[Bug 1308378] Re: No working network after 12.04 LTS -> 14.04 LTS upgrade if no /etc/resolv.conf in precise

[Timo Jyrinki]

If an admin does rm /etc/resolv.conf on the 14.04, name resolving stops
working and no upgrade will undo that change, since the preinst check is
only there when upgrading from the 12.04's version of the package. This
can also be tested by installing the new package multiple times,
removing resolv.conf in between - it does not get back in that case,
because the upgrade) is called with the 1.69 version which is higher
than what's checked.

If admin does rm /etc/resolv.conf in 12.04, name resolving continues to
work because the name resolving does not stop working at that like it
does on 14.04 (possibly because /run/resolvconf/resolv.conf is always
watched in 12.04 instead or some other detail that has changed?). If the
system is then upgraded to 14.04, either: a) without this fix name
resolving will simply stop working, because of the difference in
behavior in 14.04, b) with this fix the symlink gets reinstated and name
resolving continues to work after the upgrade.

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https://bugs.launchpad.net/bugs/1308378

Title:
  No working network after 12.04 LTS -> 14.04 LTS upgrade if no
  /etc/resolv.conf in precise

Status in “resolvconf” package in Ubuntu:
  New

Bug description:
  [Impact]

  Name resolving does not work after upgrading from 12.04 (precise) to
  14.04 (trusty) in the corner case of not having /etc/resolv.conf in
  precise. precise however works fine with that file absent, while
  trusty does not, so it increases the likelihood of having these kind
  of machines (including some people from bug #1000244 probably).

  [Test Case]

  1. Install 12.04 LTS
  2. rm /etc/resolv.conf and observe everything works fine
  3. update-manager -c -d
  4. Observe that in 14.04 LTS name resolving does not work.

  [Test Case for testing the fix on 14.04]

  1. wget http://de.archive.ubuntu.com/ubuntu/pool/main/r/resolvconf/resolvconf_1.63ubuntu16_all.deb
  2. dpkg -i resolvconf_1.63ubuntu16_all.deb
  3. Running that precise version, test the possible cases like rm /etc/resolv.conf (note you lose name resolving at this point in 14.04)
  4. Run 'bzr bd' (apt-get install bzr-builddeb first) in the proposed branch, dpkg -i ../build-area/*.deb
  5. Check how either, depending on the case being tested, /etc/resolv.conf is intact / non-touched or it's created when not-existing, restoring network connectivity

  [Regression Potential]

  Seems low considering the limiting of the proposed fix to 12.04 ->
  14.04 upgrades only, and only in the case of the user/3rd party
  app/something intentionally removed /etc/resolv.conf (instead of just
  modifying it).

  ---

  I fixed it manually with:
  cd /etc
  sudo ln -s /run/resolvconf/resolv.conf .

  It turns out 12.04 LTS works fine without /etc/resolv.conf, so that
  increases the likelihoods that 12.04 LTS systems do not have that
  file. When such a system is upgraded to 14.04 LTS, network seemingly
  stops working to the user.

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