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

2010-02-09 10:41:03
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(_at_)yahoo(_dot_)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(_at_)yahoo(_dot_)com
_report.pop-smtp.bt.mail.yahoo.com IN TXT abuse-report(_at_)yahoo(_dot_)com
_report.mail.btinternet.com IN TXT abuse-report(_at_)yahoo(_dot_)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(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
"I dropped the toothpaste", said Tom, crestfallenly.
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