[Bug 1668771] Re: systemd-resolved negative caching for extended period of time

[Jorge Niedbalski]

I've made a proposal to change the resolved.conf Cache option to a tri-
state "no, no-negative, yes" values. [0]

If a lookup returns SERVFAIL systemd-resolved will cache the result for 30s (See 201d995),
however, there are several use cases on which this condition is not acceptable (See #5552 comments)
and the only workaround would be to disable cache entirely or flush it , which isn't optimal.

This change adds the 'no-negative' option when set it avoids putting in cache
negative answers but still works the same heuristics for positive answers.

[0] https://github.com/systemd/systemd/pull/13047

** Changed in: systemd (Ubuntu)
   Importance: Undecided => High

** Changed in: systemd (Ubuntu)
       Status: Confirmed => In Progress

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Title:
  systemd-resolved negative caching for extended period of time

Status in systemd:
  New
Status in systemd package in Ubuntu:
  In Progress

Bug description:
  231-9ubuntu3

  If a DNS lookup returns SERVFAIL, systemd-resolved seems to cache the
  result for very long (infinity?). I have to restart systemd-resolved
  to have the negative caching purged.

  After SERVFAIL DNS server issue has been resolved, chromium/firefox
  still returns DNS error despite host can correctly resolve the name.

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