Q: double cname reference and resulting mx

rgvt at gmx.net rgvt at gmx.net
Sat Jan 18 15:43:57 UTC 2003


> 
> rgvt at gmx.net wrote:
> 
> >>>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.
> >>
> >>    
> >>
> >
> >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?
> >
> In general, chaining CNAMEs is disrecommended. Pointing a CNAME at an A 
> or MX record is fine, but pointing an MX at a CNAME is illegal.
> 
> What Mark was mostly talking about was whether MTAs will rewrite 
> addresses when CNAMEs are encountered. Note that this behavior would be 
> above and beyond the actual mail routing function. Such rewriting 
> behavior can cause mail-handling problems, especially if exotic forms of 
> aliasing and/or forwarding are in effect.
> 

the iteresting part of mark's answer is marked with "***".
it's not about the re-writing or the "cnaming" of an mx.
how many cname redirections to an A record having an mx associated with are
allowed?
i think unlimited.
where can i find a reference for this?
i think i found it in section 3.6 / 5 of rfc 2821.
(or better: a reference not talking about a limit.)
am i right?

-rgvt-


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