about the additional section

Daniel McDonald dan.mcdonald at austinenergy.com
Thu Sep 1 14:41:54 UTC 2011


On 8/31/11 10:13 PM, "风河" <shorttag at gmail.com> wrote:

> Hello,
> 
> I found that some queries have got the response which has additional
> section, but some haven't.
> For example, this query with www.google.com got the answer with
> additional section set:
> 
> $ dig www.google.com
> ;; MSG SIZE  rcvd: 236
> $ dig www.google.com
> 
> ;; MSG SIZE  rcvd: 220

> My question is under what condition the nameserver answers the query
> with additional section set?

When there is sufficient space for the additional information in the packet,
it is sent.

If the answer and authority were too big to fit in the packet, then the DNS
server is supposed to set the "truncated data" bit so that the resolver
knows it is supposed to try TCP or EDNS0 to get a larger response.  But the
truncated data bit is not sent when the only things truncated are additional
information records.

When querying google.com A using edns=0, the size of the message is 271
bytes, including all 4 RRs in the additional section.


-- 
Daniel J McDonald, CCIE # 2495, CISSP # 78281





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