Re: [dmarc-ietf] Nonexistent Domain Policy was: Re: Working Group Last Call: draft-ietf-dmarc-psd

Ian Levy <> Mon, 15 July 2019 07:08 UTC

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From: Ian Levy <>
To: Scott Kitterman <>, "" <>
Thread-Topic: [dmarc-ietf] Nonexistent Domain Policy was: Re: Working Group Last Call: draft-ietf-dmarc-psd
Date: Mon, 15 Jul 2019 07:08:04 +0000
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Subject: Re: [dmarc-ietf] Nonexistent Domain Policy was: Re: Working Group Last Call: draft-ietf-dmarc-psd
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Sorry for not contributing more to this thread - please don't take it as any indication of lack of interest. For UK NCSC specifically, I think we'd prefer NXDOMAIN rather than NODATA, given it's more constrained and this is an experiment. My view would be that if we've published a name under, even with no valid (in the eyes of the receiver) associated records, then *someone* is responsible for it and we can go find and educate them. They may even believe they have a valid reason for doing so that may outweigh any email authentication concerns. But there's a conversation to be had. If there's no published name, then there's no-one responsible, so it should default to the top-level policy. 
We've learned a lot running our DMARC processing for over the last couple of years and a lot about the consistency of how non-existent domains are treated, or lack thereof more to the point. If I'm honest, we see a lot of inconsistency in how DMARC is processed more generally that makes analysis harder than it needs to be. A lot of receivers have obviously made optimizations for their own specific circumstances. I'm just a little nervous of starting an experiment on a live and (some may argue) quite important PSD in anything other than a constrained manner. 

Incidentally, we *should* be publishing on our 2nd annual report on our Active Cyber Defence programme tomorrow (Tuesday). This includes a chapter on our DMARC experiences, including a bit of data relevant to this discussion, as well as some novel data science work on our DMARC report archive. As a preview, from July when we set the 'synthetic DMARC' record to p=reject, we've had this many reports : 
Month (2018)	Total reports
July		5,764
August 		274,532 
September 	127,901 
October 	17,553 
November 	17,191 
December 	105,078 

We'll also publish a couple of examples of where synthesizing DMARC/SPF records for non-existent domains has helped stop abuse. However, it's clear that this method isn't universally accepted. 
Here's the volume of reports received on our normal DMARC processing chain in January 2019 (noting Microsoft are one of the bigger providers in the UK and *still* don't generate any reports): 

Reporter 	Total Reports 	61,363,605 
Yahoo! Inc. 	18,876,201 
Mail.Ru 	699,554 227,587 
AMAZON-SES 	178,262

And here's the volume for the same month for the synthetic DMARC reports : 
Reporter 		Total Reports 		23,745 
Yahoo! Inc. 		1,060 		64 	37 	30

Just from that, it's pretty clear that the synthesized DMARC records are not universally processed, which gives weight to completing this work and starting to try things out. Given the level of inconsistency we see in receiver behaviour, I think it'd be easier to start with NXDOMAIN and see what that actually achieves. 

I may well be missing something subtle, so please correct me if I've got this wrong. 


Dr Ian Levy
Technical Director
National Cyber Security Centre

Staff Officer : Kate Atkins,

(I work stupid hours and weird times - that doesn't mean you have to. If this arrives outside your normal working hours, don't feel compelled to respond immediately!)

-----Original Message-----
From: dmarc <>; On Behalf Of Scott Kitterman
Sent: 13 July 2019 07:04
Subject: Re: [dmarc-ietf] Nonexistent Domain Policy was: Re: Working Group Last Call: draft-ietf-dmarc-psd

On Saturday, July 13, 2019 12:34:09 AM EDT John Levine wrote:
> In article <2902055.CzhLQO0xIX@l5580> you write:
> >Here's the definition we have in the draft now:
> >> 2.6.  Non-existent Domains
> >>
> >>    For DMARC [RFC7489] purposes, a non-existent domain is a domain name
> >>    that publishes none of A, AAAA, or MX records that the receiver is
> >>    willing to accept.  This is a broader definition than that in
> >>    NXDOMAIN [RFC8020].
> >
> >That's what I was expecting this new tag to apply to (and I think 
> >matches their expectation, but they can speak for themselves).
> That's OK.
> >Another way to say what's in 2.6 now might be:
> >
> >... a domain for which there is a NODATA response for A, AAAA, and MX 
> >records.
> Not so OK -- if there's no records at all at or below a name you 
> really will get NXDOMAIN.

Good point.  Thanks.  I'll leave it as is.

Scott K

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