Comment 0 for bug 1902960

The systemd upgrade 245.4-4ubuntu3.3 to 245.4-4ubuntu3.2 appears to have broken DNS resolution across much of our Azure fleet earlier today. We ended up mitigating this by forcing reboots on the associated instances, no combination of networkctl reload, reconfigure, systemctl daemon-reexec, systemctl daemon-reload, netplan generate, netplan apply would get resolvectl to have a DNS server again. The main symptom appears to have been systemd-networkd believing it wasn't managing the eth0 interfaces:

ubuntu@machine-1:~$ sudo networkctl
IDX LINK TYPE OPERATIONAL SETUP
  1 lo loopback carrier unmanaged
  2 eth0 ether routable unmanaged
                                                                                                                                                                                                                   2 links listed.

Which eventually made them lose their DNS resolvers:

ubuntu@machine-1:~$ sudo resolvectl dns
Global:
Link 2 (eth0):

After rebooting, we see this behaving properly:

ubuntu@machine-1:~$ sudo networkctl list
IDX LINK TYPE OPERATIONAL SETUP
  1 lo loopback carrier unmanaged
  2 eth0 ether routable configured

2 links listed.
ubuntu@machine-1:~$ sudo resolvectl dns
Global:
Link 2 (eth0): 168.63.129.16

This appears to be specifically linked to the upgrade, i.e. we were able to provoke the issue by upgrading the systemd package, so I suspect it's part of the packaging in the upgrade process.