Update CNAME resolution

[tgreenx]

Purpose

This PR slightly updates the CNAME resolution of Engine's recursor. See the Changes section below.
Updated test scenarios are defined in zonemaster/zonemaster#1477.

Context

zonemaster/zonemaster#1477

Changes

• Zonemaster::Engine::Recursor::_resolve_cname() now returns undef when the CNAME resolution fails, along with a new message tag: CNAME_UNRESOLVABLE
• Update unit tests:
  • Move the content of scenario TOO-LONG-CNAME-CHAIN to a new scenario TOO-MANY-CNAME,
  • Update scenario TOO-LONG-CNAME-CHAIN to check for tag CNAME_CHAIN_TOO_LONG instead
  • Add scenario UNRESOLVABLE-CNAME

How to test this PR

Unit tests are updated and should pass.

[marc-vanderwal]

So far, so good.

[marc-vanderwal]

The CNAME_UNRESOLVABLE tag is output in some cases where the final query for a CNAME chain gives NODATA. That case can happen fairly frequently and should simply be treated like a negative response (i.e. NXDOMAIN), not like a resolution failure (as if we had gotten SERVFAIL).

I’d suggest a different name for that tag. How about CNAME_TO_NODATA?

[matsduf]

I’d suggest a different name for that tag. How about CNAME_TO_NODATA?

It think that is a good suggestion. Should there be another tag, CNAME_TO_NXDOMAIN?

For me CNAME_UNRESOLVABLE sounds like we cannot resolve the CNAME record.

[marc-vanderwal]

Yes, a CNAME_TO_NXDOMAIN might make sense too.

In any case, I don’t think the ERROR level is appropriate for these tags because they do not indicate a problem so severe that the zone is unresolvable.