Comment 0 for bug 1732717

Inside a docker container DNS lookups are routed to 127.0.0.11:53 which is provided by the docker environment in some way. The real DNS service is taken from the hosts /etc/resolv.conf. The docker man page claims that selecting a good automatic default would not work if the hosts uses local resolvers (127.0.0.0/8).
Since switching to netplan/systemd-resolvd there is only a local resolver (127.0.0.53) added to /etc/resolv.conf on the host. Surprisingly this does not seem to be an issue for an artful(or later) VM running on my own local desktop. But in the Canonical CI environment DNS lookups fail inside a docker container.

SRU Justification:

Impact: DNS lookups inside docker containers can fail if the host environment is set up to only use a local resolver. The built-in docker test of fanatic will always fail in that case.

Fix: Add code to built-in tests which detect systemd-resolvd being in use and pass the IP address which systemd-resolv is using as forwarder with the --dns option to the docker run command.

Testcase:
  - If the test host is using systemd-resolvd as its primary DNS
    server, there will be the following message in the test log:
      local docker test: *** Using DNS override ***
          --dns=<ip>
  - Otherwise no message is printed.
  - For all cases docker ADT test should (continue to) pass