When you have multiple network interfaces on a linux box, you can link them all together using a network bridge. This connects them all at the data-link level (OSI layer 2). To do this, use the brctl command.

brctl addbr br0
brctl addif eth0
brctl addif eth1
brctl addif eth2

This turns your linux box essentially into a hub. I don’t know how smart linux routes different packets – it’d be great if linux keeps some sort of MAC routing table.

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