Re: [dmarc-ietf] Summary comments on draft-ietf-dmarc-psd

"Murray S. Kucherawy" <superuser@gmail.com> Wed, 18 March 2020 23:29 UTC

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From: "Murray S. Kucherawy" <superuser@gmail.com>
Date: Wed, 18 Mar 2020 16:29:41 -0700
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To: Dave Crocker <dcrocker@bbiw.net>
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Subject: Re: [dmarc-ietf] Summary comments on draft-ietf-dmarc-psd
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On Sun, Mar 15, 2020 at 5:49 AM Dave Crocker <dhc@dcrocker.net> wrote:

> On 3/11/2020 8:36 PM, Murray S. Kucherawy wrote:
>
> Since we're mentioning a tree walk here, I believe we are ripe for a
> reopening of that debate.
>
> That's a shame. Besides created additional general cost, it creates an
> attack vector.
>
> Consider: From foo@bogus.bogus.bogus.bogus.bogus...bogus.bogus.example.com
>
Yeah, I'm familiar with the nature of the attack.   But based on what
amounts to the hallway track, it feels like the perspective of the DNS
community these days is that the currently deployed DNS infrastructure
could easily deal with such an attack, to at least the extent that it's not
a blocker for something potentially useful like resolving the
Organizational Domain problem for DMARC.

Tim might be able to elucidate here.

I believe where this is leading, though, is toward the notion that the use
> of the PSL should be considered external to DMARC.  I don't think anyone
> has disagreed with that assertion.
>
> And yet both DMARC and PSD have text tied to the term and its function.
>
My point here is that I believe we've already agreed that you're correct,
and this separation should happen, but there's no apparent urgency that
MUST be done before the PSD experiment can proceed.  Indeed, I'm fairly
certain the amount of code change in my own implementation in order to be
compliant with such a change in the specification is around zero.  The
audience for the experiment, by my read, also agrees, so they understand
the need, but this won't change the experiment either.


>
> > 2. Externalizing internal difficulties
>> >
>> > Some organizations have sub-groups to which various administrative
>> > responsibilities have been delegated.  In general, there can be many
>> > levels of such delegation.  Not just two.
>> >
>> > For the cases the PSD specification is intended to cover,
>> > PSD seeks to adapt to slow DMARC adoption or non-existent domains for
>> > one of its delegated sub-groups, by looking for an even-higher-level
>> > encompassing record under a next-level Organizational Domain.  That is,
>> > PSD is specifying using two different Organizational Domains.  The usual
>> > one and its parent.
>> >
>> > A difficulty within the organization is being externalized to a burden
>> > for everyone else on the net.
>>
>
> Can you expand on how you see abuse of non-existent domains to be an
> exploitation of an internal problem?  Specifically, what's the internal
> problem being exploited?
>
> I apologize for not being more clear about what I meant.  I hope that the
> above effort elaborate on this suffices.  Basically it concerns an
> organization's inability to get its subordinates to publish DMARC records
> and problems with PSLs that lead to identifying the wrong domain as an OD.
> Or somesuch...
>
The issue PSD is attempting to address is mail sent as a nonexistent
subdomain.  For example, irs.gov doesn't have a subdomain called
auditors.irs.gov, so irrespective of any irs.gov DMARC policy, I could send
email as msk@auditors.irs.gov without limitation.  The goal here is to come
up with a mechanism to prevent this.  Obviously the maintainers of irs.gov
can't anticipate all possible nonexistent names that might be used, so
either something like PSD is needed, or some kind of wildcarding mechanism
is needed by which the domain owner can block any name they're not actually
using.  What such mechanism could be developed?  Don't wildcard TXT
records, for example, have some other side effects?

-MSK