Routing specific traffic to the VPN on OS X

I have a client that requires me to use a VPN when connecting to their servers. I use OS X’s built in L2TP VPN to connect, but don’t want all my traffic going that way.

To do this, I unchecked the Advanced VPN setting “Send all traffic over VPN connection” in the Network preferences and then created the file /etc/ppp/ip-up like this:

sudo touch /etc/ppp/ip-up
sudo chmod 755 /etc/ppp/ip-up

The file itself is a bash script that runs various /sbin/route commands and looks similar to this:

/etc/ppp/ip-up:

#!/bin/sh
/sbin/route add -net 192.168.1.0/16 -interface ppp0

Now, whenever I connect to the VPN, only traffic for hosts on 192.168.1.x is sent to the client’s VPN and we’re both happy.