Na minha máquina é
/etc/udev/rules.d/70-persistent-net.rules
Esta é uma máquina squeeze Debian, mas é provavelmente similar para outras distribuições Linux. O meu parece
# This file was automatically generated by the /lib/udev/write_net_rules
# program, probably run by the persistent-net-generator.rules rules file.
#
# You can modify it, as long as you keep each rule on a single line.
# MAC addresses must be written in lowercase.
# Firewire device 00e081000026d042 (ohci1394)
SUBSYSTEM=="net", DRIVERS=="?*", ATTR{address}=="00:e0:81:00:00:26:d0:42", NAME="eth0"
# PCI device 0x10de:0x0057 (forcedeth)
SUBSYSTEM=="net", DRIVERS=="?*", ATTR{address}=="00:e0:81:70:18:22", NAME="eth1"
Dica: fazendo
/etc# grep -r eth0 * | less
lhe dará a resposta em alguns minutos, provavelmente. Foi o que eu fiz.