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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Appsawg D. Otis 3 Internet-Draft Trend Micro 4 Intended status: Experimental D. Black 5 Expires: September 4, 2015 March 3, 2015 7 Third-Party Authorization Label 8 draft-otis-tpa-label-05 10 Abstract 12 This experimental specification proposes a Third-Party Authorization 13 Label (TPA-Label) as a DNS-based method that allows Trusted Domains 14 an efficient means to authorize acceptable Third-Party Domains. This 15 method permits autonomous unilateral authorizations and uses scalable 16 individual DNS transactions. 18 A TPA-Label Resource Record transaction asserts an alignment 19 exception to convey informally Federated Domains. It affords 20 recipients a practical and safe means to extend Domain Alignment. 21 Exceptions are managed by either the Trusted Domain, or their agent, 22 seeking to avoid disruption of informal services enjoyed by their 23 users. Third-Party Authorization of a Federated Domain eliminates a 24 need to share private credentials. 26 Requirements Language 28 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 29 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 30 document are to be interpreted as described in [RFC2119]. 32 Status of this Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at http://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on September 4, 2015. 49 Copyright Notice 51 Copyright (c) 2015 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (http://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 67 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 68 3. Domain Validation Issues . . . . . . . . . . . . . . . . . . . 6 69 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 70 5. TPA-Label Listed Domain, TPA-LLD, . . . . . . . . . . . . . . 7 71 6. TPA-Label Resource Record Authorization Considerations . . . . 8 72 7. Evaluating the Third-Party Domain . . . . . . . . . . . . . . 9 73 7.1. Third Party Authorization - Closed Mailing List Example . 9 74 7.2. Third Party Authorization - Open Mailing List Example . . 10 75 7.3. Third Party Authorization Example - Sender Header Field . 10 76 7.4. Services Lacking DKIM Signatures . . . . . . . . . . . . . 10 77 7.4.1. Abuse and DSN Reporting . . . . . . . . . . . . . . . 11 78 7.4.2. Third Party Authorization Example - SMTP Host . . . . 11 79 7.4.3. Third Party Authorization Example - Return Path . . . 11 80 7.4.4. Use of Path Authorization . . . . . . . . . . . . . . 11 81 8. DNS Representation . . . . . . . . . . . . . . . . . . . . . . 12 82 9. TPA-Label and Tag Syntax Definitions . . . . . . . . . . . . . 12 83 10. TPA-Label Generation . . . . . . . . . . . . . . . . . . . . . 13 84 11. TPA-Label TXT Resource Record Structure . . . . . . . . . . . 14 85 12. TPA-Label Resource Record Definition . . . . . . . . . . . . . 15 86 13. TPA-Label Resource Record Version . . . . . . . . . . . . . . 15 87 14. Authorized Validated Domains . . . . . . . . . . . . . . . . . 15 88 14.1. TPA-Label Resource Record Param Syntax . . . . . . . . . . 16 89 14.2. Header Dependent Authorizations . . . . . . . . . . . . . 16 90 14.2.1. List-ID Header Field . . . . . . . . . . . . . . . . . 16 91 14.2.2. Sender Header Field . . . . . . . . . . . . . . . . . 17 92 14.2.3. OAR Header Field . . . . . . . . . . . . . . . . . . . 17 93 14.2.4. Combined 'L' or 'S' Params . . . . . . . . . . . . . . 17 94 14.3. DKIM signed domain . . . . . . . . . . . . . . . . . . . . 17 95 14.3.1. DKIM signed . . . . . . . . . . . . . . . . . . . . . 17 97 14.4. SMTP Host domains . . . . . . . . . . . . . . . . . . . . 17 98 14.5. SMTP Host domains . . . . . . . . . . . . . . . . . . . . 17 99 14.6. MailFrom Parameter . . . . . . . . . . . . . . . . . . . . 17 100 14.7. Not Federated . . . . . . . . . . . . . . . . . . . . . . 18 101 14.8. SMTP Host domains . . . . . . . . . . . . . . . . . . . . 18 102 15. TPA-Label Resource Record Query Transactions . . . . . . . . . 18 103 16. TPA-Label Resource Record Compliance Extension . . . . . . . . 19 104 17. Alternative Mitigation Strategies . . . . . . . . . . . . . . 20 105 17.1. Proposed DKIM-Delegate Signature survives all Message 106 modifications . . . . . . . . . . . . . . . . . . . . . . 20 107 17.2. Vouch-By-Reference . . . . . . . . . . . . . . . . . . . . 21 108 18. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 21 109 19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 110 19.1. Moving RFC6541 to historic . . . . . . . . . . . . . . . . 22 111 19.2. TPA-Label (TPA-LLD) Parameters . . . . . . . . . . . . . . 22 112 19.3. Email Authentication Method Registry . . . . . . . . . . . 23 113 19.4. Email Authentication Result Names Registry . . . . . . . . 23 114 19.5. Third Party Authorizations Labels Registry . . . . . . . . 24 115 19.6. Third Party Authorizations Param Registry . . . . . . . . 24 116 20. Security Considerations . . . . . . . . . . . . . . . . . . . 25 117 20.1. Benefits to Recipients . . . . . . . . . . . . . . . . . . 25 118 20.2. Risks to Recipients . . . . . . . . . . . . . . . . . . . 26 119 20.3. Benefits to Trusted Domains . . . . . . . . . . . . . . . 26 120 20.4. Risks to Trusted Domains . . . . . . . . . . . . . . . . . 27 121 20.5. Benefits to Third Party Signers . . . . . . . . . . . . . 28 122 20.6. Risks caused by Third Party Signers . . . . . . . . . . . 28 123 20.7. SHA-1 Collisions . . . . . . . . . . . . . . . . . . . . . 28 124 20.8. DNS Limits . . . . . . . . . . . . . . . . . . . . . . . . 28 125 21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29 126 22. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29 127 22.1. Normative References . . . . . . . . . . . . . . . . . . . 29 128 22.2. Informative References . . . . . . . . . . . . . . . . . . 30 129 Appendix A. DNS Example of TPA-Label Resource Record placement . 32 130 Appendix B. C code for label generation . . . . . . . . . . . . . 33 131 Appendix C. History of Prior Efforts . . . . . . . . . . . . . . 38 132 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40 134 1. Introduction 136 A TPA-Label Resource Record supports an authorization of separately 137 validated domains. This added authorization step avoids a need to 138 share private credentials. Also, it ensures each domain remains 139 apparent and open to validation when establishing an informal 140 federation of domains protecting Federated Domain Identities. To 141 improve security, authorization records may also limit how they are 142 to be applied. 144 With TPA-Label Resource Records, mailing lists, among similar Third- 145 Party Domain services, can indirectly assert protection of identities 146 when the source domain is within an informally Federated Domain. 147 Since mailing-lists receive differently formatted messages, a common 148 practice is to convert multi-party conversations into consistent and 149 compact formats facilitating the organization of the many multi-party 150 conversations. Such processing often breaks any meaningful message 151 signature. With the proposed scheme, trusting the federated message 152 source can supersede otherwise broken alignment validation. 154 Trusted Domains can seek to ensure the domains they federate protect 155 the Federated Identity. In situations where the Trusted Domain 156 cannot be confirmed, TPA-Labels are able to signal which domains are 157 within the Trusted Domain's informally established federation. When 158 a user wishes to utilize an informal Third-Party Domain service, it 159 is both logical and desirable to retain the original Federated 160 Identity to better convey who substantially created message content. 161 Not retaining this identity would otherwise prohibit subsequent 162 review of prior exchanges. However, when recipients wish to 163 determine whether to trust the Federated Identity, Domain Alignment 164 with the validated source may not exist. TPA-Labels can indicate 165 whether the source domain has been informally federated by the 166 Trusted Domain. 168 2. Terminology 170 Please see [RFC5598] for general email terminology. 172 The following additional terms are used: 174 Transparent Domain Authorization: Third-Party Domains validating as 175 the Trusted Domain represent a type of transparent authorization. 176 This method normally depends on securely sharing private details 177 between domain owners and providers. However, private sharing 178 between different administrative domains is expensive and carries 179 some risk a security breach may result in the wrong administration 180 being held accountable or more resources being placed in peril. 182 Trusted Domain: Often a visible domain that acts as a basis for 183 acceptance and/or subsequent actions. For DMARC, this is the 184 fully qualified domain name found in the From header field. 186 Author Domain: See Section 3 of DMARC ([I-D.kucherawy-dmarc-base] 187 specifying the domain of the From header field which represents 188 the Trusted Domain. 190 Federated Domain: A domain (among possibly others) working in 191 concert with the Trusted Domain authorized as protecting Federated 192 Identifiers. 194 Federated Identity: Identity protected by a Federated Domain. In 195 the case of DMARC, this identity is contained in the From header 196 field. 198 Domain Alignment: Strict alignment requires matching the Trusted 199 Domain. Relaxed alignment allows source domains to be a sub- 200 domain of the Trusted Domain. 202 Third-Party Authorization: A different domain authorized by the 203 Trusted Domain. 205 Informal Third-Party Service: A Service not by the Trusted Domain 206 that does not require administrative cooperation for users to 207 independently establish their own access credentials. 209 Third-Party Service: A Service not by the Trusted Domain. 211 Third-Party Domain: A domain that is not the Trusted Domain. 213 TPA-Label Authorization: The referencing domain which meets the 214 validation and header field content requirements of the resolved 215 TPA-Label Resource Record is thereby authorized by the Trusted 216 Domain. 218 TPA-Label Listed Domain, TPA-LLD: TPA-Label Listed Domain, TPA-LLD, 219 is a TXT Resource Record referenced with the hash value of the 220 domain being authorized. This Resource Record is published within 221 a Trusted Domain. When a "tpa" tag exists, the referencing domain 222 (the domain used to generate the label) must be within the listed 223 domains. When the "tpa" tag does not exist, the referenced domain 224 is presumed. The "param" tag may stipulate a required existence 225 of additional header fields, or indicate alternate domain 226 validation methods to be applied against specific elements. The 227 "param" tag may also indicate that the domain matching in the 228 prior "tpa" list validated according to "param" associated listed 229 methods, is specifically not federated by the Trusted Domain. 231 3. Domain Validation Issues 233 Changing the validated domain, the one referenced by SPF [RFC7208], 234 or the one adding a DKIM [RFC6376] signature, is not a problem since 235 it is rare for acceptance to be based on From header field Domain 236 Alignment. However, when acceptance is based on the From header 237 field alignment, as in the case of DMARC [I-D.kucherawy-dmarc-base] 238 using either SPF or DKIM, this can disrupt many Third-Party Services. 239 The disruption becomes egregious when messages from the domain's own 240 users are rejected based on the level of this domain's asserted 241 alignment practices. At the strictest alignment level, an erroneous 242 assertion not only disrupts messages from their users, it can also 243 affect subscriptions or services for other users of the Third-Party 244 Service. 246 DKIM, unlike SPF, permits better retention of From/signature 247 alignment where only the From header field could be signed. 248 Nevertheless, the integrity of the original DKIM signature is likely 249 affected by message flattening, inclusion of Subject tags, or 250 appended list footer information. Just signing the From header field 251 is not a practical solution, because it would expose this message 252 fragment to replay abuse, even when given short signature expiry. 254 TPA-Label authorization may individually authorize domain validation 255 methods. This may either increase or decrease the number of 256 validation methods normally used. For example, a virtual server may 257 share an IP address with thousands of different domains. Its 258 authorization may need to exclude IP addresses as a basis for 259 validation. In the TPA-Label scheme, unless a validation method is 260 asserted, no changes to the domain validation process should be 261 assumed. 263 TPA-Labels can minimize the number of systems involved and the 264 related deployment time before the disruption of legitimate messaging 265 is avoided. TPA-Labels should also ensure greater cooperation to 266 sustain the desired protections. These types of restrictions are 267 increasingly likely to be relied upon to help mitigate harm caused by 268 future breaches. 270 4. 272 Administrative domains, that assert all of their outbound message 273 sources can be validated as having aligned domains, offer significant 274 forensic value. However, messages where domains are not in alignment 275 remain a potential issue. Only domains offering messages of a 276 transactional nature are unlikely to benefit from the use of TPA- 277 Labels. 279 This document describes how any Trusted Domain publishing DMARC 280 records can autonomously authorize other validated domains. TPA- 281 Labels offer secondary compliance options whenever authorized 282 exceptions are needed to permit the use of Third-Party Domains. The 283 intended purpose of TPA-Label Resource Records is to improve 284 acceptance rates of genuine messages, to minimize DNS use, to 285 minimize success rates for phishing, to improve sorting protections, 286 and to minimize a recipient's administrative costs. 288 TPA-Label Resource Records authorize Third-Party Domains and services 289 to extend compliance options for asserted practices defined by 290 [I-D.kucherawy-dmarc-base]. Domains, that both reference and are 291 listed, and also comply with a TPA-Label resource record, should be 292 considered equivalent to the authorizing Trusted Domain when 293 assessing compliance with DMARC asserted practices. 295 5. TPA-Label Listed Domain, TPA-LLD, 297 TPA-Label Listed Domain, TPA-LLD, is a TXT resource record referenced 298 with a TPA-Label published within a Trusted Domain. When a "tpa" tag 299 exists within the TXT resource record located at the TPA-Label, the 300 referencing domain (the domain used to generate the label) must be 301 within the listed domain. When the "tpa" tag does not exist, the 302 referenced domain is presumed listed. The "param" tag may stipulate 303 existence of additional header fields, or indicate alternate 304 validation methods applied against specific email elements. 306 Third-Party Domain validation might use a DKIM signature or confirm 307 the Authorized Domain using specific methods with various path 308 related email elements. The default assertion for param is 'd' and 309 'm', indicating DKIM or the Mail From parameter processed by SPF 310 confirms the Authorized Domain when no other method is specified. 312 The 'S' and 'L' param do not confirm the domain, but requires at 313 least one Sender or List-ID header field to hold a TPA-LLD 314 respectively. The 'O' param can be stipulated when the Authorized 315 Domain does not offer DMARC acceptance or validate access in 316 association with From header fields. The 'e', 'h', and 't' indicate 317 specific alternative methods using message elements to confirm the 318 Authorized Domain. 320 When any param method is asserted (denoted by a lower case letter), 321 methods not listed should not be considered to provide valid results. 322 The 'n' assertion indicates that the prior domain listed in the same 323 TPA-LLD is specifically not federated as determined by either the 324 default or listed methods. Being compliant with TPA-LLD allows the 325 referencing domain to informally act on behalf of the Trusted Domain. 326 Indicating domains as not federated necessitates the use of sequence 327 sensitive "tpa" and "param" pairs within the TPA-LLD. Per [RFC5321], 328 domain name comparisons, as well as TPA-Labels, are case insensitive. 330 6. TPA-Label Resource Record Authorization Considerations 332 When a Trusted Domain is not within a DKIM or SPF validated domain, 333 the TPA-LLD scheme can extend Domain Alignment compliance. The TPA- 334 LLD scheme with an 'S', or 'L' param requires the respective Sender 335 header field or a List-ID identifier of the List-ID header field to 336 exist for at least one of the params, and to contain a domain within 337 the TPA-LLD for authorization to be valid. The 'd' param permits 338 validations based upon the DKIM signing domain. The 'm' param 339 permits validations based upon the return path (Mail From) domain. 340 The 'e', 'h', and 't' params permit acceptance based upon validation 341 of the client hostname (EHLO/HELO). 343 The 'S' and 'L' params support message sorting. Any matching header 344 field with a domain within the TPA-LLD allows recipients to 345 differentiate sources, which satisfies requirements for any other 'S' 346 or 'L' param. The 'S' and 'L' params provide Trusted Domains a means 347 to limit domain authorizations. 349 The TPA-LLD scheme plays the role of only qualifying acceptable 350 domains with the goal of improving delivery acceptance, such as 351 messages from specific mailing-lists. The TPA-LLD authorization 352 scheme only requires that DNS publications be made by the Trusted 353 Domain, even when the sending domains and the Trusted Domain differ. 354 This approach eliminates a need to exchange private information thus 355 protecting the domain's integrity. Before TPA-LLD authorization is 356 deployed, the Trusted Domain should be assured by domains being 357 authorized that appropriate measures are in place to validate those 358 submitting messages and ensure the Federated Identity is protected. 360 Retaining validation and authorization for the From, Sender, and 361 List-ID header fields, and being able to ensure Third-party inclusion 362 of a Sender or List-ID header fields, enhances protections afforded 363 by message sorting. This protection reduces susceptibility to 364 deceptive look-alike phishing attempts. Use of subdomains that 365 assert less stringent practices might inadvertently combine with 366 those having more stringent practices when sorting is based upon 367 parent domains. Consistently using the same domain prevents possible 368 confusion that could be exploited to deceive recipients. 370 TPA-Label authorization will not ensure all possible spoofing is 371 prevented. However, by permitting broader use of strict alignment 372 practices, this should generally reduce the level of spoofing over 373 what might be otherwise allowed. Authorized third party messages 374 SHOULD NOT receive annotations that indicate the message contains 375 validated identities. The TPA-LLD param SHOULD include the 'S' or 376 'L' param where appropriate to allow recipients a means to isolate 377 and distinguish different message sources. 379 7. Evaluating the Third-Party Domain 381 A Trusted Domain deploying a TPA-Label Resource Record does so on a 382 trust basis. Reasons for deploying TPA-Label Resource Records might 383 be to allow deployment of more stringent DMARC records while also 384 utilizing Third-Party Services. 386 When an authorized Third Party domain does not employ DKIM or SPF or 387 does not include Authentication-Results header fields [RFC7001] or 388 perhaps [I-D.kucherawy-original-authres] (OAR) or its "X-" version 389 could allow authorizations to be exploited. For Third Party domains 390 not applying DMARC but capture the OAR, past compliance with DMARC 391 based on the OAR can be made a requirement for authorization. 393 While conceivably Domain Alignment might just rely on the content of 394 the Original-Authentication-Results header, whether to trust this, or 395 any other message content can not be based on the mere acceptance of 396 the message alone. Whether false content even effects message 397 acceptance would be difficult to determine. Only the Trusted Domain 398 is able to make this type of determination based on their knowledge 399 of outbound messages and corrections needed based on DMARC feedback 400 which they then share in the form of a TPA-Label. 402 7.1. Third Party Authorization - Closed Mailing List Example 404 The Trusted Domain wants to deploy a TPA-Label Resource Record for a 405 mailing list with a closed posting policy. The mailing list 406 redistributes email which breaks the Trusted Domain Alignment, and 407 the mailing list offers a means to validate the mailing list domain 408 and includes an Authentication-Results header field for posted 409 messages. The closed posting policy can be enforced by requiring 410 subscribers to validate control of their Author Addresses by 411 responding to encoded "pingback" email sent to these addresses. 413 Since the mailing list validates their domain as indicated in the 414 TPA-Label, and validates control of the posted message Author 415 Address, and includes Authentication-Results header fields, and 416 includes a List-ID header field, the referenced TPA-Label Resource 417 Record can include an 'L' param value to stipulate that the Third- 418 Party Domain messages contain an authorized List-ID domain. 420 7.2. Third Party Authorization - Open Mailing List Example 422 The Trusted Domain wants to deploy a TPA-Label Resource Record for a 423 mailing list with an open posting policy. The mailing list 424 redistributes email in a way that breaks Trusted Domain alignment, 425 does not post from an Author Address not in compliance with DMARC, 426 offers a means to validate the mailing list domain, and it includes 427 an Authentication-Results header field for posted messages. 429 Since the mailing list validates the domain as indicated in the TPA- 430 Label, and is configured to include Authentication-Results header 431 fields and possibly the Original-Authentication-Results 432 [I-D.kucherawy-original-authres], and includes a List-ID header 433 field, the referenced TPA-Label Resource Record can include an 'L' 434 param value to stipulate the Third-Party Domain messages contain an 435 authorized List-ID domain. 437 7.3. Third Party Authorization Example - Sender Header Field 439 Trusted Domain "example.com" wishes to temporarily employ the service 440 agency "temp.example.org" to handle overflow secretarial support. 441 The agency "temp.example.org" sends email on behalf of the executive 442 staff of "example.com" and adds the Sender header field of 443 "secretary@temp.example.org" in the email. 445 Since "temp.example.org" only allows its own staff to email through 446 its server which adds "temp.example.org" DKIM signatures, a TPA-LLD 447 can include the "temp.example.org" domain with an 'S' and 'd' param 448 to specifically authorize DKIM signed messages containing the Sender 449 header field, to help ensure these messages are not handled as 450 phishing attempts. 452 7.4. Services Lacking DKIM Signatures 453 7.4.1. Abuse and DSN Reporting 455 There is likely little interest for an otherwise uninvolved domain to 456 receive a massive number of bogus messages being returned as 457 feedback. Often the purpose of feedback is to discover compromised 458 systems or accounts actively being exploited in some manner. Unless 459 the Trusted Domain is confirmed as having handled or authorized the 460 handling of the message, only statistics and samples should be 461 reported to the associated Autonomous System [RFC1930], and perhaps 462 to the Trusted Domain when interest is expressed. 464 The 'd', 'e', 'h', 'm', and 't' param options within the TPA-LLD 465 records allow the Trusted Domain to be associated through various 466 methods. In this case, appropriate DSN or abuse reporting to the 467 Trusted Domain is better assured as well. 469 7.4.2. Third Party Authorization Example - SMTP Host 471 Trusted Domain "example.com" makes use of invite services. This 472 service does not utilize DKIM, where the host name given by the EHLO 473 command is "invite.example.net". The Trusted Domain can authorize 474 the domain "invite.example.net" or "example.net" with the param of 475 'e' to improve acceptance of messages that are sent on behalf of 476 "example.com" from this outbound server. 478 7.4.3. Third Party Authorization Example - Return Path 480 Trusted Domain "example.com" makes use of tell-a-friend services. 481 This service does not utilize DKIM with its own return path as 482 "customer@taf.example.net" in the SMTP exchange. The Trusted Domain 483 can authorize the domain "taf.example.net" with the param of 'm' to 484 improve acceptance of messages that are sent on behalf of 485 "example.com" from this outbound server. 487 7.4.4. Use of Path Authorization 489 Those using validations related to 'e', 'h', 'm' param options should 490 not authorize domains requiring more than an average number of 491 network transactions. Those implementing DMARC should also limit the 492 number of DNS transactions attempted, otherwise this could negatively 493 impact unrelated domains when evaluating path related validation. 495 Methods that create subsequent transactions based upon the macro 496 expansion of email-address local-parts should not be used. 497 Libraries that process SPF [RFC7208] record scripts may invoke a 498 large number of DNS transactions from cached records, and target 499 unrelated domains with queries modulated by the local-part 500 component through receiver macro expansion. 502 8. DNS Representation 504 The receiver obtains domain authorizations with a DNS query for an IN 505 class TXT TPA-Label Resource Record located below the 506 "_smtp._tpa." location. The TPA-Label itself is 507 generated by processing the domain in question, which normally 508 matches the DKIM signature's "d=" parameter. The Trusted Domain 509 provides authorization for other domains with the existence of a TPA- 510 Label TXT resource record. When a "tpa" tag value exists, it MUST 511 include the referenced domain before authorization is valid. This 512 represents an informal authorization on behalf of the Trusted Domain 513 which can be limited by the "param" tag value for specific message 514 elements. 516 A Trusted Domain may wish to delegate the listing of Third-Party 517 Services to a different administrative domain. Ideally, this would 518 be accomplished by delegating the _tpa. zone to the 519 administrative entity handling publication of TPA-Label Resource 520 Records. This delegation could also be done unilaterally with a 521 DNAME [RFC6672] resource record published at _smtp._tpa.. 524 Character-strings contained within the TXT resource record are 525 concatenated into forming a single string. A character-string, as 526 defined in [RFC1035] Section 3.3 for resource records, is a single 527 length octet followed by that number of characters treated as binary 528 information. 530 The TPA-Label Resource Records should be located at these domains: 532 ._smtp._tpa.. 534 9. TPA-Label and Tag Syntax Definitions 536 Augmented BNF for Syntax Specifications: 538 asterisk = %x2A ; "*" 539 dash = %x2D ; "-" 540 dot = %x2E ; "." 541 underscore = %x5F ; "_" 542 ANY = asterisk dot ; "*." 543 dns-char = ALPHA / DIGIT / dash 544 id-prefix = ALPHA / DIGIT 545 label = id-prefix [*61dns-char id-prefix] 546 sldn = label dot label 547 base-char = (dns-char / underscore) 548 domain = *(label dot) sldn 550 FWS = ([*WSP CRLF] 1*WSP) ; omits RFC5322 obs-FWS 551 tag-sep = %x3B ; "%" 552 tag-list = tag-spec 0*( tag-sep tag-spec ) [ tag-sep ] 553 tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS] 554 tag-name = ALPHA 0*ALNUMPUNC / "v" / [["tpa"] / ["param"]] 555 tag-value = [ tval 0*( 1*(WSP / FWS) tval ) ] 556 ; WSP and FWS prohibited at beginning and end 557 tval = 1*VALCHAR 558 VALCHAR = %x21-3A / %x3C-7E 559 ; EXCLAMATION to TILDE except SEMICOLON 560 ALNUMPUNC = ALPHA / DIGIT / "_" 562 10. TPA-Label Generation 564 The TPA-Label is generated by nesting functions as follows: 566 "base32" function is defined in [RFC4648]. 568 "sha1" function is defined in [FIPS.180-2.2002]. 570 "lcase" converts upper-case ALPHA characters to lower-case. 572 "tpa-domain" is normally the "d=" tag value defined in Section 3.5 573 of [RFC6376]. 575 (underscore) base32( sha1( lcase(tpa-domain))) 577 The TPA-Label is created from the hash value returned by the "sha1" 578 function of the tpa-domain expressed in lower case ASCII. Any 579 terminating period is not included with the tpa-domain, as indicated 580 by the ABNF definition. 582 Note: No newline character, 0x0A, is to be appended to the end of 583 the domain name, as might occur with the command line generation 584 of sha1 values. For example, these command line appended newlines 585 can be avoided by using the 'echo -n" option. 587 The label encoding process inputs the hash as a byte stream of four 588 40-bit data blocks where each data block outputs 8 encoded 589 characters. Proceeding from left to right, a 40-bit input group is 590 formed by concatenating 5 bytes. The 40-bit input is then treated as 591 8 concatenated 5-bit groups, each of which is translated into a 592 single digit of the base32 alphabet. The bit stream is ordered with 593 the most-significant-bit first, being the high-order bit of the first 594 byte. The entire output is then concatenated first to last, left to 595 right, into 32 characters prefixed with an underscore. 597 11. TPA-Label TXT Resource Record Structure 599 Every TPA-Label TXT resource record MUST start with the version tag, 600 so the first six characters of the record are lowercase "v=tpa1", 601 TPA-Label syntax descriptions for additional tags follow the tag- 602 value syntax described in the ABNF below, the WSP token is inherited 603 from [RFC5322]. The ALPHA and DIGIT tokens are imported from 604 [RFC5234]. The "param" values refer only to domains previously 605 listed in the TPA-LLD which makes the "tpa" and "param" tags 606 sensitive to their sequence. 608 The tags used in TPA-Label Resource Records are as follows: 610 +------------+--------------------------------------+ 611 | Tag | Function | 612 +------------+--------------------------------------+ 613 | v | Label Version (version-tag) | 614 | tpa | Authorized Domains List (tpa-tag) | 615 | param | Authorization Param List (param-tag) | 616 +------------+--------------------------------------+ 618 TPA-Label Tags 620 +--------+------------------------------------+---------------------+ 621 | param | Field or Parameter | Method | 622 | values | | | 623 +--------+------------------------------------+---------------------+ 624 | L | List-ID Header Field | Match List-ID | 625 | | | Identifier | 626 | S | Sender Header Field | Match Address | 627 | | | Domain | 628 | O | Original Authentication Results | Match Address | 629 | | Header Field | Domain | 630 | d | DKIM Signature | Match Signature | 631 | | | Domain | 632 | e | SMTP Hostname | Resolve Hostname IP | 633 | | | Addr | 634 | h | SMTP Hostname | Pass SPF with | 635 | | | Hostname | 636 | n | Not Federated | See Other Params | 637 | m | MailFrom | Pass SPF with | 638 | | | MailFrom | 639 | t | SMTP Hostname | Cert of Hostname | 640 +--------+------------------------------------+---------------------+ 642 TPA-Label Param Values 644 12. TPA-Label Resource Record Definition 646 Tags in the TPA-Label Resourse Record are shown below. The ver-tag 647 MUST be present as the left most tag. Unrecognized tags MUST be 648 ignored. 650 TPA-Label Resource Record Definition 651 tpalabelrr = v-tag [tag-sep] 0*( 1*(WSP) tag-list) ] 653 13. TPA-Label Resource Record Version 655 Label Version (Required). This tag defines the version of the TPA- 656 Label. Only recognized tpa:param values offer DMARC authorizations 657 (except for param "n" which excludes domains validated per other 658 param values). 660 "v" tag 661 v-tag = %76.3d.74.70.61.31 ; "v=tpa1" 663 14. Authorized Validated Domains 664 Authorized validated domain list. (optional) This tag, when present, 665 MUST contain a domain that repeats all or right-most portions of the 666 domain encoded within the TPA-Label Resource Record. This option 667 ensures the proper handling of possible hash collisions. When a 668 domain is prefixed with the "*." ANY label, then all subdomains of 669 this domain are to be considered included within the list. When the 670 'tpa' tag is not present or has no value, it should be assumed to 671 compare with the domain used to generate the TPA-Label. The purpose 672 of the ANY label is to reduce the size of the resource records. 673 Containing the entire string to confirm hostnames or List-ID content 674 is unnecessary. The hash label must still be an exact match of the 675 domain authorized. Additional domains may be included as optional 676 Sender or List-ID comparison options. The tpa list is optionally 677 followed by param list. There can be multiple tpa:param sets. 679 Use of the ANY label is not intended to support wildcards for 680 referencing hash labels. No wildcard labels are to be used below the 681 "_tpa." label to access DNS resources. 683 "tpa" tag 684 ad_val = [ANY] domain 685 ad-list = %x74.70.61 *WSP "=" [ ad_val 0*( 1*(WSP) ad_val )] 687 14.1. TPA-Label Resource Record Param Syntax 689 Authorization Param List (Optional). This tag defines a list of 690 assertions for the preceding (if listed) domains which indicate 691 various email-address locations within the message and authorized 692 validation methods. Only recognized param values offer any form of 693 DMARC authorization. The "n" param however excludes the prior tpa 694 list as not being within the federation. 696 "param" tag 697 pa_val = "L" / "S" / "O" / "d" / "e" / "h" / "m" / "n" / "t" 698 pa-list = %x73.63.6f.70.65 *WSP "=" [ pa_val 0*( 1*(WSP) pa_val )] 700 14.2. Header Dependent Authorizations 702 14.2.1. List-ID Header Field 704 The "L" param asserts that authorization is valid only when a List-ID 705 identifier of the List-ID header field [RFC2919] contains a domain 706 that is within a domain listed in the TPA-LLD "tpa" tag. 708 The syntax of the List-Id header field is as follows: 709 list-id-header = "List-ID:" [phrase] "<"identifier">"CRLF 711 14.2.2. Sender Header Field 713 The "S" param asserts that authorization is valid only when the 714 domain in the Sender header field is within the TPA-LLD. 716 14.2.3. OAR Header Field 718 The "O" param asserts that authorization is valid only when the 719 domain in an Original Authentication Results header field indicates 720 it passed based on a domain within the TPA-LLD or the Trusted Domain 721 itself. 723 14.2.4. Combined 'L' or 'S' Params 725 When combined, the params 'L' and 'S' require that either a List-ID 726 identifier of the List-ID header field or the Sender header field 727 must contain a domain within the TPA-LLD for the authorization to be 728 valid. 730 14.3. DKIM signed domain 732 14.3.1. DKIM signed 734 The "d" param asserts that messages carrying the Trusted Domain 735 within the From header field are authorized to be signed by the TPA- 736 LLD. 738 14.4. SMTP Host domains 740 The "e" param asserts that host names given in [RFC5321] EHLO or HELO 741 commands within TPA-LLD is authorized when the hostname resolves the 742 server's IP address. 744 14.5. SMTP Host domains 746 The "h" param asserts that host names given in [RFC5321] EHLO or HELO 747 commands within TPA-LLD is authorized only when this hostname 748 submitted to an SPF [RFC7208] process returns pass. 750 14.6. MailFrom Parameter 752 The "m" param asserts that an email-address domain in the [RFC5321] 753 MAIL command within a TPA-LLD is authorized only when this email- 754 address submitted to an SPF [RFC7208] process returns pass. 756 14.7. Not Federated 758 The "n" param asserts that a previous "tpa" listed domain is 759 specifically not federated and not authorized. The use of this 760 parameter is to suppress subsequent processing that may otherwise be 761 needlessly repeated in the case where the third-party is being 762 detected as conveying user messages but are not authorized for policy 763 related reasons, such as not adequately protecting the Federated 764 Identity. 766 14.8. SMTP Host domains 768 The "t" param asserts that host names given in [RFC5321] EHLO command 769 after [RFC3207] negotiation where the Cert DNS-ID domain is within 770 TPA-LLD is authorized. It will also be interesting to see whether 771 [I-D.ietf-dane-smtp-with-dane] establishes a way to authenticate 772 sending domains. 774 Note to RFC Editor: Remove this comment before publishing. 775 Currently, no general practice employs certificates to confirm the 776 domain of the client initiating a connection. This may be needed 777 for clients within IPv6 IP address space where tunneling, carrier 778 grade NATs, and rapid space assignment without any practical 779 reverse mapping reduces the effectiveness of IP address based 780 reputations. 781 There is an existing TLS option for SMTP and an ongoing effort to 782 standardize automated server confirmation. It might be possible 783 to leverage this effort to establish practices used at the client. 784 See conversations defined in [RFC4954] Section 4. For information 785 related to ongoing server related efforts see: 786 [RFC6125] and [RFC6698] 788 15. TPA-Label Resource Record Query Transactions 790 The discovery of TPA-Label Resource Records need not be subsequent to 791 the discovery of the DMARC record. However, when no DMARC record is 792 discovered which includes the tag value of "tpa", the verifier MAY 793 assume no TPA-Label Resource Records have been published. Otherwise, 794 when there is no Trusted Domain validation, the discovery of TPA- 795 Label Resource Records should be attempted. 797 16. TPA-Label Resource Record Compliance Extension 798 The signing practice compliance assessment of Third Party Signatures 799 is a discretionary operation performed by the verifier. For messages 800 that do not have valid Trusted Domain alignment, a verifier may 801 decide to assess compliance for Third Party messages when there is a 802 DMARC tag of "tpa". Elements then referenced in the TPA-Label param 803 values of "d", "m", "e", "h", "t" are to be checked in their listed 804 succession. One of the following sets of conditions MUST be met for 805 the result to be considered a pass: 807 For Third Party DKIM signatures, the following represents the set of 808 conditions to be checked: 810 o The Third Party Signature MUST validate according to [RFC6376]. 811 o A TXT resource record, referenced by a TPA-Label created by the 812 DKIM signature "d=" tag, MUST exist in DNS. 813 o The discovered TPA-Label Resource Record structure MUST be valid. 814 o The domain that created the TPA-Label MUST be within the TPA-LLD. 815 o Where a param of 'd' is specified, the Trusted Domain MUST have an 816 authorized DKIM signature. 817 o Where a param of 'L' or 'S' is specified, a List-ID identifier in 818 the List-ID header field or a Sender header field MUST contain a 819 domain within the TPA-LLD. This provides Third-Party services a 820 reason to ensure their outbound messages do not spoof these 821 associated header fields. 822 o Where a param of 'O' is specified, an Original Authentication 823 Results header field MUST indicate a pass for the Trusted Domain 824 or for a domain within the TPA-LLD. This parameter requires the 825 message was received from an approved Originating source. 827 For non-DKIM validations, the TXT record discovery process continues 828 until a TPA-Label Resource Record structure is found where: 830 One of the three possible TXT resource records is checked in their 831 listed succession. Each would be referenced by an 'h' or 'e' or 't' 832 related domain given by [RFC5321] EHLO or HELO command, this domain 833 with left-most label omitted, or by an 'm' related email-address 834 domain within the [RFC5321] MAIL command. 836 o The discovered TPA-Label Resource Record Structure is valid. 837 o The domain that created the TPA-Label is within the TPA-LLD. 838 o The domain that created the TPA-Label corresponds with a listed 839 param of 'e', 'h' or 'm' or 't'. 840 o Where a param of 'L' or 'S' is specified, either the domain in 841 List-ID given by [RFC2919] in the List-ID header field is within 842 the TPA-LLD, or a Sender header field contains a domain within the 843 TPA-LLD respectively. 844 o Once these four conditions have been met, for 'h' or 'm' params 845 the domain MUST be confirmed by submitting the domain to an SPF 846 process that then returns pass. The 'e' param MUST be confirmed 847 by a forward DNS reference that resolves the address of the SMTP 848 client. The 't' param MUST be confirmed by the DNS-ID in the 849 client certificate. 851 When the TPA-Label Resource Record can not be retrieved due to some 852 error that is likely transient in nature, such as "SERVFAIL" for 853 example, the result of the TPA-Label Resource Record compliance 854 assessment is "temperror". 856 When the TPA-Label Resource Record retrieval returns a DNS "NOERROR", 857 but not with a single record, the result of the TPA-Label Resource 858 Record compliance assessment is "permerror". 860 When the TPA-Label Resource Record can not be retrieved with a DNS 861 "NXDOMAIN" response, the result of the TPA-Label Resource Record 862 compliance assessment is "nxdomain". 864 17. Alternative Mitigation Strategies 866 17.1. Proposed DKIM-Delegate Signature survives all Message 867 modifications 869 When a domain sends a message to services likely to invalidate DKIM 870 signatures, such as that of a mailing-list, some envision use of a 871 [I-D.kucherawy-dkim-delegate] header field as a type of DMARC 872 disruption prophylactic conveying signed Third-Party Authorizations 873 similar to that of TPA-LLD. 875 The DKIM-Delegate header field is not a DKIM message signature nor 876 can it generally authorize trustworthy sources. This header field 877 represents a signed domain authorization list placed in the header 878 field's 't' tag. To limit its duration, short expiry can be 879 specified in the 'x' tag. Nevertheless, validity of this header 880 field is completely independent of the message body or any other 881 message header field. 883 Since DKIM-Delegate authorization is actually unrelated to any 884 specific message, it offers less protection at the expense of a 885 higher transactional overhead compared to that of the smaller and 886 simpler TPA-LLD transactions. The TPA-LLD resource also better 887 facilitates timely authorization retractions. Before DKIM-Delegate 888 can be as effective, separate and much larger signature resources 889 would be needed for each authorized domain to match the timely de- 890 authorization selectivity the TPA-Label scheme permits. 892 Since either scheme is primarily aimed at circumventing DMARC 893 compliance issues, once adopted, changes to DMARC related validation 894 can quickly bolster DKIM/DMARC legacy shortfalls in a similar fashion 895 to that offered by TPA-LLD. It is even conceivable, that to obtain 896 an automated authorization expiry, a TPA-LLD could signal when a 897 DKIM-Delegate should be included. The TPA-LLD can still reduce the 898 authorization latitude offered by DKIM-Delegate. Unlike TPA-LLD, 899 DKIM-Delegate is unable to stipulate an additional set of required 900 conditions. 902 Since mailing-lists are normally fairly public, bad actors only need 903 to subscribe to obtain a fresh set of DKIM-Delegate header fields as 904 a means to circumvent its expiry feature. Using freshly minted DKIM- 905 Delegate header fields also avoids elements that might be identified 906 as pertaining to a specific campaign at little cost to the 907 malefactor. 909 17.2. Vouch-By-Reference 911 VBR uses a third-party domain referenced in a message header to vouch 912 for the types of messaging expected from a domain verified as part of 913 normal message handling. Since any domain other than the DMARC 914 domain attempting to guard against From header field spoofing is 915 unable to make such assessments, for the purposes of Anti-Phishing, 916 VBR lacks the necessary input and knowledge to offer timely and 917 accurate advice. The nature of Phishing abuse is often too low in 918 frequency for typical Anti-Spam policies to be effective. 920 18. Privacy Considerations 922 Unless all valid Third-Party Domains have been authorized, personally 923 identifiable information will be exchanged within the DMARC feedback. 924 This feedback can unintentionally expose private exchanges made on 925 behalf or the DMARC domain's users. To the greatest extent possible, 926 this feedback information should not be shared with other domains not 927 offering the information. This feedback can even identify mailing- 928 list subscribers that never sent any message to the list, or invoices 929 made on behalf of an accountant's client. 931 As with other authorization schemes that utilize DNS, relationships 932 are publicly revealed. This is the nature of SPF authorization which 933 reveals first party services being used. A TPA-Label on the other 934 hand can resolve a hash obscured Third-Party Service. Unlike SPF, a 935 TPA-Label does not include any user identity related parameters and 936 does not reveal any users specific relationships. In addition, these 937 relationships are accessed with a hash of the entire domain. Use of 938 a few random subdomains can inhibit discovery of these relationships. 940 However, the low latency of DNS means resource records can not be 941 assumed to remain secret. 943 Even so, disclosures of Third-Party Services might be justified by 944 dissuading malefactors who have compromised the Trusted Domain and 945 then are able to subsequently spoof the discovered personal 946 relationships. Such spoofing might be seen as causing greater harm 947 than public knowledge of possible Third-Party Services used by the 948 Trusted Domain's users. 950 It seems this can not be overstated: The overhead associated with 951 managing a "_tpa." zone is fairly small and is well offset by 952 squelching DMARC feedback generation and the remediation of a loss of 953 legitimate messages. Alternatives to TPA-Labels are likely to be the 954 dissemination of plaintext lists of domains known to cause alignment 955 failures, although operating in full compliance with SMTP protocols 956 and practices. The dissemination of lists lessens the domain's 957 privacy, and their ability to react to mitigate abuse. 959 19. IANA Considerations 961 19.1. Moving RFC6541 to historic 963 This document is seeking to replace [RFC6541] and to move it to 964 historic. 966 19.2. TPA-Label (TPA-LLD) Parameters 968 To accommodate the extensions to [RFC7001] needs the following 969 elements to be added: 971 +------+-----------------+ 972 | Type | Reference | 973 +------+-----------------+ 974 | tpa | (this document) | 975 +------+-----------------+ 977 TPA-Label Resource Record validation Method 979 19.3. Email Authentication Method Registry 981 To accommodate the method derived from TPA-Label Resource Record 982 processing, the IANA Registry "Email Authentication Method" defined 983 by Section 6.2 of [RFC7001] needs the following elements to be added: 985 +---------+-----------+--------+----------+-------------------------+ 986 | Method | Defined | ptype | property | value | 987 +---------+-----------+--------+----------+-------------------------+ 988 | tpa-lld | (this | domain | 3p-dom | Domain evaluated. The | 989 | | document) | | | method results from | 990 | | | | | [RFC7001] should also | 991 | | | | | be included in a | 992 | | | | | Authenticated Results | 993 | | | | | header field. | 994 | | | | param | value of param | 995 | | | | | (Section 19.6) tag. | 996 | | | | | (When 'param' contains | 997 | | | | | 'e', 'h' or 'm', the | 998 | | | | | iprev [RFC7001] | 999 | | | | | (Section 3) method | 1000 | | | | | results should also be | 1001 | | | | | included in the | 1002 | | | | | Authenticated-Results | 1003 | | | | | header field to capture | 1004 | | | | | the SMTP client IP | 1005 | | | | | address. | 1006 | | | | ca-param | The params | 1007 | | | | | (Section 19.6) with a | 1008 | | | | | compliance assessment | 1009 | | | | | as pass | 1010 | | | | tpa | Value of tpa | 1011 | | | | | (Section 14) tag at | 1012 | | | | | time of compliance | 1013 | | | | | assessment | 1014 +---------+-----------+--------+----------+-------------------------+ 1016 TPA-Label Resource Record validation Method 1018 19.4. Email Authentication Result Names Registry 1020 To accommodate the results derived from TPA-Label Resource Record 1021 processing, the IANA Registry "Email Authentication Method" defined 1022 by Section 6.3 of [RFC7001] needs the following elements added: 1024 +----------+----------+---------------------------------------------+ 1025 | code | method | meaning | 1026 +----------+----------+---------------------------------------------+ 1027 | none | tpa-lld | No TPA-Label was published | 1028 | pass | tpa-lld | Section 16 | 1029 | tempfail | tpa-lld | Section 16 | 1030 | permfail | tpa-lld | Section 16 | 1031 | hdrfail | tpa-lld | The TPA-Label Resource Record param values | 1032 | | | of "S" or "L" failed to match. | 1033 | nxdomain | tpa-lld | When obtaining the TPA-Label Resource | 1034 | | | Record, DNS indicated this domain does not | 1035 | | | exist. | 1036 +----------+----------+---------------------------------------------+ 1038 TPA-Label Resource Record complaince assessment Results 1040 19.5. Third Party Authorizations Labels Registry 1042 Names of tags that are valid in TPA-Label Resource Records with the 1043 exception of experimental tags Section 11 MUST be registered in this 1044 created IANA registry. 1046 New entries are assigned only for values that have been documented in 1047 a published RFC that has had IETF Review, per IANA CONSIDERATIONS 1048 [RFC5226]. 1050 Each tag registered must correspond to a definition. 1052 The initial set of values for this registry is: 1054 +-----------------+--------------+----------------------------------+ 1055 | tag | defined | definition | 1056 +-----------------+--------------+----------------------------------+ 1057 | v | Section 11 | Label Version | 1058 | tpa | Section 14 | List of Authorized Domains or | 1059 | | | Identifiers | 1060 | prior tpa param | Section 14.1 | Section 19.6 | 1061 +-----------------+--------------+----------------------------------+ 1063 TPA-Label Resource Record compliance assessment Results 1065 19.6. Third Party Authorizations Param Registry 1067 Values that correspond to Section 14.1 MUST be registered in this 1068 created registry: 1070 New entries are assigned only for values that have been documented in 1071 a published RFC that has had IETF Review, per IANA CONSIDERATIONS 1073 [RFC5226]. 1075 Each value registered must correspond to a definition. 1077 The initial set of values for this registry is: 1079 +------------+----------------+ 1080 | value | defined | 1081 +------------+----------------+ 1082 | L | Section 14.2.1 | 1083 | S | Section 14.2.2 | 1084 | O | Section 14.2.3 | 1085 | d | Section 14.3 | 1086 | h | Section 14.5 | 1087 | e | Section 14.4 | 1088 | m | Section 14.6 | 1089 | n | Section 14.7 | 1090 | t | Section 14.8 | 1091 +------------+----------------+ 1093 TPA-Label Resource Record compliance assessment Results 1095 20. Security Considerations 1097 This draft extends Domain Alignment validation practices that depend 1098 on DKIM [RFC6376] or SPF [RFC7208]. Most related security matters 1099 are discussed in those specifications. Additional considerations are 1100 also included in [RFC6377]. Security considerations for the TPA-LLD 1101 scheme are mostly related to attempts on the part of malefactors to 1102 falsely represent themselves as others, often in an attempt to 1103 defraud either the recipient or the alleged originator. Some 1104 receivers mistakenly bypass validation of the [RFC5322] header fields 1105 because a signature from a Trusted Domain had been confirmed as 1106 perhaps suggested in [RFC5863]. Do not omit the validation of header 1107 fields unless the message is not accepted for other reasons. 1109 Additional security considerations regarding DKIM signing practices 1110 may be found in the DKIM threat analysis [RFC4686]. 1112 20.1. Benefits to Recipients 1114 The verifier, after validating a Federated Domain, will have 1115 significantly greater confidence in the Third-Party, than when no 1116 TPA-Label Resource Record is obtained. This enhanced confidence may, 1117 at the receivers' discretion, cause a message to be delivered to the 1118 recipient with less stringent assessments. 1120 20.2. Risks to Recipients 1122 The decisions a recipient makes in regard to message filtering based 1123 on TPA-Label Resource Records are likely to depend on the system 1124 integrity of the Third Party with respect to the validation methods 1125 determined by authorization param labels. When the 'e', 'h', or 'm' 1126 param domain is not confirmed, or the Third-Party Domain does not 1127 validate the submitter, there is a risk of accepting potentially 1128 spoofed messages. Authentication-Results header fields then play an 1129 important role when there is no out-of-band validations confirming 1130 the submitter. Without proper Authentication-Results handling by the 1131 Third-Party, there is also risk of accepting potentially spoofed 1132 messages. 1134 With the TPA-Label specification, third party validation provides 1135 verifiable value. Implementers should consider the possibility a 1136 malefactor will send a message having a large number of valid DKIM 1137 Signatures. Verifying all the signatures may consume a large amount 1138 of processing resources. As such, it might be worth checking for the 1139 existence of a TPA-Label Resource Record first to minimize network 1140 amplification concerns. Section 15 describes a quick check to see if 1141 TPA-Label Resource Records may exist. Additionally, validating DKIM 1142 signatures and obtaining related resource records might be limited to 1143 known trustworthy domains. 1145 Services that depend only upon path authorizations might permit the 1146 Trusted Domain to be spoofed and yet obtain acceptance. During such 1147 events, the Trusted Domain might need to retract its authorization 1148 from the service. For this reason, path related validation based on 1149 IP addresses should only be used as a carefully monitored interim 1150 solution. 1152 20.3. Benefits to Trusted Domains 1154 TPA-Label Resource Records can replace domain delegations, selector/ 1155 key record mirroring, or key exchanges. A significant number of 1156 details are associated with selector/key records. These details 1157 include user limitations, suitable services, key resource record's 1158 Time-To-Live, revocation and update procedures, and how the DKIM 1159 Signature header field's 'i=' semantics are to be applied. In 1160 addition, services that depend upon DKIM keys are better secured by 1161 not delegating these DKIM keys, where instead the TPA-LLD scheme 1162 allows Trusted Domains an ability to limit the scope of their 1163 authorizations, while also not being mistaken for having validated 1164 the entity submitting the message. 1166 TPA-Label Resource Records convey which domains are authoritative 1167 even when they are not the Trusted Domain. However, Authorized 1168 Domains are unable to utilize the DKIM signature's 'i=' semantics to 1169 directly assert which identifiers on whose behalf a signature was 1170 added. As such, no domain should be authorized unless it is trusted 1171 to ensure the Federated Identity of an email undergoes validation 1172 that offers acceptable protections for the Trusted Domain. For 1173 example, such validation might ensure submitting entities have 1174 demonstrated receipt of "pingback" messages sent to the Federated 1175 Identity (Author's address) contained within the messages being 1176 signed. 1178 By deploying TPA-Label Resource Records, Trusted Domains benefit when 1179 recipients assess the senders' practice compliance by using the TPA- 1180 LLD scheme. These recipients will be less likely to drop the Trusted 1181 Domain's genuine messages, whenever the Trusted Domain attempts to 1182 restrict acceptance. Restricting acceptance of non-compliant 1183 messages is the basic motivation for publishing DMARC records. In 1184 addition, recipients are more likely to validate messages by an 1185 Authorized Domain. 1187 Broader use of strict DMARC alignment assertions provides a greater 1188 likelihood of being able to eliminate a broader range of non- 1189 compliant messages, in addition to improving acceptance from 1190 authorized sources. TPA-Labels also allow Trusted Domains to control 1191 message Sender and List-ID attributes, to exclude problematic 1192 validation methods or include others as they become available. 1194 Trusted Domains having good reputations might extend limited 1195 compliance assessment resources to otherwise unknown domains or SMTP 1196 Clients that are referenced by their TPA-LLD. Conversely, TPA-LLD 1197 resources that assert a domain as not being federated can be used to 1198 suppress any processing that might be otherwise needlessly repeated. 1200 20.4. Risks to Trusted Domains 1202 As indicated in Section 7, it is ultimately an issue of trusting the 1203 Third Party Domain to do the right thing and not generate, or allow 1204 others to generate, messages that falsely appear to be from the 1205 Trusted Domain. The validation methods in place for different email 1206 elements need to be carefully reflected in the "param" tag of the 1207 TPA-LLD. 1209 Authorization of mailing lists with TPA-LLD could cause a loss of 1210 confidentiality in mailing list participation by the Trusted Domain. 1211 This might help malefactors deduce which subscription related email 1212 the Trusted Domain may receive. Because of the hashing function in 1213 generating the TPA-Label, anyone wishing to discover which domains 1214 are being authorized, has to probe each TPA-Label based on the exact 1215 domain. In addition, service organizations or community groups are 1216 able to share comprehensive lists. Such possible sharing means even 1217 though a domain has been authorized, that in itself does not mean the 1218 Trusted Domain is exchanging messages with the Authorized Domain. 1220 20.5. Benefits to Third Party Signers 1222 Third Party Signers benefit by allowing those using their service, 1223 the autonomy to authorize their service without needing to exchange 1224 DKIM key related details. This is particularly useful for mailing 1225 lists. 1227 20.6. Risks caused by Third Party Signers 1229 As mentioned before, Authorized Third Party Signers need to validate 1230 messages from Trusted Domains. This validation provides a safety 1231 mechanism for the Trusted Domain and their recipients. The Third 1232 Party may not be aware of the validation value or the message 1233 elements involved, and as a result make changes without understanding 1234 the impact this may have on Trusted Domains and their recipients. 1235 For example, the Third Party might stop DKIM signing or stop applying 1236 Authentication-Results header fields. The unexpected exposure that 1237 this might enable could allow abuse and prove detrimental for both 1238 the Trusted Domain and their recipients. 1240 20.7. SHA-1 Collisions 1242 The use of the SHA-1 hash algorithm does not represent a security 1243 concern. The hash simply ensures a deterministic domain-name size is 1244 achieved. Unexpected collisions can be detected and handled by using 1245 the extended TPA-Label Resource Record "tpa=" option. The use of 1246 TPA-Label Resource Records without the TPA-Label "tpa=" options does 1247 present an opportunity for an adversary to attempt to find a hash 1248 collision. Message spoofing outside the realm of DKIM protection is 1249 likely easier to achieve than finding hash collisions. There is 1250 minimal risk of TPA-Labels colliding. Listing 3 x 10^45 domains has 1251 less than a 0.1 percent risk of any two domain labels colliding. 1253 20.8. DNS Limits 1255 Use of the TPA-Label Resource Records, rather than simply listing the 1256 Authorized Domain, ensures the DNS record size is independent of the 1257 Third Party Domain. The typical domain name size has been steadily 1258 increasing. This increase has been caused by domain names that 1259 encode international character sets. Perhaps, soon there will be a 1260 further increase spurred by an expanse of TLDs having larger 1261 international labels. 1263 The maximum domain name size allowed, per [RFC1034] Section 3, is 255 1264 bytes (or octets). Each label has a byte for its length. Every 1265 domain name adds an additional byte by having a right-most label that 1266 represents the root "." signified as a zero length label. A labeling 1267 scheme that combines together a listed domain with the publishing 1268 domain separated by some label for this convention, reduces the 1269 maximal domain name in half, where the convention label reduces this 1270 further. 1272 If "_smtp._tpa." were used as the convention label with a simple 1273 listing method, the maximum domain name size this supports would be 1274 128 bytes. The suffix for TPA-Labels is "_smtp._tpa." which consumes 1275 11 bytes. The TPA-Label itself consumes 34 bytes. A domain that 1276 publishes the TPA-Labels in its domain would then have 122 bytes 1277 available for their Trusted Domain. This permits the authorization 1278 of any domain having a valid length with a deterministic amount of 1279 space available for resource records. 1281 Normally, DNS messages should not exceed 512 bytes as per Section 1282 2.3.4 of [RFC1035]. Using TPA-Label Resource Records in the DNS, as 1283 described by this document, consumes a consistent 50 bytes, in 1284 addition to the domain name publishing the TPA-Labels. With this 1285 being constant, a limit can be determined as a constraint to resource 1286 record size, to ensure a response does not exceed the maximum DNS 1287 message size. DNS servers that add additional resource records, for 1288 nameservers as an example, will further reduce available resource 1289 record capacity. Domains publishing TPA-Labels exceeding the DNS 1290 message limit will need to rely on recipients using TCP for DNS 1291 retrieval, or EDNS0 [RFC6891] for extended DNS lengths. 1293 21. Acknowledgements 1295 Jeff MacDonald, Michael Deutschmann, Frank Ellermann, Murray 1296 Kucherawy, Wietse Venema, Alessandro Vesely, and John Leslie. 1298 22. References 1300 22.1. Normative References 1302 [FIPS.180-2.2002] 1303 National Institute of Standards and Technology, "Secure 1304 Hash Standard", FIPS PUB 180-2, August 2002, . 1307 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1308 Requirement Levels", BCP 14, RFC 2119, March 1997. 1310 [RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field 1311 and Namespace for the Identification of Mailing Lists", 1312 RFC 2919, March 2001. 1314 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 1315 Transport Layer Security", RFC 3207, February 2002. 1317 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1318 Encodings", RFC 4648, October 2006. 1320 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1321 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1323 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 1324 October 2008. 1326 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 1327 October 2008. 1329 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 1330 July 2009. 1332 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 1333 Verification of Domain-Based Application Service Identity 1334 within Internet Public Key Infrastructure Using X.509 1335 (PKIX) Certificates in the Context of Transport Layer 1336 Security (TLS)", RFC 6125, March 2011. 1338 [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys 1339 Identified Mail (DKIM) Signatures", STD 76, RFC 6376, 1340 September 2011. 1342 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 1343 for DNS (EDNS(0))", STD 75, RFC 6891, April 2013. 1345 [RFC7001] Kucherawy, M., "Message Header Field for Indicating 1346 Message Authentication Status", RFC 7001, September 2013. 1348 22.2. Informative References 1350 [I-D.ietf-dane-smtp-with-dane] 1351 Dukhovni, V. and W. Hardaker, "SMTP security via 1352 opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-14 1353 (work in progress), February 2015. 1355 [I-D.kucherawy-dkim-delegate] 1356 Kucherawy, M. and D. Crocker, "Delegating DKIM Signing 1357 Authority", draft-kucherawy-dkim-delegate-01 (work in 1358 progress), June 2014. 1360 [I-D.kucherawy-dmarc-base] 1361 Kucherawy, M. and E. Zwicky, "Domain-based Message 1362 Authentication, Reporting and Conformance (DMARC)", 1363 draft-kucherawy-dmarc-base-13 (work in progress), 1364 February 2015. 1366 [I-D.kucherawy-original-authres] 1367 Chew, M. and M. Kucherawy, "Original-Authentication- 1368 Results Header Field", draft-kucherawy-original-authres-00 1369 (work in progress), February 2012. 1371 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1372 STD 13, RFC 1034, November 1987. 1374 [RFC1035] Mockapetris, P., "Domain names - implementation and 1375 specification", STD 13, RFC 1035, November 1987. 1377 [RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation, 1378 selection, and registration of an Autonomous System (AS)", 1379 BCP 6, RFC 1930, March 1996. 1381 [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys 1382 Identified Mail (DKIM)", RFC 4686, September 2006. 1384 [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension 1385 for Authentication", RFC 4954, July 2007. 1387 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1388 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1389 May 2008. 1391 [RFC5518] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By 1392 Reference", RFC 5518, April 2009. 1394 [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, 1395 "DomainKeys Identified Mail (DKIM) Development, 1396 Deployment, and Operations", RFC 5863, May 2010. 1398 [RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and 1399 Mailing Lists", BCP 167, RFC 6377, September 2011. 1401 [RFC6541] Kucherawy, M., "DomainKeys Identified Mail (DKIM) 1402 Authorized Third-Party Signatures", RFC 6541, 1403 February 2012. 1405 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 1406 DNS", RFC 6672, June 2012. 1408 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 1409 of Named Entities (DANE) Transport Layer Security (TLS) 1410 Protocol: TLSA", RFC 6698, August 2012. 1412 [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for 1413 Authorizing Use of Domains in Email, Version 1", RFC 7208, 1414 April 2014. 1416 Appendix A. DNS Example of TPA-Label Resource Record placement 1418 #### 1419 # Practices for Example.com email domain using example.com, isp.com, 1420 # and example.com.isp.com as signing domains. 1421 #### 1423 #### 5322.From authorization for 3P domains #### 1425 ## "isp.com" TPA-Label Resource Record ## 1426 _HTIE4SWL3L7G4TKAFAUA7UYJSS2BTEOV._smtp._tpa.example.com. IN TXT 1427 "v=tpa1; tpa=isp.com; param=d;" 1429 #### 5322.Sender/List-ID authorization for 3P domains #### 1431 ## "example.com.isp.com" TPA-Label Resource Record ## 1432 _6MEHLQLKWAL5HQREXWDN2TBXAJ6VZ44B._smtp._tpa.example.com. IN TXT 1433 "v=tpa1 tpa=*.isp.com; param=d L S;" 1435 Appendix B. C code for label generation 1437 The following utility can be compiled as TPA-Label.c using the 1438 following: 1440 gcc -lcrypto TPA-Label.c -o TPA-Label 1442 1443 /* 1444 * TPA-Label generation utility 1445 * Copyright (c) 2010 IETF Trust and the persons identified as the 1446 * document authors. All rights reserved. 1447 * 1448 * This document is subject to BCP 78 and the IETF Trust's Legal 1449 * Provisions Relating to IETF Documents 1450 * (http://trustee.ietf.org/license-info) in effect on the date of 1451 * publication of this document. Please review these documents 1452 * carefully, as they describe your rights and restrictions with respect 1453 * to this document. Code Components extracted from this document must 1454 * include Simplified BSD License text as described in Section 4.e of 1455 * the Trust Legal Provisions and are provided without warranty as 1456 * described in the Simplified BSD License. 1457 * 1458 * This document and the information contained herein are provided on an 1459 * "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1460 * OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 1461 * THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 1462 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 1463 * THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1464 * WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1465 */ 1466 #include 1467 #include 1468 #include 1469 #include 1470 #include 1471 #include 1472 #include 1473 #include 1474 #include 1475 #include 1476 #include 1478 #define TPA_LABEL_VERSION 102 1479 #define MAX_DOMAIN_NAME 256 1480 #define MAX_FILE_NAME 1024 1482 static char base32[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"; 1483 static char sign_on[] = 1484 {"%s v%d.%02d Copyright (C) (2014) The IETF Trust\n"}; 1485 char err_cmd[] =\ 1486 "ERR: Command error with [%s]\n"; 1487 char use_txt[]=\ 1488 "Usage: TPA-Label [-i domain_input_file] [-o label_output_file][-v]\n"; 1489 char help_txt[]=\ 1490 "The options are as follows:\n"\ 1491 "-i domain name input. Defaults to stdin. Removes trailing '.'\n"\ 1492 "-o TPA-Label output. Defaults to stdout.\n"\ 1493 "-v Specifies Verbose Mode.\n\n"; 1495 static void usage(void); 1496 /*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */ 1498 static void 1499 usage(void) 1500 { 1501 (void) fprintf(stderr, "\n%s%s", use_txt, help_txt); 1502 exit(1); 1503 } 1504 /*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */ 1506 int 1507 main (int argc, char * argv[]) 1508 { 1509 int ret_val, in_mode, out_mode, verbose, done, i, j, k; 1510 char ch; 1511 unsigned int len; 1512 unsigned long b_5; 1513 char in_fn[MAX_FILE_NAME], out_fn[MAX_FILE_NAME]; 1514 unsigned char in_buf[MAX_DOMAIN_NAME + 2]; 1515 unsigned char sha_res[20], tpa_label[33]; 1516 FILE *in_file, *out_file; 1518 ret_val = in_mode = out_mode = verbose = done = 0; 1519 len = 0; 1521 while ((ch = getopt(argc, argv, "i:o:v")) != -1) 1522 { 1523 switch (ch) 1524 { 1525 case 'i': 1526 in_mode = 1; /* input from file */ 1527 (void) strncpy(in_fn, optarg, sizeof(in_fn)); 1528 in_fn[sizeof(in_fn) - 1] = '\0'; 1529 break; 1530 case 'o': 1532 out_mode = 1; /* out to file */ 1533 (void) strncpy(out_fn, optarg, sizeof(out_fn)); 1534 out_fn[sizeof(out_fn) - 1] = '\0'; 1535 break; 1536 case 'v': 1537 verbose = 1; 1538 break; 1539 case '?': 1540 default: 1541 (void) usage(); 1542 break; 1543 } 1544 }; 1546 if (in_mode) 1547 { 1548 if ((in_file = fopen(in_fn, "r")) == NULL) 1549 { 1550 (void) fprintf(stderr, 1551 "ERR: Error opening [%s] input file.\n", 1552 in_fn); 1553 exit(2); 1554 } 1555 } 1556 else 1557 { 1558 in_file = stdin; 1559 } 1561 if (out_mode) 1562 { 1563 if ((out_file = fopen(out_fn, "w")) == NULL) 1564 { 1565 (void) fprintf(stderr, 1566 "ERR: Error opening [%s] output file.\n", 1567 out_fn); 1568 exit(3); 1569 } 1570 } 1571 else 1572 { 1573 out_file = stdout; 1574 } 1576 if (out_mode && verbose) 1577 { 1578 (void) printf(sign_on, "TPA-Label utility", 1579 TPA_LABEL_VERSION / 100, 1580 TPA_LABEL_VERSION % 100); 1581 } 1583 for (i = 0; i < MAX_DOMAIN_NAME && !done; i++) 1584 { 1585 if ((ch = fgetc(in_file)) == EOF) 1586 { 1587 ch = 0; 1588 } 1589 else if (ch == '\n' || ch == '\r') 1590 { 1591 ch = 0; 1592 } 1594 in_buf[i] = tolower(ch); 1596 if (ch == 0) 1597 { 1598 len = i; /* string length */ 1599 done = 1; 1600 } 1601 } 1603 if (!done) 1604 { 1605 (void) fprintf(stderr, "ERR: Domain name too long.\n"); 1606 exit (4); 1607 } 1609 if (len && in_buf[len - 1] == '.') /* remove any trailing "." */ 1610 { 1611 len--; 1612 in_buf[len] = 0; /* replace trailing "." with 0 */ 1613 } 1615 in_buf[len] = 0; /* terminate string */ 1617 if (len < 2) 1618 { 1619 (void) 1620 fprintf(stderr, 1621 "ERR: Domain name [%s] too short with %d length.\n", 1622 in_buf, 1623 len); 1624 exit (5); 1625 } 1627 SHA1(in_buf, len, sha_res); 1628 if (verbose) 1629 { 1630 printf("Normalized Domain = [%s] %d, SHA-1 = ", in_buf, len); 1632 for (i = 0; i < 20; i++) 1633 { 1634 printf("%02x", sha_res[i]); 1635 } 1636 printf("\nTPA-Label 5 bit intervals left to right.\n"); 1637 } 1639 /* process sha1 results 4 times by 40 bits (160 bits) */ 1640 for (i = 0, j = 0; i < 4 ; i++) 1641 { 1642 b_5 = (unsigned long long) sha_res[(i * 5)] << 32; 1643 b_5 |= (unsigned long long) sha_res[(i * 5) + 1] << 24; 1644 b_5 |= (unsigned long long) sha_res[(i * 5) + 2] << 16; 1645 b_5 |= (unsigned long long) sha_res[(i * 5) + 3] << 8; 1646 b_5 |= (unsigned long long) sha_res[(i * 5) + 4]; 1648 if (verbose) 1649 { 1650 printf(" {%010llX}->", b_5); 1651 } 1653 for (k = 35; k >= 0; k-= 5, j++) /* convert 40 bits (5x8) */ 1654 { 1655 tpa_label[j] = base32[(b_5 >> k) & 0x1F]; 1657 if (verbose) 1658 { 1659 printf(" %02X:%c", 1660 (unsigned int)(b_5 >> k) & 0x1F, 1661 tpa_label[j]); 1662 } 1663 } 1664 if (verbose) 1665 { 1666 printf ("\n"); 1667 } 1668 } 1669 if (verbose) 1670 { 1671 printf("\n"); 1672 } 1673 tpa_label[j] = 0; /* terminate label string */ 1674 fprintf(out_file, "_%s", tpa_label); 1675 printf("\n"); 1676 /* close */ 1677 if (out_mode) 1678 { 1679 if (fclose (out_file) != 0) 1680 { 1681 (void) fprintf(stderr, 1682 "ERR: Unable to close %s output file.\n", 1683 out_fn); 1684 ret_val = 6; 1685 } 1686 } 1687 if (in_mode) 1688 { 1689 if (fclose (in_file) != 0) 1690 { 1691 (void) fprintf(stderr, 1692 "ERR: Unable to close %s input file.\n", 1693 in_fn); 1694 ret_val = 7; 1695 } 1696 } 1697 return (ret_val); 1698 } 1699 1701 Appendix C. History of Prior Efforts 1703 To withstand asserting strict alignment practices, a scheme was 1704 devised that transferred the burden of a resulting disruption from 1705 receivers back to the Trusted Domains making the stringent requests. 1706 As such, a method to authorize other validated domains to establish 1707 informally Federated Third-Party Services, such as mailing-lists was 1708 developed. This initial scheme was then modified and proposed by 1709 ATPS [RFC6541]. Unlike the initial scheme, ATPS required Third-Party 1710 Services to use specific non-standard DKIM signatures to signal use 1711 of the ATPS authorization strategy. ATPS also required the DKIM 1712 signatures used by Third-Party Services to somehow determine the 1713 different label encoding employed by the many Trusted Domains without 1714 any defined discovery or exchange method. 1716 Both of these changes made deployment impractical by impacting 1717 systems not benefiting from additional alignment requirements. 1718 Third-parties have often been offering free services for decades. 1719 Even renaming From headers would impair normal handling. Those 1720 offering these services should not be expected to carry the burden of 1721 enabling a new Trusted Domain compliance scheme. Trusted Domains 1722 should offer the information needed to avoid disrupting these 1723 services instead, which is the purpose of TPA-Labels. 1725 Rather, the Trusted Domain seeking cooperative handling and receiving 1726 receiver feedback necessary to mitigate disruption should handle this 1727 burden instead. It is the Trusted Domain that directly benefits 1728 after all. There should not be unnecessary and problematic encoding 1729 schemes or assertions of delivery chains being expected of any Third- 1730 Party Service. Such matters are simply not their concern nor in 1731 their benefit. 1733 It seems the added complexities found in ATPS were to defend against 1734 a single DNS transaction. However, before this transaction occurs, 1735 the Third-Party must permit the validation of their own domain. Even 1736 then, a Third-Party checking transaction only occur after the domain 1737 is not within the Trusted Domain's alignment assertions. An 1738 assertion that can always be removed at any point. It is clearly in 1739 the interest of the Trusted Domain where the checking transaction 1740 represents a very minor contribution in support of desired receiver 1741 cooperation. 1743 Tailoring their TPA-Label list to suit their own users should 1744 discourage non-cooperative references to their domain. As more 1745 domains reference a common "_tpa." zone, the clout of that zone 1746 increases at a very moderate cost. This additional clout better 1747 ensures timely responses to abuse notifications. In this manner, 1748 DMARC/TPA-Labels would be helping to improve anti-abuse cooperation. 1749 In that light, TPA-Labels should be considered a sound investment and 1750 not an unwanted burden. 1752 ATPS required new tags be included in Third-Party DKIM signatures. 1753 These were "atps" and "atpsh" to construct a chain of "d=" and 1754 "atps=". This added complexity without any immediate benefit. 1755 Determining optional label encoding without any defined discovery 1756 method overlooks that authorization is only possible after the Third- 1757 Party Domain has been validated. A complete lack of ATPS deployment 1758 should have been expected since necessary changes did not align with 1759 benefits. 1761 In contrast, TPA-Labels do not require ANY change be made by 1762 authorized third-parties. Disrupting legitimate communications 1763 imposes inordinate support costs as a result of erroneously asserting 1764 strict alignment practices. The resulting disruption will eventually 1765 cause the domain's assertions to be ignored. If this disruption 1766 becomes endemic, assertions of other domains will become ignored as 1767 well. Domains wishing to benefit from their handling advice being 1768 employed while sending legitimate messages that may not retain their 1769 asserted alignment practices, should be offering the needed TPA-Label 1770 exception information. This information is essential and is only 1771 known by the Trusted Domain through their DMARC feedback. 1773 At this time, it is not practical for large ISPs to make strict DMARC 1774 assertions. Strict alignment assertions exclude normal Third-Party 1775 Services that modify the requisite alignment. TPA-Label lists 1776 specifically tailored to handle their users' desired Third-Party 1777 Services will permit their users to have normal email use. While 1778 entailing some administrative effort, TPA-Labels will generally be of 1779 benefit to their users by widely discouraging any spoofing of their 1780 messages. This affords greater protection for the users and the 1781 user's recipients. 1783 SPF purported to provide an anti-spoofing feature for an unseen 1784 parameter. Nevertheless, its strict IP address authorization causes 1785 problems and is largely disregarded for anything other than limiting 1786 the sending of DSNs or the scoring of messages. Many institutions 1787 will benefit by ensuring their strict DMARC assertions are not 1788 disruptive. Exercising this care will help retain recipient's trust 1789 in their assertions and the veracity of their messages. TPA-Labels 1790 would allow these institutions a means to use informal Third-Party 1791 Services with minimal administrative effort. Rather than using 1792 subdomains that lack DMARC restrictions, suitable Third-Party 1793 Services can be authorized by TPA-Labels. This approach offers a 1794 proactive method for recipients to better filter possible phishing 1795 attempts by not exposing them to unrestricted subdomain abuse. 1797 TPA-Label publishing is similar to VBR ([RFC5518]). However, it 1798 leverages Third-Party validation confirmed by labels held in the 1799 Trusted Domain. DNS also permits the information to be transparently 1800 made available from other domains whenever desired. TPA-Labels 1801 provide domains a means to protect their recipients while still 1802 permitting the use of legitimate SMTP exchanges. By implementing 1803 DMARC/TPA-Labels, these domains should be better able to stand on 1804 their own merit. 1806 Authors' Addresses 1808 Douglas Otis 1809 Trend Micro 1810 10101 N. De Anza Blvd 1811 Cupertino, CA 95014 1812 USA 1814 Phone: +1.408.257-1500 1815 Email: doug_otis@trendmicro.com 1816 Daniel Black 1817 Canberra ACT 1818 Australia 1820 Email: daniel.subs@internode.on.net