Bind response to query's very small edns udp payload size

Barry Margolin barmar at
Fri Apr 15 15:47:06 UTC 2016

In article <mailman.564.1460733871.73610.bind-users at>,
 John Wobus <jw354 at> wrote:

> >> What does bind try to do if the client specifies a udp size of less than 
> >> 512?
> > 
> > From RFC 6891:
> > 
> > Values lower than 512 MUST be treated as equal to 512.
> Doh.
> The behavior I saw was a shorter authority section and no additional section 
> (or TO)
> when I specified a UDP buffer of 200 as opposed to sending a non-EDNS 
> request.
> But I do see that an EDS request with UDP buffer size 512 gives me
> exactly the same result as with buffer size 200.  I speculate
> bind will skip part of its usual authority section and provide no
> TC bit as long as it can give the full answer section.

RFC 2181 says:

"The TC bit should be set in responses only when an RRSet is required 
as a part of the response, but could not be included in its entirety. 
The TC bit should not be set merely because some extra information could 
have been included, but there was insufficient room."

So if there are optional records that could be included in the Authority 
section, but they aren't required, it can leave them out without setting 

Barry Margolin
Arlington, MA

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