Re: MAC address of bridge interfaces

[Steve McIntyre <steve@einval.com>]

Steve wrote:
>I upgraded a Debian machine from stretch to bullseye and see
>a change of the IP address of a ethernet bridge interface.
>
>The bridge has a physical LAN interface as one fixed bridge port
>and additional ports for kvm virtual machines I may start.
>
>Before the upgrade the bridge interface got its MAC address from
>the physical LAN interface.  With Bullseye this is no longer the
>case.  Instead, the MAC address seems to be generated "somehow".
>At least, this new address is fixed and doesn't change on each
>reboot.
>
>But how is that MAC address generated?  Is it stored somewhere?
>Can I set it to an address of my own preference?
>And why was this changed, why don't we still use the address of
>the physical port connected to it?

In my experience, the bridge may end up advertising the MAC of any/all
of the underlying interfaces, and that behaviour can be racy
sometimes. I noticed locally that *sometimes* I'd lose IPv6
connectivity from my workstation when I started bridge VMs. Eventually
I worked out that the MACs from the VMs were causing my bridge to
change IP. Now I've modified things:

auto br0
iface br0 inet dhcp
      hwaddress XX:XX:XX:XX:XX:XX

(substitute your own MAC address here) and it all works flawlessly
now.

-- 
Steve McIntyre, Cambridge, UK.                                steve@einval.com
"We're the technical experts.  We were hired so that management could
 ignore our recommendations and tell us how to do our jobs."  -- Mike Andrews