Re: [dmarc-ietf] Using CNAME records to DMARC templates causes issues

jbouwh <dmarc-list@jbsoft.nl> Tue, 02 March 2021 12:09 UTC

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Date: Tue, 02 Mar 2021 13:09:25 +0100
From: jbouwh <dmarc-list@jbsoft.nl>
To: Douglas Foster <dougfoster.emailstandards@gmail.com>
Cc: Tim Wicinski <tjw.ietf@gmail.com>, Henning Krause <mail=40henningkrause.eu@dmarc.ietf.org>, dmarc@ietf.org
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Subject: Re: [dmarc-ietf] Using CNAME records to DMARC templates causes issues
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L.S.
I would suggest update the DMARC standard make explicit how CNAME can be 
used or not.
Beside of that, the opendmarc software should address this as a bug in 
some way. Their opendmarc-check tool shows the correct policy that fails 
from the opendmarc service when used on a CNAME-ed DMARC policy.
The usecase I have seen failing only was one CNAME level deep.
The failing mechanism in opendmarc for fetching the DMARC policy now 
seems to leed leads to a 'none' policy.

Regards, Jan

Douglas Foster schreef op 2021-03-02 12:51:
> Because CNAME usage was not mentioned in the previous DMARC document,
> existing implementations may not have tested this configuration.   For
> the policy publishing organization, this increases the possibility
> that some recipients may treat the mail as not protected by DMARC.
> As with any deployment issue, the publishing organization has no
> reliable way to know if the deployment of DMARC implementations with
> full CNAME support is "essentially complete".  This uncertainty may be
> acceptable for some organizations, but may be an obstacle for others,
> depending on their motivations for implementing DMARC.
> 
> On the implementation side, the use of CNAME will introduce the
> possibility of referral errors, which may or may not require
> mentioning in the DMARC specification, since such issues have probably
> been addressed in core DNS documents.   The issues that come to mind
> are:
> CNAME referrals to non-existent names
> Nested CNAME referrals (what depth is allowed?)
> CNAME referrals that produce loops or excessive nesting depth.
> 
> DF
> 
> On Tue, Mar 2, 2021 at 6:12 AM Tim Wicinski <tjw.ietf@gmail.com>
> wrote:
> 
>> Using a CNAME at  _dmarc.example should not be a problem, as long as
>> 
>> the CNAME target is a TXT record.  The DNS resolver functions should
>> 
>> should handle this seamlessly. This does sound like a vendor
>> software
>> problem.
>> 
>> I am aware of DKIM records being deployed using CNAMEs pointing to a
>> TXT record target.
>> Has anyone seen the above error condition when testing DKIM records?
>> 
>> 
>> This definitely sounds like an issue with the software.
>> 
>> Nobody should shy away from publishing DMARC records that are CNAMEs
>> to DMARC
>> TXT records elsewhere. Using this design should be strongly
>> encouraged.
>> 
>> tim _______________________________________________
>> dmarc mailing list
>> dmarc@ietf.org
>> https://www.ietf.org/mailman/listinfo/dmarc
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