OLD_PID=`cat ${LOCK_DIR}/vpnd.pid`
      if [ -d /proc/${OLD_PID} ]; then
         echo "vpnd is already running on process ${OLD_PID}."
         exit 1
      else
         echo "removing stale pid file."
         rm -rf ${LOCK_DIR}/vpnd.pid
         echo $$ > ${LOCK_DIR}/vpnd.pid
         echo "checking tunnel state."
         eval checktunnel
      fi
   else
      echo $$ > ${LOCK_DIR}/vpnd.pid
      eval starttunnel
   fi
}

case $1 in
    check)  if [ -d /proc/`cat ${LOCK_DIR}/vpnd.pid` ]; then
               kill -USR1 `cat ${LOCK_DIR}/vpnd.pid`
               exit 0
            else
               echo "vpnd is not running."
               exit 1
            fi ;;

    reset)  if [ -d /proc/`cat ${LOCK_DIR}/vpnd.pid` ]; then
               kill -HUP `cat ${LOCK_DIR}/vpnd.pid`
               exit 0
            else
               echo "vpnd is not running."
               exit 1
            fi ;;

   --help | -h)
            echo "Usage: vpnd [ check | reset ]"
            echo "Options:"
            echo "     check    Sends running vpnd a USR1 signal, telling it to check"
            echo "              the tunnel state, and restart if neccesary."
            echo "     reset    Sends running vpnd a HUP signal, telling it to reset"
            echo "              it's tunnel connection." ;; 
esac

ln -sf $ORIG_SSH $SSH
settraps
runchecks

while true; do
   i=0
   while [ $i -lt 600 ]; do
      i=((i+1))
      sleep 1
   done
   eval checktunnel
done

4.4 LRP - Linux Router Project

I actually run this setup on pentium 90's running the LRP distribution of Linux. LRP is a distribution of Linux that fits in, and boots off of a single floppy disk. You can learn more about it at http://www.linuxrouter.org/ You can download my LRP package for the VPN client from here. You will also need both the ppp and ssh packages from the LRP site.

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