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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 March 23, 2015 4 Obsoletes: 7001, 7410 5 (if approved) 6 Intended status: Standards Track 7 Expires: September 24, 2015 9 Message Header Field for Indicating Message Authentication Status 10 draft-ietf-appsawg-rfc7001bis-04 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 September 24, 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 . . . . . . . . . . . . . . . . . . . . . . 5 58 1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 59 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 60 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6 61 1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 62 1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . . 7 63 1.5.3. Email Architecture . . . . . . . . . . . . . . . . . . 7 64 1.5.4. Other Terms . . . . . . . . . . . . . . . . . . . . . 8 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 . . . . . . . . . . . . 15 74 2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16 75 2.7.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 17 76 2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 18 77 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 19 78 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 20 79 2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21 80 2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 21 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 . . . . . . . . . 24 84 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 25 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 . . . 27 89 6.3. "Email Authentication Methods" Registry Update . . . . . . 28 90 6.4. "Email Authentication Property Types" Registry . . . . . . 30 91 6.5. "Email Authentication Result Names" Description . . . . . 30 92 6.6. "Email Authentication Result Names" Update . . . . . . . . 30 93 7. Security Considerations . . . . . . . . . . . . . . . . . . . 31 94 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 31 95 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 33 96 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 33 97 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 34 98 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 34 99 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 34 100 7.7. Attacks against Authentication Methods . . . . . . . . . . 34 101 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 34 102 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 35 103 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 35 104 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 35 105 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35 106 8.1. Normative References . . . . . . . . . . . . . . . . . . . 35 107 8.2. Informative References . . . . . . . . . . . . . . . . . . 36 108 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 38 109 Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 38 110 Appendix C. Authentication-Results Examples . . . . . . . . . . . 39 111 C.1. Trivial Case; Header Field Not Present . . . . . . . . . . 39 112 C.2. Nearly Trivial Case; Service Provided, but No 113 Authentication Done . . . . . . . . . . . . . . . . . . . 40 114 C.3. Service Provided, Authentication Done . . . . . . . . . . 41 115 C.4. Service Provided, Several Authentications Done, Single 116 MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 117 C.5. Service Provided, Several Authentications Done, 118 Different MTAs . . . . . . . . . . . . . . . . . . . . . . 43 119 C.6. Service Provided, Multi-Tiered Authentication Done . . . . 45 120 C.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 46 121 Appendix D. Operational Considerations about Message 122 Authentication . . . . . . . . . . . . . . . . . . . 47 123 Appendix E. Change History . . . . . . . . . . . . . . . . . . . 48 124 E.1. RFC7001 to -00 . . . . . . . . . . . . . . . . . . . . . . 48 125 E.2. -00 to -01 . . . . . . . . . . . . . . . . . . . . . . . . 49 126 E.3. -01 to -02 . . . . . . . . . . . . . . . . . . . . . . . . 49 127 E.4. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . . 49 128 E.5. -03 to -04 . . . . . . . . . . . . . . . . . . . . . . . . 49 130 1. Introduction 132 This document describes a header field called Authentication-Results 133 for electronic mail messages that presents the results of a message 134 authentication effort in a machine-readable format. The intent of 135 the header field is to create a place to collect such data when 136 message authentication mechanisms are in use so that a Mail User 137 Agent (MUA) and downstream filters can make filtering decisions 138 and/or provide a recommendation to the user as to the validity of the 139 message's origin and possibly the safety and integrity of its 140 content. 142 This document revises the original definition found in [RFC5451] 143 based upon various authentication protocols in current use and 144 incorporates errata logged since the publication of the original 145 specification. 147 End users are not expected to be direct consumers of this header 148 field. This header field is intended for consumption by programs 149 that will then use such data or render it in a human-usable form. 151 This document specifies the format of this header field and discusses 152 the implications of its presence or absence. However, it does not 153 discuss how the data contained in the header field ought to be used, 154 such as what filtering decisions are appropriate or how an MUA might 155 render those results, as these are local policy and/or user interface 156 design questions that are not appropriate for this document. 158 At the time of publication of this document, the following are 159 published, domain-level email authentication methods in common use: 161 o Author Domain Signing Practices ([ADSP]) 163 o SMTP Service Extension for Authentication ([AUTH]) 165 o DomainKeys Identified Mail Signatures ([DKIM]) 167 o Sender Policy Framework ([SPF]) 169 o Vouch By Reference ([VBR]) 171 o reverse IP address name validation ("iprev", defined in Section 3) 173 In addition, the following are non-standard methods recognized by 174 this specification that are no longer common: 176 o DomainKeys ([DOMAINKEYS]) (Historic) 177 o Sender ID ([SENDERID]) (Experimental) 179 This specification is not intended to be restricted to domain-based 180 authentication schemes, but the existing schemes in that family have 181 proven to be a good starting point for implementations. The goal is 182 to give current and future authentication schemes a common framework 183 within which to deliver their results to downstream agents and 184 discourage the creation of unique header fields for each. 186 Although SPF defined a header field called "Received-SPF" and the 187 historic DomainKeys defined one called "DomainKey-Status" for this 188 purpose, those header fields are specific to the conveyance of their 189 respective results only and thus are insufficient to satisfy the 190 requirements enumerated below. In addition, many SPF implementations 191 have adopted the header field specified here at least as an option, 192 and DomainKeys has been obsoleted by DKIM. 194 1.1. Purpose 196 The header field defined in this document is expected to serve 197 several purposes: 199 1. Convey the results of various message authentication checks, 200 which are applied by upstream filters and Mail Transfer Agents 201 (MTAs) and then passed to MUAs and downstream filters within the 202 same "trust domain". Such agents might wish to render those 203 results to end users or to use those data to apply more or less 204 stringent content checks based on authentication results; 206 2. Provide a common location within a message for this data; 208 3. Create an extensible framework for reporting new authentication 209 methods as they emerge. 211 In particular, the mere presence of this header field does not mean 212 its contents are valid. Rather, the header field is reporting 213 assertions made by one or more authentication schemes (supposedly) 214 applied somewhere upstream. For an MUA or downstream filter to treat 215 the assertions as actually valid, there must be an assessment of the 216 trust relationship among such agents, the validating MTA, and the 217 mechanism for conveying the information. 219 1.2. Trust Boundary 221 This document makes several references to the "trust boundary" of an 222 administrative management domain (ADMD). Given the diversity among 223 existing mail environments, a precise definition of this term isn't 224 possible. 226 Simply put, a transfer from the producer of the header field to the 227 consumer must occur within a context that permits the consumer to 228 treat assertions by the producer as being reliable and accurate 229 (trustworthy). How this trust is obtained is outside the scope of 230 this document. It is entirely a local matter. 232 Thus, this document defines a "trust boundary" as the delineation 233 between "external" and "internal" entities. Services that are 234 internal -- within the trust boundary -- are provided by the ADMD's 235 infrastructure for its users. Those that are external are outside of 236 the authority of the ADMD. By this definition, hosts that are within 237 a trust boundary are subject to the ADMD's authority and policies, 238 independent of their physical placement or their physical operation. 239 For example, a host within a trust boundary might actually be 240 operated by a remote service provider and reside physically within 241 its data center. 243 It is possible for a message to be evaluated inside a trust boundary 244 but then depart and re-enter the trust boundary. An example might be 245 a forwarded message such as a message/rfc822 attachment (see 246 Multipurpose Internet Mail Extensions [MIME]) or one that is part of 247 a multipart/digest. The details reported by this field cannot be 248 trusted in that case. Thus, this field found within one of those 249 media types is typically ignored. 251 1.3. Processing Scope 253 The content of this header field is meant to convey to message 254 consumers that authentication work on the message was already done 255 within its trust boundary, and those results are being presented. It 256 is not intended to provide message parameters to consumers so that 257 they can perform authentication protocols on their own. 259 1.4. Requirements 261 This document establishes no new requirements on existing protocols 262 or servers. 264 In particular, this document establishes no requirement on MTAs to 265 reject or filter arriving messages that do not pass authentication 266 checks. The data conveyed by the specified header field's contents 267 are for the information of MUAs and filters and are to be used at 268 their discretion. 270 1.5. Definitions 272 This section defines various terms used throughout this document. 274 1.5.1. Key Words 276 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 277 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 278 document are to be interpreted as described in [KEYWORDS]. 280 1.5.2. Security 282 "Guidelines for Writing RFC Text on Security Considerations" 283 ([SECURITY]) discusses authentication and authorization and the 284 conflation of the two concepts. The use of those terms within the 285 context of recent message security work has given rise to slightly 286 different definitions, and this document reflects those current 287 usages, as follows: 289 o "Authorization" is the establishment of permission to use a 290 resource or represent an identity. In this context, authorization 291 indicates that a message from a particular ADMD arrived via a 292 route the ADMD has explicitly approved. 294 o "Authentication" is the assertion of validity of a piece of data 295 about a message (such as the sender's identity) or the message in 296 its entirety. 298 As examples: SPF and Sender ID are authorization mechanisms in that 299 they express a result that shows whether or not the ADMD that 300 apparently sent the message has explicitly authorized the connecting 301 Simple Mail Transfer Protocol ([SMTP]) client to relay messages on 302 its behalf, but they do not actually validate any other property of 303 the message itself. By contrast, DKIM is agnostic as to the routing 304 of a message but uses cryptographic signatures to authenticate 305 agents, assign (some) responsibility for the message (which implies 306 authorization), and ensure that the listed portions of the message 307 were not modified in transit. Since the signatures are not tied to 308 SMTP connections, they can be added by either the ADMD of origin, 309 intermediate ADMDs (such as a mailing list server), other handling 310 agents, or any combination. 312 Rather than create a separate header field for each class of 313 solution, this proposal groups them both into a single header field. 315 1.5.3. Email Architecture 317 o A "border MTA" is an MTA that acts as a gateway between the 318 general Internet and the users within an organizational boundary. 319 (See also Section 1.2.) 321 o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that 322 actually enacts delivery of a message to a user's inbox or other 323 final delivery. 325 o An "intermediate MTA" is any MTA that is not a delivery MTA and is 326 also not the first MTA to handle the message. 328 The following diagram illustrates the flow of mail among these 329 defined components. See Internet Mail Architecture [EMAIL-ARCH] for 330 further discussion on general email system architecture, which 331 includes detailed descriptions of these components, and Appendix D of 332 this document for discussion about the common aspects of email 333 authentication in current environments. 335 +-----+ +-----+ +------------+ 336 | MUA |-->| MSA |-->| Border MTA | 337 +-----+ +-----+ +------------+ 338 | 339 | 340 V 341 +----------+ 342 | Internet | 343 +----------+ 344 | 345 | 346 V 347 +-----+ +-----+ +------------------+ +------------+ 348 | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA | 349 +-----+ +-----+ +------------------+ +------------+ 351 Generally, it is assumed that the work of applying message 352 authentication schemes takes place at a border MTA or a delivery MTA. 353 This specification is written with that assumption in mind. However, 354 there are some sites at which the entire mail infrastructure consists 355 of a single host. In such cases, such terms as "border MTA" and 356 "delivery MTA" might well apply to the same machine or even the very 357 same agent. It is also possible that some message authentication 358 tests could take place on an intermediate MTA. Although this 359 document doesn't specifically describe such cases, they are not meant 360 to be excluded. 362 1.5.4. Other Terms 364 In this document, the term "producer" refers to any component that 365 adds this header field to messages it is handling, and "consumer" 366 refers to any component that identifies, extracts, and parses the 367 header field to use as part of a handling decision. 369 1.6. Trust Environment 371 This header field permits one or more message validation mechanisms 372 to communicate output to one or more separate assessment mechanisms. 373 These mechanisms operate within a unified trust boundary that defines 374 an Administrative Management Domain (ADMD). An ADMD contains one or 375 more entities that perform validation and generate the header field 376 and one or more that consume it for some type of assessment. The 377 field often contains no integrity or validation mechanism of its own, 378 so its presence must be trusted implicitly. Hence, valid use of the 379 header field requires removing any occurrences of it that are present 380 when the message enters the ADMD. This ensures that later 381 occurrences have been added within the trust boundary of the ADMD. 383 The authserv-id token defined in Section 2.2 can be used to reference 384 an entire ADMD or a specific validation engine within an ADMD. 385 Although the labeling scheme is left as an operational choice, some 386 guidance for selecting a token is provided in later sections of this 387 document. 389 2. Definition and Format of the Header Field 391 This section gives a general overview of the format of the header 392 field being defined and then provides more formal specification. 394 2.1. General Description 396 The header field specified here is called Authentication-Results. It 397 is a Structured Header Field as defined in Internet Message Format 398 ([MAIL]), and thus all of the related definitions in that document 399 apply. 401 This header field is added at the top of the message as it transits 402 MTAs that do authentication checks, so some idea of how far away the 403 checks were done can be inferred. It is therefore considered to be a 404 trace field as defined in [MAIL], and thus all of the related 405 definitions in that document apply. 407 The value of the header field (after removing comments) consists of 408 an authentication identifier, an optional version, and then a series 409 of statements and supporting data. The statements are of the form 410 "method=result" and indicate which authentication method(s) were 411 applied and their respective results. For each such statement, the 412 supporting data can include a "reason" string and one or more 413 "property=value" statements indicating which message properties were 414 evaluated to reach that conclusion. 416 The header field can appear more than once in a single message, more 417 than one result can be represented in a single header field, or a 418 combination of these can be applied. 420 2.2. Formal Definition 422 Formally, the header field is specified as follows using Augmented 423 Backus-Naur Form ([ABNF]): 425 authres-header = "Authentication-Results:" [CFWS] authserv-id 426 [ CFWS authres-version ] 427 ( no-result / 1*resinfo ) [CFWS] CRLF 429 authserv-id = value 430 ; see below for a description of this element 432 authres-version = 1*DIGIT [CFWS] 433 ; indicates which version of this specification is in use; 434 ; this specification is version "1", and the absence of a 435 ; version implies this version of the specification 437 no-result = [CFWS] ";" [CFWS] "none" 438 ; the special case of "none" is used to indicate that no 439 ; message authentication was performed 441 resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ] 442 *( CFWS propspec ) 444 methodspec = [CFWS] method [CFWS] "=" [CFWS] result 445 ; indicates which authentication method was evaluated 446 ; and what its output was 448 reasonspec = "reason" [CFWS] "=" [CFWS] value 449 ; a free-form comment on the reason the given result 450 ; was returned 452 propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue 453 ; an indication of which properties of the message 454 ; were evaluated by the authentication scheme being 455 ; applied to yield the reported result 457 method = Keyword [ [CFWS] "/" [CFWS] method-version ] 458 ; a method indicates which method's result is 459 ; represented by "result", and is one of the methods 460 ; explicitly defined as valid in this document 461 ; or is an extension method as defined below 463 method-version = 1*DIGIT [CFWS] 464 ; indicates which version of the method specification is 465 ; in use, corresponding to the matching entry in the IANA 466 ; "Email Authentication Methods" registry; a value of "1" 467 ; is assumed if this version string is absent 469 result = Keyword 470 ; indicates the results of the attempt to authenticate 471 ; the message; see below for details 473 ptype = Keyword 474 ; indicates whether the property being evaluated was 475 ; a parameter to an [SMTP] command, was a value taken 476 ; from a message header field, was some property of 477 ; the message body, or was some other property evaluated by 478 ; the receiving MTA; expected to be one of the "property 479 ; types" explicitly defined as valid, or an extension 480 ; ptype, as defined below 482 property = special-smtp-verb / Keyword 483 ; indicates more specifically than "ptype" what the 484 ; source of the evaluated property is; the exact meaning 485 ; is specific to the method whose result is being reported 486 ; and is defined more clearly below 488 special-smtp-verb = "mailfrom" / "rcptto" 489 ; special cases of [SMTP] commands that are made up 490 ; of multiple words 492 pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) 493 [CFWS] 494 ; the value extracted from the message property defined 495 ; by the "ptype.property" construction 497 "local-part" is defined in Section 3.4.1 of [MAIL], and "CFWS" is 498 defined in Section 3.2.2 of [MAIL]. 500 "Keyword" is defined in Section 4.1.2 of [SMTP]. 502 The "value" is as defined in Section 5.1 of [MIME]. 504 The "domain-name" is as defined in Section 3.5 of [DKIM]. 506 The "Keyword" used in "result" above is further constrained by the 507 necessity of being enumerated in Section 2.7. 509 See Section 2.5 for a description of the authserv-id element. 511 If the value portion of a "pvalue" construction identifies something 512 intended to be an e-mail identity, then it MUST use the right hand 513 portion of that ABNF definition. 515 The list of commands eligible for use with the "smtp" ptype can be 516 found in Section 4.1 of [SMTP]. 518 The "propspec" may be omitted if, for example, the method was unable 519 to extract any properties to do its evaluation yet has a result to 520 report. 522 Where an SMTP command name is being reported as a "property", the 523 agent generating the header field represents that command by 524 converting it to lowercase and dropping any spaces (e.g., "MAIL FROM" 525 becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.). 527 A "ptype" value of "policy" indicates a policy decision about the 528 message not specific to a property of the message that could be 529 extracted. See Section 2.4 for details. 531 Examples of complete messages using this header field can be found in 532 Appendix C. 534 2.3. Property Types (ptypes) and Properties 536 The "ptype" in the ABNF above indicates the general type of property 537 being described by the result being reported, upon which the reported 538 result was based. Coupled with the "property", which is more 539 specific, they inidcate from which particular part of the message the 540 reported data were extracted. 542 Combinations of ptypes and properties are registered and described in 543 the "Email Authentication Methods" registry, coupled with the 544 authentication methods with which they are used. This is further 545 described in Section 6. 547 Legal values of "ptype" are as defined in the IANA "Email 548 Authentication Property Types" registry, created by 549 [PTYPES-REGISTRY]. The initial values and what they typically 550 indicate are as follows, copied from [RFC7001]: 552 body: Information that was extracted from the body of the message. 553 This might be an arbitrary string of bytes, a hash of a string of 554 bytes, a Uniform Resource Identifier, or some other content of 555 interest. The "property" is an indication of where within the 556 message body the extracted content was found, and can indicate an 557 offset, identify a MIME part, or 559 header: Indicates information that was extracted from the header of 560 the message. This might be the value of a header field or some 561 portion of a header field. The "property" gives a more precise 562 indication of the place in the header from which the extraction 563 took place. 565 policy: A local policy mechanism was applied that augments or 566 overrides the result returned by the authentication mechanism. 567 (See Section 2.4.) 569 smtp: Indicates information that was extracted from an SMTP command 570 that was used to relay the message. The "property" indicates 571 which SMTP command included the extracted content as a parameter. 573 When a consumer of this header field encounters a "ptype" that it 574 does not understand, it ignores the result reported with that 575 "ptype". 577 Entries in the "Email Authentication Methods" registry can define 578 properties that deviate from these definitions when appropriate. 579 Such deviations need to be clear in the registry and/or in the 580 defining document. See Section 2.7.1 for an example. 582 2.4. The "policy" ptype 584 A special ptype value of "policy" is also defined. This ptype is 585 provided to indicate that some local policy mechanism was applied 586 that augments or even replaces (i.e., overrides) the result returned 587 by the authentication mechanism. The property and value in this case 588 identify the local policy that was applied and the result it 589 returned. 591 For example, a DKIM signature is not required to include the Subject 592 header field in the set of fields that are signed. An ADMD receiving 593 such a message might decide that such a signature is unacceptable, 594 even if it passes, because the content of the Subject header field 595 could be altered post-signing without invalidating the signature. 596 Such an ADMD could replace the DKIM "pass" result with a "policy" 597 result and then also include the following in the corresponding 598 Authentication-Result field: 600 ... dkim=fail policy.dkim-rules=unsigned-subject ... 602 In this case, the property is "dkim-rules", indicating some local 603 check by that name took place and that check returned a result of 604 "unsigned-subject". These are arbitrary names selected by (and 605 presumably used within) the ADMD making use of them, so they are not 606 normally registered with IANA or otherwise specified apart from 607 setting syntax restrictions that allow for easy parsing within the 608 rest of the header field. 610 This ptype existed in the original specification for this header 611 field, but without a complete description or example of intended use. 612 As a result, it has not seen any practical use to date that matches 613 its intended purpose. These added details are provided to guide 614 implementers toward proper use. 616 2.5. Authentication Identifier Field 618 Every Authentication-Results header field has an authentication 619 service identifier field (authserv-id above). Specifically, this is 620 any string intended to identify the authentication service within the 621 ADMD that conducted authentication checks on the message. This 622 identifier is intended to be machine-readable and not necessarily 623 meaningful to users. 625 Since agents consuming this field will use this identifier to 626 determine whether its contents are of interest (and are safe to use), 627 the uniqueness of the identifier MUST be guaranteed by the ADMD that 628 generates it and MUST pertain to that ADMD. MUAs or downstream 629 filters SHOULD use this identifier to determine whether or not the 630 data contained in an Authentication-Results header field ought to be 631 used or ignored. 633 For simplicity and scalability, the authentication service identifier 634 SHOULD be a common token used throughout the ADMD. Common practice 635 is to use the DNS domain name used by or within that ADMD, sometimes 636 called the "organizational domain", but this is not strictly 637 necessary. 639 For tracing and debugging purposes, the authentication identifier can 640 instead be the specific hostname of the MTA performing the 641 authentication check whose result is being reported. Moreover, some 642 implementations define a substructure to the identifier; these are 643 outside of the scope of this specification. 645 Note, however, that using a local, relative identifier like a flat 646 hostname, rather than a hierarchical and globally unique ADMD 647 identifier like a DNS domain name, makes configuration more difficult 648 for large sites. The hierarchical identifier permits aggregating 649 related, trusted systems together under a single, parent identifier, 650 which in turn permits assessing the trust relationship with a single 651 reference. The alternative is a flat namespace requiring 652 individually listing each trusted system. Since consumers will use 653 the identifier to determine whether to use the contents of the header 654 field: 656 o Changes to the identifier impose a large, centralized 657 administrative burden. 659 o Ongoing administrative changes require constantly updating this 660 centralized table, making it difficult to ensure that an MUA or 661 downstream filter will have access to accurate information for 662 assessing the usability of the header field's content. In 663 particular, consumers of the header field will need to know not 664 only the current identifier(s) in use but previous ones as well to 665 account for delivery latency or later re-assessment of the header 666 field's contents. 668 Examples of valid authentication identifiers are "example.com", 669 "mail.example.org", "ms1.newyork.example.com", and "example-auth". 671 2.6. Version Tokens 673 The grammar above provides for the optional inclusion of versions on 674 both the header field itself (attached to the authserv-id token) and 675 on each of the methods being reported. The method version refers to 676 the method itself, which is specified in the documents describing 677 those methods, while the authserv-id version refers to this document 678 and thus the syntax of this header field. 680 The purpose of including these is to avoid misinterpretation of the 681 results. That is, if a parser finds a version after an authserv-id 682 that it does not explicitly know, it can immediately discontinue 683 trying to parse since what follows might not be in an expected 684 format. For a method version, the parser SHOULD ignore a method 685 result if the version is not supported in case the semantics of the 686 result have a different meaning than what is expected. For example, 687 if a hypothetical DKIM version 2 yielded a "pass" result for 688 different reasons than version 1 does, a consumer of this field might 689 not want to use the altered semantics. Allowing versions in the 690 syntax is a way to indicate this and let the consumer of the header 691 field decide. 693 2.7. Defined Methods and Result Values 695 Each individual authentication method returns one of a set of 696 specific result values. The subsections below provide references to 697 the documents defining the authentication methods specifically 698 supported by this document, and their corresponding result values. 699 Verifiers SHOULD use these values as described below. New methods 700 not specified in this document, but intended to be supported by the 701 header field defined here, MUST include a similar result table either 702 in their defining documents or in supplementary ones. 704 2.7.1. DKIM and DomainKeys 706 DKIM is represented by the "dkim" method and is defined in [DKIM]. 707 DomainKeys is defined in [DOMAINKEYS] and is represented by the 708 "domainkeys" method. 710 Section 3.8 of [DOMAINKEYS] enumerates some possible results of a 711 DomainKeys evaluation. Those results are not used when generating 712 this header field; rather, the results returned are listed below. 714 A signature is "acceptable to the ADMD" if it passes local policy 715 checks (or there are no specific local policy checks). For example, 716 an ADMD policy might require that the signature(s) on the message be 717 added using the DNS domain present in the From header field of the 718 message, thus making third-party signatures unacceptable even if they 719 verify. 721 Both DKIM and DomainKeys use the same result set, as follows: 723 none: The message was not signed. 725 pass: The message was signed, the signature or signatures were 726 acceptable to the ADMD, and the signature(s) passed verification 727 tests. 729 fail: The message was signed and the signature or signatures were 730 acceptable to the ADMD, but they failed the verification test(s). 732 policy: The message was signed, but some aspect of the signature or 733 signatures was not acceptable to the ADMD. 735 neutral: The message was signed, but the signature or signatures 736 contained syntax errors or were not otherwise able to be 737 processed. This result is also used for other failures not 738 covered elsewhere in this list. 740 temperror: The message could not be verified due to some error that 741 is likely transient in nature, such as a temporary inability to 742 retrieve a public key. A later attempt may produce a final 743 result. 745 permerror: The message could not be verified due to some error that 746 is unrecoverable, such as a required header field being absent. A 747 later attempt is unlikely to produce a final result. 749 DKIM results are reported using a ptype of "header". The property, 750 however, represents one of the tags found in the DKIM-Signature 751 header field rather than a distinct header field. For example, the 752 ptype-property combination "header.d" refers to the content of the 753 "d" (signing domain) tag from within the signature header field, and 754 not a distinct header field called "d". 756 [DKIM] advises that if a message fails verification, it is to be 757 treated as an unsigned message. A report of "fail" here permits the 758 receiver of the report to decide how to handle the failure. A report 759 of "neutral" or "none" preempts that choice, ensuring the message 760 will be treated as if it had not been signed. 762 Section 3.1 of [DOMAINKEYS] describes a process by which the sending 763 address of the message is determined. DomainKeys results are thus 764 reported along with the signing domain name, the sending address of 765 the message, and the name of the header field from which the latter 766 was extracted. This means that a DomainKeys result includes a ptype- 767 property combination of "header.d", plus one of "header.from" and 768 "header.sender". The sending address extracted from the header is 769 included with any [MAIL]-style comments removed; moreover, the local- 770 part of the address is removed if it has not been authenticated in 771 some way. 773 2.7.2. SPF and Sender ID 775 SPF and Sender ID use the "spf" and "sender-id" method names, 776 respectively. The result values for SPF are defined in Section 2.6 777 of [SPF], and those definitions are included here by reference: 779 +-----------+--------------------------------+ 780 | Code | Meaning | 781 +-----------+--------------------------------+ 782 | none | [RFC7208], Section 2.6.1 | 783 +-----------+--------------------------------+ 784 | pass | [RFC7208], Section 2.6.3 | 785 +-----------+--------------------------------+ 786 | fail | [RFC7208], Section 2.6.4 | 787 +-----------+--------------------------------+ 788 | softfail | [RFC7208], Section 2.6.5 | 789 +-----------+--------------------------------+ 790 | policy | [this RFC], Section 2.4 | 791 +-----------+--------------------------------+ 792 | neutral | [RFC7208], Section 2.6.2 | 793 +-----------+--------------------------------+ 794 | temperror | [RFC7208], Section 2.6.6 | 795 +-----------+--------------------------------+ 796 | permerror | [RFC7208], Section 2.6.7 | 797 +-----------+--------------------------------+ 799 These result codes are used in the context of this specification to 800 reflect the result returned by the component conducting SPF 801 evaluation. 803 For SPF, the ptype used is "smtp", and the property is any of 804 "mailfrom", "helo", and "ehlo", since those values are the ones SPF 805 can evaluate. 807 The "sender-id" method is described in [SENDERID]. For this method, 808 the ptype used is "header" and the property will be the name of the 809 header field from which the Purported Responsible address (see [PRA]) 810 was extracted, namely one of "Resent-Sender", "Resent-From", 811 "Sender", or "From". 813 The results for Sender ID are listed and described in Section 4.2 of 814 [SENDERID], but for the purposes of this specification, the SPF 815 definitions enumerated above are used instead. 817 Note that both of those documents specify result codes that use mixed 818 case, but they are typically used all lowercase in this context. 820 For both methods, an additional result of "policy" is defined, which 821 means the client was authorized to inject or relay mail on behalf of 822 the sender's DNS domain according to the authentication method's 823 algorithm, but local policy dictates that the result is unacceptable. 824 For example, "policy" might be used if SPF returns a "pass" result, 825 but a local policy check matches the sending DNS domain to one found 826 in an explicit list of unacceptable DNS domains (e.g., spammers). 828 If the retrieved sender policies used to evaluate SPF and Sender ID 829 do not contain explicit provisions for authenticating the local-part 830 (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported 831 along with results for these mechanisms SHOULD NOT include the local- 832 part. 834 2.7.3. "iprev" 836 The result values used by the "iprev" method, defined in Section 3, 837 are as follows: 839 pass: The DNS evaluation succeeded, i.e., the "reverse" and 840 "forward" lookup results were returned and were in agreement. 842 fail: The DNS evaluation failed. In particular, the "reverse" and 843 "forward" lookups each produced results, but they were not in 844 agreement, or the "forward" query completed but produced no 845 result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an 846 RCODE of 0 (NOERROR) in a reply containing no answers, was 847 returned. 849 temperror: The DNS evaluation could not be completed due to some 850 error that is likely transient in nature, such as a temporary DNS 851 error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or 852 other error condition resulted. A later attempt may produce a 853 final result. 855 permerror: The DNS evaluation could not be completed because no PTR 856 data are published for the connecting IP address, e.g., a DNS 857 RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) 858 in a reply containing no answers, was returned. This prevented 859 completion of the evaluation. A later attempt is unlikely to 860 produce a final result. 862 There is no "none" for this method since any TCP connection 863 delivering email has an IP address associated with it, so some kind 864 of evaluation will always be possible. 866 The result is reported using a ptype of "policy" (as this is not part 867 of any established protocol) and a property of "iprev". 869 For discussion of the format of DNS replies, see "Domain Names - 870 Implementation and Specification" ([DNS]). 872 2.7.4. SMTP AUTH 874 SMTP AUTH (defined in [AUTH]) is represented by the "auth" method, 875 and its result values are as follows: 877 none: SMTP authentication was not attempted. 879 pass: The SMTP client authenticated to the server reporting the 880 result using the protocol described in [AUTH]. 882 fail: The SMTP client attempted to authenticate to the server using 883 the protocol described in [AUTH] but was not successful, yet 884 continued to send the message about which a result is being 885 reported. 887 temperror: The SMTP client attempted to authenticate using the 888 protocol described in [AUTH] but was not able to complete the 889 attempt due to some error that is likely transient in nature, such 890 as a temporary directory service lookup error. A later attempt 891 may produce a final result. 893 permerror: The SMTP client attempted to authenticate using the 894 protocol described in [AUTH] but was not able to complete the 895 attempt due to some error that is likely not transient in nature, 896 such as a permanent directory service lookup error. A later 897 attempt is not likely to produce a final result. 899 The result of AUTH is reported using a ptype of "smtp" and a property 900 of either: 902 o "auth", in which case the value is the authorization identity 903 generated by the exchange initiated by the AUTH command; or 905 o "mailfrom", in which case the value is the mailbox identified by 906 the AUTH parameter used with the MAIL FROM command. 908 If both identities are available, both can be reported. For example, 909 consider this command issued by a client that has completed session 910 authentication with the AUTH command resulting in an authorized 911 identity of "client@c.example": 913 MAIL FROM: AUTH= 915 This could result in a resinfo construction like so: 917 ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example 919 An agent making use of the data provided by this header field SHOULD 920 consider "fail" and "temperror" to be synonymous in terms of message 921 authentication, i.e., the client did not authenticate in either case. 923 2.7.5. Other Registered Codes 925 Result codes were also registered in other RFCs as follows: 927 o Vouch By Reference (in [AR-VBR], represented by "vbr"); 929 o Authorized Third-Party Signatures (in [ATPS], represented by 930 "dkim-atps"); 932 o Author Domain Signing Practices (in [ADSP], represented by "dkim- 933 adsp"); 935 o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs"); 937 o S/MIME (in [SMIME-REG], represented by "smime"). 939 o The ability to report different DKIM results for a multiply-signed 940 message (in [RFC6008]). 942 2.7.6. Extension Methods 944 Additional authentication method identifiers (extension methods) may 945 be defined in the future by later revisions or extensions to this 946 specification. These method identifiers are registered with the 947 Internet Assigned Numbers Authority (IANA) and, preferably, published 948 in an RFC. See Section 6 for further details. 950 Extension methods can be defined for the following reasons: 952 1. To allow additional information from new authentication systems 953 to be communicated to MUAs or downstream filters. The names of 954 such identifiers ought to reflect the name of the method being 955 defined but ought not be needlessly long. 957 2. To allow the creation of "sub-identifiers" that indicate 958 different levels of authentication and differentiate between 959 their relative strengths, e.g., "auth1-weak" and "auth1-strong". 961 Authentication method implementers are encouraged to provide adequate 962 information, via message header field comments if necessary, to allow 963 an MUA developer to understand or relay ancillary details of 964 authentication results. For example, if it might be of interest to 965 relay what data was used to perform an evaluation, such information 966 could be relayed as a comment in the header field, such as: 968 Authentication-Results: example.com; 969 foo=pass bar.baz=blob (2 of 3 tests OK) 971 Experimental method identifiers MUST only be used within ADMDs that 972 have explicitly consented to use them. These method identifiers and 973 the parameters associated with them are not documented in RFCs. 974 Therefore, they are subject to change at any time and not suitable 975 for production use. Any MTA, MUA, or downstream filter intended for 976 production use SHOULD ignore or delete any Authentication-Results 977 header field that includes an experimental (unknown) method 978 identifier. 980 2.7.7. Extension Result Codes 982 Additional result codes (extension results) might be defined in the 983 future by later revisions or extensions to this specification. 984 Result codes MUST be registered with the Internet Assigned Numbers 985 Authority (IANA) and preferably published in an RFC. See Section 6 986 for further details. 988 Experimental results MUST only be used within ADMDs that have 989 explicitly consented to use them. These results and the parameters 990 associated with them are not formally documented. Therefore, they 991 are subject to change at any time and not suitable for production 992 use. Any MTA, MUA, or downstream filter intended for production use 993 SHOULD ignore or delete any Authentication-Results header field that 994 includes an extension result. 996 3. The "iprev" Authentication Method 998 This section defines an additional authentication method called 999 "iprev". 1001 "iprev" is an attempt to verify that a client appears to be valid 1002 based on some DNS queries, which is to say that the IP address is 1003 explicitly associated with a domain name. Upon receiving a session 1004 initiation of some kind from a client, the IP address of the client 1005 peer is queried for matching names (i.e., a number-to-name 1006 translation, also known as a "reverse lookup" or a "PTR" record 1007 query). Once that result is acquired, a lookup of each of the names 1008 (i.e., a name-to-number translation, or an "A" or "AAAA" record 1009 query) thus retrieved is done. The response to this second check 1010 will typically result in at least one mapping back to the client's IP 1011 address. 1013 Expressed as an algorithm: If the client peer's IP address is I, the 1014 list of names to which I maps (after a "PTR" query) is the set N, and 1015 the union of IP addresses to which each member of N maps (after 1016 corresponding "A" and "AAAA" queries) is L, then this test is 1017 successful if I is an element of L. 1019 The response to a PTR query could contain multiple names. To prevent 1020 heavy DNS loads, agents performing these queries MUST be implemented 1021 such that the number of names evaluated by generation of 1022 corresponding A or AAAA queries is limited so as not to be unduly 1023 taxing to the DNS infrastructure, though it MAY be configurable by an 1024 administrator. As an example, Section 4.6.4 of [SPF] chose a limit 1025 of 10 for its implementation of this algorithm. 1027 "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the 1028 query formats for the IPv6 case. 1030 There is some contention regarding the wisdom and reliability of this 1031 test. For example, in some regions, it can be difficult for this 1032 test ever to pass because the practice of arranging to match the 1033 forward and reverse DNS is infrequently observed. Therefore, the 1034 precise implementation details of how a verifier performs an "iprev" 1035 test are not specified here. The verifier MAY report a successful or 1036 failed "iprev" test at its discretion having done some kind of check 1037 of the validity of the connection's identity using DNS. It is 1038 incumbent upon an agent making use of the reported "iprev" result to 1039 understand what exactly that particular verifier is attempting to 1040 report. 1042 Extensive discussion of reverse DNS mapping and its implications can 1043 be found in "Considerations for the use of DNS Reverse Mapping" 1044 ([DNSOP-REVERSE]). In particular, it recommends that applications 1045 avoid using this test as a means of authentication or security. Its 1046 presence in this document is not an endorsement but is merely 1047 acknowledgement that the method remains common and provides the means 1048 to relay the results of that test. 1050 4. Adding the Header Field to a Message 1052 This specification makes no attempt to evaluate the relative 1053 strengths of various message authentication methods that may become 1054 available. The methods listed are an order-independent set; their 1055 sequence does not indicate relative strength or importance of one 1056 method over another. Instead, the MUA or downstream filter consuming 1057 this header field is to interpret the result of each method based on 1058 its own knowledge of what that method evaluates. 1060 Each "method" MUST refer to an authentication method declared in the 1061 IANA registry or an extension method as described in Section 2.7.6, 1062 and each "result" MUST refer to a result code declared in the IANA 1063 registry or an extension result code as defined in Section 2.7.7. 1064 See Section 6 for further information about the registered methods 1065 and result codes. 1067 An MTA compliant with this specification adds this header field 1068 (after performing one or more message authentication tests) to 1069 indicate which MTA or ADMD performed the test, which test got 1070 applied, and what the result was. If an MTA applies more than one 1071 such test, it adds this header field either once per test or once 1072 indicating all of the results. An MTA MUST NOT add a result to an 1073 existing header field. 1075 An MTA MAY add this header field containing only the authentication 1076 identifier portion and the "none" token (see Section 2.2) to indicate 1077 explicitly that no message authentication schemes were applied prior 1078 to delivery of this message. 1080 An MTA adding this header field has to take steps to identify it as 1081 legitimate to the MUAs or downstream filters that will ultimately 1082 consume its content. One process to do so is described in Section 5. 1083 Further measures may be necessary in some environments. Some 1084 possible solutions are enumerated in Section 7.1. This document does 1085 not mandate any specific solution to this issue as each environment 1086 has its own facilities and limitations. 1088 Most known message authentication methods focus on a particular 1089 identifier to evaluate. SPF and Sender ID differ in that they can 1090 yield a result based on more than one identifier; specifically, SPF 1091 can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom 1092 parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter 1093 or the Purported Responsible Address (PRA) identity. When generating 1094 this field to report those results, only the parameter that yielded 1095 the result is included. 1097 For MTAs that add this header field, adding header fields in order 1098 (at the top), per Section 3.6 of [MAIL], is particularly important. 1099 Moreover, this header field SHOULD be inserted above any other trace 1100 header fields such MTAs might prepend. This placement allows easy 1101 detection of header fields that can be trusted. 1103 End users making direct use of this header field might inadvertently 1104 trust information that has not been properly vetted. If, for 1105 example, a basic SPF result were to be relayed that claims an 1106 authenticated addr-spec, the local-part of that addr-spec has 1107 actually not been authenticated. Thus, an MTA adding this header 1108 field SHOULD NOT include any data that has not been authenticated by 1109 the method(s) being applied. Moreover, MUAs SHOULD NOT render to 1110 users such information if it is presented by a method known not to 1111 authenticate it. 1113 4.1. Header Field Position and Interpretation 1115 In order to ensure non-ambiguous results and avoid the impact of 1116 false header fields, MUAs and downstream filters SHOULD NOT interpret 1117 this header field unless specifically configured to do so by the user 1118 or administrator. That is, this interpretation should not be "on by 1119 default". Naturally then, users or administrators ought not activate 1120 such a feature unless they are certain the header field will be 1121 validly added by an agent within the ADMD that accepts the mail that 1122 is ultimately read by the MUA, and instances of the header field 1123 appearing to originate within the ADMD but are actually added by 1124 foreign MTAs will be removed before delivery. 1126 Furthermore, MUAs and downstream filters SHOULD NOT interpret this 1127 header field unless the authentication service identifier it bears 1128 appears to be one used within its own ADMD as configured by the user 1129 or administrator. 1131 MUAs and downstream filters MUST ignore any result reported using a 1132 "result" not specified in the IANA "Result Code" registry or a 1133 "ptype" not listed in the corresponding registry for such values as 1134 defined in Section 6. Moreover, such agents MUST ignore a result 1135 indicated for any "method" they do not specifically support. 1137 An MUA SHOULD NOT reveal these results to end users, absent careful 1138 human factors design considerations and testing, for the presentation 1139 of trust-related materials. For example, an attacker could register 1140 examp1e.com (note the digit "one") and send signed mail to intended 1141 victims; a verifier would detect that the signature was valid and 1142 report a "pass" even though it's clear the DNS domain name was 1143 intended to mislead. See Section 7.2 for further discussion. 1145 As stated in Section 2.1, this header field MUST be treated as though 1146 it were a trace header field as defined in Section 3.6.7 of [MAIL] 1147 and hence MUST NOT be reordered and MUST be prepended to the message, 1148 so that there is generally some indication upon delivery of where in 1149 the chain of handling MTAs the message authentication was done. 1151 Note that there are a few message handlers that are only capable of 1152 appending new header fields to a message. Strictly speaking, these 1153 handlers are not compliant with this specification. They can still 1154 add the header field to carry authentication details, but any signal 1155 about where in the handling chain the work was done may be lost. 1156 Consumers SHOULD be designed such that this can be tolerated, 1157 especially from a producer known to have this limitation. 1159 MUAs SHOULD ignore instances of this header field discovered within 1160 message/rfc822 MIME attachments. 1162 Further discussion of these topics can be found in Section 7 below. 1164 4.2. Local Policy Enforcement 1166 Some sites have a local policy that considers any particular 1167 authentication policy's non-recoverable failure results (typically 1168 "fail" or similar) as justification for rejecting the message. In 1169 such cases, the border MTA SHOULD issue an SMTP rejection response to 1170 the message, rather than adding this header field and allowing the 1171 message to proceed toward delivery. This is more desirable than 1172 allowing the message to reach an internal host's MTA or spam filter, 1173 thus possibly generating a local rejection such as a Delivery Status 1174 Notification (DSN) [DSN] to a forged originator. Such generated 1175 rejections are colloquially known as "backscatter". 1177 The same MAY also be done for local policy decisions overriding the 1178 results of the authentication methods (e.g., the "policy" result 1179 codes described in Section 2.7). 1181 Such rejections at the SMTP protocol level are not possible if local 1182 policy is enforced at the MUA and not the MTA. 1184 5. Removing Existing Header Fields 1186 For security reasons, any MTA conforming to this specification MUST 1187 delete any discovered instance of this header field that claims, by 1188 virtue of its authentication service identifier, to have been added 1189 within its trust boundary but that did not come directly from another 1190 trusted MTA. For example, an MTA for example.com receiving a message 1191 MUST delete or otherwise obscure any instance of this header field 1192 bearing an authentication service identifier indicating that the 1193 header field was added within example.com prior to adding its own 1194 header fields. This could mean each MTA will have to be equipped 1195 with a list of internal MTAs known to be compliant (and hence 1196 trustworthy). 1198 For simplicity and maximum security, a border MTA could remove all 1199 instances of this header field on mail crossing into its trust 1200 boundary. However, this may conflict with the desire to access 1201 authentication results performed by trusted external service 1202 providers. It may also invalidate signed messages whose signatures 1203 cover external instances of this header field. A more robust border 1204 MTA could allow a specific list of authenticating MTAs whose 1205 information is to be admitted, removing the header field originating 1206 from all others. 1208 As stated in Section 1.2, a formal definition of "trust boundary" is 1209 deliberately not made here. It is entirely possible that a border 1210 MTA for example.com will explicitly trust authentication results 1211 asserted by upstream host example.net even though they exist in 1212 completely disjoint administrative boundaries. In that case, the 1213 border MTA MAY elect not to delete those results; moreover, the 1214 upstream host doing some authentication work could apply a signing 1215 technology such as [DKIM] on its own results to assure downstream 1216 hosts of their authenticity. An example of this is provided in 1217 Appendix C. 1219 Similarly, in the case of messages signed using [DKIM] or other 1220 message-signing methods that sign header fields, this removal action 1221 could invalidate one or more signatures on the message if they 1222 covered the header field to be removed. This behavior can be 1223 desirable since there's little value in validating the signature on a 1224 message with forged header fields. However, signing agents MAY 1225 therefore elect to omit these header fields from signing to avoid 1226 this situation. 1228 An MTA SHOULD remove any instance of this header field bearing a 1229 version (express or implied) that it does not support. However, an 1230 MTA MUST remove such a header field if the [SMTP] connection relaying 1231 the message is not from a trusted internal MTA. This means the MTA 1232 needs to be able to understand versions of this header field at least 1233 as late as the ones understood by the MUAs or other consumers within 1234 its ADMD. 1236 6. IANA Considerations 1238 IANA has registered the defined header field and created two tables 1239 as described below. These registry actions were originally defined 1240 by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created 1241 registries are being further updated here to increase their 1242 completeness. 1244 6.1. The Authentication-Results Header Field 1246 [RFC5451] added the Authentication-Results header field to the IANA 1247 "Permanent Message Header Field Names" registry, per the procedure 1248 found in [IANA-HEADERS]. That entry is to be updated to reference 1249 this document. The following is the registration template: 1251 Header field name: Authentication-Results 1252 Applicable protocol: mail ([MAIL]) 1253 Status: Standard 1254 Author/Change controller: IETF 1255 Specification document(s): [this RFC] 1256 Related information: 1257 Requesting review of any proposed changes and additions to 1258 this field is recommended. 1260 6.2. "Email Authentication Methods" Registry Description 1262 Names of message authentication methods supported by this 1263 specification are to be registered with IANA, with the exception of 1264 experimental names as described in Section 2.7.6. Along with each 1265 method is recorded the properties that accompany the method's result. 1267 The "Email Authentication Parameters" group, and within it the "Email 1268 Authentication Methods" registry, were created by [RFC5451] for this 1269 purpose. [RFC6577] added a "status" field for each entry. [RFC7001] 1270 amended the rules governing that registry, and also added a "version" 1271 field to the registry. 1273 The reference for that registry shall be updated to reference this 1274 document. 1276 New entries are assigned only for values that have received Expert 1277 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1278 appointed by the IESG. The designated expert has discretion to 1279 request that a publication be referenced if a clear, concise 1280 definition of the authentication method cannot be provided such that 1281 interoperability is assured. Registrations should otherwise be 1282 permitted. The designated expert can also handle requests to mark 1283 any current registration as "deprecated". 1285 No two entries can have the same combination of method, ptype, and 1286 property. 1288 An entry in this registry contains the following: 1290 Method: the name of the method; 1292 Defined: a reference to the document that created this entry, if any 1293 (see below); 1295 ptype: a "ptype" value appropriate for use with that method; 1297 property: a "property" value matching that "ptype" also appropriate 1298 for use with that method; 1300 Value: a brief description of the value to be supplied with that 1301 method/ptype/property tuple; 1303 Status: the status of this entry, which is either: 1305 active: The entry is in current use. 1307 deprecated: The entry is no longer in current use. 1309 Version: a version number associated with the method (preferably 1310 starting at "1"). 1312 The "Defined" field will typically refer to a permanent document, or 1313 at least some descriptive text, where additional information about 1314 the entry being added can be found. This in turn would reference the 1315 document where the method is defined so that all of the semantics 1316 around creating or interpreting an Authentication-Results header 1317 field using this method, ptype, and property can be understood. 1319 6.3. "Email Authentication Methods" Registry Update 1321 The following changes are to be made to this registry upon approval 1322 of this document: 1324 1. The current entry for the "auth" method shall have its "property" 1325 field changed to "mailfrom", and its "Defined" field changed to 1326 this document. 1328 2. The entry for the "dkim" method, "header" ptype and "b" property 1329 shall now reference [RFC6008] as its defining document, and the 1330 reference shall be removed from the description. 1332 3. All other "dkim", "domainkeys", "iprev", "sender-id", and "spf" 1333 method entries shall have their "Defined" fields changed to this 1334 document. 1336 4. All "smime" entries have their "Defined" fields changed to 1337 [SMIME-REG]. 1339 5. The "value" field of the "smime" entry using property "smime- 1340 part" shall be changed to read "A reference to the MIME body part 1341 that contains the signature." The redundant reference is thus 1342 removed. 1344 6. The following entry is to be added: 1346 Method: auth 1348 Defined: [this document] 1350 ptype: smtp 1352 property: auth 1354 Value: identity confirmed by the AUTH command 1356 Status: active 1358 Version: 1 1360 7. The values of the "domainkeys" entries for ptype "header" are 1361 updated as follows: 1363 from: contents of the [MAIL] From: header field, after removing 1364 comments, and removing the local-part if not authenticated 1366 sender: contents of the [MAIL] Sender: header field, after 1367 removing comments, and removing the local-part if not 1368 authenticated 1370 8. All entries for "dkim-adsp" and "domainkeys" shall have their 1371 Status values changed to "deprecated", reflecting the fact that 1372 the corresponding specifications now have Historical status. 1374 6.4. "Email Authentication Property Types" Registry 1376 [PTYPES-REGISTRY] created the Email Authentication Property Types 1377 registry. IANA shall update this registry to show Section 2.3 of 1378 this document as the current definitions for the "body", "header", 1379 "policy" and "smtp" entries of that registry. 1381 6.5. "Email Authentication Result Names" Description 1383 Names of message authentication result codes supported by this 1384 specification must be registered with IANA, with the exception of 1385 experimental codes as described in Section 2.7.7. A registry was 1386 created by [RFC5451] for this purpose. [RFC6577] added the "status" 1387 column, and [RFC7001] updated the rules governing that registry. 1389 New entries are assigned only for values that have received Expert 1390 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1391 appointed by the IESG. The designated expert has discretion to 1392 request that a publication be referenced if a clear, concise 1393 definition of the authentication result cannot be provided such that 1394 interoperability is assured. Registrations should otherwise be 1395 permitted. The designated expert can also handle requests to mark 1396 any current registration as "deprecated". 1398 No two entries can have the same combination of method and code. 1400 An entry in this registry contains the following: 1402 Auth Method: an authentication method for which results are being 1403 returned using the header field defined in this document; 1405 Code: a result code that can be returned for this authentication 1406 method; 1408 Specification: either free form text explaining the meaning of this 1409 method-code combination, or a reference to such a definition. 1411 6.6. "Email Authentication Result Names" Update 1413 The following changes are to be made to this registry on publication 1414 of this document: 1416 o The "Defined" field shall be removed. 1418 o The "Meaning" field shall be renamed to "Specification", as 1419 described above. 1421 o The "Auth Method" field shall appear before the "Code" field. 1423 o For easier searching, the table shall be arranged such that it is 1424 sorted first by Auth Method, then by Code within each Auth Method 1425 grouping. 1427 o All entries for the "dkim", "domainkeys", "spf", "sender-id", 1428 "auth", and "iprev" methods shall have their "Specification" 1429 fields changed to refer to this document, as follows: 1431 dkim: [this document] Section 2.7.1 1433 domainkeys: [this document] Section 2.7.1 1435 spf: [this document] Section 2.7.2 1437 sender-id: [this document] Section 2.7.2 1439 auth: [this document] Section 2.7.4 1441 iprev: [this document] Section 2.7.3 1443 o All entries for "dkim-adsp" that are missing an explicit reference 1444 to a defining document shall have [ADSP] added to their 1445 Specification fields. 1447 o All entries for "dkim-adsp" and "domainkeys" shall have their 1448 Status values changed to "deprecated", reflecting the fact that 1449 the corresponding specifications now have Historical status. 1451 7. Security Considerations 1453 The following security considerations apply when adding or processing 1454 the Authentication-Results header field: 1456 7.1. Forged Header Fields 1458 An MUA or filter that accesses a mailbox whose messages are handled 1459 by a non-conformant MTA, and understands Authentication-Results 1460 header fields, could potentially make false conclusions based on 1461 forged header fields. A malicious user or agent could forge a header 1462 field using the DNS domain of a receiving ADMD as the authserv-id 1463 token in the value of the header field and, with the rest of the 1464 value, claim that the message was properly authenticated. The non- 1465 conformant MTA would fail to strip the forged header field, and the 1466 MUA could inappropriately trust it. 1468 For this reason, it is best not to have processing of the 1469 Authentication-Results header field enabled by default; instead, it 1470 should be ignored, at least for the purposes of enacting filtering 1471 decisions, unless specifically enabled by the user or administrator 1472 after verifying that the border MTA is compliant. It is acceptable 1473 to have an MUA aware of this specification but have an explicit list 1474 of hostnames whose Authentication-Results header fields are 1475 trustworthy; however, this list should initially be empty. 1477 Proposed alternative solutions to this problem were made some time 1478 ago and are listed below. To date, they have not been developed due 1479 to lack of demand but are documented here should the information be 1480 useful at some point in the future: 1482 1. Possibly the simplest is a digital signature protecting the 1483 header field, such as using [DKIM], that can be verified by an 1484 MUA by using a posted public key. Although one of the main 1485 purposes of this document is to relieve the burden of doing 1486 message authentication work at the MUA, this only requires that 1487 the MUA learn a single authentication scheme even if a number of 1488 them are in use at the border MTA. Note that [DKIM] requires 1489 that the From header field be signed, although in this 1490 application, the signing agent (a trusted MTA) likely cannot 1491 authenticate that value, so the fact that it is signed should be 1492 ignored. Where the authserv-id is the ADMD's domain name, the 1493 authserv-id matching this valid internal signature's "d=" DKIM 1494 value is sufficient. 1496 2. Another would be a means to interrogate the MTA that added the 1497 header field to see if it is actually providing any message 1498 authentication services and saw the message in question, but this 1499 isn't especially palatable given the work required to craft and 1500 implement such a scheme. 1502 3. Yet another might be a method to interrogate the internal MTAs 1503 that apparently handled the message (based on Received header 1504 fields) to determine whether any of them conform to Section 5 of 1505 this memo. This, too, has potentially high barriers to entry. 1507 4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to 1508 allow an MUA or filtering agent to acquire the authserv-id in use 1509 within an ADMD, thus allowing it to identify which 1510 Authentication-Results header fields it can trust. 1512 5. On the presumption that internal MTAs are fully compliant with 1513 Section 3.6 of [MAIL] and the compliant internal MTAs are using 1514 their own hostnames or the ADMD's DNS domain name as the 1515 authserv-id token, the header field proposed here should always 1516 appear above a Received header added by a trusted MTA. This can 1517 be used as a test for header field validity. 1519 Support for some of these is being considered for future work. 1521 In any case, a mechanism needs to exist for an MUA or filter to 1522 verify that the host that appears to have added the header field (a) 1523 actually did so and (b) is legitimately adding that header field for 1524 this delivery. Given the variety of messaging environments deployed 1525 today, consensus appears to be that specifying a particular mechanism 1526 for doing so is not appropriate for this document. 1528 Mitigation of the forged header field attack can also be accomplished 1529 by moving the authentication results data into metadata associated 1530 with the message. In particular, an [SMTP] extension could be 1531 established to communicate authentication results from the border MTA 1532 to intermediate and delivery MTAs; the latter of these could arrange 1533 to store the authentication results as metadata retrieved and 1534 rendered along with the message by an [IMAP] client aware of a 1535 similar extension in that protocol. The delivery MTA would be told 1536 to trust data via this extension only from MTAs it trusts, and border 1537 MTAs would not accept data via this extension from any source. There 1538 is no vector in such an arrangement for forgery of authentication 1539 data by an outside agent. 1541 7.2. Misleading Results 1543 Until some form of service for querying the reputation of a sending 1544 agent is widely deployed, the existence of this header field 1545 indicating a "pass" does not render the message trustworthy. It is 1546 possible for an arriving piece of spam or other undesirable mail to 1547 pass checks by several of the methods enumerated above (e.g., a piece 1548 of spam signed using [DKIM] by the originator of the spam, which 1549 might be a spammer or a compromised system). In particular, this 1550 issue is not resolved by forged header field removal discussed above. 1552 Hence, MUAs and downstream filters must take some care with use of 1553 this header even after possibly malicious headers are scrubbed. 1555 7.3. Header Field Position 1557 Despite the requirements of [MAIL], header fields can sometimes be 1558 reordered en route by intermediate MTAs. The goal of requiring 1559 header field addition only at the top of a message is an 1560 acknowledgement that some MTAs do reorder header fields, but most do 1561 not. Thus, in the general case, there will be some indication of 1562 which MTAs (if any) handled the message after the addition of the 1563 header field defined here. 1565 7.4. Reverse IP Query Denial-of-Service Attacks 1567 Section 4.6.4 of [SPF] describes a DNS-based denial-of-service attack 1568 for verifiers that attempt DNS-based identity verification of 1569 arriving client connections. A verifier wishing to do this check and 1570 report this information needs to take care not to go to unbounded 1571 lengths to resolve "A" and "PTR" queries. MUAs or other filters 1572 making use of an "iprev" result specified by this document need to be 1573 aware of the algorithm used by the verifier reporting the result and, 1574 especially, its limitations. 1576 7.5. Mitigation of Backscatter 1578 Failing to follow the instructions of Section 4.2 can result in a 1579 denial-of-service attack caused by the generation of [DSN] messages 1580 (or equivalent) to addresses that did not send the messages being 1581 rejected. 1583 7.6. Internal MTA Lists 1585 Section 5 describes a procedure for scrubbing header fields that may 1586 contain forged authentication results about a message. A compliant 1587 installation will have to include, at each MTA, a list of other MTAs 1588 known to be compliant and trustworthy. Failing to keep this list 1589 current as internal infrastructure changes may expose an ADMD to 1590 attack. 1592 7.7. Attacks against Authentication Methods 1594 If an attack becomes known against an authentication method, clearly 1595 then the agent verifying that method can be fooled into thinking an 1596 inauthentic message is authentic, and thus the value of this header 1597 field can be misleading. It follows that any attack against the 1598 authentication methods supported by this document is also a security 1599 consideration here. 1601 7.8. Intentionally Malformed Header Fields 1603 It is possible for an attacker to add an Authentication-Results 1604 header field that is extraordinarily large or otherwise malformed in 1605 an attempt to discover or exploit weaknesses in header field parsing 1606 code. Implementers must thoroughly verify all such header fields 1607 received from MTAs and be robust against intentionally as well as 1608 unintentionally malformed header fields. 1610 7.9. Compromised Internal Hosts 1612 An internal MUA or MTA that has been compromised could generate mail 1613 with a forged From header field and a forged Authentication-Results 1614 header field that endorses it. Although it is clearly a larger 1615 concern to have compromised internal machines than it is to prove the 1616 value of this header field, this risk can be mitigated by arranging 1617 that internal MTAs will remove this header field if it claims to have 1618 been added by a trusted border MTA (as described above), yet the 1619 [SMTP] connection is not coming from an internal machine known to be 1620 running an authorized MTA. However, in such a configuration, 1621 legitimate MTAs will have to add this header field when legitimate 1622 internal-only messages are generated. This is also covered in 1623 Section 5. 1625 7.10. Encapsulated Instances 1627 MIME messages can contain attachments of type "message/rfc822", which 1628 contain other messages. Such an encapsulated message can also 1629 contain an Authentication-Results header field. Although the 1630 processing of these is outside of the intended scope of this document 1631 (see Section 1.3), some early guidance to MUA developers is 1632 appropriate here. 1634 Since MTAs are unlikely to strip Authentication-Results header fields 1635 after mailbox delivery, MUAs are advised in Section 4.1 to ignore 1636 such instances within MIME attachments. Moreover, when extracting a 1637 message digest to separate mail store messages or other media, such 1638 header fields should be removed so that they will never be 1639 interpreted improperly by MUAs that might later consume them. 1641 7.11. Reverse Mapping 1643 Although Section 3 of this memo includes explicit support for the 1644 "iprev" method, its value as an authentication mechanism is limited. 1645 Implementers of both this proposal and agents that use the data it 1646 relays are encouraged to become familiar with the issues raised by 1647 [DNSOP-REVERSE] when deciding whether or not to include support for 1648 "iprev". 1650 8. References 1652 8.1. Normative References 1654 [ABNF] Crocker, D. and P. Overell, "Augmented BNF for 1655 Syntax Specifications: ABNF", STD 68, 1656 RFC 5234, January 2008. 1658 [IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul, 1659 "Registration Procedures for Message Header 1660 Fields", BCP 90, RFC 3864, September 2004. 1662 [KEYWORDS] Bradner, S., "Key words for use in RFCs to 1663 Indicate Requirement Levels", BCP 14, 1664 RFC 2119. 1666 [MAIL] Resnick, P., Ed., "Internet Message Format", 1667 RFC 5322, October 2008. 1669 [MIME] Freed, N. and N. Borenstein, "Multipurpose 1670 Internet Mail Extensions (MIME) Part One: 1671 Format of Internet Message Bodies", RFC 2045, 1672 November 1996. 1674 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", 1675 RFC 5321, October 2008. 1677 8.2. Informative References 1679 [ADSP] Allman, E., Fenton, J., Delany, M., and J. 1680 Levine, "DomainKeys Identified Mail (DKIM) 1681 Author Domain Signing Practices (ADSP)", 1682 RFC 5617, August 2009. 1684 [AR-VBR] Kucherawy, M., "Authentication-Results 1685 Registration for Vouch by Reference Results", 1686 RFC 6212, April 2011. 1688 [ATPS] Kucherawy, M., "DomainKeys Identified Mail 1689 (DKIM) Authorized Third-Party Signatures", 1690 RFC 6541, February 2012. 1692 [AUTH] Siemborski, R. and A. Melnikov, "SMTP Service 1693 Extension for Authentication", RFC 4954, 1694 July 2007. 1696 [DKIM] Crocker, D., Hansen, T., and M. Kucherawy, 1697 "DomainKeys Identified Mail (DKIM) 1698 Signatures", STD 76, RFC 6376, September 2011. 1700 [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., 1701 "Domain-based Message Authentication, 1702 Reporting and Conformance (DMARC)", 1703 I-D draft-kucherawy-dmarc-base, February 2015. 1705 [DNS] Mockapetris, P., "Domain names - 1706 Implementation and Specification", STD 13, 1707 RFC 1035, November 1987. 1709 [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. 1710 Souissi, "DNS Extensions to Support IP Version 1711 6", RFC 3596, October 2003. 1713 [DNSOP-REVERSE] Senie, D. and A. Sullivan, "Considerations for 1714 the use of DNS Reverse Mapping", Work 1715 in Progress, March 2008. 1717 [DOMAINKEYS] Delany, M., "Domain-Based Email Authentication 1718 Using Public Keys Advertised in the DNS 1719 (DomainKeys)", RFC 4870, May 2007. 1721 [DSN] Moore, K. and G. Vaudreuil, "An Extensible 1722 Message Format for Delivery Status 1723 Notifications", RFC 3464, January 2003. 1725 [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", 1726 RFC 5598, July 2009. 1728 [IANA-CONSIDERATIONS] Narten, T. and H. Alvestrand, "Guidelines for 1729 Writing an IANA Considerations Section in 1730 RFCs", BCP 26, RFC 5226, May 2008. 1732 [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL 1733 - VERSION 4rev1", RFC 3501, March 2003. 1735 [POP3] Myers, J. and M. Rose, "Post Office Protocol - 1736 Version 3", STD 53, RFC 1939, May 1996. 1738 [PRA] Lyon, J., "Purported Responsible Address in 1739 E-Mail Messages", RFC 4407, April 2006. 1741 [PTYPES-REGISTRY] Kucherawy, M., "A Property Types Registry for 1742 the Authentication-Results Header Field", 1743 RFC 7410, December 2014. 1745 [RFC5451] Kucherawy, M., "Message Header Field for 1746 Indicating Message Authentication Status", 1747 RFC 5451, April 2009. 1749 [RFC6008] Kucherawy, M., "Authentication-Results 1750 Registration for Differentiating among 1751 Cryptographic Results", RFC 6008, 1752 September 2010. 1754 [RFC6577] Kucherawy, M., "Authentication-Results 1755 Registration Update for Sender Policy 1756 Framework (SPF) Results", RFC 6577, 1757 March 2012. 1759 [RFC7001] Kucherawy, M., "Message Header Field for 1760 Indicating Message Authentication Status", 1761 RFC 7001, September 2013. 1763 [RRVS] Mills, W. and M. Kucherawy, "The Require- 1764 Recipient-Valid-Since Header Field and SMTP 1765 Service Extension", RFC 7293, July 2014. 1767 [SECURITY] Rescorla, E. and B. Korver, "Guidelines for 1768 Writing RFC Text on Security Considerations", 1769 BCP 72, RFC 3552, July 2003. 1771 [SENDERID] Lyon, J. and M. Wong, "Sender ID: 1772 Authenticating E-Mail", RFC 4406, April 2006. 1774 [SMIME-REG] Melnikov, A., "Authentication-Results 1775 Registration for S/MIME Signature 1776 Verification", RFC 7281, June 2014. 1778 [SPF] Kitterman, S., "Sender Policy Framework (SPF) 1779 for Authorizing Use of Domains in E-Mail, 1780 Version 1", RFC 7208, April 2014. 1782 [VBR] Hoffman, P., Levine, J., and A. Hathcock, 1783 "Vouch By Reference", RFC 5518, April 2009. 1785 Appendix A. Acknowledgements 1787 The author wishes to acknowledge the following individuals for their 1788 review and constructive criticism of this document: Stephane 1789 Bortzmeyer, Scott Kitterman, John Levine, and Tom Petch. 1791 Appendix B. Legacy MUAs 1793 Implementers of this protocol should be aware that many MUAs are 1794 unlikely to be retrofitted to support the new header field and its 1795 semantics. In the interests of convenience and quicker adoption, a 1796 delivery MTA might want to consider adding things that are processed 1797 by existing MUAs in addition to the Authentication-Results header 1798 field. One suggestion is to include a Priority header field, on 1799 messages that don't already have such a header field, containing a 1800 value that reflects the strength of the authentication that was 1801 accomplished, e.g., "low" for weak or no authentication, "normal" or 1802 "high" for good or strong authentication. 1804 Some modern MUAs can already filter based on the content of this 1805 header field. However, there is keen interest in having MUAs make 1806 some kind of graphical representation of this header field's meaning 1807 to end users. Until this capability is added, other interim means of 1808 conveying authentication results may be necessary while this proposal 1809 and its successors are adopted. 1811 Appendix C. Authentication-Results Examples 1813 This section presents some examples of the use of this header field 1814 to indicate authentication results. 1816 C.1. Trivial Case; Header Field Not Present 1818 The trivial case: 1820 Received: from mail-router.example.com 1821 (mail-router.example.com [192.0.2.1]) 1822 by server.example.org (8.11.6/8.11.6) 1823 with ESMTP id g1G0r1kA003489; 1824 Fri, Feb 15 2002 17:19:07 -0800 1825 From: sender@example.com 1826 Date: Fri, Feb 15 2002 16:54:30 -0800 1827 To: receiver@example.org 1828 Message-Id: <12345.abc@example.com> 1829 Subject: here's a sample 1831 Hello! Goodbye! 1833 Example 1: Trivial Case 1835 The Authentication-Results header field is completely absent. The 1836 MUA may make no conclusion about the validity of the message. This 1837 could be the case because the message authentication services were 1838 not available at the time of delivery, or no service is provided, or 1839 the MTA is not in compliance with this specification. 1841 C.2. Nearly Trivial Case; Service Provided, but No Authentication Done 1843 A message that was delivered by an MTA that conforms to this 1844 specification but provides no actual message authentication service: 1846 Authentication-Results: example.org 1; none 1847 Received: from mail-router.example.com 1848 (mail-router.example.com [192.0.2.1]) 1849 by server.example.org (8.11.6/8.11.6) 1850 with ESMTP id g1G0r1kA003489; 1851 Fri, Feb 15 2002 17:19:07 -0800 1852 From: sender@example.com 1853 Date: Fri, Feb 15 2002 16:54:30 -0800 1854 To: receiver@example.org 1855 Message-Id: <12345.abc@example.com> 1856 Subject: here's a sample 1858 Hello! Goodbye! 1860 Example 2: Header Present but No Authentication Done 1862 The Authentication-Results header field is present, showing that the 1863 delivering MTA conforms to this specification. It used its DNS 1864 domain name as the authserv-id. The presence of "none" (and the 1865 absence of any method and result tokens) indicates that no message 1866 authentication was done. The version number of the specification to 1867 which the field's content conforms is explicitly provided. 1869 C.3. Service Provided, Authentication Done 1871 A message that was delivered by an MTA that conforms to this 1872 specification and applied some message authentication: 1874 Authentication-Results: example.com; 1875 spf=pass smtp.mailfrom=example.net 1876 Received: from dialup-1-2-3-4.example.net 1877 (dialup-1-2-3-4.example.net [192.0.2.200]) 1878 by mail-router.example.com (8.11.6/8.11.6) 1879 with ESMTP id g1G0r1kA003489; 1880 Fri, Feb 15 2002 17:19:07 -0800 1881 From: sender@example.net 1882 Date: Fri, Feb 15 2002 16:54:30 -0800 1883 To: receiver@example.com 1884 Message-Id: <12345.abc@example.net> 1885 Subject: here's a sample 1887 Hello! Goodbye! 1889 Example 3: Header Reporting Results 1891 The Authentication-Results header field is present, indicating that 1892 the border MTA conforms to this specification. The authserv-id is 1893 once again the DNS domain name. Furthermore, the message was 1894 authenticated by that MTA via the method specified in [SPF]. Note 1895 that since that method cannot authenticate the local-part, it has 1896 been omitted from the result's value. The MUA could extract and 1897 relay this extra information if desired. 1899 C.4. Service Provided, Several Authentications Done, Single MTA 1901 A message that was relayed inbound via a single MTA that conforms to 1902 this specification and applied three different message authentication 1903 checks: 1905 Authentication-Results: example.com; 1906 auth=pass (cram-md5) smtp.auth=sender@example.net; 1907 spf=pass smtp.mailfrom=example.net 1908 Authentication-Results: example.com; 1909 sender-id=pass header.from=example.net 1910 Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6) 1911 (dialup-1-2-3-4.example.net [192.0.2.200]) 1912 by mail-router.example.com (8.11.6/8.11.6) 1913 with ESMTP id g1G0r1kA003489; 1914 Fri, Feb 15 2002 17:19:07 -0800 1915 Date: Fri, Feb 15 2002 16:54:30 -0800 1916 To: receiver@example.com 1917 From: sender@example.net 1918 Message-Id: <12345.abc@example.net> 1919 Subject: here's a sample 1921 Hello! Goodbye! 1923 Example 4: Headers Reporting Results from One MTA 1925 The Authentication-Results header field is present, indicating that 1926 the delivering MTA conforms to this specification. Once again, the 1927 receiving DNS domain name is used as the authserv-id. Furthermore, 1928 the sender authenticated herself/himself to the MTA via a method 1929 specified in [AUTH], and both SPF and Sender ID checks were done and 1930 passed. The MUA could extract and relay this extra information if 1931 desired. 1933 Two Authentication-Results header fields are not required since the 1934 same host did all of the checking. The authenticating agent could 1935 have consolidated all the results into one header field. 1937 This example illustrates a scenario in which a remote user on a 1938 dialup connection (example.net) sends mail to a border MTA 1939 (example.com) using SMTP authentication to prove identity. The 1940 dialup provider has been explicitly authorized to relay mail as 1941 example.com resulting in passes by the SPF and Sender ID checks. 1943 C.5. Service Provided, Several Authentications Done, Different MTAs 1945 A message that was relayed inbound by two different MTAs that conform 1946 to this specification and applied multiple message authentication 1947 checks: 1949 Authentication-Results: example.com; 1950 sender-id=fail header.from=example.com; 1951 dkim=pass (good signature) header.d=example.com 1952 Received: from mail-router.example.com 1953 (mail-router.example.com [192.0.2.1]) 1954 by auth-checker.example.com (8.11.6/8.11.6) 1955 with ESMTP id i7PK0sH7021929; 1956 Fri, Feb 15 2002 17:19:22 -0800 1957 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com; 1958 t=1188964191; c=simple/simple; h=From:Date:To:Subject: 1959 Message-Id:Authentication-Results; 1960 bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70; 1961 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 1962 Authentication-Results: example.com; 1963 auth=pass (cram-md5) smtp.auth=sender@example.com; 1964 spf=fail smtp.mailfrom=example.com 1965 Received: from dialup-1-2-3-4.example.net 1966 (dialup-1-2-3-4.example.net [192.0.2.200]) 1967 by mail-router.example.com (8.11.6/8.11.6) 1968 with ESMTP id g1G0r1kA003489; 1969 Fri, Feb 15 2002 17:19:07 -0800 1970 From: sender@example.com 1971 Date: Fri, Feb 15 2002 16:54:30 -0800 1972 To: receiver@example.com 1973 Message-Id: <12345.abc@example.com> 1974 Subject: here's a sample 1976 Hello! Goodbye! 1978 Example 5: Headers Reporting Results from Multiple MTAs 1980 The Authentication-Results header field is present, indicating 1981 conformance to this specification. Once again, the authserv-id used 1982 is the recipient's DNS domain name. The header field is present 1983 twice because two different MTAs in the chain of delivery did 1984 authentication tests. The first MTA, mail-router.example.com, 1985 reports that SMTP AUTH and SPF were both used and that the former 1986 passed while the latter failed. In the SMTP AUTH case, additional 1987 information is provided in the comment field, which the MUA can 1988 choose to render if desired. 1990 The second MTA, auth-checker.example.com, reports that it did a 1991 Sender ID test (which failed) and a DKIM test (which passed). Again, 1992 additional data about one of the tests is provided as a comment, 1993 which the MUA may choose to render. Also noteworthy here is the fact 1994 that there is a DKIM signature added by example.com that assured the 1995 integrity of the lower Authentication-Results field. 1997 Since different hosts did the two sets of authentication checks, the 1998 header fields cannot be consolidated in this example. 2000 This example illustrates more typical transmission of mail into 2001 example.com from a user on a dialup connection example.net. The user 2002 appears to be legitimate as he/she had a valid password allowing 2003 authentication at the border MTA using SMTP AUTH. The SPF and Sender 2004 ID tests failed since example.com has not granted example.net 2005 authority to relay mail on its behalf. However, the DKIM test passed 2006 because the sending user had a private key matching one of 2007 example.com's published public keys and used it to sign the message. 2009 C.6. Service Provided, Multi-Tiered Authentication Done 2011 A message that had authentication done at various stages, one of 2012 which was outside the receiving ADMD: 2014 Authentication-Results: example.com; 2015 dkim=pass reason="good signature" 2016 header.i=@mail-router.example.net; 2017 dkim=fail reason="bad signature" 2018 header.i=@newyork.example.com 2019 Received: from mail-router.example.net 2020 (mail-router.example.net [192.0.2.250]) 2021 by chicago.example.com (8.11.6/8.11.6) 2022 for 2023 with ESMTP id i7PK0sH7021929; 2024 Fri, Feb 15 2002 17:19:22 -0800 2025 DKIM-Signature: v=1; a=rsa-sha256; s=furble; 2026 d=mail-router.example.net; t=1188964198; c=relaxed/simple; 2027 h=From:Date:To:Message-Id:Subject:Authentication-Results; 2028 bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=; 2029 b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= 2030 Authentication-Results: example.net; 2031 dkim=pass (good signature) header.i=@newyork.example.com 2032 Received: from smtp.newyork.example.com 2033 (smtp.newyork.example.com [192.0.2.220]) 2034 by mail-router.example.net (8.11.6/8.11.6) 2035 with ESMTP id g1G0r1kA003489; 2036 Fri, Feb 15 2002 17:19:07 -0800 2037 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; 2038 d=newyork.example.com; 2039 t=1188964191; c=simple/simple; 2040 h=From:Date:To:Message-Id:Subject; 2041 bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; 2042 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 2043 From: sender@newyork.example.com 2044 Date: Fri, Feb 15 2002 16:54:30 -0800 2045 To: meetings@example.net 2046 Message-Id: <12345.abc@newyork.example.com> 2047 Subject: here's a sample 2049 Example 6: Headers Reporting Results from Multiple MTAs in Different 2050 ADMDs 2052 In this example, we see multi-tiered authentication with an extended 2053 trust boundary. 2055 The message was sent from someone at example.com's New York office 2056 (newyork.example.com) to a mailing list managed at an intermediary. 2058 The message was signed at the origin using DKIM. 2060 The message was sent to a mailing list service provider called 2061 example.net, which is used by example.com. There, 2062 meetings@example.net is expanded to a long list of recipients, one of 2063 whom is at the Chicago office. In this example, we will assume that 2064 the trust boundary for chicago.example.com includes the mailing list 2065 server at example.net. 2067 The mailing list server there first authenticated the message and 2068 affixed an Authentication-Results header field indicating such using 2069 its DNS domain name for the authserv-id. It then altered the message 2070 by affixing some footer text to the body, including some 2071 administrivia such as unsubscription instructions. Finally, the 2072 mailing list server affixes a second DKIM signature and begins 2073 distribution of the message. 2075 The border MTA for chicago.example.com explicitly trusts results from 2076 mail-router.example.net, so that header field is not removed. It 2077 performs evaluation of both signatures and determines that the first 2078 (most recent) is a "pass" but, because of the aforementioned 2079 modifications, the second is a "fail". However, the first signature 2080 included the Authentication-Results header added at mail- 2081 router.example.net that validated the second signature. Thus, 2082 indirectly, it can be determined that the authentications claimed by 2083 both signatures are indeed valid. 2085 Note that two styles of presenting metadata about the result are in 2086 use here. In one case, the "reason=" clause is present, which is 2087 intended for easy extraction by parsers; in the other case, the CFWS 2088 production of the ABNF is used to include such data as a header field 2089 comment. The latter can be harder for parsers to extract given the 2090 varied supported syntaxes of mail header fields. 2092 C.7. Comment-Heavy Example 2094 The formal syntax permits comments within the content in a number of 2095 places. For the sake of illustration, this example is also legal: 2097 Authentication-Results: foo.example.net (foobar) 1 (baz); 2098 dkim (Because I like it) / 1 (One yay) = (wait for it) fail 2099 policy (A dot can go here) . (like that) expired 2100 (this surprised me) = (as I wasn't expecting it) 1362471462 2102 Example 7: A Very Comment-Heavy but Perfectly Legal Example 2104 Appendix D. Operational Considerations about Message Authentication 2106 This protocol is predicated on the idea that authentication (and 2107 presumably in the future, reputation) work is typically done by 2108 border MTAs rather than MUAs or intermediate MTAs; the latter merely 2109 make use of the results determined by the former. Certainly this is 2110 not mandatory for participation in electronic mail or message 2111 authentication, but this protocol and its deployment to date are 2112 based on that model. The assumption satisfies several common ADMD 2113 requirements: 2115 1. Service operators prefer to resolve the handling of problem 2116 messages as close to the border of the ADMD as possible. This 2117 enables, for example, rejection of messages at the SMTP level 2118 rather than generating a DSN internally. Thus, doing any of the 2119 authentication or reputation work exclusively at the MUA or 2120 intermediate MTA renders this desire unattainable. 2122 2. Border MTAs are more likely to have direct access to external 2123 sources of authentication or reputation information since modern 2124 MUAs are more likely to be heavily firewalled. Thus, some MUAs 2125 might not even be able to complete the task of performing 2126 authentication or reputation evaluations without complex proxy 2127 configurations or similar burdens. 2129 3. MUAs rely upon the upstream MTAs within their trust boundaries to 2130 make correct (as much as is possible) evaluations about the 2131 message's envelope, header, and content. Thus, MUAs don't need 2132 to know how to do the work that upstream MTAs do; they only need 2133 the results of that work. 2135 4. Evaluations about the quality of a message, from simple token 2136 matching (e.g., a list of preferred DNS domains) to cryptanalysis 2137 (e.g., public/private key work), are at least a little bit 2138 expensive and thus need to be minimized. To that end, performing 2139 those tests at the border MTA is far preferred to doing that work 2140 at each MUA that handles a message. If an ADMD's environment 2141 adheres to common messaging protocols, a reputation query or an 2142 authentication check performed by a border MTA would return the 2143 same result as the same query performed by an MUA. By contrast, 2144 in an environment where the MUA does the work, a message arriving 2145 for multiple recipients would thus cause authentication or 2146 reputation evaluation to be done more than once for the same 2147 message (i.e., at each MUA), causing needless amplification of 2148 resource use and creating a possible denial-of-service attack 2149 vector. 2151 5. Minimizing change is good. As new authentication and reputation 2152 methods emerge, the list of methods supported by this header 2153 field would presumably be extended. If MUAs simply consume the 2154 contents of this header field rather than actually attempt to do 2155 authentication and/or reputation work, then MUAs only need to 2156 learn to parse this header field once; emergence of new methods 2157 requires only a configuration change at the MUAs and software 2158 changes at the MTAs (which are presumably fewer in number). When 2159 choosing to implement these functions in MTAs vs. MUAs, the 2160 issues of individual flexibility, infrastructure inertia, and 2161 scale of effort must be considered. It is typically easier to 2162 change a single MUA than an MTA because the modification affects 2163 fewer users and can be pursued with less care. However, changing 2164 many MUAs is more effort than changing a smaller number of MTAs. 2166 6. For decisions affecting message delivery and display, assessment 2167 based on authentication and reputation is best performed close to 2168 the time of message transit, as a message makes its journey 2169 toward a user's inbox, not afterwards. DKIM keys and IP address 2170 reputations, etc., can change over time or even become invalid, 2171 and users can take a long time to read a message once delivered. 2172 The value of this work thus degrades, perhaps quickly, once the 2173 delivery process has completed. This seriously diminishes the 2174 value of this work when done other than at MTAs. 2176 Many operational choices are possible within an ADMD, including the 2177 venue for performing authentication and/or reputation assessment. 2178 The current specification does not dictate any of those choices. 2179 Rather, it facilitates those cases in which information produced by 2180 one stage of analysis needs to be transported with the message to the 2181 next stage. 2183 Appendix E. Change History 2185 E.1. RFC7001 to -00 2187 o Remove "Changes since RFC5451" section; add this "Change History" 2188 section. 2190 o Restore XML to previous format. (No visible changes). 2192 o Reset "Acknowledgments". 2194 o Add "To-Do" section. 2196 E.2. -00 to -01 2198 o Apply RFC7410. 2200 o Update all the RFC4408 references to RFC7208. 2202 o Add section explaining "property" values. (Errata #4201) 2204 o Remove "To-Do" section. 2206 E.3. -01 to -02 2208 o Consolidate new sections. 2210 E.4. -02 to -03 2212 o Move the DKIM exception text down to where the DKIM results are 2213 defined, and add a forward reference to them. 2215 o More detail about registry creation and previous augmentations. 2217 o Add text explaining each of the method-ptype-property tuples 2218 registered by this document. 2220 o Change the meaning of the "Defined" column of the methods registry 2221 to be the place where each entry was created and described; it is 2222 expected that this will then refer to the method's defining 2223 document. Provide IANA with corresponding update instructions. 2225 o Add references: [DMARC], [PRA], [RFC6577], [RRVS], [SMIME-REG]. 2227 E.5. -03 to -04 2229 o Add specific update instructions for the "dkim"/"header"/"b" entry 2230 in IANA Considerations. 2232 o Add description of values that can be extracted from SMTP AUTH 2233 sessions and an example. 2235 o Much more complete descriptions of reporting DomainKeys results. 2237 o Minor editorial adjustments. 2239 o Fix up "smime" entries. 2241 o Update current definitions for the Email Authentication Property 2242 Types registry to point to this document. 2244 o Rework the Email Authentication Result Names registry. 2246 o Add more detail about Sender ID. 2248 o Mark all ADSP and DomainKeys entries as deprecated since their 2249 defining documents are as well. 2251 o Add references: [RFC6008]. 2253 Author's Address 2255 Murray S. Kucherawy 2256 270 Upland Drive 2257 San Francisco, CA 94127 2258 US 2260 EMail: superuser@gmail.com