[Bug 1880258] Re: Add trailing dot to make connectivity-check.ubuntu.com. absolute and reduce NXDOMAIN warning noise

[MarcH]

I ran tcpdump 'host ubuntu20 and port domain' on my router and looked at
all the other NXDOMAIN at boot time besides this one.

1. PTR request on the IPv4 address of my virbr0 interface. Gone after
uninstalling a couple libvirt-* packages

2. PTR request on ONE of my IPv6 addresses. It's not obvious to me which
address gets picked for that PTR query. I disabled IPv6 ULA on my router
and the PTR request just moved to a different address.

3. SOA local. ??


I also found that NetworkManager needs to be told "connectivity-check.ubuntu.com has no AAAA record" about 12 times during boot, that seems a bit excessive? (this is not NXDOMAIN)

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Title:
  Add trailing dot to make connectivity-check.ubuntu.com. absolute and
  reduce NXDOMAIN warning noise

Status in network-manager package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  Triaged
Status in network-manager source package in Focal:
  Confirmed

Bug description:
  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.

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