[dns-operations] MX record definition?

Mark Andrews Mark_Andrews at isc.org
Thu Dec 18 01:36:59 UTC 2008

In message <a06240800c56f3fcb2752@[]>, Edward Lewis writes:
> That's the place for the definition.
> Don't know if there's a document that says why/that they are bad.
> I can only venture a guess - probably because the reason they are bad 
> is weakly written application code that assumes the name in the MX 
> record is a host name and not a domain name.  (Host names in RFC 
> 1123.)  I.e., what do you get if you try to look up the A record for 
> 123.321.23.123?

	Well the description pretty clearly says it it a host.

EXCHANGE        A <domain-name> which specifies a host willing to act as
                a mail exchange for the owner name.

	Seeing a IP address in the exchange field of a MX record
	in a master file is, almost always, a configuration error.

	Seeing a IP address with a period at the end in the exchange
	field of a MX record in a master file is also, almost always,
	a configuration error

	MTA that treat a "123.321.23.123." as a IP address are broken.

> As far as being a no-no: they aren't prohibited by the DNS 
> specification.  Perhaps there's a mail protocol restriction.
> At 15:21 -0800 12/17/08, Seth Mattinen wrote:
> >Does anyone know offhand where the MX record is defined? The best I 
> >can find is section 3.3.9 in RFC 1035. I'm trying to find a 
> >reference that says numeric MX records are bad. I'm assuming they're 
> >a no-no, at least that's what I've always been told.
> >
> >~Seth
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> -- 
> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
> Edward Lewis
> NeuStar                    You can leave a voice message at +1-571-434-5468
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Mark Andrews, ISC
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PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at isc.org

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