Network Working Group M. Kucherawy Internet-Draft August 5, 2014 Updates: 7208 (if approved) Intended status: Standards Track Expires: February 6, 2015 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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. 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 Kucherawy Expires February 6, 2015 [Page 1] Internet-Draft Email Auth Status Codes August 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 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 . . . . . . . . . . . . . . . . . . . 7 Kucherawy Expires February 6, 2015 [Page 2] Internet-Draft Email Auth Status Codes August 2014 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: a. The signature passed the basic DKIM verification algorithm as defined in [RFC6376]; and b. 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.). Kucherawy Expires February 6, 2015 [Page 3] Internet-Draft Email Auth Status Codes August 2014 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 Kucherawy Expires February 6, 2015 [Page 4] Internet-Draft Email Auth Status Codes August 2014 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 Kucherawy Expires February 6, 2015 [Page 5] Internet-Draft Email Auth Status Codes August 2014 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 Kucherawy Expires February 6, 2015 [Page 6] Internet-Draft Email Auth Status Codes August 2014 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: superuser@gmail.com Kucherawy Expires February 6, 2015 [Page 7]