NOTIMP for unrecognized qtypes

[Robert Edmonds]

Jacob Hoffman-Andrews via Unbound-users wrote:
> I'm trying to write some documentation for users of Let's Encrypt about
> CAA. I believe it's the case that standards-conformant authoritative
> resolvers should return NOERROR for qtypes they don't recognize, rather
> than NOTIMP. Is this correct? If so, what is the relevant standard? I
> haven't been able to find a citation in
> https://tools.ietf.org/html/rfc3597,
> https://tools.ietf.org/html/rfc6895, or https://tools.ietf.org/html/rfc1035.

RFC 1035 seems to be pretty clear that NOTIMP applies to the OPCODE, not
the QTYPE.

    Mockapetris                                                    [Page 25]

    RFC 1035        Domain Implementation and Specification    November 1987


    4.1.1. Header section format

    […]

    OPCODE          A four bit field that specifies kind of query in this
                    message.  This value is set by the originator of a query
                    and copied into the response.  The values are:

                    0               a standard query (QUERY)
    […]

    RCODE           Response code - this 4 bit field is set as part of
                    responses.  The values have the following
                    interpretation:
    […]
                    4               Not Implemented - The name server does
                                    not support the requested kind of query.
    […]

That is, OPCODE specifies the "kind of query", and NOTIMP indicates that
the "kind of query" (= OPCODE) isn't supported.

-- 
Robert Edmonds
edmonds at debian.org