From owner-freebsd-net@FreeBSD.ORG Wed Oct 13 01:57:35 2010 Return-Path: Delivered-To: net@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 435B9106564A for ; Wed, 13 Oct 2010 01:57:34 +0000 (UTC) (envelope-from smithi@nimnet.asn.au) Received: from sola.nimnet.asn.au (paqi.nimnet.asn.au [115.70.110.159]) by mx1.freebsd.org (Postfix) with ESMTP id 8A3BC8FC0A for ; Wed, 13 Oct 2010 01:57:33 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by sola.nimnet.asn.au (8.14.2/8.14.2) with ESMTP id o9D1vUhd087294; Wed, 13 Oct 2010 12:57:31 +1100 (EST) (envelope-from smithi@nimnet.asn.au) Date: Wed, 13 Oct 2010 12:57:30 +1100 (EST) From: Ian Smith To: Tom Evans In-Reply-To: Message-ID: <20101013103715.V2036@sola.nimnet.asn.au> References: <4CB2AF28.30309@rdtc.ru> <20101012152857.X2036@sola.nimnet.asn.au> <20101013010757.N2036@sola.nimnet.asn.au> MIME-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="0-1776953488-1286935050=:2036" Cc: net@freebsd.org Subject: Re: strange resolver behavour X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 13 Oct 2010 01:57:35 -0000 This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --0-1776953488-1286935050=:2036 Content-Type: TEXT/PLAIN; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT On Tue, 12 Oct 2010, Tom Evans wrote: > On Tue, Oct 12, 2010 at 3:39 PM, Ian Smith wrote: > > On Tue, 12 Oct 2010, Tom Evans wrote: > >  > On Tue, Oct 12, 2010 at 10:05 AM, Ian Smith wrote: [..] > >  > > If a domain has no MX server, how's an MTA supposed to do mail with it? > >  > > > >  > > >  > The same way as has been done since they invented the MX record type - > >  > if no MX record exists, fallback to an A record. See RFC 5321, section > >  > 5.1. > > > > Well thanks Tom, I did - but which A record? > > > > Taking the '5.3. Master file example' in RFC1035, what is the A response > > for 'ISI.EDU.' where the domain itself has no specific A RR?  Would it > > be that of VENERA.ISI.EDU, or that of the first A listed, ie A.ISI.EDU? > > That domain has an MX record, so it wouldn't do either. When I do a > dig isi.edu, I just get a single A record, so I would assume an SMTP > server would attempt to deliver mail there. Sorry, I didn't express that very well; I meant to literally use that example as written, um, 23 years ago, when the example had no A record for the domain itself as hostname, only for various specific hosts. It does these days of course, but it was a valid example to hand of a domain having no specific A record for the domain; not something I do, nor probably so common nowadays, though I not that infrequently find websites that only resolve with the www hostname. And then to compound the confusion, I suggested imagining a domain setup like that - having no A RR - but having no MX RRs either. From memory, I don't think an 'A' query for such a domain returns anything, but I didn't know where to find one to test offhand. > > And in either case - assuming a domain without any MX RR as above - why > > would that A response be expected to address a mail server? > > Initially, email came about just before people started using DNS, so > mail servers were found using hostnames and directly delivering to the > host. > > DNS then came into being, and you could look up a MD or MF record to > find the mail host. This didn't work too well, which is why we MX > records were invented. By that point, people had been relying on mail > servers looking up an A record if MD/MF didn't exist, so the behaviour > was preserved. Thanks. I guess if there's no A returned for the domain, or no mailserver on the A returned, our MTA will find out soon enough .. cheers, Ian --0-1776953488-1286935050=:2036--