Internet Engineering Task Force (IETF) M. Kucherawy Request for Comments: 6212 Cloudmark, Inc. Category: Standards Track April 2011 ISSN: 2070-1721
Authentication-Results Registration for Vouch by Reference Results
This memo updates the registry of properties in Authentication- Results: message header fields to allow relaying of the results of a Vouch By Reference query.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
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[AUTHRES] defined a new header field for electronic mail messages that presents the results of a message authentication effort in a machine-readable format. In the interim, a proposal for rudimentary domain-level reputation assessment, called Vouch By Reference, [VBR] was published and is now beginning to see popular use.
This memo thus registers an additional reporting property allowing a VBR result to be relayed as an annotation in a message header.
Vouch By Reference [VBR] introduced a mechanism by which a message receiver can query a "vouching" service to determine whether or not a trusted third party is willing to state that mail from a particular source can be considered legitimate. When this assessment is done at an inbound border mail gateway, it would be useful to relay the result of that assessment to internal mail entities such as filters or user agents.
Reactions to the information contained in an Authentication-Results header field that contains VBR (or any) results are not specified here, as they are entirely a matter of local policy at the receiver.
This memo adds to the "Email Authentication Methods" registry, created by IANA upon publication of [AUTHRES], the following:
o The method "vbr"; and
o Associated with that method, the properties (reporting items) "header.md" and "header.mv".
If "header.md" is present, its value MUST be the DNS domain name about which a VBR query was made. If "header.mv" is present, its value MUST be the DNS domain name that was queried as the potential voucher for the "header.md" domain.
If the VBR query was made based on the content of a "VBR-Info" header field present on an incoming message, "header.md" is typically taken from the "md" tag of the "VBR-Info" header field, and "header.mv" is typically one of the values of the "mv" tag in the "VBR-Info" header field on that message. However, [VBR] permits a different mechanism for selection of the subject domain and/or list of vouchers, ignoring those present in any "VBR-Info" header field the message might have included. A server could even conduct a VBR query when no "VBR-Info" field was present, based on locally configured policy options. Where such mechanisms are applied, the verifying server MAY generate an Authentication-Results field to relay the results of the VBR query.
This memo also adds to the "Email Authentication Result Names" registry the following result codes and definitions:
none: No valid VBR-Info header was found in the message, or a domain name to be queried could not be determined.
pass: A VBR query was completed, and the vouching service queried gave a positive response.
fail: A VBR query was completed, and the vouching service queried did not give a positive response, or the message contained multiple VBR-Info header fields with different "mc" values (see [VBR]).
temperror: A VBR query was attempted but could not be completed due to some error that is likely transient in nature, such as a temporary DNS error. A later attempt may produce a final result.
Kucherawy Standards Track [Page 3]
RFC 6212 Auth-Results VBR Registration April 2011
permerror: A VBR query was attempted but could not be completed due to some error that is likely not transient in nature, such as a permanent DNS error. A later attempt is unlikely to produce a final result.
A message that triggered a VBR query, returning a result:
Authentication-Results: mail-router.example.net; dkim=pass (good signature) header.d=newyork.example.com header.b=oINEO8hg; vbr=pass (voucher.example.net) header.md=newyork.example.com header.mv=voucher.example.org Received: from newyork.example.com (newyork.example.com [192.0.2.250]) by mail-router.example.net (8.11.6/8.11.6) for <email@example.com> with ESMTP id i7PK0sH7021929; Fri, Feb 15 2002 17:19:22 -0800 DKIM-Signature: v=1; a=rsa-sha256; s=rashani; d=newyork.example.com; t=1188964191; c=relaxed/simple; h=From:Date:To:VBR-Info:Message-Id:Subject; bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= From: firstname.lastname@example.org Date: Fri, Feb 15 2002 16:54:30 -0800 To: email@example.com VBR-Info: md=newyork.example.com; mc=list; mv=voucher.example.org Message-Id: <firstname.lastname@example.org> Subject: here's a sample
Example 1: Header Field Reporting Results from a VBR Query
Here we see an example of a message that was signed using DomainKeys Identified Mail (DKIM) and that also included a VBR-Info header field. On receipt, it is found that the "md=" field in the latter and the "d=" field in the former matched, and also that the DKIM signature verified, so a VBR query was performed. The vouching service, voucher.example.org, indicated that the sender can be trusted, so a "pass" result is included in the Authentication-Results field affixed prior to delivery.