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.
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Contents
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.
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.
New Enhanced Status Codes
The new enhanced status codes are defined in the following subsections.
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
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
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
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
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.
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.
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.
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]
