This can break things easily in remove environments where it was normally easy to convert a normal eth0 to a bridge under br0, and that bridge would normally have the same MAC address by default, which is desirable for most situations.

In Debian 11 this is different for some reason now.

https://unix.stackexchange.com/questions/681013/bridge-gets-random-mac-instead-of-port-address

One simple solution is to set the hwaddress in /etc/network/interfaces:

iface br0 inet static
bridge_ports eth0
    address 172.16.1.3
    netmask 255.255.255.0
    gateway 172.16.1.1
    hwaddress 00:1d:e0:00:13:58
 

Set the hwaddress to the mac address of eth0 to solve this problem.  What happens is that if you don't do this, the server will become inaccessible after some random amount of time, at least initially until a reboot or network restart.

This may be a result of poor cloning without regenerating the machine-id:

#possibly related, we need to delete the machine-id and generate a new one
rm /var/lib/dbus/machine-id
rm /etc/machine-id

Regenerate new machine-id:

The file lives in /etc and /var/lib/dbus and both must match.

dbus-uuidgen --ensure=/etc/machine-id


cp /etc/machine-id /var/lib/dbus/

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