Re: [dmarc-ietf] Forensic report loops are a problem

Alessandro Vesely <vesely@tana.it> Wed, 27 January 2021 11:59 UTC

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To: "Murray S. Kucherawy" <superuser@gmail.com>, John Levine <johnl@taugh.com>, IETF DMARC WG <dmarc@ietf.org>
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From: Alessandro Vesely <vesely@tana.it>
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Date: Wed, 27 Jan 2021 12:58:56 +0100
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Subject: Re: [dmarc-ietf] Forensic report loops are a problem
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On Sat 16/Jan/2021 22:30:56 +0100 Murray S. Kucherawy wrote:
> On Fri, Jan 15, 2021 at 7:40 PM John Levine <johnl@taugh.com> wrote:
> 
>> I still don't understand why anyone thinks there is a problem to be
>> fixed. If you don't want reports, don't ask for them. If you think the
>> mail you send shouldn't be provoking DMARC failure reports, adjust
>> whatever is sending the mail the mail is aligned, or get rid of the
>> ruf= that asks for the reports. What am I missing here?
> 
> You might be right, so I'll go back and ask this use case what the
> specifics are.
> 
> What I'm concerned about is that since this has come up before N times
> (for, admittedly, some currently small value of N), we've seen enough
> disparate cases of it that we may be missing something bigger, and if so,
> doing something defensive in the specification would be prudent.  There's
> smoke here, and there may be fire.
> 
> Will report back.


Examining my report folder, I note I'm sending one-liner aggregate reports to 
domains I never wrote to.  The pattern is their sending me feedback for one or 
more mailing list posts, followed by my one-liner acknowledging their report 
later on the same day or on the next day, depending on their sending time.

It is not a loop, because I send from noloop.tana.it, which has no rua=.  I'm 
not alone using this trick.  Two naive report generators would continue to send 
one-liners indefinitely.

While this is a minor problem for aggregate reports, it can be a real problem 
for naive failure reports generators.  Juri reported he had to target a 
specific address, attributing the loop to a remote misconfiguration.  However, 
if it is possible to screw up authentications, the probability to meet a loop 
is just its square, times the number of generators.

The only prevention in the spec is to apply rate limiting.

I'd add a couple of loop-specific provisions:

* send reports from a subdomain having a DMARC record without ruf=,

* configure the verifier to not generate failures of messages destined to one's 
own ruf= address.

I shouldn't hurt, should it?

Best
Ale
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