Handle DNS queries of type MX

[ddebroy]

The Docker embedded server was not handling DNS queries of type MX for containers. They were instead being passed down to the host leading to NXDOMAIN responses not compliant with SMTP RFC 5321 (in case someone wanted to host a mail server in a container and deliver emails to the server). This PR adds support to respond to MX queries in a way compliant with RFC 5321 for containers.

Note that providing a full mechanism for starting a mail server in a container/service and registering MX RRs within the Docker resolver is beyond the scope of this PR.

[ddebroy]

Addresses #2026

[thaJeztah]

Do we need a unit-test for this?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@83862f4). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #2041   +/-   ##
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  Coverage          ?   40.4%           
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  Files             ?     138           
  Lines             ?   22141           
  Branches          ?       0           
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  Hits              ?    8947           
  Misses            ?   11880           
  Partials          ?    1314

Impacted Files	Coverage Δ
resolver.go	36.64% <100%> (ø)

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[ddebroy]

@thaJeztah good point. I was indeed looking for unit-tests around the resolver to add something for this but could not find anything. If there is a suite already, let me know. Otherwise we will need something like resolver_test.go to cover all the name query scenarios.

[thaJeztah]

Yes, I noticed the same (no _test.go); would probably be good to have, to also encourage adding tests (where they make sense of course)

[fcrisciani]

I see this comment but not sure how is it implemented below? does it mean that is enough to just reply with the same records?

[ddebroy]

If we are able to resolve the hostname, for MX, we need to reply with a DNS msg with empty answers section (with status set to default RcodeSuccess). createRespMsg generates the DNS msg with empty answers section and status set to RcodeSuccess. This is very similar to what we are doing down below in handleIPQuery for ipv6 AAAA querries that resolve to a ipv4 address.

[ddebroy]

I have added the unit test for resolver.

[selansen]

LGTM

[fcrisciani]

LGTM