Re: [Qemu-devel] [RFC] default mac address issue

[Anthony Liguori]

Hi Bruce,

On 05/10/2010 02:07 PM, Bruce Rogers wrote:
> I know this behavior has worked this way all along, but I wanted to bring up 
> the following concern and float a few ideas about possible solutions. Please 
> provide your perspective, opinion, etc.
>
> qemu (or qemu-kvm) users can easily get into trouble when they don't specifying 
> the mac address for their vm's nic and don't realize that multiple vm's running 
> this way on the same network segment are colliding, since they all get a 
> default mac address that is the same. They may be under the assumption that a 
> random mac would be the default, as in many higher level tools for vm creation
>    

This is certainly an important issue but it's one that's difficult to 
resolve.

> Does it make sense to do any of the following:
>
> 1) have qemu print a warning to stdout/stderr that the default mac address is 
> being used and that it will interfere with other vms running the same way on a 
> common network segment
>    

This is definitely reasonable.

> 2) what about changing the default behavior to randomizing the mac, and provide the legacy behavior 
> with "-net nic,macaddr=default" or just "-use-default-mac"
>
> (or, as a flip side to #2):
>
> 3) to at least make it easy for people to get around the problem, and just
> use qem directly (without additional tools to launch qemu), add an option such as "-net 
> nic,macaddr=randomize" or "-use-random-mac" which randomizes the mac for you
> each time the machine is brought up, and hence avoids possible collisions.
>    

A random mac address is almost always wrong.  If you run a guest twice 
with this option, it's usually enough to trigger a new network detection 
which which rename the network device to ethN + 1.  The result would be 
broken networking for naive users since distros don't bother configuring 
interfaces that weren't present during installation.

Regards,

Anthony Liguori

> Of course having the same vm come up with a different mac each time is an issue 
> for some guest os's, but for some usages, that is not a problem.
>
> I imagine the concensus would be that the current behavior is appropriate, and 
> it's up to management tools to do the randomization, but I think there is 
> perhaps a place for randomization within qemu itself.  I'm interested to hear 
> what you think.
>
> Bruce
>
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