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List:       djbdns
Subject:    Re: Malformed Zone?
From:       Jonathan Duncan <jonathan () nacnud ! com>
Date:       2010-04-28 22:24:54
Message-ID: 97524CB5-1755-4BA6-ABB7-4B20FE739853 () nacnud ! com
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On 28 Apr 2010, at 16:09, Paul Jarc wrote:

> Jonathan Duncan <jonathan@nacnud.com> wrote:
>> Just for my education, why would the CNAME record and the MX record
>> clash?  I know that Matthew said, "CNAME records can't coexist at
>> the same name with other records."  Why is this?  The A record
>> "+pennypix.net" is the same name as "@pennypix.net" but they do not
>> clash.  What is different about CNAME records?
> 
> The short answer is "because the RFC says so".
> 
> A CNAME isn't just a subsitute for an A record; it substitutes for all
> records--when a resolver is looking for a record of type X for the
> name foo, and it sees a CNAME for foo pointing to bar (possibly
> already cached, from earlier when it was looking for a Y record for
> foo), then it looks for a record of type X for bar instead.  If foo
> also had its own X record, the resolver wouldn't be able to use its
> cached knowledge; it would have to check with foo's server
> specifically about the X record.  So this requirement saves on network
> traffic and resolution time.
> 

Good to know.  Thanks for the lesson.  One more reason not to use CNAME's.

Jonathan
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