IPSec-VPN on Stock Kernel

Attention: What ever you do, you do it on your own risk

Prerequisite

You have to have firmware 1.34 already installed and opened for telnet access. If not, you can find a guide here: Open Stock Firmware LS-XHL

For Firmwares before 1.34 this also may work, but it's unknown if the IPSEC-XL2TP Packages are also in there.

What's the aim ?

The aim is to realize a VPN-Server that uses L2TP-IPSec as tunneling technology.

Why this, and not PPTP ? The issue with PPTP is, that it needs MPPE support within the kernel, which is simply not there.

Therefore we are going to use IPSec & L2TP, as they are more secury in most scenarios in any way.

What is needed ?

The good news are: everything is already on the box, you don't have to install any external software-package at all.

The bad news: The packages are configured to be used for a service called PocketU (only in Japan). As a matter of fact all boxes outside of Japan are not using those things at all.

As a general guidline for an IPSec-L2TP Server we need: IPSec - Package (here OpenSwan with pluto), an IPSec-Configuration and a tunnel-configuration L2TP - Package (here xl2tp), and xl2tp-Configuration and ppp.xl2tp options

How does it work

The VPN works as follows:

1) An IPSec tunnel will be opened (using a preshared-key or certificates)

2) Within the tunnel L2TP is used to authenticate a user and do IP-adressing with PPP in there

Configuration

Needed files to be touched / modified:

/etc/init.d/xl2tpd.sh

/etc/ipsec.conf

/etc/ipsec.d/l2tp.conf

/etc/ipsec.d/l2tp.secrets

/etc/xl2tpd/xl2tpd.conf

/etc/ppp/options.xl2tpd

As all these files are already there, make sure, you are backing them up.

Configuring IPSec: Modify /etc/ipsec.conf to enable NAT-Traversal (NAT-T) and to define private networks:

 # basic configuration
 config setup
    # Debug-logging controls:  "none" for (almost) none, "all" for lots.
    # klipsdebug=none
    # plutodebug=all
    protostack=netkey
    nat_traversal=yes
    virtual_private=%v4:10.0.0.0/8,%v4:172.16.0.0/12,%v4:192.168.0.0/16,%v4:!YOUR.PRIVATE:NETWORK:ADD/24
    oe=off
    # Enable this if you see "failed to find any available worker"
    nhelpers=0
    forwardcontrol=no
 #You may put your configuration (.conf) file in the "/etc/ipsec.d/" and uncomment this.
    include /etc/ipsec.d/l2tp.conf

Based on this we are adding in /etc/ipsec.d/ a file called l2tp.conf to configure our connection:

  conn L2TP-PSK
       #type=tunnel
       left=YOUR.LS.IP.ADDR       #Put the address of your linkstation here
       #leftnexthop=              #Put your (NAT)gateway address here
       leftprotoport=udp/l2tp
       right=%any
       rightsubnet=vhost:%no,%priv
       rightprotoport=udp/l2tp
       auto=add
       authby=secret
       ike=3des-sha1-modp2048,3des-sha1-modp1024
       pfs=no
       rekey=no
       keyingtries=3

Now let's define secrets (preshared-key in this case) for the tunnel in /etc/ipsec.d/l2tp.secrets

  : PSK "PRESHARED-KEY" # you can add addresses before the ":" to define for which addresses this key is used

This should be sufficient for IPSec itself. You could try to start this with /etc/init.d/ipsec start

For L2TP we need to modify the file /etc/xl2tpd/xl2tpd.conf

  [global]
    listen-addr = YOUR.LS:IP.ADDR   #Put your Linkstation-IP Address here
    ;
    ; debug tunnel = yes
    ; debug packet = yes
  [lns default]
    ip range = 192.168.XX.YYY-192.168.XX.ZZZ  # Address-range for clients
    local ip = YOUR.LS:IP.ADDR   #Put your Linkstation-IP Address here
    require chap = yes
    refuse pap = yes
    require authentication = yes
    name = YOUR-VPN-SERVER NAME
    pppoptfile = /etc/ppp/options.xl2tpd
    length bit = yes
    ppp debug = yes

Now let's tune the options for PPP in /etc/ppp/options.xl2tpd

  ipcp-accept-local
  ipcp-accept-remote
  #ms-dns YOUR.DNS.SERVER.ADDR
  #refuse-eap
  require-mschap
  noccp
  #nopcomp
  #noaccomp
  #noauth
  auth
  crtscts
  idle 1800
  mtu 1410
  mru 1410
  defaultroute
  debug
  lock
  connect-delay 5000
  #record /root/pppd.log
  #usepeerdns
  proxyarp

You need to add chap-secrets for L2TP in /etc/ppp/chap-secrets. This can be done as you like (and as something is also there)

Now we need to modify the XL2TPD start-script as this one is specific for pocketU. So generate a new start-script for xl2tpd in /etc/init.d and put there:

  VARRUNDIR=/var/run/xl2tpd
  LOGTAG=xl2tpd.sh
  LOGFACILITY=local0.info
  CreateRunDir()
  {
       if [ -d "${VARRUNDIR}" ] ; then
               return 0;
       fi
       mkdir "${VARRUNDIR}" -p
       return $?
  }
  Start()
  {
       CreateRunDir
       xl2tpd
       logger -s -t ${LOGTAG} -p ${LOGFACILITY} "Started!"
       #sysctl -p    #Add this if you want to enable IP_FORWARDING in sysctl.conf
  }
  Stop()
  {
       killall xl2tpd
       logger -s -t ${LOGTAG} -p ${LOGFACILITY} "Stopped!"
  }
  case $1 in
    start)
       Start
    ;;
    stop)
       Stop
    ;;
    restart)
       Stop
       sleep 1
       Start
    ;;
  *)

OK, you're done. To start the story you need to do the following:

/etc/init.d/ipsec start

/etc/init.d/xl2tpd.sh start

Make sure that the start-script from pocketU didn't cross your way (/etc/rc.d/extensions.d).

What is left to do for you?

On the client-side you have to put in the pre-shared key from l2tp.secrets and UName/PW from /etc/ppp/chap-sectrets.

That should do the job.

Changes

2010.09.08: Initial Version