Public bug reported:

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.

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1902960

Title:
  Upgrade from 245.4-4ubuntu3.3 to 245.4-4ubuntu3.2 appears to break DNS
  resolution in some cases

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