Re: [dmarc-ietf] tree walk and Org and PSD, Second WGLC for draft-ietf-dmarc-psd

John Levine <johnl@taugh.com> Sun, 22 November 2020 02:23 UTC

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From: "John Levine" <johnl@taugh.com>
To: dmarc@ietf.org
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In-Reply-To: <CAL0qLwYgTiHW5XXt3PTUMOiSHV0wUt_fRLyZS7D5v1ZH_WUCNg@mail.gmail.com>
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Subject: Re: [dmarc-ietf] tree walk and Org and PSD, Second WGLC for draft-ietf-dmarc-psd
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In article <CAL0qLwYgTiHW5XXt3PTUMOiSHV0wUt_fRLyZS7D5v1ZH_WUCNg@mail.gmail.com> you write:
>Someone in DNSOP, I think, proposed doing the tree walk in the other
>direction.

Turns out that won't work because here's what you'd be checking:

> _dmarc.paypal.com
> _dmarc.baz.paypal.com
> _dmarc.bar.baz.paypal.com
> _dmarc.foo.bar.baz.paypal.com

You can have a NXDOMAIN at _dmarc.paypal.com but a TXT record at
_dmarc.bar.baz.paypal.com. You could certainly add heuristics and
check plain baz.paypal.com to see if it gives you an NXDOMAIN stop but
I have no reason to think that on average it'd actually save queries.

It is my impression that most real From: domains are pretty short. I
don't think I've ever seen one more than four labels long that wasn't
deliberately contrived. Anyone got data on that?

R's,
John
root@18.183.57.64.in-addr.arpa