[Bug 1874337] Re: resolvconf should never touch /etc/resolv.conf in Ubuntu; all DNS configuration from resolvconf, ifupdown, and dhclient should always be fed directly to systemd-resolved.

[Dimitri John Ledkov]

I have prepared xenial containers (last ones that used ifupdown / resolvconf by default) and configured it with:
- default resolvconf & dhcp acquired dns
- above, but with an extra dns nameserver specified in eni
- with resolvconf removed and static resolv.conf configuration
- with dhclient removed and static ip configuration

I've staged above updates in a bileto PPA and will execute upgrade to
bionic, then upgrade to focal + bileto PPA.

The expectation is for every configuration above, that the configuration
survives the upgrade, and dns configurations still work.

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https://bugs.launchpad.net/bugs/1874337

Title:
  resolvconf should never touch /etc/resolv.conf in Ubuntu; all DNS
  configuration from resolvconf, ifupdown, and dhclient should always be
  fed directly to systemd-resolved.

Status in ifupdown package in Ubuntu:
  New
Status in resolvconf package in Ubuntu:
  New
Status in systemd package in Ubuntu:
  New

Bug description:
  systemd-resolved is a standard component of Ubuntu in 20.04.  We
  should not have packages in the archive - some of which may be
  installed on users' systems as a result of upgrading from previous
  releases - that cause the handling of DNS resolution to diverge from
  the default.

  This means that:

  - the dhclient hook that picks up DNS settings should only feed settings directly to resolved, and not via resolvconf.
  - resolvconf must not change the target of /etc/resolv.conf and on upgrade must correct it.
  - resolvconf must feed its settings reliably into resolved rather than pulling resolved settings into resolvconf.
  - systemd should not ship a dhclient hook at all because dhclient is not used by any systems that use netplan for network management, it is only needed for compatibility on upgrade with ifupdown; so move the hook to the ifupdown package.

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