[Bug 1895418] Re: systemd-resolved default config for Caching is still "yes"

[Cameron Steel]

Testing systemd 245.4-4ubuntu3.3 on Focal, I can verify that the default
value of the "Cache" directive is "no-negative" in the man page, default
config file, and actual program behaviour.

While doing this, I was reminded that the default value of the "DNSSEC"
directive is "no" in both the default config file and program behaviour,
however the man page lists the default as "allow-downgrade". As a user
looking to change this setting would do so by editing the config file
anyway, I don't personally think this is of great importance to fix.

** Tags removed: verification-needed-focal
** Tags added: verification-done-focal

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Title:
  systemd-resolved default config for Caching is still "yes"

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Focal:
  Fix Committed
Status in systemd source package in Groovy:
  Fix Released

Bug description:
  [impact]

  /etc/systemd/resolved.conf file indicates the default value for
  'Cache=' is 'yes', but the actual default is 'no-negative'

  [test case]

  $ grep Cache /etc/systemd/resolved.conf
  #Cache=yes

  With the version of the package from -proposed it'll be "#Cache=no-
  negative".

  [regression potential]

  any regression would likely result in a mismatch between the commented
  default value of the Cache parameter and the actual build-time default
  value, or possibly a problem while systemd-resolved is parsing the
  resolved.conf file.

  [scope]

  this is needed only in focal.

  this was changed already in groovy as indicated in comment 1.

  bionic and earlier still use 'yes' as the default value of Cache=,
  which matches the resolved.conf file.

  [original description]

  Back in December, the default for systemd-resolved caching in Ubuntu
  systemd was changed to "no-negative" from the upstream default "yes"
  [0]

  In this change, the default value in the resolved.conf file was
  missed. As the defaults in this file are commented, the effective
  default is still "no-negative", however when viewing the config file,
  the commented default "yes" is at odds with the man page
  resolved.conf(5), which correctly states the default as "no-negative".

  This was an issue for me as I set DNSSEC to "yes", and expected
  Caching to also be "yes". Running DNSSEC with the default "no-
  negative" Caching is detrimental to performance resolving unsigned
  zones, as the non-existence of DNSSEC RRs must be looked up every
  time.

  The issue with the intersection of DNSSEC and Caching is for upstream,
  but the least that needs to be done here is updating the resolved.conf
  template with "Caching=no-negative" to match the man page and
  behaviour, and perhaps even adding a note to the "DNSSEC=" section of
  resolved.conf(5) that Caching should be enabled. Now that I'm looking
  at that man page, the default for DNSSEC is also listed as "allow-
  downgrade", whereas the default for Ubuntu is "no".

  [0] https://git.launchpad.net/~ubuntu-core-
  dev/ubuntu/+source/systemd/commit/?id=b42658843a9496d6b6bb68ac159f2a9f0a8ba9db&h
  =ubuntu-focal

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