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

[Steve Langasek]

I'm not happy with the proposed change to make resolvconf recreate the
missing symlink on upgrade from precise to trusty without understanding
why the symlink was absent in the first place.  Timo, why was this
missing for you?  How do we know that whatever caused this problem for
you, won't happen again after upgrade to trusty?

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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:
  Triaged
Status in “resolvconf” source package in Trusty:
  Triaged

Bug description:
  [Impact]

  Name resolving using the NetworkManager-controlled nameserver
  discontinues working after upgrading from 12.04 (precise) to 14.04
  (trusty) in the case where /etc/resolv.conf is absent.

  Precise works fine with that file absent because the resolver defaults
  to using "nameserver 127.0.0.1" — so it is possible to run Precise for
  a long time without even noticing that the file is absent.

  In Trusty, resolving does not work if /etc/resolv.conf is absent
  because the NetworkManager-controlled nameserver listens at 127.0.1.1.

  People suffering from bug #1000244 may lack /etc/resolv.conf on
  upgrade.

  [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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