I have two LANS, each with their own OS X Server, that need to talk to each other through a low bandwidth, secure, dedicated connection. There are various ways to do this, but I opted for using a pair of EtherBridges, which are devices that bridge over a dialup modem connection. So, each server has three ethernet cards: one for the Internet (en0), one for the LAN (en1), and one for the bridging connection between the LANs (en2). The problem was how to configure a route such that traffic for the "other" LAN goes through en2. Note that I did not want simply to bridge the two LANs into a single subnetwork. I needed each to be its own distinct subnet, but with traffic routed between them.
After a bunch of research on the problem, I concluded that the OS X GUI configuration package simply doesn't understand about any kind of routing except a single default route. The traditional BSD way to deal with this problem is to insert code into rc.local, but if I added a route command there, it had no effect or a bizarre effect. I experimented with LaunchDaemons and StartupItems, but they also had no effect or the wrong effect.
To make a long story (several days) short, the trick here is that you must use an ifconfig command in the rc.local script, even though at a later point in the process, the GUI-specified interface configuration will be done redundantly. If you do not do this, the route command will fail, because there will be no device configured for the bridging subnet.
Here is the critical portion of the rc.local script:
ifconfig en2 inet 11.22.33.5 netmask 255.255.255.252
route -n add 11.22.33.64/26 111.222.333.6
Note that 11.22.33 represents my Class C network; the "other" LAN subnet is .64/26 off of that, and the bridging subnet is 4/30 (a point-to-point subnet) of of the same Class C. On the other server, the script is:
ifconfig en2 inet 11.22.33.6 netmask 255.255.255.252
route -n add 11.22.33.32/27 111.222.333.5
Note that the LAN subnet is different, the bridging subnet is the same, but the two ports are swapped in terms of direction.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060329085850170