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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 7001 (Obsoleted by RFC 7601) Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Individual submission M. Kucherawy 3 Internet-Draft February 19, 2015 4 Obsoletes: 7001, 7410 5 (if approved) 6 Intended status: Standards Track 7 Expires: August 23, 2015 9 Message Header Field for Indicating Message Authentication Status 10 draft-ietf-appsawg-rfc7001bis-01 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 August 23, 2015. 38 Copyright Notice 40 Copyright (c) 2015 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 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) . . . . . . . . . . . . . . . . . 12 70 2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 13 71 2.5. Properties . . . . . . . . . . . . . . . . . . . . . . . . 13 72 2.6. Authentication Identifier Field . . . . . . . . . . . . . 14 73 2.7. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 15 74 2.8. Defined Methods and Result Values . . . . . . . . . . . . 16 75 2.8.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16 76 2.8.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 17 77 2.8.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 18 78 2.8.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 19 79 2.8.5. Other Registered Codes . . . . . . . . . . . . . . . . 19 80 2.8.6. Extension Methods . . . . . . . . . . . . . . . . . . 19 81 2.8.7. Extension Result Codes . . . . . . . . . . . . . . . . 20 82 3. The "iprev" Authentication Method . . . . . . . . . . . . . . 21 83 4. Adding the Header Field to a Message . . . . . . . . . . . . . 22 84 4.1. Header Field Position and Interpretation . . . . . . . . . 23 85 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 24 86 5. Removing Existing Header Fields . . . . . . . . . . . . . . . 24 87 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 88 6.1. The Authentication-Results Header Field . . . . . . . . . 26 89 6.2. "Email Authentication Methods" Registry . . . . . . . . . 26 90 6.3. "Email Authentication Result Names" Registry . . . . . . . 27 91 7. Security Considerations . . . . . . . . . . . . . . . . . . . 27 92 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 27 93 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 29 94 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 29 95 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 29 96 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 30 97 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 30 98 7.7. Attacks against Authentication Methods . . . . . . . . . . 30 99 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 30 100 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 30 101 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 31 102 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 31 103 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 104 8.1. Normative References . . . . . . . . . . . . . . . . . . . 31 105 8.2. Informative References . . . . . . . . . . . . . . . . . . 32 106 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 34 107 Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 34 108 Appendix C. Authentication-Results Examples . . . . . . . . . . . 34 109 C.1. Trivial Case; Header Field Not Present . . . . . . . . . . 35 110 C.2. Nearly Trivial Case; Service Provided, but No 111 Authentication Done . . . . . . . . . . . . . . . . . . . 35 112 C.3. Service Provided, Authentication Done . . . . . . . . . . 36 113 C.4. Service Provided, Several Authentications Done, Single 114 MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 115 C.5. Service Provided, Several Authentications Done, 116 Different MTAs . . . . . . . . . . . . . . . . . . . . . . 38 117 C.6. Service Provided, Multi-Tiered Authentication Done . . . . 40 118 C.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 41 119 Appendix D. Operational Considerations about Message 120 Authentication . . . . . . . . . . . . . . . . . . . 42 121 Appendix E. Change History . . . . . . . . . . . . . . . . . . . 43 122 E.1. RFC7001 to -00 . . . . . . . . . . . . . . . . . . . . . . 43 123 E.2. -00 to -01 . . . . . . . . . . . . . . . . . . . . . . . . 44 125 1. Introduction 127 This document describes a header field called Authentication-Results 128 for electronic mail messages that presents the results of a message 129 authentication effort in a machine-readable format. The intent of 130 the header field is to create a place to collect such data when 131 message authentication mechanisms are in use so that a Mail User 132 Agent (MUA) and downstream filters can make filtering decisions 133 and/or provide a recommendation to the user as to the validity of the 134 message's origin and possibly the safety and integrity of its 135 content. 137 This document revises the original definition found in [RFC5451] 138 based upon various authentication protocols in current use and 139 incorporates errata logged since the publication of the original 140 specification. 142 End users are not expected to be direct consumers of this header 143 field. This header field is intended for consumption by programs 144 that will then use such data or render it in a human-usable form. 146 This document specifies the format of this header field and discusses 147 the implications of its presence or absence. However, it does not 148 discuss how the data contained in the header field ought to be used, 149 such as what filtering decisions are appropriate or how an MUA might 150 render those results, as these are local policy and/or user interface 151 design questions that are not appropriate for this document. 153 At the time of publication of this document, the following are 154 published, domain-level email authentication methods in common use: 156 o Author Domain Signing Practices ([ADSP]) 158 o SMTP Service Extension for Authentication ([AUTH]) 160 o DomainKeys Identified Mail Signatures ([DKIM]) 162 o Sender Policy Framework ([SPF]) 164 o Vouch By Reference ([VBR]) 166 o reverse IP address name validation ("iprev", defined in Section 3) 168 In addition, the following are non-standard methods recognized by 169 this specification that are no longer common: 171 o DomainKeys ([DOMAINKEYS]) (Historic) 172 o Sender ID ([SENDERID]) (Experimental) 174 This specification is not intended to be restricted to domain-based 175 authentication schemes, but the existing schemes in that family have 176 proven to be a good starting point for implementations. The goal is 177 to give current and future authentication schemes a common framework 178 within which to deliver their results to downstream agents and 179 discourage the creation of unique header fields for each. 181 Although SPF defined a header field called "Received-SPF" and the 182 historic DomainKeys defined one called "DomainKey-Status" for this 183 purpose, those header fields are specific to the conveyance of their 184 respective results only and thus are insufficient to satisfy the 185 requirements enumerated below. In addition, many SPF implementations 186 have adopted the header field specified here at least as an option, 187 and DomainKeys has been obsoleted by DKIM. 189 1.1. Purpose 191 The header field defined in this document is expected to serve 192 several purposes: 194 1. Convey the results of various message authentication checks, 195 which are applied by upstream filters and Mail Transfer Agents 196 (MTAs) and then passed to MUAs and downstream filters within the 197 same "trust domain". Such agents might wish to render those 198 results to end users or to use those data to apply more or less 199 stringent content checks based on authentication results; 201 2. Provide a common location within a message for this data; 203 3. Create an extensible framework for reporting new authentication 204 methods as they emerge. 206 In particular, the mere presence of this header field does not mean 207 its contents are valid. Rather, the header field is reporting 208 assertions made by one or more authentication schemes (supposedly) 209 applied somewhere upstream. For an MUA or downstream filter to treat 210 the assertions as actually valid, there must be an assessment of the 211 trust relationship among such agents, the validating MTA, and the 212 mechanism for conveying the information. 214 1.2. Trust Boundary 216 This document makes several references to the "trust boundary" of an 217 administrative management domain (ADMD). Given the diversity among 218 existing mail environments, a precise definition of this term isn't 219 possible. 221 Simply put, a transfer from the producer of the header field to the 222 consumer must occur within a context that permits the consumer to 223 treat assertions by the producer as being reliable and accurate 224 (trustworthy). How this trust is obtained is outside the scope of 225 this document. It is entirely a local matter. 227 Thus, this document defines a "trust boundary" as the delineation 228 between "external" and "internal" entities. Services that are 229 internal -- within the trust boundary -- are provided by the ADMD's 230 infrastructure for its users. Those that are external are outside of 231 the authority of the ADMD. By this definition, hosts that are within 232 a trust boundary are subject to the ADMD's authority and policies, 233 independent of their physical placement or their physical operation. 234 For example, a host within a trust boundary might actually be 235 operated by a remote service provider and reside physically within 236 its data center. 238 It is possible for a message to be evaluated inside a trust boundary 239 but then depart and re-enter the trust boundary. An example might be 240 a forwarded message such as a message/rfc822 attachment (see 241 Multipurpose Internet Mail Extensions [MIME]) or one that is part of 242 a multipart/digest. The details reported by this field cannot be 243 trusted in that case. Thus, this field found within one of those 244 media types is typically ignored. 246 1.3. Processing Scope 248 The content of this header field is meant to convey to message 249 consumers that authentication work on the message was already done 250 within its trust boundary, and those results are being presented. It 251 is not intended to provide message parameters to consumers so that 252 they can perform authentication protocols on their own. 254 1.4. Requirements 256 This document establishes no new requirements on existing protocols 257 or servers. 259 In particular, this document establishes no requirement on MTAs to 260 reject or filter arriving messages that do not pass authentication 261 checks. The data conveyed by the specified header field's contents 262 are for the information of MUAs and filters and are to be used at 263 their discretion. 265 1.5. Definitions 267 This section defines various terms used throughout this document. 269 1.5.1. Key Words 271 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 272 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 273 document are to be interpreted as described in [KEYWORDS]. 275 1.5.2. Security 277 "Guidelines for Writing RFC Text on Security Considerations" 278 ([SECURITY]) discusses authentication and authorization and the 279 conflation of the two concepts. The use of those terms within the 280 context of recent message security work has given rise to slightly 281 different definitions, and this document reflects those current 282 usages, as follows: 284 o "Authorization" is the establishment of permission to use a 285 resource or represent an identity. In this context, authorization 286 indicates that a message from a particular ADMD arrived via a 287 route the ADMD has explicitly approved. 289 o "Authentication" is the assertion of validity of a piece of data 290 about a message (such as the sender's identity) or the message in 291 its entirety. 293 As examples: SPF and Sender ID are authorization mechanisms in that 294 they express a result that shows whether or not the ADMD that 295 apparently sent the message has explicitly authorized the connecting 296 Simple Mail Transfer Protocol ([SMTP]) client to relay messages on 297 its behalf, but they do not actually validate any other property of 298 the message itself. By contrast, DKIM is agnostic as to the routing 299 of a message but uses cryptographic signatures to authenticate 300 agents, assign (some) responsibility for the message (which implies 301 authorization), and ensure that the listed portions of the message 302 were not modified in transit. Since the signatures are not tied to 303 SMTP connections, they can be added by either the ADMD of origin, 304 intermediate ADMDs (such as a mailing list server), other handling 305 agents, or any combination. 307 Rather than create a separate header field for each class of 308 solution, this proposal groups them both into a single header field. 310 1.5.3. Email Architecture 312 o A "border MTA" is an MTA that acts as a gateway between the 313 general Internet and the users within an organizational boundary. 314 (See also Section 1.2.) 316 o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that 317 actually enacts delivery of a message to a user's inbox or other 318 final delivery. 320 o An "intermediate MTA" is any MTA that is not a delivery MTA and is 321 also not the first MTA to handle the message. 323 The following diagram illustrates the flow of mail among these 324 defined components. See Internet Mail Architecture [EMAIL-ARCH] for 325 further discussion on general email system architecture, which 326 includes detailed descriptions of these components, and Appendix D of 327 this document for discussion about the common aspects of email 328 authentication in current environments. 330 +-----+ +-----+ +------------+ 331 | MUA |-->| MSA |-->| Border MTA | 332 +-----+ +-----+ +------------+ 333 | 334 | 335 V 336 +----------+ 337 | Internet | 338 +----------+ 339 | 340 | 341 V 342 +-----+ +-----+ +------------------+ +------------+ 343 | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA | 344 +-----+ +-----+ +------------------+ +------------+ 346 Generally, it is assumed that the work of applying message 347 authentication schemes takes place at a border MTA or a delivery MTA. 348 This specification is written with that assumption in mind. However, 349 there are some sites at which the entire mail infrastructure consists 350 of a single host. In such cases, such terms as "border MTA" and 351 "delivery MTA" might well apply to the same machine or even the very 352 same agent. It is also possible that some message authentication 353 tests could take place on an intermediate MTA. Although this 354 document doesn't specifically describe such cases, they are not meant 355 to be excluded. 357 1.5.4. Other Terms 359 In this document, the term "producer" refers to any component that 360 adds this header field to messages it is handling, and "consumer" 361 refers to any component that identifies, extracts, and parses the 362 header field to use as part of a handling decision. 364 1.6. Trust Environment 366 This header field permits one or more message validation mechanisms 367 to communicate output to one or more separate assessment mechanisms. 368 These mechanisms operate within a unified trust boundary that defines 369 an Administrative Management Domain (ADMD). An ADMD contains one or 370 more entities that perform validation and generate the header field 371 and one or more that consume it for some type of assessment. The 372 field often contains no integrity or validation mechanism of its own, 373 so its presence must be trusted implicitly. Hence, valid use of the 374 header field requires removing any occurrences of it that are present 375 when the message enters the ADMD. This ensures that later 376 occurrences have been added within the trust boundary of the ADMD. 378 The authserv-id token defined in Section 2.2 can be used to reference 379 an entire ADMD or a specific validation engine within an ADMD. 380 Although the labeling scheme is left as an operational choice, some 381 guidance for selecting a token is provided in later sections of this 382 document. 384 2. Definition and Format of the Header Field 386 This section gives a general overview of the format of the header 387 field being defined and then provides more formal specification. 389 2.1. General Description 391 The header field specified here is called Authentication-Results. It 392 is a Structured Header Field as defined in Internet Message Format 393 ([MAIL]), and thus all of the related definitions in that document 394 apply. 396 This header field is added at the top of the message as it transits 397 MTAs that do authentication checks, so some idea of how far away the 398 checks were done can be inferred. It is therefore considered to be a 399 trace field as defined in [MAIL], and thus all of the related 400 definitions in that document apply. 402 The value of the header field (after removing comments) consists of 403 an authentication identifier, an optional version, and then a series 404 of statements and supporting data. The statements are of the form 405 "method=result" and indicate which authentication method(s) were 406 applied and their respective results. For each such statement, the 407 supporting data can include a "reason" string and one or more 408 "property=value" statements indicating which message properties were 409 evaluated to reach that conclusion. 411 The header field can appear more than once in a single message, more 412 than one result can be represented in a single header field, or a 413 combination of these can be applied. 415 2.2. Formal Definition 417 Formally, the header field is specified as follows using Augmented 418 Backus-Naur Form ([ABNF]): 420 authres-header = "Authentication-Results:" [CFWS] authserv-id 421 [ CFWS authres-version ] 422 ( no-result / 1*resinfo ) [CFWS] CRLF 424 authserv-id = value 425 ; see below for a description of this element 427 authres-version = 1*DIGIT [CFWS] 428 ; indicates which version of this specification is in use; 429 ; this specification is version "1", and the absence of a 430 ; version implies this version of the specification 432 no-result = [CFWS] ";" [CFWS] "none" 433 ; the special case of "none" is used to indicate that no 434 ; message authentication was performed 436 resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ] 437 *( CFWS propspec ) 439 methodspec = [CFWS] method [CFWS] "=" [CFWS] result 440 ; indicates which authentication method was evaluated 441 ; and what its output was 443 reasonspec = "reason" [CFWS] "=" [CFWS] value 444 ; a free-form comment on the reason the given result 445 ; was returned 447 propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue 448 ; an indication of which properties of the message 449 ; were evaluated by the authentication scheme being 450 ; applied to yield the reported result 452 method = Keyword [ [CFWS] "/" [CFWS] method-version ] 453 ; a method indicates which method's result is 454 ; represented by "result", and is one of the methods 455 ; explicitly defined as valid in this document 456 ; or is an extension method as defined below 458 method-version = 1*DIGIT [CFWS] 459 ; indicates which version of the method specification is 460 ; in use, corresponding to the matching entry in the IANA 461 ; "Email Authentication Methods" registry; a value of "1" 462 ; is assumed if this version string is absent 464 result = Keyword 465 ; indicates the results of the attempt to authenticate 466 ; the message; see below for details 468 ptype = Keyword 469 ; indicates whether the property being evaluated was 470 ; a parameter to an [SMTP] command, was a value taken 471 ; from a message header field, was some property of 472 ; the message body, or was some other property evaluated by 473 ; the receiving MTA; expected to be one of the "property 474 ; types" explicitly defined as valid, or an extension 475 ; ptype, as defined below 477 property = special-smtp-verb / Keyword 478 ; indicates more specifically than "ptype" what the 479 ; source of the evaluated property is; the exact meaning 480 ; is specific to the method whose result is being reported 481 ; and is defined more clearly below 483 special-smtp-verb = "mailfrom" / "rcptto" 484 ; special cases of [SMTP] commands that are made up 485 ; of multiple words 487 pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) 488 [CFWS] 489 ; the value extracted from the message property defined 490 ; by the "ptype.property" construction 492 "local-part" is defined in Section 3.4.1 of [MAIL], and "CFWS" is 493 defined in Section 3.2.2 of [MAIL]. 495 "Keyword" is defined in Section 4.1.2 of [SMTP]. 497 The "value" is as defined in Section 5.1 of [MIME]. 499 The "domain-name" is as defined in Section 3.5 of [DKIM]. 501 The "Keyword" used in "result" above is further constrained by the 502 necessity of being enumerated in Section 2.8. 504 See Section 2.6 for a description of the authserv-id element. 506 If the value portion of a "pvalue" construction identifies something 507 intended to be an e-mail identity, then it MUST use the right hand 508 portion of that ABNF definition. 510 The list of commands eligible for use with the "smtp" ptype can be 511 found in Section 4.1 of [SMTP]. 513 The "propspec" may be omitted if, for example, the method was unable 514 to extract any properties to do its evaluation yet has a result to 515 report. 517 Where an SMTP command name is being reported as a "property", the 518 agent generating the header field represents that command by 519 converting it to lowercase and dropping any spaces (e.g., "MAIL FROM" 520 becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.). 522 A "ptype" value of "policy" indicates a policy decision about the 523 message not specific to a property of the message that could be 524 extracted. See Section 2.4 for details. 526 Examples of complete messages using this header field can be found in 527 Appendix C. 529 2.3. Property Types (ptypes) 531 The "ptype" in the ABNF above indicates the type of property being 532 described by the result being reported, upon which the reported 533 result was based. Coupled with the "property", it inidcates from 534 which part of the message the reported data were extracted. 536 Legal values of "ptype" are as defined in the IANA "Email 537 Authentication Property Types" registry, created by 538 [PTYPES-REGISTRY]. The initial values are as follows, copied from 539 [RFC7001]: 541 body: Indicates information that was extracted from the body of the 542 message. This might be an arbitrary string of bytes, a hash of a 543 string of bytes, a Uniform Resource Identifier, or some other 544 content of interest. 546 header: Indicates information that was extracted from the header of 547 the message. This might be the value of a header field or some 548 portion of a header field. 550 policy: A local policy mechanism was applied that augments or 551 overrides the result returned by the authentication mechanism. 552 (See Section 2.4.) 554 smtp: Indicates information that was extracted from an SMTP command 555 that was used to relay the message. 557 When a consumer of this header field encounters a "ptype" that it 558 does not understand, it ignores the result reported with that 559 "ptype". 561 2.4. The "policy" ptype 563 A special ptype value of "policy" is also defined. This ptype is 564 provided to indicate that some local policy mechanism was applied 565 that augments or even replaces (i.e., overrides) the result returned 566 by the authentication mechanism. The property and value in this case 567 identify the local policy that was applied and the result it 568 returned. 570 For example, a DKIM signature is not required to include the Subject 571 header field in the set of fields that are signed. An ADMD receiving 572 such a message might decide that such a signature is unacceptable, 573 even if it passes, because the content of the Subject header field 574 could be altered post-signing without invalidating the signature. 575 Such an ADMD could replace the DKIM "pass" result with a "policy" 576 result and then also include the following in the corresponding 577 Authentication-Result field: 579 ... dkim=fail policy.dkim-rules=unsigned-subject ... 581 In this case, the property is "dkim-rules", indicating some local 582 check by that name took place and that check returned a result of 583 "unsigned-subject". These are arbitrary names selected by (and 584 presumably used within) the ADMD making use of them, so they are not 585 normally registered with IANA or otherwise specified apart from 586 setting syntax restrictions that allow for easy parsing within the 587 rest of the header field. 589 This ptype existed in the original specification for this header 590 field, but without a complete description or example of intended use. 591 As a result, it has not seen any practical use to date that matches 592 its intended purpose. These added details are provided to guide 593 implementers toward proper use. 595 2.5. Properties 597 While the "ptype" names the general area from which some data of 598 interest were extracted to perform authentication, the associated 599 "property" is more specific. Its meaning, however, can also vary 600 slightly depeneding on which authentication method's result is being 601 reported. 603 Generally, the properties are defined as follows: 605 header: Indicates from which header field the value being evaluated 606 was extracted. 608 body: Indicates where in the message body a value being evaluated 609 can be found (e.g., a specific offset into the message or a 610 reference to a MIME part). 612 policy: Indicates the local name of the policy that caused this 613 header field to be added (see Section 2.4). 615 smtp: Indicates which [SMTP] command provided the value that was 616 evaluated by the authentication scheme being applied. 618 Entries in the "Email Authentication Methods" registry can define 619 properties that deviate from these definitions when appropriate. For 620 example, when reporting the result of a [DKIM] evaluation, it would 621 be redundant to report the name of the header field from which 622 details were extracted. Thus, instead, DKIM results use the "header" 623 property to name the signature tag from which a detail of interest 624 was extracted. 626 2.6. Authentication Identifier Field 628 Every Authentication-Results header field has an authentication 629 service identifier field (authserv-id above). Specifically, this is 630 any string intended to identify the authentication service within the 631 ADMD that conducted authentication checks on the message. This 632 identifier is intended to be machine-readable and not necessarily 633 meaningful to users. 635 Since agents consuming this field will use this identifier to 636 determine whether its contents are of interest (and are safe to use), 637 the uniqueness of the identifier MUST be guaranteed by the ADMD that 638 generates it and MUST pertain to that ADMD. MUAs or downstream 639 filters SHOULD use this identifier to determine whether or not the 640 data contained in an Authentication-Results header field ought to be 641 used or ignored. 643 For simplicity and scalability, the authentication service identifier 644 SHOULD be a common token used throughout the ADMD. Common practice 645 is to use the DNS domain name used by or within that ADMD, sometimes 646 called the "organizational domain", but this is not strictly 647 necessary. 649 For tracing and debugging purposes, the authentication identifier can 650 instead be the specific hostname of the MTA performing the 651 authentication check whose result is being reported. Moreover, some 652 implementations define a substructure to the identifier; these are 653 outside of the scope of this specification. 655 Note, however, that using a local, relative identifier like a flat 656 hostname, rather than a hierarchical and globally unique ADMD 657 identifier like a DNS domain name, makes configuration more difficult 658 for large sites. The hierarchical identifier permits aggregating 659 related, trusted systems together under a single, parent identifier, 660 which in turn permits assessing the trust relationship with a single 661 reference. The alternative is a flat namespace requiring 662 individually listing each trusted system. Since consumers will use 663 the identifier to determine whether to use the contents of the header 664 field: 666 o Changes to the identifier impose a large, centralized 667 administrative burden. 669 o Ongoing administrative changes require constantly updating this 670 centralized table, making it difficult to ensure that an MUA or 671 downstream filter will have access to accurate information for 672 assessing the usability of the header field's content. In 673 particular, consumers of the header field will need to know not 674 only the current identifier(s) in use but previous ones as well to 675 account for delivery latency or later re-assessment of the header 676 field's contents. 678 Examples of valid authentication identifiers are "example.com", 679 "mail.example.org", "ms1.newyork.example.com", and "example-auth". 681 2.7. Version Tokens 683 The grammar above provides for the optional inclusion of versions on 684 both the header field itself (attached to the authserv-id token) and 685 on each of the methods being reported. The method version refers to 686 the method itself, which is specified in the documents describing 687 those methods, while the authserv-id version refers to this document 688 and thus the syntax of this header field. 690 The purpose of including these is to avoid misinterpretation of the 691 results. That is, if a parser finds a version after an authserv-id 692 that it does not explicitly know, it can immediately discontinue 693 trying to parse since what follows might not be in an expected 694 format. For a method version, the parser SHOULD ignore a method 695 result if the version is not supported in case the semantics of the 696 result have a different meaning than what is expected. For example, 697 if a hypothetical DKIM version 2 yielded a "pass" result for 698 different reasons than version 1 does, a consumer of this field might 699 not want to use the altered semantics. Allowing versions in the 700 syntax is a way to indicate this and let the consumer of the header 701 field decide. 703 2.8. Defined Methods and Result Values 705 Each individual authentication method returns one of a set of 706 specific result values. The subsections below provide references to 707 the documents defining the authentication methods specifically 708 supported by this document, and their corresponding result values. 709 Verifiers SHOULD use these values as described below. New methods 710 not specified in this document, but intended to be supported by the 711 header field defined here, MUST include a similar result table either 712 in their defining documents or in supplementary ones. 714 2.8.1. DKIM and DomainKeys 716 DKIM is represented by the "dkim" method and is defined in [DKIM]. 717 DomainKeys is defined in [DOMAINKEYS] and is represented by the 718 "domainkeys" method. 720 A signature is "acceptable to the ADMD" if it passes local policy 721 checks (or there are no specific local policy checks). For example, 722 an ADMD policy might require that the signature(s) on the message be 723 added using the DNS domain present in the From header field of the 724 message, thus making third-party signatures unacceptable even if they 725 verify. 727 Both DKIM and DomainKeys use the same result set, as follows: 729 none: The message was not signed. 731 pass: The message was signed, the signature or signatures were 732 acceptable to the ADMD, and the signature(s) passed verification 733 tests. 735 fail: The message was signed and the signature or signatures were 736 acceptable to the ADMD, but they failed the verification test(s). 738 policy: The message was signed, but some aspect of the signature or 739 signatures was not acceptable to the ADMD. 741 neutral: The message was signed, but the signature or signatures 742 contained syntax errors or were not otherwise able to be 743 processed. This result is also used for other failures not 744 covered elsewhere in this list. 746 temperror: The message could not be verified due to some error that 747 is likely transient in nature, such as a temporary inability to 748 retrieve a public key. A later attempt may produce a final 749 result. 751 permerror: The message could not be verified due to some error that 752 is unrecoverable, such as a required header field being absent. A 753 later attempt is unlikely to produce a final result. 755 [DKIM] advises that if a message fails verification, it is to be 756 treated as an unsigned message. A report of "fail" here permits the 757 receiver of the report to decide how to handle the failure. A report 758 of "neutral" or "none" preempts that choice, ensuring the message 759 will be treated as if it had not been signed. 761 2.8.2. SPF and Sender ID 763 SPF and Sender ID use the "spf" and "sender-id" method names, 764 respectively. The result values for SPF are defined in Section 2.6 765 of [SPF], and those definitions are included here by reference: 767 +-----------+--------------------------------+ 768 | Code | Meaning | 769 +-----------+--------------------------------+ 770 | none | [RFC7208], Section 2.6.1 | 771 +-----------+--------------------------------+ 772 | pass | [RFC7208], Section 2.6.3 | 773 +-----------+--------------------------------+ 774 | fail | [RFC7208], Section 2.6.4 | 775 +-----------+--------------------------------+ 776 | softfail | [RFC7208], Section 2.6.5 | 777 +-----------+--------------------------------+ 778 | policy | [this RFC], Section 2.4 | 779 +-----------+--------------------------------+ 780 | neutral | [RFC7208], Section 2.6.2 | 781 +-----------+--------------------------------+ 782 | temperror | [RFC7208], Section 2.6.6 | 783 +-----------+--------------------------------+ 784 | permerror | [RFC7208], Section 2.6.7 | 785 +-----------+--------------------------------+ 787 These result codes are used in the context of this specification to 788 reflect the result returned by the component conducting SPF 789 evaluation. 791 Similarly, the results for Sender ID are listed and described in 792 Section 4.2 of [SENDERID], which in turn uses the SPF definitions 793 that now appear in [SPF]. 795 Note that both of those documents specify result codes that use mixed 796 case, but they are typically used all lowercase in this context. 798 In both cases, an additional result of "policy" is defined, which 799 means the client was authorized to inject or relay mail on behalf of 800 the sender's DNS domain according to the authentication method's 801 algorithm, but local policy dictates that the result is unacceptable. 802 For example, "policy" might be used if SPF returns a "pass" result, 803 but a local policy check matches the sending DNS domain to one found 804 in an explicit list of unacceptable DNS domains (e.g., spammers). 806 If the retrieved sender policies used to evaluate SPF and Sender ID 807 do not contain explicit provisions for authenticating the local-part 808 (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported 809 along with results for these mechanisms SHOULD NOT include the local- 810 part. 812 2.8.3. "iprev" 814 The result values used by the "iprev" method, defined in Section 3, 815 are as follows: 817 pass: The DNS evaluation succeeded, i.e., the "reverse" and 818 "forward" lookup results were returned and were in agreement. 820 fail: The DNS evaluation failed. In particular, the "reverse" and 821 "forward" lookups each produced results, but they were not in 822 agreement, or the "forward" query completed but produced no 823 result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an 824 RCODE of 0 (NOERROR) in a reply containing no answers, was 825 returned. 827 temperror: The DNS evaluation could not be completed due to some 828 error that is likely transient in nature, such as a temporary DNS 829 error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or 830 other error condition resulted. A later attempt may produce a 831 final result. 833 permerror: The DNS evaluation could not be completed because no PTR 834 data are published for the connecting IP address, e.g., a DNS 835 RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) 836 in a reply containing no answers, was returned. This prevented 837 completion of the evaluation. A later attempt is unlikely to 838 produce a final result. 840 There is no "none" for this method since any TCP connection 841 delivering email has an IP address associated with it, so some kind 842 of evaluation will always be possible. 844 For discussion of the format of DNS replies, see "Domain Names - 845 Implementation and Specification" ([DNS]). 847 2.8.4. SMTP AUTH 849 SMTP AUTH (defined in [AUTH]) is represented by the "auth" method, 850 and its result values are as follows: 852 none: SMTP authentication was not attempted. 854 pass: The SMTP client authenticated to the server reporting the 855 result using the protocol described in [AUTH]. 857 fail: The SMTP client attempted to authenticate to the server using 858 the protocol described in [AUTH] but was not successful, yet 859 continued to send the message about which a result is being 860 reported. 862 temperror: The SMTP client attempted to authenticate using the 863 protocol described in [AUTH] but was not able to complete the 864 attempt due to some error that is likely transient in nature, such 865 as a temporary directory service lookup error. A later attempt 866 may produce a final result. 868 permerror: The SMTP client attempted to authenticate using the 869 protocol described in [AUTH] but was not able to complete the 870 attempt due to some error that is likely not transient in nature, 871 such as a permanent directory service lookup error. A later 872 attempt is not likely to produce a final result. 874 An agent making use of the data provided by this header field SHOULD 875 consider "fail" and "temperror" to be synonymous in terms of message 876 authentication, i.e., the client did not authenticate in either case. 878 2.8.5. Other Registered Codes 880 Result codes were also registered in other RFCs for Vouch By 881 Reference (in [AR-VBR], represented by "vbr"), Authorized Third-Party 882 Signatures (in [ATPS], represented by "dkim-atps"), and the DKIM- 883 related Author Domain Signing Practices (in [ADSP], represented by 884 "dkim-adsp"). 886 2.8.6. Extension Methods 888 Additional authentication method identifiers (extension methods) may 889 be defined in the future by later revisions or extensions to this 890 specification. These method identifiers are registered with the 891 Internet Assigned Numbers Authority (IANA) and, preferably, published 892 in an RFC. See Section 6 for further details. 894 Extension methods can be defined for the following reasons: 896 1. To allow additional information from new authentication systems 897 to be communicated to MUAs or downstream filters. The names of 898 such identifiers ought to reflect the name of the method being 899 defined but ought not be needlessly long. 901 2. To allow the creation of "sub-identifiers" that indicate 902 different levels of authentication and differentiate between 903 their relative strengths, e.g., "auth1-weak" and "auth1-strong". 905 Authentication method implementers are encouraged to provide adequate 906 information, via message header field comments if necessary, to allow 907 an MUA developer to understand or relay ancillary details of 908 authentication results. For example, if it might be of interest to 909 relay what data was used to perform an evaluation, such information 910 could be relayed as a comment in the header field, such as: 912 Authentication-Results: example.com; 913 foo=pass bar.baz=blob (2 of 3 tests OK) 915 Experimental method identifiers MUST only be used within ADMDs that 916 have explicitly consented to use them. These method identifiers and 917 the parameters associated with them are not documented in RFCs. 918 Therefore, they are subject to change at any time and not suitable 919 for production use. Any MTA, MUA, or downstream filter intended for 920 production use SHOULD ignore or delete any Authentication-Results 921 header field that includes an experimental (unknown) method 922 identifier. 924 2.8.7. Extension Result Codes 926 Additional result codes (extension results) might be defined in the 927 future by later revisions or extensions to this specification. 928 Result codes MUST be registered with the Internet Assigned Numbers 929 Authority (IANA) and preferably published in an RFC. See Section 6 930 for further details. 932 Experimental results MUST only be used within ADMDs that have 933 explicitly consented to use them. These results and the parameters 934 associated with them are not formally documented. Therefore, they 935 are subject to change at any time and not suitable for production 936 use. Any MTA, MUA, or downstream filter intended for production use 937 SHOULD ignore or delete any Authentication-Results header field that 938 includes an extension result. 940 3. The "iprev" Authentication Method 942 This section defines an additional authentication method called 943 "iprev". 945 "iprev" is an attempt to verify that a client appears to be valid 946 based on some DNS queries, which is to say that the IP address is 947 explicitly associated with a domain name. Upon receiving a session 948 initiation of some kind from a client, the IP address of the client 949 peer is queried for matching names (i.e., a number-to-name 950 translation, also known as a "reverse lookup" or a "PTR" record 951 query). Once that result is acquired, a lookup of each of the names 952 (i.e., a name-to-number translation, or an "A" or "AAAA" record 953 query) thus retrieved is done. The response to this second check 954 will typically result in at least one mapping back to the client's IP 955 address. 957 Expressed as an algorithm: If the client peer's IP address is I, the 958 list of names to which I maps (after a "PTR" query) is the set N, and 959 the union of IP addresses to which each member of N maps (after 960 corresponding "A" and "AAAA" queries) is L, then this test is 961 successful if I is an element of L. 963 The response to a PTR query could contain multiple names. To prevent 964 heavy DNS loads, agents performing these queries MUST be implemented 965 such that the number of names evaluated by generation of 966 corresponding A or AAAA queries is limited so as not to be unduly 967 taxing to the DNS infrastructure, though it MAY be configurable by an 968 administrator. As an example, Section 4.6.4 of [SPF] chose a limit 969 of 10 for its implementation of this algorithm. 971 "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the 972 query formats for the IPv6 case. 974 There is some contention regarding the wisdom and reliability of this 975 test. For example, in some regions, it can be difficult for this 976 test ever to pass because the practice of arranging to match the 977 forward and reverse DNS is infrequently observed. Therefore, the 978 precise implementation details of how a verifier performs an "iprev" 979 test are not specified here. The verifier MAY report a successful or 980 failed "iprev" test at its discretion having done some kind of check 981 of the validity of the connection's identity using DNS. It is 982 incumbent upon an agent making use of the reported "iprev" result to 983 understand what exactly that particular verifier is attempting to 984 report. 986 Extensive discussion of reverse DNS mapping and its implications can 987 be found in "Considerations for the use of DNS Reverse Mapping" 988 ([DNSOP-REVERSE]). In particular, it recommends that applications 989 avoid using this test as a means of authentication or security. Its 990 presence in this document is not an endorsement but is merely 991 acknowledgement that the method remains common and provides the means 992 to relay the results of that test. 994 4. Adding the Header Field to a Message 996 This specification makes no attempt to evaluate the relative 997 strengths of various message authentication methods that may become 998 available. The methods listed are an order-independent set; their 999 sequence does not indicate relative strength or importance of one 1000 method over another. Instead, the MUA or downstream filter consuming 1001 this header field is to interpret the result of each method based on 1002 its own knowledge of what that method evaluates. 1004 Each "method" MUST refer to an authentication method declared in the 1005 IANA registry or an extension method as described in Section 2.8.6, 1006 and each "result" MUST refer to a result code declared in the IANA 1007 registry or an extension result code as defined in Section 2.8.7. 1008 See Section 6 for further information about the registered methods 1009 and result codes. 1011 An MTA compliant with this specification adds this header field 1012 (after performing one or more message authentication tests) to 1013 indicate which MTA or ADMD performed the test, which test got 1014 applied, and what the result was. If an MTA applies more than one 1015 such test, it adds this header field either once per test or once 1016 indicating all of the results. An MTA MUST NOT add a result to an 1017 existing header field. 1019 An MTA MAY add this header field containing only the authentication 1020 identifier portion and the "none" token (see Section 2.2) to indicate 1021 explicitly that no message authentication schemes were applied prior 1022 to delivery of this message. 1024 An MTA adding this header field has to take steps to identify it as 1025 legitimate to the MUAs or downstream filters that will ultimately 1026 consume its content. One process to do so is described in Section 5. 1027 Further measures may be necessary in some environments. Some 1028 possible solutions are enumerated in Section 7.1. This document does 1029 not mandate any specific solution to this issue as each environment 1030 has its own facilities and limitations. 1032 Most known message authentication methods focus on a particular 1033 identifier to evaluate. SPF and Sender ID differ in that they can 1034 yield a result based on more than one identifier; specifically, SPF 1035 can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom 1036 parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter 1037 or the Purported Responsible Address (PRA) identity. When generating 1038 this field to report those results, only the parameter that yielded 1039 the result is included. 1041 For MTAs that add this header field, adding header fields in order 1042 (at the top), per Section 3.6 of [MAIL], is particularly important. 1043 Moreover, this header field SHOULD be inserted above any other trace 1044 header fields such MTAs might prepend. This placement allows easy 1045 detection of header fields that can be trusted. 1047 End users making direct use of this header field might inadvertently 1048 trust information that has not been properly vetted. If, for 1049 example, a basic SPF result were to be relayed that claims an 1050 authenticated addr-spec, the local-part of that addr-spec has 1051 actually not been authenticated. Thus, an MTA adding this header 1052 field SHOULD NOT include any data that has not been authenticated by 1053 the method(s) being applied. Moreover, MUAs SHOULD NOT render to 1054 users such information if it is presented by a method known not to 1055 authenticate it. 1057 4.1. Header Field Position and Interpretation 1059 In order to ensure non-ambiguous results and avoid the impact of 1060 false header fields, MUAs and downstream filters SHOULD NOT interpret 1061 this header field unless specifically configured to do so by the user 1062 or administrator. That is, this interpretation should not be "on by 1063 default". Naturally then, users or administrators ought not activate 1064 such a feature unless they are certain the header field will be 1065 validly added by an agent within the ADMD that accepts the mail that 1066 is ultimately read by the MUA, and instances of the header field 1067 appearing to originate within the ADMD but are actually added by 1068 foreign MTAs will be removed before delivery. 1070 Furthermore, MUAs and downstream filters SHOULD NOT interpret this 1071 header field unless the authentication service identifier it bears 1072 appears to be one used within its own ADMD as configured by the user 1073 or administrator. 1075 MUAs and downstream filters MUST ignore any result reported using a 1076 "result" not specified in the IANA "Result Code" registry or a 1077 "ptype" not listed in the corresponding registry for such values as 1078 defined in Section 6. Moreover, such agents MUST ignore a result 1079 indicated for any "method" they do not specifically support. 1081 An MUA SHOULD NOT reveal these results to end users, absent careful 1082 human factors design considerations and testing, for the presentation 1083 of trust-related materials. For example, an attacker could register 1084 examp1e.com (note the digit "one") and send signed mail to intended 1085 victims; a verifier would detect that the signature was valid and 1086 report a "pass" even though it's clear the DNS domain name was 1087 intended to mislead. See Section 7.2 for further discussion. 1089 As stated in Section 2.1, this header field MUST be treated as though 1090 it were a trace header field as defined in Section 3.6.7 of [MAIL] 1091 and hence MUST NOT be reordered and MUST be prepended to the message, 1092 so that there is generally some indication upon delivery of where in 1093 the chain of handling MTAs the message authentication was done. 1095 Note that there are a few message handlers that are only capable of 1096 appending new header fields to a message. Strictly speaking, these 1097 handlers are not compliant with this specification. They can still 1098 add the header field to carry authentication details, but any signal 1099 about where in the handling chain the work was done may be lost. 1100 Consumers SHOULD be designed such that this can be tolerated, 1101 especially from a producer known to have this limitation. 1103 MUAs SHOULD ignore instances of this header field discovered within 1104 message/rfc822 MIME attachments. 1106 Further discussion of these topics can be found in Section 7 below. 1108 4.2. Local Policy Enforcement 1110 Some sites have a local policy that considers any particular 1111 authentication policy's non-recoverable failure results (typically 1112 "fail" or similar) as justification for rejecting the message. In 1113 such cases, the border MTA SHOULD issue an SMTP rejection response to 1114 the message, rather than adding this header field and allowing the 1115 message to proceed toward delivery. This is more desirable than 1116 allowing the message to reach an internal host's MTA or spam filter, 1117 thus possibly generating a local rejection such as a Delivery Status 1118 Notification (DSN) [DSN] to a forged originator. Such generated 1119 rejections are colloquially known as "backscatter". 1121 The same MAY also be done for local policy decisions overriding the 1122 results of the authentication methods (e.g., the "policy" result 1123 codes described in Section 2.8). 1125 Such rejections at the SMTP protocol level are not possible if local 1126 policy is enforced at the MUA and not the MTA. 1128 5. Removing Existing Header Fields 1130 For security reasons, any MTA conforming to this specification MUST 1131 delete any discovered instance of this header field that claims, by 1132 virtue of its authentication service identifier, to have been added 1133 within its trust boundary but that did not come directly from another 1134 trusted MTA. For example, an MTA for example.com receiving a message 1135 MUST delete or otherwise obscure any instance of this header field 1136 bearing an authentication service identifier indicating that the 1137 header field was added within example.com prior to adding its own 1138 header fields. This could mean each MTA will have to be equipped 1139 with a list of internal MTAs known to be compliant (and hence 1140 trustworthy). 1142 For simplicity and maximum security, a border MTA could remove all 1143 instances of this header field on mail crossing into its trust 1144 boundary. However, this may conflict with the desire to access 1145 authentication results performed by trusted external service 1146 providers. It may also invalidate signed messages whose signatures 1147 cover external instances of this header field. A more robust border 1148 MTA could allow a specific list of authenticating MTAs whose 1149 information is to be admitted, removing the header field originating 1150 from all others. 1152 As stated in Section 1.2, a formal definition of "trust boundary" is 1153 deliberately not made here. It is entirely possible that a border 1154 MTA for example.com will explicitly trust authentication results 1155 asserted by upstream host example.net even though they exist in 1156 completely disjoint administrative boundaries. In that case, the 1157 border MTA MAY elect not to delete those results; moreover, the 1158 upstream host doing some authentication work could apply a signing 1159 technology such as [DKIM] on its own results to assure downstream 1160 hosts of their authenticity. An example of this is provided in 1161 Appendix C. 1163 Similarly, in the case of messages signed using [DKIM] or other 1164 message-signing methods that sign header fields, this removal action 1165 could invalidate one or more signatures on the message if they 1166 covered the header field to be removed. This behavior can be 1167 desirable since there's little value in validating the signature on a 1168 message with forged header fields. However, signing agents MAY 1169 therefore elect to omit these header fields from signing to avoid 1170 this situation. 1172 An MTA SHOULD remove any instance of this header field bearing a 1173 version (express or implied) that it does not support. However, an 1174 MTA MUST remove such a header field if the [SMTP] connection relaying 1175 the message is not from a trusted internal MTA. This means the MTA 1176 needs to be able to understand versions of this header field at least 1177 as late as the ones understood by the MUAs or other consumers within 1178 its ADMD. 1180 6. IANA Considerations 1182 IANA has registered the defined header field and created two tables 1183 as described below. These registry actions were originally defined 1184 by [RFC5451] and are repeated here to provide a single, current 1185 reference. 1187 6.1. The Authentication-Results Header Field 1189 [RFC5451] added the Authentication-Results header field to the IANA 1190 "Permanent Message Header Field Names" registry, per the procedure 1191 found in [IANA-HEADERS]. That entry has been updated to reference 1192 this document. The following is the registration template: 1194 Header field name: Authentication-Results 1195 Applicable protocol: mail ([MAIL]) 1196 Status: Standard 1197 Author/Change controller: IETF 1198 Specification document(s): [this RFC] 1199 Related information: 1200 Requesting review of any proposed changes and additions to 1201 this field is recommended. 1203 6.2. "Email Authentication Methods" Registry 1205 Names of message authentication methods supported by this 1206 specification are to be registered with IANA, with the exception of 1207 experimental names as described in Section 2.8.6. A registry was 1208 created by [RFC5451] for this purpose. [RFC7001] amended ther rules 1209 governing that registry, and also added a "version" field to the 1210 registry. 1212 New entries are assigned only for values that have received Expert 1213 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1214 appointed by the IESG. The designated expert has discretion to 1215 request that a publication be referenced if a clear, concise 1216 definition of the authentication method cannot be provided such that 1217 interoperability is assured. Registrations should otherwise be 1218 permitted. The designated expert can also handle requests to mark 1219 any current registration as "deprecated". 1221 Each method must register a name, the specification that defines it, 1222 a version number associated with the method being registered 1223 (preferably starting at "1"), zero or more "ptype" values appropriate 1224 for use with that method, which "property" value(s) should be 1225 reported by that method, and a description of the "value" to be used 1226 with each. 1228 All existing registry entries that reference [RFC7001] are to be 1229 updated to reference this document, except where entries have already 1230 been deprecated. 1232 6.3. "Email Authentication Result Names" Registry 1234 Names of message authentication result codes supported by this 1235 specification must be registered with IANA, with the exception of 1236 experimental codes as described in Section 2.8.7. A registry was 1237 created by [RFC5451] for this purpose. [RFC7001] updated the rules 1238 governing that registry. 1240 New entries are assigned only for values that have received Expert 1241 Review, per [IANA-CONSIDERATIONS]. The designated expert shall be 1242 appointed by the IESG. The designated expert has discretion to 1243 request that a publication be referenced if a clear, concise 1244 definition of the authentication result cannot be provided such that 1245 interoperability is assured. Registrations should otherwise be 1246 permitted. The designated expert can also handle requests to mark 1247 any current registration as "deprecated". 1249 All existing registry entries that reference [RFC7001] are to be 1250 updated to reference this document. 1252 The definitions for the SPF and Sender ID authentication methods are 1253 updated using the references found in Section 2.8.2. 1255 7. Security Considerations 1257 The following security considerations apply when adding or processing 1258 the Authentication-Results header field: 1260 7.1. Forged Header Fields 1262 An MUA or filter that accesses a mailbox whose messages are handled 1263 by a non-conformant MTA, and understands Authentication-Results 1264 header fields, could potentially make false conclusions based on 1265 forged header fields. A malicious user or agent could forge a header 1266 field using the DNS domain of a receiving ADMD as the authserv-id 1267 token in the value of the header field and, with the rest of the 1268 value, claim that the message was properly authenticated. The non- 1269 conformant MTA would fail to strip the forged header field, and the 1270 MUA could inappropriately trust it. 1272 For this reason, it is best not to have processing of the 1273 Authentication-Results header field enabled by default; instead, it 1274 should be ignored, at least for the purposes of enacting filtering 1275 decisions, unless specifically enabled by the user or administrator 1276 after verifying that the border MTA is compliant. It is acceptable 1277 to have an MUA aware of this specification but have an explicit list 1278 of hostnames whose Authentication-Results header fields are 1279 trustworthy; however, this list should initially be empty. 1281 Proposed alternative solutions to this problem were made some time 1282 ago and are listed below. To date, they have not been developed due 1283 to lack of demand but are documented here should the information be 1284 useful at some point in the future: 1286 1. Possibly the simplest is a digital signature protecting the 1287 header field, such as using [DKIM], that can be verified by an 1288 MUA by using a posted public key. Although one of the main 1289 purposes of this document is to relieve the burden of doing 1290 message authentication work at the MUA, this only requires that 1291 the MUA learn a single authentication scheme even if a number of 1292 them are in use at the border MTA. Note that [DKIM] requires 1293 that the From header field be signed, although in this 1294 application, the signing agent (a trusted MTA) likely cannot 1295 authenticate that value, so the fact that it is signed should be 1296 ignored. Where the authserv-id is the ADMD's domain name, the 1297 authserv-id matching this valid internal signature's "d=" DKIM 1298 value is sufficient. 1300 2. Another would be a means to interrogate the MTA that added the 1301 header field to see if it is actually providing any message 1302 authentication services and saw the message in question, but this 1303 isn't especially palatable given the work required to craft and 1304 implement such a scheme. 1306 3. Yet another might be a method to interrogate the internal MTAs 1307 that apparently handled the message (based on Received header 1308 fields) to determine whether any of them conform to Section 5 of 1309 this memo. This, too, has potentially high barriers to entry. 1311 4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to 1312 allow an MUA or filtering agent to acquire the authserv-id in use 1313 within an ADMD, thus allowing it to identify which 1314 Authentication-Results header fields it can trust. 1316 5. On the presumption that internal MTAs are fully compliant with 1317 Section 3.6 of [MAIL] and the compliant internal MTAs are using 1318 their own hostnames or the ADMD's DNS domain name as the 1319 authserv-id token, the header field proposed here should always 1320 appear above a Received header added by a trusted MTA. This can 1321 be used as a test for header field validity. 1323 Support for some of these is being considered for future work. 1325 In any case, a mechanism needs to exist for an MUA or filter to 1326 verify that the host that appears to have added the header field (a) 1327 actually did so and (b) is legitimately adding that header field for 1328 this delivery. Given the variety of messaging environments deployed 1329 today, consensus appears to be that specifying a particular mechanism 1330 for doing so is not appropriate for this document. 1332 Mitigation of the forged header field attack can also be accomplished 1333 by moving the authentication results data into metadata associated 1334 with the message. In particular, an [SMTP] extension could be 1335 established to communicate authentication results from the border MTA 1336 to intermediate and delivery MTAs; the latter of these could arrange 1337 to store the authentication results as metadata retrieved and 1338 rendered along with the message by an [IMAP] client aware of a 1339 similar extension in that protocol. The delivery MTA would be told 1340 to trust data via this extension only from MTAs it trusts, and border 1341 MTAs would not accept data via this extension from any source. There 1342 is no vector in such an arrangement for forgery of authentication 1343 data by an outside agent. 1345 7.2. Misleading Results 1347 Until some form of service for querying the reputation of a sending 1348 agent is widely deployed, the existence of this header field 1349 indicating a "pass" does not render the message trustworthy. It is 1350 possible for an arriving piece of spam or other undesirable mail to 1351 pass checks by several of the methods enumerated above (e.g., a piece 1352 of spam signed using [DKIM] by the originator of the spam, which 1353 might be a spammer or a compromised system). In particular, this 1354 issue is not resolved by forged header field removal discussed above. 1356 Hence, MUAs and downstream filters must take some care with use of 1357 this header even after possibly malicious headers are scrubbed. 1359 7.3. Header Field Position 1361 Despite the requirements of [MAIL], header fields can sometimes be 1362 reordered en route by intermediate MTAs. The goal of requiring 1363 header field addition only at the top of a message is an 1364 acknowledgement that some MTAs do reorder header fields, but most do 1365 not. Thus, in the general case, there will be some indication of 1366 which MTAs (if any) handled the message after the addition of the 1367 header field defined here. 1369 7.4. Reverse IP Query Denial-of-Service Attacks 1371 Section 4.6.4 of [SPF] describes a DNS-based denial-of-service attack 1372 for verifiers that attempt DNS-based identity verification of 1373 arriving client connections. A verifier wishing to do this check and 1374 report this information needs to take care not to go to unbounded 1375 lengths to resolve "A" and "PTR" queries. MUAs or other filters 1376 making use of an "iprev" result specified by this document need to be 1377 aware of the algorithm used by the verifier reporting the result and, 1378 especially, its limitations. 1380 7.5. Mitigation of Backscatter 1382 Failing to follow the instructions of Section 4.2 can result in a 1383 denial-of-service attack caused by the generation of [DSN] messages 1384 (or equivalent) to addresses that did not send the messages being 1385 rejected. 1387 7.6. Internal MTA Lists 1389 Section 5 describes a procedure for scrubbing header fields that may 1390 contain forged authentication results about a message. A compliant 1391 installation will have to include, at each MTA, a list of other MTAs 1392 known to be compliant and trustworthy. Failing to keep this list 1393 current as internal infrastructure changes may expose an ADMD to 1394 attack. 1396 7.7. Attacks against Authentication Methods 1398 If an attack becomes known against an authentication method, clearly 1399 then the agent verifying that method can be fooled into thinking an 1400 inauthentic message is authentic, and thus the value of this header 1401 field can be misleading. It follows that any attack against the 1402 authentication methods supported by this document is also a security 1403 consideration here. 1405 7.8. Intentionally Malformed Header Fields 1407 It is possible for an attacker to add an Authentication-Results 1408 header field that is extraordinarily large or otherwise malformed in 1409 an attempt to discover or exploit weaknesses in header field parsing 1410 code. Implementers must thoroughly verify all such header fields 1411 received from MTAs and be robust against intentionally as well as 1412 unintentionally malformed header fields. 1414 7.9. Compromised Internal Hosts 1416 An internal MUA or MTA that has been compromised could generate mail 1417 with a forged From header field and a forged Authentication-Results 1418 header field that endorses it. Although it is clearly a larger 1419 concern to have compromised internal machines than it is to prove the 1420 value of this header field, this risk can be mitigated by arranging 1421 that internal MTAs will remove this header field if it claims to have 1422 been added by a trusted border MTA (as described above), yet the 1423 [SMTP] connection is not coming from an internal machine known to be 1424 running an authorized MTA. However, in such a configuration, 1425 legitimate MTAs will have to add this header field when legitimate 1426 internal-only messages are generated. This is also covered in 1427 Section 5. 1429 7.10. Encapsulated Instances 1431 MIME messages can contain attachments of type "message/rfc822", which 1432 contain other messages. Such an encapsulated message can also 1433 contain an Authentication-Results header field. Although the 1434 processing of these is outside of the intended scope of this document 1435 (see Section 1.3), some early guidance to MUA developers is 1436 appropriate here. 1438 Since MTAs are unlikely to strip Authentication-Results header fields 1439 after mailbox delivery, MUAs are advised in Section 4.1 to ignore 1440 such instances within MIME attachments. Moreover, when extracting a 1441 message digest to separate mail store messages or other media, such 1442 header fields should be removed so that they will never be 1443 interpreted improperly by MUAs that might later consume them. 1445 7.11. Reverse Mapping 1447 Although Section 3 of this memo includes explicit support for the 1448 "iprev" method, its value as an authentication mechanism is limited. 1449 Implementers of both this proposal and agents that use the data it 1450 relays are encouraged to become familiar with the issues raised by 1451 [DNSOP-REVERSE] when deciding whether or not to include support for 1452 "iprev". 1454 8. References 1456 8.1. Normative References 1458 [ABNF] Crocker, D. and P. Overell, "Augmented BNF for 1459 Syntax Specifications: ABNF", STD 68, 1460 RFC 5234, January 2008. 1462 [IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul, 1463 "Registration Procedures for Message Header 1464 Fields", BCP 90, RFC 3864, September 2004. 1466 [KEYWORDS] Bradner, S., "Key words for use in RFCs to 1467 Indicate Requirement Levels", BCP 14, 1468 RFC 2119. 1470 [MAIL] Resnick, P., Ed., "Internet Message Format", 1471 RFC 5322, October 2008. 1473 [MIME] Freed, N. and N. Borenstein, "Multipurpose 1474 Internet Mail Extensions (MIME) Part One: 1475 Format of Internet Message Bodies", RFC 2045, 1476 November 1996. 1478 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", 1479 RFC 5321, October 2008. 1481 8.2. Informative References 1483 [ADSP] Allman, E., Fenton, J., Delany, M., and J. 1484 Levine, "DomainKeys Identified Mail (DKIM) 1485 Author Domain Signing Practices (ADSP)", 1486 RFC 5617, August 2009. 1488 [AR-VBR] Kucherawy, M., "Authentication-Results 1489 Registration for Vouch by Reference Results", 1490 RFC 6212, April 2011. 1492 [ATPS] Kucherawy, M., "DomainKeys Identified Mail 1493 (DKIM) Authorized Third-Party Signatures", 1494 RFC 6541, February 2012. 1496 [AUTH] Siemborski, R. and A. Melnikov, "SMTP Service 1497 Extension for Authentication", RFC 4954, 1498 July 2007. 1500 [DKIM] Crocker, D., Hansen, T., and M. Kucherawy, 1501 "DomainKeys Identified Mail (DKIM) 1502 Signatures", STD 76, RFC 6376, September 2011. 1504 [DNS] Mockapetris, P., "Domain names - 1505 Implementation and Specification", STD 13, 1506 RFC 1035, November 1987. 1508 [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. 1509 Souissi, "DNS Extensions to Support IP Version 1510 6", RFC 3596, October 2003. 1512 [DNSOP-REVERSE] Senie, D. and A. Sullivan, "Considerations for 1513 the use of DNS Reverse Mapping", Work 1514 in Progress, March 2008. 1516 [DOMAINKEYS] Delany, M., "Domain-Based Email Authentication 1517 Using Public Keys Advertised in the DNS 1518 (DomainKeys)", RFC 4870, May 2007. 1520 [DSN] Moore, K. and G. Vaudreuil, "An Extensible 1521 Message Format for Delivery Status 1522 Notifications", RFC 3464, January 2003. 1524 [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", 1525 RFC 5598, July 2009. 1527 [IANA-CONSIDERATIONS] Narten, T. and H. Alvestrand, "Guidelines for 1528 Writing an IANA Considerations Section in 1529 RFCs", BCP 26, RFC 5226, May 2008. 1531 [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL 1532 - VERSION 4rev1", RFC 3501, March 2003. 1534 [POP3] Myers, J. and M. Rose, "Post Office Protocol - 1535 Version 3", STD 53, RFC 1939, May 1996. 1537 [PTYPES-REGISTRY] Kucherawy, M., "A Property Types Registry for 1538 the Authentication-Results Header Field", 1539 RFC 7410, December 2014. 1541 [RFC5451] Kucherawy, M., "Message Header Field for 1542 Indicating Message Authentication Status", 1543 RFC 5451, April 2009. 1545 [RFC7001] Kucherawy, M., "Message Header Field for 1546 Indicating Message Authentication Status", 1547 RFC 7001, September 2013. 1549 [SECURITY] Rescorla, E. and B. Korver, "Guidelines for 1550 Writing RFC Text on Security Considerations", 1551 BCP 72, RFC 3552, July 2003. 1553 [SENDERID] Lyon, J. and M. Wong, "Sender ID: 1554 Authenticating E-Mail", RFC 4406, April 2006. 1556 [SPF] Kitterman, S., "Sender Policy Framework (SPF) 1557 for Authorizing Use of Domains in E-Mail, 1558 Version 1", RFC 7208, April 2014. 1560 [VBR] Hoffman, P., Levine, J., and A. Hathcock, 1561 "Vouch By Reference", RFC 5518, April 2009. 1563 Appendix A. Acknowledgements 1565 The author wishes to acknowledge the following individuals for their 1566 review and constructive criticism of this document: (names) 1568 Appendix B. Legacy MUAs 1570 Implementers of this protocol should be aware that many MUAs are 1571 unlikely to be retrofitted to support the new header field and its 1572 semantics. In the interests of convenience and quicker adoption, a 1573 delivery MTA might want to consider adding things that are processed 1574 by existing MUAs in addition to the Authentication-Results header 1575 field. One suggestion is to include a Priority header field, on 1576 messages that don't already have such a header field, containing a 1577 value that reflects the strength of the authentication that was 1578 accomplished, e.g., "low" for weak or no authentication, "normal" or 1579 "high" for good or strong authentication. 1581 Some modern MUAs can already filter based on the content of this 1582 header field. However, there is keen interest in having MUAs make 1583 some kind of graphical representation of this header field's meaning 1584 to end users. Until this capability is added, other interim means of 1585 conveying authentication results may be necessary while this proposal 1586 and its successors are adopted. 1588 Appendix C. Authentication-Results Examples 1590 This section presents some examples of the use of this header field 1591 to indicate authentication results. 1593 C.1. Trivial Case; Header Field Not Present 1595 The trivial case: 1597 Received: from mail-router.example.com 1598 (mail-router.example.com [192.0.2.1]) 1599 by server.example.org (8.11.6/8.11.6) 1600 with ESMTP id g1G0r1kA003489; 1601 Fri, Feb 15 2002 17:19:07 -0800 1602 From: sender@example.com 1603 Date: Fri, Feb 15 2002 16:54:30 -0800 1604 To: receiver@example.org 1605 Message-Id: <12345.abc@example.com> 1606 Subject: here's a sample 1608 Hello! Goodbye! 1610 Example 1: Trivial Case 1612 The Authentication-Results header field is completely absent. The 1613 MUA may make no conclusion about the validity of the message. This 1614 could be the case because the message authentication services were 1615 not available at the time of delivery, or no service is provided, or 1616 the MTA is not in compliance with this specification. 1618 C.2. Nearly Trivial Case; Service Provided, but No Authentication Done 1620 A message that was delivered by an MTA that conforms to this 1621 specification but provides no actual message authentication service: 1623 Authentication-Results: example.org 1; none 1624 Received: from mail-router.example.com 1625 (mail-router.example.com [192.0.2.1]) 1626 by server.example.org (8.11.6/8.11.6) 1627 with ESMTP id g1G0r1kA003489; 1628 Fri, Feb 15 2002 17:19:07 -0800 1629 From: sender@example.com 1630 Date: Fri, Feb 15 2002 16:54:30 -0800 1631 To: receiver@example.org 1632 Message-Id: <12345.abc@example.com> 1633 Subject: here's a sample 1635 Hello! Goodbye! 1637 Example 2: Header Present but No Authentication Done 1639 The Authentication-Results header field is present, showing that the 1640 delivering MTA conforms to this specification. It used its DNS 1641 domain name as the authserv-id. The presence of "none" (and the 1642 absence of any method and result tokens) indicates that no message 1643 authentication was done. The version number of the specification to 1644 which the field's content conforms is explicitly provided. 1646 C.3. Service Provided, Authentication Done 1648 A message that was delivered by an MTA that conforms to this 1649 specification and applied some message authentication: 1651 Authentication-Results: example.com; 1652 spf=pass smtp.mailfrom=example.net 1653 Received: from dialup-1-2-3-4.example.net 1654 (dialup-1-2-3-4.example.net [192.0.2.200]) 1655 by mail-router.example.com (8.11.6/8.11.6) 1656 with ESMTP id g1G0r1kA003489; 1657 Fri, Feb 15 2002 17:19:07 -0800 1658 From: sender@example.net 1659 Date: Fri, Feb 15 2002 16:54:30 -0800 1660 To: receiver@example.com 1661 Message-Id: <12345.abc@example.net> 1662 Subject: here's a sample 1664 Hello! Goodbye! 1666 Example 3: Header Reporting Results 1668 The Authentication-Results header field is present, indicating that 1669 the border MTA conforms to this specification. The authserv-id is 1670 once again the DNS domain name. Furthermore, the message was 1671 authenticated by that MTA via the method specified in [SPF]. Note 1672 that since that method cannot authenticate the local-part, it has 1673 been omitted from the result's value. The MUA could extract and 1674 relay this extra information if desired. 1676 C.4. Service Provided, Several Authentications Done, Single MTA 1678 A message that was relayed inbound via a single MTA that conforms to 1679 this specification and applied three different message authentication 1680 checks: 1682 Authentication-Results: example.com; 1683 auth=pass (cram-md5) smtp.auth=sender@example.net; 1684 spf=pass smtp.mailfrom=example.net 1685 Authentication-Results: example.com; 1686 sender-id=pass header.from=example.net 1687 Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6) 1688 (dialup-1-2-3-4.example.net [192.0.2.200]) 1689 by mail-router.example.com (8.11.6/8.11.6) 1690 with ESMTP id g1G0r1kA003489; 1691 Fri, Feb 15 2002 17:19:07 -0800 1692 Date: Fri, Feb 15 2002 16:54:30 -0800 1693 To: receiver@example.com 1694 From: sender@example.net 1695 Message-Id: <12345.abc@example.net> 1696 Subject: here's a sample 1698 Hello! Goodbye! 1700 Example 4: Headers Reporting Results from One MTA 1702 The Authentication-Results header field is present, indicating that 1703 the delivering MTA conforms to this specification. Once again, the 1704 receiving DNS domain name is used as the authserv-id. Furthermore, 1705 the sender authenticated herself/himself to the MTA via a method 1706 specified in [AUTH], and both SPF and Sender ID checks were done and 1707 passed. The MUA could extract and relay this extra information if 1708 desired. 1710 Two Authentication-Results header fields are not required since the 1711 same host did all of the checking. The authenticating agent could 1712 have consolidated all the results into one header field. 1714 This example illustrates a scenario in which a remote user on a 1715 dialup connection (example.net) sends mail to a border MTA 1716 (example.com) using SMTP authentication to prove identity. The 1717 dialup provider has been explicitly authorized to relay mail as 1718 example.com resulting in passes by the SPF and Sender ID checks. 1720 C.5. Service Provided, Several Authentications Done, Different MTAs 1722 A message that was relayed inbound by two different MTAs that conform 1723 to this specification and applied multiple message authentication 1724 checks: 1726 Authentication-Results: example.com; 1727 sender-id=fail header.from=example.com; 1728 dkim=pass (good signature) header.d=example.com 1729 Received: from mail-router.example.com 1730 (mail-router.example.com [192.0.2.1]) 1731 by auth-checker.example.com (8.11.6/8.11.6) 1732 with ESMTP id i7PK0sH7021929; 1733 Fri, Feb 15 2002 17:19:22 -0800 1734 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com; 1735 t=1188964191; c=simple/simple; h=From:Date:To:Subject: 1736 Message-Id:Authentication-Results; 1737 bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70; 1738 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 1739 Authentication-Results: example.com; 1740 auth=pass (cram-md5) smtp.auth=sender@example.com; 1741 spf=fail smtp.mailfrom=example.com 1742 Received: from dialup-1-2-3-4.example.net 1743 (dialup-1-2-3-4.example.net [192.0.2.200]) 1744 by mail-router.example.com (8.11.6/8.11.6) 1745 with ESMTP id g1G0r1kA003489; 1746 Fri, Feb 15 2002 17:19:07 -0800 1747 From: sender@example.com 1748 Date: Fri, Feb 15 2002 16:54:30 -0800 1749 To: receiver@example.com 1750 Message-Id: <12345.abc@example.com> 1751 Subject: here's a sample 1753 Hello! Goodbye! 1755 Example 5: Headers Reporting Results from Multiple MTAs 1757 The Authentication-Results header field is present, indicating 1758 conformance to this specification. Once again, the authserv-id used 1759 is the recipient's DNS domain name. The header field is present 1760 twice because two different MTAs in the chain of delivery did 1761 authentication tests. The first MTA, mail-router.example.com, 1762 reports that SMTP AUTH and SPF were both used and that the former 1763 passed while the latter failed. In the SMTP AUTH case, additional 1764 information is provided in the comment field, which the MUA can 1765 choose to render if desired. 1767 The second MTA, auth-checker.example.com, reports that it did a 1768 Sender ID test (which failed) and a DKIM test (which passed). Again, 1769 additional data about one of the tests is provided as a comment, 1770 which the MUA may choose to render. Also noteworthy here is the fact 1771 that there is a DKIM signature added by example.com that assured the 1772 integrity of the lower Authentication-Results field. 1774 Since different hosts did the two sets of authentication checks, the 1775 header fields cannot be consolidated in this example. 1777 This example illustrates more typical transmission of mail into 1778 example.com from a user on a dialup connection example.net. The user 1779 appears to be legitimate as he/she had a valid password allowing 1780 authentication at the border MTA using SMTP AUTH. The SPF and Sender 1781 ID tests failed since example.com has not granted example.net 1782 authority to relay mail on its behalf. However, the DKIM test passed 1783 because the sending user had a private key matching one of 1784 example.com's published public keys and used it to sign the message. 1786 C.6. Service Provided, Multi-Tiered Authentication Done 1788 A message that had authentication done at various stages, one of 1789 which was outside the receiving ADMD: 1791 Authentication-Results: example.com; 1792 dkim=pass reason="good signature" 1793 header.i=@mail-router.example.net; 1794 dkim=fail reason="bad signature" 1795 header.i=@newyork.example.com 1796 Received: from mail-router.example.net 1797 (mail-router.example.net [192.0.2.250]) 1798 by chicago.example.com (8.11.6/8.11.6) 1799 for 1800 with ESMTP id i7PK0sH7021929; 1801 Fri, Feb 15 2002 17:19:22 -0800 1802 DKIM-Signature: v=1; a=rsa-sha256; s=furble; 1803 d=mail-router.example.net; t=1188964198; c=relaxed/simple; 1804 h=From:Date:To:Message-Id:Subject:Authentication-Results; 1805 bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=; 1806 b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= 1807 Authentication-Results: example.net; 1808 dkim=pass (good signature) header.i=@newyork.example.com 1809 Received: from smtp.newyork.example.com 1810 (smtp.newyork.example.com [192.0.2.220]) 1811 by mail-router.example.net (8.11.6/8.11.6) 1812 with ESMTP id g1G0r1kA003489; 1813 Fri, Feb 15 2002 17:19:07 -0800 1814 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; 1815 d=newyork.example.com; 1816 t=1188964191; c=simple/simple; 1817 h=From:Date:To:Message-Id:Subject; 1818 bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; 1819 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 1820 From: sender@newyork.example.com 1821 Date: Fri, Feb 15 2002 16:54:30 -0800 1822 To: meetings@example.net 1823 Message-Id: <12345.abc@newyork.example.com> 1824 Subject: here's a sample 1826 Example 6: Headers Reporting Results from Multiple MTAs in Different 1827 ADMDs 1829 In this example, we see multi-tiered authentication with an extended 1830 trust boundary. 1832 The message was sent from someone at example.com's New York office 1833 (newyork.example.com) to a mailing list managed at an intermediary. 1835 The message was signed at the origin using DKIM. 1837 The message was sent to a mailing list service provider called 1838 example.net, which is used by example.com. There, 1839 meetings@example.net is expanded to a long list of recipients, one of 1840 whom is at the Chicago office. In this example, we will assume that 1841 the trust boundary for chicago.example.com includes the mailing list 1842 server at example.net. 1844 The mailing list server there first authenticated the message and 1845 affixed an Authentication-Results header field indicating such using 1846 its DNS domain name for the authserv-id. It then altered the message 1847 by affixing some footer text to the body, including some 1848 administrivia such as unsubscription instructions. Finally, the 1849 mailing list server affixes a second DKIM signature and begins 1850 distribution of the message. 1852 The border MTA for chicago.example.com explicitly trusts results from 1853 mail-router.example.net, so that header field is not removed. It 1854 performs evaluation of both signatures and determines that the first 1855 (most recent) is a "pass" but, because of the aforementioned 1856 modifications, the second is a "fail". However, the first signature 1857 included the Authentication-Results header added at mail- 1858 router.example.net that validated the second signature. Thus, 1859 indirectly, it can be determined that the authentications claimed by 1860 both signatures are indeed valid. 1862 Note that two styles of presenting metadata about the result are in 1863 use here. In one case, the "reason=" clause is present, which is 1864 intended for easy extraction by parsers; in the other case, the CFWS 1865 production of the ABNF is used to include such data as a header field 1866 comment. The latter can be harder for parsers to extract given the 1867 varied supported syntaxes of mail header fields. 1869 C.7. Comment-Heavy Example 1871 The formal syntax permits comments within the content in a number of 1872 places. For the sake of illustration, this example is also legal: 1874 Authentication-Results: foo.example.net (foobar) 1 (baz); 1875 dkim (Because I like it) / 1 (One yay) = (wait for it) fail 1876 policy (A dot can go here) . (like that) expired 1877 (this surprised me) = (as I wasn't expecting it) 1362471462 1879 Example 7: A Very Comment-Heavy but Perfectly Legal Example 1881 Appendix D. Operational Considerations about Message Authentication 1883 This protocol is predicated on the idea that authentication (and 1884 presumably in the future, reputation) work is typically done by 1885 border MTAs rather than MUAs or intermediate MTAs; the latter merely 1886 make use of the results determined by the former. Certainly this is 1887 not mandatory for participation in electronic mail or message 1888 authentication, but this protocol and its deployment to date are 1889 based on that model. The assumption satisfies several common ADMD 1890 requirements: 1892 1. Service operators prefer to resolve the handling of problem 1893 messages as close to the border of the ADMD as possible. This 1894 enables, for example, rejection of messages at the SMTP level 1895 rather than generating a DSN internally. Thus, doing any of the 1896 authentication or reputation work exclusively at the MUA or 1897 intermediate MTA renders this desire unattainable. 1899 2. Border MTAs are more likely to have direct access to external 1900 sources of authentication or reputation information since modern 1901 MUAs are more likely to be heavily firewalled. Thus, some MUAs 1902 might not even be able to complete the task of performing 1903 authentication or reputation evaluations without complex proxy 1904 configurations or similar burdens. 1906 3. MUAs rely upon the upstream MTAs within their trust boundaries to 1907 make correct (as much as is possible) evaluations about the 1908 message's envelope, header, and content. Thus, MUAs don't need 1909 to know how to do the work that upstream MTAs do; they only need 1910 the results of that work. 1912 4. Evaluations about the quality of a message, from simple token 1913 matching (e.g., a list of preferred DNS domains) to cryptanalysis 1914 (e.g., public/private key work), are at least a little bit 1915 expensive and thus need to be minimized. To that end, performing 1916 those tests at the border MTA is far preferred to doing that work 1917 at each MUA that handles a message. If an ADMD's environment 1918 adheres to common messaging protocols, a reputation query or an 1919 authentication check performed by a border MTA would return the 1920 same result as the same query performed by an MUA. By contrast, 1921 in an environment where the MUA does the work, a message arriving 1922 for multiple recipients would thus cause authentication or 1923 reputation evaluation to be done more than once for the same 1924 message (i.e., at each MUA), causing needless amplification of 1925 resource use and creating a possible denial-of-service attack 1926 vector. 1928 5. Minimizing change is good. As new authentication and reputation 1929 methods emerge, the list of methods supported by this header 1930 field would presumably be extended. If MUAs simply consume the 1931 contents of this header field rather than actually attempt to do 1932 authentication and/or reputation work, then MUAs only need to 1933 learn to parse this header field once; emergence of new methods 1934 requires only a configuration change at the MUAs and software 1935 changes at the MTAs (which are presumably fewer in number). When 1936 choosing to implement these functions in MTAs vs. MUAs, the 1937 issues of individual flexibility, infrastructure inertia, and 1938 scale of effort must be considered. It is typically easier to 1939 change a single MUA than an MTA because the modification affects 1940 fewer users and can be pursued with less care. However, changing 1941 many MUAs is more effort than changing a smaller number of MTAs. 1943 6. For decisions affecting message delivery and display, assessment 1944 based on authentication and reputation is best performed close to 1945 the time of message transit, as a message makes its journey 1946 toward a user's inbox, not afterwards. DKIM keys and IP address 1947 reputations, etc., can change over time or even become invalid, 1948 and users can take a long time to read a message once delivered. 1949 The value of this work thus degrades, perhaps quickly, once the 1950 delivery process has completed. This seriously diminishes the 1951 value of this work when done other than at MTAs. 1953 Many operational choices are possible within an ADMD, including the 1954 venue for performing authentication and/or reputation assessment. 1955 The current specification does not dictate any of those choices. 1956 Rather, it facilitates those cases in which information produced by 1957 one stage of analysis needs to be transported with the message to the 1958 next stage. 1960 Appendix E. Change History 1962 E.1. RFC7001 to -00 1964 o Remove "Changes since RFC5451" section; add this "Change History" 1965 section. 1967 o Restore XML to previous format. (No visible changes). 1969 o Reset "Acknowledgments". 1971 o Add "To-Do" section. 1973 E.2. -00 to -01 1975 o Apply RFC7410. 1977 o Update all the RFC4408 references to RFC7208. 1979 o Add section explaining "property" values. (Errata #4201) 1981 Author's Address 1983 Murray S. Kucherawy 1984 270 Upland Drive 1985 San Francisco, CA 94127 1986 US 1988 EMail: superuser@gmail.com