[dns-operations] Non-EDNS FORMERR with qdcount==0?
ietf-dane at dukhovni.org
Mon Nov 18 10:42:27 UTC 2019
On Mon, Nov 18, 2019 at 03:54:36PM +0530, Mukund Sivaraman wrote:
> > MUST servers reflect the question (on error?) or can they leave it
> > out?
> It would depend on how much of the question was syntactically parsable.
My example queries had a well-formed question, along with an EDNS(0)
OPT record, but the FORMERR response had an empty question section.
So, whether that's valid or not, I guess I'll have to accept that
as a matching response that indicates lack of EDNS(0) support, and
retry without EDNS.
> > Is FORMERR special in this regard (not being an answer to a question),
> > but an error processing my query packet?
> Maybe the outlook.com implementation thinks this question is
> syntactically incorrect, and so it can't use it in the reply.
It groks the same question once the OPT record is left out.
> > FWIW, "unbound-host" handles the "empty" FORMERR response, and retries the
> > query without EDNS. Is unbound-host doing what's expected, or employing
> > a work-around for known breakage?
> Loop's resolver does the same too, and appears to be a workaround (the
> code is from 2000 by Bob Halley written for BIND, and it describes the
I pushed a bugfix:
+-- When the response 'RCODE' is 'FormatErr', the server did not understand our
+-- query packet, and so is not expected to return a matching question.
checkRespM :: Question -> Identifier -> DNSMessage -> Maybe DNSError
checkRespM q seqno resp
| identifier (header resp) /= seqno = Just SequenceNumberMismatch
+ | FormatErr <- rcode $ flags $ header resp
+ ,  <- question resp = Nothing
| [q] /= question resp = Just QuestionMismatch
| otherwise = Nothing
More information about the dns-operations