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

Daniel Feenberg <feenberg@nber.org> Tue, 09 February 2010 12:12 UTC

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Date: Tue, 9 Feb 2010 07:11:58 -0500 (EST)
From: Daniel Feenberg <feenberg@nber.org>
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Subject: Re: [Asrg] overloading server names doesn't work, was who has the message
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On Mon, 8 Feb 2010, John R Levine wrote:

>>> ISP in the UK.  Can you describe the DNS changes needed if they were
>>> publishing a spam button address?
>>> $ dig  mail.btinternet.com a
>>> ;; ANSWER SECTION:
>>> 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.
>
>> _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.
>
> By the way, I was wrong about SRV records.  This DNS hack just doesn't work, 
> but it won't work much more elegantly with RP records.  See RFC 1183.
>

What is the objection to attaching the TXT record to the A record? I 
realize that it means that a single arf-reporting system must serve 
several CNAMEs, but that is a common requirement for many network 
services, and I rarely see complaints about it.

Daniel Feenberg

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