Re: Mac-Address uniqueness

["Daniel P. Berrange" <berrange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Thu, May 31, 2007 at 11:43:43PM -0700, H. Peter Anvin wrote:
> Avi Kivity wrote:
> > H. Peter Anvin wrote:
> >> Axel Kittenberger wrote:
> >>   
> >>> Unfortunally all machines detect the same ethernet address 
> >>> '52:54:00:12:34:56'. Which you can guess what i means, networking comes 
> >>> and goes whatever machine last the ethernet address got hold of from the 
> >>> gateway. I tried specifing an ethernet-adress with "-net 
> >>> nic,macaddr=$MAC" but this also didn't work through.
> >>>
> >>> For now I just hardseted the mac in all machines to 52:54:00:12:34:57, 
> >>> 52:54:00:12:34:58 and so on.
> >>>
> >>>     
> >> This is a Qemuism.  I always thought it was dumb, but I guess Qemu
> >> wanted reproducibility over everything.
> >>
> >> IMNSHO it would have been much better to default to a random value
> >> (meaning that all except the bottom 2 bits of the first octet are
> >> random, those bits should be set to 10 binary.)
> >>
> >>   
> > 
> > That tends to consume dhcp leases quickly, if you start guests often (as
> > I do).  Also, some distributions use the mac address as a key for naming
> > interfaces; if it changes, the guest gets confused
> > 
> 
> The right solution to that, of course, is a VM definitions file, so the
> random Ethernet address is only generated once.  For Qemu/KVM, that
> could at least in theory simply be a shell script.

This is the approach we use in libvirt. If the user defines a guest and
does not provide a mac address, we generate one for each NIC and store
it in the config file, so every time it gets the same MAC. 

Dan.
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