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7Internet Engineering Task Force (IETF) M. Kucherawy
8Request for Comments: 7372 September 2014
9Updates: 7208
10Category: Standards Track
11ISSN: 2070-1721
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14 Email Authentication Status Codes
15
16Abstract
17
18 This document registers code points to allow status codes to be
19 returned to an email client to indicate that a message is being
20 rejected or deferred specifically because of email authentication
21 failures.
22
23 This document updates RFC 7208, since some of the code points
24 registered replace the ones recommended for use in that document.
25
26Status of This Memo
27
28 This is an Internet Standards Track document.
29
30 This document is a product of the Internet Engineering Task Force
31 (IETF). It represents the consensus of the IETF community. It has
32 received public review and has been approved for publication by the
33 Internet Engineering Steering Group (IESG). Further information on
34 Internet Standards is available in Section 2 of RFC 5741.
35
36 Information about the current status of this document, any errata,
37 and how to provide feedback on it may be obtained at
38 http://www.rfc-editor.org/info/rfc7372.
39
40Copyright Notice
41
42 Copyright (c) 2014 IETF Trust and the persons identified as the
43 document authors. All rights reserved.
44
45 This document is subject to BCP 78 and the IETF Trust's Legal
46 Provisions Relating to IETF Documents
47 (http://trustee.ietf.org/license-info) in effect on the date of
48 publication of this document. Please review these documents
49 carefully, as they describe your rights and restrictions with respect
50 to this document. Code Components extracted from this document must
51 include Simplified BSD License text as described in Section 4.e of
52 the Trust Legal Provisions and are provided without warranty as
53 described in the Simplified BSD License.
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58Kucherawy Standards Track [Page 1]
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60RFC 7372 Email Auth Status Codes September 2014
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63Table of Contents
64
65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
66 2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . 2
67 3. New Enhanced Status Codes . . . . . . . . . . . . . . . . . . 3
68 3.1. DKIM Failure Codes . . . . . . . . . . . . . . . . . . . 3
69 3.2. SPF Failure Codes . . . . . . . . . . . . . . . . . . . . 4
70 3.3. Reverse DNS Failure Code . . . . . . . . . . . . . . . . 5
71 3.4. Multiple Authentication Failures Code . . . . . . . . . . 5
72 4. General Considerations . . . . . . . . . . . . . . . . . . . 5
73 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
74 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
75 7. Normative References . . . . . . . . . . . . . . . . . . . . 7
76 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 8
77
781. Introduction
79
80 [RFC3463] introduced Enhanced Mail System Status Codes, and [RFC5248]
81 created an IANA registry for these.
82
83 [RFC6376] and [RFC7208] introduced, respectively, DomainKeys
84 Identified Mail (DKIM) and Sender Policy Framework (SPF), two
85 protocols for conducting message authentication. Another common
86 email acceptance test is the reverse Domain Name System (DNS) check
87 on an email client's IP address, as described in Section 3 of
88 [RFC7001].
89
90 The current set of enhanced status codes does not include any code
91 for indicating that a message is being rejected or deferred due to
92 local policy reasons related to any of these mechanisms. This is
93 potentially useful information to agents that need more than
94 rudimentary handling information about the reason a message was
95 rejected on receipt. This document introduces enhanced status codes
96 for reporting those cases to clients.
97
98 Section 3.2 updates [RFC7208], as new enhanced status codes relevant
99 to that specification are being registered and recommended for use.
100
1012. Key Words
102
103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
105 "OPTIONAL" in this document are to be interpreted as described in
106 [RFC2119].
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1193. New Enhanced Status Codes
120
121 The new enhanced status codes are defined in the following
122 subsections.
123
1243.1. DKIM Failure Codes
125
126 In the code point definitions below, the following definitions are
127 used:
128
129 passing: A signature is "passing" if the basic DKIM verification
130 algorithm, as defined in [RFC6376], succeeds.
131
132 acceptable: A signature is "acceptable" if it satisfies all locally
133 defined requirements (if any) in addition to passing the basic
134 DKIM verification algorithm (e.g., certain header fields are
135 included in the signed content, no partial signatures, etc.).
136
137 Code: X.7.20 ../smtp/codes.go:135
138 Sample Text: No passing DKIM signature found
139 Associated basic status code: 550
140 Description: This status code is returned when a message
141 did not contain any passing DKIM
142 signatures. (This violates the
143 advice of Section 6.1 of RFC 6376.)
144 Reference: [RFC7372]; [RFC6376]
145 Submitter: M. Kucherawy
146 Change controller: IESG
147
148 Code: X.7.21 ../smtp/codes.go:136
149 Sample Text: No acceptable DKIM signature found
150 Associated basic status code: 550
151 Description: This status code is returned when a message
152 contains one or more passing DKIM signatures,
153 but none are acceptable. (This violates the
154 advice of Section 6.1 of RFC 6376.)
155 Reference: [RFC7372]; [RFC6376]
156 Submitter: M. Kucherawy
157 Change controller: IESG
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172RFC 7372 Email Auth Status Codes September 2014
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174
175 Code: X.7.22 ../smtp/codes.go:137
176 Sample Text: No valid author-matched DKIM signature found
177 Associated basic status code: 550
178 Description: This status code is returned when a message
179 contains one or more passing DKIM
180 signatures, but none are acceptable because
181 none have an identifier(s)
182 that matches the author address(es) found in
183 the From header field. This is a special
184 case of X.7.21. (This violates the advice
185 of Section 6.1 of RFC 6376.)
186 Reference: [RFC7372]; [RFC6376]
187 Submitter: M. Kucherawy
188 Change controller: IESG
189
1903.2. SPF Failure Codes
191
192 Code: X.7.23 ../smtp/codes.go:138
193 Sample Text: SPF validation failed
194 Associated basic status code: 550
195 Description: This status code is returned when a message
196 completed an SPF check that produced a
197 "fail" result, contrary to local policy
198 requirements. Used in place of 5.7.1, as
199 described in Section 8.4 of RFC 7208.
200 Reference: [RFC7372]; [RFC7208]
201 Submitter: M. Kucherawy
202 Change controller: IESG
203
204 Code: X.7.24 ../smtp/codes.go:139
205 Sample Text: SPF validation error
206 Associated basic status code: 451/550
207 Description: This status code is returned when evaluation
208 of SPF relative to an arriving message
209 resulted in an error. Used in place of
210 4.4.3 or 5.5.2, as described in Sections
211 8.6 and 8.7 of RFC 7208.
212 Reference: [RFC7372]; [RFC7208]
213 Submitter: M. Kucherawy
214 Change controller: IESG
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2313.3. Reverse DNS Failure Code
232
233 Code: X.7.25 ../smtp/codes.go:140
234 Sample Text: Reverse DNS validation failed
235 Associated basic status code: 550
236 Description: This status code is returned when an SMTP
237 client's IP address failed a reverse DNS
238 validation check, contrary to local policy
239 requirements.
240 Reference: [RFC7372]; Section 3 of [RFC7001]
241 Submitter: M. Kucherawy
242 Change controller: IESG
243
2443.4. Multiple Authentication Failures Code
245
246 Code: X.7.26 ../smtp/codes.go:141
247 Sample Text: Multiple authentication checks failed
248 Associated basic status code: 550
249 Description: This status code is returned when a message
250 failed more than one message authentication
251 check, contrary to local policy requirements.
252 The particular mechanisms that failed are not
253 specified.
254 Reference: [RFC7372]
255 Submitter: M. Kucherawy
256 Change controller: IESG
257
2584. General Considerations
259
260 By the nature of the Simple Mail Transfer Protocol (SMTP), only one
261 enhanced status code can be returned for a given exchange between
262 client and server. However, an operator might decide to defer or
263 reject a message for a plurality of reasons. Clients receiving these
264 codes need to consider that the failure reflected by one of these
265 status codes might not reflect the only reason, or the most important
266 reason, for non-acceptance of the message or command.
267
268 It is important to note that Section 6.1 of [RFC6376] discourages
269 special treatment of messages bearing no valid DKIM signature. There
270 are some operators that disregard this advice, a few of which go so
271 far as to require a valid Author Domain Signature (that is, one
272 matching the domain(s) in the From header field) in order to accept
273 the message. Moreover, some nascent technologies built atop SPF and
274 DKIM depend on such authentications. This work does not endorse
275 configurations that violate DKIM's recommendations but rather
276 acknowledges that they do exist and merely seeks to provide for
277 improved interoperability with such operators.
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287 A specific use case for these codes is mailing list software, which
288 processes rejections in order to remove from the subscriber set those
289 addresses that are no longer valid. There is a need in that case to
290 distinguish authentication failures from indications that the
291 recipient address is no longer valid.
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293 If a receiving server performs multiple authentication checks and
294 more than one of them fails, thus warranting rejection of the
295 message, the SMTP server SHOULD use the code that indicates multiple
296 methods failed rather than only reporting the first one that failed.
297 It may be the case that one method is always expected to fail; thus,
298 returning that method's specific code is not information useful to
299 the sending agent.
300
301 The reverse IP DNS check is defined in Section 3 of [RFC7001].
302
303 Any message authentication or policy enforcement technologies
304 developed in the future should also include registration of their own
305 enhanced status codes so that this kind of specific reporting is
306 available to operators that wish to use them.
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3085. Security Considerations
309
310 Use of these codes reveals local policy with respect to email
311 authentication, which can be useful information to actors attempting
312 to deliver undesired mail. It should be noted that there is no
313 specific obligation to use these codes; if an operator wishes not to
314 reveal this aspect of local policy, it can continue using a generic
315 result code such as 5.7.7, 5.7.1, or even 5.7.0.
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3176. IANA Considerations
318
319 Registration of new enhanced status codes, for addition to the
320 Enumerated Status Codes sub-registry of the SMTP Enhanced Status
321 Codes Registry, can be found in Section 3.
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3437. Normative References
344
345 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
346 Requirement Levels", BCP 14, RFC 2119, March 1997.
347
348 [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
349 3463, January 2003.
350
351 [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
352 Mail System Status Codes", BCP 138, RFC 5248, June 2008.
353
354 [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
355 Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
356 September 2011.
357
358 [RFC7001] Kucherawy, M., "Message Header Field for Indicating
359 Message Authentication Status", RFC 7001, September 2013.
360
361 [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
362 Authorizing Use of Domains in Email, Version 1", RFC 7208,
363 April 2014.
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399Appendix A. Acknowledgments
400
401 Claudio Allocchio, Dave Crocker, Ned Freed, Arnt Gulbrandsen, Scott
402 Kitterman, Barry Leiba, Alexey Melnikov, S. Moonesamy, Hector Santos,
403 and Stephen Turnbull contributed to this work.
404
405Author's Address
406
407 Murray S. Kucherawy
408 270 Upland Drive
409 San Francisco, CA 94127
410 USA
411
412 EMail: superuser@gmail.com
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