Comment 0 for bug 1880258

I normally don't like this, but it's a one-character change so it's easier to start with the solution:

diff -u -r1.1 /usr/lib/NetworkManager/conf.d/20-connectivity-ubuntu.conf
--- /usr/lib/NetworkManager/conf.d/20-connectivity-ubuntu.conf
+++ /usr/lib/NetworkManager/conf.d/20-connectivity-ubuntu.conf
@@ -1,2 +1,2 @@
 [connectivity]
-uri=http://connectivity-check.ubuntu.com/
+uri=http://connectivity-check.ubuntu.com./

Making this name absolute instead of relative avoids spurious resolutions of "connectivity-check.ubuntu.com.your_domain." This removes a fair amount of NXDOMAIN error noise in journalctl.

Observing the issue and the fix requires 3 terminals:

1. tcpdump -i any 'port domain'
2. journalctl --boot -u systemd-resolved -f

3. nmcli c down "Wired connection 1"; nmcli c up "Wired connection 1"
 => observe the NXDOMAIN noise over a couple few minutes

Now make the hostname absolute with the trailing dot above and run:
   systemctl reload NetworkManager
Wait 1 min for things to stabilize. Test again:

nmcli c down "Wired connection 1"; nmcli c up "Wired connection 1"
 => observe non-zero but significantly reduced NXDOMAIN noise over a couple few minutes

Originally reported at https://askubuntu.com/a/1242611/117217

Plenty of people annoyed by NXDOMAIN warnings, just Google it.