idnits 2.17.1 draft-ietf-dmarc-psd-10.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (January 23, 2021) is 1189 days in the past. Is this intentional? Checking references for intended status: Experimental ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 5226 (Obsoleted by RFC 8126) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group S. Kitterman 3 Internet-Draft fTLD Registry Services 4 Intended status: Experimental January 23, 2021 5 Expires: July 27, 2021 7 Experimental DMARC Extension For Public Suffix Domains 8 draft-ietf-dmarc-psd-10 10 Abstract 12 DMARC (Domain-based Message Authentication, Reporting, and 13 Conformance) is a scalable mechanism by which a mail-originating 14 organization can express domain-level policies and preferences for 15 message validation, disposition, and reporting, that a mail-receiving 16 organization can use to improve mail handling. The design of DMARC 17 presumes that domain names represent either nodes in the tree below 18 which registrations occur, or nodes where registrations have 19 occurred; it does not permit a domain name to have both of these 20 properties simultaneously. Since its deployment in 2015, use of 21 DMARC has shown a clear need for the ability to express policy for 22 these domains as well. 24 Domains at which registrations can occur are referred to as Public 25 Suffix Domains (PSDs). This document describes an extension to DMARC 26 to enable DMARC functionality for PSDs. 28 This document also seeks to address implementations that consider a 29 domain on a public Suffix list to be ineligible for DMARC 30 enforcement. 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 https://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 July 27, 2021. 49 Copyright Notice 51 Copyright (c) 2021 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 (https://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 . . . . . . . . . . . . . . . . . . . . . . . . 3 67 1.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3 68 1.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . 4 69 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 5 70 2.1. Conventions Used in This Document . . . . . . . . . . . . 5 71 2.2. Public Suffix Domain (PSD) . . . . . . . . . . . . . . . 5 72 2.3. Organizational Domain . . . . . . . . . . . . . . . . . . 6 73 2.4. Longest PSD . . . . . . . . . . . . . . . . . . . . . . . 6 74 2.5. Public Suffix Operator (PSO) . . . . . . . . . . . . . . 6 75 2.6. PSO Controlled Domain Names . . . . . . . . . . . . . . . 6 76 2.7. Non-existent Domains . . . . . . . . . . . . . . . . . . 6 77 3. PSD DMARC Updates to DMARC Requirements . . . . . . . . . . . 6 78 3.1. General Updates . . . . . . . . . . . . . . . . . . . . . 7 79 3.2. Changes in Section 6.3 "General Record Format" . . . . . 7 80 3.3. Changes in Section 6.5 "Domain Owner Actions" . . . . . . 7 81 3.4. Changes in Section 6.6.1 "Extract Author Domain" . . . . 7 82 3.5. Changes in Section 6.6.3 "Policy Discovery" . . . . . . . 8 83 3.6. Changes in Section 7 "DMARC Feedback" . . . . . . . . . . 8 84 4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8 85 4.1. Feedback leakage . . . . . . . . . . . . . . . . . . . . 9 86 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 87 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 88 6.1. Subdomain Policy Tag . . . . . . . . . . . . . . . . . . 10 89 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 90 7.1. Normative References . . . . . . . . . . . . . . . . . . 10 91 7.2. Informative References . . . . . . . . . . . . . . . . . 11 92 Appendix A. PSD DMARC Privacy Concern Mitigation Experiment . . 12 93 Appendix B. DMARC PSD Registry Examples . . . . . . . . . . . . 12 94 B.1. DMARC PSD DNS Query Service . . . . . . . . . . . . . . . 12 95 B.2. DMARC Public Suffix Domain (PSD) Registry . . . . . . . . 12 96 B.3. DMARC PSD PSL Extension . . . . . . . . . . . . . . . . . 13 98 Appendix C. Implementations . . . . . . . . . . . . . . . . . . 13 99 C.1. Authheaders Module . . . . . . . . . . . . . . . . . . . 13 100 C.2. Zdkimfilter Module . . . . . . . . . . . . . . . . . . . 14 101 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14 102 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 104 1. Introduction 106 DMARC [RFC7489] provides a mechanism for publishing organizational 107 policy information to email receivers. DMARC allows policy to be 108 specified for both individual domains and for organizational domains 109 and their sub-domains within a single organization. 111 To determine the organizational domain for a message under 112 evaluation, and thus where to look for a policy statement, DMARC 113 makes use of a Public Suffix List. The process for doing this can be 114 found in Section 3.2 of the DMARC specification. 116 DMARC as specified presumes that domain names present in a PSL are 117 not organizational domains and thus not subject to DMARC processing; 118 domains are either organizational domains, sub-domains of 119 organizational domains, or listed on a PSL. For domains listed in a 120 PSL, i.e., TLDs and domains that exist between TLDs and organization 121 level domains, policy can only be published for the exact domain. No 122 method is available for these domains to express policy or receive 123 feedback reporting for sub-domains. This missing method allows for 124 the abuse of non-existent organizational-level domains and prevents 125 identification of domain abuse in email. 127 This document specifies experimental updates to the DMARC and PSL 128 algorithm cited above, in an attempt to mitigate this abuse. 130 1.1. Example 132 As an example, imagine a country code TLD (ccTLD) which has public 133 subdomains for government and commercial use (".gov.example" and 134 ".com.example"). A PSL whose maintainer is aware of this country's 135 domain structurewould include entries for both of these in the PSL, 136 indicating that they are PSDs below which registrations can occur. 137 Suppose further that there exists a domain "tax.gov.example", 138 registered within ".gov.example", that is responsible for taxation in 139 this imagined country. 141 However, by exploiting the typically unauthenticated nature of email, 142 there are regular malicious campaigns to impersonate this 143 organization that use similar-looking ("cousin") domains such as 144 "t4x.gov.example". Such domains are not registered. 146 Within the ".gov.example" public suffix, use of DMARC has been 147 mandated, so "gov.example" publishes the following DMARC DNS record: 149 _dmarc.gov.example. IN TXT ( "v=DMARC1; p=reject; " 150 "rua=mailto:dmc@dmarc.svc.gov.example" ) 152 This DMARC record provides policy and a reporting destination for 153 mail sent from @gov.example. However, due to DMARC's current method 154 of discovering and applying policy at the organizational domain 155 level, the non-existent organizational domain of @t4x.gov.example 156 does not and cannot fall under a DMARC policy. 158 Defensively registering all variants of "tax" is obviously not a 159 scalable strategy. The intent of this specification, therefore, is 160 to enhance the DMARC algorithm by enabling an agent receiving such a 161 message to be able to determine that a relevant policy is present at 162 "gov.example", which is precluded by the current DMARC algorithm. 164 1.2. Discussion 166 This document provides a simple extension to DMARC [RFC7489] to allow 167 operators of Public Suffix Domains (PSDs) to: 169 o Express policy at the level of the PSD that covers all 170 organizational domains that do not explicitly publish DMARC 171 records 173 o Extends the DMARC policy query functionality to detect and process 174 such a policy 176 o Describes receiver feedback for such policies 178 o Provides controls to mitigate potential privacy considerations 179 associated with this extension 181 This document also provides a new DMARC tag to indicate requested 182 handling policy for non-existent subdommains. This is provided 183 specifically to support phased deployment of PSD DMARC, but is 184 expected to be useful more generally. Undesired rejection risks for 185 mail purporting to be from domains that do not exist are 186 substantially lower than for those that do, so the operational risk 187 of requesting harsh policy treatment (e.g. reject) is lower. 189 As an additional benefit, the PSD DMARC extension clarifies existing 190 requirements. Based on the requirements of DMARC [RFC7489], DMARC 191 should function above the organizational level for exact domain 192 matches (i.e. if a DMARC record were published for 'example', then 193 mail from example@example should be subject to DMARC processing). 195 Testing had revealed that this is not consistently applied in 196 different implementations. 198 There are two types of Public Suffix Operators (PSOs) for which this 199 extension would be useful and appropriate: 201 o Branded PSDs (e.g., ".google"): These domains are effectively 202 Organizational Domains as discussed in DMARC [RFC7489]. They 203 control all subdomains of the tree. These are effectively private 204 domains, but listed in the Public Suffix List. They are treated 205 as Public for DMARC purposes. They require the same protections 206 as DMARC Organizational Domains, but are currently unable to 207 benefit from DMARC. 209 o Multi-organization PSDs that require DMARC usage (e.g., ".bank"): 210 Because existing Organizational Domains using this PSD have their 211 own DMARC policy, the applicability of this extension is for non- 212 existent domains. The extension allows the brand protection 213 benefits of DMARC to extend to the entire PSD, including cousin 214 domains of registered organizations. 216 Due to the design of DMARC and the nature of the Internet email 217 architecture [RFC5598], there are interoperability issues associated 218 with DMARC [RFC7489] deployment. These are discussed in 219 Interoperability Issues between DMARC and Indirect Email Flows 220 [RFC7960]. These issues are not typically applicable to PSDs, since 221 they (e.g., the ".gov.example" used above) do not typically send 222 mail. 224 2. Terminology and Definitions 226 This section defines terms used in the rest of the document. 228 2.1. Conventions Used in This Document 230 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 231 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 232 "OPTIONAL" in this document are to be interpreted as described in 233 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 234 capitals, as shown here. 236 2.2. Public Suffix Domain (PSD) 238 The global Internet Domain Name System (DNS) is documented in 239 numerous Requests for Comment (RFC). It defines a tree of names 240 starting with root, ".", immediately below which are Top Level Domain 241 names such as ".com" and ".us". The domain name structure consists 242 of a tree of names, each of which is made of a sequence of words 243 ("labels") separated by period characters. The root of the tree is 244 simply called ".". The Internet community at large, through 245 processes and policies external to this work, selects points in this 246 tree at which to register domain names "owned" by independent 247 organizations. Real-world examples are ".com", ".org", ".us", and 248 ".gov.uk". Names at which such registrations occur are called Public 249 Suffix Domains (PSDs), and a registration consists of a label 250 selected by the registrant to which a desirable PSD is appended. For 251 example, "ietf.org" is a registered domain name, and ".org" is its 252 PSD. 254 2.3. Organizational Domain 256 The term Organizational Domains is defined in DMARC [RFC7489] 257 Section 3.2. 259 2.4. Longest PSD 261 The longest PSD is the Organizational Domain with one label removed. 262 It names the immediate parent node of the Organizational Domain in 263 the DNS namespace tree. 265 2.5. Public Suffix Operator (PSO) 267 A Public Suffix Operator is an organization which manages operations 268 within a PSD, particularly the DNS records published for names at and 269 under that domain name. 271 2.6. PSO Controlled Domain Names 273 PSO Controlled Domain Names are names in the DNS that are managed by 274 a PSO and are not available for use as Organizational Domains. PSO 275 Controlled Domain Names may have one (e.g., ".com") or more (e.g., 276 ".co.uk") name components, depending on PSD policy. 278 2.7. Non-existent Domains 280 For DMARC purposes, a non-existent domain is a domain for which there 281 is an NXDOMAIN or NODATA response for A, AAAA, and MX records. This 282 is a broader definition than that in NXDOMAIN [RFC8020]. 284 3. PSD DMARC Updates to DMARC Requirements 286 This document updates DMARC as follows: 288 3.1. General Updates 290 References to "Domain Owners" also apply to PSOs. 292 3.2. Changes in Section 6.3 "General Record Format" 294 A new tag is added after "fo": 296 np: Requested Mail Receiver policy for non-existent subdomains 297 (plain-text; OPTIONAL). Indicates the policy to be enacted by the 298 Receiver at the request of the Domain Owner. It applies only to 299 non-existent subdomains of the domain queried and not to either 300 existing subdomains or the domain itself. Its syntax is identical 301 to that of the "p" tag defined below. If the 'np' tag is absent, 302 the policy specified by the "sp" tag (if the 'sp' tag is present) 303 or the policy specified by the "p" tag, if the 'sp' tag is not 304 present, MUST be applied for non-existent subdomains. Note that 305 "np" will be ignored for DMARC records published on subdomains of 306 Organizational Domains and PSDs due to the effect of the DMARC 307 policy discovery mechanism described in DMARC Section 6.6.3. 309 The following tag definitions from DMARC are updated: 311 p: The sentence 'Policy applies to the domain queried and to 312 subdomains, unless subdomain policy is explicitly described using 313 the "sp" tag' is updated to read 'Policy applies to the domain 314 queried and to subdomains, unless subdomain policy is explicitly 315 described using the "sp" or "np" tags.' 317 sp: The sentence 'If absent, the policy specified by the "p" tag 318 MUST be applied for subdomains' is updated to read 'If both the 319 'sp' tag is absent and the 'np' tag is either absent or not 320 applicable, the policy specified by the "p" tag MUST be applied 321 for subdomains. 323 3.3. Changes in Section 6.5 "Domain Owner Actions" 325 In addition to the DMARC domain owner actions, PSOs that require use 326 of DMARC and participate in PSD DMARC ought to make that information 327 available to receivers. This document is an experimental mechanism 328 for doing so. See the [this document] experiment description 329 (Appendix A). 331 3.4. Changes in Section 6.6.1 "Extract Author Domain" 333 Experience with DMARC has shown that some implementations short- 334 circuit messages, bypassing DMARC policy application, when the domain 335 name extracted by the receiver (from the RFC5322.From) is on the 336 public suffix list used by the receiver. This negates the capability 337 being created by this specification. Therefore, the following 338 paragraph is appended to Section 6.6.1 of DMARC: 340 Note that domain names that appear on a public suffix list are not 341 exempt from DMARC policy application and reporting. 343 3.5. Changes in Section 6.6.3 "Policy Discovery" 345 A new step between step 3 and 4 is added: 347 3A. If the set is now empty and the longest PSD (Section 2.4) of the 348 Organizational Domain is one that the receiver has determined is 349 acceptable for PSD DMARC (discussed in the [this document] 350 experiment description (Appendix A)), the Mail Receiver MUST query 351 the DNS for a DMARC TXT record at the DNS domain matching the 352 [this document] longest PSD (Section 2.4) in place of the 353 RFC5322.From domain in the message (if different). A possibly 354 empty set of records is returned. 356 As an example, for a message with the Organizational Domain of 357 "example.compute.cloudcompany.com.example", the query for PSD DMARC 358 would use "compute.cloudcompany.com.example" as the [this document] 359 longest PSD (Section 2.4). The receiver would check to see if that 360 PSD is listed in the DMARC PSD Registry, and if so, perform the 361 policy lookup at "_dmarc.compute.cloudcompany.com.example". 363 Note: Because the PSD policy query comes after the Organizational 364 Domain policy query, PSD policy is not used for Organizational 365 domains that have published a DMARC policy. Specifically, this is 366 not a mechanism to provide feedback addresses (RUA/RUF) when an 367 Organizational Domain has declined to do so. 369 3.6. Changes in Section 7 "DMARC Feedback" 371 Operational note for PSD DMARC: For PSOs, feedback for non-existent 372 domains is desirable and useful, just as it is for org-level DMARC 373 operators. See Section 4 of [this document] for discussion of 374 Privacy Considerations for PSD DMARC. 376 4. Privacy Considerations 378 These privacy considerations are developed based on the requirements 379 of [RFC6973]. Additionally, the Privacy Considerations of [RFC7489] 380 apply to the mechanisms described by this document. 382 4.1. Feedback leakage 384 Providing feedback reporting to PSOs can, in some cases, cause 385 information to leak out of an organization to the PSO. This leakage 386 could potentially be utilized as part of a program of pervasive 387 surveillance (See [RFC7624]). There are roughly three cases to 388 consider: 390 o Single Organization PSDs (e.g., ".google"), RUA and RUF reports 391 based on PSD DMARC have the potential to contain information about 392 emails related to entities managed by the organization. Since 393 both the PSO and the Organizational Domain owners are common, 394 there is no additional privacy risk for either normal or non- 395 existent Domain reporting due to PSD DMARC. 397 o Multi-organization PSDs that require DMARC usage (e.g., ".bank"): 398 PSD DMARC based reports will only be generated for domains that do 399 not publish a DMARC policy at the organizational or host level. 400 For domains that do publish the required DMARC policy records, the 401 feedback reporting addresses (RUA and RUF) of the organization (or 402 hosts) will be used. The only direct feedback leakage risk for 403 these PSDs are for Organizational Domains that are out of 404 compliance with PSD policy. Data on non-existent cousin domains 405 would be sent to the PSO. 407 o Multi-organization PSDs (e.g., ".com") that do not mandate DMARC 408 usage: Privacy risks for Organizational Domains that have not 409 deployed DMARC within such PSDs are significant. For non-DMARC 410 Organizational Domains, all DMARC feedback will be directed to the 411 PSO. PSD DMARC is opt-out (by publishing a DMARC record at the 412 Organizational Domain level) vice opt-in, which would be the more 413 desirable characteristic. This means that any non-DMARC 414 organizational domain would have its feedback reports redirected 415 to the PSO. The content of such reports, particularly for 416 existing domains, is privacy sensitive. 418 PSOs will receive feedback on non-existent domains, which may be 419 similar to existing Organizational Domains. Feedback related to such 420 cousin domains have a small risk of carrying information related to 421 an actual Organizational Domain. To minimize this potential concern, 422 PSD DMARC feedback MUST be limited to Aggregate Reports. Feedback 423 Reports carry more detailed information and present a greater risk. 425 Due to the inherent Privacy and Security risks associated with PSD 426 DMARC for Organizational Domains in multi-organization PSDs that do 427 not particpate in DMARC, any Feedback Reporting related to multi- 428 organizational PSDs MUST be limited to non-existent domains except in 429 cases where the reporter knows that PSO requires use of DMARC. 431 5. Security Considerations 433 This document does not change the Security Considerations of 434 [RFC7489] and [RFC7960]. 436 The risks of the issues identified in [RFC7489], Section 12.3, DNS 437 Security, are amplified by PSD DMARC. In particular, DNS cache 438 poisoning (or Name Chaining), see [RFC3833] for details, consequences 439 are increased because a successful attack would potentially have a 440 much wider scope. 442 The risks of the issues identified in [RFC7489], Section 12.5, 443 External Reporting Addresses, are amplified by PSD DMARC. By design, 444 PSD DMARC causes unrequested reporting of feedback to entities 445 external to the Organizational Domain. This is discussed in more 446 detail in Section 4. 448 6. IANA Considerations 450 This section describes actions requested to be completed by IANA. 452 6.1. Subdomain Policy Tag 454 IANA is requested to add a new tag to DMARC Tag Registry in the 455 Domain-based Message Authentication, Reporting, and Conformance 456 (DMARC) Parameters Registry. 458 The entry is as follows: 460 +----------+-----------+---------+-------------------------------+ 461 | Tag Name | Reference | Status | Description | 462 +----------+-----------+---------+-------------------------------+ 463 | np | this | current | Requested handling policy for | 464 | | document | | non-existent subdomains | 465 +----------+-----------+---------+-------------------------------+ 467 7. References 469 7.1. Normative References 471 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 472 Requirement Levels", BCP 14, RFC 2119, 473 DOI 10.17487/RFC2119, March 1997, 474 . 476 [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based 477 Message Authentication, Reporting, and Conformance 478 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, 479 . 481 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 482 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 483 May 2017, . 485 7.2. Informative References 487 [psddmarc.org] 488 multiple, "PSD DMARC Web Site", April 2019, 489 . 491 [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain 492 Name System (DNS)", RFC 3833, DOI 10.17487/RFC3833, August 493 2004, . 495 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 496 IANA Considerations Section in RFCs", RFC 5226, 497 DOI 10.17487/RFC5226, May 2008, 498 . 500 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 501 DOI 10.17487/RFC5598, July 2009, 502 . 504 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 505 Morris, J., Hansen, M., and R. Smith, "Privacy 506 Considerations for Internet Protocols", RFC 6973, 507 DOI 10.17487/RFC6973, July 2013, 508 . 510 [RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T., 511 Trammell, B., Huitema, C., and D. Borkmann, 512 "Confidentiality in the Face of Pervasive Surveillance: A 513 Threat Model and Problem Statement", RFC 7624, 514 DOI 10.17487/RFC7624, August 2015, 515 . 517 [RFC7960] Martin, F., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky, 518 E., Ed., and K. Andersen, Ed., "Interoperability Issues 519 between Domain-based Message Authentication, Reporting, 520 and Conformance (DMARC) and Indirect Email Flows", 521 RFC 7960, DOI 10.17487/RFC7960, September 2016, 522 . 524 [RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is 525 Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020, 526 November 2016, . 528 Appendix A. PSD DMARC Privacy Concern Mitigation Experiment 530 The experiment being performed has three different questions which 531 are looking to be addressed in this document. 533 o Section 3.2 modifies policy discovery to add an additional DNS 534 lookup. To determine if this lookup is useful, PSDs will add 535 additional DMARC records in place, and will analyze the DMARC 536 reports. Success will be determined if a consensus of PSDs that 537 publish DMARC records are able to collect useful data. 539 o Section 3.2 adds the "np" tag for non-existent subdomains (DNS 540 NXDOMAIN). PSOs wishing to test this will add this flag to their 541 DMARC record, and will analyze DMARC reports for deployment. 542 Success will be determined if organizations find explicitly 543 blocking non-existent subdomains domains desirable and provide 544 added value. 546 o Section 4.1 discusses three cases where providing feedback could 547 cause information to leak out of an organization. This experiment 548 will analyze the feedback reports generated for each case to 549 determine if there is information leakage. 551 Appendix B. DMARC PSD Registry Examples 553 To facilitate experimentation around data leakage mitigation, samples 554 of the DNS based and IANA like registries are available at 555 [psddmarc.org]. 557 B.1. DMARC PSD DNS Query Service 559 A sample stand-alone DNS query service is available at 560 [psddmarc.org]. It was developed based on the contents suggested for 561 an IANA registry in an earlier revision of this draft. Usage of the 562 service is described on the web site. 564 B.2. DMARC Public Suffix Domain (PSD) Registry 566 [psddmarc.org] provides an IANA like DMARC Public Suffix Domain (PSD) 567 Registry as a stand-alone DNS query service. It follows the contents 568 and structure described below. There is a Comma Separated Value 569 (CSV) version of the listed PSD domains which is suitable for use in 570 build updates for PSD DMARC capable software. 572 Names of PSDs participating in PSD DMARC must be registered this new 573 registry. New entries are assigned only for PSDs that require use of 574 DMARC. The requirement has to be documented in a manner that 575 satisfies the terms of Expert Review,per [RFC5226]. The Designated 576 Expert needs to confirm that provided documentation adequately 577 describes PSD policy to require domain owners to use DMARC or that 578 all domain owners are part of a single organization with the PSO. 580 The initial set of entries in this registry is as follows: 582 +-------------+---------------+ 583 | PSD | Status | 584 +-------------+---------------+ 585 | .bank | current | 586 +-------------+---------------+ 587 | .insurance | current | 588 +-------------+---------------+ 589 | .gov.uk | current | 590 +-------------+---------------+ 591 | .mil | current | 592 +-------------+---------------+ 594 B.3. DMARC PSD PSL Extension 596 [psddmarc.org] provides a PSL like file to enable to facilitate 597 identification of PSD DMARC participants. Contents are functionally 598 identical to the IANA like registry, but presented in a different 599 format. 601 When using this approach, the input domain of the extension lookup is 602 supposed to be the output domain of the regular PSL lookup, i.e. the 603 organizational domain. This alternative data approach is potentially 604 useful since DMARC implementations already need to be able to parse 605 the data format, so it should be easier to implement. 607 Appendix C. Implementations 609 There are two known implementations of PSD DMARC available for 610 testing. 612 C.1. Authheaders Module 614 The authheaders Python module and command line tool is available for 615 download or installation from Pypi (Python Packaging Index). 617 It supports both use of the DNS based query service and download of 618 the CSV registry file from [psddmarc.org]. 620 C.2. Zdkimfilter Module 622 The zdkimfilter module is a separately available add-on to Courier- 623 MTA. 625 Mostly used for DKIM signing, it can be configured to also verify, 626 apply DMARC policies, and send aggregate reports. For PSD DMARC it 627 uses the PSL extension list approach, which is available from from 628 [psddmarc.org] 630 Acknowledgements 632 Thanks to the following individuals for their contributions (both 633 public and private) to improving this document. Special shout out to 634 Dave Crocker for naming the beast. 636 Kurt Andersen, Seth Blank, Dave Crocker, Heather Diaz, Tim Draegen, 637 Zeke Hendrickson, Andrew Kennedy, John Levine, Dr Ian Levy, Craig 638 Schwartz, Alessandro Vesely, and Tim Wicinski 640 Author's Address 642 Scott Kitterman 643 fTLD Registry Services 644 600 13th Street, NW, Suite 400 645 Washington, DC 20005 646 United States of America 648 Phone: +1 301 325-5475 649 Email: scott@kitterman.com