It's actually quite simple, this is allowed:
example.com. IN MX 10 mail.example.com.
$ORIGIN example.com.
mail IN A 10.5.2.69
This is not allowed:
example.com. IN MX 10 mail.example.com
example.com. IN A 10.5.2.69
$ORIGIN example.com.
mail IN CNAME example.com
This is not allowed either:
example.com IN MX 10 10.5.2.69
Thus, a domain may have an MX record without having an A record for the zone
itself, but, the MX record must point to a host, and that host must have an
A record. In orther words, in extremely odd set-ups, this is also allowed:
example.com. IN A 10.5.2.69
example.com. IN MX 10 example.com.
This would work, because example.com has an A record, and is thus,
considered a host, instead of a zone. Any host (with an A record) may act as
an MX for any host or any zone.
Infact, you could have an MX for a non-existant host, though stupid, it is
legal:
$ORIGIN example.com.
foo IN MX 10 mail
mail IN A 10.5.2.69
However, in order to comply with most registrar's standards, ie, domains
must have an MX for the entire zone, this would probably be more correct:
$ORIGIN example.com.
@ IN MX 10 mail
foo IN MX 10 mail
mail IN A 10.5.2.69
In this example, example.com and foo.example.com have no A records, but MX
records pointing to mail.example.com, which has an A record, and can thus be
an MX for the zone and the non-existant host foo.example.com.
You could even do this:
$ORIGIN example.com.
@ IN MX 10 mail
* IN MX 10 mail
mail IN A 10.5.2.69
In this case, mail for both example.com, which has no A record, and any host
or subdomain under example.com, even non-existant, will be handled by
mail.example.com, which must ofcourse have the A record.
Will
----- Original Message -----
From: "Tony Finch" <dot@???>
To: <jpdalbec@???>
Cc: <exim-users@???>
Sent: Thursday, November 20, 2003 11:52 AM
Subject: Re: [Exim] MX without A allowed by which RFC's?
> John Dalbec <jpdalbec@???> wrote:
> >What RFC allows an MX record (example.com -> mail.example.com say) to
exist
> >without an A record for example.com existing?
>
> RFC 1123 says "With the advent of the domain system and of mail routing
> using mail-exchange (MX) resource records, implementors should now think
> of delivering mail to a user at a domain, which may or may not be a
> particular host."
>
> Tony.
> --
> f.a.n.finch <dot@???> http://dotat.at/
> MULL OF KINTYRE TO ARDNAMURCHAN POINT: SOUTH OR SOUTHWEST 4 OR 5,
OCCASIONALLY
> 3 IN SHELTER, INCREASING 5 OR 6 ON THURSDAY. SHOWERS. MAINLY GOOD.
MODERATE OR
> ROUGH, BUT SLIGHT IN SHELTER.
>
> --
>
> ## List details at http://www.exim.org/mailman/listinfo/exim-users Exim
details at
http://www.exim.org/ ##
>
>
>
