Re: interop problems with getaddrinfo() address selection

Tony Finch <dot@dotat.at> Mon, 10 December 2007 19:46 UTC

Return-path: <discuss-bounces@apps.ietf.org>
Received: from [127.0.0.1] (helo=stiedprmman1.va.neustar.com) by megatron.ietf.org with esmtp (Exim 4.43) id 1J1oad-0006yP-Ft; Mon, 10 Dec 2007 14:46:55 -0500
Received: from discuss by megatron.ietf.org with local (Exim 4.43) id 1J1oac-0006vf-OK for discuss-confirm+ok@megatron.ietf.org; Mon, 10 Dec 2007 14:46:54 -0500
Received: from [10.91.34.44] (helo=ietf-mx.ietf.org) by megatron.ietf.org with esmtp (Exim 4.43) id 1J1oac-0006tQ-DN for discuss@apps.ietf.org; Mon, 10 Dec 2007 14:46:54 -0500
Received: from ppsw-6.csi.cam.ac.uk ([131.111.8.136]) by ietf-mx.ietf.org with esmtp (Exim 4.43) id 1J1oab-0002Dc-Rq for discuss@apps.ietf.org; Mon, 10 Dec 2007 14:46:54 -0500
X-Cam-SpamDetails: Not scanned
X-Cam-AntiVirus: No virus found
X-Cam-ScannerInfo: http://www.cam.ac.uk/cs/email/scanner/
Received: from hermes-1.csi.cam.ac.uk ([131.111.8.51]:54268) by ppsw-6.csi.cam.ac.uk (smtp.hermes.cam.ac.uk [131.111.8.156]:25) with esmtpa (EXTERNAL:fanf2) id 1J1oaa-0000MK-Js (Exim 4.67) (return-path <fanf2@hermes.cam.ac.uk>); Mon, 10 Dec 2007 19:46:52 +0000
Received: from fanf2 (helo=localhost) by hermes-1.csi.cam.ac.uk (hermes.cam.ac.uk) with local-esmtp id 1J1oaa-0001JN-4o (Exim 4.67) (return-path <fanf2@hermes.cam.ac.uk>); Mon, 10 Dec 2007 19:46:52 +0000
Date: Mon, 10 Dec 2007 19:46:52 +0000
From: Tony Finch <dot@dotat.at>
X-X-Sender: fanf2@hermes-1.csi.cam.ac.uk
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
Subject: Re: interop problems with getaddrinfo() address selection
In-Reply-To: <200712101815.NAA27674@Sparkle.Rodents.Montreal.QC.CA>
Message-ID: <Pine.LNX.4.64.0712101935070.24448@hermes-1.csi.cam.ac.uk>
References: <Pine.LNX.4.64.0712061901020.24448@hermes-1.csi.cam.ac.uk> <200712062020.PAA16213@Sparkle.Rodents.Montreal.QC.CA> <Pine.LNX.4.64.0712071735070.24448@hermes-1.csi.cam.ac.uk> <475B0ABC.5090806@cs.utk.edu> <Pine.LNX.4.64.0712092000150.29087@hermes-1.csi.cam.ac.uk> <475C5047.3010807@cs.utk.edu> <Pine.LNX.4.64.0712100739410.29087@hermes-1.csi.cam.ac.uk> <200712101815.NAA27674@Sparkle.Rodents.Montreal.QC.CA>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
X-Spam-Score: -4.0 (----)
X-Scan-Signature: 2409bba43e9c8d580670fda8b695204a
Cc: discuss@apps.ietf.org
X-BeenThere: discuss@apps.ietf.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: general discussion of application-layer protocols <discuss.apps.ietf.org>
List-Unsubscribe: <https://www1.ietf.org/mailman/listinfo/discuss>, <mailto:discuss-request@apps.ietf.org?subject=unsubscribe>
List-Post: <mailto:discuss@apps.ietf.org>
List-Help: <mailto:discuss-request@apps.ietf.org?subject=help>
List-Subscribe: <https://www1.ietf.org/mailman/listinfo/discuss>, <mailto:discuss-request@apps.ietf.org?subject=subscribe>
Errors-To: discuss-bounces@apps.ietf.org

On Mon, 10 Dec 2007, der Mouse wrote:
>
> But whose definition of "equivalent" applies?  From a client point of
> view, close addresses are not equivalent to distant addresses - the
> client wants a nearby server, not unreasonably - making sorting by
> match length at least moderately reasonable.

In practice, services advertise server-specific names as well as umbrella
names, so that clients that care about which server they use can ask for
it.

> Any architecture that depends on the DNS preserving record ordering
> (never mind depending on clients doing anything in particular with the
> order the DNS happens to give them) is already broken.

Services that use round robin DNS do not depend on preserved record
ordering: quite the opposite, they depend on disordering. (Bind does
round-robin when acting as a cache as well as an authoritative server.)
You can claim it's broken until you are blue in the face, but it works in
practice and it is widely documented (though not by the IETF) without
caveats.

Tony.
-- 
f.a.n.finch  <dot@dotat.at>  http://dotat.at/
TYNE DOGGER: NORTHERLY 5 TO 7 BECOMING VARIABLE 4. ROUGH OR VERY ROUGH
DECREASING MODERATE OR ROUGH. SHOWERS. MAINLY GOOD.