[Mimedefang] Domain canonifyin?g and RFCs

[Les Mikesell]

On Wed, Oct 19, 2011 at 11:04 AM, Joseph Brennan <brennan at columbia.edu> wrote:
>
> Check this though- section 5 of RFC 2821:--
>
>                            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.  If
>  no MX records are found, but an A RR is found, the A RR is treated as
>  if it was associated with an implicit MX RR, with a preference of 0,
>  pointing to that host.
>
> So prefer MX, and if there is no MX, an A record is an implicit MX. OK,
> we all know this.
>
> But if a CNAME is found "instead"-- and this seems to imply, as RFC 1034
> also implies, that you can't have both CNAME and MX together-- then the
> "resulting name" is processed. What's the "resulting name"? The result
> of canonifying the CNAME to an A record?
>
> Similar in RFC 5321 section 5.1, without "instead":--
>
>  The lookup first attempts to locate an MX record associated with the
>  name.  If a CNAME record is found, the resulting name is processed as
>  if it were the initial name.
>
> Both say that in the absence of an MX record, an A record is an implicit
> MX record, but both do *not* say that a CNAME can be an implicit MX. So
> we have a logical problem:
>
> (a) If there is a CNAME, there can't be an MX record.
>
> (b) If there is no MX record, a CNAME is not an implicit MX record.
>
> So where would mail go?

A CNAME is an alias to some other name.  It can't have a separate MX
associated with the CNAME, but if there is one for the real name, the
CNAME (being simply an alias) inherits it.  And if there isn't an MX,
the real name's A record is used as it would be if you had used the
real name.

-- 
  Les Mikesell
    lesmikesell at gmail.com