The scenario:

iMac with two active network interfaces, Airport & Ethernet

Airport = DHCP, 10.0.0.x address (cable modem gateway)
Ethernet = Static 66.93.x.x address (DSL gateway)

Airport is first in the “Service Order” list and so that interface serves as the default gateway.

Web browsing and other internet traffic which originates on the iMac travels over the Airport and through the cable modem gateway, as expected.

However, pings to the 66.93.x.x address from the outside world are not returned.

By watching tcpdump on both en0 and en1, I can see that these pings are in fact reaching the iMac on en0 (as expected), but the iMac is trying to return them on en1 (which is not expected).

How can I fix this? I am fairly certain that this behavior is new to Leopard. (ie: I am fairly certain that under Tiger, this Mac would properly respond on the interface and network which it was contacted, rather than trying to redirect its responses through the default gateway.)

Help!