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'IMAP') (Obsoleted by RFC 9051) -- Obsolete informational reference (is this intentional?): RFC 5451 (Obsoleted by RFC 7001) -- Obsolete informational reference (is this intentional?): RFC 6577 (Obsoleted by RFC 7001) -- Obsolete informational reference (is this intentional?): RFC 7001 (Obsoleted by RFC 7601) -- Obsolete informational reference (is this intentional?): RFC 7410 (Obsoleted by RFC 7601) Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Individual submission M. Kucherawy 3 Internet-Draft January 28, 2019 4 Obsoletes: 7601 (if approved) 5 Intended status: Standards Track 6 Expires: August 1, 2019 8 Message Header Field for Indicating Message Authentication Status 9 draft-ietf-dmarc-rfc7601bis-06 11 Abstract 13 This document specifies a message header field called Authentication- 14 Results for use with electronic mail messages to indicate the results 15 of message authentication efforts. Any receiver-side software, such 16 as mail filters or Mail User Agents (MUAs), can use this header field 17 to relay that information in a convenient and meaningful way to users 18 or to make sorting and filtering decisions. 20 This document obsoletes [RFC7601]. 22 Status of this Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on August 1, 2019. 39 Copyright Notice 41 Copyright (c) 2019 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 57 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6 59 1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 60 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 61 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 62 1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 63 1.5.2. Internationalized Email . . . . . . . . . . . . . . . 7 64 1.5.3. Security . . . . . . . . . . . . . . . . . . . . . . . 7 65 1.5.4. Email Architecture . . . . . . . . . . . . . . . . . . 8 66 1.5.5. Other Terms . . . . . . . . . . . . . . . . . . . . . 9 67 1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9 68 2. Definition and Format of the Header Field . . . . . . . . . . 10 69 2.1. General Description . . . . . . . . . . . . . . . . . . . 10 70 2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10 71 2.3. Property Types (ptypes) and Properties . . . . . . . . . . 13 72 2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 14 73 2.5. Authentication Identifier Field . . . . . . . . . . . . . 15 74 2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 16 75 2.7. Defined Methods and Result Values . . . . . . . . . . . . 16 76 2.7.1. DKIM . . . . . . . . . . . . . . . . . . . . . . . . . 17 77 2.7.2. SPF . . . . . . . . . . . . . . . . . . . . . . . . . 18 78 2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 19 79 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 20 80 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 21 81 2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21 82 2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 22 83 3. The "iprev" Authentication Method . . . . . . . . . . . . . . 23 84 4. Adding the Header Field to a Message . . . . . . . . . . . . . 24 85 4.1. Header Field Position and Interpretation . . . . . . . . . 25 86 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 26 87 5. Removing Existing Header Fields . . . . . . . . . . . . . . . 27 88 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 89 6.1. The Authentication-Results Header Field . . . . . . . . . 28 90 6.2. "Email Authentication Methods" Registry Description . . . 29 91 6.3. "Email Authentication Methods" Registry Update . . . . . . 30 92 6.3.1. 'header.a' for DKIM . . . . . . . . . . . . . . . . . 30 93 6.3.2. 'header.s' for DKIM . . . . . . . . . . . . . . . . . 31 94 6.4. "Email Authentication Property Types" Registry 95 Description . . . . . . . . . . . . . . . . . . . . . . . 31 97 6.5. "Email Authentication Property Types" Registry Update . . 32 98 6.6. "Email Authentication Result Names" Registry 99 Description . . . . . . . . . . . . . . . . . . . . . . . 32 100 6.7. "Email Authentication Result Names" Registry Update . . . 33 101 6.8. SMTP Enhanced Status Codes . . . . . . . . . . . . . . . . 33 102 7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 103 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 34 104 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 35 105 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 36 106 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 36 107 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 36 108 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 36 109 7.7. Attacks against Authentication Methods . . . . . . . . . . 36 110 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 37 111 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 37 112 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 37 113 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 38 114 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38 115 8.1. Normative References . . . . . . . . . . . . . . . . . . . 38 116 8.2. Informative References . . . . . . . . . . . . . . . . . . 39 117 Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 42 118 Appendix B. Authentication-Results Examples . . . . . . . . . . . 42 119 B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 43 120 B.2. Nearly Trivial Case; Service Provided, but No 121 Authentication Done . . . . . . . . . . . . . . . . . . . 43 122 B.3. Service Provided, Authentication Done . . . . . . . . . . 44 123 B.4. Service Provided, Several Authentications Done, Single 124 MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 125 B.5. Service Provided, Several Authentications Done, 126 Different MTAs . . . . . . . . . . . . . . . . . . . . . . 46 127 B.6. Service Provided, Multi-tiered Authentication Done . . . . 48 128 B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 49 129 Appendix C. Operational Considerations about Message 130 Authentication . . . . . . . . . . . . . . . . . . . 50 131 Appendix D. Changes since RFC7601 . . . . . . . . . . . . . . . . 51 132 Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 52 133 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 52 135 1. Introduction 137 This document describes a header field called Authentication-Results 138 for electronic mail messages that presents the results of a message 139 authentication effort in a machine-readable format. The intent of 140 the header field is to create a place to collect such data when 141 message authentication mechanisms are in use so that a Mail User 142 Agent (MUA) and downstream filters can make filtering decisions 143 and/or provide a recommendation to the user as to the validity of the 144 message's origin and possibly the safety and integrity of its 145 content. 147 End users are not expected to be direct consumers of this header 148 field. This header field is intended for consumption by programs 149 that will then use such data or render it in a human-usable form. 151 This document specifies the format of this header field and discusses 152 the implications of its presence or absence. However, it does not 153 discuss how the data contained in the header field ought to be used, 154 such as what filtering decisions are appropriate or how an MUA might 155 render those results, as these are local policy and/or user interface 156 design questions that are not appropriate for this document. 158 At the time of publication of this document, the following are 159 published email authentication methods: 161 o SMTP Service Extension for Authentication ([AUTH]) 163 o DomainKeys Identified Mail Signatures ([DKIM]) 165 o Domain-based Message Authentication, Reporting and Conformance 166 ([DMARC]) 168 o Sender Policy Framework ([SPF]) 170 o reverse IP address name validation ("iprev", defined in Section 3) 172 o Require-Recipient-Valid-Since Header Field and SMTP Service 173 Extension ([RRVS]) 175 o S/MIME Signature Verification ([SMIME-REG]) 177 o Vouch By Reference ([VBR]) 179 The following historic specifications were previously supported by 180 this framework, but have since become obsolete: 182 o Author Domain Signing Practices ([ADSP]) (Historic) 184 o DomainKeys ([DOMAINKEYS]) (Historic) 186 o Sender ID ([SENDERID]) (Historic) 188 There exist registries for tokens used within this header field that 189 refer to the specifications listed above. Section 6 describes the 190 registries and their contents and specifies the process by which 191 entries are added or updated. It also updates the existing contents 192 to match the current states of these specifications. 194 The goal of this work is to give current and future authentication 195 schemes a common framework within which to deliver their results to 196 downstream agents and discourage the creation of unique header fields 197 for each. 199 Although SPF defined a header field called "Received-SPF" and the 200 historic DomainKeys defined one called "DomainKey-Status" for this 201 purpose, those header fields are specific to the conveyance of their 202 respective results only and thus are insufficient to satisfy the 203 requirements enumerated below. In addition, many SPF implementations 204 have adopted the header field specified here at least as an option, 205 and DomainKeys has been obsoleted by DKIM. 207 1.1. Purpose 209 The header field defined in this document is expected to serve 210 several purposes: 212 1. Convey the results of various message authentication checks, 213 which are applied by upstream filters and Mail Transfer Agents 214 (MTAs) and then passed to MUAs and downstream filters within the 215 same "trust domain". Such agents might wish to render those 216 results to end users or to use those data to apply more or less 217 stringent content checks based on authentication results; 219 2. Provide a common location within a message for this data; 221 3. Create an extensible framework for reporting new authentication 222 methods as they emerge. 224 In particular, the mere presence of this header field does not mean 225 its contents are valid. Rather, the header field is reporting 226 assertions made by one or more authentication schemes applied 227 somewhere upstream. For an MUA or downstream filter to treat the 228 assertions as actually valid, there must be an assessment of the 229 trust relationship among such agents, the validating MTA, the paths 230 between them, and the mechanism for conveying the information. 232 1.2. Trust Boundary 234 This document makes several references to the "trust boundary" of an 235 administrative management domain (ADMD). Given the diversity among 236 existing mail environments, a precise definition of this term isn't 237 possible. 239 Simply put, a transfer from the producer of the header field to the 240 consumer must occur within a context that permits the consumer to 241 treat assertions by the producer as being reliable and accurate 242 (trustworthy). How this trust is obtained is outside the scope of 243 this document. It is entirely a local matter. 245 Thus, this document defines a "trust boundary" as the delineation 246 between "external" and "internal" entities. Services that are 247 internal -- within the trust boundary -- are provided by the ADMD's 248 infrastructure for its users. Those that are external are outside of 249 the authority of the ADMD. By this definition, hosts that are within 250 a trust boundary are subject to the ADMD's authority and policies, 251 independent of their physical placement or their physical operation. 252 For example, a host within a trust boundary might actually be 253 operated by a remote service provider and reside physically within 254 its data center. 256 It is possible for a message to be evaluated inside a trust boundary 257 but then depart and re-enter the trust boundary. An example might be 258 a forwarded message such as a message/rfc822 attachment (see 259 Multipurpose Internet Mail Extensions [MIME]) or one that is part of 260 a multipart/digest. The details reported by this field cannot be 261 trusted in that case. Thus, this field found within one of those 262 media types is typically ignored. 264 Note that an MUA could be configured to retrieve messages from a 265 Receiver yet not be within the Receiver's ADMD. In this case, for 266 the purposes of this work, that MUA is considered to be within the 267 Receiver's ADMD if is configured to identify and ascribe value to 268 authentication results recorded by that ADMD. 270 1.3. Processing Scope 272 The content of this header field is meant to convey to message 273 consumers that authentication work on the message was already done 274 within its trust boundary, and those results are being presented. It 275 is not intended to provide message parameters to consumers so that 276 they can perform authentication protocols on their own. 278 1.4. Requirements 280 This document establishes no new requirements on existing protocols, 281 insofar as a non-participating service will continue to interoperate 282 with the deployed messaging infrastructure. 284 In particular, this document establishes no requirement on MTAs to 285 reject or filter arriving messages that do not pass authentication 286 checks. The data conveyed by the specified header field's contents 287 are for the information of MUAs and filters and are to be used at 288 their discretion. 290 A participating ADMD does undertake some filtering and message 291 modification obligations described in Section 5. 293 1.5. Definitions 295 This section defines various terms used throughout this document. 297 1.5.1. Key Words 299 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 300 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 301 document are to be interpreted as described in BCP 14 [RFC2119] 302 [RFC8174] when, and only when, they appear in all capitals, as shown 303 here. 305 1.5.2. Internationalized Email 307 In this document, there are references to messages formatted to 308 support Email Address Internationalization (EAI). Reference material 309 for this can be found in [RFC6530], [RFC6531], and [RFC6532]. 310 Generally speaking, these documents allow UTF-8 in most places that 311 free-form text can be found and U-labels where domain names can be 312 used, and this document extends Authentication-Results accordingly. 314 1.5.3. Security 316 "Guidelines for Writing RFC Text on Security Considerations" 317 ([SECURITY]) discusses authentication and authorization and the 318 conflation of the two concepts. The use of those terms within the 319 context of recent message security work has given rise to slightly 320 different definitions, and this document reflects those current 321 usages, as follows: 323 o "Authorization" is the establishment of permission to use a 324 resource or represent an identity. In this context, authorization 325 indicates that a message from a particular ADMD arrived via a 326 route the ADMD has explicitly approved. 328 o "Authentication" is the assertion of validity of a piece of data 329 about a message (such as the sender's identity) or the message in 330 its entirety. 332 As examples: SPF is an authorization mechanism in that it expresses a 333 result that shows whether the ADMD that apparently sent the message 334 has explicitly authorized the connecting Simple Mail Transfer 335 Protocol ([SMTP]) client to relay messages on its behalf, but they do 336 not actually validate any other property of the message itself. By 337 contrast, DKIM is agnostic as to the routing of a message but uses 338 cryptographic signatures to authenticate agents, assign (some) 339 responsibility for the message (which implies authorization), and 340 ensure that the listed portions of the message were not modified in 341 transit. Since the signatures are not tied to SMTP connections, they 342 can be added by either the ADMD of origin, intermediate ADMDs (such 343 as a mailing list server), other handling agents, or any combination. 345 Rather than create a separate header field for each class of 346 solution, this specification groups them both into a single header 347 field. 349 1.5.4. Email Architecture 351 o A "border MTA" is an MTA that acts as a gateway between the 352 general Internet and the users within an organizational boundary. 353 (See also Section 1.2.) 355 o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that 356 actually enacts delivery of a message to a user's inbox or other 357 final delivery. 359 o An "intermediate MTA" is any MTA that is not a delivery MTA and is 360 also not the first MTA to handle the message. 362 The following diagram illustrates the flow of mail among these 363 defined components. See Internet Mail Architecture [EMAIL-ARCH] for 364 further discussion on general email system architecture, which 365 includes detailed descriptions of these components, and Appendix C of 366 this document for discussion about the common aspects of email 367 authentication in current environments. 369 +-----+ +-----+ +------------+ 370 | MUA |-->| MSA |-->| Border MTA | 371 +-----+ +-----+ +------------+ 372 | 373 | 374 V 375 +----------+ 376 | Internet | 377 +----------+ 378 | 379 | 380 V 381 +-----+ +-----+ +------------------+ +------------+ 382 | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA | 383 +-----+ +-----+ +------------------+ +------------+ 385 Generally, it is assumed that the work of applying message 386 authentication schemes takes place at a border MTA or a delivery MTA. 387 This specification is written with that assumption in mind. However, 388 there are some sites at which the entire mail infrastructure consists 389 of a single host. In such cases, such terms as "border MTA" and 390 "delivery MTA" might well apply to the same machine or even the very 391 same agent. It is also possible that some message authentication 392 tests could take place on an intermediate MTA. Although this 393 document doesn't specifically describe such cases, they are not meant 394 to be excluded. 396 1.5.5. Other Terms 398 In this document, the term "producer" refers to any component that 399 adds this header field to messages it is handling, and "consumer" 400 refers to any component that identifies, extracts, and parses the 401 header field to use as part of a handling decision. 403 1.6. Trust Environment 405 This header field permits one or more message validation mechanisms 406 to communicate output to one or more separate assessment mechanisms. 407 These mechanisms operate within a unified trust boundary that defines 408 an Administrative Management Domain (ADMD). An ADMD contains one or 409 more entities that perform validation and generate the header field 410 and one or more that consume it for some type of assessment. The 411 field often contains no integrity or validation mechanism of its own, 412 so its presence must be trusted implicitly. Hence, valid use of the 413 header field requires removing any occurrences of it that claim to be 414 associated with the ADMD when the message enters the ADMD. This 415 ensures that later occurrences have been added within the trust 416 boundary of the ADMD. 418 The authserv-id token defined in Section 2.2 can be used to reference 419 an entire ADMD or a specific validation engine within an ADMD. 420 Although the labeling scheme is left as an operational choice, some 421 guidance for selecting a token is provided in later sections of this 422 document. 424 2. Definition and Format of the Header Field 426 This section gives a general overview of the format of the header 427 field being defined and then provides a formal specification. 429 2.1. General Description 431 The header field specified here is called Authentication-Results. It 432 is a Structured Header Field as defined in Internet Message Format 433 ([MAIL]), and thus all of the related definitions in that document 434 apply. 436 This header field is added at the top of the message as it transits 437 MTAs that do authentication checks, so some idea of how far away the 438 checks were done can be inferred. It is therefore considered to be a 439 trace field as defined in [MAIL], and thus all of the related 440 definitions in that document apply. 442 The value of the header field (after removing comments) consists of 443 an authentication identifier, an optional version, and then a series 444 of statements and supporting data. The statements are of the form 445 "method=result" and indicate which authentication method(s) were 446 applied and their respective results. For each such statement, the 447 supporting data can include a "reason" string and one or more 448 "property=value" statements indicating which message properties were 449 evaluated to reach that conclusion. 451 The header field can appear more than once in a single message, more 452 than one result can be represented in a single header field, or a 453 combination of these can be applied. 455 2.2. Formal Definition 457 Formally, the header field is specified as shown below using 458 Augmented Backus-Naur Form ([ABNF]). Examples of valid header fields 459 with explanations of their semantics can be found in Appendix B. 461 authres-header-field = "Authentication-Results:" authres-payload 463 authres-payload = [CFWS] authserv-id 464 [ CFWS authres-version ] 465 ( no-result / 1*resinfo ) [CFWS] CRLF 467 authserv-id = value 468 ; see below for a description of this element 470 authres-version = 1*DIGIT [CFWS] 471 ; indicates which version of this specification is in use; 472 ; this specification is version "1", and the absence of a 473 ; version implies this version of the specification 475 no-result = [CFWS] ";" [CFWS] "none" 476 ; the special case of "none" is used to indicate that no 477 ; message authentication was performed 479 resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ] 480 *( CFWS propspec ) 482 methodspec = [CFWS] method [CFWS] "=" [CFWS] result 483 ; indicates which authentication method was evaluated 484 ; and what its output was 486 reasonspec = "reason" [CFWS] "=" [CFWS] value 487 ; a free-form comment on the reason the given result 488 ; was returned 490 propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue 491 ; an indication of which properties of the message 492 ; were evaluated by the authentication scheme being 493 ; applied to yield the reported result 495 method = Keyword [ [CFWS] "/" [CFWS] method-version ] 496 ; a method indicates which method's result is 497 ; represented by "result", and is one of the methods 498 ; explicitly defined as valid in this document 499 ; or is an extension method as defined below 501 method-version = 1*DIGIT [CFWS] 502 ; indicates which version of the method specification is 503 ; in use, corresponding to the matching entry in the IANA 504 ; "Email Authentication Methods" registry; a value of "1" 505 ; is assumed if this version string is absent 507 result = Keyword 508 ; indicates the results of the attempt to authenticate 509 ; the message; see below for details 511 ptype = Keyword 512 ; indicates whether the property being evaluated was 513 ; a parameter to an [SMTP] command, was a value taken 514 ; from a message header field, was some property of 515 ; the message body, or was some other property evaluated by 516 ; the receiving MTA; expected to be one of the "property 517 ; types" explicitly defined as valid, or an extension 518 ; ptype, as defined below 520 property = special-smtp-verb / Keyword 521 ; indicates more specifically than "ptype" what the 522 ; source of the evaluated property is; the exact meaning 523 ; is specific to the method whose result is being reported 524 ; and is defined more clearly below 526 special-smtp-verb = "mailfrom" / "rcptto" 527 ; special cases of [SMTP] commands that are made up 528 ; of multiple words 530 pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) 531 [CFWS] 532 ; the value extracted from the message property defined 533 ; by the "ptype.property" construction 535 "local-part" is defined in Section 3.4.1 of [MAIL], as modified by 536 [RFC6531]. 538 "CFWS" is defined in Section 3.2.2 of [MAIL]. 540 "Keyword" is defined in Section 4.1.2 of [SMTP]. It is further 541 constrained by the necesity of being registered in the IANA registry 542 relevant to the context in which it it is used. See Section 2.7, and 543 Section 2.3, and Section 6. 545 The "value" is as defined in Section 5.1 of [MIME], with "quoted- 546 string" updated as specified in [RFC6532]. 548 The "domain-name" is as defined in Section 3.5 of [DKIM]. 550 The "Keyword" used in "result" above is further constrained by the 551 necessity of being enumerated in Section 2.7. 553 See Section 2.5 for a description of the authserv-id element. 555 If the value portion of a "pvalue" construction identifies something 556 intended to be an email identity, then it MUST use the right hand 557 portion of that ABNF definition. 559 The list of commands eligible for use with the "smtp" ptype can be 560 found in Section 4.1 of [SMTP]. 562 The "propspec" may be omitted if, for example, the method was unable 563 to extract any properties to do its evaluation yet still has a result 564 to report. It may also be omitted if the agent generating this 565 result wishes not to reveal such properties to downstream agents. 567 Where an SMTP command name is being reported as a "property", the 568 agent generating the header field represents that command by 569 converting it to lowercase and dropping any spaces (e.g., "MAIL FROM" 570 becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.). 572 A "ptype" value of "policy" indicates a policy decision about the 573 message not specific to a property of the message that could be 574 extracted. See Section 2.4 for details. 576 Examples of complete messages using this header field can be found in 577 Appendix B. 579 2.3. Property Types (ptypes) and Properties 581 The "ptype" in the ABNF above indicates the general type of property 582 being described by the result being reported, upon which the reported 583 result was based. Coupled with the "property", which is more 584 specific, they indicate from where the reported data were extracted. 585 This can include a particular part of the message header or body, 586 some part of the SMTP session, a secondary output of an 587 authentication method (apart from its pure result), or some other 588 aspect of the message's handling. 590 Combinations of ptypes and properties are registered and described in 591 the "Email Authentication Methods" registry, coupled with the 592 authentication methods with which they are used. This is further 593 described in Section 6. 595 Legal values of "ptype" are as defined in the IANA "Email 596 Authentication Property Types" registry, created by [RFC7410]. The 597 initial values and what they typically indicate are as follows, based 598 on [RFC7001]: 600 body: Information that was extracted from the body of the message. 601 This might be an arbitrary string of bytes, a hash of a string of 602 bytes, a Uniform Resource Identifier, or some other content of 603 interest. The "property" is an indication of where within the 604 message body the extracted content was found, and can indicate an 605 offset, identify a MIME part, etc. (At the time of this revision, 606 no properties matching this ptype have been registered. 607 Accordingly, this ptype may be deprecated in the future.) 609 header: Indicates information that was extracted from the header of 610 the message. This might be the value of a header field or some 611 portion of a header field. The "property" gives a more precise 612 indication of the place in the header from which the extraction 613 took place. 615 policy: A local policy mechanism was applied that augments or 616 overrides the result returned by the authentication mechanism. 617 (See Section 2.4.) 619 smtp: Indicates information that was extracted from an SMTP command 620 that was used to relay the message. The "property" indicates 621 which SMTP command included the extracted content as a parameter. 623 Results reported using unknown ptypes MUST NOT be used in making 624 handling decisions. They can be safely ignored by consumers. 626 Entries in the "Email Authentication Methods" registry can define 627 properties that deviate from these definitions when appropriate. 628 Such deviations need to be clear in the registry and/or in the 629 defining document. See Section 2.7.1 for an example. 631 2.4. The "policy" ptype 633 A special ptype value of "policy" is also defined. This ptype is 634 provided to indicate that some local policy mechanism was applied 635 that augments or even replaces (i.e., overrides) the result returned 636 by the authentication mechanism. The property and value in this case 637 identify the local policy that was applied and the result it 638 returned. 640 For example, a DKIM signature is not required to include the Subject 641 header field in the set of fields that are signed. An ADMD receiving 642 such a message might decide that such a signature is unacceptable, 643 even if it passes, because the content of the Subject header field 644 could be altered post-signing without invalidating the signature. 645 Such an ADMD could replace the DKIM "pass" result with a "policy" 646 result and then also include the following in the corresponding 647 Authentication-Result field: 649 ... dkim=policy policy.dkim-rules=unsigned-subject ... 651 In this case, the property is "dkim-rules", indicating some local 652 check by that name took place and that check returned a result of 653 "unsigned-subject". These are arbitrary names selected by (and 654 presumably used within) the ADMD making use of them, so they are not 655 normally registered with IANA or otherwise specified apart from 656 setting syntax restrictions that allow for easy parsing within the 657 rest of the header field. 659 This ptype existed in the original specification for this header 660 field ([RFC5451]), but without a complete description or example of 661 intended use. As a result, it has not seen any practical use to date 662 that matches its intended purpose. These added details are provided 663 to guide implementers toward proper use. 665 2.5. Authentication Identifier Field 667 Every Authentication-Results header field has an authentication 668 service identifier field (authserv-id above). Specifically, this is 669 any string intended to identify the authentication service within the 670 ADMD that conducted authentication checks on the message. This 671 identifier is intended to be machine-readable and not necessarily 672 meaningful to users. 674 Note that in an EAI-formatted message, this identifier may be 675 expressed in UTF-8. 677 Since agents consuming this field will use this identifier to 678 determine whether its contents are of interest (and are safe to use), 679 the uniqueness of the identifier MUST be guaranteed by the ADMD that 680 generates it and MUST pertain to that ADMD. MUAs or downstream 681 filters SHOULD use this identifier to determine whether or not the 682 data contained in an Authentication-Results header field ought to be 683 used or ignored. 685 For simplicity and scalability, the authentication service identifier 686 SHOULD be a common token used throughout the ADMD. Common practice 687 is to use the DNS domain name used by or within that ADMD, sometimes 688 called the "organizational domain", but this is not strictly 689 necessary. 691 For tracing and debugging purposes, the authentication identifier can 692 instead be the specific hostname of the MTA performing the 693 authentication check whose result is being reported. Moreover, some 694 implementations define a substructure to the identifier; such 695 structures are outside of the scope of this specification. 697 Note, however, that using a local, relative identifier like a flat 698 hostname, rather than a hierarchical and globally unique ADMD 699 identifier like a DNS domain name, makes configuration more difficult 700 for large sites. The hierarchical identifier permits aggregating 701 related, trusted systems together under a single, parent identifier, 702 which in turn permits assessing the trust relationship with a single 703 reference. The alternative is a flat namespace requiring 704 individually listing each trusted system. Since consumers will use 705 the identifier to determine whether to use the contents of the header 706 field: 708 o Changes to the identifier impose a large, centralized 709 administrative burden. 711 o Ongoing administrative changes require constantly updating this 712 centralized table, making it difficult to ensure that an MUA or 713 downstream filter will have access to accurate information for 714 assessing the usability of the header field's content. In 715 particular, consumers of the header field will need to know not 716 only the current identifier(s) in use but previous ones as well to 717 account for delivery latency or later re-assessment of the header 718 field's contents. 720 Examples of valid authentication identifiers are "example.com", 721 "mail.example.org", "ms1.newyork.example.com", and "example-auth". 723 2.6. Version Tokens 725 The grammar above provides for the optional inclusion of versions on 726 both the header field itself (attached to the authserv-id token) and 727 on each of the methods being reported. The method version refers to 728 the method itself, which is specified in the documents describing 729 those methods, while the authserv-id version refers to this document 730 and thus the syntax of this header field. 732 The purpose of including these is to avoid misinterpretation of the 733 results. That is, if a parser finds a version after an authserv-id 734 that it does not explicitly know, it can immediately discontinue 735 trying to parse since what follows might not be in an expected 736 format. For a method version, the parser SHOULD ignore a method 737 result if the version is not supported in case the semantics of the 738 result have a different meaning than what is expected. For example, 739 if a hypothetical DKIM version 2 yielded a "pass" result for 740 different reasons than version 1 does, a consumer of this field might 741 not want to use the altered semantics. Allowing versions in the 742 syntax is a way to indicate this and let the consumer of the header 743 field decide. 745 2.7. Defined Methods and Result Values 747 Each individual authentication method returns one of a set of 748 specific result values. The subsections below provide references to 749 the documents defining the authentication methods specifically 750 supported by this document, and their corresponding result values. 751 Verifiers SHOULD use these values as described below. New methods 752 not specified in this document, but intended to be supported by the 753 header field defined here, MUST include a similar result table either 754 in their defining documents or in supplementary ones. 756 2.7.1. DKIM 758 DKIM is represented by the "dkim" method and is defined in [DKIM]. 760 A signature is "acceptable to the ADMD" if it passes local policy 761 checks (or there are no specific local policy checks). For example, 762 an ADMD policy might require that the signature(s) on the message be 763 added using the DNS domain present in the From header field of the 764 message, thus making third-party signatures unacceptable even if they 765 verify. 767 The DKIM result set is as follows: 769 none: The message was not signed. 771 pass: The message was signed, the signature or signatures were 772 acceptable to the ADMD, and the signature(s) passed verification 773 tests. 775 fail: The message was signed and the signature or signatures were 776 acceptable to the ADMD, but they failed the verification test(s). 778 policy: The message was signed, but some aspect of the signature or 779 signatures was not acceptable to the ADMD. 781 neutral: The message was signed, but the signature or signatures 782 contained syntax errors or were not otherwise able to be 783 processed. This result is also used for other failures not 784 covered elsewhere in this list. 786 temperror: The message could not be verified due to some error that 787 is likely transient in nature, such as a temporary inability to 788 retrieve a public key. A later attempt may produce a final 789 result. 791 permerror: The message could not be verified due to some error that 792 is unrecoverable, such as a required header field being absent. A 793 later attempt is unlikely to produce a final result. 795 DKIM results are reported using a ptype of "header". The property, 796 however, represents one of the tags found in the DKIM-Signature 797 header field rather than a distinct header field. For example, the 798 ptype-property combination "header.d" refers to the content of the 799 "d" (signing domain) tag from within the signature header field, and 800 not a distinct header field called "d". 802 Note that in an EAI-formatted message, the values of the "d" and "i" 803 properties can be expressed in UTF-8. 805 In addition to previous registrations, this document registers the 806 DKIM tags "a" (cryptographic algorithm used to sign the message) and 807 "s" (selector) as reportable properties. This can be used to aid 808 receivers during post-verification processing. In particular, 809 [RFC8301] obsoleted use of the "rsa-sha1" algorithm in DKIM, so it is 810 important to be able to distinguish such signatures from those using 811 preferred algorithms. 813 The ability to report different DKIM results for a message with 814 multiple signatures is described in [RFC6008]. 816 [DKIM] advises that if a message fails verification, it is to be 817 treated as an unsigned message. A report of "fail" here permits the 818 receiver of the report to decide how to handle the failure. A report 819 of "neutral" or "none" preempts that choice, ensuring the message 820 will be treated as if it had not been signed. 822 2.7.2. SPF 824 SPF uses the "spf" method name. The result values for SPF are 825 defined in Section 2.6 of [SPF], and those definitions are included 826 here by reference: 828 +-----------+------------------------------+ 829 | Code | Meaning | 830 +-----------+------------------------------+ 831 | none | [SPF], Section 2.6.1 | 832 +-----------+------------------------------+ 833 | pass | [SPF], Section 2.6.3 | 834 +-----------+------------------------------+ 835 | fail | [SPF], Section 2.6.4 | 836 +-----------+------------------------------+ 837 | softfail | [SPF], Section 2.6.5 | 838 +-----------+------------------------------+ 839 | policy | [this document], Section 2.4 | 840 +-----------+------------------------------+ 841 | neutral | [SPF], Section 2.6.2 | 842 +-----------+------------------------------+ 843 | temperror | [SPF], Section 2.6.6 | 844 +-----------+------------------------------+ 845 | permerror | [SPF], Section 2.6.7 | 846 +-----------+------------------------------+ 848 These result codes are used in the context of this specification to 849 reflect the result returned by the component conducting SPF 850 evaluation. 852 For SPF, the ptype used is "smtp", and the property is either 853 "mailfrom" or "helo", since those values are the ones SPF can 854 evaluate. (If the SMTP client issued the EHLO command instead of 855 HELO, the property used is "helo".) 857 Note that in an EAI-formatted message, the local-part of the 858 "mailfrom" can be expressed in UTF-8 and the domain part can be 859 expressed as a U-label. 861 For this method, an additional result of "policy" is defined, which 862 means the client was authorized to inject or relay mail on behalf of 863 the sender's DNS domain according to the authentication method's 864 algorithm, but local policy dictates that the result is unacceptable. 865 For example, "policy" might be used if SPF returns a "pass" result, 866 but a local policy check matches the sending DNS domain to one found 867 in an explicit list of unacceptable DNS domains (e.g., spammers). 869 If the retrieved sender policies used to evaluate SPF do not contain 870 explicit provisions for authenticating the local-part (see Section 871 3.4.1 of [MAIL]) of an address, the "pvalue" reported along with 872 results for this mechanism SHOULD NOT include the local-part or the 873 following "@" character. 875 2.7.3. "iprev" 877 The result values used by the "iprev" method, defined in Section 3, 878 are as follows: 880 pass: The DNS evaluation succeeded, i.e., the "reverse" and 881 "forward" lookup results were returned and were in agreement. 883 fail: The DNS evaluation failed. In particular, the "reverse" and 884 "forward" lookups each produced results, but they were not in 885 agreement, or the "forward" query completed but produced no 886 result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an 887 RCODE of 0 (NOERROR) in a reply containing no answers, was 888 returned. 890 temperror: The DNS evaluation could not be completed due to some 891 error that is likely transient in nature, such as a temporary DNS 892 error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or 893 other error condition resulted. A later attempt may produce a 894 final result. 896 permerror: The DNS evaluation could not be completed because no PTR 897 data are published for the connecting IP address, e.g., a DNS 898 RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) 899 in a reply containing no answers, was returned. This prevented 900 completion of the evaluation. A later attempt is unlikely to 901 produce a final result. 903 There is no "none" for this method since any TCP connection 904 delivering email has an IP address associated with it, so some kind 905 of evaluation will always be possible. 907 The result is reported using a ptype of "policy" (as this is not part 908 of any established protocol) and a property of "iprev". 910 For discussion of the format of DNS replies, see "Domain Names - 911 Implementation and Specification" ([DNS]). 913 2.7.4. SMTP AUTH 915 SMTP AUTH (defined in [AUTH]) is represented by the "auth" method. 916 Its result values are as follows: 918 none: SMTP authentication was not attempted. 920 pass: The SMTP client authenticated to the server reporting the 921 result using the protocol described in [AUTH]. 923 fail: The SMTP client attempted to authenticate to the server using 924 the protocol described in [AUTH] but was not successful (such as 925 providing a valid identity but an incorrect password). 927 temperror: The SMTP client attempted to authenticate using the 928 protocol described in [AUTH] but was not able to complete the 929 attempt due to some error that is likely transient in nature, such 930 as a temporary directory service lookup error. A later attempt 931 may produce a final result. 933 permerror: The SMTP client attempted to authenticate using the 934 protocol described in [AUTH] but was not able to complete the 935 attempt due to some error that is likely not transient in nature, 936 such as a permanent directory service lookup error. A later 937 attempt is not likely to produce a final result. 939 The result of AUTH is reported using a ptype of "smtp" and a property 940 of either: 942 o "auth", in which case the value is the authorization identity 943 generated by the exchange initiated by the AUTH command; or 945 o "mailfrom", in which case the value is the mailbox identified by 946 the AUTH parameter used with the MAIL FROM command. 948 Note that in an EAI-formatted message, the local-part can be 949 expressed in UTF-8, and the domain can be expressed as a U-label. 951 If both identities are available, both can be reported. For example, 952 consider this command issued by a client that has completed session 953 authentication with the AUTH command resulting in an authorized 954 identity of "client@c.example": 956 MAIL FROM: AUTH= 958 This could result in a "resinfo" construction like so: 960 ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example 962 Note that in all cases other than "pass", the message was sent by an 963 unauthenticated client. All non-"pass" cases SHOULD thus be treated 964 as equivalent with respect to this method. 966 2.7.5. Other Registered Codes 968 Result codes were also registered in other RFCs as follows: 970 o Vouch By Reference (in [AR-VBR], represented by "vbr"); 972 o Authorized Third-Party Signatures (in [ATPS], represented by 973 "dkim-atps"); 975 o Author Domain Signing Practices (in [ADSP], represented by "dkim- 976 adsp"); 978 o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs"); 980 o S/MIME (in [SMIME-REG], represented by "smime"). 982 Note that in an EAI-formatted message, "vbr.mv" and "vbr.md", which 983 are already registered, can be expressed as U-labels. 985 2.7.6. Extension Methods 987 Additional authentication method identifiers (extension methods) may 988 be defined in the future by later revisions or extensions to this 989 specification. These method identifiers are registered with the 990 Internet Assigned Numbers Authority (IANA) and, preferably, published 991 in an RFC. See Section 6 for further details. 993 Extension methods can be defined for the following reasons: 995 1. To allow additional information from new authentication systems 996 to be communicated to MUAs or downstream filters. The names of 997 such identifiers ought to reflect the name of the method being 998 defined but ought not be needlessly long. 1000 2. To allow the creation of "sub-identifiers" that indicate 1001 different levels of authentication and differentiate between 1002 their relative strengths, e.g., "auth1-weak" and "auth1-strong". 1004 Authentication method implementers are encouraged to provide adequate 1005 information, via message header field comments if necessary, to allow 1006 an MUA developer to understand or relay ancillary details of 1007 authentication results. For example, if it might be of interest to 1008 relay what data was used to perform an evaluation, such information 1009 could be relayed as a comment in the header field, such as: 1011 Authentication-Results: example.com; 1012 foo=pass bar.baz=blob (2 of 3 tests OK) 1014 Experimental method identifiers MUST only be used within ADMDs that 1015 have explicitly consented to use them. These method identifiers and 1016 the parameters associated with them are not documented formally. 1017 Therefore, they are subject to change at any time and not suitable 1018 for production use. Any MTA, MUA, or downstream filter intended for 1019 production use SHOULD ignore or delete any Authentication-Results 1020 header field that includes an experimental (unknown) method 1021 identifier. 1023 2.7.7. Extension Result Codes 1025 Additional result codes (extension results) might be defined in the 1026 future by later revisions or extensions to this specification. Non- 1027 experimental result codes MUST be registered with the Internet 1028 Assigned Numbers Authority (IANA) and preferably published in an RFC. 1029 See Section 6 for further details. 1031 Experimental results MUST only be used within ADMDs that have 1032 explicitly consented to use them. These results and the parameters 1033 associated with them are not formally documented. Therefore, they 1034 are subject to change at any time and not suitable for production 1035 use. Any MTA, MUA, or downstream filter intended for production use 1036 SHOULD ignore or delete any Authentication-Results header field that 1037 includes an extension result. 1039 3. The "iprev" Authentication Method 1041 This section defines an additional authentication method called 1042 "iprev". 1044 "iprev" is an attempt to verify that a client appears to be valid 1045 based on some DNS queries, which is to say that the IP address is 1046 explicitly associated with a domain name. Upon receiving a session 1047 initiation of some kind from a client, the IP address of the client 1048 peer is queried for matching names (i.e., a number-to-name 1049 translation, also known as a "reverse lookup" or a "PTR" record 1050 query). Once that result is acquired, a lookup of each of the names 1051 (i.e., a name-to-number translation, or an "A" or "AAAA" record 1052 query) thus retrieved is done. The response to this second check 1053 will typically result in at least one mapping back to the client's IP 1054 address. 1056 Expressed as an algorithm: If the client peer's IP address is I, the 1057 list of names to which I maps (after a "PTR" query) is the set N, and 1058 the union of IP addresses to which each member of N maps (after 1059 corresponding "A" and "AAAA" queries) is L, then this test is 1060 successful if I is an element of L. 1062 Often an MTA receiving a connection that fails this test will simply 1063 reject the connection using the enhanced status code defined in 1064 [AUTH-ESC]. If an operator instead wishes to make this information 1065 available to downstream agents as a factor in handling decisions, it 1066 records a result in accordance with Section 2.7.3. 1068 The response to a PTR query could contain multiple names. To prevent 1069 heavy DNS loads, agents performing these queries MUST be implemented 1070 such that the number of names evaluated by generation of 1071 corresponding A or AAAA queries is limited so as not to be unduly 1072 taxing to the DNS infrastructure, though it MAY be configurable by an 1073 administrator. As an example, Section 4.6.4 of [SPF] chose a limit 1074 of 10 for its implementation of this algorithm. 1076 "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the 1077 query formats for the IPv6 case. 1079 There is some contention regarding the wisdom and reliability of this 1080 test. For example, in some regions, it can be difficult for this 1081 test ever to pass because the practice of arranging to match the 1082 forward and reverse DNS is infrequently observed. Therefore, the 1083 precise implementation details of how a verifier performs an "iprev" 1084 test are not specified here. The verifier MAY report a successful or 1085 failed "iprev" test at its discretion having done some kind of check 1086 of the validity of the connection's identity using DNS. It is 1087 incumbent upon an agent making use of the reported "iprev" result to 1088 understand what exactly that particular verifier is attempting to 1089 report. 1091 Extensive discussion of reverse DNS mapping and its implications can 1092 be found in "Considerations for the use of DNS Reverse Mapping" 1093 ([DNSOP-REVERSE]). In particular, it recommends that applications 1094 avoid using this test as a means of authentication or security. Its 1095 presence in this document is not an endorsement but is merely 1096 acknowledgment that the method remains common and provides the means 1097 to relay the results of that test. 1099 4. Adding the Header Field to a Message 1101 This specification makes no attempt to evaluate the relative 1102 strengths of various message authentication methods that may become 1103 available. The methods listed are an order-independent set; their 1104 sequence does not indicate relative strength or importance of one 1105 method over another. Instead, the MUA or downstream filter consuming 1106 this header field is to interpret the result of each method based on 1107 its own knowledge of what that method evaluates. 1109 Each "method" MUST refer to an authentication method declared in the 1110 IANA registry or an extension method as described in Section 2.7.6, 1111 and each "result" MUST refer to a result code declared in the IANA 1112 registry or an extension result code as defined in Section 2.7.7. 1113 See Section 6 for further information about the registered methods 1114 and result codes. 1116 An MTA compliant with this specification adds this header field 1117 (after performing one or more message authentication tests) to 1118 indicate which MTA or ADMD performed the test, which test was 1119 applied, and what the result was. If an MTA applies more than one 1120 such test, it adds this header field either once per test or once 1121 indicating all of the results. An MTA MUST NOT add a result to an 1122 existing header field. 1124 An MTA MAY add this header field containing only the authentication 1125 identifier portion and the "none" token (see Section 2.2) to indicate 1126 explicitly that no message authentication schemes were applied prior 1127 to delivery of this message. 1129 An MTA adding this header field has to take steps to identify it as 1130 legitimate to the MUAs or downstream filters that will ultimately 1131 consume its content. One process to do so is described in Section 5. 1132 Further measures may be necessary in some environments. Some 1133 possible solutions are enumerated in Section 7.1. This document does 1134 not mandate any specific solution to this issue as each environment 1135 has its own facilities and limitations. 1137 Most known message authentication methods focus on a particular 1138 identifier to evaluate. SPF differs in that it can yield a result 1139 based on more than one identifier; specifically, SPF can evaluate the 1140 RFC5321.HELO parameter or the RFC5321.MailFrom parameter. When 1141 generating this field to report those results, only the parameter 1142 that yielded the result is included. 1144 For MTAs that add this header field, adding header fields in order 1145 (at the top), per Section 3.6 of [MAIL], is particularly important. 1146 Moreover, this header field SHOULD be inserted above any other trace 1147 header fields such MTAs might prepend. This placement allows easy 1148 detection of header fields that can be trusted. 1150 End users making direct use of this header field might inadvertently 1151 trust information that has not been properly vetted. If, for 1152 example, a basic SPF result were to be relayed that claims an 1153 authenticated addr-spec, the local-part of that addr-spec has 1154 actually not been authenticated. Thus, an MTA adding this header 1155 field SHOULD NOT include any data that has not been authenticated by 1156 the method(s) being applied. Moreover, MUAs SHOULD NOT render to 1157 users such information if it is presented by a method known not to 1158 authenticate it. 1160 4.1. Header Field Position and Interpretation 1162 In order to ensure non-ambiguous results and avoid the impact of 1163 false header fields, MUAs and downstream filters SHOULD NOT interpret 1164 this header field unless specifically configured to do so by the user 1165 or administrator. That is, this interpretation should not be "on by 1166 default". Naturally then, users or administrators ought not activate 1167 such a feature unless (1) they are certain the header field will be 1168 validly added by an agent within the ADMD that accepts the mail that 1169 is ultimately read by the MUA, and (2) instances of the header field 1170 that appear to originate within the ADMD but are actually added by 1171 foreign MTAs will be removed before delivery. 1173 Furthermore, MUAs and downstream filters SHOULD NOT interpret this 1174 header field unless the authentication service identifier of the 1175 header field is used within the ADMD as configured by the user or 1176 administrator. 1178 MUAs and downstream filters MUST ignore any result reported using a 1179 "result" not specified in the IANA "Result Code" registry or a 1180 "ptype" not listed in the "Email Authentication Property Types" 1181 registry for such values as defined in Section 6. Moreover, such 1182 agents MUST ignore a result indicated for any "method" they do not 1183 specifically support. The exception to this is experimental methods 1184 as discussed in Section 2.7.6. 1186 An MUA SHOULD NOT reveal these results to end users, absent careful 1187 human factors design considerations and testing, for the presentation 1188 of trust-related materials. For example, an attacker could register 1189 examp1e.com (note the digit "1" (one)) and send signed mail to 1190 intended victims; a verifier would detect that the signature was 1191 valid and report a "pass" even though it's clear the DNS domain name 1192 was intended to mislead. See Section 7.2 for further discussion. 1194 As stated in Section 2.1, this header field MUST be treated as though 1195 it were a trace header field as defined in Section 3.6.7 of [MAIL] 1196 and hence MUST NOT be reordered and MUST be prepended to the message, 1197 so that there is generally some indication upon delivery of where in 1198 the chain of handling MTAs the message authentication was done. 1200 Note that there are a few message handlers that are only capable of 1201 appending new header fields to a message. Strictly speaking, these 1202 handlers are not compliant with this specification. They can still 1203 add the header field to carry authentication details, but any signal 1204 about where in the handling chain the work was done may be lost. 1205 Consumers SHOULD be designed such that this can be tolerated, 1206 especially from a producer known to have this limitation. 1208 MUAs SHOULD ignore instances of this header field discovered within 1209 message/rfc822 MIME attachments. They are likely to contain the 1210 results of authentication checks done in the past, possibly long ago, 1211 and have no contemporary value. Due caution to this needs to be 1212 taken when choosing to consume them. 1214 Further discussion of these topics can be found in Section 7 below. 1216 4.2. Local Policy Enforcement 1218 Some sites have a local policy that considers any particular 1219 authentication policy's non-recoverable failure results (typically 1220 "fail" or similar) as justification for rejecting the message. In 1221 such cases, the border MTA SHOULD issue an SMTP rejection response to 1222 the message, rather than adding this header field and allowing the 1223 message to proceed toward delivery. This is more desirable than 1224 allowing the message to reach an internal host's MTA or spam filter, 1225 thus possibly generating a local rejection such as a Delivery Status 1226 Notification (DSN) [DSN] to a forged originator. Such generated 1227 rejections are colloquially known as "backscatter". 1229 The same MAY also be done for local policy decisions overriding the 1230 results of the authentication methods (e.g., the "policy" result 1231 codes described in Section 2.7). 1233 Such rejections at the SMTP protocol level are not possible if local 1234 policy is enforced at the MUA and not the MTA. 1236 5. Removing Existing Header Fields 1238 To mitigate the impact of forged header fields, any MTA conforming to 1239 this specification MUST delete any discovered instance of this header 1240 field that claims, by virtue of its authentication service 1241 identifier, to have been added within its trust boundary but that did 1242 not come directly from another trusted MTA. For example, an MTA for 1243 example.com receiving a message MUST delete or otherwise obscure any 1244 instance of this header field bearing an authentication service 1245 identifier indicating that the header field was added within 1246 example.com prior to adding its own header fields. This could mean 1247 each internal MTA will need to be configured with a list of other 1248 known, trusted MTAs that are thus expected to be using that same 1249 identifier. 1251 For messages that are EAI-formatted messages, this test is done after 1252 converting A-labels into U-labels. 1254 For simplicity and maximum security, a border MTA could remove all 1255 instances of this header field on mail crossing into its trust 1256 boundary. However, this may conflict with the desire to access 1257 authentication results performed by trusted external service 1258 providers. It may also invalidate signed messages whose signatures 1259 cover external instances of this header field. A more robust border 1260 MTA could allow a specific list of authenticating MTAs whose 1261 information is to be admitted, removing the header field originating 1262 from all others. 1264 As stated in Section 1.2, a formal definition of "trust boundary" is 1265 deliberately not made here. It is entirely possible that a border 1266 MTA for example.com will explicitly trust authentication results 1267 asserted by upstream host example.net even though they exist in 1268 completely disjoint administrative boundaries. In that case, the 1269 border MTA MAY elect not to delete those results; moreover, the 1270 upstream host doing some authentication work could apply a signing 1271 technology such as [DKIM] on its own results to assure downstream 1272 hosts of their authenticity. An example of this is provided in 1273 Appendix B. 1275 Similarly, in the case of messages signed using [DKIM] or other 1276 message-signing methods that sign header fields, this removal action 1277 could invalidate one or more signatures on the message if they 1278 covered the header field to be removed. This behavior can be 1279 desirable since there's little value in validating the signature on a 1280 message with forged header fields. However, signing agents MAY 1281 therefore elect to omit these header fields from signing to avoid 1282 this situation. 1284 An MTA SHOULD remove any instance of this header field bearing a 1285 version (express or implied) that it does not support. However, an 1286 MTA MUST remove such a header field if the [SMTP] connection relaying 1287 the message is not from a trusted internal MTA. (As discussed above, 1288 this too can result in invalidation of signatures.) This means the 1289 MTA needs to be able to understand versions of this header field at 1290 least as late as the ones understood by the MUAs or other consumers 1291 within its ADMD. 1293 6. IANA Considerations 1295 IANA has registered the defined header field and created registries 1296 as described below. These registry actions were originally defined 1297 by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created 1298 registries were further updated in [RFC7601] to make them more 1299 complete. 1301 Each registry has two related sections below. The first describes 1302 the registry and its update procedures, which are unchanged from 1303 [RFC7601]. The second enumerates changes to entries that are 1304 relevant to this document. 1306 6.1. The Authentication-Results Header Field 1308 The Authentication-Results header field was added to the IANA 1309 "Permanent Message Header Field Names" registry, per the procedure 1310 found in [IANA-HEADERS]. That entry will be updated to reference 1311 this document. The following is the registration template: 1313 Header field name: Authentication-Results 1314 Applicable protocol: mail ([MAIL]) 1315 Status: Standard 1316 Author/Change controller: IETF 1317 Specification document(s): [this document] 1318 Related information: none 1320 6.2. "Email Authentication Methods" Registry Description 1322 Names of message authentication methods supported by this 1323 specification have been registered with IANA, with the exception of 1324 experimental names as described in Section 2.7.6. Along with each 1325 method is recorded the properties that accompany the method's result. 1327 The "Email Authentication Parameters" group, and within it the "Email 1328 Authentication Methods" registry, were created by [RFC5451] for this 1329 purpose. [RFC6577] added a "status" field for each entry. [RFC7001] 1330 amended the rules governing that registry and also added a "version" 1331 field to the registry. 1333 The reference for that registry will be updated to reference this 1334 document. 1336 New entries are assigned only for values that have received Expert 1337 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1338 appointed by the IESG. The designated expert has discretion to 1339 request that a publication be referenced if a clear, concise 1340 definition of the authentication method cannot be provided such that 1341 interoperability is assured. Registrations should otherwise be 1342 permitted. The designated expert can also handle requests to mark 1343 any current registration as "deprecated". 1345 No two entries can have the same combination of method, ptype, and 1346 property. 1348 An entry in this registry contains the following: 1350 Method: the name of the method. 1352 Definition: a reference to the document that created this entry, if 1353 any (see below). 1355 ptype: a "ptype" value appropriate for use with that method. 1357 property: a "property" value matching that "ptype" also appropriate 1358 for use with that method. 1360 Value: a brief description of the value to be supplied with that 1361 method/ptype/property tuple. 1363 Status: the status of this entry, which is either: 1365 active: The entry is in current use. 1367 deprecated: The entry is no longer in current use. 1369 Version: a version number associated with the method (preferably 1370 starting at "1"). 1372 The "Definition" field will typically refer to a permanent document, 1373 or at least some descriptive text, where additional information about 1374 the entry being added can be found. This might in turn reference the 1375 document where the method is defined so that all of the semantics 1376 around creating or interpreting an Authentication-Results header 1377 field using this method, ptype, and property can be understood. 1379 6.3. "Email Authentication Methods" Registry Update 1381 The following entries in this registry are to be updated to replace 1382 [RFC7601] with this document: 1384 +------------+--------+----------------------------------+ 1385 | Method | ptype | Property | 1386 +------------+--------+----------------------------------+ 1387 | auth | smtp | auth | 1388 +------------+--------+----------------------------------+ 1389 | auth | smtp | mailfrom | 1390 +------------+--------+----------------------------------+ 1391 | dkim | header | d | 1392 +------------+--------+----------------------------------+ 1393 | dkim | header | i | 1394 +------------+--------+----------------------------------+ 1395 | iprev | policy | iprev | 1396 +------------+--------+----------------------------------+ 1397 | spf | smtp | mailfrom | 1398 +------------+--------+----------------------------------+ 1399 | spf | smtp | helo | 1400 +------------+--------+----------------------------------+ 1402 Notably, the DomainKeys and Sender ID entries are not updated to 1403 refer to this revised specification, as they are considered obsolete. 1404 IANA is accordingly asked to change the Status of the "sender-id" 1405 entry in this table to "deprecated". 1407 Finally, two new entries are added to this registry, as follows: 1409 6.3.1. 'header.a' for DKIM 1410 Method: dkim 1412 Definition: [this document] 1414 ptype: header 1416 property: a 1418 Description: value of signature "a" tag 1420 Status: active 1422 Version: 1 1424 6.3.2. 'header.s' for DKIM 1426 Method: dkim 1428 Definition: [this document] 1430 ptype: header 1432 property: s 1434 Description: value of signature "s" tag 1436 Status: active 1438 Version: 1 1440 6.4. "Email Authentication Property Types" Registry Description 1442 [RFC7410] created the "Email Authentication Property Types" registry. 1444 Entries in this registry are subject to the Expert Review rules as 1445 described in [IANA-CONSIDERATIONS]. Each entry in the registry 1446 requires the following values: 1448 ptype: The name of the ptype being registered, which must fit within 1449 the ABNF described in Section 2.2. 1451 Definition: An optional reference to a defining specification. 1453 Description: A brief description of what sort of information this 1454 "ptype" is meant to cover. 1456 For new entries, the Designated Expert needs to assure that the 1457 description provided for the new entry adequately describes the 1458 intended use. An example would be helpful to include in the entry's 1459 defining document, if any, although entries in the "Email 1460 Authentication Methods" registry or the "Email Authentication Result 1461 Names" registry might also serve as examples of intended use. 1463 As this is a complete restatement of the definition and rules for 1464 this registry, IANA will update this registry to show Section 2.3 of 1465 this document as the current definitions for the "body", "header", 1466 "policy", and "smtp" entries of that registry. References to other 1467 documents will be removed. 1469 6.5. "Email Authentication Property Types" Registry Update 1471 All current entries in this registry are to be updated to replace 1472 [RFC7601] with this document. 1474 6.6. "Email Authentication Result Names" Registry Description 1476 Names of message authentication result codes supported by this 1477 specification must be registered with IANA, with the exception of 1478 experimental codes as described in Section 2.7.7. 1480 New entries are assigned only for values that have received Expert 1481 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1482 appointed by the IESG. The designated expert has discretion to 1483 request that a publication be referenced if a clear, concise 1484 definition of the authentication result cannot be provided such that 1485 interoperability is assured. Registrations should otherwise be 1486 permitted. The designated expert can also handle requests to mark 1487 any current registration as "deprecated". 1489 No two entries can have the same combination of method and code. 1491 An entry in this registry contains the following: 1493 Auth Method: an authentication method for which results are being 1494 returned using the header field defined in this document. 1496 Code: a result code that can be returned for this authentication 1497 method. 1499 Specification: either free form text explaining the meaning of this 1500 method-code combination, or a reference to such a definition. 1502 Status: the status of this entry, which is either: 1504 active: The entry is in current use. 1506 deprecated: The entry is no longer in current use. 1508 6.7. "Email Authentication Result Names" Registry Update 1510 The following entries in this registry are to be updated to reflect 1511 this new Specification as follows: 1513 o All "auth" method result codes ("fail", "none", "pass", 1514 "permerror", "temperror") are now specified in Section 2.7.4 of 1515 this document. 1517 o All "dkim" method result names ("fail", "neutral", "none", "pass", 1518 "permerror", "policy", "temperror") are now specified in 1519 Section 2.7.1 of this document. 1521 o All "iprev" method result names ("fail", "pass", "permerror", 1522 "temperror") are now specified in Section 2.7.3 of this document. 1524 o The "spf" method result names "fail", "neutral", "none", "pass", 1525 "permerror", "policy", "softfail", and "temperror" are now 1526 specified in Section 2.7.2 of this document. The registration for 1527 result name "hardfail" is not updated. 1529 The following entries in this registry are to be updated with a new 1530 Status of "deprecated", with no change to the Specification as they 1531 reference historic protocols: 1533 o All "domainkeys" method result names ("fail", "neutral", "none", 1534 "pass", "permerror", "policy", and "temperror"). 1536 o All "sender-id" method result names ("fail", "neutral", "none", 1537 "pass", "permerror", "policy", "softfail", and "temperror"). 1539 6.8. SMTP Enhanced Status Codes 1541 The entry for X.7.25 in the "Enumerated Status Codes" sub-registry of 1542 the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes 1543 Registry" is to be updated to refer only to Section 3.3 of [AUTH-ESC] 1544 as that is where that registration was done. 1546 7. Security Considerations 1548 The following security considerations apply when adding or processing 1549 the Authentication-Results header field: 1551 7.1. Forged Header Fields 1553 An MTA not applying the filtering discussed in Section 5 exposes MUAs 1554 to false conclusions based on forged header fields. A malicious user 1555 or agent could forge a header field using the DNS domain of a 1556 receiving ADMD as the authserv-id token in the value of the header 1557 field and, with the rest of the value, claim that the message was 1558 properly authenticated. The non-conformant MTA would fail to strip 1559 the forged header field, and the MUA could inappropriately trust it. 1561 For this reason, it is best not to have processing of the 1562 Authentication-Results header field enabled by default; instead, it 1563 should be ignored, at least for the purposes of enacting filtering 1564 decisions, unless specifically enabled by the user or administrator 1565 after verifying that the border MTA is compliant. It is acceptable 1566 to have an MUA aware of this specification but have an explicit list 1567 of hostnames whose Authentication-Results header fields are 1568 trustworthy; however, this list should initially be empty. 1570 Proposed alternative solutions to this problem were made some time 1571 ago and are listed below. To date, they have not been developed due 1572 to lack of demand but are documented here should the information be 1573 useful at some point in the future: 1575 1. Possibly the simplest is a digital signature protecting the 1576 header field, such as using [DKIM], that can be verified by an 1577 MUA by using a posted public key. Although one of the main 1578 purposes of this document is to relieve the burden of doing 1579 message authentication work at the MUA, this only requires that 1580 the MUA learn a single authentication scheme even if a number of 1581 them are in use at the border MTA. Note that [DKIM] requires 1582 that the From header field be signed, although in this 1583 application, the signing agent (a trusted MTA) likely cannot 1584 authenticate that value, so the fact that it is signed should be 1585 ignored. Where the authserv-id is the ADMD's domain name, the 1586 authserv-id matching this valid internal signature's "d=" DKIM 1587 value is sufficient. 1589 2. Another would be a means to interrogate the MTA that added the 1590 header field to see if it is actually providing any message 1591 authentication services and saw the message in question, but this 1592 isn't especially palatable given the work required to craft and 1593 implement such a scheme. 1595 3. Yet another might be a method to interrogate the internal MTAs 1596 that apparently handled the message (based on Received header 1597 fields) to determine whether any of them conform to Section 5 of 1598 this memo. This, too, has potentially high barriers to entry. 1600 4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to 1601 allow an MUA or filtering agent to acquire the authserv-id in use 1602 within an ADMD, thus allowing it to identify which 1603 Authentication-Results header fields it can trust. 1605 5. On the presumption that internal MTAs are fully compliant with 1606 Section 3.6 of [MAIL] and the compliant internal MTAs are using 1607 their own hostnames or the ADMD's DNS domain name as the 1608 authserv-id token, the header field proposed here should always 1609 appear above a Received header added by a trusted MTA. This can 1610 be used as a test for header field validity. 1612 Support for some of these is being considered for future work. 1614 In any case, a mechanism needs to exist for an MUA or filter to 1615 verify that the host that appears to have added the header field (a) 1616 actually did so and (b) is legitimately adding that header field for 1617 this delivery. Given the variety of messaging environments deployed 1618 today, consensus appears to be that specifying a particular mechanism 1619 for doing so is not appropriate for this document. 1621 Mitigation of the forged header field attack can also be accomplished 1622 by moving the authentication results data into metadata associated 1623 with the message. In particular, an [SMTP] extension could be 1624 established to communicate authentication results from the border MTA 1625 to intermediate and delivery MTAs; the latter of these could arrange 1626 to store the authentication results as metadata retrieved and 1627 rendered along with the message by an [IMAP] client aware of a 1628 similar extension in that protocol. The delivery MTA would be told 1629 to trust data via this extension only from MTAs it trusts, and border 1630 MTAs would not accept data via this extension from any source. There 1631 is no vector in such an arrangement for forgery of authentication 1632 data by an outside agent. 1634 7.2. Misleading Results 1636 Until some form of service for querying the reputation of a sending 1637 agent is widely deployed, the existence of this header field 1638 indicating a "pass" does not render the message trustworthy. It is 1639 possible for an arriving piece of spam or other undesirable mail to 1640 pass checks by several of the methods enumerated above (e.g., a piece 1641 of spam signed using [DKIM] by the originator of the spam, which 1642 might be a spammer or a compromised system). In particular, this 1643 issue is not resolved by forged header field removal discussed above. 1645 Hence, MUAs and downstream filters must take some care with use of 1646 this header even after possibly malicious headers are scrubbed. 1648 7.3. Header Field Position 1650 Despite the requirements of [MAIL], header fields can sometimes be 1651 reordered en route by intermediate MTAs. The goal of requiring 1652 header field addition only at the top of a message is an 1653 acknowledgment that some MTAs do reorder header fields, but most do 1654 not. Thus, in the general case, there will be some indication of 1655 which MTAs (if any) handled the message after the addition of the 1656 header field defined here. 1658 7.4. Reverse IP Query Denial-of-Service Attacks 1660 Section 4.6.4 of [SPF] describes a DNS-based denial-of-service attack 1661 for verifiers that attempt DNS-based identity verification of 1662 arriving client connections. A verifier wishing to do this check and 1663 report this information needs to take care not to go to unbounded 1664 lengths to resolve "A" and "PTR" queries. MUAs or other filters 1665 making use of an "iprev" result specified by this document need to be 1666 aware of the algorithm used by the verifier reporting the result and, 1667 especially, its limitations. 1669 7.5. Mitigation of Backscatter 1671 Failing to follow the instructions of Section 4.2 can result in a 1672 denial-of-service attack caused by the generation of [DSN] messages 1673 (or equivalent) to addresses that did not send the messages being 1674 rejected. 1676 7.6. Internal MTA Lists 1678 Section 5 describes a procedure for scrubbing header fields that may 1679 contain forged authentication results about a message. A compliant 1680 installation will have to include, at each MTA, a list of other MTAs 1681 known to be compliant and trustworthy. Failing to keep this list 1682 current as internal infrastructure changes may expose an ADMD to 1683 attack. 1685 7.7. Attacks against Authentication Methods 1687 If an attack becomes known against an authentication method, clearly 1688 then the agent verifying that method can be fooled into thinking an 1689 inauthentic message is authentic, and thus the value of this header 1690 field can be misleading. It follows that any attack against the 1691 authentication methods supported by this document is also a security 1692 consideration here. 1694 7.8. Intentionally Malformed Header Fields 1696 As with any other header field found in the message, it is possible 1697 for an attacker to add an Authentication-Results header field that is 1698 extraordinarily large or otherwise malformed in an attempt to 1699 discover or exploit weaknesses in header field parsing code. 1700 Implementers must thoroughly verify all such header fields received 1701 from MTAs and be robust against intentionally as well as 1702 unintentionally malformed header fields. 1704 7.9. Compromised Internal Hosts 1706 An internal MUA or MTA that has been compromised could generate mail 1707 with a forged From header field and a forged Authentication-Results 1708 header field that endorses it. Although it is clearly a larger 1709 concern to have compromised internal machines than it is to prove the 1710 value of this header field, this risk can be mitigated by arranging 1711 that internal MTAs will remove this header field if it claims to have 1712 been added by a trusted border MTA (as described above), yet the 1713 [SMTP] connection is not coming from an internal machine known to be 1714 running an authorized MTA. However, in such a configuration, 1715 legitimate MTAs will have to add this header field when legitimate 1716 internal-only messages are generated. This is also covered in 1717 Section 5. 1719 7.10. Encapsulated Instances 1721 MIME messages can contain attachments of type "message/rfc822", which 1722 contain other messages. Such an encapsulated message can also 1723 contain an Authentication-Results header field. Although the 1724 processing of these is outside of the intended scope of this document 1725 (see Section 1.3), some simple guidance to MUA developers is 1726 appropriate here. 1728 Since MTAs are generally unlikely to strip Authentication-Results 1729 header fields during mailbox delivery, normative language exists in 1730 Section 4.1 cautioning MUAs to ignore such instances within MIME 1731 attachments, as might be included when a message is forwarded. 1732 Moreover, when extracting a message digest to separate mail store 1733 messages or other media, such header fields should be removed so that 1734 they will never be interpreted improperly by MUAs that might later 1735 consume them. 1737 There can be cases where these header fields included as part of 1738 encapsulated messages might actually be of value, such as when they 1739 are taken from messages within the same ADMD where they will be 1740 consumed. Caution must be taken that the consumer fully understands 1741 the semantics of what the header field is indicating and the 1742 message's handling history before ascribing any value, positive or 1743 negative, to such data. 1745 7.11. Reverse Mapping 1747 Although Section 3 of this memo includes explicit support for the 1748 "iprev" method, its value as an authentication mechanism is limited. 1749 Implementers of both this specification and agents that use the data 1750 it relays are encouraged to become familiar with the issues raised by 1751 [DNSOP-REVERSE] when deciding whether or not to include support for 1752 "iprev". 1754 8. References 1756 8.1. Normative References 1758 [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1759 Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ 1760 RFC5234, January 2008, 1761 . 1763 [IANA-HEADERS] 1764 Klyne, G., Nottingham, M., and J. Mogul, "Registration 1765 Procedures for Message Header Fields", BCP 90, RFC 3864, 1766 DOI 10.17487/RFC3864, September 2004, 1767 . 1769 [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, 1770 DOI 10.17487/RFC5322, October 2008, 1771 . 1773 [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1774 Extensions (MIME) Part One: Format of Internet Message 1775 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 1776 . 1778 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1779 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 1780 RFC2119, March 1997, 1781 . 1783 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 1784 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, 1785 February 2012, . 1787 [RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized 1788 Email", RFC 6531, DOI 10.17487/RFC6531, February 2012, 1789 . 1791 [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized 1792 Email Headers", RFC 6532, DOI 10.17487/RFC6532, 1793 February 2012, . 1795 [RFC7601] Kucherawy, M., "Message Header Field for Indicating 1796 Message Authentication Status", RFC 7601, DOI 10.17487/ 1797 RFC7601, August 2015, 1798 . 1800 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1801 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1802 May 2017, . 1804 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 1805 DOI 10.17487/RFC5321, October 2008, 1806 . 1808 8.2. Informative References 1810 [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, 1811 "DomainKeys Identified Mail (DKIM) Author Domain Signing 1812 Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, 1813 August 2009, . 1815 [AR-VBR] Kucherawy, M., "Authentication-Results Registration for 1816 Vouch by Reference Results", RFC 6212, DOI 10.17487/ 1817 RFC6212, April 2011, 1818 . 1820 [ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM) 1821 Authorized Third-Party Signatures", RFC 6541, 1822 DOI 10.17487/RFC6541, February 2012, 1823 . 1825 [AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service 1826 Extension for Authentication", RFC 4954, DOI 10.17487/ 1827 RFC4954, July 2007, 1828 . 1830 [AUTH-ESC] 1831 Kucherawy, M., "Email Authentication Status Codes", 1832 RFC 7372, DOI 10.17487/RFC7372, September 2014, 1833 . 1835 [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 1836 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 1837 RFC 6376, DOI 10.17487/RFC6376, September 2011, 1838 . 1840 [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based 1841 Message Authentication, Reporting, and Conformance 1842 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, 1843 . 1845 [DNS] Mockapetris, P., "Domain names - implementation and 1846 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1847 November 1987, . 1849 [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 1850 "DNS Extensions to Support IP Version 6", RFC 3596, 1851 DOI 10.17487/RFC3596, October 2003, 1852 . 1854 [DNSOP-REVERSE] 1855 Senie, D. and A. Sullivan, "Considerations for the use of 1856 DNS Reverse Mapping", Work in Progress, draft-ietf-dnsop- 1857 reverse-mapping-considerations-06, March 2008. 1859 [DOMAINKEYS] 1860 Delany, M., "Domain-Based Email Authentication Using 1861 Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, 1862 DOI 10.17487/RFC4870, May 2007, 1863 . 1865 [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format 1866 for Delivery Status Notifications", RFC 3464, 1867 DOI 10.17487/RFC3464, January 2003, 1868 . 1870 [EMAIL-ARCH] 1871 Crocker, D., "Internet Mail Architecture", RFC 5598, 1872 DOI 10.17487/RFC5598, July 2009, 1873 . 1875 [IANA-CONSIDERATIONS] 1876 Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1877 Writing an IANA Considerations Section in RFCs", BCP 26, 1878 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1879 . 1881 [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 1882 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 1883 . 1885 [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 1886 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 1887 . 1889 [RFC5451] Kucherawy, M., "Message Header Field for Indicating 1890 Message Authentication Status", RFC 5451, DOI 10.17487/ 1891 RFC5451, April 2009, 1892 . 1894 [RFC6008] Kucherawy, M., "Authentication-Results Registration for 1895 Differentiating among Cryptographic Results", RFC 6008, 1896 DOI 10.17487/RFC6008, September 2010, 1897 . 1899 [RFC6577] Kucherawy, M., "Authentication-Results Registration Update 1900 for Sender Policy Framework (SPF) Results", RFC 6577, 1901 DOI 10.17487/RFC6577, March 2012, 1902 . 1904 [RFC7001] Kucherawy, M., "Message Header Field for Indicating 1905 Message Authentication Status", RFC 7001, DOI 10.17487/ 1906 RFC7001, September 2013, 1907 . 1909 [RFC7410] Kucherawy, M., "A Property Types Registry for the 1910 Authentication-Results Header Field", RFC 7410, 1911 DOI 10.17487/RFC7410, December 2014, 1912 . 1914 [RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage 1915 Update to DomainKeys Identified Mail (DKIM)", RFC 8301, 1916 DOI 10.17487/RFC8301, January 2018, 1917 . 1919 [RRVS] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid- 1920 Since Header Field and SMTP Service Extension", RFC 7293, 1921 DOI 10.17487/RFC7293, July 2014, 1922 . 1924 [SECURITY] 1925 Rescorla, E. and B. Korver, "Guidelines for Writing RFC 1926 Text on Security Considerations", BCP 72, RFC 3552, 1927 DOI 10.17487/RFC3552, July 2003, 1928 . 1930 [SENDERID] 1931 Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", 1932 RFC 4406, DOI 10.17487/RFC4406, April 2006, 1933 . 1935 [SMIME-REG] 1936 Melnikov, A., "Authentication-Results Registration for 1937 S/MIME Signature Verification", RFC 7281, DOI 10.17487/ 1938 RFC7281, June 2014, 1939 . 1941 [SPF] Kitterman, S., "Sender Policy Framework (SPF) for 1942 Authorizing Use of Domains in Email, Version 1", RFC 7208, 1943 DOI 10.17487/RFC7208, April 2014, 1944 . 1946 [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By 1947 Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009, 1948 . 1950 Appendix A. Legacy MUAs 1952 Implementers of this protocol should be aware that many MUAs are 1953 unlikely to be retrofitted to support the new header field and its 1954 semantics. In the interests of convenience and quicker adoption, a 1955 delivery MTA might want to consider adding things that are processed 1956 by existing MUAs in addition to the Authentication-Results header 1957 field. One suggestion is to include a Priority header field, on 1958 messages that don't already have such a header field, containing a 1959 value that reflects the strength of the authentication that was 1960 accomplished, e.g., "low" for weak or no authentication, "normal" or 1961 "high" for good or strong authentication. 1963 Some modern MUAs can already filter based on the content of this 1964 header field. However, there is keen interest in having MUAs make 1965 some kind of graphical representation of this header field's meaning 1966 to end users. Until this capability is added (i.e., while this 1967 specification and its successors continue to be adopted), other 1968 interim means of conveying authentication results may be necessary. 1970 Appendix B. Authentication-Results Examples 1972 This section presents some examples of the use of this header field 1973 to indicate authentication results. 1975 B.1. Trivial Case; Header Field Not Present 1977 The trivial case: 1979 Received: from mail-router.example.com 1980 (mail-router.example.com [192.0.2.1]) 1981 by server.example.org (8.11.6/8.11.6) 1982 with ESMTP id g1G0r1kA003489; 1983 Fri, Feb 15 2002 17:19:07 -0800 1984 From: sender@example.com 1985 Date: Fri, Feb 15 2002 16:54:30 -0800 1986 To: receiver@example.org 1987 Message-Id: <12345.abc@example.com> 1988 Subject: here's a sample 1990 Hello! Goodbye! 1992 Example 1: Trivial Case 1994 The Authentication-Results header field is completely absent. The 1995 MUA may make no conclusion about the validity of the message. This 1996 could be the case because the message authentication services were 1997 not available at the time of delivery, or no service is provided, or 1998 the MTA is not in compliance with this specification. 2000 B.2. Nearly Trivial Case; Service Provided, but No Authentication Done 2002 A message that was delivered by an MTA that conforms to this 2003 specification but provides no actual message authentication service: 2005 Authentication-Results: example.org 1; none 2006 Received: from mail-router.example.com 2007 (mail-router.example.com [192.0.2.1]) 2008 by server.example.org (8.11.6/8.11.6) 2009 with ESMTP id g1G0r1kA003489; 2010 Fri, Feb 15 2002 17:19:07 -0800 2011 From: sender@example.com 2012 Date: Fri, Feb 15 2002 16:54:30 -0800 2013 To: receiver@example.org 2014 Message-Id: <12345.abc@example.com> 2015 Subject: here's a sample 2017 Hello! Goodbye! 2019 Example 2: Header Present but No Authentication Done 2021 The Authentication-Results header field is present, showing that the 2022 delivering MTA conforms to this specification. It used its DNS 2023 domain name as the authserv-id. The presence of "none" (and the 2024 absence of any method or result tokens) indicates that no message 2025 authentication was done. The version number of the specification to 2026 which the field's content conforms is explicitly provided. 2028 B.3. Service Provided, Authentication Done 2030 A message that was delivered by an MTA that conforms to this 2031 specification and applied some message authentication: 2033 Authentication-Results: example.com; 2034 spf=pass smtp.mailfrom=example.net 2035 Received: from dialup-1-2-3-4.example.net 2036 (dialup-1-2-3-4.example.net [192.0.2.200]) 2037 by mail-router.example.com (8.11.6/8.11.6) 2038 with ESMTP id g1G0r1kA003489; 2039 Fri, Feb 15 2002 17:19:07 -0800 2040 From: sender@example.net 2041 Date: Fri, Feb 15 2002 16:54:30 -0800 2042 To: receiver@example.com 2043 Message-Id: <12345.abc@example.net> 2044 Subject: here's a sample 2046 Hello! Goodbye! 2048 Example 3: Header Reporting Results 2050 The Authentication-Results header field is present, indicating that 2051 the border MTA conforms to this specification. The authserv-id is 2052 once again the DNS domain name. Furthermore, the message was 2053 authenticated by that MTA via the method specified in [SPF]. Note 2054 that since that method cannot authenticate the local-part, it has 2055 been omitted from the result's value. The MUA could extract and 2056 relay this extra information if desired. 2058 B.4. Service Provided, Several Authentications Done, Single MTA 2060 A message that was relayed inbound via a single MTA that conforms to 2061 this specification and applied three different message authentication 2062 checks: 2064 Authentication-Results: example.com; 2065 auth=pass (cram-md5) smtp.auth=sender@example.net; 2066 spf=pass smtp.mailfrom=example.net 2067 Authentication-Results: example.com; iprev=pass 2068 policy.iprev=192.0.2.200 2069 Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6) 2070 (dialup-1-2-3-4.example.net [192.0.2.200]) 2071 by mail-router.example.com (8.11.6/8.11.6) 2072 with ESMTPA id g1G0r1kA003489; 2073 Fri, Feb 15 2002 17:19:07 -0800 2074 Date: Fri, Feb 15 2002 16:54:30 -0800 2075 To: receiver@example.com 2076 From: sender@example.net 2077 Message-Id: <12345.abc@example.net> 2078 Subject: here's a sample 2080 Hello! Goodbye! 2082 Example 4: Headers Reporting Results from One MTA 2084 The Authentication-Results header field is present, indicating that 2085 the delivering MTA conforms to this specification. Once again, the 2086 receiving DNS domain name is used as the authserv-id. Furthermore, 2087 the sender authenticated herself/himself to the MTA via a method 2088 specified in [AUTH], and both SPF and "iprev" checks were done and 2089 passed. The MUA could extract and relay this extra information if 2090 desired. 2092 Two Authentication-Results header fields are not required since the 2093 same host did all of the checking. The authenticating agent could 2094 have consolidated all the results into one header field. 2096 This example illustrates a scenario in which a remote user on a 2097 dial-up connection (example.net) sends mail to a border MTA 2098 (example.com) using SMTP authentication to prove identity. The 2099 dial-up provider has been explicitly authorized to relay mail as 2100 example.net, producing a "pass" result from the SPF check. 2102 B.5. Service Provided, Several Authentications Done, Different MTAs 2104 A message that was relayed inbound by two different MTAs that conform 2105 to this specification and applied multiple message authentication 2106 checks: 2108 Authentication-Results: example.com; 2109 dkim=pass (good signature) header.d=example.com 2110 Received: from mail-router.example.com 2111 (mail-router.example.com [192.0.2.1]) 2112 by auth-checker.example.com (8.11.6/8.11.6) 2113 with ESMTP id i7PK0sH7021929; 2114 Fri, Feb 15 2002 17:19:22 -0800 2115 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com; 2116 t=1188964191; c=simple/simple; h=From:Date:To:Subject: 2117 Message-Id:Authentication-Results; 2118 bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70; 2119 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 2120 Authentication-Results: example.com; 2121 auth=pass (cram-md5) smtp.auth=sender@example.com; 2122 spf=fail smtp.mailfrom=example.com 2123 Received: from dialup-1-2-3-4.example.net 2124 (dialup-1-2-3-4.example.net [192.0.2.200]) 2125 by mail-router.example.com (8.11.6/8.11.6) 2126 with ESMTPA id g1G0r1kA003489; 2127 Fri, Feb 15 2002 17:19:07 -0800 2128 From: sender@example.com 2129 Date: Fri, Feb 15 2002 16:54:30 -0800 2130 To: receiver@example.com 2131 Message-Id: <12345.abc@example.com> 2132 Subject: here's a sample 2134 Hello! Goodbye! 2136 Example 5: Headers Reporting Results from Multiple MTAs 2138 The Authentication-Results header field is present, indicating 2139 conformance to this specification. Once again, the authserv-id used 2140 is the recipient's DNS domain name. The header field is present 2141 twice because two different MTAs in the chain of delivery did 2142 authentication tests. The first MTA, mail-router.example.com, 2143 reports that SMTP AUTH and SPF were both used and that the former 2144 passed while the latter failed. In the SMTP AUTH case, additional 2145 information is provided in the comment field, which the MUA can 2146 choose to render if desired. 2148 The second MTA, auth-checker.example.com, reports that it did a DKIM 2149 test (which passed). Again, additional data about one of the tests 2150 is provided as a comment, which the MUA may choose to render. Also 2151 noteworthy here is the fact that there is a DKIM signature added by 2152 example.com that assured the integrity of the lower Authentication- 2153 Results field. 2155 Since different hosts did the two sets of authentication checks, the 2156 header fields cannot be consolidated in this example. 2158 This example illustrates more typical transmission of a message into 2159 example.com from a user on a dial-up connection example.net. The 2160 user appears to be legitimate as he/she had a valid password allowing 2161 authentication at the border MTA using SMTP AUTH. The SPF test 2162 failed since example.com has not granted example.net's dial-up 2163 network authority to relay mail on its behalf. The DKIM test passed 2164 because the sending user had a private key matching one of 2165 example.com's published public keys and mail-router.example.com used 2166 it to sign the message. 2168 B.6. Service Provided, Multi-tiered Authentication Done 2170 A message that had authentication done at various stages, one of 2171 which was outside the receiving ADMD: 2173 Authentication-Results: example.com; 2174 dkim=pass reason="good signature" 2175 header.i=@mail-router.example.net; 2176 dkim=fail reason="bad signature" 2177 header.i=@newyork.example.com 2178 Received: from mail-router.example.net 2179 (mail-router.example.net [192.0.2.250]) 2180 by chicago.example.com (8.11.6/8.11.6) 2181 for 2182 with ESMTP id i7PK0sH7021929; 2183 Fri, Feb 15 2002 17:19:22 -0800 2184 DKIM-Signature: v=1; a=rsa-sha256; s=furble; 2185 d=mail-router.example.net; t=1188964198; c=relaxed/simple; 2186 h=From:Date:To:Message-Id:Subject:Authentication-Results; 2187 bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=; 2188 b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= 2189 Authentication-Results: example.net; 2190 dkim=pass (good signature) header.i=@newyork.example.com 2191 Received: from smtp.newyork.example.com 2192 (smtp.newyork.example.com [192.0.2.220]) 2193 by mail-router.example.net (8.11.6/8.11.6) 2194 with ESMTP id g1G0r1kA003489; 2195 Fri, Feb 15 2002 17:19:07 -0800 2196 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; 2197 d=newyork.example.com; 2198 t=1188964191; c=simple/simple; 2199 h=From:Date:To:Message-Id:Subject; 2200 bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; 2201 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 2202 From: sender@newyork.example.com 2203 Date: Fri, Feb 15 2002 16:54:30 -0800 2204 To: meetings@example.net 2205 Message-Id: <12345.abc@newyork.example.com> 2206 Subject: here's a sample 2208 Example 6: Headers Reporting Results from Multiple MTAs in Different 2209 ADMDs 2211 In this example, we see multi-tiered authentication with an extended 2212 trust boundary. 2214 The message was sent from someone at example.com's New York office 2215 (newyork.example.com) to a mailing list managed at an intermediary. 2217 The message was signed at the origin using DKIM. 2219 The message was sent to a mailing list service provider called 2220 example.net, which is used by example.com. There, 2221 meetings@example.net is expanded to a long list of recipients, one of 2222 whom is at the Chicago office. In this example, we will assume that 2223 the trust boundary for chicago.example.com includes the mailing list 2224 server at example.net. 2226 The mailing list server there first authenticated the message and 2227 affixed an Authentication-Results header field indicating such using 2228 its DNS domain name for the authserv-id. It then altered the message 2229 by affixing some footer text to the body, including some 2230 administrivia such as unsubscription instructions. Finally, the 2231 mailing list server affixes a second DKIM signature and begins 2232 distribution of the message. 2234 The border MTA for chicago.example.com explicitly trusts results from 2235 mail-router.example.net, so that header field is not removed. It 2236 performs evaluation of both signatures and determines that the first 2237 (most recent) is a "pass" but, because of the aforementioned 2238 modifications, the second is a "fail". However, the first signature 2239 included the Authentication-Results header added at mail- 2240 router.example.net that validated the second signature. Thus, 2241 indirectly, it can be determined that the authentications claimed by 2242 both signatures are indeed valid. 2244 Note that two styles of presenting metadata about the result are in 2245 use here. In one case, the "reason=" clause is present, which is 2246 intended for easy extraction by parsers; in the other case, the CFWS 2247 production of the ABNF is used to include such data as a header field 2248 comment. The latter can be harder for parsers to extract given the 2249 varied supported syntaxes of mail header fields. 2251 B.7. Comment-Heavy Example 2253 The formal syntax permits comments within the content in a number of 2254 places. For the sake of illustration, this example is also legal: 2256 Authentication-Results: foo.example.net (foobar) 1 (baz); 2257 dkim (Because I like it) / 1 (One yay) = (wait for it) fail 2258 policy (A dot can go here) . (like that) expired 2259 (this surprised me) = (as I wasn't expecting it) 1362471462 2261 Example 7: A Very Comment-Heavy but Perfectly Legal Example 2263 Appendix C. Operational Considerations about Message Authentication 2265 This protocol is predicated on the idea that authentication (and 2266 presumably in the future, reputation) work is typically done by 2267 border MTAs rather than MUAs or intermediate MTAs; the latter merely 2268 make use of the results determined by the former. Certainly this is 2269 not mandatory for participation in electronic mail or message 2270 authentication, but this protocol and its deployment to date are 2271 based on that model. The assumption satisfies several common ADMD 2272 requirements: 2274 1. Service operators prefer to resolve the handling of problem 2275 messages as close to the border of the ADMD as possible. This 2276 enables, for example, rejection of messages at the SMTP level 2277 rather than generating a DSN internally. Thus, doing any of the 2278 authentication or reputation work exclusively at the MUA or 2279 intermediate MTA renders this desire unattainable. 2281 2. Border MTAs are more likely to have direct access to external 2282 sources of authentication or reputation information since modern 2283 MUAs inside of an ADMD are more likely to be heavily firewalled. 2284 Thus, some MUAs might not even be able to complete the task of 2285 performing authentication or reputation evaluations without 2286 complex proxy configurations or similar burdens. 2288 3. MUAs rely upon the upstream MTAs within their trust boundaries to 2289 make correct (as much as is possible) evaluations about the 2290 message's envelope, header, and content. Thus, MUAs don't need 2291 to know how to do the work that upstream MTAs do; they only need 2292 the results of that work. 2294 4. Evaluations about the quality of a message, from simple token 2295 matching (e.g., a list of preferred DNS domains) to cryptographic 2296 verification (e.g., public/private key work), do have a cost and 2297 thus need to be minimized. To that end, performing those tests 2298 at the border MTA is far preferred to doing that work at each MUA 2299 that handles a message. If an ADMD's environment adheres to 2300 common messaging protocols, a reputation query or an 2301 authentication check performed by a border MTA would return the 2302 same result as the same query performed by an MUA. By contrast, 2303 in an environment where the MUA does the work, a message arriving 2304 for multiple recipients would thus cause authentication or 2305 reputation evaluation to be done more than once for the same 2306 message (i.e., at each MUA), causing needless amplification of 2307 resource use and creating a possible denial-of-service attack 2308 vector. 2310 5. Minimizing change is good. As new authentication and reputation 2311 methods emerge, the list of methods supported by this header 2312 field would presumably be extended. If MUAs simply consume the 2313 contents of this header field rather than actually attempt to do 2314 authentication and/or reputation work, then MUAs only need to 2315 learn to parse this header field once; emergence of new methods 2316 requires only a configuration change at the MUAs and software 2317 changes at the MTAs (which are presumably fewer in number). When 2318 choosing to implement these functions in MTAs vs. MUAs, the 2319 issues of individual flexibility, infrastructure inertia, and 2320 scale of effort must be considered. It is typically easier to 2321 change a single MUA than an MTA because the modification affects 2322 fewer users and can be pursued with less care. However, changing 2323 many MUAs is more effort than changing a smaller number of MTAs. 2325 6. For decisions affecting message delivery and display, assessment 2326 based on authentication and reputation is best performed close to 2327 the time of message transit, as a message makes its journey 2328 toward a user's inbox, not afterwards. DKIM keys and IP address 2329 reputations, etc., can change over time or even become invalid, 2330 and users can take a long time to read a message once delivered. 2331 The value of this work thus degrades, perhaps quickly, once the 2332 delivery process has completed. This seriously diminishes the 2333 value of this work when done elsewhere than at MTAs. 2335 Many operational choices are possible within an ADMD, including the 2336 venue for performing authentication and/or reputation assessment. 2337 The current specification does not dictate any of those choices. 2338 Rather, it facilitates those cases in which information produced by 2339 one stage of analysis needs to be transported with the message to the 2340 next stage. 2342 Appendix D. Changes since RFC7601 2344 o Added IANA registration for DKIM "a" and "s" properties. 2346 o Include EAI guidance. 2348 o Adjust some ABNF tokes and names for easier inclusion by other 2349 documents. 2351 o Made minor editorial adjustments. 2353 o Deprecate entries from RFCs that are now Historic. 2355 Appendix E. Acknowledgments 2357 The author wishes to acknowledge the following individuals for their 2358 review and constructive criticism of this document: Kurt Andersen, 2359 Seth Blank, Tim Draegen, Scott Kitterman, John Levine, and Alessandro 2360 Vesely. 2362 Author's Address 2364 Murray S. Kucherawy 2365 270 Upland Drive 2366 San Francisco, CA 94127 2367 United States 2369 Email: superuser@gmail.com