[Bug 1988300] Re: systemd-resolved in kinetic image tries to replace resolv.conf

[Athos Ribeiro]

Thanks for filing this bug, Peter.

The issue here is not related to /etc/resolv.conf being read-only (it is
actually a RW file) [1]. The issue lies in the fact that it is always
mounted in a running container by docker itself [2].

During a package installation process, dpkg performs backups of existing
files through hard links. Hard links cannot be performed across mounts,
hence, dpkg fails to install systemd-resolved when it tried to backup
/etc/resolv.conf

$ apt-file search /etc/resolv.conf
...
systemd-resolved: /etc/resolv.conf

Now, while I am unsure where this bug belongs, this should be a good
place to start a discussion.

For further context, this is the bug where systemd-resolved split and
/etc/resolv.conf ownership were discussed [3].

[1] https://github.com/moby/moby/pull/5129/files
[2] https://docs.docker.com/storage/#good-use-cases-for-bind-mounts
[3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=939904

** Bug watch added: Debian Bug tracker #939904
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=939904

** Also affects: dpkg (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu)
   Importance: Undecided
       Status: New

** Summary changed:

- systemd-resolved in kinetic image tries to replace resolv.conf
+ systemd-resolved is not installable in Docker images

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https://bugs.launchpad.net/bugs/1988300

Title:
  systemd-resolved is not installable in Docker images

Status in Ubuntu Docker Images:
  New
Status in dpkg package in Ubuntu:
  New
Status in systemd package in Ubuntu:
  New

Bug description:
  Since 30 August (going by my daily CI builds, I do see the changelog
  entry for resolved is a few days older), I get this:

  Selecting previously unselected package systemd-resolved.
  Preparing to unpack .../321-systemd-resolved_251.4-1ubuntu1_amd64.deb ...
  Unpacking systemd-resolved (251.4-1ubuntu1) ...
  dpkg: error processing archive /tmp/apt-dpkg-install-NS2Yvi/321-systemd-resolved_251.4-1ubuntu1_amd64.deb (--unpack):
   unable to make backup link of './etc/resolv.conf' before installing new version: Invalid cross-device link

  The reason this fails is that Docker mounts resolv.conf, readonly,
  from the host system, so dpkg is not allowed to move/replace it.

  (To be clear, I do not need systemd-resolved in my container. "apt
  install devscripts" pulled it in, and debtree does not tell me why.
  "apt install --no-install-recommends devscripts" does not pull it in,
  and I'll likely adjust my builds [for PowerDNS] to do that because
  it's a good idea anyway).

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