Fixing Ubuntu's ethN device names when you swap hardware
If you swap an Ubuntu 6.06 system disk between hardware units (even
nominally identical hardware units, like from one Dell 2950 to another
PE2950), the system will come up scrambled Ethernet devices and you
won't be on the network. (As we found out the hard way today.)
This happens because the only way Ethernet devices get consistent
numbers on Ubuntu machines is because Ubuntu remembers the mapping
between hardware ethernet addresses and the ethN name that they should
get. When you transplant hardware, the hardware addresses change,
nothing matches up any more, and your Ethernet NICs get random names.
(This also means that
eth0 in kernel messages is usually not
the device that
ifconfig eth0 is talking about.)
/etc/iftab has the ethN ↔ hardware ethernet address mapping;
unknown hardware addresses will be given ethN names that are not in use
/etc/network/interfaces tells you what ethN names you are using for
what actual network connections.
Because Ubuntu seems to more or less randomly renumbers ethN devices on
each reboot, you can't just remove
/etc/iftab, fix your
file to use the 'natural' names, and be done. Instead you have to
reconstruct it with the right MAC addresses for any interfaces you care
about. Steps to do this:
- look at
/etc/network/interfaces to find out what ethN names you
- find out what current ethN names are connected to the network:
for i in eth*; do ifconfig $i up; done
dmesg | tail -10
Now look for devices that have reported that their link is up.
If you are lucky, the machine only has one network connection
and you are done.
If the machine has multiple network connections you will need to
use various means (eg, unplugging and replugging network cables and
watching kernel messages) to figure out which ethN name is which
- look up the MAC of each interface you care about with
ethN, and edit
/etc/iftab to update the MAC for the 'correct'
name for that interface. Delete every other ethN name in
just so they don't confuse anyone.
Once you have updated everything you care about, reboot.
Optionally and possibly later, add name/MAC mappings for everything
iftab, so that if you add another network connection in the future
it's sure to have a stable name. If you do this you will want to map out
what each physical port's Ethernet address is, so that you can assign
the names in a consistent and logical way and do things like make sure
that a card with two NICs gets two consecutive ethN names.
For bonus charm
iftab is apparently being deprecated in favour of
another way of doing this, so the next Ubuntu LTS release will probably
require an entirely different way of fixing this.
(Of course, Ubuntu does not supply a convenient program to (re)build an
iftab for you.)
Sidebar: on the random renumbering issue
If a kernel device driver is responsible for more than one Ethernet
device, it always reports them in a consistent internal order. If you
iftab file, Ubuntu will probably only reorder blocks of ethN
devices served by different drivers. Eg, if you have four Intel NICs and
two Broadcom NICs, the internal order in the Intel NICs and the Broadcom
NICs is probably always going to be consistent, and the first Intel NIC
will be either eth0 (Intel driver loaded first) or eth2 (Broadcom driver
loaded first). However, which driver is loaded first seems to be more or
If Ubuntu does have an
iftab file it will rename the kernel's initial
ethN names to not use any ethN name claimed by
iftab, and I believe
the order it does this renaming is really random and can completely
shuffle ethN names, so that if
iftab claims eth0 through eth3, you
could have eth4 being the third Intel NIC, eth5 being the first Broadcom
NIC, and so on.