DOH: retrieving an RRset from a child node

Hello.

Some data is published in the DNS at a child node of the node
which corresponds to the hostname, as documented in
[RFC8552](https://datatracker.ietf.org/doc/rfc8552/).

In particular, current (experimental) ESNI deployment uses
TXT records and child-prefix "_esni".

I've been thinking about how to extend dohprobe() to query for
an RRset containing such data, which action may be useful for
other use cases involving existing TXT, SRV, TLSA, and URI
RR types, as well as future RRtypes, such as SVCB.

Here's what I'm minded to do.

- Introduce new functions dohprobepfx() and doh_encodepfx(),
  corresponding to dohprobe() and doh_encode respectively,
  each with an additional `const char *prefix` parameter.

- Retain dohprobe() and doh_encode(), but as trivial
  wrappers around the "pfx" functions, each passing NULL
  as the argument to the corresponding "pfx" function, so
  that existing invoking code need not be modified, and
  duplication of code can be avoided.

If this doesn't seem too daft, I'll work up a PR.

I expect to deal with reception of the TXT response in a
separate PR, so as to keep changes small and modular.

/Niall

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Received on 2019-11-18