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

jbouwh <> Tue, 02 March 2021 12:09 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 680C13A16B7 for <>; Tue, 2 Mar 2021 04:09:30 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -2.1
X-Spam-Status: No, score=-2.1 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_PASS=-0.001, URIBL_BLOCKED=0.001] autolearn=ham autolearn_force=no
Authentication-Results: (amavisd-new); dkim=pass (2048-bit key)
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id T5hNryokgu1B for <>; Tue, 2 Mar 2021 04:09:27 -0800 (PST)
Received: from ( []) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPS id AA7643A16B5 for <>; Tue, 2 Mar 2021 04:09:27 -0800 (PST)
Received: from localhost (localhost.localdomain []) by (Postfix) with ESMTP id 9D73E27FE71A; Tue, 2 Mar 2021 13:09:25 +0100 (CET)
X-Virus-Scanned: Debian amavisd-new at
Received: from ([]) by localhost ( []) (amavisd-new, port 10026) with ESMTP id QDuiyd_b9RR5; Tue, 2 Mar 2021 13:09:25 +0100 (CET)
Received: from ( []) (using TLSv1.2 with cipher DHE-RSA-CAMELLIA256-SHA256 (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPSA id 77AC927FE3F2; Tue, 2 Mar 2021 13:09:25 +0100 (CET)
DKIM-Filter: OpenDKIM Filter v2.11.0 77AC927FE3F2
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=201607; t=1614686965; bh=4kqGoqftd7a5yZAoDZVjGoIRdNOQvzNhWc8M+06hB54=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=KjZ6kGY21tedSom0W7S24xKX4bF/GHC2zOGSCcI+ZihBnq80FPXR5ny98HgxbNzPI zFBjP9WrFzujfrtPadYYEfhbUMzmYZI1jFCTebOmf9Mrhh4FSMRA862FwQhaz0LV7k Nlt0pK2LVSSJrUGixQE3j6igUwNr2GFE+6dZv5KGAHqUHbFzeTHhP2VWdMVI4vr3nr mCwkj5TWNLSIr9WOM75v54MeEBOylwmLCqfn+MNsuquCtgQjhQMnycAIADbP1NTrzp VuX/1vxY3Opofi92LBzaWK/eT/ycmoh6Y/BddDaHYlBVz3eGoJoD36xm6f9TeIyFH9 uIfRjIYAvcqZA==
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII; format=flowed
Content-Transfer-Encoding: 7bit
Date: Tue, 02 Mar 2021 13:09:25 +0100
From: jbouwh <>
To: Douglas Foster <>
Cc: Tim Wicinski <>, Henning Krause <>,
In-Reply-To: <>
References: <> <> <> <> <>
Message-ID: <>
User-Agent: Roundcube Webmail/1.3.16
Archived-At: <>
Subject: Re: [dmarc-ietf] Using CNAME records to DMARC templates causes issues
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "Domain-based Message Authentication, Reporting, and Compliance \(DMARC\)" <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Tue, 02 Mar 2021 12:09:30 -0000

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 <>
> 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 mailing list