Whether or not /e/n/i supports something correctly or just happens to
work by sheer luck has no bearing on what is technically correct and
sensical -- let's abstract this, what we need to concern ourselves with
here is netplan, cloud-init and maas.

In the network world, it is absolutely true that DNS nameservers only
make sense on a per-interface basis: nothing guarantees that a
"globally-set" nameserver of 8.8.8.8 is reachable through all
interfaces. It just happens to be that way on 99% of systems because
they only connect to one network, through one interface.

So, in light of the nameserver setting in MaaS under Settings being
unspecified, there's /no other way/ but to either set the nameserver
globally or try to guess the right interface (either by which is
connected to MaaS, or which gets the default gateway, or which matches
subnets). None of the detection options I could come up with strike me
as particularly solid and foolproof.

Next steps for netplan strike me as being to add support for global
nameservers even if it's silly; we shouldn't expect users to have to
have the level of knowledge of networking to know to set it up on a
particular interface when it will work correctly as "global" for 99% of
cases.

In MaaS, my opinion is that every effort should be taken to allow
network administrators to set things up correctly -- nothing guarantees
that the systems deployed will remain accessible to MaaS, or that it
will be their main interface to the network. Only allowing global DNS is
brittle at best, and breaks at least what *I* have been taught were best
practices, to have a separate "public" network from the authenticated
"private" side where monitoring and/or deployment happens. Maybe I'm
just the one who's particular in doing this.

I'm not sure what next steps are for cloud-init -- given a valid
configuration, I assumed it should just pass it through to netplan if
it's v2.

I'll take care of the "global DNS" implementation in netplan.

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https://bugs.launchpad.net/bugs/1750884

Title:
  [2.4, bionic] /etc/resolv.conf not configured correctly in Bionic,
  leads to no DNS resolution

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