Inherit MAC address for bridge from physical device

[olafhering]

With ifcfg or NetworkManager it is possible to have generic config files for every system to configure a bridge on the one and only physical network interface. See the examples below how such configuration looks like.

With systemd-networkd it is apparently required to specify the MAC in the configuration. As a result, such configuration is per host and not generic anymore:

==> /etc/systemd/network/br0.netdev <==
[NetDev]
Name=br0
Kind=bridge
==> /etc/systemd/network/br0.network <==
[Match]
Name=br0
[Link]
MACAddress=b8:ac:6f:a3:7d:0a
[Network]
DHCP=ipv4
==> /etc/systemd/network/em1.network <==
[Match]
MACAddress=b8:ac:6f:a3:7d:0a
[Network]
Bridge=br0

Here the physical link b8:ac:6f:a3:7d:0a should be part of bridge br0. Without [Link] MACAddress=b8:ac:6f:a3:7d:0a in br0.network the assigned MAC will be random. If there is just one single interface for the bridge, the MAC should be inherited. With more physical interfaces, like in the examples provided by systemd.network(5), the decision what MAC should be assigned to br0 is not straight forward.

==> /etc/sysconfig/network/ifcfg-br0 <==  
BOOTPROTO='dhcp'
BRIDGE='yes'
BRIDGE_PORTS='eth0'
BRIDGE_STP='off'
STARTMODE='auto'
==> /etc/sysconfig/network/ifcfg-eth0 <==  
BOOTPROTO='none'
STARTMODE='auto'

==> /etc/NetworkManager/system-connections/eth <==  
[connection]
id=eth
uuid=11111111-1111-1111-1111-111111111111
type=ethernet
interface-name=eth0
master=br0
permissions=
slave-type=bridge
[ethernet]
mac-address-blacklist=
==> /etc/NetworkManager/system-connections/br <==  
[connection]
autoconnect-slaves=1
id=br
interface-name=br0
permissions=
type=bridge
uuid=22222222-2222-2222-2222-222222222222
[bridge]
stp=false
[ipv4]
dhcp-timeout=2147483647
dns-search=
method=auto
[ipv6]
addr-gen-mode=stable-privacy
dns-search=
ip6-privacy=0
method=auto