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

John Wobus jw354 at
Fri Apr 15 15:24:25 UTC 2016

>> 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.


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.

The actual issue I was investigating is indeed unrelated.  I was merely
eliminating possibilities.

John Wobus
Cornell IT

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