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

[Al Stu]

RFC 2821 is much more recent and clearly documents in sections 3.5 and 5 
that CNAME MX RR are permitted and are to be handled by SMTP MTA's.

3.6 Domains
"Only resolvable, fully-qualified, domain names (FQDNs) are permitted when 
domain names are used in SMTP.  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."


This is also backed up by the older RFC 974.
"There is one other special case.  If the response contains an answer which 
is a CNAME RR, it indicates that REMOTE is actually an alias for some other 
domain name. The query should be repeated with the canonical domain name."

So it is clear there should be no problem with using CNAME MX RR for mail 
systems that conform to these RFC's, and therefore no need for enforcing the 
use of only A RR, or even outputting an error/warning.