[dns-operations] Name server turning off RD bit in response - just curious

Robert Edmonds edmonds at isc.org
Tue Aug 7 17:39:32 UTC 2012

Anand Buddhdev wrote:
> On 07/08/2012 13:40, Faasen, Craig wrote:
> > RD is set to 1 in the query, but is 0 in the response.
> > Which is not compliant with RFC 1035: "RD Recursion Desired - this
> > bit may be set in a query and is copied into the response."
> > 
> > Out of curiosity, any idea why a name server would want to change
> > the RD bit ? (except to break an unsuspecting script ;)
> Hi Craig,
> In my opinion, the RD flag has no value in a response really. It's just
> a way for a client to signal to a server that it wants the server to
> perform recursive queries. While RFC 1035 requires the server to copy
> the value of RD from the query into the response, a client shouldn't
> even be bothering to look at RD in a response, and certainly should not
> break if its value in the response doesn't match the query.

indeed, and for an example of the opposite behavior, see
ns[1-4].google.com, which set the RD bit in responses regardless of the
RD bit in the query.

Robert Edmonds
edmonds at isc.org

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