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

[Al Stu]

"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."

Correct.  And when a that domain name is a CNAME pointing to an A RR the 
query returns not only the alias but also the real name and the IP address 
from the A RR.  Thus meeting the requirements to "return at least one 
address record (e.t., A or AAAA RR)".  But yet ISC seems to find it 
necessary to throw a message that it is "illegal", when it clearly is not.


----- Original Message ----- 
From: "SM" <sm at resistor.net>
To: <bind-users at lists.isc.org>
Sent: Sunday, January 25, 2009 12:23 AM
Subject: Re: BIND 9.6 Flaw - CNAME vs. A Record in MX Records are NOT 
"Illegal"


> 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
>
>
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