Q: double cname reference and resulting mx

[Mark_Andrews at isc.org]

> 
> hello,
> 
> following scenario:
> 
> s3.dom.com -(cname)-> s2.dom.com -(cname)-> s1.dom.com
> 
> s2 & s3 do not have an associated mx.  only s1 has an mx.
> 
> what is the correct behaviour when sending mail to s3?
> 
> - the mx of s1 will be used and mail will be delivered
> - or mail will not come through
> 
> in any case it would be great if somebody could point to a
> location within an rfc where this will be allowed or denied.
> i need this because someone who hosts a mail server
> seems to believe that it conforms to the rfcs not to deliver
> mail sent to s3.


	RFC 2821 is the current proposed standard.

	user at s3.dom.com should be delivered to the host referenced
	in the MX record.  Older MTAs may re-write user at s3.dom.com
	as user at s1.dom.com in the SMTP transaction as RFC 821 didn't
	allow aliases (owners of CNAMES) to be used.  I can't see
	that restriction in RFC 2821.

	RFC 2821 expects *local* aliases to be re-written to their
	fully qualified forms.  CNAME are not *local* aliases.

	I would not depend upon user at s3.dom.com not being re-written
	to user at s1.dom.com.  If you need user at s3.dom.com to be
	differnet to user at s1.dom.com use a MX record for s3.dom.com
	along with any other records at s1.dom.com that are required.

	Not delivering mail that referenced a CNAME doesn't pass
	the giggle test.  CNAMEs were designed to allow machined
	to be renamed and to have the old names work until they
	were no longer needed.

	Mark

> thanks a lot,
> -rgvt-
> 
> 
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--
Mark Andrews, Internet Software Consortium
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark.Andrews at isc.org