Set Default Network Route

Most servers sit behind your company firewall, reachable only through NAT and port forwarding or protected from the outside world entirely. If you've got a machine that needs full access to both the internet and your local network, however, getting both interfaces up and running can seem like a crapshoot. New servers will usually work fine, while those configured on a second network later on will often fail. XServes and Mac Pros come with two ethernet ports, so you'd figure setting them up on two separate networks wouldn't be much of a challenge. And it isn't, if you know the trick.

Unix operating systems (including Mac OS X) can only have one "default route" at a time, the path of last resort for data headed outside your local network. Mac OS X uses whatever you've configured on "Ethernet 1" in the "Network" pane of System Preferences as the default route. That's usually the internal IP on your firewall, proxy server, or router. If you later configure an external IP as "Ethernet 2", your data won't be routed properly and the machine won't respond on the outside interface.

The trick to getting your Mac routing both networks is to set up "Ethernet 1" as your external (or WAN) interface, using the information provided to you by your internet service provider. Mac OS X will then set the router setting from this connection as the default route for the machine. If, for some reason, you have to use your internal interface as "Ethernet 1", remove the IP from the "Router" field. This will force the machine to use the router information from "Ethernet 2" as the default.

Special Thanks: This tip came in a crunch from our colleague Jared Reimer, founder of Cascadeo Corporation, an excellent Seattle-based network consultancy.