Juju's MAAS bridging script needs to de-duplicate dns-* iface options

[Andrew McDermott]

When the bridge script renders an original /etc/network/interfaces file like:

  auto eth0
  iface eth0 inet static
      gateway 10.245.168.1
      address 10.245.168.11/21
      mtu 1500

  auto eth1
  iface eth1 inet static
      dns-nameservers 10.245.168.2
      gateway 10.245.168.1
      address 10.245.168.12/21
      mtu 1500

  dns-nameservers 192.168.1.1

into ->>>>>>>>>>>>>>>>>>>>>

  iface eth0 inet manual

  auto br-eth0
  iface br-eth0 inet static
      gateway 10.245.168.1
      address 10.245.168.11/21
      mtu 1500
      bridge_ports eth0

  iface eth1 inet manual

  auto br-eth1
  iface br-eth1 inet static
      dns-nameservers 10.245.168.2
      gateway 10.245.168.1
      address 10.245.168.12/21
      mtu 1500
      dns-nameservers 192.168.1.1
      bridge_ports eth1

the script needs to ensure that the dns-* options are de-duplicated and become a single line. This knowledge gained from our understanding and resolution to:

  https://bugs.launchpad.net/juju-core/+bug/1534795

The resolvconf(8) man page also spells this out:

     To add nameserver addresses, add a single line beginning with dns-name‐
     servers.

           dns-nameservers 192.168.1.254 8.8.8.8

     Note that one or more addresses can be given on this line, separated by
     spaces, and note the "s" at the end of the option name, in contrast
     with the resolv.conf(5) option name nameserver. Remember that in
     interfaces(5) each option can be used at most once per stanza.

Taking the 'iface br-eth1 inet static' example it should become:

  auto br-eth1
  iface br-eth1 inet static
      gateway 10.245.168.1
      address 10.245.168.12/21
      mtu 1500
      dns-nameservers 10.245.168.2 192.168.1.1
      bridge_ports eth1

This also holds true for dns-sortlist and dns-search options. This needs to be fixed in 1.25 and maas-spaces.

[Andrew McDermott (frobware)]

Initial implementation and PR is in maas-spaces:

  https://github.com/juju/juju/pull/4189