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After spending almost a day in trying to configuring my dd-wrt router as an OpenVPN client and reading many complicated guides, I found a really simple way to configure OpenVPN. It is necessary that you have optware set up on your router. For setting up Optware, kindly read Optware, The Right Way.

This configuration has been tested with DD-WRT K26 svn 14569 mega build (of Brainslayer) on a WRT610Nv1 router. Configuration of OpenVPN server is beyond the scope of this article. But here is an excellent article on how to setup OpenVPN server in Debian.

1. Install openvpn using ipkg.
   

   # ipkg-opt update
   # ipkg-opt install openvpn
   

2. Dump your vpn configuration file in /opt/etc/openvpn/. Let us assume the name of the configuratoin file is example.vpn.conf. You might have to make certain changes in the configuration file such as:
   • Setting the name of the user and group in the config file to that which are existing on the router. 'nobody' user and 'nobody' group exist on the system so you can use these. If you intend to use the route-down.sh script (see below), you will have to set these to 'root' user and 'root' group. For route-up.sh script though, it is fine if you use 'nobody' user and 'nobody' group because the downgrade of privileges by openvpn client happens after the route-up.sh script is called.
   • Setting the correct paths for key and certificate files in example.vpn.conf.
3. Test if openvpn connection is being established by running the below command.
   

   # /opt/sbin/openvpn --cd /opt/etc/openvpn --config example.vpn.conf
   

   If the connection is being established properly you may proceed to the next step.

4. Set up the scripts for allowing machines behind the dd-wrt router to access machines in the VPN network and for machines in the VPN network to access the dd-wrt router.

   /opt/etc/openvpn/route-up.sh:

   # Enable clients behind the router to access the machines in VPN network
   /usr/sbin/iptables -I POSTROUTING -t nat -o tun0 -j MASQUERADE
   
   # Enable the machines in the VPN network to access the router
   /usr/sbin/iptables -I INPUT -t filter -i tun0 -j ACCEPT
   

   /opt/etc/openvpn/route-down.sh:

   # Disable clients behind the router to access the machines in VPN network
   /usr/sbin/iptables -D POSTROUTING -t nat -o tun0 -j MASQUERADE
   
   # Disable the machines in the VPN network to access the router
   /usr/sbin/iptables -D INPUT -t filter -i tun0 -j ACCEPT
   

   Test the connection again.

   # chmod 755 route-up.sh route-down.sh
   # /opt/sbin/openvpn --cd /opt/etc/openvpn --config example.vpn.conf --script-security 2 system \
   --route-up /opt/etc/openvpn/route-up.sh --down /opt/etc/openvpn/route-down.sh
   

5. In this last step, we enable openvpn client to connect automatically during startup. Edit /opt/etc/init.d/S20openvpn and make the following changes:
   • Comment the statement "return 0" so that we can keep run openvpn as a daemon.
   • Set the proper path for killall command (/usr/bin/killall).
   • Add the line "/opt/sbin/openvpn --daemon --cd /opt/etc/openvpn --config example.vpn.conf --script-security 2 system --route-up /opt/etc/openvpn/route-up.sh --down /opt/etc/openvpn/route-down.sh" at the bottom.

   /opt/etc/init.d/S20openvpn:

   #!/bin/sh
   #
   # Startup script for openvpn as standalone server
   #
   
   # Make sure IP forwarding is enabled
   echo 1 > /proc/sys/net/ipv4/ip_forward
   
   # Make device if not present (not devfs)
   if ( [ ! -c /dev/net/tun ] ) then
     # Make /dev/net directory if needed
     if ( [ ! -d /dev/net ] ) then
           mkdir -m 755 /dev/net
     fi
     mknod /dev/net/tun c 10 200
   fi
   
   # Make sure the tunnel driver is loaded
   if ( !(lsmod | grep -q "^tun") ); then
           insmod /opt/lib/modules/tun.o
   fi
   
   # I you want a standalone server (not xinetd), comment out the return statement below
   #return 0
   
   
   ## This is for standalone servers only!!!!
   # Kill old server if still there
   if [ -n "`pidof openvpn`" ]; then
       /usr/bin/killall openvpn 2>/dev/null
   fi
   
   # Start afresh - add as many daemons as you want
   #/opt/sbin/openvpn --daemon --cd /opt/etc/openvpn --config openvpn.conf
   /opt/sbin/openvpn --daemon --cd /opt/etc/openvpn --config example.vpn.conf --script-security 2 \
                     --route-up /opt/etc/openvpn/route-up.sh --down /opt/etc/openvpn/route-down.sh
   
   # [EOF]
   

   Test the script.

   # chmod u+x /opt/etc/init.d/S20openvpn
   # /opt/etc/init.d/S20openvpn
   

6. Rebooting the router should now get you connected automatically to your VPN network.