Re: [Asrg] overloading server names doesn't work, was who has the message

"John R Levine" <johnl@taugh.com> Tue, 09 February 2010 16:39 UTC

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From: "John R Levine" <johnl@taugh.com>
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Subject: Re: [Asrg] overloading server names doesn't work, was who has the message
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>>>> mail.btinternet.com. 600 IN CNAME pop-smtp.bt.mail.yahoo.com.
>>>> pop-smtp.bt.mail.yahoo.com. 1800 IN CNAME pop-smtp.bt.mail.fy5.b.yahoo.com.
>>>> pop-smtp.bt.mail.fy5.b.yahoo.com. 300 IN A 217.12.13.134
>>>> pop-smtp.bt.mail.fy5.b.yahoo.com. 300 IN A 217.146.188.192
>>> 
>>> I don't hack DNS records enought to be sure, but it appears to need
>>> exactly one new record:
>>> 
>>> _report.pop-smtp.bt.mail.fy5.b.yahoo.com IN TXT abuse-report@yahoo.com
>> 
>> Nope, that won't work. CNAMEs don't do a partial match.
> Then I guess it's lucky I didn't specify one.

But you did.  The CNAME only matches its exact name, and doesen't do a 
partial match of names below it.  RFC 2181 explains this, albeit not very 
clearly.  DNAME does what you think CNAME does, but almost nobody 
implements it because the record synthesis it requires is so complex.

>>> _report.pop-smtp.bt.mail.fy5.b.yahoo.com IN TXT abuse-report@yahoo.com
>>> _report.pop-smtp.bt.mail.yahoo.com IN TXT abuse-report@yahoo.com
>>> _report.mail.btinternet.com IN TXT abuse-report@yahoo.com
>> 
>> That won't work, either. You can't have DNS records below a CNAME.
>
> I'm not finding where that restriction specified among the list of 
> restrictions I can find, and apparently all of popular the summaries written 
> about CNames have missed it too.
>
> Please cite the standards text that specifies it.

Also 2181.  I'll go see if there's a clearer explanation somewhere else.

Regards,
John Levine, johnl@taugh.com, Taughannock Networks, Trumansburg NY
"I dropped the toothpaste", said Tom, crestfallenly.