[dns-operations] MX record definition?

bmanning at vacation.karoshi.com bmanning at vacation.karoshi.com
Thu Dec 18 14:24:45 UTC 2008

On Thu, Dec 18, 2008 at 05:57:31AM +0000, Paul Vixie wrote:
> > > 	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.

	as you point out below - none of this is germane to the
	format of an MX - the DNS is a bearer channel.  
> > > 	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.

	if something is legal then why would it be 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.
> 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.

	if an MTA treats 123.321.23.123. as an IP address, i'd claim 
	sloppy parsing...  but then i've always been in favor of FQDNs.
	123.321.23.123 would be an IP address and if used where it
	would be parsed as a domain name, we are stuck w/ the rule on
	appending the domain suffix.   which brings in search rules again.

	123.321.23.123.gen.ca		<---  this might be fun


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