We are trying to put together a box with dual LAN cards (let's say Outer and Inner), where the Inner LAN card is supposed to act as a default gateway on the network it is connected to.

This box is running Ubuntu. The basic purpose for this box is to take messages generated on the inner network, do some work with them and forward them out the Outer LAN card to a server. The inner network is completely isolated with simply a regular switch connecting the Inner LAN Card with two other boxes.

These other boxes either throw out multi-cast messages (which the Inner LAN Card is listening to), or send out unicast messages meant for the server which is not on this inner network. So, we need the Inner LAN Card to act as a default gateway, where these unicast messages will then be sent, and the code on the dual-LAN Card box can then intercept and forward these messages to the server.

Question: 1. How do we setup the LAN Card to be default gateway (does it need some configuration on Ubuntu)? 2. Once we have this setup, is it a simple matter of listening to the interface to intercept the incoming messages?

Any help (pointers in the right direction) is appreciated.

Thanks.