NOTIMP for unrecognized qtypes

[Petr Špaček]

On 28.7.2017 00:15, Jacob Hoffman-Andrews via Unbound-users wrote:
> On 07/27/2017 01:28 PM, Robert Edmonds wrote:
>> 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.
> 
> Thanks, this is very helpful! Adding in a little more from RFC 1035:
> 
> 
> QTYPE           a two octet code which specifies the type of the query.
>                 The values for this field include all codes valid for a
>                 TYPE field, together with some more general codes which
>                 can match more than one type of RR.
> 
> So I take it the distinction is that QTYPE represents the "type of" the
> query, while OPCODE represents the "kind of" the query, and since RCODE
> refers to "kind of" it only affects OPCODE? I can sort of see the
> distinction, but since "type of" and "kind of" have almost identical
> colloquial meanings I'm surprised that the distinction is not called out
> in more detail.

Well, the spec is from 1987. Even the meaning of MUST/SHOULD etc. was
not standardized yet back then ...

Anyway, I will try to give you some examples to help you understand this:

Imagine that term "query" in OPCODE decription refers to "type of
request", i.e. the term "query" here means "packet sent to server". In
other words, OPCODE affects semantics of the DNS message.

Exaple: One of OPCODEs is UPDATE which modifies data on authoritative
server.

RCODE=NOTIMP is suitable e.g. for situation when a resolver got UPDATE
request etc. I.e. the resolver is replying with "I have no idea what you
operation are requesting from me".


On the other hand, RR type in general do not affect message semantics,
so there is no reason to return NOTIMP based on RR type.


I hope it helps.

-- 
Petr Špaček  @  CZ.NIC