idnits 2.17.1 draft-kucherawy-dmarc-rfc7601bis-00.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- -- The draft header indicates that this document obsoletes RFC7601, but the abstract doesn't seem to mention this, which it should. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (January 18, 2018) is 2289 days in the past. Is this intentional? -- Found something which looks like a code comment -- if you have code sections in the document, please surround them with '' and '' lines. Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'CFWS' is mentioned on line 501, but not defined == Missing Reference: 'RFC7208' is mentioned on line 807, but not defined ** Obsolete normative reference: RFC 5451 (Obsoleted by RFC 7001) ** Obsolete normative reference: RFC 6577 (Obsoleted by RFC 7001) ** Obsolete normative reference: RFC 7001 (Obsoleted by RFC 7601) -- Obsolete informational reference (is this intentional?): RFC 4870 (ref. 'DOMAINKEYS') (Obsoleted by RFC 4871) -- Obsolete informational reference (is this intentional?): RFC 5226 (ref. 'IANA-CONSIDERATIONS') (Obsoleted by RFC 8126) -- Obsolete informational reference (is this intentional?): RFC 3501 (ref. 'IMAP') (Obsoleted by RFC 9051) -- Obsolete informational reference (is this intentional?): RFC 7410 (Obsoleted by RFC 7601) Summary: 3 errors (**), 0 flaws (~~), 3 warnings (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DMARC Working Group M. Kucherawy 3 Internet-Draft January 18, 2018 4 Obsoletes: 7601 (if approved) 5 Intended status: Standards Track 6 Expires: July 22, 2018 8 Message Header Field for Indicating Message Authentication Status 9 draft-kucherawy-dmarc-rfc7601bis-00 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 Status of this Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on July 22, 2018. 37 Copyright Notice 39 Copyright (c) 2018 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 55 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 56 1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6 57 1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 58 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 59 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 60 1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 61 1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . . 7 62 1.5.3. Email Architecture . . . . . . . . . . . . . . . . . . 8 63 1.5.4. Other Terms . . . . . . . . . . . . . . . . . . . . . 9 64 1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9 65 2. Definition and Format of the Header Field . . . . . . . . . . 9 66 2.1. General Description . . . . . . . . . . . . . . . . . . . 9 67 2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10 68 2.3. Property Types (ptypes) and Properties . . . . . . . . . . 12 69 2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 13 70 2.5. Authentication Identifier Field . . . . . . . . . . . . . 14 71 2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 15 72 2.7. Defined Methods and Result Values . . . . . . . . . . . . 16 73 2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16 74 2.7.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 17 75 2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 19 76 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 19 77 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 21 78 2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21 79 2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 22 80 3. The "iprev" Authentication Method . . . . . . . . . . . . . . 22 81 4. Adding the Header Field to a Message . . . . . . . . . . . . . 23 82 4.1. Header Field Position and Interpretation . . . . . . . . . 25 83 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 26 84 5. Removing Existing Header Fields . . . . . . . . . . . . . . . 26 85 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 86 6.1. The Authentication-Results Header Field . . . . . . . . . 27 87 6.2. "Email Authentication Methods" Registry Description . . . 28 88 6.3. "Email Authentication Methods" Registry Update . . . . . . 29 89 6.4. "Email Authentication Property Types" Registry . . . . . . 30 90 6.5. "Email Authentication Result Names" Description . . . . . 31 91 6.6. "Email Authentication Result Names" Update . . . . . . . . 32 92 6.7. SMTP Enhanced Status Codes . . . . . . . . . . . . . . . . 33 93 7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 94 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 33 95 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 35 96 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 35 97 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 35 98 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 36 99 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 36 100 7.7. Attacks against Authentication Methods . . . . . . . . . . 36 101 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 36 102 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 36 103 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 37 104 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 37 105 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37 106 8.1. Normative References . . . . . . . . . . . . . . . . . . . 37 107 8.2. Informative References . . . . . . . . . . . . . . . . . . 38 108 Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 41 109 Appendix B. Authentication-Results Examples . . . . . . . . . . . 41 110 B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 42 111 B.2. Nearly Trivial Case; Service Provided, but No 112 Authentication Done . . . . . . . . . . . . . . . . . . . 42 113 B.3. Service Provided, Authentication Done . . . . . . . . . . 43 114 B.4. Service Provided, Several Authentications Done, Single 115 MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 116 B.5. Service Provided, Several Authentications Done, 117 Different MTAs . . . . . . . . . . . . . . . . . . . . . . 45 118 B.6. Service Provided, Multi-tiered Authentication Done . . . . 47 119 B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 48 120 Appendix C. Operational Considerations about Message 121 Authentication . . . . . . . . . . . . . . . . . . . 49 122 Appendix D. Changes since RFC 7001 . . . . . . . . . . . . . . . 50 123 Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 52 124 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 52 126 1. Introduction 128 This document describes a header field called Authentication-Results 129 for electronic mail messages that presents the results of a message 130 authentication effort in a machine-readable format. The intent of 131 the header field is to create a place to collect such data when 132 message authentication mechanisms are in use so that a Mail User 133 Agent (MUA) and downstream filters can make filtering decisions 134 and/or provide a recommendation to the user as to the validity of the 135 message's origin and possibly the safety and integrity of its 136 content. 138 This document revises the original definition found in [RFC5451] 139 based upon various authentication protocols in current use and 140 incorporates errata logged since the publication of the original 141 specification. 143 End users are not expected to be direct consumers of this header 144 field. This header field is intended for consumption by programs 145 that will then use such data or render it in a human-usable form. 147 This document specifies the format of this header field and discusses 148 the implications of its presence or absence. However, it does not 149 discuss how the data contained in the header field ought to be used, 150 such as what filtering decisions are appropriate or how an MUA might 151 render those results, as these are local policy and/or user interface 152 design questions that are not appropriate for this document. 154 At the time of publication of this document, the following are 155 published email authentication methods: 157 o Author Domain Signing Practices ([ADSP]) (Historic) 159 o SMTP Service Extension for Authentication ([AUTH]) 161 o DomainKeys Identified Mail Signatures ([DKIM]) 163 o Domain-based Message Authentication, Reporting and Conformance 164 ([DMARC]) 166 o Sender Policy Framework ([SPF]) 168 o reverse IP address name validation ("iprev", defined in Section 3) 170 o Require-Recipient-Valid-Since Header Field and SMTP Service 171 Extension ([RRVS]) 173 o S/MIME Signature Verification ([SMIME-REG]) 175 o Vouch By Reference ([VBR]) 177 o DomainKeys ([DOMAINKEYS]) (Historic) 179 o Sender ID ([SENDERID]) (Experimental) 181 There exist registries for tokens used within this header field that 182 refer to the specifications listed above. Section 6 describes the 183 registries and their contents and specifies the process by which 184 entries are added or updated. It also updates the existing contents 185 to match the current states of these specifications. 187 This specification is not intended to be restricted to domain-based 188 authentication schemes, but the existing schemes in that family have 189 proven to be a good starting point for implementations. The goal is 190 to give current and future authentication schemes a common framework 191 within which to deliver their results to downstream agents and 192 discourage the creation of unique header fields for each. 194 Although SPF defined a header field called "Received-SPF" and the 195 historic DomainKeys defined one called "DomainKey-Status" for this 196 purpose, those header fields are specific to the conveyance of their 197 respective results only and thus are insufficient to satisfy the 198 requirements enumerated below. In addition, many SPF implementations 199 have adopted the header field specified here at least as an option, 200 and DomainKeys has been obsoleted by DKIM. 202 1.1. Purpose 204 The header field defined in this document is expected to serve 205 several purposes: 207 1. Convey the results of various message authentication checks, 208 which are applied by upstream filters and Mail Transfer Agents 209 (MTAs) and then passed to MUAs and downstream filters within the 210 same "trust domain". Such agents might wish to render those 211 results to end users or to use those data to apply more or less 212 stringent content checks based on authentication results; 214 2. Provide a common location within a message for this data; 216 3. Create an extensible framework for reporting new authentication 217 methods as they emerge. 219 In particular, the mere presence of this header field does not mean 220 its contents are valid. Rather, the header field is reporting 221 assertions made by one or more authentication schemes (supposedly) 222 applied somewhere upstream. For an MUA or downstream filter to treat 223 the assertions as actually valid, there must be an assessment of the 224 trust relationship among such agents, the validating MTA, and the 225 mechanism for conveying the information. 227 1.2. Trust Boundary 229 This document makes several references to the "trust boundary" of an 230 administrative management domain (ADMD). Given the diversity among 231 existing mail environments, a precise definition of this term isn't 232 possible. 234 Simply put, a transfer from the producer of the header field to the 235 consumer must occur within a context that permits the consumer to 236 treat assertions by the producer as being reliable and accurate 237 (trustworthy). How this trust is obtained is outside the scope of 238 this document. It is entirely a local matter. 240 Thus, this document defines a "trust boundary" as the delineation 241 between "external" and "internal" entities. Services that are 242 internal -- within the trust boundary -- are provided by the ADMD's 243 infrastructure for its users. Those that are external are outside of 244 the authority of the ADMD. By this definition, hosts that are within 245 a trust boundary are subject to the ADMD's authority and policies, 246 independent of their physical placement or their physical operation. 247 For example, a host within a trust boundary might actually be 248 operated by a remote service provider and reside physically within 249 its data center. 251 It is possible for a message to be evaluated inside a trust boundary 252 but then depart and re-enter the trust boundary. An example might be 253 a forwarded message such as a message/rfc822 attachment (see 254 Multipurpose Internet Mail Extensions [MIME]) or one that is part of 255 a multipart/digest. The details reported by this field cannot be 256 trusted in that case. Thus, this field found within one of those 257 media types is typically ignored. 259 1.3. Processing Scope 261 The content of this header field is meant to convey to message 262 consumers that authentication work on the message was already done 263 within its trust boundary, and those results are being presented. It 264 is not intended to provide message parameters to consumers so that 265 they can perform authentication protocols on their own. 267 1.4. Requirements 269 This document establishes no new requirements on existing protocols 270 or servers. 272 In particular, this document establishes no requirement on MTAs to 273 reject or filter arriving messages that do not pass authentication 274 checks. The data conveyed by the specified header field's contents 275 are for the information of MUAs and filters and are to be used at 276 their discretion. 278 1.5. Definitions 280 This section defines various terms used throughout this document. 282 1.5.1. Key Words 284 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 285 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 286 document are to be interpreted as described in [KEYWORDS]. 288 1.5.2. Security 290 "Guidelines for Writing RFC Text on Security Considerations" 291 ([SECURITY]) discusses authentication and authorization and the 292 conflation of the two concepts. The use of those terms within the 293 context of recent message security work has given rise to slightly 294 different definitions, and this document reflects those current 295 usages, as follows: 297 o "Authorization" is the establishment of permission to use a 298 resource or represent an identity. In this context, authorization 299 indicates that a message from a particular ADMD arrived via a 300 route the ADMD has explicitly approved. 302 o "Authentication" is the assertion of validity of a piece of data 303 about a message (such as the sender's identity) or the message in 304 its entirety. 306 As examples: SPF and Sender ID are authorization mechanisms in that 307 they express a result that shows whether or not the ADMD that 308 apparently sent the message has explicitly authorized the connecting 309 Simple Mail Transfer Protocol ([SMTP]) client to relay messages on 310 its behalf, but they do not actually validate any other property of 311 the message itself. By contrast, DKIM is agnostic as to the routing 312 of a message but uses cryptographic signatures to authenticate 313 agents, assign (some) responsibility for the message (which implies 314 authorization), and ensure that the listed portions of the message 315 were not modified in transit. Since the signatures are not tied to 316 SMTP connections, they can be added by either the ADMD of origin, 317 intermediate ADMDs (such as a mailing list server), other handling 318 agents, or any combination. 320 Rather than create a separate header field for each class of 321 solution, this proposal groups them both into a single header field. 323 1.5.3. Email Architecture 325 o A "border MTA" is an MTA that acts as a gateway between the 326 general Internet and the users within an organizational boundary. 327 (See also Section 1.2.) 329 o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that 330 actually enacts delivery of a message to a user's inbox or other 331 final delivery. 333 o An "intermediate MTA" is any MTA that is not a delivery MTA and is 334 also not the first MTA to handle the message. 336 The following diagram illustrates the flow of mail among these 337 defined components. See Internet Mail Architecture [EMAIL-ARCH] for 338 further discussion on general email system architecture, which 339 includes detailed descriptions of these components, and Appendix C of 340 this document for discussion about the common aspects of email 341 authentication in current environments. 343 +-----+ +-----+ +------------+ 344 | MUA |-->| MSA |-->| Border MTA | 345 +-----+ +-----+ +------------+ 346 | 347 | 348 V 349 +----------+ 350 | Internet | 351 +----------+ 352 | 353 | 354 V 355 +-----+ +-----+ +------------------+ +------------+ 356 | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA | 357 +-----+ +-----+ +------------------+ +------------+ 359 Generally, it is assumed that the work of applying message 360 authentication schemes takes place at a border MTA or a delivery MTA. 361 This specification is written with that assumption in mind. However, 362 there are some sites at which the entire mail infrastructure consists 363 of a single host. In such cases, such terms as "border MTA" and 364 "delivery MTA" might well apply to the same machine or even the very 365 same agent. It is also possible that some message authentication 366 tests could take place on an intermediate MTA. Although this 367 document doesn't specifically describe such cases, they are not meant 368 to be excluded. 370 1.5.4. Other Terms 372 In this document, the term "producer" refers to any component that 373 adds this header field to messages it is handling, and "consumer" 374 refers to any component that identifies, extracts, and parses the 375 header field to use as part of a handling decision. 377 1.6. Trust Environment 379 This header field permits one or more message validation mechanisms 380 to communicate output to one or more separate assessment mechanisms. 381 These mechanisms operate within a unified trust boundary that defines 382 an Administrative Management Domain (ADMD). An ADMD contains one or 383 more entities that perform validation and generate the header field 384 and one or more that consume it for some type of assessment. The 385 field often contains no integrity or validation mechanism of its own, 386 so its presence must be trusted implicitly. Hence, valid use of the 387 header field requires removing any occurrences of it that are present 388 when the message enters the ADMD. This ensures that later 389 occurrences have been added within the trust boundary of the ADMD. 391 The authserv-id token defined in Section 2.2 can be used to reference 392 an entire ADMD or a specific validation engine within an ADMD. 393 Although the labeling scheme is left as an operational choice, some 394 guidance for selecting a token is provided in later sections of this 395 document. 397 2. Definition and Format of the Header Field 399 This section gives a general overview of the format of the header 400 field being defined and then provides more formal specification. 402 2.1. General Description 404 The header field specified here is called Authentication-Results. It 405 is a Structured Header Field as defined in Internet Message Format 406 ([MAIL]), and thus all of the related definitions in that document 407 apply. 409 This header field is added at the top of the message as it transits 410 MTAs that do authentication checks, so some idea of how far away the 411 checks were done can be inferred. It is therefore considered to be a 412 trace field as defined in [MAIL], and thus all of the related 413 definitions in that document apply. 415 The value of the header field (after removing comments) consists of 416 an authentication identifier, an optional version, and then a series 417 of statements and supporting data. The statements are of the form 418 "method=result" and indicate which authentication method(s) were 419 applied and their respective results. For each such statement, the 420 supporting data can include a "reason" string and one or more 421 "property=value" statements indicating which message properties were 422 evaluated to reach that conclusion. 424 The header field can appear more than once in a single message, more 425 than one result can be represented in a single header field, or a 426 combination of these can be applied. 428 2.2. Formal Definition 430 Formally, the header field is specified as follows using Augmented 431 Backus-Naur Form ([ABNF]): 433 authres-header = "Authentication-Results:" [CFWS] authserv-id 434 [ CFWS authres-version ] 435 ( no-result / 1*resinfo ) [CFWS] CRLF 437 authserv-id = value 438 ; see below for a description of this element 440 authres-version = 1*DIGIT [CFWS] 441 ; indicates which version of this specification is in use; 442 ; this specification is version "1", and the absence of a 443 ; version implies this version of the specification 445 no-result = [CFWS] ";" [CFWS] "none" 446 ; the special case of "none" is used to indicate that no 447 ; message authentication was performed 449 resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ] 450 *( CFWS propspec ) 452 methodspec = [CFWS] method [CFWS] "=" [CFWS] result 453 ; indicates which authentication method was evaluated 454 ; and what its output was 456 reasonspec = "reason" [CFWS] "=" [CFWS] value 457 ; a free-form comment on the reason the given result 458 ; was returned 460 propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue 461 ; an indication of which properties of the message 462 ; were evaluated by the authentication scheme being 463 ; applied to yield the reported result 465 method = Keyword [ [CFWS] "/" [CFWS] method-version ] 466 ; a method indicates which method's result is 467 ; represented by "result", and is one of the methods 468 ; explicitly defined as valid in this document 469 ; or is an extension method as defined below 471 method-version = 1*DIGIT [CFWS] 472 ; indicates which version of the method specification is 473 ; in use, corresponding to the matching entry in the IANA 474 ; "Email Authentication Methods" registry; a value of "1" 475 ; is assumed if this version string is absent 477 result = Keyword 478 ; indicates the results of the attempt to authenticate 479 ; the message; see below for details 481 ptype = Keyword 482 ; indicates whether the property being evaluated was 483 ; a parameter to an [SMTP] command, was a value taken 484 ; from a message header field, was some property of 485 ; the message body, or was some other property evaluated by 486 ; the receiving MTA; expected to be one of the "property 487 ; types" explicitly defined as valid, or an extension 488 ; ptype, as defined below 490 property = special-smtp-verb / Keyword 491 ; indicates more specifically than "ptype" what the 492 ; source of the evaluated property is; the exact meaning 493 ; is specific to the method whose result is being reported 494 ; and is defined more clearly below 496 special-smtp-verb = "mailfrom" / "rcptto" 497 ; special cases of [SMTP] commands that are made up 498 ; of multiple words 500 pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) 501 [CFWS] 502 ; the value extracted from the message property defined 503 ; by the "ptype.property" construction 505 "local-part" is defined in Section 3.4.1 of [MAIL], and "CFWS" is 506 defined in Section 3.2.2 of [MAIL]. 508 "Keyword" is defined in Section 4.1.2 of [SMTP]. 510 The "value" is as defined in Section 5.1 of [MIME]. 512 The "domain-name" is as defined in Section 3.5 of [DKIM]. 514 The "Keyword" used in "result" above is further constrained by the 515 necessity of being enumerated in Section 2.7. 517 See Section 2.5 for a description of the authserv-id element. 519 If the value portion of a "pvalue" construction identifies something 520 intended to be an email identity, then it MUST use the right hand 521 portion of that ABNF definition. 523 The list of commands eligible for use with the "smtp" ptype can be 524 found in Section 4.1 of [SMTP]. 526 The "propspec" may be omitted if, for example, the method was unable 527 to extract any properties to do its evaluation yet has a result to 528 report. 530 Where an SMTP command name is being reported as a "property", the 531 agent generating the header field represents that command by 532 converting it to lowercase and dropping any spaces (e.g., "MAIL FROM" 533 becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.). 535 A "ptype" value of "policy" indicates a policy decision about the 536 message not specific to a property of the message that could be 537 extracted. See Section 2.4 for details. 539 Examples of complete messages using this header field can be found in 540 Appendix B. 542 2.3. Property Types (ptypes) and Properties 544 The "ptype" in the ABNF above indicates the general type of property 545 being described by the result being reported, upon which the reported 546 result was based. Coupled with the "property", which is more 547 specific, they indicate from which particular part of the message the 548 reported data were extracted. 550 Combinations of ptypes and properties are registered and described in 551 the "Email Authentication Methods" registry, coupled with the 552 authentication methods with which they are used. This is further 553 described in Section 6. 555 Legal values of "ptype" are as defined in the IANA "Email 556 Authentication Property Types" registry, created by [RFC7410]. The 557 initial values and what they typically indicate are as follows, based 558 on [RFC7001]: 560 body: Information that was extracted from the body of the message. 561 This might be an arbitrary string of bytes, a hash of a string of 562 bytes, a Uniform Resource Identifier, or some other content of 563 interest. The "property" is an indication of where within the 564 message body the extracted content was found, and can indicate an 565 offset, identify a MIME part, etc. 567 header: Indicates information that was extracted from the header of 568 the message. This might be the value of a header field or some 569 portion of a header field. The "property" gives a more precise 570 indication of the place in the header from which the extraction 571 took place. 573 policy: A local policy mechanism was applied that augments or 574 overrides the result returned by the authentication mechanism. 575 (See Section 2.4.) 577 smtp: Indicates information that was extracted from an SMTP command 578 that was used to relay the message. The "property" indicates 579 which SMTP command included the extracted content as a parameter. 581 Results reported using unknown ptypes MUST NOT be used in making 582 handling decisions. They can be safely ignored by consumers. 584 Entries in the "Email Authentication Methods" registry can define 585 properties that deviate from these definitions when appropriate. 586 Such deviations need to be clear in the registry and/or in the 587 defining document. See Section 2.7.1 for an example. 589 2.4. The "policy" ptype 591 A special ptype value of "policy" is also defined. This ptype is 592 provided to indicate that some local policy mechanism was applied 593 that augments or even replaces (i.e., overrides) the result returned 594 by the authentication mechanism. The property and value in this case 595 identify the local policy that was applied and the result it 596 returned. 598 For example, a DKIM signature is not required to include the Subject 599 header field in the set of fields that are signed. An ADMD receiving 600 such a message might decide that such a signature is unacceptable, 601 even if it passes, because the content of the Subject header field 602 could be altered post-signing without invalidating the signature. 604 Such an ADMD could replace the DKIM "pass" result with a "policy" 605 result and then also include the following in the corresponding 606 Authentication-Result field: 608 ... dkim=fail policy.dkim-rules=unsigned-subject ... 610 In this case, the property is "dkim-rules", indicating some local 611 check by that name took place and that check returned a result of 612 "unsigned-subject". These are arbitrary names selected by (and 613 presumably used within) the ADMD making use of them, so they are not 614 normally registered with IANA or otherwise specified apart from 615 setting syntax restrictions that allow for easy parsing within the 616 rest of the header field. 618 This ptype existed in the original specification for this header 619 field, but without a complete description or example of intended use. 620 As a result, it has not seen any practical use to date that matches 621 its intended purpose. These added details are provided to guide 622 implementers toward proper use. 624 2.5. Authentication Identifier Field 626 Every Authentication-Results header field has an authentication 627 service identifier field (authserv-id above). Specifically, this is 628 any string intended to identify the authentication service within the 629 ADMD that conducted authentication checks on the message. This 630 identifier is intended to be machine-readable and not necessarily 631 meaningful to users. 633 Since agents consuming this field will use this identifier to 634 determine whether its contents are of interest (and are safe to use), 635 the uniqueness of the identifier MUST be guaranteed by the ADMD that 636 generates it and MUST pertain to that ADMD. MUAs or downstream 637 filters SHOULD use this identifier to determine whether or not the 638 data contained in an Authentication-Results header field ought to be 639 used or ignored. 641 For simplicity and scalability, the authentication service identifier 642 SHOULD be a common token used throughout the ADMD. Common practice 643 is to use the DNS domain name used by or within that ADMD, sometimes 644 called the "organizational domain", but this is not strictly 645 necessary. 647 For tracing and debugging purposes, the authentication identifier can 648 instead be the specific hostname of the MTA performing the 649 authentication check whose result is being reported. Moreover, some 650 implementations define a substructure to the identifier; these are 651 outside of the scope of this specification. 653 Note, however, that using a local, relative identifier like a flat 654 hostname, rather than a hierarchical and globally unique ADMD 655 identifier like a DNS domain name, makes configuration more difficult 656 for large sites. The hierarchical identifier permits aggregating 657 related, trusted systems together under a single, parent identifier, 658 which in turn permits assessing the trust relationship with a single 659 reference. The alternative is a flat namespace requiring 660 individually listing each trusted system. Since consumers will use 661 the identifier to determine whether to use the contents of the header 662 field: 664 o Changes to the identifier impose a large, centralized 665 administrative burden. 667 o Ongoing administrative changes require constantly updating this 668 centralized table, making it difficult to ensure that an MUA or 669 downstream filter will have access to accurate information for 670 assessing the usability of the header field's content. In 671 particular, consumers of the header field will need to know not 672 only the current identifier(s) in use but previous ones as well to 673 account for delivery latency or later re-assessment of the header 674 field's contents. 676 Examples of valid authentication identifiers are "example.com", 677 "mail.example.org", "ms1.newyork.example.com", and "example-auth". 679 2.6. Version Tokens 681 The grammar above provides for the optional inclusion of versions on 682 both the header field itself (attached to the authserv-id token) and 683 on each of the methods being reported. The method version refers to 684 the method itself, which is specified in the documents describing 685 those methods, while the authserv-id version refers to this document 686 and thus the syntax of this header field. 688 The purpose of including these is to avoid misinterpretation of the 689 results. That is, if a parser finds a version after an authserv-id 690 that it does not explicitly know, it can immediately discontinue 691 trying to parse since what follows might not be in an expected 692 format. For a method version, the parser SHOULD ignore a method 693 result if the version is not supported in case the semantics of the 694 result have a different meaning than what is expected. For example, 695 if a hypothetical DKIM version 2 yielded a "pass" result for 696 different reasons than version 1 does, a consumer of this field might 697 not want to use the altered semantics. Allowing versions in the 698 syntax is a way to indicate this and let the consumer of the header 699 field decide. 701 2.7. Defined Methods and Result Values 703 Each individual authentication method returns one of a set of 704 specific result values. The subsections below provide references to 705 the documents defining the authentication methods specifically 706 supported by this document, and their corresponding result values. 707 Verifiers SHOULD use these values as described below. New methods 708 not specified in this document, but intended to be supported by the 709 header field defined here, MUST include a similar result table either 710 in their defining documents or in supplementary ones. 712 2.7.1. DKIM and DomainKeys 714 DKIM is represented by the "dkim" method and is defined in [DKIM]. 715 DomainKeys is defined in [DOMAINKEYS] and is represented by the 716 "domainkeys" method. 718 Section 3.8 of [DOMAINKEYS] enumerates some possible results of a 719 DomainKeys evaluation. Those results are not used when generating 720 this header field; rather, the results returned are listed below. 722 A signature is "acceptable to the ADMD" if it passes local policy 723 checks (or there are no specific local policy checks). For example, 724 an ADMD policy might require that the signature(s) on the message be 725 added using the DNS domain present in the From header field of the 726 message, thus making third-party signatures unacceptable even if they 727 verify. 729 Both DKIM and DomainKeys use the same result set, as follows: 731 none: The message was not signed. 733 pass: The message was signed, the signature or signatures were 734 acceptable to the ADMD, and the signature(s) passed verification 735 tests. 737 fail: The message was signed and the signature or signatures were 738 acceptable to the ADMD, but they failed the verification test(s). 740 policy: The message was signed, but some aspect of the signature or 741 signatures was not acceptable to the ADMD. 743 neutral: The message was signed, but the signature or signatures 744 contained syntax errors or were not otherwise able to be 745 processed. This result is also used for other failures not 746 covered elsewhere in this list. 748 temperror: The message could not be verified due to some error that 749 is likely transient in nature, such as a temporary inability to 750 retrieve a public key. A later attempt may produce a final 751 result. 753 permerror: The message could not be verified due to some error that 754 is unrecoverable, such as a required header field being absent. A 755 later attempt is unlikely to produce a final result. 757 DKIM results are reported using a ptype of "header". The property, 758 however, represents one of the tags found in the DKIM-Signature 759 header field rather than a distinct header field. For example, the 760 ptype-property combination "header.d" refers to the content of the 761 "d" (signing domain) tag from within the signature header field, and 762 not a distinct header field called "d". 764 The ability to report different DKIM results for a message with 765 multiple signatures is described in [RFC6008]. 767 [DKIM] advises that if a message fails verification, it is to be 768 treated as an unsigned message. A report of "fail" here permits the 769 receiver of the report to decide how to handle the failure. A report 770 of "neutral" or "none" preempts that choice, ensuring the message 771 will be treated as if it had not been signed. 773 Section 3.1 of [DOMAINKEYS] describes a process by which the sending 774 address of the message is determined. DomainKeys results are thus 775 reported along with the signing domain name, the sending address of 776 the message, and the name of the header field from which the latter 777 was extracted. This means that a DomainKeys result includes a ptype- 778 property combination of "header.d", plus one of "header.from" and 779 "header.sender". The sending address extracted from the header is 780 included with any [MAIL]-style comments removed; moreover, the local- 781 part of the address and the "@" character are removed if it has not 782 been authenticated in some way. 784 2.7.2. SPF and Sender ID 786 SPF and Sender ID use the "spf" and "sender-id" method names, 787 respectively. The result values for SPF are defined in Section 2.6 788 of [SPF], and those definitions are included here by reference: 790 +-----------+--------------------------------+ 791 | Code | Meaning | 792 +-----------+--------------------------------+ 793 | none | [RFC7208], Section 2.6.1 | 794 +-----------+--------------------------------+ 795 | pass | [RFC7208], Section 2.6.3 | 796 +-----------+--------------------------------+ 797 | fail | [RFC7208], Section 2.6.4 | 798 +-----------+--------------------------------+ 799 | softfail | [RFC7208], Section 2.6.5 | 800 +-----------+--------------------------------+ 801 | policy | RFC 7601, Section 2.4 | 802 +-----------+--------------------------------+ 803 | neutral | [RFC7208], Section 2.6.2 | 804 +-----------+--------------------------------+ 805 | temperror | [RFC7208], Section 2.6.6 | 806 +-----------+--------------------------------+ 807 | permerror | [RFC7208], Section 2.6.7 | 808 +-----------+--------------------------------+ 810 These result codes are used in the context of this specification to 811 reflect the result returned by the component conducting SPF 812 evaluation. 814 For SPF, the ptype used is "smtp", and the property is either 815 "mailfrom" or "helo", since those values are the ones SPF can 816 evaluate. (If the SMTP client issued the EHLO command instead of 817 HELO, the property used is "helo".) 819 The "sender-id" method is described in [SENDERID]. For this method, 820 the ptype used is "header" and the property will be the name of the 821 header field from which the Purported Responsible Address (see [PRA]) 822 was extracted -- namely, one of "Resent-Sender", "Resent-From", 823 "Sender", or "From". 825 The results for Sender ID are listed and described in Section 4.2 of 826 [SENDERID], but for the purposes of this specification, the SPF 827 definitions enumerated above are used instead. Also, [SENDERID] 828 specifies result codes that use mixed case, but they are typically 829 used all lowercase in this context. 831 For both methods, an additional result of "policy" is defined, which 832 means the client was authorized to inject or relay mail on behalf of 833 the sender's DNS domain according to the authentication method's 834 algorithm, but local policy dictates that the result is unacceptable. 835 For example, "policy" might be used if SPF returns a "pass" result, 836 but a local policy check matches the sending DNS domain to one found 837 in an explicit list of unacceptable DNS domains (e.g., spammers). 839 If the retrieved sender policies used to evaluate SPF and Sender ID 840 do not contain explicit provisions for authenticating the local-part 841 (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported 842 along with results for these mechanisms SHOULD NOT include the local- 843 part or the following "@" character. 845 2.7.3. "iprev" 847 The result values used by the "iprev" method, defined in Section 3, 848 are as follows: 850 pass: The DNS evaluation succeeded, i.e., the "reverse" and 851 "forward" lookup results were returned and were in agreement. 853 fail: The DNS evaluation failed. In particular, the "reverse" and 854 "forward" lookups each produced results, but they were not in 855 agreement, or the "forward" query completed but produced no 856 result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an 857 RCODE of 0 (NOERROR) in a reply containing no answers, was 858 returned. 860 temperror: The DNS evaluation could not be completed due to some 861 error that is likely transient in nature, such as a temporary DNS 862 error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or 863 other error condition resulted. A later attempt may produce a 864 final result. 866 permerror: The DNS evaluation could not be completed because no PTR 867 data are published for the connecting IP address, e.g., a DNS 868 RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) 869 in a reply containing no answers, was returned. This prevented 870 completion of the evaluation. A later attempt is unlikely to 871 produce a final result. 873 There is no "none" for this method since any TCP connection 874 delivering email has an IP address associated with it, so some kind 875 of evaluation will always be possible. 877 The result is reported using a ptype of "policy" (as this is not part 878 of any established protocol) and a property of "iprev". 880 For discussion of the format of DNS replies, see "Domain Names - 881 Implementation and Specification" ([DNS]). 883 2.7.4. SMTP AUTH 885 SMTP AUTH (defined in [AUTH]) is represented by the "auth" method. 886 Its result values are as follows: 888 none: SMTP authentication was not attempted. 890 pass: The SMTP client authenticated to the server reporting the 891 result using the protocol described in [AUTH]. 893 fail: The SMTP client attempted to authenticate to the server using 894 the protocol described in [AUTH] but was not successful (such as 895 providing a valid identity but an incorrect password). 897 temperror: The SMTP client attempted to authenticate using the 898 protocol described in [AUTH] but was not able to complete the 899 attempt due to some error that is likely transient in nature, such 900 as a temporary directory service lookup error. A later attempt 901 may produce a final result. 903 permerror: The SMTP client attempted to authenticate using the 904 protocol described in [AUTH] but was not able to complete the 905 attempt due to some error that is likely not transient in nature, 906 such as a permanent directory service lookup error. A later 907 attempt is not likely to produce a final result. 909 The result of AUTH is reported using a ptype of "smtp" and a property 910 of either: 912 o "auth", in which case the value is the authorization identity 913 generated by the exchange initiated by the AUTH command; or 915 o "mailfrom", in which case the value is the mailbox identified by 916 the AUTH parameter used with the MAIL FROM command. 918 If both identities are available, both can be reported. For example, 919 consider this command issued by a client that has completed session 920 authentication with the AUTH command resulting in an authorized 921 identity of "client@c.example": 923 MAIL FROM: AUTH= 925 This could result in a "resinfo" construction like so: 927 ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example 929 Note that in all cases other than "pass", the message was sent by an 930 unauthenticated client. All non-"pass" cases SHOULD thus be treated 931 as equivalent with respect to this method. 933 2.7.5. Other Registered Codes 935 Result codes were also registered in other RFCs as follows: 937 o Vouch By Reference (in [AR-VBR], represented by "vbr"); 939 o Authorized Third-Party Signatures (in [ATPS], represented by 940 "dkim-atps"); 942 o Author Domain Signing Practices (in [ADSP], represented by "dkim- 943 adsp"); 945 o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs"); 947 o S/MIME (in [SMIME-REG], represented by "smime"). 949 2.7.6. Extension Methods 951 Additional authentication method identifiers (extension methods) may 952 be defined in the future by later revisions or extensions to this 953 specification. These method identifiers are registered with the 954 Internet Assigned Numbers Authority (IANA) and, preferably, published 955 in an RFC. See Section 6 for further details. 957 Extension methods can be defined for the following reasons: 959 1. To allow additional information from new authentication systems 960 to be communicated to MUAs or downstream filters. The names of 961 such identifiers ought to reflect the name of the method being 962 defined but ought not be needlessly long. 964 2. To allow the creation of "sub-identifiers" that indicate 965 different levels of authentication and differentiate between 966 their relative strengths, e.g., "auth1-weak" and "auth1-strong". 968 Authentication method implementers are encouraged to provide adequate 969 information, via message header field comments if necessary, to allow 970 an MUA developer to understand or relay ancillary details of 971 authentication results. For example, if it might be of interest to 972 relay what data was used to perform an evaluation, such information 973 could be relayed as a comment in the header field, such as: 975 Authentication-Results: example.com; 976 foo=pass bar.baz=blob (2 of 3 tests OK) 978 Experimental method identifiers MUST only be used within ADMDs that 979 have explicitly consented to use them. These method identifiers and 980 the parameters associated with them are not documented in RFCs. 982 Therefore, they are subject to change at any time and not suitable 983 for production use. Any MTA, MUA, or downstream filter intended for 984 production use SHOULD ignore or delete any Authentication-Results 985 header field that includes an experimental (unknown) method 986 identifier. 988 2.7.7. Extension Result Codes 990 Additional result codes (extension results) might be defined in the 991 future by later revisions or extensions to this specification. 992 Result codes MUST be registered with the Internet Assigned Numbers 993 Authority (IANA) and preferably published in an RFC. See Section 6 994 for further details. 996 Experimental results MUST only be used within ADMDs that have 997 explicitly consented to use them. These results and the parameters 998 associated with them are not formally documented. Therefore, they 999 are subject to change at any time and not suitable for production 1000 use. Any MTA, MUA, or downstream filter intended for production use 1001 SHOULD ignore or delete any Authentication-Results header field that 1002 includes an extension result. 1004 3. The "iprev" Authentication Method 1006 This section defines an additional authentication method called 1007 "iprev". 1009 "iprev" is an attempt to verify that a client appears to be valid 1010 based on some DNS queries, which is to say that the IP address is 1011 explicitly associated with a domain name. Upon receiving a session 1012 initiation of some kind from a client, the IP address of the client 1013 peer is queried for matching names (i.e., a number-to-name 1014 translation, also known as a "reverse lookup" or a "PTR" record 1015 query). Once that result is acquired, a lookup of each of the names 1016 (i.e., a name-to-number translation, or an "A" or "AAAA" record 1017 query) thus retrieved is done. The response to this second check 1018 will typically result in at least one mapping back to the client's IP 1019 address. 1021 Expressed as an algorithm: If the client peer's IP address is I, the 1022 list of names to which I maps (after a "PTR" query) is the set N, and 1023 the union of IP addresses to which each member of N maps (after 1024 corresponding "A" and "AAAA" queries) is L, then this test is 1025 successful if I is an element of L. 1027 Often an MTA receiving a connection that fails this test will simply 1028 reject the connection using the enhanced status code defined in 1030 [AUTH-ESC]. If an operator instead wishes to make this information 1031 available to downstream agents as a factor in handling decisions, it 1032 records a result in accordance with Section 2.7.3. 1034 The response to a PTR query could contain multiple names. To prevent 1035 heavy DNS loads, agents performing these queries MUST be implemented 1036 such that the number of names evaluated by generation of 1037 corresponding A or AAAA queries is limited so as not to be unduly 1038 taxing to the DNS infrastructure, though it MAY be configurable by an 1039 administrator. As an example, Section 4.6.4 of [SPF] chose a limit 1040 of 10 for its implementation of this algorithm. 1042 "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the 1043 query formats for the IPv6 case. 1045 There is some contention regarding the wisdom and reliability of this 1046 test. For example, in some regions, it can be difficult for this 1047 test ever to pass because the practice of arranging to match the 1048 forward and reverse DNS is infrequently observed. Therefore, the 1049 precise implementation details of how a verifier performs an "iprev" 1050 test are not specified here. The verifier MAY report a successful or 1051 failed "iprev" test at its discretion having done some kind of check 1052 of the validity of the connection's identity using DNS. It is 1053 incumbent upon an agent making use of the reported "iprev" result to 1054 understand what exactly that particular verifier is attempting to 1055 report. 1057 Extensive discussion of reverse DNS mapping and its implications can 1058 be found in "Considerations for the use of DNS Reverse Mapping" 1059 ([DNSOP-REVERSE]). In particular, it recommends that applications 1060 avoid using this test as a means of authentication or security. Its 1061 presence in this document is not an endorsement but is merely 1062 acknowledgment that the method remains common and provides the means 1063 to relay the results of that test. 1065 4. Adding the Header Field to a Message 1067 This specification makes no attempt to evaluate the relative 1068 strengths of various message authentication methods that may become 1069 available. The methods listed are an order-independent set; their 1070 sequence does not indicate relative strength or importance of one 1071 method over another. Instead, the MUA or downstream filter consuming 1072 this header field is to interpret the result of each method based on 1073 its own knowledge of what that method evaluates. 1075 Each "method" MUST refer to an authentication method declared in the 1076 IANA registry or an extension method as described in Section 2.7.6, 1077 and each "result" MUST refer to a result code declared in the IANA 1078 registry or an extension result code as defined in Section 2.7.7. 1079 See Section 6 for further information about the registered methods 1080 and result codes. 1082 An MTA compliant with this specification adds this header field 1083 (after performing one or more message authentication tests) to 1084 indicate which MTA or ADMD performed the test, which test got 1085 applied, and what the result was. If an MTA applies more than one 1086 such test, it adds this header field either once per test or once 1087 indicating all of the results. An MTA MUST NOT add a result to an 1088 existing header field. 1090 An MTA MAY add this header field containing only the authentication 1091 identifier portion and the "none" token (see Section 2.2) to indicate 1092 explicitly that no message authentication schemes were applied prior 1093 to delivery of this message. 1095 An MTA adding this header field has to take steps to identify it as 1096 legitimate to the MUAs or downstream filters that will ultimately 1097 consume its content. One process to do so is described in Section 5. 1098 Further measures may be necessary in some environments. Some 1099 possible solutions are enumerated in Section 7.1. This document does 1100 not mandate any specific solution to this issue as each environment 1101 has its own facilities and limitations. 1103 Most known message authentication methods focus on a particular 1104 identifier to evaluate. SPF and Sender ID differ in that they can 1105 yield a result based on more than one identifier; specifically, SPF 1106 can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom 1107 parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter 1108 or the Purported Responsible Address (PRA) identity. When generating 1109 this field to report those results, only the parameter that yielded 1110 the result is included. 1112 For MTAs that add this header field, adding header fields in order 1113 (at the top), per Section 3.6 of [MAIL], is particularly important. 1114 Moreover, this header field SHOULD be inserted above any other trace 1115 header fields such MTAs might prepend. This placement allows easy 1116 detection of header fields that can be trusted. 1118 End users making direct use of this header field might inadvertently 1119 trust information that has not been properly vetted. If, for 1120 example, a basic SPF result were to be relayed that claims an 1121 authenticated addr-spec, the local-part of that addr-spec has 1122 actually not been authenticated. Thus, an MTA adding this header 1123 field SHOULD NOT include any data that has not been authenticated by 1124 the method(s) being applied. Moreover, MUAs SHOULD NOT render to 1125 users such information if it is presented by a method known not to 1126 authenticate it. 1128 4.1. Header Field Position and Interpretation 1130 In order to ensure non-ambiguous results and avoid the impact of 1131 false header fields, MUAs and downstream filters SHOULD NOT interpret 1132 this header field unless specifically configured to do so by the user 1133 or administrator. That is, this interpretation should not be "on by 1134 default". Naturally then, users or administrators ought not activate 1135 such a feature unless (1) they are certain the header field will be 1136 validly added by an agent within the ADMD that accepts the mail that 1137 is ultimately read by the MUA, and (2) instances of the header field 1138 that appear to originate within the ADMD but are actually added by 1139 foreign MTAs will be removed before delivery. 1141 Furthermore, MUAs and downstream filters SHOULD NOT interpret this 1142 header field unless the authentication service identifier it bears 1143 appears to be one used within its own ADMD as configured by the user 1144 or administrator. 1146 MUAs and downstream filters MUST ignore any result reported using a 1147 "result" not specified in the IANA "Result Code" registry or a 1148 "ptype" not listed in the "Email Authentication Property Types" 1149 registry for such values as defined in Section 6. Moreover, such 1150 agents MUST ignore a result indicated for any "method" they do not 1151 specifically support. 1153 An MUA SHOULD NOT reveal these results to end users, absent careful 1154 human factors design considerations and testing, for the presentation 1155 of trust-related materials. For example, an attacker could register 1156 examp1e.com (note the digit "1" (one)) and send signed mail to 1157 intended victims; a verifier would detect that the signature was 1158 valid and report a "pass" even though it's clear the DNS domain name 1159 was intended to mislead. See Section 7.2 for further discussion. 1161 As stated in Section 2.1, this header field MUST be treated as though 1162 it were a trace header field as defined in Section 3.6.7 of [MAIL] 1163 and hence MUST NOT be reordered and MUST be prepended to the message, 1164 so that there is generally some indication upon delivery of where in 1165 the chain of handling MTAs the message authentication was done. 1167 Note that there are a few message handlers that are only capable of 1168 appending new header fields to a message. Strictly speaking, these 1169 handlers are not compliant with this specification. They can still 1170 add the header field to carry authentication details, but any signal 1171 about where in the handling chain the work was done may be lost. 1172 Consumers SHOULD be designed such that this can be tolerated, 1173 especially from a producer known to have this limitation. 1175 MUAs SHOULD ignore instances of this header field discovered within 1176 message/rfc822 MIME attachments. 1178 Further discussion of these topics can be found in Section 7 below. 1180 4.2. Local Policy Enforcement 1182 Some sites have a local policy that considers any particular 1183 authentication policy's non-recoverable failure results (typically 1184 "fail" or similar) as justification for rejecting the message. In 1185 such cases, the border MTA SHOULD issue an SMTP rejection response to 1186 the message, rather than adding this header field and allowing the 1187 message to proceed toward delivery. This is more desirable than 1188 allowing the message to reach an internal host's MTA or spam filter, 1189 thus possibly generating a local rejection such as a Delivery Status 1190 Notification (DSN) [DSN] to a forged originator. Such generated 1191 rejections are colloquially known as "backscatter". 1193 The same MAY also be done for local policy decisions overriding the 1194 results of the authentication methods (e.g., the "policy" result 1195 codes described in Section 2.7). 1197 Such rejections at the SMTP protocol level are not possible if local 1198 policy is enforced at the MUA and not the MTA. 1200 5. Removing Existing Header Fields 1202 For security reasons, any MTA conforming to this specification MUST 1203 delete any discovered instance of this header field that claims, by 1204 virtue of its authentication service identifier, to have been added 1205 within its trust boundary but that did not come directly from another 1206 trusted MTA. For example, an MTA for example.com receiving a message 1207 MUST delete or otherwise obscure any instance of this header field 1208 bearing an authentication service identifier indicating that the 1209 header field was added within example.com prior to adding its own 1210 header fields. This could mean each MTA will have to be equipped 1211 with a list of internal MTAs known to be compliant (and hence 1212 trustworthy). 1214 For simplicity and maximum security, a border MTA could remove all 1215 instances of this header field on mail crossing into its trust 1216 boundary. However, this may conflict with the desire to access 1217 authentication results performed by trusted external service 1218 providers. It may also invalidate signed messages whose signatures 1219 cover external instances of this header field. A more robust border 1220 MTA could allow a specific list of authenticating MTAs whose 1221 information is to be admitted, removing the header field originating 1222 from all others. 1224 As stated in Section 1.2, a formal definition of "trust boundary" is 1225 deliberately not made here. It is entirely possible that a border 1226 MTA for example.com will explicitly trust authentication results 1227 asserted by upstream host example.net even though they exist in 1228 completely disjoint administrative boundaries. In that case, the 1229 border MTA MAY elect not to delete those results; moreover, the 1230 upstream host doing some authentication work could apply a signing 1231 technology such as [DKIM] on its own results to assure downstream 1232 hosts of their authenticity. An example of this is provided in 1233 Appendix B. 1235 Similarly, in the case of messages signed using [DKIM] or other 1236 message-signing methods that sign header fields, this removal action 1237 could invalidate one or more signatures on the message if they 1238 covered the header field to be removed. This behavior can be 1239 desirable since there's little value in validating the signature on a 1240 message with forged header fields. However, signing agents MAY 1241 therefore elect to omit these header fields from signing to avoid 1242 this situation. 1244 An MTA SHOULD remove any instance of this header field bearing a 1245 version (express or implied) that it does not support. However, an 1246 MTA MUST remove such a header field if the [SMTP] connection relaying 1247 the message is not from a trusted internal MTA. This means the MTA 1248 needs to be able to understand versions of this header field at least 1249 as late as the ones understood by the MUAs or other consumers within 1250 its ADMD. 1252 6. IANA Considerations 1254 IANA has registered the defined header field and created tables as 1255 described below. These registry actions were originally defined by 1256 [RFC5451] and updated by [RFC6577] and [RFC7001]. The created 1257 registries are being further updated here to increase their 1258 completeness. 1260 6.1. The Authentication-Results Header Field 1262 [RFC5451] added the Authentication-Results header field to the IANA 1263 "Permanent Message Header Field Names" registry, per the procedure 1264 found in [IANA-HEADERS]. That entry has been updated to reference 1265 this document. The following is the registration template: 1267 Header field name: Authentication-Results 1268 Applicable protocol: mail ([MAIL]) 1269 Status: Standard 1270 Author/Change controller: IETF 1271 Specification document(s): RFC 7601 1272 Related information: none 1274 6.2. "Email Authentication Methods" Registry Description 1276 Names of message authentication methods supported by this 1277 specification have been registered with IANA, with the exception of 1278 experimental names as described in Section 2.7.6. Along with each 1279 method is recorded the properties that accompany the method's result. 1281 The "Email Authentication Parameters" group, and within it the "Email 1282 Authentication Methods" registry, were created by [RFC5451] for this 1283 purpose. [RFC6577] added a "status" field for each entry. [RFC7001] 1284 amended the rules governing that registry and also added a "version" 1285 field to the registry. 1287 The reference for that registry has been updated to reference this 1288 document. 1290 New entries are assigned only for values that have received Expert 1291 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1292 appointed by the IESG. The designated expert has discretion to 1293 request that a publication be referenced if a clear, concise 1294 definition of the authentication method cannot be provided such that 1295 interoperability is assured. Registrations should otherwise be 1296 permitted. The designated expert can also handle requests to mark 1297 any current registration as "deprecated". 1299 No two entries can have the same combination of method, ptype, and 1300 property. 1302 An entry in this registry contains the following: 1304 Method: the name of the method. 1306 Definition: a reference to the document that created this entry, if 1307 any (see below). 1309 ptype: a "ptype" value appropriate for use with that method. 1311 property: a "property" value matching that "ptype" also appropriate 1312 for use with that method. 1314 Value: a brief description of the value to be supplied with that 1315 method/ptype/property tuple. 1317 Status: the status of this entry, which is either: 1319 active: The entry is in current use. 1321 deprecated: The entry is no longer in current use. 1323 Version: a version number associated with the method (preferably 1324 starting at "1"). 1326 The "Definition" field will typically refer to a permanent document, 1327 or at least some descriptive text, where additional information about 1328 the entry being added can be found. This might in turn reference the 1329 document where the method is defined so that all of the semantics 1330 around creating or interpreting an Authentication-Results header 1331 field using this method, ptype, and property can be understood. 1333 6.3. "Email Authentication Methods" Registry Update 1335 The following changes have been made to this registry per this 1336 document: 1338 1. The "Defined" field has been renamed "Definition", to be 1339 consistent with the other registries in this group. 1341 2. The entry for the "dkim" method, "header" ptype, and "b" property 1342 now reference [RFC6008] as the defining document, and the 1343 reference has be removed from the description. 1345 3. All other "dkim", "domainkeys", "iprev", "sender-id", and "spf" 1346 method entries have had their "Definition" fields changed to 1347 refer to this document, as this document contains a complete 1348 description of the registry and these corresponding values. 1350 4. All "smime" entries have had their "Definition" fields changed to 1351 [SMIME-REG]. 1353 5. The "value" field of the "smime" entry using property "smime- 1354 part" has been changed to read: "The MIME body part reference 1355 that contains the S/MIME signature. See Section 3.2.1 of RFC 1356 7281 for full syntax." 1358 6. The single entry for the "auth" method was intended to reflect 1359 the identity indicated by the "AUTH" parameter to the SMTP "MAIL 1360 FROM" command verb. However, there is also an "AUTH" command 1361 verb. To clarify this ambiguity, the entry for the "auth" method 1362 has had its "property" field changed to "mailfrom", and its 1363 "Definition" field changed to this document. 1365 7. The following entry has been added: 1367 Method: auth 1369 Definition: this document (RFC 7601) 1371 ptype: smtp 1373 property: auth 1375 Value: identity confirmed by the AUTH command 1377 Status: active 1379 Version: 1 1381 8. The values of the "domainkeys" entries for ptype "header" have 1382 been updated as follows: 1384 from: contents of the [MAIL] From: header field, after removing 1385 comments, and removing the local-part and following "@" if not 1386 authenticated 1388 sender: contents of the [MAIL] Sender: header field, after 1389 removing comments, and removing the local-part and following 1390 "@" if not authenticated 1392 9. For all entries for "dkim-adsp" and "domainkeys", their Status 1393 values have been changed to "deprecated", reflecting the fact 1394 that the corresponding specifications now have Historic status. 1395 Their "Definition" fields have also been modified to include a 1396 reference to this document. 1398 6.4. "Email Authentication Property Types" Registry 1400 [RFC7410] created the "Email Authentication Property Types" registry. 1402 Entries in this registry are subject to the Expert Review rules as 1403 described in [IANA-CONSIDERATIONS]. Each entry in the registry 1404 requires the following values: 1406 ptype: The name of the ptype being registered, which must fit within 1407 the ABNF described in Section 2.2. 1409 Definition: An optional reference to a defining specification. 1411 Description: A brief description of what sort of information this 1412 "ptype" is meant to cover. 1414 For new entries, the Designated Expert needs to assure that the 1415 description provided for the new entry adequately describes the 1416 intended use. An example would be helpful to include in the entry's 1417 defining document, if any, although entries in the "Email 1418 Authentication Methods" registry or the "Email Authentication Result 1419 Names" registry might also serve as examples of intended use. 1421 As this is a complete restatement of the definition and rules for 1422 this registry, IANA has updated this registry to show Section 2.3 of 1423 this document as the current definitions for the "body", "header", 1424 "policy", and "smtp" entries of that registry. References to 1425 [RFC7001] and [RFC7410] have been removed. 1427 6.5. "Email Authentication Result Names" Description 1429 Names of message authentication result codes supported by this 1430 specification must be registered with IANA, with the exception of 1431 experimental codes as described in Section 2.7.7. A registry was 1432 created by [RFC5451] for this purpose. [RFC6577] added the "status" 1433 column and [RFC7001] updated the rules governing that registry. 1435 New entries are assigned only for values that have received Expert 1436 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1437 appointed by the IESG. The designated expert has discretion to 1438 request that a publication be referenced if a clear, concise 1439 definition of the authentication result cannot be provided such that 1440 interoperability is assured. Registrations should otherwise be 1441 permitted. The designated expert can also handle requests to mark 1442 any current registration as "deprecated". 1444 No two entries can have the same combination of method and code. 1446 An entry in this registry contains the following: 1448 Auth Method: an authentication method for which results are being 1449 returned using the header field defined in this document. 1451 Code: a result code that can be returned for this authentication 1452 method. 1454 Specification: either free form text explaining the meaning of this 1455 method-code combination, or a reference to such a definition. 1457 Status: the status of this entry, which is either: 1459 active: The entry is in current use. 1461 deprecated: The entry is no longer in current use. 1463 6.6. "Email Authentication Result Names" Update 1465 This document includes a complete description of the registry, 1466 obsoleting [RFC7001]. Accordingly, the following changes have been 1467 made to this registry per this document: 1469 o The "Defined" field has been removed. 1471 o The "Meaning" field has been renamed "Specification", as described 1472 above. 1474 o The "Auth Method" field now appears before the "Code" field. 1476 o For easier searching, the table has been arranged such that it is 1477 sorted first by Auth Method, then by Code within each Auth Method 1478 grouping. 1480 o All entries for the "dkim", "domainkeys", "spf", "sender-id", 1481 "auth", and "iprev" methods have had their "Specification" fields 1482 replaced as follows: 1484 dkim: Section 2.7.1 of this document (RFC 7601) 1486 domainkeys: Section 2.7.1 of this document (RFC 7601) 1488 spf: for "hardfail", Section 2.4.2 of [RFC5451]; for all others, 1489 Section 2.7.2 of this document (RFC 7601) 1491 sender-id: for "hardfail", Section 2.4.2 of [RFC5451]; for all 1492 others, Section 2.7.2 of this document (RFC 7601) 1494 auth: Section 2.7.4 of this document (RFC 7601) 1496 iprev: Section 2.7.3 of this document (RFC 7601) 1498 o All entries for "dkim-adsp" that were missing an explicit 1499 reference to a defining document now reference [ADSP] in their 1500 "Specification" fields. 1502 o All entries for "dmarc" have had their "Specification" fields 1503 changed to reference Section 11.2 of [DMARC]. 1505 o All entries for "dkim-adsp" and "domainkeys" have had their Status 1506 values changed to "deprecated", reflecting the fact that the 1507 corresponding specifications now have Historic status. Their 1508 "Specification" fields have also been modified to include a 1509 reference to this document. 1511 6.7. SMTP Enhanced Status Codes 1513 The entry for X.7.25 in the "Enumerated Status Codes" sub-registry of 1514 the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes 1515 Registry" has been updated to refer to this document instead of 1516 [RFC7001]. 1518 7. Security Considerations 1520 The following security considerations apply when adding or processing 1521 the Authentication-Results header field: 1523 7.1. Forged Header Fields 1525 An MUA or filter that accesses a mailbox whose messages are handled 1526 by a non-conformant MTA, and understands Authentication-Results 1527 header fields, could potentially make false conclusions based on 1528 forged header fields. A malicious user or agent could forge a header 1529 field using the DNS domain of a receiving ADMD as the authserv-id 1530 token in the value of the header field and, with the rest of the 1531 value, claim that the message was properly authenticated. The non- 1532 conformant MTA would fail to strip the forged header field, and the 1533 MUA could inappropriately trust it. 1535 For this reason, it is best not to have processing of the 1536 Authentication-Results header field enabled by default; instead, it 1537 should be ignored, at least for the purposes of enacting filtering 1538 decisions, unless specifically enabled by the user or administrator 1539 after verifying that the border MTA is compliant. It is acceptable 1540 to have an MUA aware of this specification but have an explicit list 1541 of hostnames whose Authentication-Results header fields are 1542 trustworthy; however, this list should initially be empty. 1544 Proposed alternative solutions to this problem were made some time 1545 ago and are listed below. To date, they have not been developed due 1546 to lack of demand but are documented here should the information be 1547 useful at some point in the future: 1549 1. Possibly the simplest is a digital signature protecting the 1550 header field, such as using [DKIM], that can be verified by an 1551 MUA by using a posted public key. Although one of the main 1552 purposes of this document is to relieve the burden of doing 1553 message authentication work at the MUA, this only requires that 1554 the MUA learn a single authentication scheme even if a number of 1555 them are in use at the border MTA. Note that [DKIM] requires 1556 that the From header field be signed, although in this 1557 application, the signing agent (a trusted MTA) likely cannot 1558 authenticate that value, so the fact that it is signed should be 1559 ignored. Where the authserv-id is the ADMD's domain name, the 1560 authserv-id matching this valid internal signature's "d=" DKIM 1561 value is sufficient. 1563 2. Another would be a means to interrogate the MTA that added the 1564 header field to see if it is actually providing any message 1565 authentication services and saw the message in question, but this 1566 isn't especially palatable given the work required to craft and 1567 implement such a scheme. 1569 3. Yet another might be a method to interrogate the internal MTAs 1570 that apparently handled the message (based on Received header 1571 fields) to determine whether any of them conform to Section 5 of 1572 this memo. This, too, has potentially high barriers to entry. 1574 4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to 1575 allow an MUA or filtering agent to acquire the authserv-id in use 1576 within an ADMD, thus allowing it to identify which 1577 Authentication-Results header fields it can trust. 1579 5. On the presumption that internal MTAs are fully compliant with 1580 Section 3.6 of [MAIL] and the compliant internal MTAs are using 1581 their own hostnames or the ADMD's DNS domain name as the 1582 authserv-id token, the header field proposed here should always 1583 appear above a Received header added by a trusted MTA. This can 1584 be used as a test for header field validity. 1586 Support for some of these is being considered for future work. 1588 In any case, a mechanism needs to exist for an MUA or filter to 1589 verify that the host that appears to have added the header field (a) 1590 actually did so and (b) is legitimately adding that header field for 1591 this delivery. Given the variety of messaging environments deployed 1592 today, consensus appears to be that specifying a particular mechanism 1593 for doing so is not appropriate for this document. 1595 Mitigation of the forged header field attack can also be accomplished 1596 by moving the authentication results data into metadata associated 1597 with the message. In particular, an [SMTP] extension could be 1598 established to communicate authentication results from the border MTA 1599 to intermediate and delivery MTAs; the latter of these could arrange 1600 to store the authentication results as metadata retrieved and 1601 rendered along with the message by an [IMAP] client aware of a 1602 similar extension in that protocol. The delivery MTA would be told 1603 to trust data via this extension only from MTAs it trusts, and border 1604 MTAs would not accept data via this extension from any source. There 1605 is no vector in such an arrangement for forgery of authentication 1606 data by an outside agent. 1608 7.2. Misleading Results 1610 Until some form of service for querying the reputation of a sending 1611 agent is widely deployed, the existence of this header field 1612 indicating a "pass" does not render the message trustworthy. It is 1613 possible for an arriving piece of spam or other undesirable mail to 1614 pass checks by several of the methods enumerated above (e.g., a piece 1615 of spam signed using [DKIM] by the originator of the spam, which 1616 might be a spammer or a compromised system). In particular, this 1617 issue is not resolved by forged header field removal discussed above. 1619 Hence, MUAs and downstream filters must take some care with use of 1620 this header even after possibly malicious headers are scrubbed. 1622 7.3. Header Field Position 1624 Despite the requirements of [MAIL], header fields can sometimes be 1625 reordered en route by intermediate MTAs. The goal of requiring 1626 header field addition only at the top of a message is an 1627 acknowledgment that some MTAs do reorder header fields, but most do 1628 not. Thus, in the general case, there will be some indication of 1629 which MTAs (if any) handled the message after the addition of the 1630 header field defined here. 1632 7.4. Reverse IP Query Denial-of-Service Attacks 1634 Section 4.6.4 of [SPF] describes a DNS-based denial-of-service attack 1635 for verifiers that attempt DNS-based identity verification of 1636 arriving client connections. A verifier wishing to do this check and 1637 report this information needs to take care not to go to unbounded 1638 lengths to resolve "A" and "PTR" queries. MUAs or other filters 1639 making use of an "iprev" result specified by this document need to be 1640 aware of the algorithm used by the verifier reporting the result and, 1641 especially, its limitations. 1643 7.5. Mitigation of Backscatter 1645 Failing to follow the instructions of Section 4.2 can result in a 1646 denial-of-service attack caused by the generation of [DSN] messages 1647 (or equivalent) to addresses that did not send the messages being 1648 rejected. 1650 7.6. Internal MTA Lists 1652 Section 5 describes a procedure for scrubbing header fields that may 1653 contain forged authentication results about a message. A compliant 1654 installation will have to include, at each MTA, a list of other MTAs 1655 known to be compliant and trustworthy. Failing to keep this list 1656 current as internal infrastructure changes may expose an ADMD to 1657 attack. 1659 7.7. Attacks against Authentication Methods 1661 If an attack becomes known against an authentication method, clearly 1662 then the agent verifying that method can be fooled into thinking an 1663 inauthentic message is authentic, and thus the value of this header 1664 field can be misleading. It follows that any attack against the 1665 authentication methods supported by this document is also a security 1666 consideration here. 1668 7.8. Intentionally Malformed Header Fields 1670 It is possible for an attacker to add an Authentication-Results 1671 header field that is extraordinarily large or otherwise malformed in 1672 an attempt to discover or exploit weaknesses in header field parsing 1673 code. Implementers must thoroughly verify all such header fields 1674 received from MTAs and be robust against intentionally as well as 1675 unintentionally malformed header fields. 1677 7.9. Compromised Internal Hosts 1679 An internal MUA or MTA that has been compromised could generate mail 1680 with a forged From header field and a forged Authentication-Results 1681 header field that endorses it. Although it is clearly a larger 1682 concern to have compromised internal machines than it is to prove the 1683 value of this header field, this risk can be mitigated by arranging 1684 that internal MTAs will remove this header field if it claims to have 1685 been added by a trusted border MTA (as described above), yet the 1686 [SMTP] connection is not coming from an internal machine known to be 1687 running an authorized MTA. However, in such a configuration, 1688 legitimate MTAs will have to add this header field when legitimate 1689 internal-only messages are generated. This is also covered in 1690 Section 5. 1692 7.10. Encapsulated Instances 1694 MIME messages can contain attachments of type "message/rfc822", which 1695 contain other messages. Such an encapsulated message can also 1696 contain an Authentication-Results header field. Although the 1697 processing of these is outside of the intended scope of this document 1698 (see Section 1.3), some early guidance to MUA developers is 1699 appropriate here. 1701 Since MTAs are unlikely to strip Authentication-Results header fields 1702 after mailbox delivery, MUAs are advised in Section 4.1 to ignore 1703 such instances within MIME attachments. Moreover, when extracting a 1704 message digest to separate mail store messages or other media, such 1705 header fields should be removed so that they will never be 1706 interpreted improperly by MUAs that might later consume them. 1708 7.11. Reverse Mapping 1710 Although Section 3 of this memo includes explicit support for the 1711 "iprev" method, its value as an authentication mechanism is limited. 1712 Implementers of both this proposal and agents that use the data it 1713 relays are encouraged to become familiar with the issues raised by 1714 [DNSOP-REVERSE] when deciding whether or not to include support for 1715 "iprev". 1717 8. References 1719 8.1. Normative References 1721 [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1722 Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ 1723 RFC5234, January 2008, 1724 . 1726 [IANA-HEADERS] 1727 Klyne, G., Nottingham, M., and J. Mogul, "Registration 1728 Procedures for Message Header Fields", BCP 90, RFC 3864, 1729 DOI 10.17487/RFC3864, September 2004, 1730 . 1732 [KEYWORDS] 1733 Bradner, S., "Key words for use in RFCs to Indicate 1734 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 1735 RFC2119, March 1997, 1736 . 1738 [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, 1739 DOI 10.17487/RFC5322, October 2008, 1740 . 1742 [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1743 Extensions (MIME) Part One: Format of Internet Message 1744 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 1745 . 1747 [RFC5451] Kucherawy, M., "Message Header Field for Indicating 1748 Message Authentication Status", RFC 5451, DOI 10.17487/ 1749 RFC5451, April 2009, 1750 . 1752 [RFC6008] Kucherawy, M., "Authentication-Results Registration for 1753 Differentiating among Cryptographic Results", RFC 6008, 1754 DOI 10.17487/RFC6008, September 2010, 1755 . 1757 [RFC6577] Kucherawy, M., "Authentication-Results Registration Update 1758 for Sender Policy Framework (SPF) Results", RFC 6577, 1759 DOI 10.17487/RFC6577, March 2012, 1760 . 1762 [RFC7001] Kucherawy, M., "Message Header Field for Indicating 1763 Message Authentication Status", RFC 7001, DOI 10.17487/ 1764 RFC7001, September 2013, 1765 . 1767 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 1768 DOI 10.17487/RFC5321, October 2008, 1769 . 1771 8.2. Informative References 1773 [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, 1774 "DomainKeys Identified Mail (DKIM) Author Domain Signing 1775 Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, 1776 August 2009, . 1778 [AR-VBR] Kucherawy, M., "Authentication-Results Registration for 1779 Vouch by Reference Results", RFC 6212, DOI 10.17487/ 1780 RFC6212, April 2011, 1781 . 1783 [ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM) 1784 Authorized Third-Party Signatures", RFC 6541, 1785 DOI 10.17487/RFC6541, February 2012, 1786 . 1788 [AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service 1789 Extension for Authentication", RFC 4954, DOI 10.17487/ 1790 RFC4954, July 2007, 1791 . 1793 [AUTH-ESC] 1794 Kucherawy, M., "Email Authentication Status Codes", 1795 RFC 7372, DOI 10.17487/RFC7372, September 2014, 1796 . 1798 [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 1799 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 1800 RFC 6376, DOI 10.17487/RFC6376, September 2011, 1801 . 1803 [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based 1804 Message Authentication, Reporting, and Conformance 1805 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, 1806 . 1808 [DNS] Mockapetris, P., "Domain names - implementation and 1809 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1810 November 1987, . 1812 [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 1813 "DNS Extensions to Support IP Version 6", RFC 3596, 1814 DOI 10.17487/RFC3596, October 2003, 1815 . 1817 [DNSOP-REVERSE] 1818 Senie, D. and A. Sullivan, "Considerations for the use of 1819 DNS Reverse Mapping", Work in Progress, draft-ietf-dnsop- 1820 reverse-mapping-considerations-06, March 2008. 1822 [DOMAINKEYS] 1823 Delany, M., "Domain-Based Email Authentication Using 1824 Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, 1825 DOI 10.17487/RFC4870, May 2007, 1826 . 1828 [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format 1829 for Delivery Status Notifications", RFC 3464, 1830 DOI 10.17487/RFC3464, January 2003, 1831 . 1833 [EMAIL-ARCH] 1834 Crocker, D., "Internet Mail Architecture", RFC 5598, 1835 DOI 10.17487/RFC5598, July 2009, 1836 . 1838 [IANA-CONSIDERATIONS] 1839 Narten, T. and H. Alvestrand, "Guidelines for Writing an 1840 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1841 DOI 10.17487/RFC5226, May 2008, 1842 . 1844 [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 1845 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 1846 . 1848 [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 1849 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 1850 . 1852 [PRA] Lyon, J., "Purported Responsible Address in E-Mail 1853 Messages", RFC 4407, DOI 10.17487/RFC4407, April 2006, 1854 . 1856 [RFC7410] Kucherawy, M., "A Property Types Registry for the 1857 Authentication-Results Header Field", RFC 7410, 1858 DOI 10.17487/RFC7410, December 2014, 1859 . 1861 [RRVS] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid- 1862 Since Header Field and SMTP Service Extension", RFC 7293, 1863 DOI 10.17487/RFC7293, July 2014, 1864 . 1866 [SECURITY] 1867 Rescorla, E. and B. Korver, "Guidelines for Writing RFC 1868 Text on Security Considerations", BCP 72, RFC 3552, 1869 DOI 10.17487/RFC3552, July 2003, 1870 . 1872 [SENDERID] 1873 Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", 1874 RFC 4406, DOI 10.17487/RFC4406, April 2006, 1875 . 1877 [SMIME-REG] 1878 Melnikov, A., "Authentication-Results Registration for 1879 S/MIME Signature Verification", RFC 7281, DOI 10.17487/ 1880 RFC7281, June 2014, 1881 . 1883 [SPF] Kitterman, S., "Sender Policy Framework (SPF) for 1884 Authorizing Use of Domains in Email, Version 1", RFC 7208, 1885 DOI 10.17487/RFC7208, April 2014, 1886 . 1888 [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By 1889 Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009, 1890 . 1892 Appendix A. Legacy MUAs 1894 Implementers of this protocol should be aware that many MUAs are 1895 unlikely to be retrofitted to support the new header field and its 1896 semantics. In the interests of convenience and quicker adoption, a 1897 delivery MTA might want to consider adding things that are processed 1898 by existing MUAs in addition to the Authentication-Results header 1899 field. One suggestion is to include a Priority header field, on 1900 messages that don't already have such a header field, containing a 1901 value that reflects the strength of the authentication that was 1902 accomplished, e.g., "low" for weak or no authentication, "normal" or 1903 "high" for good or strong authentication. 1905 Some modern MUAs can already filter based on the content of this 1906 header field. However, there is keen interest in having MUAs make 1907 some kind of graphical representation of this header field's meaning 1908 to end users. Until this capability is added (i.e., while this 1909 proposal and its successors are being adopted), other interim means 1910 of conveying authentication results may be necessary. 1912 Appendix B. Authentication-Results Examples 1914 This section presents some examples of the use of this header field 1915 to indicate authentication results. 1917 B.1. Trivial Case; Header Field Not Present 1919 The trivial case: 1921 Received: from mail-router.example.com 1922 (mail-router.example.com [192.0.2.1]) 1923 by server.example.org (8.11.6/8.11.6) 1924 with ESMTP id g1G0r1kA003489; 1925 Fri, Feb 15 2002 17:19:07 -0800 1926 From: sender@example.com 1927 Date: Fri, Feb 15 2002 16:54:30 -0800 1928 To: receiver@example.org 1929 Message-Id: <12345.abc@example.com> 1930 Subject: here's a sample 1932 Hello! Goodbye! 1934 Example 1: Trivial Case 1936 The Authentication-Results header field is completely absent. The 1937 MUA may make no conclusion about the validity of the message. This 1938 could be the case because the message authentication services were 1939 not available at the time of delivery, or no service is provided, or 1940 the MTA is not in compliance with this specification. 1942 B.2. Nearly Trivial Case; Service Provided, but No Authentication Done 1944 A message that was delivered by an MTA that conforms to this 1945 specification but provides no actual message authentication service: 1947 Authentication-Results: example.org 1; none 1948 Received: from mail-router.example.com 1949 (mail-router.example.com [192.0.2.1]) 1950 by server.example.org (8.11.6/8.11.6) 1951 with ESMTP id g1G0r1kA003489; 1952 Fri, Feb 15 2002 17:19:07 -0800 1953 From: sender@example.com 1954 Date: Fri, Feb 15 2002 16:54:30 -0800 1955 To: receiver@example.org 1956 Message-Id: <12345.abc@example.com> 1957 Subject: here's a sample 1959 Hello! Goodbye! 1961 Example 2: Header Present but No Authentication Done 1963 The Authentication-Results header field is present, showing that the 1964 delivering MTA conforms to this specification. It used its DNS 1965 domain name as the authserv-id. The presence of "none" (and the 1966 absence of any method or result tokens) indicates that no message 1967 authentication was done. The version number of the specification to 1968 which the field's content conforms is explicitly provided. 1970 B.3. Service Provided, Authentication Done 1972 A message that was delivered by an MTA that conforms to this 1973 specification and applied some message authentication: 1975 Authentication-Results: example.com; 1976 spf=pass smtp.mailfrom=example.net 1977 Received: from dialup-1-2-3-4.example.net 1978 (dialup-1-2-3-4.example.net [192.0.2.200]) 1979 by mail-router.example.com (8.11.6/8.11.6) 1980 with ESMTP id g1G0r1kA003489; 1981 Fri, Feb 15 2002 17:19:07 -0800 1982 From: sender@example.net 1983 Date: Fri, Feb 15 2002 16:54:30 -0800 1984 To: receiver@example.com 1985 Message-Id: <12345.abc@example.net> 1986 Subject: here's a sample 1988 Hello! Goodbye! 1990 Example 3: Header Reporting Results 1992 The Authentication-Results header field is present, indicating that 1993 the border MTA conforms to this specification. The authserv-id is 1994 once again the DNS domain name. Furthermore, the message was 1995 authenticated by that MTA via the method specified in [SPF]. Note 1996 that since that method cannot authenticate the local-part, it has 1997 been omitted from the result's value. The MUA could extract and 1998 relay this extra information if desired. 2000 B.4. Service Provided, Several Authentications Done, Single MTA 2002 A message that was relayed inbound via a single MTA that conforms to 2003 this specification and applied three different message authentication 2004 checks: 2006 Authentication-Results: example.com; 2007 auth=pass (cram-md5) smtp.auth=sender@example.net; 2008 spf=pass smtp.mailfrom=example.net 2009 Authentication-Results: example.com; 2010 sender-id=pass header.from=example.net 2011 Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6) 2012 (dialup-1-2-3-4.example.net [192.0.2.200]) 2013 by mail-router.example.com (8.11.6/8.11.6) 2014 with ESMTPA id g1G0r1kA003489; 2015 Fri, Feb 15 2002 17:19:07 -0800 2016 Date: Fri, Feb 15 2002 16:54:30 -0800 2017 To: receiver@example.com 2018 From: sender@example.net 2019 Message-Id: <12345.abc@example.net> 2020 Subject: here's a sample 2022 Hello! Goodbye! 2024 Example 4: Headers Reporting Results from One MTA 2026 The Authentication-Results header field is present, indicating that 2027 the delivering MTA conforms to this specification. Once again, the 2028 receiving DNS domain name is used as the authserv-id. Furthermore, 2029 the sender authenticated herself/himself to the MTA via a method 2030 specified in [AUTH], and both SPF and Sender ID checks were done and 2031 passed. The MUA could extract and relay this extra information if 2032 desired. 2034 Two Authentication-Results header fields are not required since the 2035 same host did all of the checking. The authenticating agent could 2036 have consolidated all the results into one header field. 2038 This example illustrates a scenario in which a remote user on a 2039 dial-up connection (example.net) sends mail to a border MTA 2040 (example.com) using SMTP authentication to prove identity. The 2041 dial-up provider has been explicitly authorized to relay mail as 2042 example.com, producing "pass" results from the SPF and Sender ID 2043 checks. 2045 B.5. Service Provided, Several Authentications Done, Different MTAs 2047 A message that was relayed inbound by two different MTAs that conform 2048 to this specification and applied multiple message authentication 2049 checks: 2051 Authentication-Results: example.com; 2052 sender-id=fail header.from=example.com; 2053 dkim=pass (good signature) header.d=example.com 2054 Received: from mail-router.example.com 2055 (mail-router.example.com [192.0.2.1]) 2056 by auth-checker.example.com (8.11.6/8.11.6) 2057 with ESMTP id i7PK0sH7021929; 2058 Fri, Feb 15 2002 17:19:22 -0800 2059 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com; 2060 t=1188964191; c=simple/simple; h=From:Date:To:Subject: 2061 Message-Id:Authentication-Results; 2062 bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70; 2063 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 2064 Authentication-Results: example.com; 2065 auth=pass (cram-md5) smtp.auth=sender@example.com; 2066 spf=fail smtp.mailfrom=example.com 2067 Received: from dialup-1-2-3-4.example.net 2068 (dialup-1-2-3-4.example.net [192.0.2.200]) 2069 by mail-router.example.com (8.11.6/8.11.6) 2070 with ESMTPA id g1G0r1kA003489; 2071 Fri, Feb 15 2002 17:19:07 -0800 2072 From: sender@example.com 2073 Date: Fri, Feb 15 2002 16:54:30 -0800 2074 To: receiver@example.com 2075 Message-Id: <12345.abc@example.com> 2076 Subject: here's a sample 2078 Hello! Goodbye! 2080 Example 5: Headers Reporting Results from Multiple MTAs 2082 The Authentication-Results header field is present, indicating 2083 conformance to this specification. Once again, the authserv-id used 2084 is the recipient's DNS domain name. The header field is present 2085 twice because two different MTAs in the chain of delivery did 2086 authentication tests. The first MTA, mail-router.example.com, 2087 reports that SMTP AUTH and SPF were both used and that the former 2088 passed while the latter failed. In the SMTP AUTH case, additional 2089 information is provided in the comment field, which the MUA can 2090 choose to render if desired. 2092 The second MTA, auth-checker.example.com, reports that it did a 2093 Sender ID test (which failed) and a DKIM test (which passed). Again, 2094 additional data about one of the tests is provided as a comment, 2095 which the MUA may choose to render. Also noteworthy here is the fact 2096 that there is a DKIM signature added by example.com that assured the 2097 integrity of the lower Authentication-Results field. 2099 Since different hosts did the two sets of authentication checks, the 2100 header fields cannot be consolidated in this example. 2102 This example illustrates more typical transmission of mail into 2103 example.com from a user on a dial-up connection example.net. The 2104 user appears to be legitimate as he/she had a valid password allowing 2105 authentication at the border MTA using SMTP AUTH. The SPF and Sender 2106 ID tests failed since example.com has not granted example.net 2107 authority to relay mail on its behalf. However, the DKIM test passed 2108 because the sending user had a private key matching one of 2109 example.com's published public keys and used it to sign the message. 2111 B.6. Service Provided, Multi-tiered Authentication Done 2113 A message that had authentication done at various stages, one of 2114 which was outside the receiving ADMD: 2116 Authentication-Results: example.com; 2117 dkim=pass reason="good signature" 2118 header.i=@mail-router.example.net; 2119 dkim=fail reason="bad signature" 2120 header.i=@newyork.example.com 2121 Received: from mail-router.example.net 2122 (mail-router.example.net [192.0.2.250]) 2123 by chicago.example.com (8.11.6/8.11.6) 2124 for 2125 with ESMTP id i7PK0sH7021929; 2126 Fri, Feb 15 2002 17:19:22 -0800 2127 DKIM-Signature: v=1; a=rsa-sha256; s=furble; 2128 d=mail-router.example.net; t=1188964198; c=relaxed/simple; 2129 h=From:Date:To:Message-Id:Subject:Authentication-Results; 2130 bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=; 2131 b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= 2132 Authentication-Results: example.net; 2133 dkim=pass (good signature) header.i=@newyork.example.com 2134 Received: from smtp.newyork.example.com 2135 (smtp.newyork.example.com [192.0.2.220]) 2136 by mail-router.example.net (8.11.6/8.11.6) 2137 with ESMTP id g1G0r1kA003489; 2138 Fri, Feb 15 2002 17:19:07 -0800 2139 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; 2140 d=newyork.example.com; 2141 t=1188964191; c=simple/simple; 2142 h=From:Date:To:Message-Id:Subject; 2143 bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; 2144 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 2145 From: sender@newyork.example.com 2146 Date: Fri, Feb 15 2002 16:54:30 -0800 2147 To: meetings@example.net 2148 Message-Id: <12345.abc@newyork.example.com> 2149 Subject: here's a sample 2151 Example 6: Headers Reporting Results from Multiple MTAs in Different 2152 ADMDs 2154 In this example, we see multi-tiered authentication with an extended 2155 trust boundary. 2157 The message was sent from someone at example.com's New York office 2158 (newyork.example.com) to a mailing list managed at an intermediary. 2160 The message was signed at the origin using DKIM. 2162 The message was sent to a mailing list service provider called 2163 example.net, which is used by example.com. There, 2164 meetings@example.net is expanded to a long list of recipients, one of 2165 whom is at the Chicago office. In this example, we will assume that 2166 the trust boundary for chicago.example.com includes the mailing list 2167 server at example.net. 2169 The mailing list server there first authenticated the message and 2170 affixed an Authentication-Results header field indicating such using 2171 its DNS domain name for the authserv-id. It then altered the message 2172 by affixing some footer text to the body, including some 2173 administrivia such as unsubscription instructions. Finally, the 2174 mailing list server affixes a second DKIM signature and begins 2175 distribution of the message. 2177 The border MTA for chicago.example.com explicitly trusts results from 2178 mail-router.example.net, so that header field is not removed. It 2179 performs evaluation of both signatures and determines that the first 2180 (most recent) is a "pass" but, because of the aforementioned 2181 modifications, the second is a "fail". However, the first signature 2182 included the Authentication-Results header added at mail- 2183 router.example.net that validated the second signature. Thus, 2184 indirectly, it can be determined that the authentications claimed by 2185 both signatures are indeed valid. 2187 Note that two styles of presenting metadata about the result are in 2188 use here. In one case, the "reason=" clause is present, which is 2189 intended for easy extraction by parsers; in the other case, the CFWS 2190 production of the ABNF is used to include such data as a header field 2191 comment. The latter can be harder for parsers to extract given the 2192 varied supported syntaxes of mail header fields. 2194 B.7. Comment-Heavy Example 2196 The formal syntax permits comments within the content in a number of 2197 places. For the sake of illustration, this example is also legal: 2199 Authentication-Results: foo.example.net (foobar) 1 (baz); 2200 dkim (Because I like it) / 1 (One yay) = (wait for it) fail 2201 policy (A dot can go here) . (like that) expired 2202 (this surprised me) = (as I wasn't expecting it) 1362471462 2204 Example 7: A Very Comment-Heavy but Perfectly Legal Example 2206 Appendix C. Operational Considerations about Message Authentication 2208 This protocol is predicated on the idea that authentication (and 2209 presumably in the future, reputation) work is typically done by 2210 border MTAs rather than MUAs or intermediate MTAs; the latter merely 2211 make use of the results determined by the former. Certainly this is 2212 not mandatory for participation in electronic mail or message 2213 authentication, but this protocol and its deployment to date are 2214 based on that model. The assumption satisfies several common ADMD 2215 requirements: 2217 1. Service operators prefer to resolve the handling of problem 2218 messages as close to the border of the ADMD as possible. This 2219 enables, for example, rejection of messages at the SMTP level 2220 rather than generating a DSN internally. Thus, doing any of the 2221 authentication or reputation work exclusively at the MUA or 2222 intermediate MTA renders this desire unattainable. 2224 2. Border MTAs are more likely to have direct access to external 2225 sources of authentication or reputation information since modern 2226 MUAs are more likely to be heavily firewalled. Thus, some MUAs 2227 might not even be able to complete the task of performing 2228 authentication or reputation evaluations without complex proxy 2229 configurations or similar burdens. 2231 3. MUAs rely upon the upstream MTAs within their trust boundaries to 2232 make correct (as much as is possible) evaluations about the 2233 message's envelope, header, and content. Thus, MUAs don't need 2234 to know how to do the work that upstream MTAs do; they only need 2235 the results of that work. 2237 4. Evaluations about the quality of a message, from simple token 2238 matching (e.g., a list of preferred DNS domains) to cryptanalysis 2239 (e.g., public/private key work), do have a cost and thus need to 2240 be minimized. To that end, performing those tests at the border 2241 MTA is far preferred to doing that work at each MUA that handles 2242 a message. If an ADMD's environment adheres to common messaging 2243 protocols, a reputation query or an authentication check 2244 performed by a border MTA would return the same result as the 2245 same query performed by an MUA. By contrast, in an environment 2246 where the MUA does the work, a message arriving for multiple 2247 recipients would thus cause authentication or reputation 2248 evaluation to be done more than once for the same message (i.e., 2249 at each MUA), causing needless amplification of resource use and 2250 creating a possible denial-of-service attack vector. 2252 5. Minimizing change is good. As new authentication and reputation 2253 methods emerge, the list of methods supported by this header 2254 field would presumably be extended. If MUAs simply consume the 2255 contents of this header field rather than actually attempt to do 2256 authentication and/or reputation work, then MUAs only need to 2257 learn to parse this header field once; emergence of new methods 2258 requires only a configuration change at the MUAs and software 2259 changes at the MTAs (which are presumably fewer in number). When 2260 choosing to implement these functions in MTAs vs. MUAs, the 2261 issues of individual flexibility, infrastructure inertia, and 2262 scale of effort must be considered. It is typically easier to 2263 change a single MUA than an MTA because the modification affects 2264 fewer users and can be pursued with less care. However, changing 2265 many MUAs is more effort than changing a smaller number of MTAs. 2267 6. For decisions affecting message delivery and display, assessment 2268 based on authentication and reputation is best performed close to 2269 the time of message transit, as a message makes its journey 2270 toward a user's inbox, not afterwards. DKIM keys and IP address 2271 reputations, etc., can change over time or even become invalid, 2272 and users can take a long time to read a message once delivered. 2273 The value of this work thus degrades, perhaps quickly, once the 2274 delivery process has completed. This seriously diminishes the 2275 value of this work when done elsewhere than at MTAs. 2277 Many operational choices are possible within an ADMD, including the 2278 venue for performing authentication and/or reputation assessment. 2279 The current specification does not dictate any of those choices. 2280 Rather, it facilitates those cases in which information produced by 2281 one stage of analysis needs to be transported with the message to the 2282 next stage. 2284 Appendix D. Changes since RFC 7001 2286 o Applied RFC 7410. 2288 o Updated all references to RFC 4408 with RFC 7208. 2290 o Added section explaining "property" values. (Addressed Erratum 2291 #4201.) 2293 o Did some minor text reorganization. 2295 o Gave registry history -- enough that this is now the authoritative 2296 registry definition. 2298 o Added text explaining each of the method-ptype-property tuples 2299 registered by this document. 2301 o Changed the meaning of the "Defined" column of the methods 2302 registry to be the place where each entry was created and 2303 described; it is expected that this will then refer to the 2304 method's defining document. Provided IANA with corresponding 2305 update instructions. 2307 o Cleaned up registry structure and content, and replaced all 2308 references to RFC 7001 with pointers to this document. 2310 o Added references: [DMARC], [PRA], [RFC6008], [RFC6577], [RRVS], 2311 [SMIME-REG]. 2313 o Added description of values that can be extracted from SMTP AUTH 2314 sessions and an example. 2316 o Provided much more complete descriptions of reporting DomainKeys 2317 results. 2319 o Added more detail about Sender ID. 2321 o Marked all ADSP and DomainKeys entries as deprecated since their 2322 defining documents are as well. 2324 o Reworked some text around ignoring unknown ptypes. 2326 o Completely described the ptypes registry. 2328 o Mentioned that EHLO is mapped to HELO for SPF. 2330 o RFC 7208 uses all-lowercase result strings now, so adjusted prose 2331 accordingly. 2333 o Updated list of supported methods, and mentioned the registries 2334 immediately below. 2336 o Mentioned that when a local-part is removed, the "@" goes with it. 2338 o Referred to RFC 7328 in the "iprev" definition. 2340 o Corrected the "smime-part" prose. 2342 o Updated examples that use SMTP AUTH to claim "with ESMTPA" in the 2343 Received fields. 2345 o Made minor editorial adjustments. 2347 Appendix E. Acknowledgments 2349 The author wishes to acknowledge the following individuals for their 2350 review and constructive criticism of this document: TBD 2352 Author's Address 2354 Murray S. Kucherawy 2355 270 Upland Drive 2356 San Francisco, CA 94127 2357 United States 2359 Email: superuser@gmail.com