[Bug 2008964] [NEW] resolved falls back to a non-preferred name server when the preferred name server appears to be working.

[Frank Trampe]

Public bug reported:

This is split from bug #2007728.

On a network with multiple DNS servers provided by DHCP, the "Current
DNS Server" shown by `resolvectl status` is sometimes not the first
server or even the second server, even when those servers appear to be
working (and other hosts continue to use them). This appears to occur on
Ubuntu 22.04 but not on Ubuntu 20.04, Ubuntu 18.04, or Windows 10.

RFC 2132 section 3.8 provides that servers are listed in order of
preference.

It seems that the correct behavior is that resolved picks as its
"Current DNS Server" the first reachable server in the list provided by
the DHCP server. The observed behavior is that resolved sometimes picks
as its "Current DNS Server" some server other than the first reachable
server in the list.

My hypothesis is that there is some name server availability check that
is too stringent and that there is no mechanism to retry the preferred
server after that check fails. I have not looked at the code or captured
packets.

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

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https://bugs.launchpad.net/bugs/2008964

Title:
  resolved falls back to a non-preferred name server when the preferred
  name server appears to be working.

Status in systemd package in Ubuntu:
  New

Bug description:
  This is split from bug #2007728.

  On a network with multiple DNS servers provided by DHCP, the "Current
  DNS Server" shown by `resolvectl status` is sometimes not the first
  server or even the second server, even when those servers appear to be
  working (and other hosts continue to use them). This appears to occur
  on Ubuntu 22.04 but not on Ubuntu 20.04, Ubuntu 18.04, or Windows 10.

  RFC 2132 section 3.8 provides that servers are listed in order of
  preference.

  It seems that the correct behavior is that resolved picks as its
  "Current DNS Server" the first reachable server in the list provided
  by the DHCP server. The observed behavior is that resolved sometimes
  picks as its "Current DNS Server" some server other than the first
  reachable server in the list.

  My hypothesis is that there is some name server availability check
  that is too stringent and that there is no mechanism to retry the
  preferred server after that check fails. I have not looked at the code
  or captured packets.

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