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Kitterman 3 Internet-Draft fTLD Registry Services 4 Intended status: Experimental April 9, 2019 5 Expires: October 11, 2019 7 DMARC (Domain-based Message Authentication, Reporting, and Conformance) 8 Extension For PSDs (Public Suffix Domains) 9 draft-ietf-dmarc-psd-02 11 Abstract 13 DMARC (Domain-based Message Authentication, Reporting, and 14 Conformance) is a scalable mechanism by which a mail-originating 15 organization can express domain-level policies and preferences for 16 message validation, disposition, and reporting, that a mail-receiving 17 organization can use to improve mail handling. DMARC policies can be 18 applied at the individual domain level or for a set of domains at the 19 organizational level. The design of DMARC precludes grouping 20 policies for a set of domains above the organizational level, such as 21 TLDs (Top Level Domains). These types of domains (which are not all 22 at the top level of the DNS tree) can be collectively referred to as 23 Public Suffix Domains (PSDs). For the subset of PSDs that require 24 DMARC usage, this memo describes an extension to DMARC to enable 25 DMARC functionality for such domains. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on October 11, 2019. 44 Copyright Notice 46 Copyright (c) 2019 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (https://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 4 63 2.1. Conventions Used in This Document . . . . . . . . . . . . 4 64 2.2. Public Suffix Domain (PSD) . . . . . . . . . . . . . . . 4 65 2.3. Longest PSD . . . . . . . . . . . . . . . . . . . . . . . 4 66 2.4. Public Suffix Operator (PSO) . . . . . . . . . . . . . . 4 67 2.5. PSO Controlled Domain Names . . . . . . . . . . . . . . . 4 68 2.6. Non-existent Domains . . . . . . . . . . . . . . . . . . 5 69 3. PSD DMARC Updates to DMARC Requirements . . . . . . . . . . . 5 70 3.1. General Updates . . . . . . . . . . . . . . . . . . . . . 5 71 3.2. Section 6.1 DMARC Policy Record . . . . . . . . . . . . . 5 72 3.3. Section 6.5. Domain Owner Actions . . . . . . . . . . . 5 73 3.4. Section 6.6.3. Policy Discovery . . . . . . . . . . . . 5 74 3.5. Section 7. DMARC Feedback . . . . . . . . . . . . . . . 6 75 4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6 76 4.1. Feedback leakage . . . . . . . . . . . . . . . . . . . . 6 77 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 78 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 79 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 80 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 81 7.2. Informative References . . . . . . . . . . . . . . . . . 8 82 Appendix A. The Experiment . . . . . . . . . . . . . . . . . . . 9 83 Appendix B. DMARC PSD Registry Example . . . . . . . . . . . . . 9 84 B.1. DMARC Public Suffix Domain (PSD) Registry . . . . . . . . 10 85 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10 86 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 88 1. Introduction 90 DMARC [RFC7489] provides a mechanism for publishing organizational 91 policy information to email receivers. DMARC [RFC7489] allows policy 92 to be specified for both individual domains and sets of domains 93 within a single organization. For domains above the organizational 94 level in the DNS tree, policy can only be published for the exact 95 domain. There is no method available to such domains to express 96 lower level policy or receive feedback reporting for sets of domains. 98 This prevents policy application to non-existent domains and 99 identification of domain abuse in email, which can be important for 100 brand and consumer protection. 102 As an example, imagine a country code TLD (ccTLD) which has public 103 subdomains for government and commercial use (.gov.example and 104 .com.example). Within the .gov.example public suffix, use of DMARC 105 [RFC7489] has been mandated and .gov.example has published its own 106 DMARC [RFC7489] record: 108 "v=DMARC1;p=reject;rua=mailto:dmarc@dmarc.service.gov.example" 110 at 112 _dmarc.gov.example. 114 This would provide policy and feedback for mail sent from 115 @gov.example, but not @tax.gov.example and there is no way to publish 116 an organizational level policy that would do so. While, in theory, 117 receivers could reject mail from non-existent domains, not all 118 receivers do so. Non-existence of the sending domain can be a factor 119 in a mail delivery decision, but is not generally treated as 120 definitive on its own. 122 This memo provides a simple extension to DMARC [RFC7489] to allow 123 operators of Public Suffix Domains (PSDs) to express policy for 124 groups of subdomains, extends the DMARC [RFC7489] policy query 125 functionality to detect and process such a policy, describes receiver 126 feedback for such policies, and provides controls to mitigate 127 potential privacy considerations associated with this extension. 129 There are two types of Public Suffix Operators (PSOs) for which this 130 extension would be useful and appropriate: 132 o Branded PSDs (e.g., ".google"): These domains are effectively 133 Organizational Domains as discussed in DMARC [RFC7489]. They 134 control all subdomains of the tree. These are effectively private 135 domains, but listed in the Public Suffix List. They are treated 136 as Public for DMARC [RFC7489] purposes. They require the same 137 protections as DMARC [RFC7489] Organizational Domains, but are 138 currently excluded. 140 o Multi-organization PSDs that require DMARC usage (e.g., ".bank"): 141 Because existing Organizational Domains using this PSD have their 142 own DMARC policy, the applicability of this extension is for non- 143 existent domains. The extension allows the brand protection 144 benefits of DMARC [RFC7489] to extend to the entire PSD, including 145 cousin domains of registered organizations. 147 Due to the design of DMARC [RFC7489] and the nature of the Internet 148 email architecture [RFC5598], there are interoperability issues 149 associated with DMARC [RFC7489] deployment. These are discussed in 150 Interoperability Issues between DMARC and Indirect Email Flows 151 [RFC7960]. These issues are not applicable to PSDs, since they 152 (e.g., the ".gov.example" used above) do not send mail. 154 DMARC [RFC7489], by design, does not support usage by PSOs. For PSDs 155 that require use of DMARC [RFC7489], an extension of DMARC reporting 156 and enforcement capability is needed for PSO to effectively manage 157 and monitor implementation of PSD requirements. 159 2. Terminology and Definitions 161 This section defines terms used in the rest of the document. 163 2.1. Conventions Used in This Document 165 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 166 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 167 "OPTIONAL" in this document are to be interpreted as described in 168 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 169 capitals, as shown here. 171 2.2. Public Suffix Domain (PSD) 173 The global Internet Domain Name System (DNS) is documented in 174 numerous Requests for Comment (RFC). It defines a tree of names 175 starting with root, ".", immediately below which are Top Level Domain 176 names such as ".com" and ".us". They are not available for private 177 registration. In many cases the public portion of the DNS tree is 178 more than one level deep. PSD DMARC includes all public domains 179 above the organizational level in the tree, e.g., ".gov.uk". 181 2.3. Longest PSD 183 Organizational Domain (DMARC [RFC7489] Section 3.2) with one label 184 removed. 186 2.4. Public Suffix Operator (PSO) 188 A Public Suffix Operator manages operations within their PSD. 190 2.5. PSO Controlled Domain Names 192 PSO Controlled Domain Names are names in the DNS that are managed by 193 a PSO and are not available for use as Organizational Domains (the 194 term Organizational Domains is defined in DMARC [RFC7489] 195 Section 3.2). Depending on PSD policy, these will have one (e.g., 196 ".com") or more (e.g., ".co.uk") name components. 198 2.6. Non-existent Domains 200 For DMARC [RFC7489] purposes, a non-existent domain is a domain name 201 that publishes none of A, AAAA, or MX records that the receiver is 202 willing to accept. This is a broader definition than that in 203 NXDOMAIN [RFC8020]. 205 3. PSD DMARC Updates to DMARC Requirements 207 This document updates DMARC [RFC7489] as follows: 209 3.1. General Updates 211 References to "Domain Owners" also apply to PSOs. 213 3.2. Section 6.1 DMARC Policy Record 215 PSD DMARC records are published as a subdomain of the PSD. For the 216 PSD ".example", the PSO would post DMARC policy in a TXT record at 217 "_dmarc.example". 219 3.3. Section 6.5. Domain Owner Actions 221 In addition to the DMARC [RFC7489] domain owner actions, PSOs that 222 require use of DMARC ought to make that information available to 223 receivers. 225 3.4. Section 6.6.3. Policy Discovery 227 A new step between step 3 and 4 is added: 229 3A. If the set is now empty and the longest PSD (Section 2.3) of the 230 Organizational Domain is one that the receiver has determined is 231 acceptable for PSD DMARC, the Mail Receiver MUST query the DNS for 232 a DMARC TXT record at the DNS domain matching the longest PSD 233 (Section 2.3) in place of the RFC5322.From domain in the message 234 (if different). A possibly empty set of records is returned. 236 As an example, for a message with the Organizational Domain of 237 "example.compute.cloudcompany.com.cctld", the query for PSD DMARC 238 would use "compute.cloudcompany.com.cctld" as the longest PSD 239 (Section 2.3). The receiver would check to see if that PSD is listed 240 in the DMARC PSD Registry, and if so, perform the policy lookup at 241 "_dmarc.compute.cloudcompany.com.cctld". 243 Note: Because the PSD policy query comes after the Organizational 244 Domain policy query, PSD policy is not used for Organizational 245 domains that have published a DMARC [RFC7489] policy. Specifically, 246 this is not a mechanism to provide feedback addresses (RUA/RUF) when 247 an Organizational Domain has declined to do so. 249 3.5. Section 7. DMARC Feedback 251 Operational note for PSD DMARC: For PSOs, feedback for non-existent 252 domains is desired and useful. See Section 4 for discussion of 253 Privacy Considerations. 255 4. Privacy Considerations 257 These privacy considerations are developed based on the requiremetns 258 of [RFC6973]. The Privacy Considerations of [RFC7489] apply to this 259 document. 261 4.1. Feedback leakage 263 Providing feedback reporting to PSOs can, in some cases, create 264 leakage of information outside of an organization to the PSO. This 265 leakage could be potentially be utilized as part of a program of 266 pervasive surveillance (See [RFC7624]). There are roughly three 267 cases to consider: 269 o Single Organization PSDs (e.g., ".google"), RUA and RUF reports 270 based on PSD DMARC have the potential to contain information about 271 emails related to entities managed by the organization. Since 272 both the PSO and the Organizational Domain owners are common, 273 there is no additional privacy risk for either normal or non- 274 existent Domain reporting due to PSD DMARC. 276 o Multi-organization PSDs that require DMARC usage (e.g., ".bank"): 277 PSD DMARC based reports will only be generated for domains that do 278 not publish a DMARC policy at the organizational or host level. 279 For domains that do publish the required DMARC policy records, the 280 feedback reporting addresses (RUA and RUF) of the organization (or 281 hosts) will be used. The only direct feedback leakage risk for 282 these PSDs are for Organizational Domains that are out of 283 compliance with PSD policy. Data on non-existent cousin domains 284 would be sent to the PSO. 286 o Multi-organization PSDs (e.g., ".com") that do not mandate DMARC 287 usage: Privacy risks for Organizational Domains that have not 288 deployed DMARC within such PSDs are significant. For non-DMARC 289 Organizational Domains, all DMARC feedback will be directed to the 290 PSO. PSD DMARC is opt-out (by publishing a DMARC record at the 291 Organizational Domain level) vice opt-in, which would be the more 292 desirable characteristic. This means that any non-DMARC 293 organizational domain would have it's feedback reports redirected 294 to the PSDo. The content of such reports, particularly for 295 existing domains, is privacy sensitive. 297 PSOs will receive feedback on non-existent domains, which may be 298 similar to existing Organizational Domains. Feedback related to such 299 cousin domains have a small risk of carrying information related to 300 an actual Organizational Domain. To minimize this potential concern, 301 PSD DMARC feedback is best limited to Aggregate Reports. Feedback 302 Reports carry more detailed information and present a greater risk. 304 Due to the inherent Privacy and Security risks associated with PSD 305 DMARC for Organizational Domains in multi-organization PSDs that do 306 not particpate in DMARC, any Feedback Reporting related to multi- 307 organizational PSDs ought to be limited to non-existent domains 308 except in cases where the reporter knows that PSO requires use of 309 DMARC. 311 5. Security Considerations 313 This document does not change the Security Considerations of 314 [RFC7489] and [RFC7960]. 316 The risks of the issues identified in [RFC7489], Section 12.5, 317 External Reporting Addresses, are amplified by PSD DMARC. By design, 318 PSD DMARC causes unrequested reporting of feedback to entities 319 external to the Organizational Domain. This is discussed in more 320 detail in Section 4. 322 6. IANA Considerations 324 This document does not require any IANA actions. 326 7. References 328 7.1. Normative References 330 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 331 Requirement Levels", BCP 14, RFC 2119, 332 DOI 10.17487/RFC2119, March 1997, 333 . 335 [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based 336 Message Authentication, Reporting, and Conformance 337 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, 338 . 340 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 341 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 342 May 2017, . 344 7.2. Informative References 346 [psddmarc.org] 347 multiple, "PSD DMARC Web Site", April 2019, 348 . 350 [PSL] multiple, "Public Suffix List", April 2019, 351 . 353 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 354 IANA Considerations Section in RFCs", RFC 5226, 355 DOI 10.17487/RFC5226, May 2008, 356 . 358 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 359 DOI 10.17487/RFC5598, July 2009, 360 . 362 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 363 Morris, J., Hansen, M., and R. Smith, "Privacy 364 Considerations for Internet Protocols", RFC 6973, 365 DOI 10.17487/RFC6973, July 2013, 366 . 368 [RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T., 369 Trammell, B., Huitema, C., and D. Borkmann, 370 "Confidentiality in the Face of Pervasive Surveillance: A 371 Threat Model and Problem Statement", RFC 7624, 372 DOI 10.17487/RFC7624, August 2015, 373 . 375 [RFC7960] Martin, F., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky, 376 E., Ed., and K. Andersen, Ed., "Interoperability Issues 377 between Domain-based Message Authentication, Reporting, 378 and Conformance (DMARC) and Indirect Email Flows", 379 RFC 7960, DOI 10.17487/RFC7960, September 2016, 380 . 382 [RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is 383 Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020, 384 November 2016, . 386 Appendix A. The Experiment 388 To mitigate the privacy concerns associated with Multi-organization 389 PSDs that do not mandate DMARC usage, see Section 4.1, a mechanism to 390 indicate which PSDs do not present this privacy risk is appropriate. 391 There are multiple approaches that are possible. 393 The experiment is to evaluate different possible approaches. The 394 experiment will be complete when there is rough consensus on a 395 technical approach that is demonstrated to be operationally usable 396 and effective at mitigating the privacy concern. 398 The mechanism needs the following attributes: 400 o Be reliably, publicly accessible 402 o Be under configuration control based on a public set of criteria 404 o List PSDs that either mandate DMARC for their registrants or for 405 which all lower level domains are controlled by the PSDo and that 406 the relevant PSDo has indicated a desire for the PSD to 407 participate in PSD DMARC 409 o Have a small operational footprint (e.g. provide a documented, 410 lightweight mechanism for developers and operators to retrieve the 411 list of PSD DMARC participants) 413 o Not allow PSDos to add PSDs to the PSD DMARC participants list 414 without third party review 416 As of this writing, three approaches have been proposed. None of 417 them are ideal: 419 o An IANA registry 421 o An extension to the Public Suffix List at [PSL] 423 o A dedicated registry queried via DNS - an example of such a 424 service is described in Appendix B below 426 Appendix B. DMARC PSD Registry Example 428 To faciliate experimentation around data leakage mitigation, a sample 429 service is available at [psddmarc.org]. It was developed based on 430 the requirements suggested for an IANA registry in an earlier 431 revision of this draft. Usage of the service is described on the web 432 site. 434 B.1. DMARC Public Suffix Domain (PSD) Registry 436 [psddmarc.org] provides a DMARC Public Suffix Domain (PSD) Registry 437 as a stand-alone DNS query service. 439 Names of PSDs participating in PSD DMARC must be registered this new 440 registry. New entries are assigned only for PSDs that require use of 441 DMARC. The requirement has to be documented in a manner that 442 satisfies the terms of Expert Review,per [RFC5226]. The Designated 443 Expert needs to confirm that provided documentation adequately 444 describes PSD policy to require domain owners to use DMARC or that 445 all domain owners are part of a single organization with the PSO. 447 The initial set of entries in this registry is as follows: 449 +-------------+---------------+ 450 | PSD | Status | 451 +-------------+---------------+ 452 | .bank | current | 453 +-------------+---------------+ 454 | .insurance | current | 455 +-------------+---------------+ 456 | .gov.uk | current | 457 +-------------+---------------+ 459 Acknowledgements 461 Thanks to the following individuals for their contributions (both 462 public and private) to improving this document. Special shout out to 463 Dave Crocker for naming the beast. 465 Kurt Andersen, Seth Blank, Dave Crocker, Heather Diaz, Tim Draegen, 466 Zeke Hendrickson, Andrew Kennedy, John Levine, Dr Ian Levy, Craig 467 Schwartz, Alessandro Vesely, and Tim Wicinski 469 Author's Address 471 Scott Kitterman 472 fTLD Registry Services 473 600 13th Street, NW, Suite 400 474 Washington, DC 20005 475 United States of America 477 Phone: +1 301 325-5475 478 Email: scott@kitterman.com