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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]