strange resolver behavour

[Tom Evans]

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:
>  > > On Mon, 11 Oct 2010, Eugene Grosbein wrote:
>  > >  > Hi!
>  > >  >
>  > >  > FreeBSD 8.1-STABLE:
>  > >  >
>  > >  > # host koin-nkz.com.
>  > >  > koin-nkz.com has address 62.231.164.101
>  > >  > Host koin-nkz.com not found: 3(NXDOMAIN)
>  > >  >
>  > >  > This domain does not have MX records but NXDOMAIN seems to wrong return
>  > >  > code to me. Think about MTA that does look-up for MX first,
>  > >  > obtains NXDOMAIN and rejects mail.
>  > >
>  > > 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.

>
> 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.

>
> cc Doug for obvious reasons, ie I'm likely missing something obvious :)
>
> cheers, Ian
>
> (BTW I'd begun to reply off-list, as neither your private copy nor your
> reply to the list indicated cc'ing the other .. googlemail quaintness?)

No gmail quaintness, PEBKAC. I replied off-list first, meaning to
reply on-list, so I re-sent to list :/

Cheers

Tom