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

[MarcH]

> the vast, VAST majority of the time you see this message it is
actually not a NXDOMAIN error.

I don't have any extensive data as you seem to have but _for me at home_
(= office now!) and without zero captive portal involved, adding this
trailing dot reduces the NXDOMAIN frequency from one message every 5
minutes to a few messages _per day_ (NM's default interval is 300
seconds)

The "journalctl --boot -p notice" total also becomes about twice
smaller. I filed this bug for a reason.


> the NXDOMAIN returned by the upstream nameserver is the *correct* response - that hostname connectivity-check.ubuntu.com.your_domain really, actually doesn't exist.

Yes of course, hence this one character fix which IMHO also removes one
large "distraction" from the main, captive portal issue!


> if you see this NXDOMAIN "error" [outside a captive portal] it is almost always just a normal lookup of a domain that doesn't exist,

... which is for sure very rare on my (admittedly headless) system. Not
sure why querying inexistent domains would be frequent on others'
systems?

> and the error message is simply wrong

It says "mitigating _potential_..." so it's not strictly speaking wrong.
Agreed it can be misleading but summarizing this fairly complex issue in
just one line is quite a challenge.


> This is due to a Ubuntu-only patch to systemd 

Care to share the "best" link(s) about this? I mean links to best bugs
and/or relevant source code. An internet search returns too many hits!
BTW a good and authoritative bug number in the error message would
probably not hurt either, see why above.


> network-manager could work around this problematic Ubuntu-only systemd patch,

With an Ubuntu-only network-manager patch maybe? Sorry couldn't resist
:-)

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https://bugs.launchpad.net/bugs/1880258

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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