DNS protocol cleanup: require correct AA bit

Description

Allegedly different resolvers treat AA bit in responses differently, and this is causing different operational problems for each implementation. PowerDNS and Knot Resolver have had issues with that.

Proposal by Peter van Dijk is to be strict on AA bit and punish non-compliance. Main motivation seems to be code simplification when it comes various combinations of NXDOMAIN/NOERROR without SOA RR and/or "extra" NS records in authority which are sometimes added as "good measure" but do not actually mean a referral.

Anecdotes from the field: a) Ralf Weber from Akamai has some reservations:

Given that a lot of people use resolvers in front of their authoritative servers who don't send AA I fail to envision what resolvers should do. If we drop non AA answers I expect huge portion of the Internet to go dark, though I don't have hard numbers on that.

b) Recent versions of PowerDNS switched to stricter mode and insist on AA bit being correct. A person from Deutsche Telecom claims this:

To give a sense of possible impact, we have tens of millions of subscribers and only 5-10 cases per year estimated. So I guess nothing would "go dark" 🙂

Links / references

Thread https://chat.dns-oarc.net/community/pl/57pcpenfkf86tr8onmhn1q5a4a

Personally I argue this is a) not significant enough b) not widespread enough to warrant full fledged flag day, but we can start being stricter on AA bit if we decide to do so. PowerDNS already went in that direction so first-mover disadvantage is already paid :-)