VBR results: Authentication-Results Registration for Vouch by Reference Results
A small registry update that lets a mail system record the outcome of a Vouch By Reference check in a message header.
Kucherawy, M., "Authentication-Results Registration for Vouch by Reference Results", RFC 6212, DOI 10.17487/RFC6212, April 2011, <https://www.rfc-editor.org/info/rfc6212>.
@misc{rfc6212,
series = {Request for Comments},
number = 6212,
howpublished = {RFC 6212},
publisher = {RFC Editor},
doi = {10.17487/RFC6212},
url = {https://www.rfc-editor.org/info/rfc6212},
author = {M. Kucherawy},
title = {{Authentication-Results Registration for Vouch by Reference Results}},
pagetotal = 7,
year = 2011,
month = apr,
} RFC text
Contents
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 Abstract 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. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6212. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction ....................................................2 2. Keywords ........................................................2 3. Discussion ......................................................2 4. Definition ......................................................3 5. IANA Considerations .............................................4 6. Security Considerations .........................................5 7. References ......................................................5 7.1. Normative References .......................................5 7.2. Informative References .....................................5 Appendix A. Authentication-Results Examples .......................6 A.1. VBR Results ................................................6 Appendix B. Acknowledgements ......................................7 1. Introduction [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. 2. Keywords The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS]. 3. Discussion 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. 4. Definition 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. 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. 5. IANA Considerations Per [IANA], the following items have been added to the "Email Authentication Methods" registry: +------------+----------+--------+----------------+-----------------+ | Method | Defined | ptype | property | value | +------------+----------+--------+----------------+-----------------+ | vbr | RFC 6212 | header | md | DNS domain name | | | | | | used as the | | | | | | subject of a | | | | | | VBR query | +------------+----------+--------+----------------+-----------------+ | vbr | RFC 6212 | header | mv | DNS domain name | | | | | | of the entity | | | | | | acting as | | | | | | the voucher | +------------+----------+--------+----------------+-----------------+ Also, the following items have been added to the "Email Authentication Result Names" registry: +-----------+--------------+------------+---------+-----------------+ | Code | Existing/New | Defined In | Method | Meaning | +-----------+--------------+------------+---------+-----------------+ | none | existing | RFC 5451 | vbr | Section 4 of | | | | | (added) | RFC 6212 | +-----------+--------------+------------+---------+-----------------+ | pass | existing | RFC 5451 | vbr | Section 4 of | | | | | (added) | RFC 6212 | +-----------+--------------+------------+---------+-----------------+ | fail | existing | RFC 5451 | vbr | Section 4 of | | | | | (added) | RFC 6212 | +-----------+--------------+------------+---------+-----------------+ | temperror | existing | RFC 5451 | vbr | Section 4 of | | | | | (added) | RFC 6212 | +-----------+--------------+------------+---------+-----------------+ | permerror | existing | RFC 5451 | vbr | Section 4 of | | | | | (added) | RFC 6212 | +-----------+--------------+------------+---------+-----------------+ 6. Security Considerations This memo creates a mechanism for relaying [VBR] results using the structure already defined by [AUTHRES]. The Security Considerations sections of those documents should be consulted. 7. References 7.1. Normative References [AUTHRES] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 5451, April 2009. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By Reference", RFC 5518, April 2009. 7.2. Informative References [IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. Appendix A. Authentication-Results Examples This section presents an example of the use of this new header field to indicate VBR results. A.1. VBR Results 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 <recipient@example.net> 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: sender@newyork.example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: meetings@example.net VBR-Info: md=newyork.example.com; mc=list; mv=voucher.example.org Message-Id: <12345.abc@newyork.example.com> 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. Appendix B. Acknowledgements The author wishes to acknowledge the following for their review and constructive criticism of this proposal: JD Falk, John Levine, and Alessandro Vesely. Author's Address Murray S. Kucherawy Cloudmark, Inc. 128 King St., 2nd Floor San Francisco, CA 94107 US Phone: +1 415 946 3800 EMail: msk@cloudmark.com
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§1Introduction
RFC 5451 defined the Authentication-Results header field, which presents the results of a message authentication effort in a machine-readable format. Meanwhile, a proposal for rudimentary domain-level reputation assessment called Vouch By Reference (VBR) had been published as RFC 5518 and was, in the words of this memo, beginning to see popular use.
RFC 6212 is deliberately small. It registers one more reporting property so that a VBR result can be relayed as an annotation in a message header.
There was already a place on a message to write down what checks it passed. This memo adds one more kind of check to that list.
§2Vouch By Reference and Why Relay It
Vouch By Reference lets a message receiver query a "vouching" service to find out whether a trusted third party is willing to state that mail from a particular source can be considered legitimate.
When that assessment is done at an inbound border mail gateway, it is useful to relay the result to internal mail entities such as filters or user agents. The Authentication-Results field already exists for this kind of reporting, so the memo reuses it rather than inventing a new field.
The memo does not say how to react to a result. Reactions to an Authentication-Results field containing VBR (or any) results are entirely a matter of local policy at the receiver.
A guard at the front door asks a trusted friend, "is this visitor okay?" and then wants to tell the people inside the house what the friend said. How the people inside act on it is up to them.
§3The vbr Method and Its Properties
The memo adds the method vbr to the "Email Authentication Methods" registry that IANA created when RFC 5451 was published. Two properties (reporting items) go with it: header.md and header.mv.
- If
header.mdis present, its value MUST be the DNS domain name about which a VBR query was made. - If
header.mvis present, its value MUST be the DNS domain name that was queried as the potential voucher for theheader.mddomain.
When the query was based on a VBR-Info header field in the incoming message, header.md is typically taken from its md tag and header.mv is typically one of the values of its mv tag. RFC 5518 allows other ways to choose the subject domain or vouchers, and a server could even run a VBR query when no VBR-Info field is present, based on local policy. In any of these cases the verifying server MAY generate an Authentication-Results field to relay the result.
The note has two blanks: which sender's name was checked, and which trusted friend was asked. Fill in the two names and you are done.
§4VBR Result Codes
The memo also adds the method vbr to five existing result names in the "Email Authentication Result Names" registry:
none: no validVBR-Infoheader was found in the message, or a domain name to query could not be determined.pass: a VBR query was completed and the vouching service gave a positive response.fail: a VBR query was completed and the service did not give a positive response, or the message contained multipleVBR-Infoheader fields with differentmcvalues.temperror: the query could not be completed because of an error that is likely transient, such as a temporary DNS error. A later attempt may produce a final result.permerror: the query could not be completed because of an error that is likely not transient, such as a permanent DNS error. A later attempt is unlikely to produce a final result.
The note can say "no answer needed," "the friend said yes," "the friend said no," "the phone line was busy, try again," or "the number is broken."
- noneNo query — No valid VBR-Info header, or no domain to query.
- passVouched — Query completed with a positive response.
- failNot vouched — Query completed without a positive response, or conflicting mc values.
- temperrorTransient error — Query could not complete; a later attempt may succeed.
- permerrorPermanent error — Query could not complete; a later attempt is unlikely to succeed.
§5An Example Header
The appendix shows a message signed with DomainKeys Identified Mail (DKIM) that also carried a VBR-Info header field. On receipt, the md= value in VBR-Info matched the d= value in the DKIM signature, and the 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 was included in the Authentication-Results field added before delivery. It reads vbr=pass with header.md=newyork.example.com and header.mv=voucher.example.org.
A message is checked two ways, and the trusted friend says yes, so the mailroom writes "pass" next to both checks.
§6Security Considerations
The memo creates a mechanism for relaying VBR results using the structure already defined by RFC 5451. It adds no security analysis of its own and points readers to the Security Considerations sections of RFC 5451 and RFC 5518.
This memo only adds a new kind of note. For safety advice, read the rules for the notes themselves and for the vouching system.