Delegation NS-records when zones share an authority server

[Fred Morris]

TLDR: NS records occur above and below zone cuts.

On Wed, 12 Apr 2023, John Thurston wrote:
>
> We have authority over state.ak.us, which we publish as a public zone. We 
> also publish challenge.state.ak.us as a public zone.
>
> The public NS records for state.ak.us are: ns4.state.ak.us and 
> ns3.state.ak.us The NS records for challenge.state.ak.us are the same.
>
> I recently noticed there were no NS records _in the state.ak.us zone_ for 
> challenge.state.ak.us.

So nothing above the zone cut == there is no zone cut. (IMO)

> This had me scratching my head . . "how can this be 
> working?", until I remembered the same instances of BIND were serving out 
> both zones. There _were_ NS records in the challenge.state.ak.us zone, BIND 
> had them, was authoritative, so would answer with them; BIND didn't need to 
> look in the state.ak.us zone to find them.

Yup.

> Some experimentation shows that even if I insert NS records into state.ak.us 
> (for challenge.state.ak.us), BIND does not add them to its answer when asked 
> "dig NS challenge.state.ak.us". I interpret this to mean that while this 
> instance of BIND is authoritative for both zones, it answers with information 
> from the most specific zone it has, and ignores values in the delegating 
> zone. And that makes sense to me.

Yup.

> Now the question is, should I insert NS records into state.ak.us (for 
> challenge.state.ak.us) anyway?
> [...]
>
> Unknown:
>
> * Does the answer change if we want to start signing either zone?

I suspect you don't need the NS records in challenge.state.ak.us and if 
you remove them then the records in challenge.state.ak.us are simply part 
of the state.ak.us zone since they're served off of the same server. Glue 
records (above the cut) are essentially the same NS record(s) published on 
nameservers above the zone cut as within the zone on the nameservers for 
the zone proper (below the cut).

On the other hand maybe whatever software you're using to manage / serve 
DNS does something with those records (or requires them since / if the two 
namespaces are loaded as separate zones).

In terms of NXDOMAIN and SOA queries, both state.ak.us and 
challenge.state.ak.us seem to do the right thing in terms of pretending to 
be separate zones, e.g. in the first case returning the correct domain in 
the AUTHORITY and in the second case returning the relevant SOA records 
directly in the ANSWER.

--

Fred Morris, internet plumber