I’ve been having some issues with my virtual machines running Debian Etch coming up with their primary interface as eth2 or eth3, even though there is only one “physical” (it’s virtual, actually) NIC in the system. Well, another kind soul has had the exact same problem: Primary interface not eth0. Debian caches the MAC address used so that interface numbers are somewhat persistent. The fix? Look at the contents of /etc/udev/rules.d/z25_persistent-net.rules. If you’re too lazy for that, just delete the file. Don’t forget to reboot.