Subject: [DNSOP] why no QDCOUNT>1

From: Paul Vixie <[email-masked]>
Date: Sun, 21 Jul 2019 17:29:22 +0000

someone here recently asked why multiple questions are allowed by the DNS 
header format but not implemented. this was in the context of performance 
comparisons between tcp/53 and udp/53, vs. DoT, vs. DoH.

the reason it's not implemented is that there's only one RCODE in the 
response, so if one question results in RCODE=0 but another is RCODE=3, there 
is no way to signal this. there's a similar issue with AA. and if there's a 
delegation response for one but not the other, and one is the parent of the 
other, ambiguity runs rampant.

the reason this hasn't been repaired via a massive protocol overhaul is that 
it's a trivial matter to pipeline multiple questions, either on udp/53, DoT, 
or even DoH. the extra DNS headers cost ten (10) octets per question, which is 
insignificant.

here's some undocumented advice: don't initiate transactions via UDP/53 back 
to back, because this microburst will probably overflow a queue somewhere. if 
pipelining, either use DoT, or make sure there's a small delay between 
subsequent UDP/53 transactions. 1.5 milliseconds is enough.

-- 
Paul

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From: Mark Andrews <[email-masked]>
Date: Mon, 22 Jul 2019 03:49:39 +1000

Also with IQUERY (obsolete) you could get multiple QNAMEs in the response.

> On 22 Jul 2019, at 3:29 am, Paul Vixie <[email-masked]> wrote:
> 
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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: [email-masked]