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 © 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.

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].

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
      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

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.

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.

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.

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:

          [email protected]