[dns-operations] MX record definition?

Paul Vixie vixie at isc.org
Thu Dec 18 05:57:31 UTC 2008

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

in that it will never work reliably and is rarely what's intended,
that's a literally true statement, but in no way germane to the topic.

> > 	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

even moreso than the above, on all counts.

> 	sounds to me like -in some cases- a fully numeric string that 
> 	-could- be parsed as an IP address would be legal.

now you're changing the subject from whether it's a config error to
whether it's legal.  it's always legal, but can be meaningless in a
way that does not look meaningless, and therefore always dangerous.

> 	i've seen and used fully numeric (domain names) in the past
> 	and may do so in the future... 

no doubt.

> > 	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.

DNS is just a bearer channel for this.  RFC 974, RFC 1123, and others
talk about the content and meaning of the MX RDATA.  an MTA who treats
the "123.321.23.123." domain in an MX RDATA as an IP address isn't so
much broken as it is acting outside of the internet standards.  the
reason that such MX RDATAs are consider configuration errors is that
since treatment of same as an IP address cannot be depended upon and
is somewhat rare.

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