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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: February 6, 2009 Cisco Systems, Inc. 6 M. Delany 7 Yahoo! Inc. 8 J. Levine 9 Taughannock Networks 10 August 5, 2008 12 DKIM Author Domain Signing Practices (ADSP) 13 draft-ietf-dkim-ssp-05 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 February 6, 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 a domain signs its outgoing mail, 48 and how other hosts can access that record. 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 . . . . . . . . . . . . . . . 7 68 4.3. ADSP Lookup Procedure . . . . . . . . . . . . . . . . . . 7 69 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 70 5.1. ADSP Specification Tag Registry . . . . . . . . . . . . . 9 71 5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 9 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 73 6.1. ADSP Threat Model . . . . . . . . . . . . . . . . . . . . 10 74 6.2. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 10 75 6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 11 76 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 77 7.1. References - Normative . . . . . . . . . . . . . . . . . . 11 78 7.2. References - Informative . . . . . . . . . . . . . . . . . 12 79 Appendix A. Usage Examples . . . . . . . . . . . . . . . . . . . 12 80 A.1. Single Location Domains . . . . . . . . . . . . . . . . . 12 81 A.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 13 82 A.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 13 83 A.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 13 84 A.5. Domains with Independent Users and Liberal Use Policies . 14 85 A.6. Non-email Domains . . . . . . . . . . . . . . . . . . . . 14 86 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 14 87 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 14 88 C.1. Changes since -ietf-dkim-04 . . . . . . . . . . . . . . . 14 89 C.2. Changes since -ietf-dkim-03 . . . . . . . . . . . . . . . 14 90 C.3. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 15 91 C.4. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 16 92 C.5. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 17 93 C.6. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 17 94 C.7. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 18 95 C.8. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 18 96 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 97 Intellectual Property and Copyright Statements . . . . . . . . . . 20 99 1. Introduction 101 DomainKeys Identified Mail (DKIM) defines a mechanism by which email 102 messages can be cryptographically signed, permitting a signing domain 103 to claim responsibility for the introduction of a message into the 104 mail stream. Message recipients can verify the signature by querying 105 the signer's domain directly to retrieve the appropriate public key, 106 and thereby confirm that the message was attested to by a party in 107 possession of the private key for the signing domain. 109 However, the legacy of the Internet is such that not all messages 110 will be signed, and the absence of a signature on a message is not an 111 a priori indication of forgery. In fact, during early phases of 112 deployment it is very likely that most messages will remain unsigned. 113 However, some domains might decide to sign all of their outgoing 114 mail, for example, to protect their brand names. It is desirable for 115 such domains to be able to advertise that fact to other hosts. This 116 is the topic of Author Domain Signing Practices (ADSP). 118 Hosts implementing this specification can inquire what Author Signing 119 Practices a domain advertises. This inquiry is called an Author 120 Signing Practices check. 122 The basic requirements for ADSP are given in [RFC5016]. This 123 document refers extensively to [RFC4871] and assumes the reader is 124 familiar with it. 126 Requirements Notation: The key words "MUST", "MUST NOT", 127 "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", 128 "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be 129 interpreted as described in [RFC2119] 131 2. Language and Terminology 133 2.1. Terms Imported from DKIM Signatures Specification 135 Some terminology used herein is derived directly from [RFC4871]. In 136 several cases, references in that document to Sender have been 137 changed to Author here, to emphasize the relationship to the Author 138 address(es) in the From: header field described in [RFC2822]. 139 Briefly, 141 o A "Signer" is the agent that signs a message, as defined in 142 section 2.1 of [RFC4871]. 144 o A "Local-part" is the part of an address preceding the @ 145 character, as defined in [RFC2822] and used in [RFC4871]. 147 2.2. Valid Signature 149 A "Valid Signature" is any signature on a message which correctly 150 verifies using the procedure described in section 6.1 of [RFC4871]. 152 2.3. Author Address 154 An "Author Address" is an email address in the From header field of a 155 message [RFC2822]. If the From header field contains multiple 156 addresses, the message has multiple Author Addresses. 158 2.4. Author Domain 160 An "Author Domain" is everything to the right of the "@" in an Author 161 Address (excluding the "@" itself). 163 2.5. Alleged Author 165 An "Alleged Author" is an Author Address of a message; it is 166 "alleged" because it has not yet been checked. 168 2.6. Author Domain Signing Practices 170 "Author Domain Signing Practices" (or just "practices") consist of a 171 machine-readable record published by the domain of an Alleged Author 172 which includes statements about the domain's practices with respect 173 to mail it sends with its domain in the From: line. 175 2.7. Author Signature 177 An "Author Signature" is any Valid Signature where the identity of 178 the user or agent on behalf of which the message is signed (listed in 179 the "i=" tag or its default value from the "d=" tag) matches an 180 Author Address in the message. When the identity of the user or 181 agent includes a Local-part, the identities match if the Local-parts 182 are the same string, and the domains are the same string. Otherwise, 183 the identities match if the domains are the same string. Following 184 [RFC2821], Local-part comparisons are case sensitive, domain 185 comparisons are case insensitive. 187 For example, if a message has a Valid Signature, with the DKIM- 188 Signature field containing "i=a@domain.example", then domain.example 189 is asserting that it takes responsibility for the message. If the 190 message's From: field contains the address "b@domain.example" and an 191 ADSP query produces a "dkim=all" or "dkim=discardable" result, that 192 would mean that the message does not have a valid Author Signature. 193 Even though the message is signed by the same domain, it fails to 194 satisfy ADSP. 196 3. Operation Overview 198 Domain owners can publish ADSP information via a query mechanism such 199 as the Domain Name System; specific details are given in Section 4.1. 201 Hosts can look up the ADSP information of the domain(s) specified by 202 the Author Address(es) as described in Section 4.3. If a message has 203 multiple Author Addresses the ADSP lookups SHOULD be performed 204 independently on each address. This standard does not address the 205 process a host might use to combine the lookup results. 207 3.1. ADSP Applicability 209 ADSP as defined in this document is bound to DNS. For this reason, 210 ADSP is applicable only to Author Domains with appropriate DNS 211 records (see Note below). The handling of other Author Domains is 212 outside the scope of this document. However, attackers may use such 213 domain names in a deliberate attempt to sidestep an organization's 214 ADSP policy statements. It is up to the ADSP checker implementation 215 to return an appropriate error result for Author Domains outside the 216 scope of ADSP. 218 Note: The results from DNS queries that are intended to validate a 219 domain name unavoidably approximate the set of Author Domains that 220 can appear in legitimate email. For example, a DNS A record could 221 belong to a device that does not even have an email 222 implementation. It is up to the checker to decide what degree of 223 approximation is acceptable. 225 3.2. ADSP Usage 227 Depending on the Author Domain(s) and the signatures in a message, a 228 recipient gets varying amounts of useful information from each ADSP 229 lookup. 231 o If a message has no Valid Signature, the ADSP result is directly 232 relevant to the message. 234 o If a message has a Valid Signature from an Author Domain, ADSP 235 provides no benefit relative to that domain since the message is 236 already known to be compliant with any possible ADSP for that 237 domain. 239 o If a message has a Valid Signature from a domain other than an 240 Author Domain, the receiver can use both the Signature and the 241 ADSP result in its evaluation of the message. 243 3.3. ADSP Results 245 An ADSP lookup for an Author Address produces one of four possible 246 results: 248 o Messages from this domain might or might not have an author 249 signature. This is the default if the domain exists in the DNS 250 but no record is found. 252 o All messages from this domain are signed. 254 o All messages from this domain are signed and discardable. 256 o The domain is not a valid mail domain. 258 4. Detailed Description 260 4.1. DNS Representation 262 ADSP records are published using the DNS TXT resource record type. 264 The RDATA for ADSP resource records is textual in format, with 265 specific syntax and semantics relating to their role in describing 266 ADSP. The "Tag=Value List" syntax described in section 3.2 of 267 [RFC4871] is used. Records not in compliance with that syntax or the 268 syntax of individual tags described in Section 4.3 MUST be ignored 269 (considered equivalent to a NODATA result) for purposes of ADSP, 270 although they MAY cause the logging of warning messages via an 271 appropriate system logging mechanism. If the RDATA contains multiple 272 character strings, the strings are logically concatenated with no 273 delimiters between the strings. 275 The ADSP record for a domain is published at a location in the 276 domain's DNS hierarchy prefixed by _adsp._domainkey.; e.g., the ADSP 277 record for example.com would be a TXT record that is published at 278 "_adsp._domainkey.example.com". A domain MUST NOT publish more than 279 one ADSP record; the semantics of an ADSP lookup that returns 280 multiple ADSP records for a single domain are undefined. (Note that 281 example.com and mail.example.com are different domains.) 283 ADSP records MUST NOT be published at any location other than 284 the_adsp._domainkey subdomain of the domain for which they are 285 expressing signing practices. In particular, domains MUST NOT 286 publish ADSP records with wildcard names. Wildcards within a domain 287 publishing ADSP records pose a particular problem, as discussed in 288 more detail in Section 6.3. 290 4.2. Publication of ADSP Records 292 ADSP is intended to apply to all mail sent using the domain name 293 string of an Alleged Author. 295 4.2.1. Record Syntax 297 ADSP records use the "tag=value" syntax described in section 3.2 of 298 [RFC4871]. Every ADSP record MUST start with an outbound signing 299 practices tag, so the first four characters of the record are lower 300 case "dkim". 302 Tags used in ADSP records are described below. Unrecognized tags 303 MUST be ignored. In the ABNF below, the WSP token, and the ALPHA and 304 DIGIT tokens are imported from [RFC5234]. 306 dkim= Outbound signing practices for the domain (plain-text; 307 REQUIRED). Possible values are as follows: 309 unknown The domain might sign some or all email. 311 all All mail from the domain is signed with an Author 312 Signature. 314 discardable All mail from the domain is signed with an Author 315 Signature. Furthermore, if a message arrives without a valid 316 Author Signature due to modification in transit, submission via 317 a path without access to a signing key, or other reason, the 318 domain encourages the recipient(s) to discard it. 320 ABNF: 321 adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP 322 ("unknown" / "all" / "discardable") 324 4.3. ADSP Lookup Procedure 326 Hosts doing an ADSP lookup MUST produce a result that is semantically 327 equivalent to applying the following steps in the order listed below. 328 In practice, these steps can be performed in parallel in order to 329 improve performance. However, implementations SHOULD avoid doing 330 unnecessary DNS lookups. 332 For the purposes of this section a "valid ADSP record" is one that is 333 both syntactically and semantically correct; in particular, it 334 matches the ABNF for a "tag-list" and includes a valid "dkim" tag. 336 Check Domain Scope: An ADSP checker implementation MUST determine 337 whether a given Author Domain is within scope for ADSP. Given the 338 background in Section 3.1 the checker MUST decide which degree of 339 approximation is acceptable. The checker MUST return an 340 appropriate error result for Author Domains that are outside the 341 scope of ADSP. 343 The host MUST perform a DNS query for a record corresponding to 344 the Author Domain (with no prefix). The type of the query can be 345 of any type, since this step is only to determine if the domain 346 itself exists in DNS. This query MAY be done in parallel with the 347 query to fetch the Named ADSP Record. If the result of this query 348 is that the Author domain does not exist in the DNS (often called 349 an "NXDOMAIN" error), the algorithm MUST terminate with an error 350 indicating that the domain is out of scope. 352 NON-NORMATIVE DISCUSSION: Any resource record type could be 353 used for this query since the existence of a resource record of 354 any type will prevent an "NXDOMAIN" error. MX is a reasonable 355 choice for this purpose because this record type is thought to 356 be the most common for domains used in e-mail, and will 357 therefore produce a result which can be more readily cached 358 than a negative result. 360 If the domain does exist, the checker MAY make more extensive 361 checks to verify the existence of the domain, such as the ones 362 described in Section 5 of [RFC2821]. If those checks indicate 363 that the Author domain does not exist for mail, e.g., the domain 364 has no MX, A, or AAAA record, the checker SHOULD terminate with an 365 error indicating that the domain is out of scope. 367 Fetch Named ADSP Record: The host MUST query DNS for a TXT record 368 corresponding to the Author Domain prefixed by "_adsp._domainkey." 369 (note the trailing dot). 371 If the result of this query is a "NOERROR" response with an answer 372 which is a valid ADSP record, use that record, and the algorithm 373 terminates. 375 If a query results in a "SERVFAIL" error response, the algorithm 376 terminates without returning a result; possible actions include 377 queuing the message or returning an SMTP error indicating a 378 temporary failure. 380 5. IANA Considerations 382 ADSP adds the following namespaces to the IANA registry. In all 383 cases, new values are assigned only for values that have been 384 documented in a published RFC that has IETF Consensus [RFC5226]. 386 5.1. ADSP Specification Tag Registry 388 An ADSP record provides for a list of specification tags. IANA has 389 established the ADSP Specification Tag Registry for specification 390 tags that can be used in ADSP fields. 392 The initial entry in the registry is: 393 +------+-----------------+ 394 | TYPE | REFERENCE | 395 +------+-----------------+ 396 | dkim | (this document) | 397 +------+-----------------+ 398 ADSP Specification Tag Registry Initial Values 400 5.2. ADSP Outbound Signing Practices Registry 402 The "dkim=" tag spec, defined in Section 4.2.1, provides for a value 403 specifying Outbound Signing Practices. IANA has established the ADSP 404 Outbound Signing Practices Registry for Outbound Signing Practices. 406 The initial entries in the registry comprise: 407 +-------------+-----------------+ 408 | TYPE | REFERENCE | 409 +-------------+-----------------+ 410 | unknown | (this document) | 411 | all | (this document) | 412 | discardable | (this document) | 413 +-------------+-----------------+ 414 ADSP Outbound Signing Practices Registry Initial Values 416 6. Security Considerations 418 Security considerations in the ADSP are mostly related to attempts on 419 the part of malicious senders to represent themselves as authors for 420 whom they are not authorized to send mail, often in an attempt to 421 defraud either the recipient or an Alleged Author. 423 Additional security considerations regarding Author Domain Signing 424 Practices are found in the DKIM threat analysis [RFC4686]. 426 6.1. ADSP Threat Model 428 Email recipients often have a core set of content authors that they 429 already trust. Common examples include financial institutions with 430 which they have an existing relationship and Internet web transaction 431 sites with which they conduct business. 433 Email abuse often seeks to exploit a legitimate email author's name- 434 recognition among recipients, by using the author's domain name in 435 the From: header field. Especially since many popular MUAs do not 436 display the author's email address, there is no empirical evidence of 437 the extent that this particular unauthorized use of a domain name 438 contributes to recipient deception or that eliminating it will have 439 significant effect. 441 However, closing this exploit could facilitate some types of 442 optimized processing by receive-side message filtering engines, since 443 it could permit them to maintain higher-confidence assertions about 444 From: header field uses of a domain, when the occurrence is 445 authorized. 447 Unauthorized uses of domain names occur elsewhere in messages, as do 448 unauthorized uses of organizations' names. These attacks are outside 449 the scope of this specification. 451 ADSP does not provide any benefit--nor, indeed, have any effect at 452 all--unless an external system acts upon the verdict, either by 453 treating the message differently during the delivery process or by 454 showing some indicator to the end recipient. Such a system is out of 455 scope for this specification. 457 ADSP checkers may perform multiple DNS lookups per Alleged Author 458 Domain. Since these lookups are driven by domain names in email 459 message headers of possibly fraudulent email, legitimate ADSP 460 checkers can become participants in traffic multiplication attacks. 462 6.2. DNS Attacks 464 An attacker might attack the DNS infrastructure in an attempt to 465 impersonate ADSP records to influence a receiver's decision on how it 466 will handle mail. However, such an attacker is more likely to attack 467 at a higher level, e.g., redirecting A or MX record lookups in order 468 to capture traffic that was legitimately intended for the target 469 domain. These DNS security issues are addressed by DNSSEC [RFC4033]. 471 Because ADSP operates within the framework of the legacy e-mail 472 system, the default result in the absence of an ADSP record is that 473 the domain does not sign all of its messages. It is therefore 474 important that the ADSP clients distinguish a DNS failure such as 475 "SERVFAIL" from other DNS errors so that appropriate actions can be 476 taken. 478 6.3. DNS Wildcards 480 DNS wildcards (described in [RFC4592]) that exist in the DNS 481 hierarchy at or above the domain being checked interfere with the 482 ability to verify the scope of the ADSP check described in 483 Section 4.3. For example, a wildcard record for *.example.com makes 484 all subdomains such as foo.example.com exist in the DNS. Domains 485 that intend to make active use of ADSP by publishing a practice other 486 than Unknown are advised to avoid the use of wildcards elsewhere in 487 their hierarchy. 489 If a domain contains wildcards, then any name that matches the 490 wildcard can appear to be a valid mail domain eligible for ADSP. But 491 the "_adsp._domainkey." prefix on ADSP records does not allow 492 publication of wildcard records that cover ADSP records without also 493 covering non-ADSP records, nor of wildcard records that cover non- 494 ADSP records without also covering ADSP records. Hence a domain MUST 495 NOT publish wildcard ADSP records. 497 7. References 499 7.1. References - Normative 501 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 502 Requirement Levels", BCP 14, RFC 2119, March 1997. 504 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, 505 April 2001. 507 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 508 Rose, "DNS Security Introduction and Requirements", 509 RFC 4033, March 2005. 511 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name 512 System", RFC 4592, July 2006. 514 [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, 515 J., and M. Thomas, "DomainKeys Identified Mail (DKIM) 516 Signatures", RFC 4871, May 2007. 518 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 519 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 520 May 2008. 522 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 523 Specifications: ABNF", STD 68, RFC 5234, January 2008. 525 7.2. References - Informative 527 [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, 528 April 2001. 530 [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys 531 Identified Mail (DKIM)", RFC 4686, September 2006. 533 [RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail 534 (DKIM) Signing Practices Protocol", RFC 5016, 535 October 2007. 537 Appendix A. Usage Examples 539 These examples are intended to illustrate typical uses of ADSP. They 540 are not intended to be exhaustive, nor to apply to every domain's or 541 mail system's individual situation. 543 Domain managers are advised to consider the ways that mail processing 544 can modify messages in ways that will invalidate an existing DKIM 545 signature, such as mailing lists, courtesy forwarders, and other 546 paths that could add or modify headers, or modify the message body. 547 In that case, if the modifications invalidate the DKIM signature, 548 recipient hosts will consider the mail not to have an Author 549 Signature, even though the signature was present when the mail was 550 originally sent. 552 A.1. Single Location Domains 554 A common mail system configuration handles all of a domain's users' 555 incoming and outgoing mail through a single MTA or group of MTAs. In 556 that case, the MTA(s) can be configured to sign outgoing mail with an 557 Author Signature. 559 In this situation it might be appropriate to publish an ADSP record 560 for the domain containing "all", depending on whether the users also 561 send mail through other paths that do not apply an Author Signature. 562 Such paths could include MTAs at hotels or hotspot networks used by 563 travelling users, web sites that provide "mail an article" features, 564 user messages sent through mailing lists, or third party mail clients 565 that support multiple user identities. 567 A.2. Bulk Mailing Domains 569 Another common configuration uses a domain solely for bulk or 570 broadcast mail, with no individual human users, again typically 571 sending all the mail through a single MTA or group of MTAs that can 572 apply an Author Signature. In this case, the domain's management can 573 be confident that all of its outgoing mail will be sent through the 574 signing MTA. Lacking individual users, the domain is unlikely to 575 participate in mailing lists, but could still send mail through other 576 paths that might invalidate signatures. 578 Domain owners often use specialist mailing providers to send their 579 bulk mail. In that case, the mailing provider needs access to a 580 suitable signing key in order to apply an Author Signature. One 581 possible route would be for the domain owner to generate the key and 582 give it to the mailing provider. Another would be for the domain to 583 delegate a subdomain to the mailing provider, for example, 584 bigbank.example might delegate email.bigbank.example to such a 585 provider. In that case, the provider can generate the keys and DKIM 586 DNS records itself and use the subdomain in the Author address in the 587 mail. 589 Regardless of the DNS and key management strategy chosen, whoever 590 maintains the DKIM records for the domain could also install an ADSP 591 record containing "all". 593 A.3. Bulk Mailing Domains with Discardable Mail 595 In some cases, a domain might sign all of its outgoing mail with an 596 Author Signature, but prefer that recipient systems discard mail 597 without a valid Author Signature to avoid confusion from mail sent 598 from sources that do not apply an Author Signature. (In the case of 599 domains with tightly controlled outgoing mail, this latter kind of 600 mail is sometimes loosely called "forgeries".) In that case, it 601 might be appropriate to publish an ADSP record containing 602 "discardable". Note that a domain SHOULD NOT publish a "discardable" 603 record if it wishes to maximize the likelihood that mail from the 604 domain is delivered, since it could cause some fraction of the mail 605 the domain sends to be discarded. 607 A.4. Third Party Senders 609 Another common use case is for a third party to enter into an 610 agreement whereby that third party will send bulk or other mail on 611 behalf of a designated author or author domain, using that domain in 612 the RFC2822 From: or other headers. Due to the many and varied 613 complexities of such agreements, third party signing is not addressed 614 in this specification. 616 A.5. Domains with Independent Users and Liberal Use Policies 618 When a domain has independent users and its usage policy does not 619 explicitly restrict them to sending mail only from designated mail 620 servers (e.g. many ISP domains and even some corporate domains), then 621 it is only appropriate to publish an ADSP record containing 622 "unknown". Publishing either "all" or "discardable" will likely 623 result in significant breakage because independent users are likely 624 to send mail from the external paths enumerated in Appendix A.1. 626 A.6. Non-email Domains 628 If a domain sends no mail at all, it can safely publish a 629 "discardable" ADSP record, since any mail with an author address in 630 the domain is a forgery. 632 Appendix B. Acknowledgements 634 This document greatly benefited from comments by Steve Atkins, Jon 635 Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael 636 Thomas, and Wietse Venema. 638 Appendix C. Change Log 640 *NOTE TO RFC EDITOR: This section may be removed upon publication of 641 this document as an RFC.* 643 C.1. Changes since -ietf-dkim-04 645 o Require dkim at the front of each record. 647 o Disparage wildcard records. 649 o Changed ABNF use of whitespace from FWS back to WSP, dkim-base is 650 wrong. 652 o RFC 2434 -> 5226, make ref to 4686 informational since it's not 653 standards track. 655 o Improve examples with material from Ellen. 657 C.2. Changes since -ietf-dkim-03 659 o Name change for title and filename, to be ADSP 660 o String changes throughout, to author Domain signing practices and 661 to aDsp. 663 o Added some keywords. 665 o Clarified comparison of local part and domain in Author Address. 667 o Streamlined the Abstract. 669 o Revised text of last bullet in Results list. 671 o Removed definitions not used in the document. 673 o Removed all specification details pertaining to sub-domains. 675 o Moved Lookup Procedure up one document level. 677 o Revised domain validity specification. Part in ADSP Usage in 678 Operations section, and part as it as first step in Lookup. 680 o Fixed xml for figures, including labeling ABNF with new xml2rfc 681 construct. 683 o Revised wildcard text. 685 o Removed 't' tag. 687 o Removed ADSP Flags Registry section. 689 o Changed ABNF use of whitespace from WSP back to FWS, for 690 consistency with dkim-base. 692 C.3. Changes since -ietf-dkim-02 694 o Merge in more text from ADSP draft. 696 o Phrase actions as host's rather than checker. 698 o Explanatory description of i= matching. 700 o Lookup procedure consistently refers to one ADSP record per 701 lookup. 703 o Update security section w/ language from W. Venema 705 o Simplify imports of terms from other RFCs, add Local-part, 4234 -> 706 5234. 708 o Add usage example appendix. 710 o Add IANA considerations. 712 o Update authors list 714 C.4. Changes since -ietf-dkim-ssp-01 716 o Reworded introduction for clarity. 718 o Various definition clarifications. 720 o Changed names of practices to unknown, all, and discardable. 722 o Removed normative language mandating use of SSP in particular 723 situations (issue 1538). 725 o Clarified possible confusion over handling of syntax errors. 727 o Removed normative language from Introduction (issue 1538). 729 o Changed "Originator" to "Author" throughout (issue 1529). 731 o Removed all references to Third-Party Signatures (issues 1512, 732 1521). 734 o Removed all mention of "Suspicious" (issues 1528, 1530). 736 o Removed "t=y" (testing) flag (issue 1540). 738 o Removed "handling" tag (issue 1513). 740 o Broke up the "Sender Signing Practices Check Procedure" into two 741 algorithms: fetching the SSP record and interpretation thereof 742 (issues 1531, 1535; partially addresses issue 1520). 743 Interpretation is now the responsibility of the Evaluator. 745 o Document restructuring for better flow and remove redundancies 746 (some may address issue 1523, but I'm not sure I understand that 747 issue completely; also issues 1532, 1537). 749 o Removed all mention of how this interacts with users, even though 750 it makes parts of the document harder to understand (issue 1526). 752 o Introduced the concepts of "SSP Checker" and "Evaluator". 754 o Multiple author case now handled my separate invocations of SSP 755 checker by Evaluator (issue 1525). 757 o Removed check to avoid querying top-level domains. 759 o Changed ABNF use of whitespace from [FWS] to *WSP (partially 760 addresses issue 1543). 762 C.5. Changes since -ietf-dkim-ssp-00 764 o Clarified Operation Overview and eliminated use of Legitimate as 765 the counterpart of Suspicious since the words have different 766 meanings. 768 o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT 769 records in DNS vs. a new RR type. 771 o Clarified publication rules for multilevel names. 773 o Better description of overall record syntax, in particular that 774 records with unknown tags are considered syntactically correct. 776 o Clarified Sender Signing Practices Check Procedure, primarily by 777 use of new term Author Domain. 779 o Eliminated section "Third-Party Signatures and Mailing Lists" that 780 is better included in the DKIM overview document. 782 o Added "handling" tag to express alleged sending domain's 783 preference about handling of Suspicious messages. 785 o Clarified handling of SERVFAIL error in SSP check. 787 o Replaced "entity" with "domain", since with the removal of user- 788 granularity SSP, the only entities having sender signing policies 789 are domains. 791 C.6. Changes since -allman-ssp-02 793 o Removed user-granularity SSP and u= tag. 795 o Replaced DKIMP resource record with a TXT record. 797 o Changed name of the primary tag from "p" to "dkim". 799 o Replaced lookup algorithm with one which traverses upward at most 800 one level. 802 o Added description of records to be published, and effect of 803 wildcard records within the domain, on SSP. 805 C.7. Changes since -allman-ssp-01 807 o Changed term "Sender Signing Policy" to "Sender Signing 808 Practices". 810 o Changed query methodology to use a separate DNS resource record 811 type, DKIMP. 813 o Changed tag values from SPF-like symbols to words. 815 o User level policies now default to that of the domain if not 816 specified. 818 o Removed the "Compliance" section since we're still not clear on 819 what goes here. 821 o Changed the "parent domain" policy to only search up one level 822 (assumes that subdomains will publish SSP records if appropriate). 824 o Added detailed description of SSP check procedure. 826 C.8. Changes since -allman-ssp-00 828 From a "diff" perspective, the changes are extensive. Semantically, 829 the changes are: 831 o Added section on "Third-Party Signatures and Mailing Lists" 833 o Added "Compliance" (transferred from -base document). I'm not 834 clear on what needs to be done here. 836 o Extensive restructuring. 838 Authors' Addresses 840 Eric Allman 841 Sendmail, Inc. 842 6475 Christie Ave, Suite 350 843 Emeryville, CA 94608 845 Phone: +1 510 594 5501 846 Email: eric+dkim@sendmail.org 847 Jim Fenton 848 Cisco Systems, Inc. 849 MS SJ-9/2 850 170 W. Tasman Drive 851 San Jose, CA 95134-1706 853 Phone: +1 408 526 5914 854 Email: fenton@cisco.com 856 Mark Delany 857 Yahoo! Inc. 858 701 First Avenue 859 Sunnyvale, CA 94089 861 Phone: +1 408 349 6831 862 Email: markd+dkim@yahoo-inc.com 864 John Levine 865 Taughannock Networks 866 PO Box 727 867 Trumansburg, NY 14886 869 Phone: +1 831 480 2300 870 Email: standards@taugh.com 871 URI: http://www.taugh.com 873 Full Copyright Statement 875 Copyright (C) The IETF Trust (2008). 877 This document is subject to the rights, licenses and restrictions 878 contained in BCP 78, and except as set forth therein, the authors 879 retain all their rights. 881 This document and the information contained herein are provided on an 882 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 883 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 884 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 885 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 886 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 887 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 889 Intellectual Property 891 The IETF takes no position regarding the validity or scope of any 892 Intellectual Property Rights or other rights that might be claimed to 893 pertain to the implementation or use of the technology described in 894 this document or the extent to which any license under such rights 895 might or might not be available; nor does it represent that it has 896 made any independent effort to identify any such rights. 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