BIND 9.6 Flaw - CNAME vs. A Record in MX Records are NOT "Illegal"

[SM]

At 22:11 24-01-2009, Al Stu wrote:
>Some people seem to think RFC 974 creates a standard which prohibits 
>the use of CNAME/alias in MX records.  But very much to the contrary 
>RFC 974 demonstrates that CNAME/alias is permitted in MX records.

RFC 974 is obsoleted by RFC 2821; the latter is obsoleted by RFC 
5321.  Quoting Section 5 of that RFC:

   "When a domain name associated with an MX RR is looked up and the
    associated data field obtained, the data field of that response MUST
    contain a domain name.  That domain name, when queried, MUST return
    at least one address record (e.g., A or AAAA RR) that gives the IP
    address of the SMTP server to which the message should be directed.
    Any other response, specifically including a value that will return a
    CNAME record when queried, lies outside the scope of this Standard.
    The prohibition on labels in the data that resolve to CNAMEs is
    discussed in more detail in RFC 2181, Section 10.3."

>ISC's message that a CNAME/alias in an MX record is illegal is 
>incorrect and just an attempt by ISC to get people to go along with 
>what is only a perceived rather than actual standard/requirement, 
>and should be removed so as not to further the fallacy of this 
>perceived perception of a standard/requirement, as it is neither a 
>standard nor a requirement, and certainly not illegal.

Pointing to a CNAME on the right-hand side of an MX record is 
incorrect and may affect mail delivery.  This is not about perceived 
perception of a requirement (see the MUST return at least one address 
record in the quoted text).

Regards,
-sm