Q: double cname reference and resulting mx

[Barry Finkel]

rgvt at gmx.net wrote (with Mark Andrews' reply indented):
> > 
> > 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.
> 

and rgvt at gmx.net replied to Mark:

>hello mark,
>
>thanks for your answer.
>now we are struggling around finding the exact location that inhibits this
>cname
>behaviour.  in rfc 2821 i found this:
>
>3.6 Domains
>[...]  In other words, names that can
>   be resolved to MX RRs or A RRs (as discussed in section 5) are
>   permitted, as are CNAME RRs whose targets can be resolved, in turn,
>   to MX or A RRs.  [...]
>
>5. Address Resolution and Mail Handling
>
>[...]  The lookup first attempts to locate an MX
>   record associated with the name.  If a CNAME record is found instead,
>   the resulting name is processed as if it were the initial name.   [...]
>
>does section 5 describe that having a cname as an mx is allowed (what
>everybody says is not recommended).  or does this phrase mean that
>sending mail to cnames is allowed as long as they end in an A record only
>or an A record that has an MX record associated (without specifying the
>number of indirections for the cnames).
>rfc 1123 states that smtp mail should not be sent to cnames but to
>canonical names only.  what does this statement in rfc 2821 mean?
>
>thanks a lot!
>-rgvt-

I do not know if the DRUMS (Draft Revision of User Mail Standards)
archive is still available either at cs.utk.edu or on some newsgroup
archive site.  I saved some of the DRUMS postings, but not all.
And I could not find in my saved archive any discussion of these 
sections of what turned out to be RFC 2821.
----------------------------------------------------------------------
Barry S. Finkel
Electronics and Computing Technologies Division
Argonne National Laboratory          Phone:    +1 (630) 252-7277
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Building 222, Room D209              Internet: BSFinkel at anl.gov
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