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'PTYPES-REGISTRY') (Obsoleted by RFC 7601) -- Obsolete informational reference (is this intentional?): RFC 5451 (Obsoleted by RFC 7001) -- Obsolete informational reference (is this intentional?): RFC 6577 (Obsoleted by RFC 7001) -- Obsolete informational reference (is this intentional?): RFC 7001 (Obsoleted by RFC 7601) Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Individual submission M. Kucherawy 3 Internet-Draft April 21, 2015 4 Obsoletes: 7001, 7410 5 (if approved) 6 Intended status: Standards Track 7 Expires: October 23, 2015 9 Message Header Field for Indicating Message Authentication Status 10 draft-ietf-appsawg-rfc7001bis-07 12 Abstract 14 This document specifies a message header field called Authentication- 15 Results for use with electronic mail messages to indicate the results 16 of message authentication efforts. Any receiver-side software, such 17 as mail filters or Mail User Agents (MUAs), can use this header field 18 to relay that information in a convenient and meaningful way to users 19 or to make sorting and filtering decisions. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on October 23, 2015. 38 Copyright Notice 40 Copyright (c) 2015 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6 58 1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 59 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 60 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 61 1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 62 1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . . 7 63 1.5.3. Email Architecture . . . . . . . . . . . . . . . . . . 8 64 1.5.4. Other Terms . . . . . . . . . . . . . . . . . . . . . 9 65 1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9 66 2. Definition and Format of the Header Field . . . . . . . . . . 9 67 2.1. General Description . . . . . . . . . . . . . . . . . . . 9 68 2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10 69 2.3. Property Types (ptypes) and Properties . . . . . . . . . . 12 70 2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 13 71 2.5. Authentication Identifier Field . . . . . . . . . . . . . 14 72 2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 15 73 2.7. Defined Methods and Result Values . . . . . . . . . . . . 16 74 2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16 75 2.7.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 17 76 2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 19 77 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 19 78 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 21 79 2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21 80 2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 22 81 3. The "iprev" Authentication Method . . . . . . . . . . . . . . 22 82 4. Adding the Header Field to a Message . . . . . . . . . . . . . 23 83 4.1. Header Field Position and Interpretation . . . . . . . . . 25 84 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 26 85 5. Removing Existing Header Fields . . . . . . . . . . . . . . . 26 86 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 87 6.1. The Authentication-Results Header Field . . . . . . . . . 27 88 6.2. "Email Authentication Methods" Registry Description . . . 28 89 6.3. "Email Authentication Methods" Registry Update . . . . . . 29 90 6.4. "Email Authentication Property Types" Registry . . . . . . 30 91 6.5. "Email Authentication Result Names" Description . . . . . 31 92 6.6. "Email Authentication Result Names" Update . . . . . . . . 31 93 6.7. SMTP Enhanced Stauts Codes . . . . . . . . . . . . . . . . 32 94 7. Security Considerations . . . . . . . . . . . . . . . . . . . 32 95 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 32 96 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 34 97 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 34 98 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 35 99 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 35 100 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 35 101 7.7. Attacks against Authentication Methods . . . . . . . . . . 35 102 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 35 103 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 36 104 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 36 105 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 36 106 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36 107 8.1. Normative References . . . . . . . . . . . . . . . . . . . 36 108 8.2. Informative References . . . . . . . . . . . . . . . . . . 37 109 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 39 110 Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 39 111 Appendix C. Authentication-Results Examples . . . . . . . . . . . 40 112 C.1. Trivial Case; Header Field Not Present . . . . . . . . . . 40 113 C.2. Nearly Trivial Case; Service Provided, but No 114 Authentication Done . . . . . . . . . . . . . . . . . . . 41 115 C.3. Service Provided, Authentication Done . . . . . . . . . . 42 116 C.4. Service Provided, Several Authentications Done, Single 117 MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 118 C.5. Service Provided, Several Authentications Done, 119 Different MTAs . . . . . . . . . . . . . . . . . . . . . . 44 120 C.6. Service Provided, Multi-Tiered Authentication Done . . . . 46 121 C.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 47 122 Appendix D. Operational Considerations about Message 123 Authentication . . . . . . . . . . . . . . . . . . . 48 124 Appendix E. Change History . . . . . . . . . . . . . . . . . . . 49 125 E.1. RFC7001 to -00 . . . . . . . . . . . . . . . . . . . . . . 49 126 E.2. -00 to -01 . . . . . . . . . . . . . . . . . . . . . . . . 50 127 E.3. -01 to -02 . . . . . . . . . . . . . . . . . . . . . . . . 50 128 E.4. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . . 50 129 E.5. -03 to -04 . . . . . . . . . . . . . . . . . . . . . . . . 50 130 E.6. -04 to -05 . . . . . . . . . . . . . . . . . . . . . . . . 51 131 E.7. -05 to -06 . . . . . . . . . . . . . . . . . . . . . . . . 51 132 E.8. -06 to -07 . . . . . . . . . . . . . . . . . . . . . . . . 51 134 1. Introduction 136 This document describes a header field called Authentication-Results 137 for electronic mail messages that presents the results of a message 138 authentication effort in a machine-readable format. The intent of 139 the header field is to create a place to collect such data when 140 message authentication mechanisms are in use so that a Mail User 141 Agent (MUA) and downstream filters can make filtering decisions 142 and/or provide a recommendation to the user as to the validity of the 143 message's origin and possibly the safety and integrity of its 144 content. 146 This document revises the original definition found in [RFC5451] 147 based upon various authentication protocols in current use and 148 incorporates errata logged since the publication of the original 149 specification. 151 End users are not expected to be direct consumers of this header 152 field. This header field is intended for consumption by programs 153 that will then use such data or render it in a human-usable form. 155 This document specifies the format of this header field and discusses 156 the implications of its presence or absence. However, it does not 157 discuss how the data contained in the header field ought to be used, 158 such as what filtering decisions are appropriate or how an MUA might 159 render those results, as these are local policy and/or user interface 160 design questions that are not appropriate for this document. 162 At the time of publication of this document, the following are 163 published email authentication methods: 165 o Author Domain Signing Practices ([ADSP]) (Historic) 167 o SMTP Service Extension for Authentication ([AUTH]) 169 o DomainKeys Identified Mail Signatures ([DKIM]) 171 o Domain-based Message Authentication, Reporting and Conformance 172 ([DMARC]) 174 o Sender Policy Framework ([SPF]) 176 o reverse IP address name validation ("iprev", defined in Section 3) 178 o Require-Recipient-Valid-Since Header Field and SMTP Service 179 Extension ([RRVS]) 181 o S/MIME Signature Verification ([SMIME-REG]) 183 o Vouch By Reference ([VBR]) 185 o DomainKeys ([DOMAINKEYS]) (Historic) 187 o Sender ID ([SENDERID]) (Experimental) 189 There exist registries for tokens used within this header field that 190 refer to the specifications listed above. Section 6 describes the 191 registries and their contents, and specifies the process by which 192 entries are added or updated. It also updates the existing contents 193 to match the current states of these specifications. 195 This specification is not intended to be restricted to domain-based 196 authentication schemes, but the existing schemes in that family have 197 proven to be a good starting point for implementations. The goal is 198 to give current and future authentication schemes a common framework 199 within which to deliver their results to downstream agents and 200 discourage the creation of unique header fields for each. 202 Although SPF defined a header field called "Received-SPF" and the 203 historic DomainKeys defined one called "DomainKey-Status" for this 204 purpose, those header fields are specific to the conveyance of their 205 respective results only and thus are insufficient to satisfy the 206 requirements enumerated below. In addition, many SPF implementations 207 have adopted the header field specified here at least as an option, 208 and DomainKeys has been obsoleted by DKIM. 210 1.1. Purpose 212 The header field defined in this document is expected to serve 213 several purposes: 215 1. Convey the results of various message authentication checks, 216 which are applied by upstream filters and Mail Transfer Agents 217 (MTAs) and then passed to MUAs and downstream filters within the 218 same "trust domain". Such agents might wish to render those 219 results to end users or to use those data to apply more or less 220 stringent content checks based on authentication results; 222 2. Provide a common location within a message for this data; 224 3. Create an extensible framework for reporting new authentication 225 methods as they emerge. 227 In particular, the mere presence of this header field does not mean 228 its contents are valid. Rather, the header field is reporting 229 assertions made by one or more authentication schemes (supposedly) 230 applied somewhere upstream. For an MUA or downstream filter to treat 231 the assertions as actually valid, there must be an assessment of the 232 trust relationship among such agents, the validating MTA, and the 233 mechanism for conveying the information. 235 1.2. Trust Boundary 237 This document makes several references to the "trust boundary" of an 238 administrative management domain (ADMD). Given the diversity among 239 existing mail environments, a precise definition of this term isn't 240 possible. 242 Simply put, a transfer from the producer of the header field to the 243 consumer must occur within a context that permits the consumer to 244 treat assertions by the producer as being reliable and accurate 245 (trustworthy). How this trust is obtained is outside the scope of 246 this document. It is entirely a local matter. 248 Thus, this document defines a "trust boundary" as the delineation 249 between "external" and "internal" entities. Services that are 250 internal -- within the trust boundary -- are provided by the ADMD's 251 infrastructure for its users. Those that are external are outside of 252 the authority of the ADMD. By this definition, hosts that are within 253 a trust boundary are subject to the ADMD's authority and policies, 254 independent of their physical placement or their physical operation. 255 For example, a host within a trust boundary might actually be 256 operated by a remote service provider and reside physically within 257 its data center. 259 It is possible for a message to be evaluated inside a trust boundary 260 but then depart and re-enter the trust boundary. An example might be 261 a forwarded message such as a message/rfc822 attachment (see 262 Multipurpose Internet Mail Extensions [MIME]) or one that is part of 263 a multipart/digest. The details reported by this field cannot be 264 trusted in that case. Thus, this field found within one of those 265 media types is typically ignored. 267 1.3. Processing Scope 269 The content of this header field is meant to convey to message 270 consumers that authentication work on the message was already done 271 within its trust boundary, and those results are being presented. It 272 is not intended to provide message parameters to consumers so that 273 they can perform authentication protocols on their own. 275 1.4. Requirements 277 This document establishes no new requirements on existing protocols 278 or servers. 280 In particular, this document establishes no requirement on MTAs to 281 reject or filter arriving messages that do not pass authentication 282 checks. The data conveyed by the specified header field's contents 283 are for the information of MUAs and filters and are to be used at 284 their discretion. 286 1.5. Definitions 288 This section defines various terms used throughout this document. 290 1.5.1. Key Words 292 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 293 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 294 document are to be interpreted as described in [KEYWORDS]. 296 1.5.2. Security 298 "Guidelines for Writing RFC Text on Security Considerations" 299 ([SECURITY]) discusses authentication and authorization and the 300 conflation of the two concepts. The use of those terms within the 301 context of recent message security work has given rise to slightly 302 different definitions, and this document reflects those current 303 usages, as follows: 305 o "Authorization" is the establishment of permission to use a 306 resource or represent an identity. In this context, authorization 307 indicates that a message from a particular ADMD arrived via a 308 route the ADMD has explicitly approved. 310 o "Authentication" is the assertion of validity of a piece of data 311 about a message (such as the sender's identity) or the message in 312 its entirety. 314 As examples: SPF and Sender ID are authorization mechanisms in that 315 they express a result that shows whether or not the ADMD that 316 apparently sent the message has explicitly authorized the connecting 317 Simple Mail Transfer Protocol ([SMTP]) client to relay messages on 318 its behalf, but they do not actually validate any other property of 319 the message itself. By contrast, DKIM is agnostic as to the routing 320 of a message but uses cryptographic signatures to authenticate 321 agents, assign (some) responsibility for the message (which implies 322 authorization), and ensure that the listed portions of the message 323 were not modified in transit. Since the signatures are not tied to 324 SMTP connections, they can be added by either the ADMD of origin, 325 intermediate ADMDs (such as a mailing list server), other handling 326 agents, or any combination. 328 Rather than create a separate header field for each class of 329 solution, this proposal groups them both into a single header field. 331 1.5.3. Email Architecture 333 o A "border MTA" is an MTA that acts as a gateway between the 334 general Internet and the users within an organizational boundary. 335 (See also Section 1.2.) 337 o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that 338 actually enacts delivery of a message to a user's inbox or other 339 final delivery. 341 o An "intermediate MTA" is any MTA that is not a delivery MTA and is 342 also not the first MTA to handle the message. 344 The following diagram illustrates the flow of mail among these 345 defined components. See Internet Mail Architecture [EMAIL-ARCH] for 346 further discussion on general email system architecture, which 347 includes detailed descriptions of these components, and Appendix D of 348 this document for discussion about the common aspects of email 349 authentication in current environments. 351 +-----+ +-----+ +------------+ 352 | MUA |-->| MSA |-->| Border MTA | 353 +-----+ +-----+ +------------+ 354 | 355 | 356 V 357 +----------+ 358 | Internet | 359 +----------+ 360 | 361 | 362 V 363 +-----+ +-----+ +------------------+ +------------+ 364 | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA | 365 +-----+ +-----+ +------------------+ +------------+ 367 Generally, it is assumed that the work of applying message 368 authentication schemes takes place at a border MTA or a delivery MTA. 369 This specification is written with that assumption in mind. However, 370 there are some sites at which the entire mail infrastructure consists 371 of a single host. In such cases, such terms as "border MTA" and 372 "delivery MTA" might well apply to the same machine or even the very 373 same agent. It is also possible that some message authentication 374 tests could take place on an intermediate MTA. Although this 375 document doesn't specifically describe such cases, they are not meant 376 to be excluded. 378 1.5.4. Other Terms 380 In this document, the term "producer" refers to any component that 381 adds this header field to messages it is handling, and "consumer" 382 refers to any component that identifies, extracts, and parses the 383 header field to use as part of a handling decision. 385 1.6. Trust Environment 387 This header field permits one or more message validation mechanisms 388 to communicate output to one or more separate assessment mechanisms. 389 These mechanisms operate within a unified trust boundary that defines 390 an Administrative Management Domain (ADMD). An ADMD contains one or 391 more entities that perform validation and generate the header field 392 and one or more that consume it for some type of assessment. The 393 field often contains no integrity or validation mechanism of its own, 394 so its presence must be trusted implicitly. Hence, valid use of the 395 header field requires removing any occurrences of it that are present 396 when the message enters the ADMD. This ensures that later 397 occurrences have been added within the trust boundary of the ADMD. 399 The authserv-id token defined in Section 2.2 can be used to reference 400 an entire ADMD or a specific validation engine within an ADMD. 401 Although the labeling scheme is left as an operational choice, some 402 guidance for selecting a token is provided in later sections of this 403 document. 405 2. Definition and Format of the Header Field 407 This section gives a general overview of the format of the header 408 field being defined and then provides more formal specification. 410 2.1. General Description 412 The header field specified here is called Authentication-Results. It 413 is a Structured Header Field as defined in Internet Message Format 414 ([MAIL]), and thus all of the related definitions in that document 415 apply. 417 This header field is added at the top of the message as it transits 418 MTAs that do authentication checks, so some idea of how far away the 419 checks were done can be inferred. It is therefore considered to be a 420 trace field as defined in [MAIL], and thus all of the related 421 definitions in that document apply. 423 The value of the header field (after removing comments) consists of 424 an authentication identifier, an optional version, and then a series 425 of statements and supporting data. The statements are of the form 426 "method=result" and indicate which authentication method(s) were 427 applied and their respective results. For each such statement, the 428 supporting data can include a "reason" string and one or more 429 "property=value" statements indicating which message properties were 430 evaluated to reach that conclusion. 432 The header field can appear more than once in a single message, more 433 than one result can be represented in a single header field, or a 434 combination of these can be applied. 436 2.2. Formal Definition 438 Formally, the header field is specified as follows using Augmented 439 Backus-Naur Form ([ABNF]): 441 authres-header = "Authentication-Results:" [CFWS] authserv-id 442 [ CFWS authres-version ] 443 ( no-result / 1*resinfo ) [CFWS] CRLF 445 authserv-id = value 446 ; see below for a description of this element 448 authres-version = 1*DIGIT [CFWS] 449 ; indicates which version of this specification is in use; 450 ; this specification is version "1", and the absence of a 451 ; version implies this version of the specification 453 no-result = [CFWS] ";" [CFWS] "none" 454 ; the special case of "none" is used to indicate that no 455 ; message authentication was performed 457 resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ] 458 *( CFWS propspec ) 460 methodspec = [CFWS] method [CFWS] "=" [CFWS] result 461 ; indicates which authentication method was evaluated 462 ; and what its output was 464 reasonspec = "reason" [CFWS] "=" [CFWS] value 465 ; a free-form comment on the reason the given result 466 ; was returned 468 propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue 469 ; an indication of which properties of the message 470 ; were evaluated by the authentication scheme being 471 ; applied to yield the reported result 473 method = Keyword [ [CFWS] "/" [CFWS] method-version ] 474 ; a method indicates which method's result is 475 ; represented by "result", and is one of the methods 476 ; explicitly defined as valid in this document 477 ; or is an extension method as defined below 479 method-version = 1*DIGIT [CFWS] 480 ; indicates which version of the method specification is 481 ; in use, corresponding to the matching entry in the IANA 482 ; "Email Authentication Methods" registry; a value of "1" 483 ; is assumed if this version string is absent 485 result = Keyword 486 ; indicates the results of the attempt to authenticate 487 ; the message; see below for details 489 ptype = Keyword 490 ; indicates whether the property being evaluated was 491 ; a parameter to an [SMTP] command, was a value taken 492 ; from a message header field, was some property of 493 ; the message body, or was some other property evaluated by 494 ; the receiving MTA; expected to be one of the "property 495 ; types" explicitly defined as valid, or an extension 496 ; ptype, as defined below 498 property = special-smtp-verb / Keyword 499 ; indicates more specifically than "ptype" what the 500 ; source of the evaluated property is; the exact meaning 501 ; is specific to the method whose result is being reported 502 ; and is defined more clearly below 504 special-smtp-verb = "mailfrom" / "rcptto" 505 ; special cases of [SMTP] commands that are made up 506 ; of multiple words 508 pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) 509 [CFWS] 510 ; the value extracted from the message property defined 511 ; by the "ptype.property" construction 513 "local-part" is defined in Section 3.4.1 of [MAIL], and "CFWS" is 514 defined in Section 3.2.2 of [MAIL]. 516 "Keyword" is defined in Section 4.1.2 of [SMTP]. 518 The "value" is as defined in Section 5.1 of [MIME]. 520 The "domain-name" is as defined in Section 3.5 of [DKIM]. 522 The "Keyword" used in "result" above is further constrained by the 523 necessity of being enumerated in Section 2.7. 525 See Section 2.5 for a description of the authserv-id element. 527 If the value portion of a "pvalue" construction identifies something 528 intended to be an e-mail identity, then it MUST use the right hand 529 portion of that ABNF definition. 531 The list of commands eligible for use with the "smtp" ptype can be 532 found in Section 4.1 of [SMTP]. 534 The "propspec" may be omitted if, for example, the method was unable 535 to extract any properties to do its evaluation yet has a result to 536 report. 538 Where an SMTP command name is being reported as a "property", the 539 agent generating the header field represents that command by 540 converting it to lowercase and dropping any spaces (e.g., "MAIL FROM" 541 becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.). 543 A "ptype" value of "policy" indicates a policy decision about the 544 message not specific to a property of the message that could be 545 extracted. See Section 2.4 for details. 547 Examples of complete messages using this header field can be found in 548 Appendix C. 550 2.3. Property Types (ptypes) and Properties 552 The "ptype" in the ABNF above indicates the general type of property 553 being described by the result being reported, upon which the reported 554 result was based. Coupled with the "property", which is more 555 specific, they indicate from which particular part of the message the 556 reported data were extracted. 558 Combinations of ptypes and properties are registered and described in 559 the "Email Authentication Methods" registry, coupled with the 560 authentication methods with which they are used. This is further 561 described in Section 6. 563 Legal values of "ptype" are as defined in the IANA "Email 564 Authentication Property Types" registry, created by 565 [PTYPES-REGISTRY]. The initial values and what they typically 566 indicate are as follows, copied from [RFC7001]: 568 body: Information that was extracted from the body of the message. 569 This might be an arbitrary string of bytes, a hash of a string of 570 bytes, a Uniform Resource Identifier, or some other content of 571 interest. The "property" is an indication of where within the 572 message body the extracted content was found, and can indicate an 573 offset, identify a MIME part, etc. 575 header: Indicates information that was extracted from the header of 576 the message. This might be the value of a header field or some 577 portion of a header field. The "property" gives a more precise 578 indication of the place in the header from which the extraction 579 took place. 581 policy: A local policy mechanism was applied that augments or 582 overrides the result returned by the authentication mechanism. 583 (See Section 2.4.) 585 smtp: Indicates information that was extracted from an SMTP command 586 that was used to relay the message. The "property" indicates 587 which SMTP command included the extracted content as a parameter. 589 Results reported using unknown ptypes MUST NOT be used in making 590 handling decisions. They can be safely ignored by consumers. 592 Entries in the "Email Authentication Methods" registry can define 593 properties that deviate from these definitions when appropriate. 594 Such deviations need to be clear in the registry and/or in the 595 defining document. See Section 2.7.1 for an example. 597 2.4. The "policy" ptype 599 A special ptype value of "policy" is also defined. This ptype is 600 provided to indicate that some local policy mechanism was applied 601 that augments or even replaces (i.e., overrides) the result returned 602 by the authentication mechanism. The property and value in this case 603 identify the local policy that was applied and the result it 604 returned. 606 For example, a DKIM signature is not required to include the Subject 607 header field in the set of fields that are signed. An ADMD receiving 608 such a message might decide that such a signature is unacceptable, 609 even if it passes, because the content of the Subject header field 610 could be altered post-signing without invalidating the signature. 611 Such an ADMD could replace the DKIM "pass" result with a "policy" 612 result and then also include the following in the corresponding 613 Authentication-Result field: 615 ... dkim=fail policy.dkim-rules=unsigned-subject ... 617 In this case, the property is "dkim-rules", indicating some local 618 check by that name took place and that check returned a result of 619 "unsigned-subject". These are arbitrary names selected by (and 620 presumably used within) the ADMD making use of them, so they are not 621 normally registered with IANA or otherwise specified apart from 622 setting syntax restrictions that allow for easy parsing within the 623 rest of the header field. 625 This ptype existed in the original specification for this header 626 field, but without a complete description or example of intended use. 627 As a result, it has not seen any practical use to date that matches 628 its intended purpose. These added details are provided to guide 629 implementers toward proper use. 631 2.5. Authentication Identifier Field 633 Every Authentication-Results header field has an authentication 634 service identifier field (authserv-id above). Specifically, this is 635 any string intended to identify the authentication service within the 636 ADMD that conducted authentication checks on the message. This 637 identifier is intended to be machine-readable and not necessarily 638 meaningful to users. 640 Since agents consuming this field will use this identifier to 641 determine whether its contents are of interest (and are safe to use), 642 the uniqueness of the identifier MUST be guaranteed by the ADMD that 643 generates it and MUST pertain to that ADMD. MUAs or downstream 644 filters SHOULD use this identifier to determine whether or not the 645 data contained in an Authentication-Results header field ought to be 646 used or ignored. 648 For simplicity and scalability, the authentication service identifier 649 SHOULD be a common token used throughout the ADMD. Common practice 650 is to use the DNS domain name used by or within that ADMD, sometimes 651 called the "organizational domain", but this is not strictly 652 necessary. 654 For tracing and debugging purposes, the authentication identifier can 655 instead be the specific hostname of the MTA performing the 656 authentication check whose result is being reported. Moreover, some 657 implementations define a substructure to the identifier; these are 658 outside of the scope of this specification. 660 Note, however, that using a local, relative identifier like a flat 661 hostname, rather than a hierarchical and globally unique ADMD 662 identifier like a DNS domain name, makes configuration more difficult 663 for large sites. The hierarchical identifier permits aggregating 664 related, trusted systems together under a single, parent identifier, 665 which in turn permits assessing the trust relationship with a single 666 reference. The alternative is a flat namespace requiring 667 individually listing each trusted system. Since consumers will use 668 the identifier to determine whether to use the contents of the header 669 field: 671 o Changes to the identifier impose a large, centralized 672 administrative burden. 674 o Ongoing administrative changes require constantly updating this 675 centralized table, making it difficult to ensure that an MUA or 676 downstream filter will have access to accurate information for 677 assessing the usability of the header field's content. In 678 particular, consumers of the header field will need to know not 679 only the current identifier(s) in use but previous ones as well to 680 account for delivery latency or later re-assessment of the header 681 field's contents. 683 Examples of valid authentication identifiers are "example.com", 684 "mail.example.org", "ms1.newyork.example.com", and "example-auth". 686 2.6. Version Tokens 688 The grammar above provides for the optional inclusion of versions on 689 both the header field itself (attached to the authserv-id token) and 690 on each of the methods being reported. The method version refers to 691 the method itself, which is specified in the documents describing 692 those methods, while the authserv-id version refers to this document 693 and thus the syntax of this header field. 695 The purpose of including these is to avoid misinterpretation of the 696 results. That is, if a parser finds a version after an authserv-id 697 that it does not explicitly know, it can immediately discontinue 698 trying to parse since what follows might not be in an expected 699 format. For a method version, the parser SHOULD ignore a method 700 result if the version is not supported in case the semantics of the 701 result have a different meaning than what is expected. For example, 702 if a hypothetical DKIM version 2 yielded a "pass" result for 703 different reasons than version 1 does, a consumer of this field might 704 not want to use the altered semantics. Allowing versions in the 705 syntax is a way to indicate this and let the consumer of the header 706 field decide. 708 2.7. Defined Methods and Result Values 710 Each individual authentication method returns one of a set of 711 specific result values. The subsections below provide references to 712 the documents defining the authentication methods specifically 713 supported by this document, and their corresponding result values. 714 Verifiers SHOULD use these values as described below. New methods 715 not specified in this document, but intended to be supported by the 716 header field defined here, MUST include a similar result table either 717 in their defining documents or in supplementary ones. 719 2.7.1. DKIM and DomainKeys 721 DKIM is represented by the "dkim" method and is defined in [DKIM]. 722 DomainKeys is defined in [DOMAINKEYS] and is represented by the 723 "domainkeys" method. 725 Section 3.8 of [DOMAINKEYS] enumerates some possible results of a 726 DomainKeys evaluation. Those results are not used when generating 727 this header field; rather, the results returned are listed below. 729 A signature is "acceptable to the ADMD" if it passes local policy 730 checks (or there are no specific local policy checks). For example, 731 an ADMD policy might require that the signature(s) on the message be 732 added using the DNS domain present in the From header field of the 733 message, thus making third-party signatures unacceptable even if they 734 verify. 736 Both DKIM and DomainKeys use the same result set, as follows: 738 none: The message was not signed. 740 pass: The message was signed, the signature or signatures were 741 acceptable to the ADMD, and the signature(s) passed verification 742 tests. 744 fail: The message was signed and the signature or signatures were 745 acceptable to the ADMD, but they failed the verification test(s). 747 policy: The message was signed, but some aspect of the signature or 748 signatures was not acceptable to the ADMD. 750 neutral: The message was signed, but the signature or signatures 751 contained syntax errors or were not otherwise able to be 752 processed. This result is also used for other failures not 753 covered elsewhere in this list. 755 temperror: The message could not be verified due to some error that 756 is likely transient in nature, such as a temporary inability to 757 retrieve a public key. A later attempt may produce a final 758 result. 760 permerror: The message could not be verified due to some error that 761 is unrecoverable, such as a required header field being absent. A 762 later attempt is unlikely to produce a final result. 764 DKIM results are reported using a ptype of "header". The property, 765 however, represents one of the tags found in the DKIM-Signature 766 header field rather than a distinct header field. For example, the 767 ptype-property combination "header.d" refers to the content of the 768 "d" (signing domain) tag from within the signature header field, and 769 not a distinct header field called "d". 771 The ability to report different DKIM results for a multiply-signed 772 message is described in [RFC6008]. 774 [DKIM] advises that if a message fails verification, it is to be 775 treated as an unsigned message. A report of "fail" here permits the 776 receiver of the report to decide how to handle the failure. A report 777 of "neutral" or "none" preempts that choice, ensuring the message 778 will be treated as if it had not been signed. 780 Section 3.1 of [DOMAINKEYS] describes a process by which the sending 781 address of the message is determined. DomainKeys results are thus 782 reported along with the signing domain name, the sending address of 783 the message, and the name of the header field from which the latter 784 was extracted. This means that a DomainKeys result includes a ptype- 785 property combination of "header.d", plus one of "header.from" and 786 "header.sender". The sending address extracted from the header is 787 included with any [MAIL]-style comments removed; moreover, the local- 788 part of the address and the "@" character are removed if it has not 789 been authenticated in some way. 791 2.7.2. SPF and Sender ID 793 SPF and Sender ID use the "spf" and "sender-id" method names, 794 respectively. The result values for SPF are defined in Section 2.6 795 of [SPF], and those definitions are included here by reference: 797 +-----------+--------------------------------+ 798 | Code | Meaning | 799 +-----------+--------------------------------+ 800 | none | [RFC7208], Section 2.6.1 | 801 +-----------+--------------------------------+ 802 | pass | [RFC7208], Section 2.6.3 | 803 +-----------+--------------------------------+ 804 | fail | [RFC7208], Section 2.6.4 | 805 +-----------+--------------------------------+ 806 | softfail | [RFC7208], Section 2.6.5 | 807 +-----------+--------------------------------+ 808 | policy | [this RFC], Section 2.4 | 809 +-----------+--------------------------------+ 810 | neutral | [RFC7208], Section 2.6.2 | 811 +-----------+--------------------------------+ 812 | temperror | [RFC7208], Section 2.6.6 | 813 +-----------+--------------------------------+ 814 | permerror | [RFC7208], Section 2.6.7 | 815 +-----------+--------------------------------+ 817 These result codes are used in the context of this specification to 818 reflect the result returned by the component conducting SPF 819 evaluation. 821 For SPF, the ptype used is "smtp", and the property is either 822 "mailfrom" or "helo", since those values are the ones SPF can 823 evaluate. (If the SMTP client issued the EHLO command instead of 824 HELO, the property used is "helo".) 826 The "sender-id" method is described in [SENDERID]. For this method, 827 the ptype used is "header" and the property will be the name of the 828 header field from which the Purported Responsible address (see [PRA]) 829 was extracted, namely one of "Resent-Sender", "Resent-From", 830 "Sender", or "From". 832 The results for Sender ID are listed and described in Section 4.2 of 833 [SENDERID], but for the purposes of this specification, the SPF 834 definitions enumerated above are used instead. Also, [SENDERID] 835 specifies result codes that use mixed case, but they are typically 836 used all lowercase in this context. 838 For both methods, an additional result of "policy" is defined, which 839 means the client was authorized to inject or relay mail on behalf of 840 the sender's DNS domain according to the authentication method's 841 algorithm, but local policy dictates that the result is unacceptable. 842 For example, "policy" might be used if SPF returns a "pass" result, 843 but a local policy check matches the sending DNS domain to one found 844 in an explicit list of unacceptable DNS domains (e.g., spammers). 846 If the retrieved sender policies used to evaluate SPF and Sender ID 847 do not contain explicit provisions for authenticating the local-part 848 (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported 849 along with results for these mechanisms SHOULD NOT include the local- 850 part or the following "@" character. 852 2.7.3. "iprev" 854 The result values used by the "iprev" method, defined in Section 3, 855 are as follows: 857 pass: The DNS evaluation succeeded, i.e., the "reverse" and 858 "forward" lookup results were returned and were in agreement. 860 fail: The DNS evaluation failed. In particular, the "reverse" and 861 "forward" lookups each produced results, but they were not in 862 agreement, or the "forward" query completed but produced no 863 result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an 864 RCODE of 0 (NOERROR) in a reply containing no answers, was 865 returned. 867 temperror: The DNS evaluation could not be completed due to some 868 error that is likely transient in nature, such as a temporary DNS 869 error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or 870 other error condition resulted. A later attempt may produce a 871 final result. 873 permerror: The DNS evaluation could not be completed because no PTR 874 data are published for the connecting IP address, e.g., a DNS 875 RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) 876 in a reply containing no answers, was returned. This prevented 877 completion of the evaluation. A later attempt is unlikely to 878 produce a final result. 880 There is no "none" for this method since any TCP connection 881 delivering email has an IP address associated with it, so some kind 882 of evaluation will always be possible. 884 The result is reported using a ptype of "policy" (as this is not part 885 of any established protocol) and a property of "iprev". 887 For discussion of the format of DNS replies, see "Domain Names - 888 Implementation and Specification" ([DNS]). 890 2.7.4. SMTP AUTH 892 SMTP AUTH (defined in [AUTH]) is represented by the "auth" method. 893 Its result values are as follows: 895 none: SMTP authentication was not attempted. 897 pass: The SMTP client authenticated to the server reporting the 898 result using the protocol described in [AUTH]. 900 fail: The SMTP client attempted to authenticate to the server using 901 the protocol described in [AUTH] but was not successful (such as 902 providing a valid identity but an incorrect password). 904 temperror: The SMTP client attempted to authenticate using the 905 protocol described in [AUTH] but was not able to complete the 906 attempt due to some error that is likely transient in nature, such 907 as a temporary directory service lookup error. A later attempt 908 may produce a final result. 910 permerror: The SMTP client attempted to authenticate using the 911 protocol described in [AUTH] but was not able to complete the 912 attempt due to some error that is likely not transient in nature, 913 such as a permanent directory service lookup error. A later 914 attempt is not likely to produce a final result. 916 The result of AUTH is reported using a ptype of "smtp" and a property 917 of either: 919 o "auth", in which case the value is the authorization identity 920 generated by the exchange initiated by the AUTH command; or 922 o "mailfrom", in which case the value is the mailbox identified by 923 the AUTH parameter used with the MAIL FROM command. 925 If both identities are available, both can be reported. For example, 926 consider this command issued by a client that has completed session 927 authentication with the AUTH command resulting in an authorized 928 identity of "client@c.example": 930 MAIL FROM: AUTH= 932 This could result in a resinfo construction like so: 934 ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example 936 Note that in all cases other than "pass", the message was sent by an 937 unauthenticated client. All non-"pass" cases SHOULD thus be treated 938 as equivalent with respect to this method. 940 2.7.5. Other Registered Codes 942 Result codes were also registered in other RFCs as follows: 944 o Vouch By Reference (in [AR-VBR], represented by "vbr"); 946 o Authorized Third-Party Signatures (in [ATPS], represented by 947 "dkim-atps"); 949 o Author Domain Signing Practices (in [ADSP], represented by "dkim- 950 adsp"); 952 o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs"); 954 o S/MIME (in [SMIME-REG], represented by "smime"). 956 2.7.6. Extension Methods 958 Additional authentication method identifiers (extension methods) may 959 be defined in the future by later revisions or extensions to this 960 specification. These method identifiers are registered with the 961 Internet Assigned Numbers Authority (IANA) and, preferably, published 962 in an RFC. See Section 6 for further details. 964 Extension methods can be defined for the following reasons: 966 1. To allow additional information from new authentication systems 967 to be communicated to MUAs or downstream filters. The names of 968 such identifiers ought to reflect the name of the method being 969 defined but ought not be needlessly long. 971 2. To allow the creation of "sub-identifiers" that indicate 972 different levels of authentication and differentiate between 973 their relative strengths, e.g., "auth1-weak" and "auth1-strong". 975 Authentication method implementers are encouraged to provide adequate 976 information, via message header field comments if necessary, to allow 977 an MUA developer to understand or relay ancillary details of 978 authentication results. For example, if it might be of interest to 979 relay what data was used to perform an evaluation, such information 980 could be relayed as a comment in the header field, such as: 982 Authentication-Results: example.com; 983 foo=pass bar.baz=blob (2 of 3 tests OK) 985 Experimental method identifiers MUST only be used within ADMDs that 986 have explicitly consented to use them. These method identifiers and 987 the parameters associated with them are not documented in RFCs. 989 Therefore, they are subject to change at any time and not suitable 990 for production use. Any MTA, MUA, or downstream filter intended for 991 production use SHOULD ignore or delete any Authentication-Results 992 header field that includes an experimental (unknown) method 993 identifier. 995 2.7.7. Extension Result Codes 997 Additional result codes (extension results) might be defined in the 998 future by later revisions or extensions to this specification. 999 Result codes MUST be registered with the Internet Assigned Numbers 1000 Authority (IANA) and preferably published in an RFC. See Section 6 1001 for further details. 1003 Experimental results MUST only be used within ADMDs that have 1004 explicitly consented to use them. These results and the parameters 1005 associated with them are not formally documented. Therefore, they 1006 are subject to change at any time and not suitable for production 1007 use. Any MTA, MUA, or downstream filter intended for production use 1008 SHOULD ignore or delete any Authentication-Results header field that 1009 includes an extension result. 1011 3. The "iprev" Authentication Method 1013 This section defines an additional authentication method called 1014 "iprev". 1016 "iprev" is an attempt to verify that a client appears to be valid 1017 based on some DNS queries, which is to say that the IP address is 1018 explicitly associated with a domain name. Upon receiving a session 1019 initiation of some kind from a client, the IP address of the client 1020 peer is queried for matching names (i.e., a number-to-name 1021 translation, also known as a "reverse lookup" or a "PTR" record 1022 query). Once that result is acquired, a lookup of each of the names 1023 (i.e., a name-to-number translation, or an "A" or "AAAA" record 1024 query) thus retrieved is done. The response to this second check 1025 will typically result in at least one mapping back to the client's IP 1026 address. 1028 Expressed as an algorithm: If the client peer's IP address is I, the 1029 list of names to which I maps (after a "PTR" query) is the set N, and 1030 the union of IP addresses to which each member of N maps (after 1031 corresponding "A" and "AAAA" queries) is L, then this test is 1032 successful if I is an element of L. 1034 Often an MTA receiving a connection that fails this test will simply 1035 reject the connection using the enhanced status code defined in 1036 [AUTH-ESC]. If an operator instead wishes to make this information 1037 available to downstream agents as a factor in handling decisions, it 1038 records a result in accordance with Section 2.7.3. 1040 The response to a PTR query could contain multiple names. To prevent 1041 heavy DNS loads, agents performing these queries MUST be implemented 1042 such that the number of names evaluated by generation of 1043 corresponding A or AAAA queries is limited so as not to be unduly 1044 taxing to the DNS infrastructure, though it MAY be configurable by an 1045 administrator. As an example, Section 4.6.4 of [SPF] chose a limit 1046 of 10 for its implementation of this algorithm. 1048 "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the 1049 query formats for the IPv6 case. 1051 There is some contention regarding the wisdom and reliability of this 1052 test. For example, in some regions, it can be difficult for this 1053 test ever to pass because the practice of arranging to match the 1054 forward and reverse DNS is infrequently observed. Therefore, the 1055 precise implementation details of how a verifier performs an "iprev" 1056 test are not specified here. The verifier MAY report a successful or 1057 failed "iprev" test at its discretion having done some kind of check 1058 of the validity of the connection's identity using DNS. It is 1059 incumbent upon an agent making use of the reported "iprev" result to 1060 understand what exactly that particular verifier is attempting to 1061 report. 1063 Extensive discussion of reverse DNS mapping and its implications can 1064 be found in "Considerations for the use of DNS Reverse Mapping" 1065 ([DNSOP-REVERSE]). In particular, it recommends that applications 1066 avoid using this test as a means of authentication or security. Its 1067 presence in this document is not an endorsement but is merely 1068 acknowledgment that the method remains common and provides the means 1069 to relay the results of that test. 1071 4. Adding the Header Field to a Message 1073 This specification makes no attempt to evaluate the relative 1074 strengths of various message authentication methods that may become 1075 available. The methods listed are an order-independent set; their 1076 sequence does not indicate relative strength or importance of one 1077 method over another. Instead, the MUA or downstream filter consuming 1078 this header field is to interpret the result of each method based on 1079 its own knowledge of what that method evaluates. 1081 Each "method" MUST refer to an authentication method declared in the 1082 IANA registry or an extension method as described in Section 2.7.6, 1083 and each "result" MUST refer to a result code declared in the IANA 1084 registry or an extension result code as defined in Section 2.7.7. 1086 See Section 6 for further information about the registered methods 1087 and result codes. 1089 An MTA compliant with this specification adds this header field 1090 (after performing one or more message authentication tests) to 1091 indicate which MTA or ADMD performed the test, which test got 1092 applied, and what the result was. If an MTA applies more than one 1093 such test, it adds this header field either once per test or once 1094 indicating all of the results. An MTA MUST NOT add a result to an 1095 existing header field. 1097 An MTA MAY add this header field containing only the authentication 1098 identifier portion and the "none" token (see Section 2.2) to indicate 1099 explicitly that no message authentication schemes were applied prior 1100 to delivery of this message. 1102 An MTA adding this header field has to take steps to identify it as 1103 legitimate to the MUAs or downstream filters that will ultimately 1104 consume its content. One process to do so is described in Section 5. 1105 Further measures may be necessary in some environments. Some 1106 possible solutions are enumerated in Section 7.1. This document does 1107 not mandate any specific solution to this issue as each environment 1108 has its own facilities and limitations. 1110 Most known message authentication methods focus on a particular 1111 identifier to evaluate. SPF and Sender ID differ in that they can 1112 yield a result based on more than one identifier; specifically, SPF 1113 can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom 1114 parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter 1115 or the Purported Responsible Address (PRA) identity. When generating 1116 this field to report those results, only the parameter that yielded 1117 the result is included. 1119 For MTAs that add this header field, adding header fields in order 1120 (at the top), per Section 3.6 of [MAIL], is particularly important. 1121 Moreover, this header field SHOULD be inserted above any other trace 1122 header fields such MTAs might prepend. This placement allows easy 1123 detection of header fields that can be trusted. 1125 End users making direct use of this header field might inadvertently 1126 trust information that has not been properly vetted. If, for 1127 example, a basic SPF result were to be relayed that claims an 1128 authenticated addr-spec, the local-part of that addr-spec has 1129 actually not been authenticated. Thus, an MTA adding this header 1130 field SHOULD NOT include any data that has not been authenticated by 1131 the method(s) being applied. Moreover, MUAs SHOULD NOT render to 1132 users such information if it is presented by a method known not to 1133 authenticate it. 1135 4.1. Header Field Position and Interpretation 1137 In order to ensure non-ambiguous results and avoid the impact of 1138 false header fields, MUAs and downstream filters SHOULD NOT interpret 1139 this header field unless specifically configured to do so by the user 1140 or administrator. That is, this interpretation should not be "on by 1141 default". Naturally then, users or administrators ought not activate 1142 such a feature unless they are certain the header field will be 1143 validly added by an agent within the ADMD that accepts the mail that 1144 is ultimately read by the MUA, and instances of the header field 1145 appearing to originate within the ADMD but are actually added by 1146 foreign MTAs will be removed before delivery. 1148 Furthermore, MUAs and downstream filters SHOULD NOT interpret this 1149 header field unless the authentication service identifier it bears 1150 appears to be one used within its own ADMD as configured by the user 1151 or administrator. 1153 MUAs and downstream filters MUST ignore any result reported using a 1154 "result" not specified in the IANA "Result Code" registry or a 1155 "ptype" not listed in the corresponding registry for such values as 1156 defined in Section 6. Moreover, such agents MUST ignore a result 1157 indicated for any "method" they do not specifically support. 1159 An MUA SHOULD NOT reveal these results to end users, absent careful 1160 human factors design considerations and testing, for the presentation 1161 of trust-related materials. For example, an attacker could register 1162 examp1e.com (note the digit "one") and send signed mail to intended 1163 victims; a verifier would detect that the signature was valid and 1164 report a "pass" even though it's clear the DNS domain name was 1165 intended to mislead. See Section 7.2 for further discussion. 1167 As stated in Section 2.1, this header field MUST be treated as though 1168 it were a trace header field as defined in Section 3.6.7 of [MAIL] 1169 and hence MUST NOT be reordered and MUST be prepended to the message, 1170 so that there is generally some indication upon delivery of where in 1171 the chain of handling MTAs the message authentication was done. 1173 Note that there are a few message handlers that are only capable of 1174 appending new header fields to a message. Strictly speaking, these 1175 handlers are not compliant with this specification. They can still 1176 add the header field to carry authentication details, but any signal 1177 about where in the handling chain the work was done may be lost. 1178 Consumers SHOULD be designed such that this can be tolerated, 1179 especially from a producer known to have this limitation. 1181 MUAs SHOULD ignore instances of this header field discovered within 1182 message/rfc822 MIME attachments. 1184 Further discussion of these topics can be found in Section 7 below. 1186 4.2. Local Policy Enforcement 1188 Some sites have a local policy that considers any particular 1189 authentication policy's non-recoverable failure results (typically 1190 "fail" or similar) as justification for rejecting the message. In 1191 such cases, the border MTA SHOULD issue an SMTP rejection response to 1192 the message, rather than adding this header field and allowing the 1193 message to proceed toward delivery. This is more desirable than 1194 allowing the message to reach an internal host's MTA or spam filter, 1195 thus possibly generating a local rejection such as a Delivery Status 1196 Notification (DSN) [DSN] to a forged originator. Such generated 1197 rejections are colloquially known as "backscatter". 1199 The same MAY also be done for local policy decisions overriding the 1200 results of the authentication methods (e.g., the "policy" result 1201 codes described in Section 2.7). 1203 Such rejections at the SMTP protocol level are not possible if local 1204 policy is enforced at the MUA and not the MTA. 1206 5. Removing Existing Header Fields 1208 For security reasons, any MTA conforming to this specification MUST 1209 delete any discovered instance of this header field that claims, by 1210 virtue of its authentication service identifier, to have been added 1211 within its trust boundary but that did not come directly from another 1212 trusted MTA. For example, an MTA for example.com receiving a message 1213 MUST delete or otherwise obscure any instance of this header field 1214 bearing an authentication service identifier indicating that the 1215 header field was added within example.com prior to adding its own 1216 header fields. This could mean each MTA will have to be equipped 1217 with a list of internal MTAs known to be compliant (and hence 1218 trustworthy). 1220 For simplicity and maximum security, a border MTA could remove all 1221 instances of this header field on mail crossing into its trust 1222 boundary. However, this may conflict with the desire to access 1223 authentication results performed by trusted external service 1224 providers. It may also invalidate signed messages whose signatures 1225 cover external instances of this header field. A more robust border 1226 MTA could allow a specific list of authenticating MTAs whose 1227 information is to be admitted, removing the header field originating 1228 from all others. 1230 As stated in Section 1.2, a formal definition of "trust boundary" is 1231 deliberately not made here. It is entirely possible that a border 1232 MTA for example.com will explicitly trust authentication results 1233 asserted by upstream host example.net even though they exist in 1234 completely disjoint administrative boundaries. In that case, the 1235 border MTA MAY elect not to delete those results; moreover, the 1236 upstream host doing some authentication work could apply a signing 1237 technology such as [DKIM] on its own results to assure downstream 1238 hosts of their authenticity. An example of this is provided in 1239 Appendix C. 1241 Similarly, in the case of messages signed using [DKIM] or other 1242 message-signing methods that sign header fields, this removal action 1243 could invalidate one or more signatures on the message if they 1244 covered the header field to be removed. This behavior can be 1245 desirable since there's little value in validating the signature on a 1246 message with forged header fields. However, signing agents MAY 1247 therefore elect to omit these header fields from signing to avoid 1248 this situation. 1250 An MTA SHOULD remove any instance of this header field bearing a 1251 version (express or implied) that it does not support. However, an 1252 MTA MUST remove such a header field if the [SMTP] connection relaying 1253 the message is not from a trusted internal MTA. This means the MTA 1254 needs to be able to understand versions of this header field at least 1255 as late as the ones understood by the MUAs or other consumers within 1256 its ADMD. 1258 6. IANA Considerations 1260 IANA has registered the defined header field and created tables as 1261 described below. These registry actions were originally defined by 1262 [RFC5451] and updated by [RFC6577] and [RFC7001]. The created 1263 registries are being further updated here to increase their 1264 completeness. 1266 6.1. The Authentication-Results Header Field 1268 [RFC5451] added the Authentication-Results header field to the IANA 1269 "Permanent Message Header Field Names" registry, per the procedure 1270 found in [IANA-HEADERS]. That entry is to be updated to reference 1271 this document. The following is the registration template: 1273 Header field name: Authentication-Results 1274 Applicable protocol: mail ([MAIL]) 1275 Status: Standard 1276 Author/Change controller: IETF 1277 Specification document(s): [this RFC] 1278 Related information: 1279 Requesting review of any proposed changes and additions to 1280 this field is recommended. 1282 6.2. "Email Authentication Methods" Registry Description 1284 Names of message authentication methods supported by this 1285 specification are to be registered with IANA, with the exception of 1286 experimental names as described in Section 2.7.6. Along with each 1287 method is recorded the properties that accompany the method's result. 1289 The "Email Authentication Parameters" group, and within it the "Email 1290 Authentication Methods" registry, were created by [RFC5451] for this 1291 purpose. [RFC6577] added a "status" field for each entry. [RFC7001] 1292 amended the rules governing that registry, and also added a "version" 1293 field to the registry. 1295 The reference for that registry shall be updated to reference this 1296 document. 1298 New entries are assigned only for values that have received Expert 1299 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1300 appointed by the IESG. The designated expert has discretion to 1301 request that a publication be referenced if a clear, concise 1302 definition of the authentication method cannot be provided such that 1303 interoperability is assured. Registrations should otherwise be 1304 permitted. The designated expert can also handle requests to mark 1305 any current registration as "deprecated". 1307 No two entries can have the same combination of method, ptype, and 1308 property. 1310 An entry in this registry contains the following: 1312 Method: the name of the method; 1314 Defined: a reference to the document that created this entry, if any 1315 (see below); 1317 ptype: a "ptype" value appropriate for use with that method; 1319 property: a "property" value matching that "ptype" also appropriate 1320 for use with that method; 1322 Value: a brief description of the value to be supplied with that 1323 method/ptype/property tuple; 1325 Status: the status of this entry, which is either: 1327 active: The entry is in current use. 1329 deprecated: The entry is no longer in current use. 1331 Version: a version number associated with the method (preferably 1332 starting at "1"). 1334 The "Defined" field will typically refer to a permanent document, or 1335 at least some descriptive text, where additional information about 1336 the entry being added can be found. This in turn would reference the 1337 document where the method is defined so that all of the semantics 1338 around creating or interpreting an Authentication-Results header 1339 field using this method, ptype, and property can be understood. 1341 6.3. "Email Authentication Methods" Registry Update 1343 The following changes are to be made to this registry upon approval 1344 of this document: 1346 1. The current entry for the "auth" method shall have its "property" 1347 field changed to "mailfrom", and its "Defined" field changed to 1348 this document. 1350 2. The entry for the "dkim" method, "header" ptype and "b" property 1351 shall now reference [RFC6008] as its defining document, and the 1352 reference shall be removed from the description. 1354 3. All other "dkim", "domainkeys", "iprev", "sender-id", and "spf" 1355 method entries shall have their "Defined" fields changed to this 1356 document. 1358 4. All "smime" entries have their "Defined" fields changed to 1359 [SMIME-REG]. 1361 5. The "value" field of the "smime" entry using property "smime- 1362 part" shall be changed to read: "The MIME body part reference 1363 that contains the S/MIME signature. See Section 3.2.1 of RFC7281 1364 for full syntax." 1366 6. The following entry is to be added: 1368 Method: auth 1369 Defined: [this document] 1371 ptype: smtp 1373 property: auth 1375 Value: identity confirmed by the AUTH command 1377 Status: active 1379 Version: 1 1381 7. The values of the "domainkeys" entries for ptype "header" are 1382 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 8. All entries for "dkim-adsp" and "domainkeys" shall have their 1393 Status values changed to "deprecated", reflecting the fact that 1394 the corresponding specifications now have Historical status. 1396 6.4. "Email Authentication Property Types" Registry 1398 [PTYPES-REGISTRY] created the Email Authentication Property Types 1399 registry. 1401 Entries in this registry are subject to the Expert Review rules as 1402 described in [IANA-CONSIDERATIONS]. Each entry in the registry 1403 requires the following values: 1405 ptype: The name of the ptype being registered, which must fit within 1406 the ABNF described in Section 2.2. 1408 Definition: An optional reference to a defining specification. 1410 Description: A brief description of what sort of information this 1411 "ptype" is meant to cover. 1413 For new entries, the Designated Expert needs to assure that the 1414 description provided for the new entry adequately describes the 1415 intended use. An example would be helpful to include in the entry's 1416 defining document, if any, although entries in the "Email 1417 Authentication Methods" registry or the "Email Authentication Result 1418 Names" registry might also serve as examples of intended use. 1420 IANA shall update this registry to show Section 2.3 of this document 1421 as the current definitions for the "body", "header", "policy" and 1422 "smtp" entries of that registry. 1424 6.5. "Email Authentication Result Names" Description 1426 Names of message authentication result codes supported by this 1427 specification must be registered with IANA, with the exception of 1428 experimental codes as described in Section 2.7.7. A registry was 1429 created by [RFC5451] for this purpose. [RFC6577] added the "status" 1430 column, and [RFC7001] updated the rules governing that registry. 1432 New entries are assigned only for values that have received Expert 1433 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1434 appointed by the IESG. The designated expert has discretion to 1435 request that a publication be referenced if a clear, concise 1436 definition of the authentication result cannot be provided such that 1437 interoperability is assured. Registrations should otherwise be 1438 permitted. The designated expert can also handle requests to mark 1439 any current registration as "deprecated". 1441 No two entries can have the same combination of method and code. 1443 An entry in this registry contains the following: 1445 Auth Method: an authentication method for which results are being 1446 returned using the header field defined in this document; 1448 Code: a result code that can be returned for this authentication 1449 method; 1451 Specification: either free form text explaining the meaning of this 1452 method-code combination, or a reference to such a definition. 1454 6.6. "Email Authentication Result Names" Update 1456 The following changes are to be made to this registry on publication 1457 of this document: 1459 o The "Defined" field shall be removed. 1461 o The "Meaning" field shall be renamed to "Specification", as 1462 described above. 1464 o The "Auth Method" field shall appear before the "Code" field. 1466 o For easier searching, the table shall be arranged such that it is 1467 sorted first by Auth Method, then by Code within each Auth Method 1468 grouping. 1470 o All entries for the "dkim", "domainkeys", "spf", "sender-id", 1471 "auth", and "iprev" methods shall have their "Specification" 1472 fields changed to refer to this document, as follows: 1474 dkim: [this document] Section 2.7.1 1476 domainkeys: [this document] Section 2.7.1 1478 spf: [this document] Section 2.7.2 1480 sender-id: [this document] Section 2.7.2 1482 auth: [this document] Section 2.7.4 1484 iprev: [this document] Section 2.7.3 1486 o All entries for "dkim-adsp" that are missing an explicit reference 1487 to a defining document shall have [ADSP] added to their 1488 Specification fields. 1490 o All entries for "dkim-adsp" and "domainkeys" shall have their 1491 Status values changed to "deprecated", reflecting the fact that 1492 the corresponding specifications now have Historical status. 1494 6.7. SMTP Enhanced Stauts Codes 1496 The entry for X.7.25 in the Enumerated Status Codes sub-registry of 1497 the SMTP Enhanced Status Codes registry is to be updated to refer to 1498 this document. 1500 7. Security Considerations 1502 The following security considerations apply when adding or processing 1503 the Authentication-Results header field: 1505 7.1. Forged Header Fields 1507 An MUA or filter that accesses a mailbox whose messages are handled 1508 by a non-conformant MTA, and understands Authentication-Results 1509 header fields, could potentially make false conclusions based on 1510 forged header fields. A malicious user or agent could forge a header 1511 field using the DNS domain of a receiving ADMD as the authserv-id 1512 token in the value of the header field and, with the rest of the 1513 value, claim that the message was properly authenticated. The non- 1514 conformant MTA would fail to strip the forged header field, and the 1515 MUA could inappropriately trust it. 1517 For this reason, it is best not to have processing of the 1518 Authentication-Results header field enabled by default; instead, it 1519 should be ignored, at least for the purposes of enacting filtering 1520 decisions, unless specifically enabled by the user or administrator 1521 after verifying that the border MTA is compliant. It is acceptable 1522 to have an MUA aware of this specification but have an explicit list 1523 of hostnames whose Authentication-Results header fields are 1524 trustworthy; however, this list should initially be empty. 1526 Proposed alternative solutions to this problem were made some time 1527 ago and are listed below. To date, they have not been developed due 1528 to lack of demand but are documented here should the information be 1529 useful at some point in the future: 1531 1. Possibly the simplest is a digital signature protecting the 1532 header field, such as using [DKIM], that can be verified by an 1533 MUA by using a posted public key. Although one of the main 1534 purposes of this document is to relieve the burden of doing 1535 message authentication work at the MUA, this only requires that 1536 the MUA learn a single authentication scheme even if a number of 1537 them are in use at the border MTA. Note that [DKIM] requires 1538 that the From header field be signed, although in this 1539 application, the signing agent (a trusted MTA) likely cannot 1540 authenticate that value, so the fact that it is signed should be 1541 ignored. Where the authserv-id is the ADMD's domain name, the 1542 authserv-id matching this valid internal signature's "d=" DKIM 1543 value is sufficient. 1545 2. Another would be a means to interrogate the MTA that added the 1546 header field to see if it is actually providing any message 1547 authentication services and saw the message in question, but this 1548 isn't especially palatable given the work required to craft and 1549 implement such a scheme. 1551 3. Yet another might be a method to interrogate the internal MTAs 1552 that apparently handled the message (based on Received header 1553 fields) to determine whether any of them conform to Section 5 of 1554 this memo. This, too, has potentially high barriers to entry. 1556 4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to 1557 allow an MUA or filtering agent to acquire the authserv-id in use 1558 within an ADMD, thus allowing it to identify which 1559 Authentication-Results header fields it can trust. 1561 5. On the presumption that internal MTAs are fully compliant with 1562 Section 3.6 of [MAIL] and the compliant internal MTAs are using 1563 their own hostnames or the ADMD's DNS domain name as the 1564 authserv-id token, the header field proposed here should always 1565 appear above a Received header added by a trusted MTA. This can 1566 be used as a test for header field validity. 1568 Support for some of these is being considered for future work. 1570 In any case, a mechanism needs to exist for an MUA or filter to 1571 verify that the host that appears to have added the header field (a) 1572 actually did so and (b) is legitimately adding that header field for 1573 this delivery. Given the variety of messaging environments deployed 1574 today, consensus appears to be that specifying a particular mechanism 1575 for doing so is not appropriate for this document. 1577 Mitigation of the forged header field attack can also be accomplished 1578 by moving the authentication results data into meta-data associated 1579 with the message. In particular, an [SMTP] extension could be 1580 established to communicate authentication results from the border MTA 1581 to intermediate and delivery MTAs; the latter of these could arrange 1582 to store the authentication results as meta-data retrieved and 1583 rendered along with the message by an [IMAP] client aware of a 1584 similar extension in that protocol. The delivery MTA would be told 1585 to trust data via this extension only from MTAs it trusts, and border 1586 MTAs would not accept data via this extension from any source. There 1587 is no vector in such an arrangement for forgery of authentication 1588 data by an outside agent. 1590 7.2. Misleading Results 1592 Until some form of service for querying the reputation of a sending 1593 agent is widely deployed, the existence of this header field 1594 indicating a "pass" does not render the message trustworthy. It is 1595 possible for an arriving piece of spam or other undesirable mail to 1596 pass checks by several of the methods enumerated above (e.g., a piece 1597 of spam signed using [DKIM] by the originator of the spam, which 1598 might be a spammer or a compromised system). In particular, this 1599 issue is not resolved by forged header field removal discussed above. 1601 Hence, MUAs and downstream filters must take some care with use of 1602 this header even after possibly malicious headers are scrubbed. 1604 7.3. Header Field Position 1606 Despite the requirements of [MAIL], header fields can sometimes be 1607 reordered en route by intermediate MTAs. The goal of requiring 1608 header field addition only at the top of a message is an 1609 acknowledgment that some MTAs do reorder header fields, but most do 1610 not. Thus, in the general case, there will be some indication of 1611 which MTAs (if any) handled the message after the addition of the 1612 header field defined here. 1614 7.4. Reverse IP Query Denial-of-Service Attacks 1616 Section 4.6.4 of [SPF] describes a DNS-based denial-of-service attack 1617 for verifiers that attempt DNS-based identity verification of 1618 arriving client connections. A verifier wishing to do this check and 1619 report this information needs to take care not to go to unbounded 1620 lengths to resolve "A" and "PTR" queries. MUAs or other filters 1621 making use of an "iprev" result specified by this document need to be 1622 aware of the algorithm used by the verifier reporting the result and, 1623 especially, its limitations. 1625 7.5. Mitigation of Backscatter 1627 Failing to follow the instructions of Section 4.2 can result in a 1628 denial-of-service attack caused by the generation of [DSN] messages 1629 (or equivalent) to addresses that did not send the messages being 1630 rejected. 1632 7.6. Internal MTA Lists 1634 Section 5 describes a procedure for scrubbing header fields that may 1635 contain forged authentication results about a message. A compliant 1636 installation will have to include, at each MTA, a list of other MTAs 1637 known to be compliant and trustworthy. Failing to keep this list 1638 current as internal infrastructure changes may expose an ADMD to 1639 attack. 1641 7.7. Attacks against Authentication Methods 1643 If an attack becomes known against an authentication method, clearly 1644 then the agent verifying that method can be fooled into thinking an 1645 inauthentic message is authentic, and thus the value of this header 1646 field can be misleading. It follows that any attack against the 1647 authentication methods supported by this document is also a security 1648 consideration here. 1650 7.8. Intentionally Malformed Header Fields 1652 It is possible for an attacker to add an Authentication-Results 1653 header field that is extraordinarily large or otherwise malformed in 1654 an attempt to discover or exploit weaknesses in header field parsing 1655 code. Implementers must thoroughly verify all such header fields 1656 received from MTAs and be robust against intentionally as well as 1657 unintentionally malformed header fields. 1659 7.9. Compromised Internal Hosts 1661 An internal MUA or MTA that has been compromised could generate mail 1662 with a forged From header field and a forged Authentication-Results 1663 header field that endorses it. Although it is clearly a larger 1664 concern to have compromised internal machines than it is to prove the 1665 value of this header field, this risk can be mitigated by arranging 1666 that internal MTAs will remove this header field if it claims to have 1667 been added by a trusted border MTA (as described above), yet the 1668 [SMTP] connection is not coming from an internal machine known to be 1669 running an authorized MTA. However, in such a configuration, 1670 legitimate MTAs will have to add this header field when legitimate 1671 internal-only messages are generated. This is also covered in 1672 Section 5. 1674 7.10. Encapsulated Instances 1676 MIME messages can contain attachments of type "message/rfc822", which 1677 contain other messages. Such an encapsulated message can also 1678 contain an Authentication-Results header field. Although the 1679 processing of these is outside of the intended scope of this document 1680 (see Section 1.3), some early guidance to MUA developers is 1681 appropriate here. 1683 Since MTAs are unlikely to strip Authentication-Results header fields 1684 after mailbox delivery, MUAs are advised in Section 4.1 to ignore 1685 such instances within MIME attachments. Moreover, when extracting a 1686 message digest to separate mail store messages or other media, such 1687 header fields should be removed so that they will never be 1688 interpreted improperly by MUAs that might later consume them. 1690 7.11. Reverse Mapping 1692 Although Section 3 of this memo includes explicit support for the 1693 "iprev" method, its value as an authentication mechanism is limited. 1694 Implementers of both this proposal and agents that use the data it 1695 relays are encouraged to become familiar with the issues raised by 1696 [DNSOP-REVERSE] when deciding whether or not to include support for 1697 "iprev". 1699 8. References 1701 8.1. Normative References 1703 [ABNF] Crocker, D. and P. Overell, "Augmented BNF for 1704 Syntax Specifications: ABNF", STD 68, 1705 RFC 5234, January 2008. 1707 [IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul, 1708 "Registration Procedures for Message Header 1709 Fields", BCP 90, RFC 3864, September 2004. 1711 [KEYWORDS] Bradner, S., "Key words for use in RFCs to 1712 Indicate Requirement Levels", BCP 14, 1713 RFC 2119. 1715 [MAIL] Resnick, P., Ed., "Internet Message Format", 1716 RFC 5322, October 2008. 1718 [MIME] Freed, N. and N. Borenstein, "Multipurpose 1719 Internet Mail Extensions (MIME) Part One: 1720 Format of Internet Message Bodies", RFC 2045, 1721 November 1996. 1723 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", 1724 RFC 5321, October 2008. 1726 8.2. Informative References 1728 [ADSP] Allman, E., Fenton, J., Delany, M., and J. 1729 Levine, "DomainKeys Identified Mail (DKIM) 1730 Author Domain Signing Practices (ADSP)", 1731 RFC 5617, August 2009. 1733 [AR-VBR] Kucherawy, M., "Authentication-Results 1734 Registration for Vouch by Reference Results", 1735 RFC 6212, April 2011. 1737 [ATPS] Kucherawy, M., "DomainKeys Identified Mail 1738 (DKIM) Authorized Third-Party Signatures", 1739 RFC 6541, February 2012. 1741 [AUTH] Siemborski, R. and A. Melnikov, "SMTP Service 1742 Extension for Authentication", RFC 4954, 1743 July 2007. 1745 [AUTH-ESC] Kucherawy, M., "Email Authentication Status 1746 Codes", RFC 7372, September 2014. 1748 [DKIM] Crocker, D., Hansen, T., and M. Kucherawy, 1749 "DomainKeys Identified Mail (DKIM) 1750 Signatures", STD 76, RFC 6376, September 2011. 1752 [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., 1753 "Domain-based Message Authentication, 1754 Reporting and Conformance (DMARC)", RFC 7489, 1755 March 2015. 1757 [DNS] Mockapetris, P., "Domain names - 1758 Implementation and Specification", STD 13, 1759 RFC 1035, November 1987. 1761 [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. 1762 Souissi, "DNS Extensions to Support IP Version 1763 6", RFC 3596, October 2003. 1765 [DNSOP-REVERSE] Senie, D. and A. Sullivan, "Considerations for 1766 the use of DNS Reverse Mapping", Work 1767 in Progress, March 2008. 1769 [DOMAINKEYS] Delany, M., "Domain-Based Email Authentication 1770 Using Public Keys Advertised in the DNS 1771 (DomainKeys)", RFC 4870, May 2007. 1773 [DSN] Moore, K. and G. Vaudreuil, "An Extensible 1774 Message Format for Delivery Status 1775 Notifications", RFC 3464, January 2003. 1777 [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", 1778 RFC 5598, July 2009. 1780 [IANA-CONSIDERATIONS] Narten, T. and H. Alvestrand, "Guidelines for 1781 Writing an IANA Considerations Section in 1782 RFCs", BCP 26, RFC 5226, May 2008. 1784 [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL 1785 - VERSION 4rev1", RFC 3501, March 2003. 1787 [POP3] Myers, J. and M. Rose, "Post Office Protocol - 1788 Version 3", STD 53, RFC 1939, May 1996. 1790 [PRA] Lyon, J., "Purported Responsible Address in 1791 E-Mail Messages", RFC 4407, April 2006. 1793 [PTYPES-REGISTRY] Kucherawy, M., "A Property Types Registry for 1794 the Authentication-Results Header Field", 1795 RFC 7410, December 2014. 1797 [RFC5451] Kucherawy, M., "Message Header Field for 1798 Indicating Message Authentication Status", 1799 RFC 5451, April 2009. 1801 [RFC6008] Kucherawy, M., "Authentication-Results 1802 Registration for Differentiating among 1803 Cryptographic Results", RFC 6008, 1804 September 2010. 1806 [RFC6577] Kucherawy, M., "Authentication-Results 1807 Registration Update for Sender Policy 1808 Framework (SPF) Results", RFC 6577, 1809 March 2012. 1811 [RFC7001] Kucherawy, M., "Message Header Field for 1812 Indicating Message Authentication Status", 1813 RFC 7001, September 2013. 1815 [RRVS] Mills, W. and M. Kucherawy, "The Require- 1816 Recipient-Valid-Since Header Field and SMTP 1817 Service Extension", RFC 7293, July 2014. 1819 [SECURITY] Rescorla, E. and B. Korver, "Guidelines for 1820 Writing RFC Text on Security Considerations", 1821 BCP 72, RFC 3552, July 2003. 1823 [SENDERID] Lyon, J. and M. Wong, "Sender ID: 1824 Authenticating E-Mail", RFC 4406, April 2006. 1826 [SMIME-REG] Melnikov, A., "Authentication-Results 1827 Registration for S/MIME Signature 1828 Verification", RFC 7281, June 2014. 1830 [SPF] Kitterman, S., "Sender Policy Framework (SPF) 1831 for Authorizing Use of Domains in E-Mail, 1832 Version 1", RFC 7208, April 2014. 1834 [VBR] Hoffman, P., Levine, J., and A. Hathcock, 1835 "Vouch By Reference", RFC 5518, April 2009. 1837 Appendix A. Acknowledgments 1839 The author wishes to acknowledge the following individuals for their 1840 review and constructive criticism of this document: Stephane 1841 Bortzmeyer, Scott Kitterman, John Levine, Tom Petch, and Pete 1842 Resnick. 1844 Appendix B. Legacy MUAs 1846 Implementers of this protocol should be aware that many MUAs are 1847 unlikely to be retrofitted to support the new header field and its 1848 semantics. In the interests of convenience and quicker adoption, a 1849 delivery MTA might want to consider adding things that are processed 1850 by existing MUAs in addition to the Authentication-Results header 1851 field. One suggestion is to include a Priority header field, on 1852 messages that don't already have such a header field, containing a 1853 value that reflects the strength of the authentication that was 1854 accomplished, e.g., "low" for weak or no authentication, "normal" or 1855 "high" for good or strong authentication. 1857 Some modern MUAs can already filter based on the content of this 1858 header field. However, there is keen interest in having MUAs make 1859 some kind of graphical representation of this header field's meaning 1860 to end users. Until this capability is added, other interim means of 1861 conveying authentication results may be necessary while this proposal 1862 and its successors are adopted. 1864 Appendix C. Authentication-Results Examples 1866 This section presents some examples of the use of this header field 1867 to indicate authentication results. 1869 C.1. Trivial Case; Header Field Not Present 1871 The trivial case: 1873 Received: from mail-router.example.com 1874 (mail-router.example.com [192.0.2.1]) 1875 by server.example.org (8.11.6/8.11.6) 1876 with ESMTP id g1G0r1kA003489; 1877 Fri, Feb 15 2002 17:19:07 -0800 1878 From: sender@example.com 1879 Date: Fri, Feb 15 2002 16:54:30 -0800 1880 To: receiver@example.org 1881 Message-Id: <12345.abc@example.com> 1882 Subject: here's a sample 1884 Hello! Goodbye! 1886 Example 1: Trivial Case 1888 The Authentication-Results header field is completely absent. The 1889 MUA may make no conclusion about the validity of the message. This 1890 could be the case because the message authentication services were 1891 not available at the time of delivery, or no service is provided, or 1892 the MTA is not in compliance with this specification. 1894 C.2. Nearly Trivial Case; Service Provided, but No Authentication Done 1896 A message that was delivered by an MTA that conforms to this 1897 specification but provides no actual message authentication service: 1899 Authentication-Results: example.org 1; none 1900 Received: from mail-router.example.com 1901 (mail-router.example.com [192.0.2.1]) 1902 by server.example.org (8.11.6/8.11.6) 1903 with ESMTP id g1G0r1kA003489; 1904 Fri, Feb 15 2002 17:19:07 -0800 1905 From: sender@example.com 1906 Date: Fri, Feb 15 2002 16:54:30 -0800 1907 To: receiver@example.org 1908 Message-Id: <12345.abc@example.com> 1909 Subject: here's a sample 1911 Hello! Goodbye! 1913 Example 2: Header Present but No Authentication Done 1915 The Authentication-Results header field is present, showing that the 1916 delivering MTA conforms to this specification. It used its DNS 1917 domain name as the authserv-id. The presence of "none" (and the 1918 absence of any method and result tokens) indicates that no message 1919 authentication was done. The version number of the specification to 1920 which the field's content conforms is explicitly provided. 1922 C.3. Service Provided, Authentication Done 1924 A message that was delivered by an MTA that conforms to this 1925 specification and applied some message authentication: 1927 Authentication-Results: example.com; 1928 spf=pass smtp.mailfrom=example.net 1929 Received: from dialup-1-2-3-4.example.net 1930 (dialup-1-2-3-4.example.net [192.0.2.200]) 1931 by mail-router.example.com (8.11.6/8.11.6) 1932 with ESMTP id g1G0r1kA003489; 1933 Fri, Feb 15 2002 17:19:07 -0800 1934 From: sender@example.net 1935 Date: Fri, Feb 15 2002 16:54:30 -0800 1936 To: receiver@example.com 1937 Message-Id: <12345.abc@example.net> 1938 Subject: here's a sample 1940 Hello! Goodbye! 1942 Example 3: Header Reporting Results 1944 The Authentication-Results header field is present, indicating that 1945 the border MTA conforms to this specification. The authserv-id is 1946 once again the DNS domain name. Furthermore, the message was 1947 authenticated by that MTA via the method specified in [SPF]. Note 1948 that since that method cannot authenticate the local-part, it has 1949 been omitted from the result's value. The MUA could extract and 1950 relay this extra information if desired. 1952 C.4. Service Provided, Several Authentications Done, Single MTA 1954 A message that was relayed inbound via a single MTA that conforms to 1955 this specification and applied three different message authentication 1956 checks: 1958 Authentication-Results: example.com; 1959 auth=pass (cram-md5) smtp.auth=sender@example.net; 1960 spf=pass smtp.mailfrom=example.net 1961 Authentication-Results: example.com; 1962 sender-id=pass header.from=example.net 1963 Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6) 1964 (dialup-1-2-3-4.example.net [192.0.2.200]) 1965 by mail-router.example.com (8.11.6/8.11.6) 1966 with ESMTPA id g1G0r1kA003489; 1967 Fri, Feb 15 2002 17:19:07 -0800 1968 Date: Fri, Feb 15 2002 16:54:30 -0800 1969 To: receiver@example.com 1970 From: sender@example.net 1971 Message-Id: <12345.abc@example.net> 1972 Subject: here's a sample 1974 Hello! Goodbye! 1976 Example 4: Headers Reporting Results from One MTA 1978 The Authentication-Results header field is present, indicating that 1979 the delivering MTA conforms to this specification. Once again, the 1980 receiving DNS domain name is used as the authserv-id. Furthermore, 1981 the sender authenticated herself/himself to the MTA via a method 1982 specified in [AUTH], and both SPF and Sender ID checks were done and 1983 passed. The MUA could extract and relay this extra information if 1984 desired. 1986 Two Authentication-Results header fields are not required since the 1987 same host did all of the checking. The authenticating agent could 1988 have consolidated all the results into one header field. 1990 This example illustrates a scenario in which a remote user on a 1991 dialup connection (example.net) sends mail to a border MTA 1992 (example.com) using SMTP authentication to prove identity. The 1993 dialup provider has been explicitly authorized to relay mail as 1994 example.com resulting in passes by the SPF and Sender ID checks. 1996 C.5. Service Provided, Several Authentications Done, Different MTAs 1998 A message that was relayed inbound by two different MTAs that conform 1999 to this specification and applied multiple message authentication 2000 checks: 2002 Authentication-Results: example.com; 2003 sender-id=fail header.from=example.com; 2004 dkim=pass (good signature) header.d=example.com 2005 Received: from mail-router.example.com 2006 (mail-router.example.com [192.0.2.1]) 2007 by auth-checker.example.com (8.11.6/8.11.6) 2008 with ESMTP id i7PK0sH7021929; 2009 Fri, Feb 15 2002 17:19:22 -0800 2010 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com; 2011 t=1188964191; c=simple/simple; h=From:Date:To:Subject: 2012 Message-Id:Authentication-Results; 2013 bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70; 2014 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 2015 Authentication-Results: example.com; 2016 auth=pass (cram-md5) smtp.auth=sender@example.com; 2017 spf=fail smtp.mailfrom=example.com 2018 Received: from dialup-1-2-3-4.example.net 2019 (dialup-1-2-3-4.example.net [192.0.2.200]) 2020 by mail-router.example.com (8.11.6/8.11.6) 2021 with ESMTPA id g1G0r1kA003489; 2022 Fri, Feb 15 2002 17:19:07 -0800 2023 From: sender@example.com 2024 Date: Fri, Feb 15 2002 16:54:30 -0800 2025 To: receiver@example.com 2026 Message-Id: <12345.abc@example.com> 2027 Subject: here's a sample 2029 Hello! Goodbye! 2031 Example 5: Headers Reporting Results from Multiple MTAs 2033 The Authentication-Results header field is present, indicating 2034 conformance to this specification. Once again, the authserv-id used 2035 is the recipient's DNS domain name. The header field is present 2036 twice because two different MTAs in the chain of delivery did 2037 authentication tests. The first MTA, mail-router.example.com, 2038 reports that SMTP AUTH and SPF were both used and that the former 2039 passed while the latter failed. In the SMTP AUTH case, additional 2040 information is provided in the comment field, which the MUA can 2041 choose to render if desired. 2043 The second MTA, auth-checker.example.com, reports that it did a 2044 Sender ID test (which failed) and a DKIM test (which passed). Again, 2045 additional data about one of the tests is provided as a comment, 2046 which the MUA may choose to render. Also noteworthy here is the fact 2047 that there is a DKIM signature added by example.com that assured the 2048 integrity of the lower Authentication-Results field. 2050 Since different hosts did the two sets of authentication checks, the 2051 header fields cannot be consolidated in this example. 2053 This example illustrates more typical transmission of mail into 2054 example.com from a user on a dialup connection example.net. The user 2055 appears to be legitimate as he/she had a valid password allowing 2056 authentication at the border MTA using SMTP AUTH. The SPF and Sender 2057 ID tests failed since example.com has not granted example.net 2058 authority to relay mail on its behalf. However, the DKIM test passed 2059 because the sending user had a private key matching one of 2060 example.com's published public keys and used it to sign the message. 2062 C.6. Service Provided, Multi-Tiered Authentication Done 2064 A message that had authentication done at various stages, one of 2065 which was outside the receiving ADMD: 2067 Authentication-Results: example.com; 2068 dkim=pass reason="good signature" 2069 header.i=@mail-router.example.net; 2070 dkim=fail reason="bad signature" 2071 header.i=@newyork.example.com 2072 Received: from mail-router.example.net 2073 (mail-router.example.net [192.0.2.250]) 2074 by chicago.example.com (8.11.6/8.11.6) 2075 for 2076 with ESMTP id i7PK0sH7021929; 2077 Fri, Feb 15 2002 17:19:22 -0800 2078 DKIM-Signature: v=1; a=rsa-sha256; s=furble; 2079 d=mail-router.example.net; t=1188964198; c=relaxed/simple; 2080 h=From:Date:To:Message-Id:Subject:Authentication-Results; 2081 bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=; 2082 b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= 2083 Authentication-Results: example.net; 2084 dkim=pass (good signature) header.i=@newyork.example.com 2085 Received: from smtp.newyork.example.com 2086 (smtp.newyork.example.com [192.0.2.220]) 2087 by mail-router.example.net (8.11.6/8.11.6) 2088 with ESMTP id g1G0r1kA003489; 2089 Fri, Feb 15 2002 17:19:07 -0800 2090 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; 2091 d=newyork.example.com; 2092 t=1188964191; c=simple/simple; 2093 h=From:Date:To:Message-Id:Subject; 2094 bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; 2095 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 2096 From: sender@newyork.example.com 2097 Date: Fri, Feb 15 2002 16:54:30 -0800 2098 To: meetings@example.net 2099 Message-Id: <12345.abc@newyork.example.com> 2100 Subject: here's a sample 2102 Example 6: Headers Reporting Results from Multiple MTAs in Different 2103 ADMDs 2105 In this example, we see multi-tiered authentication with an extended 2106 trust boundary. 2108 The message was sent from someone at example.com's New York office 2109 (newyork.example.com) to a mailing list managed at an intermediary. 2111 The message was signed at the origin using DKIM. 2113 The message was sent to a mailing list service provider called 2114 example.net, which is used by example.com. There, 2115 meetings@example.net is expanded to a long list of recipients, one of 2116 whom is at the Chicago office. In this example, we will assume that 2117 the trust boundary for chicago.example.com includes the mailing list 2118 server at example.net. 2120 The mailing list server there first authenticated the message and 2121 affixed an Authentication-Results header field indicating such using 2122 its DNS domain name for the authserv-id. It then altered the message 2123 by affixing some footer text to the body, including some 2124 administrivia such as unsubscription instructions. Finally, the 2125 mailing list server affixes a second DKIM signature and begins 2126 distribution of the message. 2128 The border MTA for chicago.example.com explicitly trusts results from 2129 mail-router.example.net, so that header field is not removed. It 2130 performs evaluation of both signatures and determines that the first 2131 (most recent) is a "pass" but, because of the aforementioned 2132 modifications, the second is a "fail". However, the first signature 2133 included the Authentication-Results header added at mail- 2134 router.example.net that validated the second signature. Thus, 2135 indirectly, it can be determined that the authentications claimed by 2136 both signatures are indeed valid. 2138 Note that two styles of presenting meta-data about the result are in 2139 use here. In one case, the "reason=" clause is present, which is 2140 intended for easy extraction by parsers; in the other case, the CFWS 2141 production of the ABNF is used to include such data as a header field 2142 comment. The latter can be harder for parsers to extract given the 2143 varied supported syntaxes of mail header fields. 2145 C.7. Comment-Heavy Example 2147 The formal syntax permits comments within the content in a number of 2148 places. For the sake of illustration, this example is also legal: 2150 Authentication-Results: foo.example.net (foobar) 1 (baz); 2151 dkim (Because I like it) / 1 (One yay) = (wait for it) fail 2152 policy (A dot can go here) . (like that) expired 2153 (this surprised me) = (as I wasn't expecting it) 1362471462 2155 Example 7: A Very Comment-Heavy but Perfectly Legal Example 2157 Appendix D. Operational Considerations about Message Authentication 2159 This protocol is predicated on the idea that authentication (and 2160 presumably in the future, reputation) work is typically done by 2161 border MTAs rather than MUAs or intermediate MTAs; the latter merely 2162 make use of the results determined by the former. Certainly this is 2163 not mandatory for participation in electronic mail or message 2164 authentication, but this protocol and its deployment to date are 2165 based on that model. The assumption satisfies several common ADMD 2166 requirements: 2168 1. Service operators prefer to resolve the handling of problem 2169 messages as close to the border of the ADMD as possible. This 2170 enables, for example, rejection of messages at the SMTP level 2171 rather than generating a DSN internally. Thus, doing any of the 2172 authentication or reputation work exclusively at the MUA or 2173 intermediate MTA renders this desire unattainable. 2175 2. Border MTAs are more likely to have direct access to external 2176 sources of authentication or reputation information since modern 2177 MUAs are more likely to be heavily firewalled. Thus, some MUAs 2178 might not even be able to complete the task of performing 2179 authentication or reputation evaluations without complex proxy 2180 configurations or similar burdens. 2182 3. MUAs rely upon the upstream MTAs within their trust boundaries to 2183 make correct (as much as is possible) evaluations about the 2184 message's envelope, header, and content. Thus, MUAs don't need 2185 to know how to do the work that upstream MTAs do; they only need 2186 the results of that work. 2188 4. Evaluations about the quality of a message, from simple token 2189 matching (e.g., a list of preferred DNS domains) to cryptanalysis 2190 (e.g., public/private key work), are at least a little bit 2191 expensive and thus need to be minimized. To that end, performing 2192 those tests at the border MTA is far preferred to doing that work 2193 at each MUA that handles a message. If an ADMD's environment 2194 adheres to common messaging protocols, a reputation query or an 2195 authentication check performed by a border MTA would return the 2196 same result as the same query performed by an MUA. By contrast, 2197 in an environment where the MUA does the work, a message arriving 2198 for multiple recipients would thus cause authentication or 2199 reputation evaluation to be done more than once for the same 2200 message (i.e., at each MUA), causing needless amplification of 2201 resource use and creating a possible denial-of-service attack 2202 vector. 2204 5. Minimizing change is good. As new authentication and reputation 2205 methods emerge, the list of methods supported by this header 2206 field would presumably be extended. If MUAs simply consume the 2207 contents of this header field rather than actually attempt to do 2208 authentication and/or reputation work, then MUAs only need to 2209 learn to parse this header field once; emergence of new methods 2210 requires only a configuration change at the MUAs and software 2211 changes at the MTAs (which are presumably fewer in number). When 2212 choosing to implement these functions in MTAs vs. MUAs, the 2213 issues of individual flexibility, infrastructure inertia, and 2214 scale of effort must be considered. It is typically easier to 2215 change a single MUA than an MTA because the modification affects 2216 fewer users and can be pursued with less care. However, changing 2217 many MUAs is more effort than changing a smaller number of MTAs. 2219 6. For decisions affecting message delivery and display, assessment 2220 based on authentication and reputation is best performed close to 2221 the time of message transit, as a message makes its journey 2222 toward a user's inbox, not afterwards. DKIM keys and IP address 2223 reputations, etc., can change over time or even become invalid, 2224 and users can take a long time to read a message once delivered. 2225 The value of this work thus degrades, perhaps quickly, once the 2226 delivery process has completed. This seriously diminishes the 2227 value of this work when done other than at MTAs. 2229 Many operational choices are possible within an ADMD, including the 2230 venue for performing authentication and/or reputation assessment. 2231 The current specification does not dictate any of those choices. 2232 Rather, it facilitates those cases in which information produced by 2233 one stage of analysis needs to be transported with the message to the 2234 next stage. 2236 Appendix E. Change History 2238 E.1. RFC7001 to -00 2240 o Remove "Changes since RFC5451" section; add this "Change History" 2241 section. 2243 o Restore XML to previous format. (No visible changes). 2245 o Reset "Acknowledgments". 2247 o Add "To-Do" section. 2249 E.2. -00 to -01 2251 o Apply RFC7410. 2253 o Update all the RFC4408 references to RFC7208. 2255 o Add section explaining "property" values. (Errata #4201) 2257 o Remove "To-Do" section. 2259 E.3. -01 to -02 2261 o Consolidate new sections. 2263 E.4. -02 to -03 2265 o Move the DKIM exception text down to where the DKIM results are 2266 defined, and add a forward reference to them. 2268 o More detail about registry creation and previous augmentations. 2270 o Add text explaining each of the method-ptype-property tuples 2271 registered by this document. 2273 o Change the meaning of the "Defined" column of the methods registry 2274 to be the place where each entry was created and described; it is 2275 expected that this will then refer to the method's defining 2276 document. Provide IANA with corresponding update instructions. 2278 o Add references: [DMARC], [PRA], [RFC6577], [RRVS], [SMIME-REG]. 2280 E.5. -03 to -04 2282 o Add specific update instructions for the "dkim"/"header"/"b" entry 2283 in IANA Considerations. 2285 o Add description of values that can be extracted from SMTP AUTH 2286 sessions and an example. 2288 o Much more complete descriptions of reporting DomainKeys results. 2290 o Minor editorial adjustments. 2292 o Fix up "smime" entries. 2294 o Update current definitions for the Email Authentication Property 2295 Types registry to point to this document. 2297 o Rework the Email Authentication Result Names registry. 2299 o Add more detail about Sender ID. 2301 o Mark all ADSP and DomainKeys entries as deprecated since their 2302 defining documents are as well. 2304 o Add references: [RFC6008]. 2306 E.6. -04 to -05 2308 o Fix typos. 2310 o Rework some text around ignoring unknown ptypes. 2312 o Completely describe the ptypes registry. 2314 o EHLO is mapped to HELO for SPF. 2316 o RFC7208 uses all-lowercase result strings now, so adjust prose 2317 accordingly. 2319 o Move the RFC6008 reference up to where the DKIM reporting is 2320 described. 2322 E.7. -05 to -06 2324 WGLC feedback: 2326 o Update list of supported methods, and mention the registries 2327 immediately below there. 2329 o Mention that when a local-part is removed, the "@" goes with it. 2331 o Refer to RFC7328 in the "iprev" definition. 2333 o Correction to "smime-part" prose. 2335 o Examples that use SMTP AUTH now claim "with ESMTPA" in the 2336 Received fields. 2338 E.8. -06 to -07 2340 o Wording around the S/MIME results. 2342 Author's Address 2344 Murray S. Kucherawy 2345 270 Upland Drive 2346 San Francisco, CA 94127 2347 US 2349 EMail: superuser@gmail.com