Email Authentication Status Codes
draft-ietf-appsawg-email-auth-codes-06

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 [RFC7208] since some of the code points registered replace the ones recommended for use in that document.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on February 6, 2015.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

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

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 following new enhanced status codes are defined:

3.1. DKIM Failure Codes

In the code point definitions below, the term "acceptable" means both of the following:

1. The signature passed the basic DKIM verification algorithm as defined in [RFC6376]; and
2. The signature satisfied any local policy requirements in addition to the basic algorithm (e.g., certain header fields included in the signed content, no partial signatures, etc.).

   Code:               X.7.20
   Sample Text:        No valid DKIM signature found
   Associated basic status code:  550
   Description:        This status code is returned when a message
                       did not contain any acceptable DKIM
                       signatures.  (Note that this violates the
                       advice of Section 6.1 of RFC6376.) 
   Reference:          [this document]; RFC6376
   Submitter:          M. Kucherawy
   Change controller:  IESG
			

   Code:               X.7.21
   Sample Text:        No valid author-matched DKIM signature found
   Associated basic status code:  550
   Description:        This status code is returned when a message
                       did not contain any acceptable DKIM
                       signatures whose identifier(s) match the
                       author address(es) found in the From header
                       field.  (Note that this violates the advice
                       of Section 6.1 of RFC6376.)  This is a
                       special case of the X.7.20 status code.
   Reference:          [this document]; RFC6376
   Submitter:          M. Kucherawy
   Change controller:  IESG
			

3.2. SPF Failure Codes

   Code:               X.7.22
   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 RFC7208.
   Reference:          [this document]; RFC7208
   Submitter:          M. Kucherawy
   Change controller:  IESG
			

   Code:               X.7.23
   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 RFC7208.
   Reference:          [this document]; RFC7208
   Submitter:          M. Kucherawy
   Change controller:  IESG
			

3.3. Reverse DNS Failure Code

   Code:               X.7.24
   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:          [this document]; Section 3 of RFC7001
   Submitter:          M. Kucherawy
   Change controller:  IESG
			

3.4. Multiple Authentication Failures Code

   Code:               X.7.25
   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 specific mechanisms that failed are not
                       specified.
   Reference:          [this document]
   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 versus 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, and thus returning that method's specific code is not information useful to the sending agent.

The reverse IP DNS check is defined in Section 2.6.3 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 Stauts Codes sub-registry of the SMTP Enhanced Status Codes Registry, can be found in Section 3.

7. Normative References

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