Multihomed Ethernet Interfaces on Mac OS X server 10.4

I had a Mac OS X 10.4 server with two ethernet interfaces configured so that eth0 connected to my internal network and eth1 connected directly to the Internet.

I could access either interface just fine from my internal network. But unbeknownst to
me, the outside world was not able to access the external interface.

Checking the firewall or even turning the firewall off completely had no effect. The output of ifconfig looked like it should, the cables were plugged in where they ought to be. You just couldn't get to the external interface from the Internet...

It turns out that Mac OS X expects eth0 to connect to the Internet and I had it connected to my internal network. The server could see the Internet via that interface. Because it passed through my restrictive firewall, the outside world could not see it but from my subnet I could see it just fine... a nasty little gotcha.

I swapped the network configuration parameters for that eth0 was the external interface and eth1 was internal and the problem was solved.

Look at the Network control panel under 'Network Status'. Both interfaces will be active in a configuration like mine but only one will state 'You are connected to the Internet via...'. That needs to be the external interface.

There must be some way to configure which interface connects to the Internet but it is easier to follow their convention and not worry about it.