[Doh] special meta QTYPEs

[Tony Finch <[e-mail]>]

I think there needs to be some specific wording about query meta-types
(i.e. 128 <= QTYPE <= 255).

Meta-types can change the framing rules of the response, e.g. IXFR and
AXFR, so a DoH server needs to understand them even if it is implemented
as a mostly-blind proxy.

My DoH implementation generates its own NOTIMP response if it gets a query
with 128 <= QTYPE <= 254 (it allows QTYPE=*/ANY). The other meta type is
OPT, but it isn't special-cased because it's safe to proxy.

BIND returns FORMERR rather than NOTIMP for unknown meta-types; it
seems to reserve NOTIMP for known but deprecated types.

I think it is reasonable for an implementation to support TKEY or MAILA or
MAILB if it wishes. I don't think it's possible to fit AXFR or IXFR into
DoH because their responses involve multiple DNS messages.

Suggested wording:

    A DoH server MUST return a DNS error response, either RCODE=1 (format
    error) or RCODE=4 (not implemented) if it receives a request with an
    unknown meta-QTYPE between 128 and 254 inclusive. The meta-QTYPE 255
    (* or ANY) SHOULD be supported. The meta-QTYPEs 251 (IXFR) and 252
    (AXFR) MUST NOT be supported and the DoH server MUST return an error
    response.

    This is because meta-QTYPEs can affect the way a DNS response is
    framed; for example, AXFR and IXFR responses can span multiple DNS
    messages, whereas A DoH response can only contain one DNS message.

Tony.
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