Date: Wed, 13 Oct 2010 12:57:30 +1100 (EST) From: Ian Smith <smithi@nimnet.asn.au> To: Tom Evans <tevans.uk@googlemail.com> Cc: net@freebsd.org Subject: Re: strange resolver behavour Message-ID: <20101013103715.V2036@sola.nimnet.asn.au> In-Reply-To: <AANLkTi=FLBtpmnB-08Xxj3=Aqe7k24qPmn0CVB9cFE6J@mail.gmail.com> References: <4CB2AF28.30309@rdtc.ru> <20101012152857.X2036@sola.nimnet.asn.au> <AANLkTikKQO%2BjQy=2pNMngdPGaY1AO7Bqwt-Dd0sK1Ufd@mail.gmail.com> <20101013010757.N2036@sola.nimnet.asn.au> <AANLkTi=FLBtpmnB-08Xxj3=Aqe7k24qPmn0CVB9cFE6J@mail.gmail.com>
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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 <smithi@nimnet.asn.au> wrote: > > On Tue, 12 Oct 2010, Tom Evans wrote: > > > On Tue, Oct 12, 2010 at 10:05 AM, Ian Smith <smithi@nimnet.asn.au> 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--
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