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
> >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
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?
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