Re: [Xen-users] in domU eth0 is now eth3?

["Nico Kadel-Garcia" <nkadel@xxxxxxxxx>]

Goetz Bock wrote:
> On Wed, Mar 14 '07 at 16:15, Rune Elvemo wrote:
> > We had a little strange experience here. We had to copy something to
> > our xen image. It was done while the domU was down, mounting using
> > the loopback device.
> > After we had finished the job, unmounted, and restarted the domU, we
> > had no eth0. Got an error message in the bootup. I thought something
> > had gone really wrong, and that we no more had network access in this
> > particular domU.
> You don't mention what distribution you're using for the domU.
>
> Debian Etch (4.0) has the unfortunate habbit of remembering what MAC
> address an ethernet card had. So on the normal Xen 3.0.3 (as shipping
> with etch) setup, where you get a new MAC on every boot, you start
> with an eth0 but after 4 reboots have only an eth4 ...
> (the MACs are recorded in /etc/udev/rules.d/z25-persistent-net (path
> from memory)
There's a reason for this sort of thing. Associating specific network ports 
with specific hardware can be.... awkward, since compiling some modules 
directly into a kernel or loading them after boot or even subtle differences 
in the modules themselves can re-order the network ports. This was even 
worse with wireless ports in the picture. And it's deadly on a firewall box, 
since the firewall rules may be very different for different ports.
So in many distributions, the network port is associated not only with a 
specific module, but with a specific MAC address to keep things consistent. 
Voila, no hopping ports. But Xen's habit of auto-assigning randomized MAC 
addresses can trip over this. 

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