macOS: Split Tunnel L2TP VPN Routing

by Christian Mohn · Read in about 1 min (200 words)

I use my Ubiquiti USG for Remote User VPN Using L2TP, but L2TP does not provide routing information to the client, so I needed a way to automatically create routes when the VPN connection fires. Thankfully, this is pretty easy in macOS (and Linux). The /etc/ppp/ip-up file, if present, triggers every time a PPP (L2TP is based on PPP) connection is made, thus making it easy to trigger a route command when a connection is established.

My /etc/ppp/ip-up looks like this:

/sbin/route add -net <my home network subnet>/24 -interface $1

Replace <my home network subnet>/24 with your network information, and you should be ready to go.

Note: This doesn't diffenciate if you have several L2TP VPN connections, and the script runs regardless of which connection you use. I am sure there are ways of doing different routes based on which connection is triggered, but I haven't had a need to do that so far

Running netstat -rn after making a connection, should now show a line similar to the one below, where is your remote subnet, routed over a ppp interface

~ netstat -rn
Routing tables

Destination        Gateway            Flags        Netif Expire
...          ppp0               USc           ppp0
Post last updated on June 5, 2020: Fix Hugo shortcode insertion.

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Christian Mohn works as a Chief Technologist SDDC for Proact in Norway.

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