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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) == Missing Reference: 'FWS' is mentioned on line 718, but not defined ** Obsolete normative reference: RFC 2434 (Obsoleted by RFC 5226) ** Obsolete normative reference: RFC 2822 (Obsoleted by RFC 5322) ** Downref: Normative reference to an Informational RFC: RFC 4686 ** Obsolete normative reference: RFC 4871 (Obsoleted by RFC 6376) Summary: 6 errors (**), 0 flaws (~~), 4 warnings (==), 7 comments (--). 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: August 26, 2008 Cisco Systems, Inc. 6 M. Delany 7 Yahoo! Inc. 8 J. Levine 9 Taughannock Networks 10 February 23, 2008 12 DKIM Author Signing Practices (ASP) 13 draft-ietf-dkim-ssp-03 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 August 26, 2008. 40 Copyright Notice 42 Copyright (C) The IETF Trust (2008). 44 Abstract 46 DomainKeys Identified Mail (DKIM) defines a domain-level 47 authentication framework for email using public-key cryptography and 48 key server technology to permit verification of the source and 49 contents of messages by either Mail Transport Agents (MTAs) or Mail 50 User Agents (MUAs). The primary DKIM protocol is described in 51 [RFC4871]. This document describes the records that authors' domains 52 can use to advertise their practices for signing their outgoing mail, 53 and how other hosts can access those records. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4 59 2.1. Terms Imported from DKIM Signatures Specification . . . . 4 60 2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5 61 2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 5 62 2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 5 63 2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5 64 2.6. Author Signing Practices . . . . . . . . . . . . . . . . . 5 65 2.7. Author Signature . . . . . . . . . . . . . . . . . . . . . 5 66 3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 6 67 3.1. ASP Usage . . . . . . . . . . . . . . . . . . . . . . . . 6 68 3.2. ASP Results . . . . . . . . . . . . . . . . . . . . . . . 6 69 4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 7 70 4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 7 71 4.2. Publication of ASP Records . . . . . . . . . . . . . . . . 7 72 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 73 5.1. ASP Specification Tag Registry . . . . . . . . . . . . . . 10 74 5.2. ASP Outbound Signing Practices Registry . . . . . . . . . 10 75 5.3. ASP Flags Registry . . . . . . . . . . . . . . . . . . . . 11 76 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 77 6.1. ASP Threat Model . . . . . . . . . . . . . . . . . . . . . 11 78 6.2. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 12 79 6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 12 80 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 81 7.1. References - Normative . . . . . . . . . . . . . . . . . . 13 82 7.2. References - Informative . . . . . . . . . . . . . . . . . 13 83 Appendix A. Usage Examples . . . . . . . . . . . . . . . . . . . 13 84 A.1. Single Location Domains . . . . . . . . . . . . . . . . . 14 85 A.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 14 86 A.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 14 87 A.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 15 88 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 15 89 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 15 90 C.1. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 15 91 C.2. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 16 92 C.3. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 17 93 C.4. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 17 94 C.5. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 18 95 C.6. 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 name. It is desirable for 115 such domains to be able to advertise that fact to other hosts. This 116 is the topic of Author Signing Practices (ASP). 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 detailed requirements for Author Signing Practices are given in 123 [RFC5016]. This document refers extensively to [RFC4871] and assumes 124 the reader is familiar with it. 126 Requirements Notation: The key words "MUST", "MUST NOT", "REQUIRED", 127 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", 128 "MAY", and "OPTIONAL" in this document are to be interpreted as 129 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 "Selector" specifies which of the keys published by a signing 145 domain is to be queried, as defined in section 3.1 of [RFC4871]. 147 o A "Local-part" is the part of an address preceding the @ sign, as 148 defined in [RFC2822] and used in [RFC4871]. 150 2.2. Valid Signature 152 A "Valid Signature" is any signature on a message which correctly 153 verifies using the procedure described in section 6.1 of [RFC4871]. 155 2.3. Author Address 157 An "Author Address" is an email address in the From header field of a 158 message [RFC2822]. If the From header field contains multiple 159 addresses, the message has multiple Author Addresses. 161 2.4. Author Domain 163 An "Author Domain" is everything to the right of the "@" in an Author 164 Address (excluding the "@" itself). 166 2.5. Alleged Author 168 An "Alleged Author" is an Author Address of a message; it is 169 "alleged" because it has not yet been verified. 171 2.6. Author Signing Practices 173 "Author Signing Practices" (or just "practices") consist of a 174 machine-readable record published by the domain of an Alleged Author 175 which includes statements about the domain's practices with respect 176 to mail it sends with its domain in the From: line. 178 2.7. Author Signature 180 An "Author Signature" is any Valid Signature where the identity of 181 the user or agent on behalf of which the message is signed (listed in 182 the ""i="" tag or its default value from the ""d="" tag) matches an 183 Author Address in the message. When the identity of the user or 184 agent includes a Local-part, the identities match if the Local-parts 185 match and the domains match. Otherwise, the identities match if the 186 domains match. 188 For example, if a message has a Valid Signature, with the DKIM- 189 Signature field containing "i=a@domain.example", then domain.example 190 is asserting that it takes responsibility for the message. If the 191 message's From: field contains the address "b@domain.example" and an 192 ASP query produces a "dkim=all" or "dkim=discardable" result, that 193 would mean that the message does not have a valid Author Signature. 194 Even though the message is signed by the same domain, its failure to 195 satisfy ASP could be problematic. 197 3. Operation Overview 199 Domain owners can publish Author Signing Practices via a query 200 mechanism such as the Domain Name System; specific details are given 201 in Section 4.1. 203 Hosts can look up the Author Signing Practices of the domain(s) 204 specified by the Author Address(es) as described in Section 4.2.2. 205 If a message has multiple Author Addresses the ASP lookups SHOULD be 206 performed independently on each address. This standard does not 207 address the process a host might use to combine the lookup results. 209 3.1. ASP Usage 211 Depending on the Author Domain(s) and the signatures in a message, a 212 recipient gets varying amounts of useful information from each ASP 213 lookup. 215 o If a message has no Valid Signature, the ASP result is directly 216 relevant to the message. 218 o If a message has a Valid Signature from an Author Domain, ASP 219 provides no benefit relative to that domain since the message is 220 already known to be compliant with any possible ASP for that 221 domain. 223 o If a message has a Valid Signature from a domain other than an 224 Author Domain, the receiver can use both the Signature and the ASP 225 result in its evaluation of the message. 227 3.2. ASP Results 229 An Author Signing Practices lookup for an Author Address produces one 230 of four possible results: 232 o Messages from this domain might or might not have an author 233 signature. This is the default if the domain exists in the DNS 234 but no record is found. 236 o All messages from this domain are signed. 238 o All messages from this domain are signed and discardable. 240 o The domain does not exist. 242 4. Detailed Description 244 4.1. DNS Representation 246 Author Signing Practices records are published using the DNS TXT 247 resource record type. 249 NON-NORMATIVE DISCUSSION [to be removed before publication]: There 250 has been considerable discussion on the DKIM WG mailing list 251 regarding the relative advantages of TXT and a new resource record 252 (RR) type. Read the archive for details. 254 The RDATA for ASP resource records is textual in format, with 255 specific syntax and semantics relating to their role in describing 256 Author Signing Practices. The "Tag=Value List" syntax described in 257 section 3.2 of [RFC4871] is used. Records not in compliance with 258 that syntax or the syntax of individual tags described in Section 4.3 259 MUST be ignored (considered equivalent to a NODATA result) for 260 purposes of ASP, although they MAY cause the logging of warning 261 messages via an appropriate system logging mechanism. If the RDATA 262 contains multiple character strings, the strings are logically 263 concatenated with no delimiters between the strings. 265 The ASP record for a domain is published at a location in the 266 domain's DNS hierarchy prefixed by _asp._domainkey.; e.g., the ASP 267 record for example.com would be a TXT record that is published at 268 "_asp._domainkey.example.com". A domain MUST NOT publish more than 269 one ASP record; the semantics of an ASP lookup that returns multiple 270 ASP records for a single domain are undefined. (Note that 271 example.com and mail.example.com are different domains.) 273 4.2. Publication of ASP Records 275 Author Signing Practices are intended to apply to all mail sent from 276 the domain of an Alleged Author. In order to ensure that ASP applies 277 to any hosts within that domain (e.g., www.example.com, 278 ftp.example.com.) the ASP lookup algorithm looks up one level in the 279 domain tree. For example, mail signed by www.example.com could be 280 covered by the ASP record for example.com. This avoids the need to 281 include an ASP record for every name within a given domain. 283 Normally, a domain expressing Author Signing Practices will want to 284 do so for both itself and all of its "descendants" (child domains at 285 all lower levels). Domains wishing to do so MUST publish ASP records 286 for the domain itself and any subdomains. 288 Wildcards within a domain publishing ASP records pose a particular 289 problem. This is discussed in more detail in Section 6.3. 291 4.2.1. Record Syntax 293 ASP records use the "tag=value" syntax described in section 3.2 of 294 [RFC4871]. 296 Tags used in ASP records are described below. Unrecognized tags MUST 297 be ignored. In the ABNF below, the WSP token is imported from 298 [RFC2822]. The ALPHA and DIGIT tokens are imported from [RFC5234]. 300 dkim= Outbound signing practices for the domain (plain-text; 301 REQUIRED). Possible values are as follows: 303 unknown The domain might sign some or all email. 305 all All mail from the domain is signed with an Author Signature. 307 discardable All mail from the domain is signed with an Author 308 Signature. Furthermore, if a message arrives without a valid 309 Author Signature due to modification in transit, submission via 310 a path without access to a signing key, or other reason, the 311 domain encourages the recipient(s) to discard it. 313 ABNF: 315 asp-dkim-tag = %x64.6b.69.6d *WSP "=" 316 *WSP ("unknown" / "all" / "discardable") 318 t= Flags, represented as a colon-separated list of names (plain-text; 319 OPTIONAL, default is that no flags are set). Flag values are: 321 s The signing practices apply only to the named domain, and not 322 to subdomains. 324 ABNF: 326 asp-t-tag = %x74 *WSP "=" *WSP { asp-t-tag-flag 327 0*( *WSP ":" *WSP asp-t-tag-flag ) 328 asp-t-tag-flag = "s" / hyphenated-word 329 ; for future extension 330 hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") 331 (ALPHA / DIGIT) ] 333 Unrecognized flags MUST be ignored. 335 4.2.2. Author Signing Practices Lookup Procedure 337 Hosts doing an ASP lookup MUST produce a result that is semantically 338 equivalent to applying the following steps in the order listed below. 339 In practice, several of these steps can be performed in parallel in 340 order to improve performance. However, implementations SHOULD avoid 341 doing unnecessary DNS lookups. For the purposes of this section a 342 "valid ASP record" is one that is both syntactically and semantically 343 correct; in particular, it matches the ABNF for a "tag-list" and 344 includes a defined "dkim=" tag. 346 1. _Fetch Named ASP Record._ The host MUST query DNS for a TXT 347 record corresponding to the Author Domain prefixed by 348 "_asp._domainkey." (note the trailing dot). If the result of 349 this query is a "NOERROR" response with an answer which is a 350 valid ASP record, use that record; otherwise, continue to the 351 next step. 353 2. _Verify Domain Exists._ The host MUST perform a DNS query for a 354 record corresponding to the Author Domain (with no prefix). The 355 type of the query can be of any type, since this step is only to 356 determine if the domain itself exists in DNS. This query MAY be 357 done in parallel with the query made in step 2. If the result of 358 this query is an "NXDOMAIN" error, the algorithm MUST terminate 359 with an appropriate error. 361 NON-NORMATIVE DISCUSSION: Any resource record type could be 362 used for this query since the existence of a resource record 363 of any type will prevent an "NXDOMAIN" error. MX is a 364 reasonable choice for this purpose is because this record type 365 is thought to be the most common for likely domains, and will 366 therefore result in a result which can be more readily cached 367 than a negative result. 369 3. _Try Parent Domain._ The host MUST query DNS for a TXT record for 370 the immediate parent domain, prefixed with "_asp._domainkey." If 371 the result of this query is anything other than a "NOERROR" 372 response with a valid ASP record, the algorithm terminates with a 373 result indicating that no ASP record was present. If the ASP "t" 374 tag exists in the response and any of the flags is "s" 375 (indicating it does not apply to a subdomain), the algorithm also 376 terminates without finding an ASP record. Otherwise, use that 377 record. 379 If any of the queries involved in the Author Signing Practices Check 380 result in a "SERVFAIL" error response, the algorithm terminates 381 without returning a result; possible actions include queuing the 382 message or returning an SMTP error indicating a temporary failure. 384 5. IANA Considerations 386 ASP introduces some new namespaces that have been registered with 387 IANA. In all cases, new values are assigned only for values that 388 have been documented in a published RFC that has IETF Consensus 389 [RFC2434]. 391 INFORMATIVE NOTE [ to be removed before publication ]: RFC 4871 392 defines a selector as a sub-domain, importing the term from RFC 2822. 393 A sub-domain starts with a letter or digit, hence names such as _asp 394 that start with an underscore cannot collide with valid selectors. 396 5.1. ASP Specification Tag Registry 398 An ASP record provides for a list of specification tags. IANA has 399 established the ASP Specification Tag Registry for specification tags 400 that can be used in ASP fields. 402 The initial entries in the registry comprise: 404 +------+-----------------+ 405 | TYPE | REFERENCE | 406 +------+-----------------+ 407 | dkim | (this document) | 408 | t | (this document) | 409 +------+-----------------+ 411 ASP Specification Tag Registry Initial Values 413 5.2. ASP Outbound Signing Practices Registry 415 The "dkim=" tag spec, defined in Section 4.2.1, provides for a value 416 specifying Outbound Signing Practices. IANA has established the ASP 417 Outbound Signing Practices Registry for Outbound Signing Practices. 419 The initial entries in the registry comprise: 421 +-------------+-----------------+ 422 | TYPE | REFERENCE | 423 +-------------+-----------------+ 424 | unknown | (this document) | 425 | all | (this document) | 426 | discardable | (this document) | 427 +-------------+-----------------+ 429 ASP Outbound Signing Practices Registry Initial Values 431 5.3. ASP Flags Registry 433 The "t=" tag spec, defined in Section 4.2.1, provides for a value 434 specifying Flags. IANA has established the ASP Flags Registry for 435 ASP Flags. 437 The initial entries in the registry comprise: 439 +------+-----------------+ 440 | TYPE | REFERENCE | 441 +------+-----------------+ 442 | s | (this document) | 443 +------+-----------------+ 445 ASP Flags Registry Initial Values 447 6. Security Considerations 449 Security considerations in the Author Signing Practices are mostly 450 related to attempts on the part of malicious senders to represent 451 themselves as authors for whom they are not authorized to send mail, 452 often in an attempt to defraud either the recipient or an Alleged 453 Author. 455 Additional security considerations regarding Author Signing Practices 456 are found in the DKIM threat analysis [RFC4686]. 458 6.1. ASP Threat Model 460 Email recipients often have a core set of content authors that they 461 already trust. Common examples include financial institutions with 462 which they have an existing relationship and Internet web transaction 463 sites with which they conduct business. 465 Email abuse often seeks to exploit the name-recognition that 466 recipients will have, for a legitimate email author, by using its 467 domain name in the From: header field. Especially since many popular 468 MUAs do not display the author's email address, there is no empirical 469 evidence of the extent that this particular unauthorized use of a 470 domain name contributes to recipient deception or that eliminating it 471 will have significant effect. 473 However, closing this exploit could facilitate some types of 474 optimized processing by receive-side message filtering engines, since 475 it could permit them to maintain higher-confidence assertions about 476 From: header field uses of a domain, when the occurrence is 477 authorized. 479 Unauthorized uses of domain names occur elsewhere in messages, as do 480 unauthorized uses of organizations' names. These attacks are outside 481 the scope of this specification. 483 ASP does not provide any benefit--nor, indeed, have any effect at 484 all--unless an external system acts upon the verdict, either by 485 treating the message differently during the delivery process or by 486 showing some indicator to the end recipient. Such a system is out of 487 scope for this specification. 489 ASP Checkers perform up to three DNS lookups per Alleged Author 490 Domain. Since these lookups are driven by domain names in email 491 message headers of possibly fraudulent email, legitimate ASP Checkers 492 can become participants in traffic multiplication attacks. 494 6.2. DNS Attacks 496 An attacker might attack the DNS infrastructure in an attempt to 497 impersonate ASP records, in an attempt to influence a receiver's 498 decision on how it will handle mail. However, such an attacker is 499 more likely to attack at a higher level, e.g., redirecting A or MX 500 record lookups in order to capture traffic that was legitimately 501 intended for the target domain. These DNS security issues are 502 addressed by DNSSEC [RFC4033]. 504 Because ASP operates within the framework of the legacy e-mail 505 system, the default result in the absence of an ASP record is that 506 the domain does not sign all of its messages. It is therefore 507 important that the ASP clients distinguish a DNS failure such as 508 "SERVFAIL" from other DNS errors so that appropriate actions can be 509 taken. 511 6.3. DNS Wildcards 513 Wildcards within a domain publishing ASP records, including but not 514 limited to wildcard MX records, pose a particular problem. While 515 referencing the immediate parent domain allows the discovery of an 516 ASP record corresponding to an unintended immediate-child subdomain, 517 wildcard records apply at multiple levels. For example, if there is 518 a wildcard MX record for "example.com", the domain 519 "foo.bar.example.com" can receive mail through the named mail 520 exchanger. Conversely, the existence of the record makes it 521 impossible to tell whether "foo.bar.example.com" is a legitimate name 522 since a query for that name will not return an "NXDOMAIN" error. For 523 that reason, ASP coverage for subdomains of domains containing a 524 wildcard record is incomplete. 526 NON-NORMATIVE NOTE: Complete ASP coverage of domains containing (or 527 where any parent contains) wildcards generally cannot be provided by 528 standard DNS servers. 530 7. References 532 7.1. References - Normative 534 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 535 Requirement Levels", BCP 14, RFC 2119, March 1997. 537 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 538 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 539 October 1998. 541 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, 542 April 2001. 544 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 545 Rose, "DNS Security Introduction and Requirements", 546 RFC 4033, March 2005. 548 [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys 549 Identified Mail (DKIM)", RFC 4686, September 2006. 551 [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, 552 J., and M. Thomas, "DomainKeys Identified Mail (DKIM) 553 Signatures", RFC 4871, May 2007. 555 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 556 Specifications: ABNF", STD 68, RFC 5234, January 2008. 558 7.2. References - Informative 560 [RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail 561 (DKIM) Signing Practices Protocol", RFC 5016, 562 October 2007. 564 Appendix A. Usage Examples 566 These examples are intended to illustrate typical uses of ASP. They 567 are not intended to be exhaustive, nor to apply to every domain's or 568 mail system's individual situation. 570 Domain managers are advised to consider the ways that mail processing 571 can modify messages in ways that will invalidate an existing DKIM 572 signature, such as mailing lists, courtesy forwarders, and other 573 paths that could add or modify headers, or modify the message body. 574 In that case, if the modifications invalidate the DKIM signature, 575 recipient hosts will consider the mail not to have an Author 576 Signature, even though the signature was present when the mail was 577 originally sent. 579 A.1. Single Location Domains 581 A common mail system configuration handles all of a domain's users' 582 incoming and outgoing mail through a single MTA or group of MTAs. In 583 that case, the MTA(s) can be configured to sign outgoing mail with an 584 Author Signature. 586 In this situation it might be appropriate to publish an ASP record 587 for the domain containing "all", depending on whether the users also 588 send mail through other paths that do not apply an Author Signature. 589 Such paths could include MTAs at hotels or hotspot networks used by 590 travelling users, or web sites that provide "mail an article" 591 features. 593 A.2. Bulk Mailing Domains 595 Another common configuration uses a domain solely for bulk or 596 broadcast mail, with no individual human users, again typically 597 sending all the mail through a single MTA or group of MTAs that can 598 apply an Author Signature. In this case, the domain's management can 599 be confident that all of its outgoing mail will be sent through the 600 signing MTA. Lacking individual users, the domain is unlikely to 601 participate in mailing lists, but could still send mail through other 602 paths that might invalidate signatures. 604 Domain owners often use specialist mailing providers to send their 605 bulk mail. In that case, the mailing provider needs access to a 606 suitable signing key in order to apply an Author Signature. One 607 possible route would be for the domain owner to generate the key and 608 give it to the mailing provider. Another would be for the domain to 609 delegate a subdomain to the mailing provider, for example, 610 bigbank.example might delegate email.bigbank.example to such a 611 provider. In that case, the provider can generate the keys and DKIM 612 DNS records itself and use the subdomain in the Author address in the 613 mail. 615 A.3. Bulk Mailing Domains with Discardable Mail 617 In some cases, a domain might sign all its outgoing mail with an 618 Author Signature, but prefer that recipient systems discard mail 619 without a valid Author Signature to avoid confusion from mail sent 620 from sources that do not apply an Author Signature. (This latter 621 kind of mail is sometimes loosely called "forgeries".) In that case, 622 it might be appropriate to publish an ASP record containing 623 "discardable". Note that a domain SHOULD NOT publish a "discardable" 624 record if it wishes to maximize the likelihood that mail from the 625 domain is delivered, since it could cause some fraction of the mail 626 the domain sends to be discarded. 628 As a special case, if a domain sends no mail at all, it can safely 629 publish a "discardable" ASP record, since any mail with an author 630 address in the domain is a forgery. 632 A.4. Third Party Senders 634 Another common use case is for a third party to enter into an 635 agreement whereby that third party will send bulk or other mail on 636 behalf of a designated author domain, using that domain in the 637 RFC2822 From: or other headers. Due to the many and varied 638 complexities of such agreements, third party signing is not addressed 639 in this specification. 641 Appendix B. Acknowledgements 643 This document greatly benefited from comments by Steve Atkins, Jon 644 Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael 645 Thomas, and Wietse Venema. 647 Appendix C. Change Log 649 *NOTE TO RFC EDITOR: This section may be removed upon publication of 650 this document as an RFC.* 652 C.1. Changes since -ietf-dkim-02 654 o Merge in more text from ASP draft. 656 o Phrase actions as host's rather than checker. 658 o Explanatory description of i= matching. 660 o Lookup procedure consistently refers to one ASP record per lookup. 662 o Update security section w/ language from W. Venema 664 o Simplify imports of terms from other RFCs, add Local-part, 4234 -> 665 5234. 667 o Add usage example appendix. 669 o Add IANA considerations. 671 o Update authors list 673 C.2. Changes since -ietf-dkim-ssp-01 675 o Reworded introduction for clarity. 677 o Various definition clarifications. 679 o Changed names of practices to unknown, all, and discardable. 681 o Removed normative language mandating use of SSP in particular 682 situations (issue 1538). 684 o Clarified possible confusion over handling of syntax errors. 686 o Removed normative language from Introduction (issue 1538). 688 o Changed "Originator" to "Author" throughout (issue 1529). 690 o Removed all references to Third-Party Signatures (issues 1512, 691 1521). 693 o Removed all mention of "Suspicious" (issues 1528, 1530). 695 o Removed "t=y" (testing) flag (issue 1540). 697 o Removed "handling" tag (issue 1513). 699 o Broke up the "Sender Signing Practices Check Procedure" into two 700 algorithms: fetching the SSP record and interpretation thereof 701 (issues 1531, 1535; partially addresses issue 1520). 702 Interpretation is now the responsibility of the Evaluator. 704 o Document restructuring for better flow and remove redundancies 705 (some may address issue 1523, but I'm not sure I understand that 706 issue completely; also issues 1532, 1537). 708 o Removed all mention of how this interacts with users, even though 709 it makes parts of the document harder to understand (issue 1526). 711 o Introduced the concepts of "SSP Checker" and "Evaluator". 713 o Multiple author case now handled my separate invocations of SSP 714 checker by Evaluator (issue 1525). 716 o Removed check to avoid querying top-level domains. 718 o Changed ABNF use of whitespace from [FWS] to *WSP (partially 719 addresses issue 1543). 721 C.3. Changes since -ietf-dkim-ssp-00 723 o Clarified Operation Overview and eliminated use of Legitimate as 724 the counterpart of Suspicious since the words have different 725 meanings. 727 o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT 728 records in DNS vs. a new RR type. 730 o Clarified publication rules for multilevel names. 732 o Better description of overall record syntax, in particular that 733 records with unknown tags are considered syntactically correct. 735 o Clarified Sender Signing Practices Check Procedure, primarily by 736 use of new term Author Domain. 738 o Eliminated section "Third-Party Signatures and Mailing Lists" that 739 is better included in the DKIM overview document. 741 o Added "handling" tag to express alleged sending domain's 742 preference about handling of Suspicious messages. 744 o Clarified handling of SERVFAIL error in SSP check. 746 o Replaced "entity" with "domain", since with the removal of user- 747 granularity SSP, the only entities having sender signing policies 748 are domains. 750 C.4. Changes since -allman-ssp-02 752 o Removed user-granularity SSP and u= tag. 754 o Replaced DKIMP resource record with a TXT record. 756 o Changed name of the primary tag from "p" to "dkim". 758 o Replaced lookup algorithm with one which traverses upward at most 759 one level. 761 o Added description of records to be published, and effect of 762 wildcard records within the domain, on SSP. 764 C.5. Changes since -allman-ssp-01 766 o Changed term "Sender Signing Policy" to "Sender Signing 767 Practices". 769 o Changed query methodology to use a separate DNS resource record 770 type, DKIMP. 772 o Changed tag values from SPF-like symbols to words. 774 o User level policies now default to that of the domain if not 775 specified. 777 o Removed the "Compliance" section since we're still not clear on 778 what goes here. 780 o Changed the "parent domain" policy to only search up one level 781 (assumes that subdomains will publish SSP records if appropriate). 783 o Added detailed description of SSP check procedure. 785 C.6. Changes since -allman-ssp-00 787 From a "diff" perspective, the changes are extensive. Semantically, 788 the changes are: 790 o Added section on "Third-Party Signatures and Mailing Lists" 792 o Added "Compliance" (transferred from -base document). I'm not 793 clear on what needs to be done here. 795 o Extensive restructuring. 797 Authors' Addresses 799 Eric Allman 800 Sendmail, Inc. 801 6475 Christie Ave, Suite 350 802 Emeryville, CA 94608 804 Phone: +1 510 594 5501 805 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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