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

[Dan Streetman]

> One thing that would help with these confusing NXDOMAIN errors in
general

the vast, VAST majority of the time you see this message it is actually
not a NXDOMAIN error. This is due to a Ubuntu-only patch to systemd to
work around some select captive portals that are slightly broken, so in
any environment outside the broken captive portals (e.g. public wifi
that you have to 'click here to accept terms' before getting internet
access - and note that not all captive portals are broken) if you see
this NXDOMAIN "error" it is almost always just a normal lookup of a
domain that doesn't exist, and the error message is simply wrong (this
also slows down dns due to forcing fallback to a lower dns protocol
level and retry of the already-failed lookup).

To clarify specifically for this bug, the lookup of "connectivity-
check.ubuntu.com.your_domain" clearly has nothing to do with any "DNS
violation", and the NXDOMAIN returned by the upstream nameserver is the
*correct* response - that hostname really, actually doesn't exist.

network-manager could work around this problematic Ubuntu-only systemd
patch, but the real problem is unquestionably that systemd should not
have the Ubuntu-only patch that's causing these messages.

Fixing this appropriately (i.e. so that systemd still works with the
broken captive portal issue) requires access to one of the broken
captive portals, so I haven't been able to work on correctly fixing this
lately, but it is something I want to do, so we can get rid of the very
unfortunate false NXDOMAIN "error" messages.

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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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