Subject: Re: DNS query lengths

From: [email-masked]
Date: Fri, 01 Jun 2001 11:55:11 +1000

See:

http://www.ietf.org/internet-drafts/draft-ietf-dnsext-message-size-04.txt

> Hello All,
> 
>  I'm working on a specialized communications system in which the length of
> the a DNS query is a matter of great importance.  I've looked at RFCs 1034
> and 1035, and understand how long a DNS query is for a given host name.
> I've also looked a bit at DNSsec.
> 
> Main question: what extensions/modifications to the DNS query format
> (described in RFC 1035), described in other RFCs, would affect the length of
> a query?  In particular, what such extensions/modifications are in common
> use today, or likely will be in use in the next few years?
> 
> Related matter: if I've addressed this question to the wrong venue, please
> suggest a better venue, and I'll stop polluting this one.
> 
> Thanks very much,
> --daniel
> 
> 
> 
> 
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--
Mark Andrews, Nominum Inc.
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: [email-masked]

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---

From: Brian Wellington <[email-masked]>
Date: Thu, 31 May 2001 20:04:22 -0700 (PDT)

> use today, or likely will be in use in the next few years?

EDNS (Extension Mechanisms for DNS, RFC 2671) capable clients send
requests with an OPT record, The OPT record, in the normal case, is 11
bytes.  EDNS also specifies a mechanism for adding additional fields in
the OPT record.  None are in common use right now, and the proposed ones
are not likely to be in widespread use.

Brian

---

From: Kevin Darcy <[email-masked]>
Date: Thu, 31 May 2001 19:11:07 -0400

Did you mean just *query* length, or query and response lengths?

The SIG and NXT records of DNSSEC will lengthen queries and responses (although
SIG records are typically seen in responses, a SIG(0) can be included in a
query as a "transaction signature", see RFC 2931). Similarly, a TSIG record
(see RFC 2845) may appear in a query or response as a transaction signature.

Moreover, the OPT record of EDNS0 (see RFC 2671) may appear in a query or
response. I seem to recall there were recent moves afoot for EDNS0 to be made
*mandatory* for all DNSSEC-aware clients and/or servers, since one of the
primary functions of EDNS0 is to advertise message buffer sizes larger than 512
bytes, which is likely to be necessary with DNSSEC in order to prevent
TCP retransmission. The size of an OPT record can be highly variable, since in
a sense it's a container for an indeterminate number of option "sub-records".

Lastly, it may come to pass that SRV records (see RFC 2782) will largely
supplant the use of A records, at least for "common" client/server interactions
such as web browsing. locating local authentication and/or directory services,
etc.. Since SRV records are larger than A records, this will tend to increase
response size.

Offhand, those are the only post-RFC-103[45] extensions I can think of that
would increase query and/or response size. But maybe others on this list can
think of more....

I think you have your work cut out for you.

- Kevin