[Bug 2007728] Re: resolved results differ from those from its current upstream server.

[Frank Trampe]

Would you describe the "as documented" behavior? It still seems wacky to
me that resolved returns the DNS result the majority of the time but not
all of the time. If the design intent is to use only mDNS for .local
domains, it ought to ignore DNS entirely for those domains. Inconsistent
behavior means that a configuration can test as correct, fail in the
field, fail to replicate the failure, and frustrate isolation of the
problem. I think that the earlier behavior makes a lot more sense and
would prefer to return to it.

Are you able to replicate the issue?

Given how closely the two possibly separate problems are related and
their similar effects, I am inclined to wait on filing a second bug
report on the server selection until it is clear that these are in fact
separate issues. The fact that no other hosts on the network exhibit the
problem (a highly symptomatic one since it breaks most services)
suggests that this is not an issue of both internal servers failing at
the same time.

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Title:
  resolved results differ from those from its current upstream server.

Status in systemd package in Ubuntu:
  Incomplete

Bug description:
  On a network with multiple DNS servers provided by DHCP, only the
  first two of which cover local names, resolved returns universally
  known names but fails to return the special names even when the
  "Current DNS Server" shown by `resolvectl status` returns the special
  names.

  Suppose that 172.16.9.5 and 172.16.10.5 are the two internal DNS
  servers with the local names. Windows servers with Active Directory
  enabled in this case. The DHCP server (a Cisco 4451 in this case)
  provides DNS servers 172.16.9.5, 172.16.10.5, 192.168.0.1, and
  8.8.8.8. `resolvectl status` shows all of these as "DNS Servers" and
  172.16.9.5 as the "Current DNS Server".

  `host localdomain.local` returns SRVFAIL, and `host localdomain.local
  127.0.0.53` returns SRVFAIL, but `host localdomain.local 172.16.9.5`
  returns the correct result. This all happens regardless of the
  "Current DNS Server".

  Sometimes the "Current DNS Server" switches to 8.8.8.8 for reasons
  that are not clear even when the other servers are working properly,
  which seems to violate the principle of RFC 2132 section 3.8 that
  servers are listed in order of preference.

  So, in short, it seems that the correct behavior is that (1) resolved
  returns results consistent with its "Current DNS Server" and (2)
  resolved picks as its "Current DNS Server" the first reachable server
  in the list. The current behavior is that (1) resolved returns results
  sometimes inconsistent with its "Current DNS Server" and (2) resolved
  sometimes picks as its "Current DNS Server" some server other than the
  first reachable server in the list. The first issue is consistently
  reproducible, and the second is readily reproducible in a short period
  of time.

  The problem appears on Ubuntu 22.04 and seems not to be present on
  Ubuntu 18.04.

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