== How Linux names network interfaces

The usual explanation of how network interfaces get their names is
that they're named based on what's in _/etc/modprobe.conf_
(_/etc/modules.conf_ if you're still running a 2.4 series kernel).
You'll have lines like:

>  alias eth0 sk98lin
>  alias eth1 3c59x
>  alias eth2 e100

And indeed this usually works, and when your system boots nicely the
3Com 'Marvell' gigabit card is _eth0_, the 3Com 'Cyclone' 100mbit card
is _eth1_, and the Intel 100mbit card is _eth2_. Then one day you boot
your system single-user without network interfaces enabled and decide
to bring up just _eth1_, so you do '_ifup eth1_' (or just '_modprobe
eth1_'). And the world explodes in your face, because the nice simple
explanation is a white lie.

How the Linux kernel really names network interfaces is that ~~network
interfaces are named in the order they're found~~: the first network
interface found is _eth0_, the second is _eth1_, and so on. The names
in the 'alias' lines in _/etc/modprobe.conf_ don't get passed to the
kernel; they just help make the normal configuration more generic.

So when you do '_modprobe eth1_' on a bare system _modprobe_ maps
'load eth1' to 'load the 3c59x driver', the driver finds a card with a
single network interface, and as the first found it becomes
_eth0_. Kaboom. (Similar things can happen if you remove a network
card.)

If you want stable names, the kernel does support renaming network
interfaces; the usual approach is to name interfaces based on MAC
addresses. On modern kernels you can use _udev_ rules (see
[[here|http://www.reactivated.net/writing_udev_rules.html#example-iface]]);
there's also _nameif_, and your distribution may already support it
with the right magic settings.

(On Red Hat and Fedora Core, I believe that you set _HWADDR_ in the
_ifcfg-[[*|]]_ file in _/etc/sysconfig/network-scripts_. I'm not sure
what Debian uses.)

The kernel uses registration order based naming for other things too;
the most famous (or infamous) case is SCSI drives, where the first one
found is _sda_, the second one _sdb_, etc. (The kernel treats SATA
drives as SCSI devices, so more and more people may have to care about
this one.)