Proper CNAME interpretation

Ronald F. Guilmette rfg at tristatelogic.com
Wed Sep 14 21:27:08 UTC 2011


In message <7D9B265C-36BF-40C1-9012-AC0A96FB88CD at sackheads.org>, you wrote:

>On Sep 14, 2011, at 4:35 PM, Ronald F. Guilmette wrote:
>
>> Is there a rule that says how a resolver should behave in cases where
>> there is both an A record and also a CNAME record for the same FQDN?
>> Which one should take precedence, the A or the CNAME?
>
>
>RFC 1034, Section 3.6.2: "If a CNAME RR is present at a node, no other data
>should be present; this ensures that the data for a canonical name and its
>aliases cannot be different. This rule also insures that a cached CNAME can
>be used without checking with an authoritative server for other RR types."

Thanks for the response John.

So, um, the first part of that just says what people should not be doing
when they are constructing sets of RRs applicable to a given domain name.

But we all know that, right?  We know you are not supposed to put a CNAME
with other stuff for the same domain.

The second part however seems to go more to my question, which is "What is
the resolver supposed to do when some knucklehead breaks the rules and puts
a CNAME in with some other stuff?"

It sure _sounds_ like that second sentence is encouraging any & all people
who are writing resolvers, or other related tools, that they should ignore
any flotsam & jetsum that appear along side a CNAME.  But is that encourage-
ment espressed anywhere as a "MUST"?

Not that I would want to deviate from common established practice... if
in fact ignoring flotsam & jetsum that appears with a CNAME is the common
practice.  I'd just like to be able to defend the "rightness" of my code...
RFC-wise... in case anybody ever presses me and says "Why did you do THAT??",
you know, after I tell them that my code ignores flotsam & jetsum that appears
along side a CNAME.


Regards,
rfg



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