strange resolver behavour

[Ian Smith]

On Tue, 12 Oct 2010, Tom Evans wrote:
 > On Tue, Oct 12, 2010 at 3:39 PM, Ian Smith <smithi at nimnet.asn.au> wrote:
 > > On Tue, 12 Oct 2010, Tom Evans wrote:
 > >  > On Tue, Oct 12, 2010 at 10:05 AM, Ian Smith <smithi at 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