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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Downref: Normative reference to an Historic RFC: RFC 5617 (ref. 'ADSP') ** Downref: Normative reference to an Informational RFC: RFC 5598 (ref. 'EMAIL-ARCH') ** Obsolete normative reference: RFC 5226 (ref. 'IANA-CONSIDERATIONS') (Obsoleted by RFC 8126) Summary: 3 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MARF Working Group M. Kucherawy 3 Internet-Draft Cloudmark 4 Intended status: Standards Track March 19, 2012 5 Expires: September 20, 2012 7 Extensions to DKIM for Failure Reporting 8 draft-ietf-marf-dkim-reporting-16 10 Abstract 12 This document presents extensions to the DomainKeys Identified Mail 13 (DKIM) specification to allow for detailed reporting of message 14 authentication failures in an on-demand fashion. 16 Status of this Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on September 20, 2012. 33 Copyright Notice 35 Copyright (c) 2012 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 52 2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 4 53 2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 4 54 2.3. Imported Definitions . . . . . . . . . . . . . . . . . . . 4 55 2.4. Other Definitions . . . . . . . . . . . . . . . . . . . . 4 56 3. Optional Reporting for DKIM . . . . . . . . . . . . . . . . . 5 57 3.1. Extension DKIM Signature Tag . . . . . . . . . . . . . . . 5 58 3.2. DKIM Reporting TXT Record . . . . . . . . . . . . . . . . 5 59 3.3. DKIM Reporting Algorithm . . . . . . . . . . . . . . . . . 7 60 4. Optional Reporting Address for DKIM-ADSP . . . . . . . . . . . 9 61 5. Requested Reports . . . . . . . . . . . . . . . . . . . . . . 11 62 5.1. Requested Reports for DKIM Failures . . . . . . . . . . . 11 63 5.2. Requested Reports for DKIM ADSP Failures . . . . . . . . . 11 64 6. Report Generation . . . . . . . . . . . . . . . . . . . . . . 13 65 6.1. Report Format . . . . . . . . . . . . . . . . . . . . . . 13 66 6.2. Other Guidance . . . . . . . . . . . . . . . . . . . . . . 13 67 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 68 7.1. DKIM Signature Tag Registration . . . . . . . . . . . . . 14 69 7.2. DKIM ADSP Tag Registration . . . . . . . . . . . . . . . . 14 70 7.3. DKIM Reporting Tag Registry . . . . . . . . . . . . . . . 14 71 8. Security Considerations . . . . . . . . . . . . . . . . . . . 16 72 8.1. Inherited Considerations . . . . . . . . . . . . . . . . . 16 73 8.2. Report Volume . . . . . . . . . . . . . . . . . . . . . . 16 74 8.3. Deliberate Misuse . . . . . . . . . . . . . . . . . . . . 16 75 8.4. Unreported Fraud . . . . . . . . . . . . . . . . . . . . . 17 76 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 77 9.1. Normative References . . . . . . . . . . . . . . . . . . . 18 78 9.2. Informative References . . . . . . . . . . . . . . . . . . 19 79 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 20 80 Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 21 81 B.1. Example Use of DKIM Signature Extension Tag . . . . . . . 21 82 B.2. Example DKIM Reporting TXT Record . . . . . . . . . . . . 21 83 B.3. Example Use of DKIM ADSP Extension Tags . . . . . . . . . 22 84 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23 86 1. Introduction 88 DomainKeys Identified Mail [DKIM] introduced a mechanism for message 89 signing and authentication. It uses digital signing to associate a 90 domain name with a message in a reliable manner. The verified domain 91 name can then be evaluated (e.g., checking advertised sender policy, 92 comparison to a known-good list, submission to a reputation service, 93 etc.). 95 Deployers of message authentication technologies are increasingly 96 seeking visibility into DKIM verification failures and conformance 97 failures involving the published signing practices (e.g., Author 98 Domain Signing Practices, [ADSP]) of an Administrative Management 99 Domain (ADMD; see [EMAIL-ARCH]). 101 This document extends [DKIM] and [ADSP] to add an optional reporting 102 address and some reporting parameters. Reports are generated using 103 the format defined in [I-D.IETF-MARF-AUTHFAILURE-REPORT]. 105 2. Definitions 107 2.1. Keywords 109 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 110 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 111 document are to be interpreted as described in [KEYWORDS]. 113 2.2. Notation 115 Certain properties of email messages described in this document are 116 referenced using notation found in [EMAIL-ARCH] (e.g., 117 "RFC5322.From"). 119 2.3. Imported Definitions 121 Numerous DKIM-specific terms used here are defined in [DKIM]. The 122 definitions of the [ABNF] tokens "domain-name" and "dkim-quoted- 123 printable" can also be found there. 125 2.4. Other Definitions 127 report generator: A report generator is an entity that generates and 128 sends reports. For the scope of this document, the term refers to 129 Verifiers, as defined in Section 2.2 of [DKIM], with the added 130 capability to generate authentication failure reports according to 131 this specification. 133 3. Optional Reporting for DKIM 135 A domain name owner employing [DKIM] for email signing and 136 authentication might want to know when signatures that ought to be 137 verifiable are not successfully verifying. Currently there is no 138 such mechanism defined. 140 This section adds optional "tags" (as defined in [DKIM]) to the DKIM- 141 Signature header field and the DKIM key record in the DNS, using the 142 formats defined in that specification. 144 3.1. Extension DKIM Signature Tag 146 The following tag is added to DKIM-Signature header fields when a 147 Signer wishes to request that reports of failed verifications be 148 generated by a Verifier: 150 r= Reporting Requested (plain-text; OPTIONAL; no default). If 151 present, this tag indicates that the Signer requests that 152 Verifiers generate a report when verification of the DKIM 153 signature fails. At present, the only legal value is the single 154 character "y". A complete description and illustration of how 155 this is applied can be found in Section 3.3. 157 ABNF: 159 sig-r-tag = %x72 *WSP "=" *WSP %x79 160 ; "r=y" (lower-case only) 162 3.2. DKIM Reporting TXT Record 164 When a Signer wishes to advertise that it wants to receive failed 165 verification reports, it places in the DNS a TXT resource record 166 (RR). The RR contains a sequence of tag-value objects in a format 167 similar to DKIM key records (see Section 3.6.1 of [DKIM]), but it is 168 entirely independent of those key records and is found at a different 169 name. The tag-value objects in this case comprise the parameters to 170 be used when generating the reports. A report generator will request 171 the content of this record when it sees an "r=" tag in a DKIM- 172 Signature header field. 174 Section 3.6.2.2 of [DKIM] provides guidance with respect to handling 175 of a TXT RR that comprises multiple distinct strings ("character- 176 strings" in the parlance of [DNS]). The same process MUST be applied 177 here. 179 Implementations MUST support all tags defined in this document, and 180 any other tag found in the content of the record that is not 181 recognized by an implementation MUST be ignored. See Section 7.3 for 182 details about finding or registering extension tags. 184 The initial list of tags supported for the reporting TXT record is as 185 follows: 187 ra= Reporting Address (plain-text; OPTIONAL). A dkim-quoted- 188 printable string (see Section 2.11 of [DKIM]) containing the 189 local-part of an email address to which a report SHOULD be sent 190 when mail fails DKIM verification for one of the reasons 191 enumerated below. The value MUST be interpreted as a local-part 192 only. To construct the actual address to which the report is 193 sent, the Verifier simply appends to this value an "@" followed by 194 the domain name found in the "d=" tag of the DKIM-Signature header 195 field. Therefore, an ADMD making use of this specification MUST 196 ensure that an email address thus constructed can receive reports 197 generated as described in Section 6. ABNF: 199 rep-ra-tag = %x72.61 *WSP "=" *WSP dkim-quoted-printable 200 ; "ra=..." (lower-case only for the tag name) 202 rp= Requested Report Percentage (plain-text; OPTIONAL; default is 203 "100"). The value is an integer from 0 to 100 inclusive that 204 indicates what percentage of incidents of signature authentication 205 failures, selected at random, are to cause reports to be 206 generated. The report generator SHOULD NOT issue reports for more 207 than the requested percentage of incidents. Report generators MAY 208 make use of the "Incidents:" field in [ARF] to indicate that there 209 are more reportable incidents than there are reports. ABNF: 211 rep-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT 212 ; "rp=..." (lower-case only) 214 rr= Requested Reports (plain-text; OPTIONAL; default is "all"). The 215 value MUST be a colon-separated list of tokens representing those 216 conditions under which a report is desired. See Section 5.1 for a 217 list of valid tokens. ABNF: 219 rep-rr-type = ( "all" / "d" / "o" / "p"/ "s" / "u" / "v" / "x" ) 220 rep-rr-tag = %x72.72 *WSP "=" *WSP rep-rr-type 221 *WSP *( ":" *WSP rep-rr-type ) 222 ; "rr=..." (lower-case only for the tag name) 224 rs= Requested SMTP Error String (text; OPTIONAL; no default). The 225 value is a dkim-quoted-printable string that the publishing ADMD 226 requests be included in [SMTP] error strings if messages are 227 rejected during the delivery SMTP session. ABNF: 229 rep-rs-tag = %x72.73 *WSP "=" dkim-quoted-printable 230 ; "rs=..." (lower-case only for the tag name) 232 In the absence of an "ra=" tag, the "rp=" and "rr=" tags MUST be 233 ignored, and the report generator MUST NOT issue a report. 235 3.3. DKIM Reporting Algorithm 237 Report generators MUST apply the following algorithm, or one 238 semantically equivalent to it, for each DKIM-Signature header field 239 whose verification fails for some reason. Note that this processing 240 is done as a reporting extension only; the outcome of the specified 241 DKIM evaluation MUST be otherwise unaffected. 243 1. If the DKIM-Signature field did not contain a valid "r=" tag, 244 terminate. 246 2. Issue a [DNS] TXT query to the name that results from appending 247 the value of the "d=" tag in the DKIM-Signature field to the 248 string "_report._domainkey.". For example, if the DKIM- 249 Signature header field contains "d=example.com", issue a DNS TXT 250 query to "_report._domainkey.example.com". 252 3. If the DNS query returns anything other than RCODE 0 (NOERROR), 253 or if multiple TXT records are returned, terminate. 255 4. If the resultant TXT is in several string fragments, concatenate 256 them as described in Section 3.6.2.2 of [DKIM]. 258 5. If the TXT content is syntactically invalid (see Section 3.2), 259 terminate. 261 6. If the reason for the signature evaluation failure does not 262 match one of the report requests found in the "rr=" tag (or its 263 default value), terminate. 265 7. If a report percentage ("rp=") tag was present, select a random 266 number between 0 and 99, inclusive; if the selected number is 267 not lower than the tag's value, terminate. 269 8. If no "ra=" tag was present, skip this step and the next one. 270 Otherwise, determine the reporting address by extracting the 271 value of the "ra=" tag and appending to it "@" followed by the 272 domain name found in the "d=" tag of the DKIM-Signature header 273 field. 275 9. Construct and send a report in compliance with Section 6 of this 276 document that includes as its intended recipient the address 277 constructed in the previous step. 279 10. If the [SMTP] session during which the DKIM signautre was 280 evaluated is still active and the SMTP server has not already 281 given its response to the DATA command that relayed the message, 282 and an "rs=" tag was present in the TXT record, the SMTP server 283 SHOULD include the decoded string found in the "rs=" tag in its 284 SMTP reply to the DATA command. 286 In order to thwart attacks that seek to convert report generators 287 into unwitting denial-of-service attack participants, a report 288 generator SHOULD NOT issue more than one report to any given domain 289 as a result of a single message. Further, a report generator SHOULD 290 establish an upper bound on the number of reports a single message 291 can generate overall. For example, a message with three invalid 292 signatures, two from example.com and one from example.net, would 293 generate at most one report to each of those domains. 295 This algorithm has the following advantages over previous pre- 296 standardization implementations, such as early versions of 297 [OPENDKIM]: 299 a. If the DKIM signature fails to verify, no additional DNS check is 300 made to see if reporting is requested; the request is active in 301 that it is included in the DKIM-Signature header field. 302 (Previous implementations included the reporting address in the 303 DKIM key record, which is not queried for certain failure cases. 304 This meant, for full reporting, that the key record had to be 305 retrieved even when it was not otherwise necessary.) 307 b. The request is confirmed by the presence of a corresponding TXT 308 record in the DNS, since the Signer thus provides the parameters 309 required to construct and send the report. This means a 310 malicious Signer cannot falsely assert that someone else wants 311 failure reports and cause unwanted mail to be generated. It can 312 cause additional DNS traffic against the domain listed in the 313 "d=" signature tag, but negative caching of the requested DNS 314 record will help to mitigate this issue. 316 c. It is not possible for a Signer to direct reports to an email 317 address outside of its own domain, preventing distributed email- 318 based denial-of-service attacks. 320 See Section 8.4 for some considerations regarding limitations of this 321 mechanism. 323 4. Optional Reporting Address for DKIM-ADSP 325 A domain name owner employing Author Domain Signing Practices [ADSP] 326 may also want to know when messages are received without valid author 327 domain signatures. Currently there is no such mechanism defined. 329 This section adds the following optional "tags" (as defined in 330 [ADSP]) to the DKIM ADSP records, using the form defined in that 331 specification: 333 ra= Reporting Address (plain-text; OPTIONAL; no default). The value 334 MUST be a dkim-quoted-printable string containing the local-part 335 of an email address to which a report SHOULD be sent when mail 336 claiming to be from this domain failed the verification algorithm 337 described in [ADSP], in particular because a message arrived 338 without a signature that validates, which contradicts what the 339 ADSP record claims. The value MUST be interpreted as a local-part 340 only. To construct the actual address to which the report is 341 sent, the Verifier simply appends to this value an "@" followed by 342 the domain whose policy was queried in order to evaluate the 343 sender's ADSP, i.e., the RFC5322.From domain of the message under 344 evaluation. Therefore, a signer making use of this extension tag 345 MUST ensure that an email address thus constructed can receive 346 reports generated as described in Section 6. ABNF: 348 adsp-ra-tag = %x72.61 *WSP "=" dkim-quoted-printable 349 ; "ra=..." (lower-case only for the tag name) 351 rp= Requested Report Percentage (plain-text; OPTIONAL; default is 352 "100"). The value is a single integer from 0 to 100 inclusive 353 that indicates what percentage of incidents of ADSP evaluation 354 failures, selected at random, should cause reports to be 355 generated. The report generator SHOULD NOT issue reports for more 356 than the requested percentage of incidents. An exception to this 357 might be some out-of-band arrangement between two parties to 358 override it with some mutually agreed value. Report generators 359 MAY make use of the "Incidents:" field in [ARF] to indicate that 360 there are more reportable incidents than there are reports. ABNF: 362 adsp-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT 363 ; "rp=..." (lower-case only) 365 rr= Requested Reports (plain-text; OPTIONAL; default is "all"). The 366 value MUST be a colon-separated list of tokens representing those 367 conditions under which a report is desired. See Section 5.2 for a 368 list of valid tokens. ABNF: 370 adsp-rr-type = ( "all" / "o" / "p" / "s" / "u" ) 371 adsp-rr-tag = %x72.72 *WSP "=" *WSP adsp-rr-type 372 *WSP *( ":" *WSP adsp-rr-type ) 373 ; "rr=..." (lower-case only for the tag name) 375 rs= Requested SMTP Error String (plain-text; OPTIONAL; no default). 376 The value is a string the signing domain requests be included in 377 [SMTP] error strings when messages are rejected during a single 378 SMTP session. ABNF: 380 adsp-rs-tag = %x72.73 *WSP "=" dkim-quoted-printable 381 ; "rs=..." (lower-case only for the tag name) 383 In the absence of an "ra=" tag, the "rp=" and "rr=" tags MUST be 384 ignored, and the report generator MUST NOT issue a report. 386 5. Requested Reports 388 The "rr" tags defined above allow a Signer to specify the types of 389 errors about which it is interested in receiving reports. This 390 section defines the error types and corresponding token values. 392 Verifiers MUST NOT generate reports for incidents that do not match a 393 requested report, and MUST ignore requests for reports not included 394 in this list. 396 5.1. Requested Reports for DKIM Failures 398 The following report requests are defined for DKIM keys: 400 all All reports are requested. 402 d Reports are requested for signature evaluation errors that 403 resulted from DNS issues (e.g., key retrieval problems). 405 o Reports are requested for any reason related to DKIM signature 406 evaluation not covered by other report requests listed here. 408 p Reports are requested for signatures that are rejected for local 409 policy reasons at the Verifier that are related to DKIM signature 410 evaluation. 412 s Reports are requested for signature or key syntax errors. 414 u Reports are requested for signatures that include unknown tags in 415 the signature field. 417 v Reports are requested for signature verification failures or body 418 hash mismatches. 420 x Reports are requested for signatures rejected by the Verifier 421 because the expiration time has passed. 423 5.2. Requested Reports for DKIM ADSP Failures 425 The following report requests are defined for ADSP records: 427 all All reports are requested. 429 o Reports are requested for any [ADSP]-related failure reason not 430 covered by other report requests listed here. 432 p Reports are requested for messages that are rejected for local 433 policy reasons at the Verifier that are related to [ADSP]. 435 s Reports are requested for messages that have a valid [DKIM] 436 signature but do not match the published [ADSP] policy. 438 u Reports are requested for messages that have no valid [DKIM] 439 signature and do not match the published [ADSP] policy. 441 6. Report Generation 443 This section describes the process for generating and sending reports 444 in accordance with the request of the signer and/or sender as 445 described above. 447 6.1. Report Format 449 All reports generated as a result of requests contained in these 450 extension parameters MUST be generated in compliance with [ARF] and 451 its extension specific to this work, 452 [I-D.IETF-MARF-AUTHFAILURE-REPORT]. Moreover, because abuse reports 453 from unverified sources might be handled with some skepticism, report 454 generators are strongly advised to use [DKIM] to sign reports they 455 generate. 457 6.2. Other Guidance 459 Additional guidance about the generation of these reports can be 460 found in [I-D.IETF-MARF-AS], especially Section 9. 462 7. IANA Considerations 464 As required by [IANA-CONSIDERATIONS], this section contains registry 465 information for the new [DKIM] signature tags, and the new [ADSP] 466 tags. It also creates a DKIM reporting tag registry. 468 7.1. DKIM Signature Tag Registration 470 IANA is requested to update the DKIM Signature Tag Specification 471 Registry to include the following new items: 473 +------+-----------------+--------+ 474 | TYPE | REFERENCE | STATUS | 475 +------+-----------------+--------+ 476 | r | (this document) | active | 477 +------+-----------------+--------+ 479 7.2. DKIM ADSP Tag Registration 481 IANA is requested to update the DKIM ADSP Specification Tag Registry 482 to include the following new items: 484 +------+-----------------+ 485 | TYPE | REFERENCE | 486 +------+-----------------+ 487 | ra | (this document) | 488 | rp | (this document) | 489 | rr | (this document) | 490 | rs | (this document) | 491 +------+-----------------+ 493 7.3. DKIM Reporting Tag Registry 495 IANA is requested to create a sub-registry of the DKIM Parameters 496 registry called "DKIM Reporting Tags". Additions to this registry 497 follow the "Specification Required" rules, with the following columns 498 required for all registrations: 500 Type: The name of the tag being used in reporting records 502 Reference: The document that specifies the tag being defined 504 Status: The status of the tag's current use, either "active" 505 indicating active use, or "historic" indicating discontinued or 506 deprecated use 508 The initial registry entries are as follows: 510 +------+-----------------+--------+ 511 | TYPE | REFERENCE | STATUS | 512 +------+-----------------+--------+ 513 | ra | (this document) | active | 514 | rp | (this document) | active | 515 | rr | (this document) | active | 516 | rs | (this document) | active | 517 +------+-----------------+--------+ 519 8. Security Considerations 521 Security issues with respect to these reports are similar to those 522 found in [DSN]. 524 8.1. Inherited Considerations 526 Implementers are advised to consider the Security Considerations 527 sections of [DKIM], [ADSP], [I-D.IETF-MARF-AS], and 528 [I-D.IETF-MARF-AUTHFAILURE-REPORT]. Many security issues related to 529 this draft are already covered in those documents. 531 8.2. Report Volume 533 It is impossible to predict the volume of reports this facility will 534 generate when enabled by a report receiver. An implementer ought to 535 anticipate substantial volume, since the amount of abuse occurring at 536 receivers cannot be known ahead of time, and may vary rapidly and 537 unpredictably. 539 8.3. Deliberate Misuse 541 Some threats caused by deliberate misuse of this error reporting 542 mechanism are discussed in Section 3.3, but they warrant further 543 discussion here. 545 The presence of the DNS record that indicates willingness to accept 546 reports opens the recipient to abuse. In particular, it is possible 547 for an attacker to attempt to cause a flood of reports toward the 548 domain identified in a signature's "d=" tag in one of these ways: 550 1. Alter existing DKIM-Signature header fields by adding an "r=y" 551 tag (and possibly altering the "d=" tag to point at the target 552 domain); 554 2. Add a new but bogus signature bearing an "r=y" tag and a "d=" tag 555 pointing at the target domain; 557 3. Generate a completely new message bearing an "r=y" tag and a "d=" 558 tag pointing at the target domain. 560 Consider, for example, the situation where an an attacker sends out a 561 multi-million-message spam run, and includes in the messages a fake 562 DKIM signature containing "d=example.com; r=y". It won't matter that 563 those signatures couldn't possibly be real: each will fail 564 verification, and any implementations that support this specification 565 will report those failures, in the millions and in short order, to 566 example.com. 568 Implementers are therefore strongly advised not to advertise the DNS 569 record specified in this document except when failure reports are 570 desired. Upon doing so, unexpected traffic volumes and attacks 571 should be anticipated. 573 Negative caching offers some protection against this pattern of 574 abuse, although it will work only as long as the negative time-to- 575 live on the relevant SOA record in the DNS. 577 Positive caching of this DNS reply also means turning off the flow of 578 reports by removing the record is not likely to have immediate 579 effect. A low time-to-live on the record needs to be considered. 581 8.4. Unreported Fraud 583 An attacker can craft fraudulent DKIM-Signature fields on messages, 584 without using "r=" tags, and avoid having these reported. The 585 procedure described in Section 3.3 does not permit the detection and 586 reporting of such cases. 588 It might be useful to some Signers to receive such reports, but the 589 mechanism does not support it. To offer such support, a Verifier 590 would have to violate the first step in the procedure and continue 591 even in the absence of an "r=" tag. Although that would enable the 592 desired report, it would also create a possible denial-of-service 593 attack: such Verifiers would always look for the reporting TXT 594 record, so a generator of fraudulent messages could simply send a 595 large volume of messages without an "r=" tag to a number of 596 destinations. To avoid that outcome, reports of fraudulent DKIM- 597 Signature header fields are not possible using the published 598 mechanism. 600 9. References 602 9.1. Normative References 604 [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax 605 Specifications: ABNF", RFC 5234, January 2008. 607 [ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, 608 "DomainKeys Identified Mail (DKIM) Author Domain Signing 609 Practices (ADSP)", RFC 5617, August 2009. 611 [ARF] Shafranovich, Y., Levine, J., and M. Kucherawy, "An 612 Extensible Format for Email Feedback Reports", RFC 5965, 613 August 2010. 615 [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 616 "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, 617 September 2011. 619 [DNS] Mockapetris, P., "Domain names - implementation and 620 specification", STD 13, RFC 1035, November 1987. 622 [EMAIL-ARCH] 623 Crocker, D., "Internet Mail Architecture", RFC 5598, 624 October 2008. 626 [I-D.IETF-MARF-AS] 627 Falk, J. and M. Kucherawy, Ed., "Creation and Use of Email 628 Feedback Reports: An Applicability Statement for the Abuse 629 Reporting Format (ARF)", draft-ietf-marf-as (work in 630 progress), January 2012. 632 [I-D.IETF-MARF-AUTHFAILURE-REPORT] 633 Fontana, H., "Authentication Failure Reporting using the 634 Abuse Report Format", draft-ietf-marf-authfailure-report 635 (work in progress), January 2012. 637 [IANA-CONSIDERATIONS] 638 Alvestrand, H. and T. Narten, "Guidelines for Writing an 639 IANA Considerations Section in RFCs", RFC 5226, May 2008. 641 [KEYWORDS] 642 Bradner, S., "Key words for use in RFCs to Indicate 643 Requirement Levels", RFC 2119, March 1997. 645 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 646 October 2008. 648 9.2. Informative References 650 [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format 651 for Delivery Status Notifications", RFC 3464, 652 January 2003. 654 [OPENDKIM] 655 Kucherawy, M., "OpenDKIM -- Open Source DKIM Library and 656 Filter", August 2009, . 658 Appendix A. Acknowledgements 660 The authors wish to acknowledge the following for their review and 661 constructive criticism of this proposal: Steve Atkins, Monica Chew, 662 Dave Crocker, Tim Draegen, Frank Ellermann, JD Falk, John Levine, 663 Scott Kitterman, and Andrew Sullivan. 665 Appendix B. Examples 667 This section contains examples of the use of each of the extensions 668 defined by this document. 670 B.1. Example Use of DKIM Signature Extension Tag 672 A DKIM-Signature field including use of the extension tag defined by 673 this document: 675 DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; 676 d=example.com; s=jan2012; r=y; 677 h=from:to:subject:date:message-id; 678 bh=YJAYwiNdc3wMh6TD8FjVhtmxaHYHo7Z/06kHQYvQ4tQ=; 679 b=jHF3tpgqr6nH/icHKIqFK2IJPtCLF0CRJaz2Hj1Y8yNwTJ 680 IMYIZtLccho3ymGF2GYqvTl2nP/cn4dH+55rH5pqkWNnuJ 681 R9z54CFcanoKKcl9wOZzK9i5KxM0DTzfs0r8 683 Example 1: DKIM-Signature field using this extension 685 This example DKIM-Signature field contains the "r=" tag that 686 indicates reports are requested on verification failure. 688 Assuming the public key retrieved from the DNS and processed 689 according to [DKIM] would determine that the signature is invalid, a 690 TXT query will be sent to "_report._domainkey.example.com" to 691 retrieve a reporting address and other report parameters as described 692 in Section 3.3. 694 B.2. Example DKIM Reporting TXT Record 696 An example DKIM Reporting TXT Record as defined by this document: 698 ra=dkim-errors; rp=100; rr=v:x 700 Example 2: Example DKIM Reporting TXT Record 702 This example, continuing from the previous one, shows a message that 703 might be found at "_report._domainkey.example.com" in a TXT record. 704 It makes the following requests: 706 o Reports about signature evaluation failures should be send to the 707 address "dkim-errors" at the signer's domain; 709 o All (100%) incidents should be reported; 711 o Only reports about signature verification failures and expired 712 signatures should be generated. 714 B.3. Example Use of DKIM ADSP Extension Tags 716 A DKIM ADSP record including use of the extensions defined by this 717 document: 719 dkim=all; ra=dkim-adsp-errors; rr=u 721 Example 3: DKIM ADSP record using these extensions 723 This example ADSP record makes the following assertions: 725 o The sending domain (i.e. the one that is advertising this policy) 726 signs all mail it sends; 728 o Reports about ADSP evaluation failures should be send to the 729 address "dkim-adsp-errors" at the Author's domain; 731 o Only reports about unsigned messages should be generated. 733 Author's Address 735 Murray S. Kucherawy 736 Cloudmark 737 128 King St., 2nd Floor 738 San Francisco, CA 94107 739 US 741 Phone: +1 415 946 3800 742 Email: msk@cloudmark.com