Q: double cname reference and resulting mx
Barry Finkel
b19141 at achilles.ctd.anl.gov
Mon Jan 20 20:49:43 UTC 2003
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
9700 South Cass Avenue Facsimile:+1 (630) 252-4601
Building 222, Room D209 Internet: BSFinkel at anl.gov
Argonne, IL 60439-4828 IBMMAIL: I1004994
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