Split tunnelling with vpnc the easy way

I recently switched to running a clean Ubuntu Server with i3 as my window manager – this meant no more network connect, and thus I couldn’t simply use the checkbox “Use this connection only for resources on its network”.

So running a barebone vpnc, I finally figured out a simple way to split the traffic, so only traffic intended for the remote network runs through the VPN.

This configuration consists of 3 files, all of which I put in “/home/steffen/Scripts”:
– vpn.sh
– vpn.conf
– vpnc-custom.sh

The two .sh files are executable shell scripts. (if that wasn’t obvious from the naming)

vpn.conf is your standard vpnc configuration file, mine looks like this:
# vpn.conf
IPSec secret SECRET
Xauth username MY_USERNAME
Xauth password MY_PASSWORD

Note: URL_FOR_GATEWAY doesn’t have to be an URL – it can also just be an IP.

Right beside it I made this simple vpn.sh script (which is executable)

# vpn.sh
sudo vpnc vpn.conf --script "/home/steffen/Scripts/vpnc-custom.sh"
sudo route del -net default dev tun0
sudo route add -net netmask dev tun0
sudo route add default gw

This establishes the VPN tunnel, then immediately removes the default route vpnc creates, which would route all traffic through the tunnel. It then adds a custom route for the subnet I need to reach through the tunnel instead. This way only traffic for 192.168.1.X is routed through the tunnel – all other traffic uses my normal default route (i.e. your normal LAN or Internet connection)
And finally I had to add the original default route back in – do note that this may wary from distribution to distribution. I didn’t need this step on Linux Mint for instance, but I do on Ubuntu Server.

Finally there’s the vpnc-custom.sh script, which is a custom instruction for vpnc:
# vpnc-custom.sh

This simply sets the variable INTERNAL_IP4_DNS to nothing, which prevents vpnc from altering your resolv.conf. This is necessary as otherwise all DNS lookups would go through the tunnel, as vpnc automatically gets a new DNS server from the VPN connection.

Lastly it runs the standard vpnc-script, without which the tunnel would not work. Do note that the location of this script varies with distributions – on Ubuntu it’s located as in the code above. If you can’t locate it, you could either do a find / -name vpnc-script or check the man page for vpnc: man vpnc

This ALSO means you need to know the IP of whatever you try to reach through the VPN, as you no longer have access to the remote DNS server. I just put the few hostnames I need into /etc/hosts and called it a day 🙂

And that was all there was to it – I saw quite a few blog posts around the web with much more complicated setups, so while this may not be the “right” way to do it – it’s certainly easier AND works just as well.