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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group E. Allman 3 Internet-Draft Sendmail, Inc. 4 Intended status: Standards Track J. Fenton 5 Expires: January 3, 2009 Cisco Systems, Inc. 6 M. Delany 7 Yahoo! Inc. 8 J. Levine 9 Taughannock Networks 10 July 2, 2008 12 DKIM Author Domain Signing Practices (ADSP) 13 draft-ietf-dkim-ssp-04 15 Status of this Memo 17 By submitting this Internet-Draft, each author represents that any 18 applicable patent or other IPR claims of which he or she is aware 19 have been or will be disclosed, and any of which he or she becomes 20 aware will be disclosed, in accordance with Section 6 of BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF), its areas, and its working groups. Note that 24 other groups may also distribute working documents as Internet- 25 Drafts. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 The list of current Internet-Drafts can be accessed at 33 http://www.ietf.org/ietf/1id-abstracts.txt. 35 The list of Internet-Draft Shadow Directories can be accessed at 36 http://www.ietf.org/shadow.html. 38 This Internet-Draft will expire on January 3, 2009. 40 Abstract 42 DomainKeys Identified Mail (DKIM) defines a domain-level 43 authentication framework for email to permit verification of the 44 source and contents of messages. This document specifies an adjunct 45 mechanism to aid in assessing messages that do not contain a DKIM 46 signature for the domain used in the author's address. It defines a 47 record that can advertise whether they sign their outgoing mail, and 48 how other hosts can access those records. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 2. Language and Terminology . . . . . . . . . . . . . . . . . . . 3 54 2.1. Terms Imported from DKIM Signatures Specification . . . . 3 55 2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 4 56 2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 4 57 2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 4 58 2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 4 59 2.6. Author Domain Signing Practices . . . . . . . . . . . . . 4 60 2.7. Author Signature . . . . . . . . . . . . . . . . . . . . . 4 61 3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 5 62 3.1. ADSP Applicability . . . . . . . . . . . . . . . . . . . . 5 63 3.2. ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . . 5 64 3.3. ADSP Results . . . . . . . . . . . . . . . . . . . . . . . 6 65 4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 6 66 4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 6 67 4.2. Publication of ADSP Records . . . . . . . . . . . . . . . 6 68 4.3. ADSP Lookup Procedure . . . . . . . . . . . . . . . . . . 7 69 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 70 5.1. ADSP Specification Tag Registry . . . . . . . . . . . . . 8 71 5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 9 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 73 6.1. ADSP Threat Model . . . . . . . . . . . . . . . . . . . . 9 74 6.2. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 10 75 6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 10 76 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 77 7.1. References - Normative . . . . . . . . . . . . . . . . . . 11 78 7.2. References - Informative . . . . . . . . . . . . . . . . . 11 79 Appendix A. Usage Examples . . . . . . . . . . . . . . . . . . . 11 80 A.1. Single Location Domains . . . . . . . . . . . . . . . . . 12 81 A.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 12 82 A.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 13 83 A.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 13 84 A.5. Non-email Domains . . . . . . . . . . . . . . . . . . . . 13 85 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 13 86 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 13 87 C.1. Changes since -ietf-dkim-03 . . . . . . . . . . . . . . . 14 88 C.2. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 14 89 C.3. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 15 90 C.4. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 16 91 C.5. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 16 92 C.6. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 17 93 C.7. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 17 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 95 Intellectual Property and Copyright Statements . . . . . . . . . . 19 97 1. Introduction 99 DomainKeys Identified Mail (DKIM) defines a mechanism by which email 100 messages can be cryptographically signed, permitting a signing domain 101 to claim responsibility for the introduction of a message into the 102 mail stream. Message recipients can verify the signature by querying 103 the signer's domain directly to retrieve the appropriate public key, 104 and thereby confirm that the message was attested to by a party in 105 possession of the private key for the signing domain. 107 However, the legacy of the Internet is such that not all messages 108 will be signed, and the absence of a signature on a message is not an 109 a priori indication of forgery. In fact, during early phases of 110 deployment it is very likely that most messages will remain unsigned. 111 However, some domains might decide to sign all of their outgoing 112 mail, for example, to protect their brand names. It is desirable for 113 such domains to be able to advertise that fact to other hosts. This 114 is the topic of Author Domain Signing Practices (ADSP). 116 Hosts implementing this specification can inquire what Author Signing 117 Practices a domain advertises. This inquiry is called an Author 118 Signing Practices check. 120 The basic requirements for ADSP are given in [RFC5016]. This 121 document refers extensively to [RFC4871] and assumes the reader is 122 familiar with it. 124 Requirements Notation: The key words "MUST", "MUST NOT", 125 "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", 126 "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be 127 interpreted as described in [RFC2119] 129 2. Language and Terminology 131 2.1. Terms Imported from DKIM Signatures Specification 133 Some terminology used herein is derived directly from [RFC4871]. In 134 several cases, references in that document to Sender have been 135 changed to Author here, to emphasize the relationship to the Author 136 address(es) in the From: header field described in [RFC2822]. 137 Briefly, 139 o A "Signer" is the agent that signs a message, as defined in 140 section 2.1 of [RFC4871]. 142 o A "Local-part" is the part of an address preceding the @ 143 character, as defined in [RFC2822] and used in [RFC4871]. 145 2.2. Valid Signature 147 A "Valid Signature" is any signature on a message which correctly 148 verifies using the procedure described in section 6.1 of [RFC4871]. 150 2.3. Author Address 152 An "Author Address" is an email address in the From header field of a 153 message [RFC2822]. If the From header field contains multiple 154 addresses, the message has multiple Author Addresses. 156 2.4. Author Domain 158 An "Author Domain" is everything to the right of the "@" in an Author 159 Address (excluding the "@" itself). 161 2.5. Alleged Author 163 An "Alleged Author" is an Author Address of a message; it is 164 "alleged" because it has not yet been verified. 166 2.6. Author Domain Signing Practices 168 "Author Domain Signing Practices" (or just "practices") consist of a 169 machine-readable record published by the domain of an Alleged Author 170 which includes statements about the domain's practices with respect 171 to mail it sends with its domain in the From: line. 173 2.7. Author Signature 175 An "Author Signature" is any Valid Signature where the identity of 176 the user or agent on behalf of which the message is signed (listed in 177 the "i=" tag or its default value from the "d=" tag) matches an 178 Author Address in the message. When the identity of the user or 179 agent includes a Local-part, the identities match if the Local-parts 180 are the same string, and the domains are the same string. Otherwise, 181 the identities match if the domains are the same string. Following 182 [RFC2821], Local-part comparisons are case sensitive, domain 183 comparisons are case insensitive. 185 For example, if a message has a Valid Signature, with the DKIM- 186 Signature field containing "i=a@domain.example", then domain.example 187 is asserting that it takes responsibility for the message. If the 188 message's From: field contains the address "b@domain.example" and an 189 ADSP query produces a "dkim=all" or "dkim=discardable" result, that 190 would mean that the message does not have a valid Author Signature. 191 Even though the message is signed by the same domain, it fails to 192 satisfy ADSP. 194 3. Operation Overview 196 Domain owners can publish ADSP information via a query mechanism such 197 as the Domain Name System; specific details are given in Section 4.1. 199 Hosts can look up the ADSP information of the domain(s) specified by 200 the Author Address(es) as described in Section 4.3. If a message has 201 multiple Author Addresses the ADSP lookups SHOULD be performed 202 independently on each address. This standard does not address the 203 process a host might use to combine the lookup results. 205 3.1. ADSP Applicability 207 ADSP as defined in this document is bound to DNS. For this reason, 208 ADSP is applicable only to Author Domains with appropriate DNS 209 records (see Note below). The handling of other Author Domains is 210 outside the scope of this document. However, attackers may use such 211 domain names in a deliberate attempt to sidestep an organization's 212 ADSP policy statements. It is up to the ADSP verifier implementation 213 to return an appropriate error result for Author Domains outside the 214 scope of ADSP. 216 Note: The results from DNS queries that are intended to validate a 217 domain name unavoidably approximate the set of Author Domains that 218 can appear in legitimate email. For example, a DNS A record could 219 belong to a device that does not even have an email 220 implementation. It is up to the verifier to decide what degree of 221 approximation is acceptable. 223 3.2. ADSP Usage 225 Depending on the Author Domain(s) and the signatures in a message, a 226 recipient gets varying amounts of useful information from each ADSP 227 lookup. 229 o If a message has no Valid Signature, the ADSP result is directly 230 relevant to the message. 232 o If a message has a Valid Signature from an Author Domain, ADSP 233 provides no benefit relative to that domain since the message is 234 already known to be compliant with any possible ADSP for that 235 domain. 237 o If a message has a Valid Signature from a domain other than an 238 Author Domain, the receiver can use both the Signature and the 239 ADSP result in its evaluation of the message. 241 3.3. ADSP Results 243 An ADSP lookup for an Author Address produces one of four possible 244 results: 246 o Messages from this domain might or might not have an author 247 signature. This is the default if the domain exists in the DNS 248 but no record is found. 250 o All messages from this domain are signed. 252 o All messages from this domain are signed and discardable. 254 o The domain is not a valid mail domain. 256 4. Detailed Description 258 4.1. DNS Representation 260 ADSP records are published using the DNS TXT resource record type. 262 The RDATA for ADSP resource records is textual in format, with 263 specific syntax and semantics relating to their role in describing 264 ADSP. The "Tag=Value List" syntax described in section 3.2 of 265 [RFC4871] is used. Records not in compliance with that syntax or the 266 syntax of individual tags described in Section 4.3 MUST be ignored 267 (considered equivalent to a NODATA result) for purposes of ADSP, 268 although they MAY cause the logging of warning messages via an 269 appropriate system logging mechanism. If the RDATA contains multiple 270 character strings, the strings are logically concatenated with no 271 delimiters between the strings. 273 The ADSP record for a domain is published at a location in the 274 domain's DNS hierarchy prefixed by _adsp._domainkey.; e.g., the ADSP 275 record for example.com would be a TXT record that is published at 276 "_adsp._domainkey.example.com". A domain MUST NOT publish more than 277 one ADSP record; the semantics of an ADSP lookup that returns 278 multiple ADSP records for a single domain are undefined. (Note that 279 example.com and mail.example.com are different domains.) 281 4.2. Publication of ADSP Records 283 ADSP is intended to apply to all mail sent using the domain name 284 string of an Alleged Author. 286 Wildcards within a domain publishing ADSP records pose a particular 287 problem. This is discussed in more detail in Section 6.3. 289 4.2.1. Record Syntax 291 ADSP records use the "tag=value" syntax described in section 3.2 of 292 [RFC4871]. 294 Tags used in ADSP records are described below. Unrecognized tags 295 MUST be ignored. In the ABNF below, the FWS token is imported from 296 [RFC4871]. The ALPHA and DIGIT tokens are imported from [RFC5234]. 298 dkim= Outbound signing practices for the domain (plain-text; 299 REQUIRED). Possible values are as follows: 301 unknown The domain might sign some or all email. 303 all All mail from the domain is signed with an Author 304 Signature. 306 discardable All mail from the domain is signed with an Author 307 Signature. Furthermore, if a message arrives without a valid 308 Author Signature due to modification in transit, submission via 309 a path without access to a signing key, or other reason, the 310 domain encourages the recipient(s) to discard it. 312 ABNF: 313 adsp-dkim-tag = %x64.6b.69.6d *FWS "=" *FWS 314 ("unknown" / "all" / "discardable") 316 4.3. ADSP Lookup Procedure 318 Hosts doing an ADSP lookup MUST produce a result that is semantically 319 equivalent to applying the following steps in the order listed below. 320 In practice, these steps can be performed in parallel in order to 321 improve performance. However, implementations SHOULD avoid doing 322 unnecessary DNS lookups. 324 For the purposes of this section a "valid ADSP record" is one that is 325 both syntactically and semantically correct; in particular, it 326 matches the ABNF for a "tag-list" and includes a defined "dkim=" tag. 328 Verify Domain Scope: An ADSP verifier implementation MUST determine 329 whether a given Author Domain is within scope for ADSP. Given the 330 background in Section 3.1 the verifier MUST decide which degree of 331 over-approximation is acceptable. The verifier MUST return an 332 appropriate error result for Author Domains that are outside the 333 scope of ADSP. 335 The host MUST perform a DNS query for a record corresponding to 336 the Author Domain (with no prefix). The type of the query can be 337 of any type, since this step is only to determine if the domain 338 itself exists in DNS. This query MAY be done in parallel with the 339 query to fetch the Named ADSP Record. If the result of this query 340 is that the Author domain does not exist in the DNS (often called 341 an "NXDOMAIN" error), the algorithm MUST terminate with an error 342 indicating that the domain is out of scope. 344 NON-NORMATIVE DISCUSSION: Any resource record type could be 345 used for this query since the existence of a resource record of 346 any type will prevent an "NXDOMAIN" error. MX is a reasonable 347 choice for this purpose because this record type is thought to 348 be the most common for domains used in e-mail, and will 349 therefore produce a result which can be more readily cached 350 than a negative result. 352 If the domain does exist, the verifier MAY make more extensive 353 checks to verify the existence of the domain, such as the ones 354 described in Section 5 of [RFC2821]. If those checks indicate 355 that the Author domain does not exist for mail, e.g., the domain 356 has no MX, A, or AAAA record, the verifier SHOULD terminate with 357 an error indicating that the domain is out of scope. 359 Fetch Named ADSP Record: The host MUST query DNS for a TXT record 360 corresponding to the Author Domain prefixed by "_adsp._domainkey." 361 (note the trailing dot). 363 If the result of this query is a "NOERROR" response with an answer 364 which is a valid ADSP record, use that record, and the algorithm 365 terminates. 367 If a query results in a "SERVFAIL" error response, the algorithm 368 terminates without returning a result; possible actions include 369 queuing the message or returning an SMTP error indicating a 370 temporary failure. 372 5. IANA Considerations 374 ADSP adds the following namespaces to the IANA registry. In all 375 cases, new values are assigned only for values that have been 376 documented in a published RFC that has IETF Consensus [RFC2434]. 378 5.1. ADSP Specification Tag Registry 380 An ADSP record provides for a list of specification tags. IANA has 381 established the ADSP Specification Tag Registry for specification 382 tags that can be used in ADSP fields. 384 The initial entry in the registry is: 385 +------+-----------------+ 386 | TYPE | REFERENCE | 387 +------+-----------------+ 388 | dkim | (this document) | 389 +------+-----------------+ 390 ADSP Specification Tag Registry Initial Values 392 5.2. ADSP Outbound Signing Practices Registry 394 The "dkim=" tag spec, defined in Section 4.2.1, provides for a value 395 specifying Outbound Signing Practices. IANA has established the ADSP 396 Outbound Signing Practices Registry for Outbound Signing Practices. 398 The initial entries in the registry comprise: 399 +-------------+-----------------+ 400 | TYPE | REFERENCE | 401 +-------------+-----------------+ 402 | unknown | (this document) | 403 | all | (this document) | 404 | discardable | (this document) | 405 +-------------+-----------------+ 406 ADSP Outbound Signing Practices Registry Initial Values 408 6. Security Considerations 410 Security considerations in the ADSP are mostly related to attempts on 411 the part of malicious senders to represent themselves as authors for 412 whom they are not authorized to send mail, often in an attempt to 413 defraud either the recipient or an Alleged Author. 415 Additional security considerations regarding Author Domain Signing 416 Practices are found in the DKIM threat analysis [RFC4686]. 418 6.1. ADSP Threat Model 420 Email recipients often have a core set of content authors that they 421 already trust. Common examples include financial institutions with 422 which they have an existing relationship and Internet web transaction 423 sites with which they conduct business. 425 Email abuse often seeks to exploit a legitimate email author's name- 426 recognition among recipients, by using the author's domain name in 427 the From: header field. Especially since many popular MUAs do not 428 display the author's email address, there is no empirical evidence of 429 the extent that this particular unauthorized use of a domain name 430 contributes to recipient deception or that eliminating it will have 431 significant effect. 433 However, closing this exploit could facilitate some types of 434 optimized processing by receive-side message filtering engines, since 435 it could permit them to maintain higher-confidence assertions about 436 From: header field uses of a domain, when the occurrence is 437 authorized. 439 Unauthorized uses of domain names occur elsewhere in messages, as do 440 unauthorized uses of organizations' names. These attacks are outside 441 the scope of this specification. 443 ADSP does not provide any benefit--nor, indeed, have any effect at 444 all--unless an external system acts upon the verdict, either by 445 treating the message differently during the delivery process or by 446 showing some indicator to the end recipient. Such a system is out of 447 scope for this specification. 449 ADSP checkers may perform multiple DNS lookups per Alleged Author 450 Domain. Since these lookups are driven by domain names in email 451 message headers of possibly fraudulent email, legitimate ADSP 452 checkers can become participants in traffic multiplication attacks. 454 6.2. DNS Attacks 456 An attacker might attack the DNS infrastructure in an attempt to 457 impersonate ADSP records to influence a receiver's decision on how it 458 will handle mail. However, such an attacker is more likely to attack 459 at a higher level, e.g., redirecting A or MX record lookups in order 460 to capture traffic that was legitimately intended for the target 461 domain. These DNS security issues are addressed by DNSSEC [RFC4033]. 463 Because ADSP operates within the framework of the legacy e-mail 464 system, the default result in the absence of an ADSP record is that 465 the domain does not sign all of its messages. It is therefore 466 important that the ADSP clients distinguish a DNS failure such as 467 "SERVFAIL" from other DNS errors so that appropriate actions can be 468 taken. 470 6.3. DNS Wildcards 472 If a domain contains wildcards, then any name that matches the 473 wildcard according to [RFC4592] is potentially a valid mail domain 474 eligible for ADSP. It is possible to add a wildcard TXT record 475 alongside a wildcard MX that will provide suitable ADSP records for 476 any domain chosen by an attacker, since if the wildcard synthesizes 477 chosen-name.example.com IN MX, it will then also synthesize 478 _adsp._domainkey.chosen-name.example.com IN TXT. However multiple 479 wildcard TXT records produce an undefined ADSP result, which means 480 you cannot also publish both ADSP records and records for any other 481 TXT-using protocol (such as SPF) for a wildcard domain. 483 7. References 485 7.1. References - Normative 487 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 488 Requirement Levels", BCP 14, RFC 2119, March 1997. 490 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 491 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 492 October 1998. 494 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, 495 April 2001. 497 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 498 Rose, "DNS Security Introduction and Requirements", 499 RFC 4033, March 2005. 501 [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys 502 Identified Mail (DKIM)", RFC 4686, September 2006. 504 [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, 505 J., and M. Thomas, "DomainKeys Identified Mail (DKIM) 506 Signatures", RFC 4871, May 2007. 508 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 509 Specifications: ABNF", STD 68, RFC 5234, January 2008. 511 7.2. References - Informative 513 [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, 514 April 2001. 516 [RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail 517 (DKIM) Signing Practices Protocol", RFC 5016, 518 October 2007. 520 Appendix A. Usage Examples 522 These examples are intended to illustrate typical uses of ADSP. They 523 are not intended to be exhaustive, nor to apply to every domain's or 524 mail system's individual situation. 526 Domain managers are advised to consider the ways that mail processing 527 can modify messages in ways that will invalidate an existing DKIM 528 signature, such as mailing lists, courtesy forwarders, and other 529 paths that could add or modify headers, or modify the message body. 530 In that case, if the modifications invalidate the DKIM signature, 531 recipient hosts will consider the mail not to have an Author 532 Signature, even though the signature was present when the mail was 533 originally sent. 535 A.1. Single Location Domains 537 A common mail system configuration handles all of a domain's users' 538 incoming and outgoing mail through a single MTA or group of MTAs. In 539 that case, the MTA(s) can be configured to sign outgoing mail with an 540 Author Signature. 542 In this situation it might be appropriate to publish an ADSP record 543 for the domain containing "all", depending on whether the users also 544 send mail through other paths that do not apply an Author Signature. 545 Such paths could include MTAs at hotels or hotspot networks used by 546 travelling users, or web sites that provide "mail an article" 547 features. 549 A.2. Bulk Mailing Domains 551 Another common configuration uses a domain solely for bulk or 552 broadcast mail, with no individual human users, again typically 553 sending all the mail through a single MTA or group of MTAs that can 554 apply an Author Signature. In this case, the domain's management can 555 be confident that all of its outgoing mail will be sent through the 556 signing MTA. Lacking individual users, the domain is unlikely to 557 participate in mailing lists, but could still send mail through other 558 paths that might invalidate signatures. 560 Domain owners often use specialist mailing providers to send their 561 bulk mail. In that case, the mailing provider needs access to a 562 suitable signing key in order to apply an Author Signature. One 563 possible route would be for the domain owner to generate the key and 564 give it to the mailing provider. Another would be for the domain to 565 delegate a subdomain to the mailing provider, for example, 566 bigbank.example might delegate email.bigbank.example to such a 567 provider. In that case, the provider can generate the keys and DKIM 568 DNS records itself and use the subdomain in the Author address in the 569 mail. 571 Regardless of the DNS and key management strategy chosen, whoever 572 maintains the DKIM records for the domain could also install an ADSP 573 record containing "all". 575 A.3. Bulk Mailing Domains with Discardable Mail 577 In some cases, a domain might sign all of its outgoing mail with an 578 Author Signature, but prefer that recipient systems discard mail 579 without a valid Author Signature to avoid confusion from mail sent 580 from sources that do not apply an Author Signature. (This latter 581 kind of mail is sometimes loosely called "forgeries".) In that case, 582 it might be appropriate to publish an ADSP record containing 583 "discardable". Note that a domain SHOULD NOT publish a "discardable" 584 record if it wishes to maximize the likelihood that mail from the 585 domain is delivered, since it could cause some fraction of the mail 586 the domain sends to be discarded. 588 A.4. Third Party Senders 590 Another common use case is for a third party to enter into an 591 agreement whereby that third party will send bulk or other mail on 592 behalf of a designated author or author domain, using that domain in 593 the RFC2822 From: or other headers. Due to the many and varied 594 complexities of such agreements, third party signing is not addressed 595 in this specification. 597 A.5. Non-email Domains 599 If a domain sends no mail at all, it can safely publish a 600 "discardable" ADSP record, since any mail with an author address in 601 the domain is a forgery. 603 Appendix B. Acknowledgements 605 This document greatly benefited from comments by Steve Atkins, Jon 606 Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael 607 Thomas, and Wietse Venema. 609 Appendix C. Change Log 611 *NOTE TO RFC EDITOR: This section may be removed upon publication of 612 this document as an RFC.* 614 C.1. Changes since -ietf-dkim-03 616 o Name change for title and filename, to be ADSP 618 o String changes throughout, to author Domain signing practices and 619 to aDsp. 621 o Added some keywords. 623 o Clarified comparison of local part and domain in Author Address. 625 o Streamlined the Abstract. 627 o Revised text of last bullet in Results list. 629 o Removed definitions not used in the document. 631 o Removed all specification details pertaining to sub-domains. 633 o Moved Lookup Procedure up one document level. 635 o Revised domain validity specification. Part in ADSP Usage in 636 Operations section, and part as it as first step in Lookup. 638 o Fixed xml for figures, including labeling ABNF with new xml2rfc 639 construct. 641 o Revised wildcard text. 643 o Removed 't' tag. 645 o Removed ADSP Flags Registry section. 647 o Changed ABNF use of whitespace from WSP back to FWS, for 648 consistency with dkim-base. 650 C.2. Changes since -ietf-dkim-02 652 o Merge in more text from ADSP draft. 654 o Phrase actions as host's rather than checker. 656 o Explanatory description of i= matching. 658 o Lookup procedure consistently refers to one ADSP record per 659 lookup. 661 o Update security section w/ language from W. Venema 663 o Simplify imports of terms from other RFCs, add Local-part, 4234 -> 664 5234. 666 o Add usage example appendix. 668 o Add IANA considerations. 670 o Update authors list 672 C.3. Changes since -ietf-dkim-ssp-01 674 o Reworded introduction for clarity. 676 o Various definition clarifications. 678 o Changed names of practices to unknown, all, and discardable. 680 o Removed normative language mandating use of SSP in particular 681 situations (issue 1538). 683 o Clarified possible confusion over handling of syntax errors. 685 o Removed normative language from Introduction (issue 1538). 687 o Changed "Originator" to "Author" throughout (issue 1529). 689 o Removed all references to Third-Party Signatures (issues 1512, 690 1521). 692 o Removed all mention of "Suspicious" (issues 1528, 1530). 694 o Removed "t=y" (testing) flag (issue 1540). 696 o Removed "handling" tag (issue 1513). 698 o Broke up the "Sender Signing Practices Check Procedure" into two 699 algorithms: fetching the SSP record and interpretation thereof 700 (issues 1531, 1535; partially addresses issue 1520). 701 Interpretation is now the responsibility of the Evaluator. 703 o Document restructuring for better flow and remove redundancies 704 (some may address issue 1523, but I'm not sure I understand that 705 issue completely; also issues 1532, 1537). 707 o Removed all mention of how this interacts with users, even though 708 it makes parts of the document harder to understand (issue 1526). 710 o Introduced the concepts of "SSP Checker" and "Evaluator". 712 o Multiple author case now handled my separate invocations of SSP 713 checker by Evaluator (issue 1525). 715 o Removed check to avoid querying top-level domains. 717 o Changed ABNF use of whitespace from [FWS] to *WSP (partially 718 addresses issue 1543). 720 C.4. Changes since -ietf-dkim-ssp-00 722 o Clarified Operation Overview and eliminated use of Legitimate as 723 the counterpart of Suspicious since the words have different 724 meanings. 726 o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT 727 records in DNS vs. a new RR type. 729 o Clarified publication rules for multilevel names. 731 o Better description of overall record syntax, in particular that 732 records with unknown tags are considered syntactically correct. 734 o Clarified Sender Signing Practices Check Procedure, primarily by 735 use of new term Author Domain. 737 o Eliminated section "Third-Party Signatures and Mailing Lists" that 738 is better included in the DKIM overview document. 740 o Added "handling" tag to express alleged sending domain's 741 preference about handling of Suspicious messages. 743 o Clarified handling of SERVFAIL error in SSP check. 745 o Replaced "entity" with "domain", since with the removal of user- 746 granularity SSP, the only entities having sender signing policies 747 are domains. 749 C.5. Changes since -allman-ssp-02 751 o Removed user-granularity SSP and u= tag. 753 o Replaced DKIMP resource record with a TXT record. 755 o Changed name of the primary tag from "p" to "dkim". 757 o Replaced lookup algorithm with one which traverses upward at most 758 one level. 760 o Added description of records to be published, and effect of 761 wildcard records within the domain, on SSP. 763 C.6. Changes since -allman-ssp-01 765 o Changed term "Sender Signing Policy" to "Sender Signing 766 Practices". 768 o Changed query methodology to use a separate DNS resource record 769 type, DKIMP. 771 o Changed tag values from SPF-like symbols to words. 773 o User level policies now default to that of the domain if not 774 specified. 776 o Removed the "Compliance" section since we're still not clear on 777 what goes here. 779 o Changed the "parent domain" policy to only search up one level 780 (assumes that subdomains will publish SSP records if appropriate). 782 o Added detailed description of SSP check procedure. 784 C.7. Changes since -allman-ssp-00 786 From a "diff" perspective, the changes are extensive. Semantically, 787 the changes are: 789 o Added section on "Third-Party Signatures and Mailing Lists" 791 o Added "Compliance" (transferred from -base document). I'm not 792 clear on what needs to be done here. 794 o Extensive restructuring. 796 Authors' Addresses 798 Eric Allman 799 Sendmail, Inc. 800 6475 Christie Ave, Suite 350 801 Emeryville, CA 94608 803 Phone: +1 510 594 5501 804 Email: eric+dkim@sendmail.org 806 Jim Fenton 807 Cisco Systems, Inc. 808 MS SJ-9/2 809 170 W. Tasman Drive 810 San Jose, CA 95134-1706 812 Phone: +1 408 526 5914 813 Email: fenton@cisco.com 815 Mark Delany 816 Yahoo! Inc. 817 701 First Avenue 818 Sunnyvale, CA 94089 820 Phone: +1 408 349 6831 821 Email: markd+dkim@yahoo-inc.com 823 John Levine 824 Taughannock Networks 825 PO Box 727 826 Trumansburg, NY 14886 828 Phone: +1 831 480 2300 829 Email: standards@taugh.com 830 URI: http://www.taugh.com 832 Full Copyright Statement 834 Copyright (C) The IETF Trust (2008). 836 This document is subject to the rights, licenses and restrictions 837 contained in BCP 78, and except as set forth therein, the authors 838 retain all their rights. 840 This document and the information contained herein are provided on an 841 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 842 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 843 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 844 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 845 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 846 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 848 Intellectual Property 850 The IETF takes no position regarding the validity or scope of any 851 Intellectual Property Rights or other rights that might be claimed to 852 pertain to the implementation or use of the technology described in 853 this document or the extent to which any license under such rights 854 might or might not be available; nor does it represent that it has 855 made any independent effort to identify any such rights. 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