Two NICs - swapped initialization

[Kevin Brown]

> Hoping someone knows this one, to save me what would probably turn into a
> couple of days of research to satisfy my curiosity :)
> 
> I have been running Gentoo for a while, and this machine is my router and
> firewall. It has two ethernet controllers in it.  When I first installed, it
> called the controller I wanted to be eth0, eth1 and vice versa (I know it
> doesn't really matter, but it's a personal idiosyncracy).  I placed the names
> for the modules in the /etc/modules.autoload.d/kernel-2.6 file, with the one
> I wanted to be eth0 first, and that got them in my preferred order so I
> assumed that the first loaded module would be eth0.
> 
> Well after an update the other day, the two swapped places and I wasn't able
> to get them back, so finally just left them.  It wasn't all bad, I discovered
> a bug in my firewall setup script when it failed, but I'm curious what
> decides which controller is which, and how it could change on it's own.

The order of the naming of the devices is based on the order they are 
discovered on the PCI bus.  It is possible that an update to the kernel 
resulted in a reversal of the discovery order and so reverse the names. 
  Back when I had a small linux router (P166 box) I had put entries in 
modprobe.conf or conf.modprobe (been so long) that dictated the naming 
of each device based on the MAC address to correct for this issue. 
Another way might be to specify which NIC is which in the config files.

This is a RH specific link, but might help you figure out how to do 
things in Gentoo.
http://linux.dell.com/files/whitepapers/nic-enum-whitepaper-v2.pdf