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Re: MX to CNAME and (mis)interptretation of 2821

2007-12-12 17:18:41

On Wed, Dec 12, 2007 at 03:33:21PM -0700, Trevor Paquette wrote:
I'm currently at odds with a few folks regarding the interpretation of
RFC 2821 and the case of MX records that resolve to CNAMEs. I'm hoping
that those here, who are authoritative when it comes to this RFC, can
shed some light.

I am not authoritative, nevertheless I wish to comment on this.

However, in talking to TrendMicro, they say that this syntax is
perfectly valid and that RFC 2821 overrides the MX to CNAME limitation.
The following website is their stance on this:

The solution mentions "Section 5" of 2821 which discusses


and they (ab)use it to defend


which is just not the same, at all.

However, in talking to others, they say that TrendMicro is
misinterpreting the RFC. I'll admit after reading 2821, I could
interpret things both ways.

I can see how 3.6 can be interpreted the wrong way, but it does
not (IMHO) overrule 2181.  Neither does 2821 claim to obsolete or
update 2181.

2821 2.3.5 says:
Domain names are used as names of hosts and of other entities in
the domain name hierarchy.  For example, a domain may refer to an
alias (label of a CNAME RR) or the label of Mail eXchanger records 

If these exist:  MX      10      CNAME

then 2.3.5 talks about mailrelay and alias, not about
mailhost01 or canonicalname.

Trendmicro uses it to defend "" being an alias.

my 2ct