Sometimes after a fresh Debian Etch install (I am not sure if this is fixed yet in Lenny or not), the order of your ethernet interfaces will be incorrect. You may also be in a position where you have more then two NIC cards and and you wish to swap eth0 and eth1 with eth2 and eth3, for consistency purposes for example.
In order to do so, you’ll need to make a change to the udev configuration file which controls which interfaces receive which names. You need to edit the following file:
Simply make sure that you match the proposed interface with the correct MAC address and you are all set.
Go ahead and restart the server and you should be all set with the correctly labeled interfaces.