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

[Al Stu]

"Thus, if an alias is used as the value of an NS or MX record, no address 
will be returned with the NS or MX value."

Above statement, belief, perception etc. has already been proven to be a 
fallacy (see the network trace attached to one of the previous messages). 
Both the CNAME and A record is in fact returned, unless the CNAME RR points 
to some other zone such as say smtp.googlemail.com.

So within the zone SMTP requirements are in fact met when the MX RR is a 
CNAME.  So there is no need to prevent this nor to label it as "illegal". 
The MX RR CNAME check should be improved to include this case and not throw 
a message if the MX RR CNAME is resolvable within the zone.


----- Original Message ----- 
From: "Matus UHLAR - fantomas" <uhlar at fantomas.sk>
To: <bind-users at lists.isc.org>
Sent: Monday, January 26, 2009 8:18 AM
Subject: Re: BIND 9.6 Flaw - CNAME vs. A Record in MX Records are NOT 
"Illegal"


> On 26.01.09 09:19, bsfinkel at anl.gov wrote:
>> If I have in DNS
>>
>>      cn IN CNAME realname
>>
>> and I query for cn, the DNS resolver will return "realname".
>> BIND also returns the "A" record for realname.  Is this a requirement?
>> If not, then
>>
>>      mx IN 10 MX cn
>>
>> will result in:
>>
>>      1) the MX query returning cn,
>>
>>      2) the cn query returning realname,
>>
>>      3) a third (and RFC-breaking) query to get the "A" for realname.
>>
>> There are only two queries if the resolver returns the "A" record along
>> with the realname of the CNAME record.
>
> according to RFC1035 sect. 3.3.9
>
> "MX records cause type A additional section processing for the host
> specified by EXCHANGE."
>
> according to RFC2181 sect 10.3.
>
> "The domain name used as the value of a NS resource record, or part of the
> value of a MX resource record must not be an alias."
>
> "It can also have other RRs, but never a CNAME RR."
>
> "Additional section processing does not include CNAME records"...
>
> "Thus, if an alias is used as the value of an NS or MX record, no address
> will be returned with the NS or MX value."
>
>
> -- 
> Matus UHLAR - fantomas, uhlar at fantomas.sk ; http://www.fantomas.sk/
> Warning: I wish NOT to receive e-mail advertising to this address.
> Varovanie: na tuto adresu chcem NEDOSTAVAT akukolvek reklamnu postu.
> "The box said 'Requires Windows 95 or better', so I bought a Macintosh".
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