Routing with a multihomed Mac

Hi,

I have a Mac running Leopard 10.5.4 with two network cards in it, and I'm trying to get it to act as a router.

The setup is very simple:

A ---- M ---- B

M is the Mac with two nics.
The nic (en0) on computer M connected to computer A has an IP of 10.0.0.1/24.
Computer A has an IP of 10.0.0.2 and a gateway of 10.0.0.1

The nic (en1) on computer M connected to computer B has an IP of 192.168.0.1/24.
Computer B's IP is 192.168.0.2 with a gateway of 192.168.0.1

Both computer A and B can ping computer M on the appropriate interface, but I cannot get computers A and B to ping each other. It sounds to me like computer M simply isn't forwarding the packets across its interfaces.

The routing table on computer M shows entries for both subnets on the appropriate en0/en1 interface.

There are no firewall rules at all on any of the machines (so nothing is blocking the ping).

Is there something I need to do to tell Leopard to forward traffic across its two network cards?

For the record: there is no NAT going here at all, and this has nothing to do with internet connection sharing. This is an isolated little network with only three machines; no net connection, no NAT, no nothing. All I want is for machine M to do some routing for me 🙂

Can anyone help?

Thanks in advance.

Hamster.