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rfc:rfc7372

Internet Engineering Task Force (IETF) M. Kucherawy Request for Comments: 7372 September 2014 Updates: 7208 Category: Standards Track ISSN: 2070-1721

                 Email Authentication Status Codes

Abstract

 This document registers code points to allow status codes to be
 returned to an email client to indicate that a message is being
 rejected or deferred specifically because of email authentication
 failures.
 This document updates RFC 7208, since some of the code points
 registered replace the ones recommended for use in that document.

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/rfc7372.

Copyright Notice

 Copyright (c) 2014 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.

Kucherawy Standards Track [Page 1] RFC 7372 Email Auth Status Codes September 2014

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Key Words . . . . . . . . . . . . . . . . . . . . . . . . . .   2
 3.  New Enhanced Status Codes . . . . . . . . . . . . . . . . . .   3
   3.1.  DKIM Failure Codes  . . . . . . . . . . . . . . . . . . .   3
   3.2.  SPF Failure Codes . . . . . . . . . . . . . . . . . . . .   4
   3.3.  Reverse DNS Failure Code  . . . . . . . . . . . . . . . .   5
   3.4.  Multiple Authentication Failures Code . . . . . . . . . .   5
 4.  General Considerations  . . . . . . . . . . . . . . . . . . .   5
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
 7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
 Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   8

1. Introduction

 [RFC3463] introduced Enhanced Mail System Status Codes, and [RFC5248]
 created an IANA registry for these.
 [RFC6376] and [RFC7208] introduced, respectively, DomainKeys
 Identified Mail (DKIM) and Sender Policy Framework (SPF), two
 protocols for conducting message authentication.  Another common
 email acceptance test is the reverse Domain Name System (DNS) check
 on an email client's IP address, as described in Section 3 of
 [RFC7001].
 The current set of enhanced status codes does not include any code
 for indicating that a message is being rejected or deferred due to
 local policy reasons related to any of these mechanisms.  This is
 potentially useful information to agents that need more than
 rudimentary handling information about the reason a message was
 rejected on receipt.  This document introduces enhanced status codes
 for reporting those cases to clients.
 Section 3.2 updates [RFC7208], as new enhanced status codes relevant
 to that specification are being registered and recommended for use.

2. Key Words

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].

Kucherawy Standards Track [Page 2] RFC 7372 Email Auth Status Codes September 2014

3. New Enhanced Status Codes

 The new enhanced status codes are defined in the following
 subsections.

3.1. DKIM Failure Codes

 In the code point definitions below, the following definitions are
 used:
 passing:  A signature is "passing" if the basic DKIM verification
    algorithm, as defined in [RFC6376], succeeds.
 acceptable:  A signature is "acceptable" if it satisfies all locally
    defined requirements (if any) in addition to passing the basic
    DKIM verification algorithm (e.g., certain header fields are
    included in the signed content, no partial signatures, etc.).
    Code:               X.7.20
    Sample Text:        No passing DKIM signature found
    Associated basic status code:  550
    Description:        This status code is returned when a message
                        did not contain any passing DKIM
                        signatures.  (This violates the
                        advice of Section 6.1 of RFC 6376.)
    Reference:          [RFC7372]; [RFC6376]
    Submitter:          M. Kucherawy
    Change controller:  IESG
    Code:               X.7.21
    Sample Text:        No acceptable DKIM signature found
    Associated basic status code:  550
    Description:        This status code is returned when a message
                        contains one or more passing DKIM signatures,
                        but none are acceptable.  (This violates the
                        advice of Section 6.1 of RFC 6376.)
    Reference:          [RFC7372]; [RFC6376]
    Submitter:          M. Kucherawy
    Change controller:  IESG

Kucherawy Standards Track [Page 3] RFC 7372 Email Auth Status Codes September 2014

    Code:               X.7.22
    Sample Text:        No valid author-matched DKIM signature found
    Associated basic status code:  550
    Description:        This status code is returned when a message
                        contains one or more passing DKIM
                        signatures, but none are acceptable because
                        none have an identifier(s)
                        that matches the author address(es) found in
                        the From header field.  This is a special
                        case of X.7.21. (This violates the advice
                        of Section 6.1 of RFC 6376.)
    Reference:          [RFC7372]; [RFC6376]
    Submitter:          M. Kucherawy
    Change controller:  IESG

3.2. SPF Failure Codes

    Code:               X.7.23
    Sample Text:        SPF validation failed
    Associated basic status code:  550
    Description:        This status code is returned when a message
                        completed an SPF check that produced a
                        "fail" result, contrary to local policy
                        requirements.  Used in place of 5.7.1, as
                        described in Section 8.4 of RFC 7208.
    Reference:          [RFC7372]; [RFC7208]
    Submitter:          M. Kucherawy
    Change controller:  IESG
    Code:               X.7.24
    Sample Text:        SPF validation error
    Associated basic status code:  451/550
    Description:        This status code is returned when evaluation
                        of SPF relative to an arriving message
                        resulted in an error.  Used in place of
                        4.4.3 or 5.5.2, as described in Sections
                        8.6 and 8.7 of RFC 7208.
    Reference:          [RFC7372]; [RFC7208]
    Submitter:          M. Kucherawy
    Change controller:  IESG

Kucherawy Standards Track [Page 4] RFC 7372 Email Auth Status Codes September 2014

3.3. Reverse DNS Failure Code

    Code:               X.7.25
    Sample Text:        Reverse DNS validation failed
    Associated basic status code:  550
    Description:        This status code is returned when an SMTP
                        client's IP address failed a reverse DNS
                        validation check, contrary to local policy
                        requirements.
    Reference:          [RFC7372]; Section 3 of [RFC7001]
    Submitter:          M. Kucherawy
    Change controller:  IESG

3.4. Multiple Authentication Failures Code

    Code:               X.7.26
    Sample Text:        Multiple authentication checks failed
    Associated basic status code:  550
    Description:        This status code is returned when a message
                        failed more than one message authentication
                        check, contrary to local policy requirements.
                        The particular mechanisms that failed are not
                        specified.
    Reference:          [RFC7372]
    Submitter:          M. Kucherawy
    Change controller:  IESG

4. General Considerations

 By the nature of the Simple Mail Transfer Protocol (SMTP), only one
 enhanced status code can be returned for a given exchange between
 client and server.  However, an operator might decide to defer or
 reject a message for a plurality of reasons.  Clients receiving these
 codes need to consider that the failure reflected by one of these
 status codes might not reflect the only reason, or the most important
 reason, for non-acceptance of the message or command.
 It is important to note that Section 6.1 of [RFC6376] discourages
 special treatment of messages bearing no valid DKIM signature.  There
 are some operators that disregard this advice, a few of which go so
 far as to require a valid Author Domain Signature (that is, one
 matching the domain(s) in the From header field) in order to accept
 the message.  Moreover, some nascent technologies built atop SPF and
 DKIM depend on such authentications.  This work does not endorse
 configurations that violate DKIM's recommendations but rather
 acknowledges that they do exist and merely seeks to provide for
 improved interoperability with such operators.

Kucherawy Standards Track [Page 5] RFC 7372 Email Auth Status Codes September 2014

 A specific use case for these codes is mailing list software, which
 processes rejections in order to remove from the subscriber set those
 addresses that are no longer valid.  There is a need in that case to
 distinguish authentication failures from indications that the
 recipient address is no longer valid.
 If a receiving server performs multiple authentication checks and
 more than one of them fails, thus warranting rejection of the
 message, the SMTP server SHOULD use the code that indicates multiple
 methods failed rather than only reporting the first one that failed.
 It may be the case that one method is always expected to fail; thus,
 returning that method's specific code is not information useful to
 the sending agent.
 The reverse IP DNS check is defined in Section 3 of [RFC7001].
 Any message authentication or policy enforcement technologies
 developed in the future should also include registration of their own
 enhanced status codes so that this kind of specific reporting is
 available to operators that wish to use them.

5. Security Considerations

 Use of these codes reveals local policy with respect to email
 authentication, which can be useful information to actors attempting
 to deliver undesired mail.  It should be noted that there is no
 specific obligation to use these codes; if an operator wishes not to
 reveal this aspect of local policy, it can continue using a generic
 result code such as 5.7.7, 5.7.1, or even 5.7.0.

6. IANA Considerations

 Registration of new enhanced status codes, for addition to the
 Enumerated Status Codes sub-registry of the SMTP Enhanced Status
 Codes Registry, can be found in Section 3.

Kucherawy Standards Track [Page 6] RFC 7372 Email Auth Status Codes September 2014

7. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
            3463, January 2003.
 [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
            Mail System Status Codes", BCP 138, RFC 5248, June 2008.
 [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
            Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
            September 2011.
 [RFC7001]  Kucherawy, M., "Message Header Field for Indicating
            Message Authentication Status", RFC 7001, September 2013.
 [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
            Authorizing Use of Domains in Email, Version 1", RFC 7208,
            April 2014.

Kucherawy Standards Track [Page 7] RFC 7372 Email Auth Status Codes September 2014

Appendix A. Acknowledgments

 Claudio Allocchio, Dave Crocker, Ned Freed, Arnt Gulbrandsen, Scott
 Kitterman, Barry Leiba, Alexey Melnikov, S. Moonesamy, Hector Santos,
 and Stephen Turnbull contributed to this work.

Author's Address

 Murray S. Kucherawy
 270 Upland Drive
 San Francisco, CA  94127
 USA
 EMail: superuser@gmail.com

Kucherawy Standards Track [Page 8]

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