edns responses not sent by DNS Server

Barry Margolin barmar at alum.mit.edu
Tue May 30 15:45:24 UTC 2017

In article <mailman.206.1496135739.702.bind-users at lists.isc.org>,
 Harshith Mulky <harshith.mulky at outlook.com> wrote:

> Hello Mark,
> Yes the client is retrying the query over TCP. 
> But initially I am getting no Answers 
> The ANSWER is as below
> ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id:  18094
> ;; flags: qr aa tc rd ad ; QUESTION: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL:
> 1
> ; EDNS: version: 0, flags: do; udp: 4096
> ;pcr21381.dflt.vzb.com.         IN      NAPTR
> Should the server be sending some responses which are truncated (or) no
> Responses in this case?

BIND will omit the Additional Section (and maybe also the Authority 
Section?) if that allows the response to fit. Otherwise I believe it 
just sends an empty response, and the client is supposed to retry with 

The problem with sending a partial Answer Section is that there's no way 
for the client to know if the omitted answers are important. So it has 
to retry anyway.

Barry Margolin
Arlington, MA

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