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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MARF Working Group J. Falk 3 Internet-Draft Return Path 4 Updates: 5965 (if approved) M. Kucherawy, Ed. 5 Intended status: Standards Track Cloudmark 6 Expires: October 27, 2012 April 25, 2012 8 Creation and Use of Email Feedback Reports: An Applicability Statement 9 for the Abuse Reporting Format (ARF) 10 draft-ietf-marf-as-15 12 Abstract 14 RFC 5965 defines an extensible, machine-readable format intended for 15 mail operators to report feedback about received email to other 16 parties. This Applicability Statement describes common methods for 17 utilizing this format for reporting both abuse and authentication 18 failure events. Mailbox Providers of any size, mail sending 19 entities, and end users can use these methods as a basis to create 20 procedures that best suit them. Some related optional mechanisms are 21 also discussed. 23 Status of this Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at http://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on October 27, 2012. 40 Copyright Notice 42 Copyright (c) 2012 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 1.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3. Solicited and Unsolicited Reports . . . . . . . . . . . . . . 4 61 4. Generating And Handling Solicited Abuse Reports . . . . . . . 4 62 4.1. General Considerations for Feedback Providers . . . . . . 5 63 4.2. Where To Send Reports . . . . . . . . . . . . . . . . . . 5 64 4.3. What To Put In Reports . . . . . . . . . . . . . . . . . . 5 65 4.4. General Considerations for Feedback Consumers . . . . . . 5 66 4.5. What To Expect . . . . . . . . . . . . . . . . . . . . . . 6 67 4.6. What To Do With Reports . . . . . . . . . . . . . . . . . 6 68 5. Generating and Handling Unsolicited Abuse Reports . . . . . . 6 69 5.1. General Considerations . . . . . . . . . . . . . . . . . . 6 70 5.2. When To Generate Reports . . . . . . . . . . . . . . . . . 7 71 5.3. Where To Send Reports . . . . . . . . . . . . . . . . . . 7 72 5.4. What To Put In Reports . . . . . . . . . . . . . . . . . . 8 73 5.5. What To Do With Reports . . . . . . . . . . . . . . . . . 9 74 6. Generating Automatic Authentication Failure Reports . . . . . 10 75 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 76 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 77 8.1. In Other Documents . . . . . . . . . . . . . . . . . . . . 11 78 8.2. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 11 79 8.3. Amplification Attacks . . . . . . . . . . . . . . . . . . 11 80 8.4. Automatic Generation . . . . . . . . . . . . . . . . . . . 12 81 8.5. Reporting Multiple Incidents . . . . . . . . . . . . . . . 12 82 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 83 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 84 10.1. Normative References . . . . . . . . . . . . . . . . . . . 13 85 10.2. Informative References . . . . . . . . . . . . . . . . . . 14 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 88 1. Introduction 90 The Abuse Reporting Format (ARF) was initially developed for two very 91 specific use cases. Initially, it was intended to be used for 92 reporting feedback between large email operators, or from large email 93 operators to end user network access operators, any of whom could be 94 presumed to have automated abuse-handling systems. Secondarily, it 95 is used by those same large mail operators to send those same reports 96 to other entities, including those involved in sending bulk email for 97 commercial purposes. In either case, the reports would be triggered 98 by direct end user action such as clicking on a "report spam" button 99 in their email client. 101 Though other uses for the ARF format defined in [RFC5965] have been 102 discussed (and may be documented similarly in the future), abuse 103 remains the primary application, with a small amount of adoption of 104 extensions that enable authentication failure reporting. 106 This Applicability Statement provides direction for using the Abuse 107 Reporting Format (ARF) in both contexts. It also includes some 108 statements about the use of ARF in conjunction with other email 109 technologies. 111 The purpose for reporting abusive messages is to stop recurrences. 112 The methods described in this document focus on automating abuse 113 reporting as much as practical, so as to minimize the work of a 114 site's abuse team. There are further reasons why abuse feedback 115 generation is worthwhile, such as instruction of mail filters or 116 reputation trackers, or to initiate investigations of particularly 117 egregious abuses. These other applications are not discussed in this 118 memo. 120 Further introduction to this topic may be found in [RFC6449], which 121 is effectively an Applicability Statement written outside of the IETF 122 and thus never achieved IETF consensus. Much of the content for that 123 document was input to this one. 125 At the time of publication of this document, five feedback types are 126 registered. This document only discusses two of them ("abuse" and 127 "auth-failure") as they are seeing sufficient use in practice that 128 applicability statements can be made about them. The others are 129 either too new or too seldomly used to be included here. 131 1.1. Discussion 133 [RFC Editor: please remove this section prior to publication.] 135 This document is being discussed within the IETF MARF Working Group, 136 on the marf@ietf.org mailing list. 138 2. Definitions 140 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 141 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 142 document are to be interpreted as described in [RFC2119], and are 143 intended to replace the Requirement Levels described in Section 3.3 144 of [RFC2026]. 146 Some of the terminology used in this document is taken from 147 [RFC5598]. 149 "Mailbox Provider" refers to an organization that accepts, stores, 150 and offers access to [RFC5322] messages ("email messages") for end 151 users. Such an organization has typically implemented SMTP 152 ([RFC5321]), and might provide access to messages through IMAP 153 ([RFC3501]), POP ([RFC1939]), a proprietary interface designed for 154 HTTP ([RFC2616]), or a proprietary protocol. 156 3. Solicited and Unsolicited Reports 158 The original application of [RFC5965], and still by far the most 159 common, is when two mail systems make a private agreement to exchange 160 abuse reports, usually reports due to recipients manually reporting 161 messages as spam. We refer to these as solicited reports. 163 Other uses for ARF involve such reports sent between parties that 164 don't know each other. These unsolicited reports are sent without 165 prior arrangement between the parties as to the context and meaning 166 of the reports, so the constraints on how these unsolicited reports 167 need to be structured such that the reports generated are likely to 168 be useful to the recipient, to what address(es) they can usefully be 169 sent, what issues the can be used to report, and how they can be 170 handled by the receiver of the report are very different. 172 The two cases are covered separately in following sections. 174 4. Generating And Handling Solicited Abuse Reports 176 [The numbered items in these subsections are not intended to be in a 177 paricular sequence. The numbers are here during document development 178 to make it easier to identify the items for discussion, and will be 179 removed before publication.] 181 4.1. General Considerations for Feedback Providers 183 1. A Mailbox Provider receives reports of abusive or unwanted mail 184 from its users, most often by providing a "report spam" button 185 (or similar nomenclature) in the MUA (Mail User Agent). The 186 method of transferring this message and any associated metadata 187 from the MUA to the Mailbox Provider's ARF processing system is 188 not defined by any standards document, but is discussed further 189 in Section 3.2 of [RFC6449]. Policy concerns related to the 190 collection of this data are discussed in Section 3.4 of 191 [RFC6449]. 192 2. To implement the recommendations of this memo, the reports are 193 formatted per [RFC5965], and transmitted as an email message 194 ([RFC5322]), typically using SMTP ([RFC5321]). 195 3. Ongoing maintenance of an ARF processing system is discussed in 196 Section 3.6 of [RFC6449]. 198 4.2. Where To Send Reports 200 1. The Mailbox Provider SHOULD NOT send reports to addresses that 201 have not explicitly requested them. A valid deviation might be 202 the result of local policy instructions. The process whereby 203 such parties may request the reports is discussed in Section 3.5 204 of [RFC6449]. 206 4.3. What To Put In Reports 208 1. The reports SHOULD use "Feedback-Type: abuse", for its type. 209 Although a Mailbox Provider generating the reports can use other 210 types appropriate to the nature of the abuse being reported, the 211 operator receiving the reports might not treat different feedback 212 types differently. 213 2. The following fields are optional in [RFC5965], but SHOULD be 214 used in this context when their corresponding values are 215 available: Original-Mail-From, Arrival-Date, Source-IP, Original- 216 Rcpt-To. Other optional fields can be included, as the 217 implementer feels is appropriate. 218 3. User-identifiable data MAY be obscured as described in [RFC6590]. 220 4.4. General Considerations for Feedback Consumers 222 1. ARF report streams are established proactively between Feedback 223 Providers and Feedback Consumers. Recommendations for preparing 224 to make that request are discussed in Section 4.1 of [RFC6449]. 225 2. Operators MUST be able to accept ARF [RFC5965] reports as email 226 messages [RFC5322] over SMTP [RFC5321]. These and other types of 227 email messages that can be received are discussed in Section 4.2 228 of [RFC6449]. 230 3. Recipients of feedback reports that are part of formal feedback 231 arrangements have to be capable of handling large volumes of 232 reports. This could require automation of report processing. 233 Discussion of this can be found in Section 4.4 of [RFC6449]. 235 4.5. What To Expect 237 1. An automated report processing system MUST accept all Feedback- 238 Types defined in [RFC5965] or extensions to it. This means the 239 implementers need to provide mechanisms to add support for new 240 types as they are defined and registered, possibly including new 241 processing steps (such as when new reporting fields are added to 242 a type). However, Mailbox Providers might only make use of the 243 "abuse" Feedback-Type. Therefore, report receivers might be 244 required to do additional analysis to separate different types of 245 abuse reports after receipt if they do not have prior specific 246 knowledge of the sender of the report. 247 2. Reports receivers MUST accept reports that have obscured their 248 user-identifiable data as described in [RFC6590]. That document 249 also discusses the handling of such reports. This technique is 250 also discussed in Section 4.4 of [RFC6449]. 252 4.6. What To Do With Reports 254 1. Section 4.3 of [RFC6449] discusses actions that mail operators 255 might take upon receiving a report (or multiple reports). 257 5. Generating and Handling Unsolicited Abuse Reports 259 [The numbered items in these subsections are not intended to be in a 260 paricular sequence. The numbers are here during document development 261 to make it easier to identify the items for discussion, and will be 262 removed before publication.] 264 5.1. General Considerations 266 1. It is essential for report recipients to be capable of throttling 267 reports being sent to avoid damage to their own installations. 268 Therefore, Feedback Providers MUST provide a way for report 269 recipients to request that no further reports be sent. 270 Unfortunately, no standardized mechanism for such requests exists 271 to date, and all existing mechanisms for meeting this requirement 272 are out-of-band. 273 2. Message authentication is generally a good idea, but it is 274 especially important to encourage credibility of and thus 275 response to unsolicited reports. Therefore, as with any other 276 message, Feedback Providers sending unsolicited reports SHOULD 277 send reports that they believe will pass Sender Policy Framework 278 ([RFC4408]) and/or DomainKeys Identified Mail ([RFC6376]) checks. 280 5.2. When To Generate Reports 282 1. Handling of unsolicited reports has a significant cost to the 283 report receiver. Senders of unsolicited reports, especially 284 those sending large volumes of them automatically SHOULD NOT send 285 reports that cannot be used as a basis for action by the 286 recipient, whether this is due to the report being sent about an 287 incident that is not abuse-related, the report being sent to an 288 email address that won't result in action, or the content or 289 format of the report being hard for the recipient to read or use. 290 2. Feedback Providers SHOULD NOT report all mail sent from a 291 particular sender merely because some of it is determined to be 292 abusive. 293 3. Mechanical reports of mail that "looks like" spam, based solely 294 on the results of inline content analysis tools, SHOULD NOT be 295 sent since, because of their subjective nature, they are unlikely 296 to provide a basis for the recipient to take action. Complaints 297 generated by end users about mail that is determined by them to 298 be abusive, or mail delivered to "spam trap" or "honeypot" 299 addresses, are far more likely to be accurate and MAY be sent. 300 4. If a Feedback Provider applies the Sender Policy Framework 301 [RFC4408] to arriving messages, a report SHOULD NOT be generated 302 to the RFC5321.MailFrom domain if the SPF evaluation produced a 303 "Fail", "SoftFail", "TempError" or "PermError" report, as no 304 reliable assertion or assumption can be made that use of the 305 domain was authorized. A valid exception would be specific 306 knowledge that the SPF result is not definitive for that domain 307 under those circumstances (for example, a message that is also 308 signed using DomainKeys Identified Mail (DKIM, [RFC6376]) by the 309 same domain, and that signature validates). 311 5.3. Where To Send Reports 313 1. Rather than generating feedback reports themselves, MUAs SHOULD 314 make abuse reports back to their mailbox providers so that they 315 can generate and send ARF messages on behalf of end users (see 316 Section 3.2 of [RFC6449]). This allows centralized processing 317 and tracking of reports, and provides training input to filtering 318 systems. There is, however, no standard mechanism for this 319 signaling between MUAs and mailbox providers to trigger abuse 320 reports. 321 2. Feedback Providers SHOULD NOT send reports to recipients that are 322 uninvolved or only peripherally involved. For example, they 323 SHOULD NOT send reports to the operator of every Autonomous 324 System in the path between the apparent originating system and 325 the operator generating the report. Instead, they need to send 326 reports to recipients that are both responsible for the messages 327 and are able to do something about them. 328 3. Deciding where to send an unsolicited report will typically rely 329 on heuristics. Abuse addresses in WHOIS ([RFC3912]) records of 330 the IP address relaying the subject message and/or of the domain 331 name found in the results of a PTR ("reverse lookup") query on 332 that address are likely reasonable candidates, as is the 333 abuse@domain role address (see [RFC2142]) of related domains. 334 Unsolicited reports SHOULD NOT be sent to email addresses that 335 are not clearly intended to handle abuse reports. Legitimate 336 candidates include those found in WHOIS records or on a web site 337 that either are explicitly described as an abuse contact, or are 338 of the form "abuse@domain". 339 4. Where an abusive message is authenticated using a domain-level 340 authentication technology such as DKIM ([RFC6376]) or SPF 341 ([RFC4408]), the domain that has been verified by the 342 authentication mechanism is often a reasonable candidate for 343 receiving feedback about the message. For DKIM, though, while 344 the authenticated domain has some responsibility for the mail 345 sent, it can be a poor contact point for abuse issues (for 346 example, it could represent the message's author but not its 347 sender, it could identify the bad actor responsible for the 348 message, or it could refer to a domain that cannot receive mail 349 at all). 350 5. Often, unsolicited reports will have no meaning if sent to abuse 351 reporting addresses belonging to the abusive parties themselves. 352 In fact, it is possible that such reports might reveal 353 information about complainants. Reports SHOULD NOT be sent to 354 such addresses if they can be identified beforehand, except where 355 the abusive party is known to be responsive to such reports. 357 5.4. What To Put In Reports 359 1. Reports SHOULD use "Feedback-Type: abuse", but can use other 360 types as appropriate. However, the Mailbox Provider generating 361 the reports cannot assume that the operator receiving the reports 362 will treat different Feedback-Types differently. 363 2. Reports SHOULD include the following optional fields whenever 364 their corresponding values are available and applicable to the 365 report: Original-Mail-From, Arrival-Date, Source-IP, Original- 366 Rcpt-To. Other optional fields can be included, as the 367 implementer feels is appropriate. 368 3. Experience suggests use of ARF is advisable in most contexts. 369 Automated recipient systems can handle abuse reports sent in ARF 370 format at least as well as any other format such as plain text, 371 with or without a copy of the message attached. That holds even 372 for systems that did not request ARF format reports, assuming 373 such reports are generated considering the possibility of 374 recipients that don't use automated ARF parsing. Anyone sending 375 unsolicited reports in ARF format can legitimately presume that 376 some recipients will only be able to access the human readable 377 (first, text/plain) part of it, and SHOULD include all 378 information needed also in this part. Further, they SHOULD 379 ensure that the report is readable when viewed as plain text, to 380 give low-end ticketing systems as much assistance as possible. 381 In extreme cases, failure to take these steps may result in the 382 report being discarded or ignored. 384 5.5. What To Do With Reports 386 1. Receivers of unsolicited reports can take advantage of the 387 standardized parts of the ARF format to automate processing. 388 Independent of the sender of the report, they can improve 389 processing by separating valid from invalid reports by, for 390 example, looking for references to IP address ranges, domains, 391 and mailboxes for which the recipient organization is responsible 392 in the copy of the reported message, and by correlating multiple 393 reports of similar messages to identify bulk email senders. 394 2. Per Section 4.4 of [RFC6449], a network service provider MAY use 395 ARF data for automated forwarding of feedback messages to the 396 originating customer. 397 3. Published abuse mailbox addresses SHOULD NOT reject non-ARF 398 messages based solely on the format, as generation of ARF 399 messages can occasionally be unavailable or not applicable. 400 Deviation from this requirement could be done due to local policy 401 decisions regarding other message criteria. 402 4. Although [RFC6449] suggests that replying to feedback is not 403 useful, in the case of receipt of ARF reports where no feedback 404 arrangement has been established, a non-automated reply might be 405 desirable to indicate what action resulted from the complaint, 406 heading off more severe filtering by the Feedback Provider. In 407 addition, using an address that cannot receive replies precludes 408 any requests for additional information, and increases the 409 likelihood that further reports will be discarded or blocked. 410 Thus, a Feedback Provider sending unsolicited reports SHOULD NOT 411 generate reports for which a reply cannot be received. Where an 412 unsolicited report results in the establishment of contact with a 413 responsible and responsive party, this can be saved for future 414 complaint handling and possible establishment of a formal 415 (solicited) feedback arrangement. See Section 3.5 of [RFC6449] 416 for a discussion of establishment of feedback arrangements. 418 6. Generating Automatic Authentication Failure Reports 420 [These numbered items are not intended to be in a paricular sequence. 421 The numbers are here during document development to make it easier to 422 identify the items for discussion, and will be removed before 423 publication.] 425 There are some cases where report generation is caused by automation 426 rather than user request. A specific example of this is reporting, 427 using the ARF format (or extensions to it), of messages that fail 428 particular message authentication checks. Examples of this include 429 [I-D.IETF-MARF-DKIM-REPORTING] and [I-D.IETF-MARF-SPF-REPORTING]. 430 The considerations presented below apply in those cases. 432 The applicability statement for this use case is somewhat smaller as 433 many of the issues associated with abuse reports are not relevant to 434 reports about authentication failures. 436 1. Automatic feedback generators MUST select actual message 437 recipients based on data provided by willing report receivers. 438 In particular, recipients MUST NOT be selected using heuristics. 439 2. If the message under evaluation by the Verifier is an ARF 440 ([RFC5965]) message, a report MUST NOT be automatically 441 generated. 442 3. The message for a new report sent via SMTP MUST be constructed so 443 as to avoid amplification attacks, deliberate or otherwise. The 444 envelope sender address of the report MUST be chosen so that 445 these reports will not generate mail loops. Similar to Section 2 446 of [RFC3464], the envelope sender address of the report MUST be 447 chosen to ensure that no feedback reports will be issued in 448 response to the report itself. Therefore, when an SMTP 449 transaction is used to send a report, the MAIL FROM command 450 SHOULD use the NULL reverse-path, i.e., "MAIL FROM:<>". An 451 exception to this would be the use of a reverse-path selected 452 such that SPF checks on the report will pass; in such cases, the 453 operator will need to make provisions to avoid the amplification 454 attack or mail loop via other means. 455 4. Reports SHOULD use "Feedback-Type: auth-failure", but MAY use 456 other types as appropriate. However, the Mailbox Provider 457 generating the reports cannot assume that the operator receiving 458 the reports will treat different Feedback-Types differently. 459 5. These reports SHOULD include the following optional fields, 460 although they are optional in [RFC5965], whenever their 461 corresponding values are available: Original-Mail-From, Arrival- 462 Date, Source-IP, Original-Rcpt-To. Other optional fields can be 463 included, as the implementer feels is appropriate. 465 7. IANA Considerations 467 [RFC Editor: please remove this section prior to publication.] 469 This document has no IANA actions. 471 8. Security Considerations 473 8.1. In Other Documents 475 Implementers are strongly urged to review, at a minimum, the Security 476 Considerations sections of [RFC5965] and [RFC6449]. 478 8.2. Forgeries 480 Feedback Providers that relay user complaints directly, rather than 481 by reference to a stored message (e.g., IMAP or POP), could be duped 482 into sending a complaint about a message that the complaining user 483 never actually received, as an attack on the purported originator of 484 the falsified message. Feedback Providers need to be resilient to 485 such attack methods. 487 Also, these reports may be forged as easily as ordinary Internet 488 electronic mail. User agents and automatic mail handling facilities 489 (such as mail distribution list exploders) that wish to make 490 automatic use of reports of any kind should take appropriate 491 precautions to minimize the potential damage from denial-of-service 492 attacks. 494 Perhaps the simplest means of mitigating this threat is to assert 495 that these reports should themselves be signed with something like 496 DKIM or authorized by SPF. On the other hand, if there is a problem 497 with the DKIM infrastructure at the Verifier, signing DKIM failure 498 reports may produce reports that aren't trusted or even accepted by 499 their intended recipients. Similar issues could exist with SPF 500 evaluation. Use of both technologies can mitigate this risk to a 501 degree. 503 8.3. Amplification Attacks 505 Failure to comply with the recommendations regarding selection of the 506 envelope sender can lead to amplification denial-of-service attacks. 507 This is discussed in Section 6 as well as in [RFC3464]. 509 8.4. Automatic Generation 511 ARF ([RFC5965]) reports have historically been generated individually 512 as a result of some kind of human request, such as someone clicking a 513 "Report Abuse" button in a mail reader. In contrast, the mechanisms 514 described in some extension documents (i.e., 515 [I-D.IETF-MARF-DKIM-REPORTING] and [I-D.IETF-MARF-SPF-REPORTING]) are 516 focused around automated reporting. This obviously implies the 517 potential for much larger volumes or higher frequency of messages, 518 and thus greater mail system load (both for Feedback Providers and 519 report receivers). 521 Those mechanisms are primarily intended for use in generating reports 522 to aid implementers of DKIM ([RFC6376]), ADSP ([RFC5617]), and SPF 523 ([RFC4408]), and other related protocols during development and 524 debugging. They are not generally intended for prolonged forensic 525 use, specifically because of these load concerns. However, extended 526 use is possible by ADMDs that want to keep a close watch for fraud or 527 infrastructure problems. It is important to consider the impact of 528 doing so on both Feedback Providers and the requesting ADMDs. 530 A sender requesting these reports can cause its mail servers to be 531 overwhelmed if it sends out signed messages whose signatures fail to 532 verify for some reason, provoking a large number of reports from 533 Feedback Providers. Similarly, a Feedback Provider could be 534 overwhelmed by a large volume of messages requesting reports whose 535 signatures fail to validate, as those now need to send reports back 536 to the signer. 538 Limiting the rate of generation of these messages may be appropriate 539 but threatens to inhibit the distribution of important and possibly 540 time-sensitive information. 542 In general ARF feedback loop terms, it is often suggested that 543 Feedback Providers only create these (or any) ARF reports after an 544 out-of-band arrangement has been made between two parties. These 545 extension mechanisms then become ways to adjust parameters of an 546 authorized abuse report feedback loop that is configured and 547 activated by private agreement rather than starting to send them 548 automatically based solely on data found in the messages, which may 549 have unintended consequences. 551 8.5. Reporting Multiple Incidents 553 If it is known that a particular host generates abuse reports upon 554 certain incidents, an attacker could forge a high volume of messages 555 that will trigger such a report. The recipient of the report could 556 then be innundated with reports. This could easily be extended to a 557 distributed denial-of-service attack by finding a number of report- 558 generating servers. 560 The incident count referenced in ARF ([RFC5965]) provides a limited 561 form of mitigation. The host generating reports can elect to send 562 reports only periodically, with each report representing a number of 563 identical or nearly-identical incidents. One might even do something 564 inverse-exponentially, sending reports for each of the first ten 565 incidents, then every tenth incident up to 100, then every 100th 566 incident up to 1000, etc., until some period of relative quiet after 567 which the limitation resets. 569 The use of this for "nearly-identical" incidents in particular causes 570 a degradation in reporting quality, however. If for example a large 571 number of pieces of spam arrive from one attacker, a reporting agent 572 could decide only to send a report about a fraction of those 573 messages. While this averts a flood of reports to a system 574 administrator, the precise details of each incident are similarly not 575 sent. 577 Other rate limiting provisions might be considered, including 578 detection of a temporary failure response from the report destination 579 and thus halting report generation to that destination for some 580 period, or simply imposing or negotiating a hard limit on the number 581 of reports to be sent to a particular receiver in a given time frame. 583 9. Acknowledgements 585 The author and editor wish to thank Steve Atkins, John Levine, Shmuel 586 Metz, S. Moonesamy, and Alessandro Vesely for their contributions to 587 this memo. 589 All of the Best Practices referenced by this document are found in 590 [RFC6449], written within the Collaboration Committee of the 591 Messaging Anti-Abuse Working Group (MAAWG). 593 Finally, the original author wishes to thank the doctors and staff at 594 the University of Texas MD Anderson Cancer Center for doing what they 595 do. 597 10. References 599 10.1. Normative References 601 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 602 Requirement Levels", BCP 14, RFC 2119, March 1997. 604 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 605 October 2008. 607 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 608 October 2008. 610 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 611 July 2009. 613 [RFC5965] Shafranovich, Y., Levine, J., and M. Kucherawy, "An 614 Extensible Format for Email Feedback Reports", RFC 5965, 615 August 2010. 617 10.2. Informative References 619 [I-D.IETF-MARF-DKIM-REPORTING] 620 Kucherawy, M., "Extensions to DKIM for Failure Reporting", 621 draft-ietf-marf-dkim-reporting (work in progress), 622 January 2012. 624 [I-D.IETF-MARF-SPF-REPORTING] 625 Kitterman, S., "SPF Authentication Failure Reporting using 626 the Abuse Report Format", draft-ietf-marf-spf-reporting 627 (work in progress), January 2012. 629 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 630 STD 53, RFC 1939, May 1996. 632 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 633 3", BCP 9, RFC 2026, October 1996. 635 [RFC2142] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND 636 FUNCTIONS", RFC 2142, May 1997. 638 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 639 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 640 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 642 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format 643 for Delivery Status Notifications", RFC 3464, 644 January 2003. 646 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 647 4rev1", RFC 3501, March 2003. 649 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, 650 September 2004. 652 [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 653 for Authorizing Use of Domains in E-Mail, Version 1", 654 RFC 4408, April 2006. 656 [RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine, 657 "DomainKeys Identified Mail (DKIM) Author Domain Signing 658 Practices (ADSP)", RFC 5617, August 2009. 660 [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys 661 Identified Mail (DKIM) Signatures", RFC 6376, 662 September 2011. 664 [RFC6449] Falk, J., "Complaint Feedback Loop Operational 665 Recommendations", RFC 6449, November 2011. 667 [RFC6590] Falk, J. and M. Kucherawy, "Redaction of Potentially 668 Sensitive Data from Mail Abuse Reports", RFC 6590, 669 April 2012. 671 Authors' Addresses 673 J.D. Falk 674 Return Path 675 100 Mathilda Street, Suite 100 676 Sunnyvale, CA 94089 677 USA 679 URI: http://www.returnpath.net/ 681 M. Kucherawy (editor) 682 Cloudmark 683 128 King St., 2nd Floor 684 San Francisco, CA 94107 685 US 687 Email: msk@cloudmark.com