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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (August 16, 2013) is 3906 days in the past. Is this intentional? 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: 'CFWS' is mentioned on line 2366, but not defined ** Obsolete normative reference: RFC 5451 (Obsoleted by RFC 7001) ** Downref: Normative reference to an Informational RFC: RFC 5598 -- Possible downref: Non-RFC (?) normative reference: ref. 'US-ASCII' -- Obsolete informational reference (is this intentional?): RFC 2671 (Obsoleted by RFC 6891) -- Obsolete informational reference (is this intentional?): RFC 4408 (Obsoleted by RFC 7208) -- Obsolete informational reference (is this intentional?): RFC 5751 (Obsoleted by RFC 8551) Summary: 2 errors (**), 0 flaws (~~), 7 warnings (==), 5 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group S. Kitterman 3 Internet-Draft Kitterman Technical Services 4 Obsoletes: 4408 (if approved) August 16, 2013 5 Intended status: Standards Track 6 Expires: February 17, 2014 8 Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, 9 Version 1 10 draft-ietf-spfbis-4408bis-18 12 Abstract 14 Email on the Internet can be forged in a number of ways. In 15 particular, existing protocols place no restriction on what a sending 16 host can use as the "MAIL FROM" of a message or the domain given on 17 the SMTP HELO/EHLO commands. This document describes version 1 of 18 the Sender Policy Framework (SPF) protocol, whereby ADministrative 19 Management Domains (ADMDs) can explicitly authorize the hosts that 20 are allowed to use its domain names, and a receiving host can check 21 such authorization. 23 This document obsoletes RFC4408. 25 Status of this Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at http://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on February 17, 2014. 42 Copyright Notice 44 Copyright (c) 2013 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 This document may contain material from IETF Documents or IETF 58 Contributions published or made publicly available before November 59 10, 2008. The person(s) controlling the copyright in some of this 60 material may not have granted the IETF Trust the right to allow 61 modifications of such material outside the IETF Standards Process. 62 Without obtaining an adequate license from the person(s) controlling 63 the copyright in such materials, this document may not be modified 64 outside the IETF Standards Process, and derivative works of it may 65 not be created outside the IETF Standards Process, except to format 66 it for publication as an RFC or to translate it into languages other 67 than English. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6 72 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 73 1.1.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . 6 74 1.1.2. Imported Definitions . . . . . . . . . . . . . . . . . 6 75 1.1.3. MAIL FROM Definition . . . . . . . . . . . . . . . . . 7 76 1.1.4. HELO Definition . . . . . . . . . . . . . . . . . . . 7 77 1.2. check_host() . . . . . . . . . . . . . . . . . . . . . . . 7 78 2. Operational Overview . . . . . . . . . . . . . . . . . . . . . 8 79 2.1. Publishing Authorization . . . . . . . . . . . . . . . . . 8 80 2.2. Checking Authorization . . . . . . . . . . . . . . . . . . 8 81 2.3. The "HELO" Identity . . . . . . . . . . . . . . . . . . . 9 82 2.4. The "MAIL FROM" Identity . . . . . . . . . . . . . . . . . 10 83 2.5. Location of Checks . . . . . . . . . . . . . . . . . . . . 10 84 2.6. Results of Evaluation . . . . . . . . . . . . . . . . . . 10 85 2.6.1. None . . . . . . . . . . . . . . . . . . . . . . . . . 11 86 2.6.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . 11 87 2.6.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . 11 88 2.6.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . 11 89 2.6.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . 11 90 2.6.6. Temperror . . . . . . . . . . . . . . . . . . . . . . 11 91 2.6.7. Permerror . . . . . . . . . . . . . . . . . . . . . . 11 92 3. SPF Records . . . . . . . . . . . . . . . . . . . . . . . . . 12 93 3.1. DNS Resource Records . . . . . . . . . . . . . . . . . . . 12 94 3.2. Multiple DNS Records . . . . . . . . . . . . . . . . . . . 13 95 3.3. Multiple Strings in a Single DNS record . . . . . . . . . 13 96 3.4. Record Size . . . . . . . . . . . . . . . . . . . . . . . 13 97 3.5. Wildcard Records . . . . . . . . . . . . . . . . . . . . . 14 98 4. The check_host() Function . . . . . . . . . . . . . . . . . . 15 99 4.1. Arguments . . . . . . . . . . . . . . . . . . . . . . . . 15 100 4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . 15 101 4.3. Initial Processing . . . . . . . . . . . . . . . . . . . . 16 102 4.4. Record Lookup . . . . . . . . . . . . . . . . . . . . . . 16 103 4.5. Selecting Records . . . . . . . . . . . . . . . . . . . . 16 104 4.6. Record Evaluation . . . . . . . . . . . . . . . . . . . . 16 105 4.6.1. Term Evaluation . . . . . . . . . . . . . . . . . . . 17 106 4.6.2. Mechanisms . . . . . . . . . . . . . . . . . . . . . . 17 107 4.6.3. Modifiers . . . . . . . . . . . . . . . . . . . . . . 18 108 4.6.4. DNS Lookup Limits . . . . . . . . . . . . . . . . . . 18 109 4.7. Default Result . . . . . . . . . . . . . . . . . . . . . . 19 110 4.8. Domain Specification . . . . . . . . . . . . . . . . . . . 19 111 5. Mechanism Definitions . . . . . . . . . . . . . . . . . . . . 21 112 5.1. "all" . . . . . . . . . . . . . . . . . . . . . . . . . . 22 113 5.2. "include" . . . . . . . . . . . . . . . . . . . . . . . . 22 114 5.3. "a" . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 115 5.4. "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 116 5.5. "ptr" (do not use) . . . . . . . . . . . . . . . . . . . . 24 117 5.6. "ip4" and "ip6" . . . . . . . . . . . . . . . . . . . . . 26 118 5.7. "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 26 119 6. Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 28 120 6.1. redirect: Redirected Query . . . . . . . . . . . . . . . . 28 121 6.2. exp: Explanation . . . . . . . . . . . . . . . . . . . . . 29 122 7. Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 123 7.1. Formal Specification . . . . . . . . . . . . . . . . . . . 31 124 7.2. Macro Definitions . . . . . . . . . . . . . . . . . . . . 31 125 7.3. Macro Processing Details . . . . . . . . . . . . . . . . . 32 126 7.4. Expansion Examples . . . . . . . . . . . . . . . . . . . . 34 127 8. Result Handling . . . . . . . . . . . . . . . . . . . . . . . 36 128 8.1. None . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 129 8.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . . . 36 130 8.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 131 8.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 132 8.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . . . 37 133 8.6. Temperror . . . . . . . . . . . . . . . . . . . . . . . . 38 134 8.7. Permerror . . . . . . . . . . . . . . . . . . . . . . . . 38 135 9. Recording the Result . . . . . . . . . . . . . . . . . . . . . 39 136 9.1. The Received-SPF Header Field . . . . . . . . . . . . . . 39 137 9.2. SPF Results in the Authentication-Results Header Field . . 41 138 10. Effects on Infrastructure . . . . . . . . . . . . . . . . . . 43 139 10.1. Sending Domains . . . . . . . . . . . . . . . . . . . . . 43 140 10.1.1. DNS Resource Considerations . . . . . . . . . . . . . 43 141 10.1.2. Administrator's Considerations . . . . . . . . . . . . 44 142 10.1.3. Bounces . . . . . . . . . . . . . . . . . . . . . . . 45 143 10.2. Receivers . . . . . . . . . . . . . . . . . . . . . . . . 45 144 10.3. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 45 145 11. Security Considerations . . . . . . . . . . . . . . . . . . . 47 146 11.1. Processing Limits . . . . . . . . . . . . . . . . . . . . 47 147 11.2. SPF-Authorized Email May Contain Other False Identities . 48 148 11.3. Spoofed DNS and IP Data . . . . . . . . . . . . . . . . . 48 149 11.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 48 150 11.5. Untrusted Information Sources . . . . . . . . . . . . . . 49 151 11.5.1. Recorded Results . . . . . . . . . . . . . . . . . . . 49 152 11.5.2. External Explanations . . . . . . . . . . . . . . . . 49 153 11.5.3. Macro Expansion . . . . . . . . . . . . . . . . . . . 50 154 11.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 50 155 11.7. Delivering Mail Producing a 'Fail' Result . . . . . . . . 50 156 12. Contributors and Acknowledgements . . . . . . . . . . . . . . 51 157 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52 158 13.1. The SPF DNS Record Type . . . . . . . . . . . . . . . . . 52 159 13.2. The Received-SPF Mail Header Field . . . . . . . . . . . . 52 160 13.3. SPF Modifier Registry . . . . . . . . . . . . . . . . . . 52 161 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53 162 14.1. Normative References . . . . . . . . . . . . . . . . . . . 53 163 14.2. Informative References . . . . . . . . . . . . . . . . . . 54 164 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 56 165 Appendix B. Extended Examples . . . . . . . . . . . . . . . . . . 59 166 B.1. Simple Examples . . . . . . . . . . . . . . . . . . . . . 59 167 B.2. Multiple Domain Example . . . . . . . . . . . . . . . . . 60 168 B.3. DNSBL Style Example . . . . . . . . . . . . . . . . . . . 61 169 B.4. Multiple Requirements Example . . . . . . . . . . . . . . 61 170 Appendix C. Changes in implementation requirements from RFC 171 4408 . . . . . . . . . . . . . . . . . . . . . . . . 62 172 Appendix D. Further Testing Advice . . . . . . . . . . . . . . . 63 173 Appendix E. SPF/Mediator Interactions . . . . . . . . . . . . . . 64 174 E.1. Originating ADMDs . . . . . . . . . . . . . . . . . . . . 64 175 E.2. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 65 176 E.3. Receving ADMDs . . . . . . . . . . . . . . . . . . . . . . 65 177 Appendix F. Mail Services . . . . . . . . . . . . . . . . . . . . 66 178 Appendix G. MTA Relays . . . . . . . . . . . . . . . . . . . . . 67 179 Appendix H. Local Policy Considerations . . . . . . . . . . . . . 68 180 H.1. Policy For SPF Pass . . . . . . . . . . . . . . . . . . . 68 181 H.2. Policy For SPF Fail . . . . . . . . . . . . . . . . . . . 68 182 H.3. Policy For SPF Permerror . . . . . . . . . . . . . . . . . 69 183 H.4. Policy For SPF Temperror . . . . . . . . . . . . . . . . . 69 184 Appendix I. Protocol Status . . . . . . . . . . . . . . . . . . . 71 185 Appendix J. Change History . . . . . . . . . . . . . . . . . . . 72 186 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 75 188 1. Introduction 190 The current email infrastructure has the property that any host 191 injecting mail into the system can use any DNS domain name it wants 192 in each of the various identifiers specified by [RFC5321] and 193 [RFC5322]. Although this feature is desirable in some circumstances, 194 it is a major obstacle to reducing Unsolicited Bulk Email (UBE, aka 195 spam). Furthermore, many domain owning ADMDs (as described in 196 [RFC5598]) are understandably concerned about the ease with which 197 other entities can make use of their domain names, often with 198 malicious intent. 200 This document defines a protocol by which ADMDs can authorize hosts 201 to use their domain names in the "MAIL FROM" or "HELO" identities. 202 Compliant ADMDs publish Sender Policy Framework (SPF) records in the 203 DNS specifying which hosts are permitted to use their names, and 204 compliant mail receivers use the published SPF records to test the 205 authorization of sending Mail Transfer Agents (MTAs) using a given 206 "HELO" or "MAIL FROM" identity during a mail transaction. 208 An additional benefit to mail receivers is that after the use of an 209 identity is verified, local policy decisions about the mail can be 210 made based on the sender's domain, rather than the host's IP address. 211 This is advantageous because reputation of domain names is likely to 212 be more accurate than reputation of host IP addresses since domains 213 are likely to be more stable over a longer period. Furthermore, if a 214 claimed identity fails verification, local policy can take stronger 215 action against such email, such as rejecting it. 217 1.1. Terminology 219 1.1.1. Keywords 221 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 222 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 223 "OPTIONAL" in this document are to be interpreted as described in 224 [RFC2119]. 226 1.1.2. Imported Definitions 228 ABNF (Augmented Backus-Naur Form) ABNF is defined in [RFC5234], as 229 are the tokens "ALPHA", "DIGIT", and "SP" (space). 231 The token "local-part" is defined in [RFC5321]. 233 "dot-atom", "quoted-string", "comment", "CFWS" (comment folded white 234 space), "FWS" (folded white space), and "CRLF" (carriage-return/ 235 line-feed) are defined in [RFC5322]. 237 1.1.3. MAIL FROM Definition 239 This document is concerned with the portion of a mail message 240 commonly called "envelope sender", "return path", "reverse path", 241 "bounce address", "5321 FROM", "MAIL FROM", or RFC5321.MailFrom. 242 Since these terms are either not well defined or often used casually, 243 this document uses "MAIL FROM" for consistency. This means the 244 RFC5321.MailFrom as defined in [RFC5598]. Note that other terms that 245 might superficially look like the common terms, such as 'reverse- 246 path', are used only as they are specified in their defining 247 documents. 249 1.1.4. HELO Definition 251 This document also makes use of the HELO/EHLO identity. The "HELO" 252 identity derives from either the SMTP HELO or EHLO command (see 253 [RFC5321]). Since HELO and EHLO can, in many cases, be used 254 interchangeably, they are identified commonly as "HELO" in this 255 document. This means RFC5321.HELO/.EHLO as defined in [RFC5598]. 256 These commands supply the identity of the SMTP client (sending host) 257 for the SMTP session. 259 1.2. check_host() 261 Section 4 introduces an algorithm to evaluate an SPF policy against 262 an arriving email transaction. In an early implementation, this 263 algorithm was encoded in a function called check_host(). That name 264 is used in this document as symbolic of the SPF evaluation algorithm, 265 but of course implementers are not required to use this name. 267 2. Operational Overview 269 2.1. Publishing Authorization 271 An SPF-compliant domain publishes valid SPF records as described in 272 Section 3. These records authorize the use of the relevant domain 273 names in the "HELO" and "MAIL FROM" identities by the MTAs specified 274 therein. 276 SPF results can be used to make both positive (source is authorized) 277 and negative (source is not authorized) determinations. If ADMDs 278 choose to publish SPF records and want to support receivers making 279 negative authorization determinations, it is necessary for them to 280 publish records that end in "-all", or redirect to other records that 281 do, otherwise, no definitive determination of authorization can be 282 made. Potential issues and mitigations associated with negative 283 determinations are discussed in Section 10. 285 ADMDs that wish to declare that no hosts are authorized to use their 286 DNS domain names in the HELO or MAIL FROM commands during SMTP 287 sessions can publish SPF records that say so for domain names that 288 are neither used in the domain part of email addresses nor expected 289 to originate mail. 291 When changing SPF records, care has to be taken to ensure that there 292 is a transition period so that the old policy remains valid until all 293 legitimate email can reasonably expect to have been checked. 294 [RFC5321] Section 4.5.4.1 discusses how long a message might be in 295 transit. While offline checks are possible, the closer to the 296 original transmission time checks are performed, the more likely they 297 are to get an SPF result that matches the sending ADMD intent at the 298 time the message was sent. 300 2.2. Checking Authorization 302 A mail receiver can perform a set of SPF checks for each mail message 303 it receives. An SPF check tests the authorization of a client host 304 to emit mail with a given identity. Typically, such checks are done 305 by a receiving MTA, but can be performed elsewhere in the mail 306 processing chain so long as the required information is available and 307 reliable. The "MAIL FROM" and "HELO" identities are checked as 308 described in Section 2.4 and Section 2.3 respectively. 310 Without explicit approval of the publishing ADMD, checking other 311 identities against SPF version 1 records is NOT RECOMMENDED because 312 there are cases that are known to give incorrect results. For 313 example, almost all mailing lists rewrite the "MAIL FROM" identity 314 (see Section 10.3), but some do not change any other identities in 315 the message. Documents that define other identities will have to 316 define the method for explicit approval. 318 It is possible that mail receivers will use the SPF check as part of 319 a larger set of tests on incoming mail. The results of other tests 320 might influence whether or not a particular SPF check is performed. 321 For example, finding the sending host's IP address on a local white 322 list might cause all other tests to be skipped and all mail from that 323 host to be accepted. 325 When a mail receiver decides to perform an SPF check, it has to use a 326 correctly-implemented check_host() function (Section 4) evaluated 327 with the correct parameters. Although the test as a whole is 328 optional, once it has been decided to perform a test it has to be 329 performed as specified so that the correct semantics are preserved 330 between publisher and receiver. 332 To make the test, the mail receiver MUST evaluate the check_host() 333 function with the arguments described in Section 4.1. 335 Although invalid, malformed, or non-existent domains cause SPF checks 336 to return "none" because no SPF record can be found, it has long been 337 the policy of many MTAs to reject email from such domains, especially 338 in the case of invalid "MAIL FROM". Rejecting email will prevent one 339 method of circumventing of SPF records. 341 Implementations have to take care to correctly extract the 342 from the data given with the SMTP MAIL FROM command as many MTAs will 343 still accept such things as source routes (see [RFC5321], Appendix 344 C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]). 345 These archaic features have been maliciously used to bypass security 346 systems. 348 2.3. The "HELO" Identity 350 It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM" 351 identity, but also separately check the "HELO" identity by applying 352 the check_host() function (Section 4) to the "HELO" identity as the 353 . Checking "HELO" promotes consistency of results and can 354 reduce DNS resource usage. If a conclusive determination about the 355 message can be made based on a check of "HELO", then the use of DNS 356 resources to process the typically more complex "MAIL FROM" can be 357 avoided. Additionally, since SPF records published for "HELO" 358 identities refer to a single host, when available, they are a very 359 reliable source of host authorization status. Checking "HELO" before 360 "MAIL FROM" is the RECOMMENDED sequence if both are checked. 362 Note that requirements for the domain presented in the EHLO or HELO 363 command are not always clear to the sending party, and SPF verifiers 364 have to be prepared for the identity to be an IP address literal (see 365 [RFC5321] section 4.1.3), or simply be malformed. This SPF check can 366 only be performed when the "HELO" string is a valid, multi-label 367 domain name. 369 2.4. The "MAIL FROM" Identity 371 SPF verifiers MUST check the "MAIL FROM" identity if a "HELO" check 372 has either not been performed or has not reached a definitive policy 373 result by applying the check_host() function to the "MAIL FROM" 374 identity as the . 376 [RFC5321] allows the reverse-path to be null (see Section 4.5.5 in 377 [RFC5321]). In this case, there is no explicit sender mailbox, and 378 such a message can be assumed to be a notification message from the 379 mail system itself. When the reverse-path is null, this document 380 defines the "MAIL FROM" identity to be the mailbox composed of the 381 local-part "postmaster" and the "HELO" identity (which might or might 382 not have been checked separately before). 384 2.5. Location of Checks 386 The authorization check SHOULD be performed during the processing of 387 the SMTP transaction that recieves the mail. This reduces the 388 complexity of determining the correct IP address to use as an input 389 to check_host() and allows errors to be returned directly to the 390 sending MTA by way of SMTP replies. Appendix C of [RFC5451] provides 391 a more thorough discussion of this topic. 393 Performing the authorization check other than using the MAIL FROM and 394 client address at the time of the MAIL command during the SMTP 395 transaction can cause problems, such as the following: (1) It might 396 be difficult to accurately extract the required information from 397 potentially deceptive headers; (2) legitimate email might fail 398 because the sender's policy had since changed. 400 Generating non-delivery notifications to forged identities that have 401 failed the authorization check often constitutes backscatter, i.e., 402 inactionable, nuisance rejection notices. Operators are strongly 403 advised to avoid such practices. Section 2 of [RFC3834] describes 404 backscatter and the problems it causes. 406 2.6. Results of Evaluation 408 Section 4 defines check_host(), a model function definition that uses 409 the inputs defined above and the sender's policy published in the DNS 410 to reach a conclusion about client authorization. An SPF verifier 411 implements something semantically equivalent to the function defined 412 there. 414 This section enumerates and briefly defines the possible outputs of 415 that function. Note, however, that the protocol establishes no 416 normative requirements for handling any particular result. 417 Discussion of handling options for each result can be found in 418 Section 8. 420 2.6.1. None 422 A result of "none" means either (a) no syntactically valid DNS domain 423 name was extracted from the SMTP session that could be used as the 424 one to be authorized, or (b) no TXT records were retrieved from the 425 DNS that appeared to be intended for use by SPF verifiers. 427 2.6.2. Neutral 429 The ADMD has explicitly stated that it is not asserting whether the 430 IP address is authorized. 432 2.6.3. Pass 434 A "pass" result means that the client is authorized to inject mail 435 with the given identity. 437 2.6.4. Fail 439 A "fail" result is an explicit statement that the client is not 440 authorized to use the domain in the given identity. 442 2.6.5. Softfail 444 The ADMD has published a weak statement that the host is probably not 445 authorized. It has not published a stronger, more definitive policy 446 that results in a "fail". 448 2.6.6. Temperror 450 A "temperror" result means the SPF verifier encountered a transient 451 (generally DNS) error while performing the check. A later retry may 452 succeed without further operator action. 454 2.6.7. Permerror 456 A "permerror" result means the domain's published records could not 457 be correctly interpreted. This signals an error condition that 458 definitely requires operator intervention to be resolved. 460 3. SPF Records 462 An SPF record is a DNS record that declares which hosts are, and are 463 not, authorized to use a domain name for the "HELO" and "MAIL FROM" 464 identities. Loosely, the record partitions hosts into permitted and 465 not-permitted sets (though some hosts might fall into neither 466 category). 468 The SPF record is expressed as a single string of text found in the 469 RDATA of a single DNS TXT resource record; multiple SPF records are 470 not permitted for the same owner name. The record format and the 471 process for selecting records is described below in Section 4. An 472 example record is the following: 474 v=spf1 +mx a:colo.example.com/28 -all 476 This record has a version of "spf1" and three directives: "+mx", 477 "a:colo.example.com/28" (the + is implied), and "-all". 479 Each SPF record is placed in the DNS tree at the owner name it 480 pertains to, not a subdomain under it, such as is done with SRV 481 records [RFC2782]. 483 The example in this section might be published via these lines in a 484 domain zone file: 486 example.com. TXT "v=spf1 +mx a:colo.example.com/28 -all" 487 smtp-out.example.com. TXT "v=spf1 a -all" 489 Since TXT records have multiple uses, beware of other TXT records 490 published there for other purposes. They might cause problems with 491 size limits (see Section 3.4) and care has to be taken to ensure only 492 SPF records are used for SPF processing. 494 ADMDs publishing SPF records ought to keep the amount of DNS 495 information needed to evaluate a record to a minimum. Section 4.6.4 496 and Section 10.1.1 provide some suggestions about "include" 497 mechanisms and chained "redirect" modifiers. 499 3.1. DNS Resource Records 501 SPF records MUST be published as a DNS TXT (type 16) Resource Record 502 (RR) [RFC1035] only. The character content of the record is encoded 503 as [US-ASCII]. Use of alternative DNS RR types was supported in 504 SPF's experimental phase, but has been discontinued. See Appendix A 505 of [RFC6686] for further information. 507 3.2. Multiple DNS Records 509 A domain name MUST NOT have multiple records that would cause an 510 authorization check to select more than one record. See Section 4.5 511 for the selection rules. 513 3.3. Multiple Strings in a Single DNS record 515 As defined in [RFC1035] sections 3.3 and 3.3.14, a single text DNS 516 record can be composed of more than one string. If a published 517 record contains multiple character-strings, then the record MUST be 518 treated as if those strings are concatenated together without adding 519 spaces. For example: 521 IN TXT "v=spf1 .... first" "second string..." 523 is equivalent to: 525 IN TXT "v=spf1 .... firstsecond string..." 527 TXT records containing multiple strings are useful in constructing 528 records that would exceed the 255-octet maximum length of a 529 character-string within a single TXT record. 531 3.4. Record Size 533 The published SPF record for a given domain name SHOULD remain small 534 enough that the results of a query for it will fit within 512 octets. 535 This UDP limit is defined in [RFC1035] section 2.3.4, although it was 536 raised by [RFC2671]. Staying below 512 octets ought to prevent older 537 DNS implementations from failing over to TCP,and will work with UDP 538 in the absence of EDNS0 [RFC6891] support. Since the answer size is 539 dependent on many things outside the scope of this document, it is 540 only possible to give this guideline: If the combined length of the 541 DNS name and the text of all the records of a given type is under 450 542 octets, then DNS answers ought to fit in UDP packets. Records that 543 are too long to fit in a single UDP packet could be silently ignored 544 by SPF verifiers due to firewall and other issues that interfere with 545 the operation of DNS over TCP or using ENDS0. 547 Note that when computing the sizes for replies to queries of the TXT 548 format, one has to take into account any other TXT records published 549 at the domain name. Similarly, the sizes for replies to all queries 550 related to SPF have to be evaluated to fit in a single 512 octet UDP 551 packet. 553 3.5. Wildcard Records 555 Use of wildcard records for publishing is discouraged and care has to 556 be taken if they are used. If a zone includes wildcard MX records, 557 it might want to publish wildcard declarations, subject to the same 558 requirements and problems. In particular, the declaration MUST be 559 repeated for any host that has any RR records at all, and for 560 subdomains thereof. Consider the example in [RFC1034], Section 561 4.3.3. Based on that, we can do the following: 563 EXAMPLE.COM. MX 10 A.EXAMPLE.COM 564 EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 566 *.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 567 *.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 569 A.EXAMPLE.COM. A 203.0.113.1 570 A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 571 A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 573 *.A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 574 *.A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 576 SPF records have to be listed twice for every name within the zone: 577 once for the name, and once with a wildcard to cover the tree under 578 the name, in order to cover all domains in use in outgoing mail. 580 4. The check_host() Function 582 This description is not an API (Application Program Interface) 583 definition, but rather a function description used to illustrate the 584 algorithm. A compliant SPF implementation MUST produce results 585 semantically equivalent to this description. 587 The check_host() function fetches SPF records, parses them, and 588 evaluates them to determine whether a particular host is or is not 589 permitted to send mail with a given identity. Receiving ADMDs that 590 perform this check MUST correctly evaluate the check_host() function 591 as described here. 593 Implementations MAY use a different algorithm than the canonical 594 algorithm defined here, so long as the results are the same in all 595 cases. 597 4.1. Arguments 599 The check_host() function takes these arguments: 601 - the IP address of the SMTP client that is emitting the 602 mail, either IPv4 or IPv6. 604 - the domain that provides the sought-after authorization 605 information; initially, the domain portion of the "MAIL 606 FROM" or "HELO" identity. 608 - the "MAIL FROM" or "HELO" identity. 610 For recursive evaluations, the domain portion of might not 611 be the same as the argument when check_host() is initially 612 evaluated. In most other cases it will be the same. (See 613 Section 5.2 below). 615 Note that the argument might not be a well-formed domain 616 name. For example, if the reverse-path was null, then the EHLO/HELO 617 domain is used, with its associated problems (see Section 2.3). In 618 these cases, check_host() is defined in Section 4.3 to return a 619 "none" result. 621 4.2. Results 623 The function check_host() can return one of several results described 624 in Section 2.6. Based on the result, the action to be taken is 625 determined by the local policies of the receiver. This is discussed 626 in Section 8. 628 4.3. Initial Processing 630 If the is malformed (e.g. label longer than 63 characters, 631 zero-length label not at the end, etc.) or is not a multi-label 632 domain name, or if the DNS lookup returns "domain does not exist" 633 (RCODE 3), check_host() immediately returns the result "none". DNS 634 RCODES are defined in [RFC1035]. Properly formed domains are fully 635 qualified domains as defined in [RFC1983]. That is, in the DNS they 636 are implicitly qualified relative to the root (see section 3.1 of 637 [RFC1034]). Internationalized domain names MUST be encoded as 638 A-labels, as described in Section 2.3 of [RFC5890]. 640 If the has no local-part, substitute the string "postmaster" 641 for the local-part. 643 4.4. Record Lookup 645 In accordance with how the records are published (see Section 3 646 above), a DNS query needs to be made for the name, querying 647 for type TXT only. 649 If the DNS lookup returns a server failure (RCODE 2), or other error 650 (RCODE other than 0 or 3), or time out, then check_host() terminates 651 immediately with the result "temperror". 653 4.5. Selecting Records 655 Records begin with a version section: 657 record = version terms *SP 658 version = "v=spf1" 660 Starting with the set of records that were returned by the lookup, 661 discard records that do not begin with a version section of exactly 662 "v=spf1". Note that the version section is terminated either by an 663 SP character or the end of the record. A record with a version 664 section of "v=spf10" does not match and is discarded. 666 If the resultant record set includes no records, check_host() 667 produces the "none" result. If the resultant record set includes 668 more than one record, check_host() produces the "permerror" result. 670 4.6. Record Evaluation 672 The check_host() function parses and interprets the SPF record to 673 find a result for the current test. If there are any syntax errors 674 anywhere in the record, check_host() returns immediately with the 675 result "permerror", without further interpretation. 677 4.6.1. Term Evaluation 679 There are two types of terms: mechanisms (defined in Section 5) and 680 modifiers (defined in Section 6). A record contains an ordered list 681 of these as specified in the following Augmented Backus-Naur Form 682 (ABNF). 684 terms = *( 1*SP ( directive / modifier ) ) 686 directive = [ qualifier ] mechanism 687 qualifier = "+" / "-" / "?" / "~" 688 mechanism = ( all / include 689 / a / mx / ptr / ip4 / ip6 / exists ) 690 modifier = redirect / explanation / unknown-modifier 691 unknown-modifier = name "=" macro-string 692 ; where name is not any known modifier 694 name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) 696 Most mechanisms allow a ":" or "/" character after the name. 698 Modifiers always contain an equals ('=') character immediately after 699 the name, and before any ":" or "/" characters that might be part of 700 the macro-string. 702 Terms that do not contain any of "=", ":", or "/" are mechanisms, as 703 defined in Section 5. 705 As per the definition of the ABNF notation in [RFC5234], mechanism 706 and modifier names are case-insensitive. 708 4.6.2. Mechanisms 710 Each mechanism is considered in turn from left to right. If there 711 are no more mechanisms, the result is the default result as described 712 in Section 4.7. 714 When a mechanism is evaluated, one of three things can happen: it can 715 match, not match, or return an exception. 717 If it matches, processing ends and the qualifier value is returned as 718 the result of that record. If it does not match, processing 719 continues with the next mechanism. If it returns an exception, 720 mechanism processing ends and the exception value is returned. 722 The possible qualifiers, and the results they cause check_host() to 723 return are as follows: 725 "+" pass 726 "-" fail 727 "~" softfail 728 "?" neutral 730 The qualifier is optional and defaults to "+". 732 When a mechanism matches and the qualifier is "-", then a "fail" 733 result is returned and the explanation string is computed as 734 described in Section 6.2. 736 The specific mechanisms are described in Section 5. 738 4.6.3. Modifiers 740 Modifiers are not mechanisms. They do not return match or not-match. 741 Instead, they provide additional information. Although modifiers do 742 not directly affect the evaluation of the record, the "redirect" 743 modifier has an effect after all the mechanisms have been evaluated. 745 4.6.4. DNS Lookup Limits 747 SPF implementations MUST limit the total number of mechanisms and 748 modifiers ("terms") that cause any DNS query to 10 during SPF 749 evaluation. Specifically, the "include", "a", "mx", "ptr", and 750 "exists" mechanisms as well as the "redirect" modifier count against 751 this collective limit. The "all", "ip4", and "ip6" mechanisms do not 752 count against this limit. If this number is exceeded during a check, 753 a "permerror" MUST be returned. The "exp" modifier does not count 754 against this limit because the DNS lookup to fetch the explanation 755 string occurs after the SPF record evaluation has been completed. 757 When evaluating the "mx" mechanism, the number of "MX" resource 758 records queried is included in the overall limit of 10 mechanisms/ 759 modifiers that cause DNS lookups described above. The evaluation of 760 each "MX" record MUST NOT result in querying more than 10 address 761 records, either "A" or "AAAA" resource records. If this limit is 762 exceeded, the "mx" mechanism MUST produce a "permerror" result. 764 When evaluating the "ptr" mechanism or the %{p} macro, the number of 765 "PTR" resource records queried is included in the overall limit of 10 766 mechanisms/modifiers that cause DNS lookups described above. The 767 evaluation of each "PTR" record MUST NOT result in querying more than 768 10 address records, either "A" or "AAAA" resource records. If this 769 limit is exceeded, all records other than the first 10 MUST be 770 ignored. 772 The reason for the disparity is that the set of and contents of the 773 MX record are under control of the publishing ADMD, while the set of 774 and contents of PTR records are under control of the owner of the IP 775 address actually making the connection. 777 These limits are per mechanism or macro in the record, and are in 778 addition to the lookup limits specified above. 780 MTAs or other processors SHOULD impose a limit on the maximum amount 781 of elapsed time to evaluate check_host(). Such a limit SHOULD allow 782 at least 20 seconds. If such a limit is exceeded, the result of 783 authorization SHOULD be "temperror". 785 As described at the end of Section 11.1, there may be cases where it 786 is useful to limit the number of "terms" for which DNS queries return 787 either a positive answer (RCODE 0) with an answer count of 0, or a no 788 such record (RCODE 3) answer. These are sometimes collectively 789 referred to as "void lookups". SPF implementations SHOULD limit 790 "void lookups" to two. An implementation MAY choose to make such a 791 limit configurable. In this case, a default of two is RECOMMENDED. 793 4.7. Default Result 795 If none of the mechanisms match and there is no "redirect" modifier, 796 then the check_host() returns a result of "neutral", just as if 797 "?all" were specified as the last directive. If there is a 798 "redirect" modifier, check_host() proceeds as defined in Section 6.1. 800 It is better to use either a "redirect" modifier or an "all" 801 mechanism to explicitly terminate processing. Although the latter 802 has a default (specifically "?all"), it aids debugging efforts if it 803 is explicitly provided. 805 For example: 807 v=spf1 +mx -all 808 or 809 v=spf1 +mx redirect=_spf.example.com 811 4.8. Domain Specification 813 Several of these mechanisms and modifiers have a domain-spec section. 814 The domain-spec string is subject to macro expansion (see Section 7). 815 The resulting string is the common presentation form of a fully- 816 qualified DNS name: a series of labels separated by periods. This 817 domain is called the in the rest of this document. 819 Note: The result of the macro expansion is not subject to any further 820 escaping. Hence, this facility cannot produce all characters that 821 are legal in a DNS label (e.g., the control characters). However, 822 this facility is powerful enough to express legal host names and 823 common utility labels (such as "_spf") that are used in DNS. 825 For several mechanisms, the domain-spec is optional. If it is not 826 provided, the from the check_host() arguments (see 827 Section 4.1) is used as the . "domain" and domain-spec 828 are syntactically identical after macro expansion. "domain" is an 829 input value for check_host() while domain-spec is computed by 830 check_host(). 832 The result of evaluating check_host() with a syntactically invalid 833 domain is undefined. 835 5. Mechanism Definitions 837 This section defines two types of mechanisms: basic language 838 framework mechanisms and designated sender mechanisms. 840 Basic mechanisms contribute to the language framework. They do not 841 specify a particular type of authorization scheme. 843 all 844 include 846 Designated sender mechanisms are used to identify a set of 847 addresses as being permitted or not permitted to use the for 848 sending mail. 850 a 851 mx 852 ptr (do not publish) 853 ip4 854 ip6 855 exists 857 The following conventions apply to all mechanisms that perform a 858 comparison between and an IP address at any point: 860 If no CIDR prefix length is given in the directive, then and the 861 IP address are compared for equality. (Here, CIDR is Classless 862 Inter-Domain Routing, described in [RFC4632].) 864 If a CIDR prefix length is specified, then only the specified number 865 of high-order bits of and the IP address are compared for 866 equality. 868 When any mechanism fetches host addresses to compare with , when 869 is an IPv4, "A" records are fetched; when is an IPv6 870 address, "AAAA" records are fetched. SPF implementations on IPv6 871 servers need to handle both "AAAA" and "A" records, for clients on 872 IPv4 mapped IPv6 addresses [RFC4291]. IPv4 addresses are only 873 listed in an SPF record using the "ip4" mechanism. 875 Several mechanisms rely on information fetched from the DNS. For 876 these DNS queries, except where noted, if the DNS server returns an 877 error (RCODE other than 0 or 3) or the query times out, the mechanism 878 stops and the topmost check_host() returns "temperror". If the 879 server returns "domain does not exist" (RCODE 3), then evaluation of 880 the mechanism continues as if the server returned no error (RCODE 0) 881 and zero answer records. 883 5.1. "all" 885 all = "all" 887 The "all" mechanism is a test that always matches. It is used as the 888 rightmost mechanism in a record to provide an explicit default. 890 For example: 892 v=spf1 a mx -all 894 Mechanisms after "all" will never be tested. Mechanisms listed after 895 "all" MUST be ignored. Any "redirect" modifier (Section 6.1) MUST be 896 ignored when there is an "all" mechanism in the record. 898 5.2. "include" 900 include = "include" ":" domain-spec 902 The "include" mechanism triggers a recursive evaluation of 903 check_host(). 905 1. The domain-spec is expanded as per Section 7. 907 2. check_host() is evaluated with the resulting string as the 908 . The and arguments remain the same as in 909 the current evaluation of check_host(). 911 3. The recursive evaluation returns either match, not match, or an 912 error. If it matches, then the appropriate result for the 913 include: mechanism is used (e.g. include or +include produces a 914 "pass" result and -include produces "fail"). 916 4. If there is no match, the parent check_host() resumes processing 917 as per the table below, with the previous value of 918 restored. 920 In hindsight, the name "include" was poorly chosen. Only the 921 evaluated result of the referenced SPF record is used, rather than 922 literally including the mechanisms of the referenced record in the 923 first. For example, evaluating a "-all" directive in the referenced 924 record does not terminate the overall processing and does not 925 necessarily result in an overall "fail". (Better names for this 926 mechanism would have been "if-match", "on-match", etc.) 928 The "include" mechanism makes it possible for one domain to designate 929 multiple administratively-independent domains. For example, a vanity 930 domain "example.net" might send mail using the servers of 931 administratively-independent domains example.com and example.org. 933 Example.net could say 935 IN TXT "v=spf1 include:example.com include:example.org -all" 937 This would direct check_host() to, in effect, check the records of 938 example.com and example.org for a "pass" result. Only if the host 939 were not permitted for either of those domains would the result be 940 "fail". 942 Whether this mechanism matches, does not match, or returns an 943 exception depends on the result of the recursive evaluation of 944 check_host(): 946 +---------------------------------+---------------------------------+ 947 | A recursive check_host() result | Causes the "include" mechanism | 948 | of: | to: | 949 +---------------------------------+---------------------------------+ 950 | pass | match | 951 | | | 952 | fail | not match | 953 | | | 954 | softfail | not match | 955 | | | 956 | neutral | not match | 957 | | | 958 | temperror | return temperror | 959 | | | 960 | permerror | return permerror | 961 | | | 962 | none | return permerror | 963 +---------------------------------+---------------------------------+ 965 The "include" mechanism is intended for crossing administrative 966 boundaries. For example, if example.com and example.org were managed 967 by the same entity, and if the permitted set of hosts for both 968 domains was "mx:example.com", it would be possible for example.org to 969 specify "include:example.com", but it would be preferable to specify 970 "redirect=example.com" or even "mx:example.com". 972 With the "include" mechanism an administratively external set of 973 hosts can be authorized, but determination of sender policy is still 974 a function of the original domain's SPF record (as determined by the 975 "all" mechanism in that record). The redirect modifier is more 976 suitable for consolidating both authorizations and policy into a 977 common set to be shared within an ADMD. Redirect is much more like a 978 common code element to be shared among records in a single ADMD. It 979 is possible to control both authorized hosts and policy for an 980 arbitrary number of domains from a single record. 982 5.3. "a" 984 This mechanism matches if is one of the 's IP 985 addresses. For clarity, this means the "a" mechanism also matches 986 AAAA records. 988 a = "a" [ ":" domain-spec ] [ dual-cidr-length ] 990 An address lookup is done on the using the type of 991 lookup (A or AAAA) appropriate for the connection type (IPv4 or 992 IPv6). The is compared to the returned address(es). If any 993 address matches, the mechanism matches. 995 5.4. "mx" 997 This mechanism matches if is one of the MX hosts for a domain 998 name. 1000 mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ] 1002 check_host() first performs an MX lookup on the . Then 1003 it performs an address lookup on each MX name returned. The is 1004 compared to each returned IP address. To prevent Denial of Service 1005 (DoS) attacks, the processing limits defined in Section 4.6.4 MUST be 1006 followed. If the MX lookup limit is exceeded, then "permerror" is 1007 returned and the evaluation is terminated. If any address matches, 1008 the mechanism matches. 1010 Note regarding implicit MXes: If the has no MX record, 1011 check_host() MUST NOT apply the implicit MX rules of[RFC5321] by 1012 querying for an A or AAAA record for the same name. 1014 5.5. "ptr" (do not use) 1016 This mechanism tests whether the DNS reverse-mapping for exists 1017 and correctly points to a domain name within a particular domain. 1018 This mechanism SHOULD NOT be published. See below for discussion. 1020 ptr = "ptr" [ ":" domain-spec ] 1022 The 's name is looked up using this procedure: 1024 o Perform a DNS reverse-mapping for : Look up the corresponding 1025 PTR record in "in-addr.arpa." if the address is an IPv4 one and in 1026 "ip6.arpa." if it is an IPv6 address. 1028 o For each record returned, validate the domain name by looking up 1029 its IP addresses. To prevent DoS attacks, the PTR processing 1030 limits defined in Section 4.6.4 MUST be applied. If they are 1031 exceeded, processing is terminated and the mechanism does not 1032 match. 1034 o If is among the returned IP addresses, then that domain name 1035 is validated. 1037 Check all validated domain names to see if they either match the 1038 domain or are a subdomain of the domain. 1039 If any do, this mechanism matches. If no validated domain name can 1040 be found, or if none of the validated domain names match or are a 1041 subdomain of the , this mechanism fails to match. If a 1042 DNS error occurs while doing the PTR RR lookup, then this mechanism 1043 fails to match. If a DNS error occurs while doing an A RR lookup, 1044 then that domain name is skipped and the search continues. 1046 Pseudocode: 1048 sending-domain_names := ptr_lookup(sending-host_IP); 1049 if more than 10 sending-domain_names are found, use at most 10. 1050 for each name in (sending-domain_names) { 1051 IP_addresses := a_lookup(name); 1052 if the sending-domain_IP is one of the IP_addresses { 1053 validated-sending-domain_names += name; 1054 } 1055 } 1057 for each name in (validated-sending-domain_names) { 1058 if name ends in , return match. 1059 if name is , return match. 1060 } 1061 return no-match. 1063 This mechanism matches if the is either a subdomain of 1064 a validated domain name or if the and a validated 1065 domain name are the same. For example: "mail.example.com" is within 1066 the domain "example.com", but "mail.bad-example.com" is not. 1068 Note: This mechanism is slow, it is not as reliable as other 1069 mechanisms in cases of DNS errors, and it places a large burden on 1070 the .arpa name servers. If used, proper PTR records have to be in 1071 place for the domain's hosts and the "ptr" mechanism SHOULD be one of 1072 the last mechanisms checked. After many years of SPF deployment 1073 experience, it has been concluded it is unnecessary and more reliable 1074 alternatives should be used instead. It is, however, still in use as 1075 part of the SPF protocol, so compliant check_host() implementations 1076 MUST support it. 1078 5.6. "ip4" and "ip6" 1080 These mechanisms test whether is contained within a given IP 1081 network. 1083 ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] 1084 ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ] 1086 ip4-cidr-length = "/" 1*DIGIT 1087 ip6-cidr-length = "/" 1*DIGIT 1088 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] 1090 ip4-network = qnum "." qnum "." qnum "." qnum 1091 qnum = DIGIT ; 0-9 1092 / %x31-39 DIGIT ; 10-99 1093 / "1" 2DIGIT ; 100-199 1094 / "2" %x30-34 DIGIT ; 200-249 1095 / "25" %x30-35 ; 250-255 1096 ; as per conventional dotted quad notation. e.g., 192.0.2.0 1097 ip6-network = 1098 ; e.g., 2001:DB8::CD30 1100 The is compared to the given network. If CIDR prefix length 1101 high-order bits match, the mechanism matches. 1103 If ip4-cidr-length is omitted, it is taken to be "/32". If 1104 ip6-cidr-length is omitted, it is taken to be "/128". It is not 1105 permitted to omit parts of the IP address instead of using CIDR 1106 notations. That is, use 192.0.2.0/24 instead of 192.0.2. 1108 5.7. "exists" 1110 This mechanism is used to construct an arbitrary domain name that is 1111 used for a DNS A record query. It allows for complicated schemes 1112 involving arbitrary parts of the mail envelope to determine what is 1113 permitted. 1115 exists = "exists" ":" domain-spec 1117 The domain-spec is expanded as per Section 7. The resulting domain 1118 name is used for a DNS A RR lookup (even when the connection type is 1119 IPv6). If any A record is returned, this mechanism matches. 1121 Domains can use this mechanism to specify arbitrarily complex 1122 queries. For example, suppose example.com publishes the record: 1124 v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all 1126 The might expand to 1127 "1.2.0.192.someuser._spf.example.com". This makes fine-grained 1128 decisions possible at the level of the user and client IP address. 1130 6. Modifier Definitions 1132 Modifiers are name/value pairs that provide additional information. 1133 Modifiers always have an "=" separating the name and the value. 1135 The modifiers defined in this document ("redirect" and "exp") SHOULD 1136 appear at the end of the record, after all mechanisms, though 1137 syntactically they can appear anywhere in the record. Ordering of 1138 these two modifiers does not matter. These two modifiers MUST NOT 1139 appear in a record more than once each. If they do, then 1140 check_host() exits with a result of "permerror". 1142 Unrecognized modifiers MUST be ignored no matter where in a record, 1143 or how often. This allows implementations of this document to 1144 gracefully handle records with modifiers that are defined in other 1145 specifications. 1147 6.1. redirect: Redirected Query 1149 The redirect modifier is intended for consolidating both 1150 authorizations and policy into a common set to be shared within a 1151 single ADMD. It is possible to control both authorized hosts and 1152 policy for an arbitrary number of domains from a single record. 1154 redirect = "redirect" "=" domain-spec 1156 If all mechanisms fail to match, and a "redirect" modifier is 1157 present, then processing proceeds as follows: 1159 The domain-spec portion of the redirect section is expanded as per 1160 the macro rules in Section 7. Then check_host() is evaluated with 1161 the resulting string as the . The and 1162 arguments remain the same as in the current evaluation of 1163 check_host(). 1165 The result of this new evaluation of check_host() is then considered 1166 the result of the current evaluation with the exception that if no 1167 SPF record is found, or if the is malformed, the result 1168 is a "permerror" rather than "none". 1170 Note that the newly-queried domain can itself specify redirect 1171 processing. 1173 This facility is intended for use by organizations that wish to apply 1174 the same record to multiple domains. For example: 1176 la.example.com. TXT "v=spf1 redirect=_spf.example.com" 1177 ny.example.com. TXT "v=spf1 redirect=_spf.example.com" 1178 sf.example.com. TXT "v=spf1 redirect=_spf.example.com" 1179 _spf.example.com. TXT "v=spf1 mx:example.com -all" 1181 In this example, mail from any of the three domains is described by 1182 the same record. This can be an administrative advantage. 1184 Note: In general, the domain "A" cannot reliably use a redirect to 1185 another domain "B" not under the same administrative control. Since 1186 the stays the same, there is no guarantee that the record at 1187 domain "B" will correctly work for mailboxes in domain "A", 1188 especially if domain "B" uses mechanisms involving local-parts. An 1189 "include" directive will generally be more appropriate. 1191 For clarity, any "redirect" modifier SHOULD appear as the very last 1192 term in a record. 1194 6.2. exp: Explanation 1196 explanation = "exp" "=" domain-spec 1198 If check_host() results in a "fail" due to a mechanism match (such as 1199 "-all"), and the "exp" modifier is present, then the explanation 1200 string returned is computed as described below. If no "exp" modifier 1201 is present, then either a default explanation string or an empty 1202 explanation string MUST be returned to the calling application. 1204 The domain-spec is macro expanded (see Section 7) and becomes the 1205 . The DNS TXT RRset for the is fetched. 1207 If there are any DNS processing errors (any RCODE other than 0), or 1208 if no records are returned, or if more than one record is returned, 1209 or if there are syntax errors in the explanation string, then proceed 1210 as if no "exp" modifier was given. 1212 The fetched TXT record's strings are concatenated with no spaces, and 1213 then treated as an explain-string, which is macro-expanded. This 1214 final result is the explanation string. Implementations MAY limit 1215 the length of the resulting explanation string to allow for other 1216 protocol constraints and/or reasonable processing limits. Since the 1217 explanation string is intended for an SMTP response and [RFC5321] 1218 Section 2.4 says that responses are in [US-ASCII], the explanation 1219 string MUST be limited to [US-ASCII]. 1221 Software evaluating check_host() can use this string to communicate 1222 information from the publishing domain in the form of a short message 1223 or URL. Software SHOULD make it clear that the explanation string 1224 comes from a third party. For example, it can prepend the macro 1225 string "%{o} explains: " to the explanation, such as shown in 1226 Section 8.4. 1228 Suppose example.com has this record: 1230 v=spf1 mx -all exp=explain._spf.%{d} 1232 Here are some examples of possible explanation TXT records at 1233 explain._spf.example.com: 1235 "Mail from example.com should only be sent by its own servers." 1236 -- a simple, constant message 1238 "%{i} is not one of %{d}'s designated mail servers." 1239 -- a message with a little more information, including the IP 1240 address that failed the check 1242 "See http://%{d}/why.html?s=%{S}&i=%{I}" 1243 -- a complicated example that constructs a URL with the 1244 arguments to check_host() so that a web page can be 1245 generated with detailed, custom instructions 1247 Note: During recursion into an "include" mechanism, an "exp" modifier 1248 from the MUST NOT be used. In contrast, when executing 1249 a "redirect" modifier, an "exp" modifier from the original domain 1250 MUST NOT be used. This is because "include" is meant to cross 1251 administrative boundaries and the explanation provided should be the 1252 one from the receiving ADMD, while "redirect" is meant to operate as 1253 a tool to consolidate policy records within an ADMD an so the 1254 redirected explanation is the one that ought to have priority. 1256 7. Macros 1258 When evaluating an SPF policy record, certain character sequences are 1259 intended to be replaced by parameters of the message or of the 1260 connection. These character sequences are referred to as "macros". 1262 7.1. Formal Specification 1264 The ABNF description for a macro is as follows: 1266 domain-spec = macro-string domain-end 1267 domain-end = ( "." toplabel [ "." ] ) / macro-expand 1269 toplabel = ( *alphanum ALPHA *alphanum ) / 1270 ( 1*alphanum "-" *( alphanum / "-" ) alphanum ) 1271 alphanum = ALPHA / DIGIT 1273 explain-string = *( macro-string / SP ) 1275 macro-string = *( macro-expand / macro-literal ) 1276 macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) 1277 / "%%" / "%_" / "%-" 1278 macro-literal = %x21-24 / %x26-7E 1279 ; visible characters except "%" 1280 macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / 1281 "c" / "r" / "t" / "v" 1282 transformers = *DIGIT [ "r" ] 1283 delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" 1285 The "toplabel" construction is subject to the LDH rule plus 1286 additional top-level domain (TLD) restrictions. See Section 2 of 1287 [RFC3696] for background. 1289 Some special cases: 1291 o A literal "%" is expressed by "%%". 1293 o "%_" expands to a single " " space. 1295 o "%-" expands to a URL-encoded space, viz., "%20". 1297 7.2. Macro Definitions 1299 The following macro letters are expanded in term arguments: 1301 s = 1302 l = local-part of 1303 o = domain of 1304 d = 1305 i = 1306 p = the validated domain name of (do not use) 1307 v = the string "in-addr" if is ipv4, or "ip6" if is ipv6 1308 h = HELO/EHLO domain 1310 , , and are defined in Section 2.2. 1312 The following macro letters are allowed only in "exp" text: 1314 c = SMTP client IP (easily readable format) 1315 r = domain name of host performing the check 1316 t = current timestamp 1318 7.3. Macro Processing Details 1320 A '%' character not followed by a '{', '%', '-', or '_' character is 1321 a syntax error. So: 1323 -exists:%(ir).sbl.example.org 1325 is incorrect and will cause check_host() to yield a "permerror". 1326 Instead, the following is legal: 1328 -exists:%{ir}.sbl.example.org 1330 Optional transformers are the following: 1332 *DIGIT = zero or more digits 1333 'r' = reverse value, splitting on dots by default 1335 If transformers or delimiters are provided, the replacement value for 1336 a macro letter is split into parts separated by one or more of the 1337 specified delimiter characters. After performing any reversal 1338 operation and/or removal of left-hand parts, the parts are rejoined 1339 using "." and not the original splitting characters. 1341 By default, strings are split on "." (dots). Note that no special 1342 treatment is given to leading, trailing, or consecutive delimiters in 1343 input strings, and so the list of parts might contain empty strings. 1344 Some older implementations of SPF prohibit trailing dots in domain 1345 names, so trailing dots SHOULD NOT be published, although they MUST 1346 be accepted by implementations conforming to this document. Macros 1347 can specify delimiter characters that are used instead of ".". 1349 The "r" transformer indicates a reversal operation: if the client IP 1350 address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1" 1351 and the macro %{ir} would expand to "1.2.0.192". 1353 The DIGIT transformer indicates the number of right-hand parts to 1354 use, after optional reversal. If a DIGIT is specified, the value 1355 MUST be nonzero. If no DIGITs are specified, or if the value 1356 specifies more parts than are available, all the available parts are 1357 used. If the DIGIT was 5, and only 3 parts were available, the macro 1358 interpreter would pretend the DIGIT was 3. Implementations MUST 1359 support at least a value of 127, as that is the maximum number of 1360 labels in a domain name (less the zero-length label at the end). 1362 The "s" macro expands to the argument. It is an email 1363 address with a local-part, an "@" character, and a domain. The "l" 1364 macro expands to just the local-part. The "o" macro expands to just 1365 the domain part. Note that these values remain the same during 1366 recursive and chained evaluations due to "include" and/or "redirect". 1367 Note also that if the original had no local-part, the local- 1368 part was set to "postmaster" in initial processing (see Section 4.3). 1370 For IPv4 addresses, both the "i" and "c" macros expand to the 1371 standard dotted-quad format. 1373 For IPv6 addresses, the "i" macro expands to a dot-format address; it 1374 is intended for use in %{ir}. The "c" macro can expand to any of the 1375 hexadecimal colon-format addresses specified in [RFC4291], Section 1376 2.2. It is intended for humans to read. 1378 The "p" macro expands to the validated domain name of . The 1379 procedure for finding the validated domain name is defined in 1380 Section 5.5. If the is present in the list of validated 1381 domains, it SHOULD be used. Otherwise, if a subdomain of the 1382 is present, it SHOULD be used. Otherwise, any name from the 1383 list can be used. If there are no validated domain names or if a DNS 1384 error occurs, the string "unknown" is used. 1386 This macro SHOULD NOT be published (see Section 5.5 for the 1387 discussion). 1389 The "h" macro expands to the parameter that was provided to the SMTP 1390 server via the HELO or EHLO SMTP verb. For sessions where that verb 1391 was provided more than once, the most recent instance is used. 1393 The "r" macro expands to the name of the receiving MTA. This SHOULD 1394 be a fully qualified domain name, but if one does not exist (as when 1395 the checking is done by a MUA) or if policy restrictions dictate 1396 otherwise, the word "unknown" SHOULD be substituted. The domain name 1397 can be different from the name found in the MX record that the client 1398 MTA used to locate the receiving MTA. 1400 The "t" macro expands to the decimal representation of the 1401 approximate number of seconds since the Epoch (Midnight, January 1, 1402 1970, UTC) at the time of the evaluation. This is the same value as 1403 is returned by the POSIX time() function in most standards-compliant 1404 libraries. 1406 When the result of macro expansion is used in a domain name query, if 1407 the expanded domain name exceeds 253 characters (the maximum length 1408 of a domain name in this format), the left side is truncated to fit, 1409 by removing successive domain labels (and their following dots) until 1410 the total length does not exceed 253 characters. 1412 Uppercased macros expand exactly as their lowercased equivalents, and 1413 are then URL escaped. URL escaping MUST be performed for characters 1414 not in the "unreserved" set, which is defined in [RFC3986]. 1416 Care has to be taken by the sending ADMD so that macro expansion for 1417 legitimate email does not exceed the 63-character limit on DNS 1418 labels. The local-part of email addresses, in particular, can have 1419 more than 63 characters between dots. 1421 To minimize DNS lookup resource requirements, it is better if sending 1422 ADMDs avoid using the "s", "l", "o", or "h" macros in conjunction 1423 with any mechanism directive. Although these macros are powerful and 1424 allow per-user records to be published, they severely limit the 1425 ability of implementations to cache results of check_host() and they 1426 reduce the effectiveness of DNS caches. 1428 If no directive processed during the evaluation of check_host() 1429 contains an "s", "l", "o", or "h" macro, then the results of the 1430 evaluation can be cached on the basis of and alone for 1431 as long as the DNS record involved with the shortest TTL has not 1432 expired. 1434 7.4. Expansion Examples 1436 The is strong-bad@email.example.com. 1437 The IPv4 SMTP client IP is 192.0.2.3. 1438 The IPv6 SMTP client IP is 2001:DB8::CB01. 1439 The PTR domain name of the client IP is mx.example.org. 1441 macro expansion 1442 ------- ---------------------------- 1443 %{s} strong-bad@email.example.com 1444 %{o} email.example.com 1445 %{d} email.example.com 1446 %{d4} email.example.com 1447 %{d3} email.example.com 1448 %{d2} example.com 1449 %{d1} com 1450 %{dr} com.example.email 1451 %{d2r} example.email 1452 %{l} strong-bad 1453 %{l-} strong.bad 1454 %{lr} strong-bad 1455 %{lr-} bad.strong 1456 %{l1r-} strong 1458 macro-string expansion 1459 -------------------------------------------------------------------- 1460 %{ir}.%{v}._spf.%{d2} 3.2.0.192.in-addr._spf.example.com 1461 %{lr-}.lp._spf.%{d2} bad.strong.lp._spf.example.com 1463 %{lr-}.lp.%{ir}.%{v}._spf.%{d2} 1464 bad.strong.lp.3.2.0.192.in-addr._spf.example.com 1466 %{ir}.%{v}.%{l1r-}.lp._spf.%{d2} 1467 3.2.0.192.in-addr.strong.lp._spf.example.com 1469 %{d2}.trusted-domains.example.net 1470 example.com.trusted-domains.example.net 1472 IPv6: 1473 %{ir}.%{v}._spf.%{d2} 1.0.B.C.0.0.0.0. 1474 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.ip6._spf.example.com 1476 8. Result Handling 1478 This section provides guidance for operators in response to the 1479 various possible outputs of check_host() on a message. Definitions 1480 of SPF results are presented in Section 2.6; this section provides 1481 more detail on each for use in developing local policy for message 1482 handling. 1484 Every operating environment is different. There are some receivers 1485 for whom strict adherence to SPF is appropriate, and definitive 1486 treatment of messages that are evaluated to be explicitly 1487 unauthorized ("fail" and sometimes "softfail") is the norm. There 1488 are others for which the "false negative" cases are more of a 1489 concern. This concern is typically handled by merely recording the 1490 result in the header and allowing the message to pass on for 1491 additional processing. There are still others where SPF is one of 1492 several inputs to the message handling decision. As such, there is 1493 no comprehensive normative requirement for message handling in 1494 response to any particular result. This section is provided to 1495 present a complete picture of the likely cause of each result and, 1496 where available, the experience gained during experimental 1497 deployment. 1499 There are essentially two classes of handling choices: 1501 o Handling within the SMTP session that attempted to deliver the 1502 message, such as by returning a permanent SMTP error (rejection) 1503 or temporary SMTP error ("try again later"); 1505 o Permitting the message to pass (a successful SMTP reply code) and 1506 adding an additional header field that indicates the result 1507 returned by check_host() and other salient details; this is 1508 discussed in more detail in Section 9. 1510 8.1. None 1512 With a "none" result, the SPF verifier has no information at all 1513 about the authorization or lack thereof of the client to use the 1514 checked identity or identities. The check_host() function completed 1515 without errors but was not able to reach any conclusion. 1517 8.2. Neutral 1519 A "neutral" result indicates that although a policy for the identity 1520 was discovered, there is no definite assertion (positive or negative) 1521 about the client. 1523 A "neutral" result MUST be treated exactly like the "none" result; 1524 the distinction exists only for informational purposes. Treating 1525 "neutral" more harshly than "none" would discourage ADMDs from 1526 testing the use of SPF records (see Section 10.1). 1528 8.3. Pass 1530 A "pass" result means that the client is authorized to inject mail 1531 with the given identity. The domain can now, in the sense of 1532 reputation, be considered responsible for sending the message. 1533 Further policy checks can now proceed with confidence in the 1534 legitimate use of the identity. This is further discussed in 1535 Appendix H.1. 1537 8.4. Fail 1539 A "fail" result is an explicit statement that the client is not 1540 authorized to use the domain in the given identity. Disposition of 1541 SPF fail messages is a matter of local policy. See Appendix H.2 for 1542 considerations on developing local policy. 1544 If the checking software chooses to reject the mail during the SMTP 1545 transaction, then it SHOULD use an SMTP reply code of 550 (see 1546 [RFC5321]) and, if supported, the 5.7.1 enhanced status code (see 1547 [RFC3463], Section 3.8), in addition to an appropriate reply text. 1548 The check_host() function will return either a default explanation 1549 string or one from the domain that published the SPF records (see 1550 Section 6.2). If the information does not originate with the 1551 checking software, it is good to make it clear that the text is 1552 provided by the sender's domain. For example: 1554 550-5.7.1 SPF MAIL FROM check failed: 1555 550-5.7.1 The domain example.com explains: 1556 550 5.7.1 Please see http://www.example.com/mailpolicy.html 1558 If the checking software chooses not to reject the mail during the 1559 SMTP transaction, then it SHOULD add a Received-SPF or 1560 Authentication-Results header field (see Section 9) to communicate 1561 this result to downstream message processors. While this is true for 1562 all SPF results, it is of particular importance for "fail" results 1563 since the message is explicitly not authorized by the ADMD. 1565 8.5. Softfail 1567 A "softfail" result ought to be treated as somewhere between "fail" 1568 and "neutral"/"none". The ADMD believes the host is not authorized 1569 but is not willing to make a strong policy statement. Receiving 1570 software SHOULD NOT reject the message based solely on this result, 1571 but MAY subject the message to closer scrutiny than normal. 1573 The ADMD wants to discourage the use of this host and thus desires 1574 limited feedback when a "softfail" result occurs. For example, the 1575 recipient's Mail User Agent (MUA) could highlight the "softfail" 1576 status, or the receiving MTA could give the sender a message using 1577 greylisting, [RFC6647], with a note the first time the message is 1578 received, but accept it on a later attempt based on receiver policy. 1580 8.6. Temperror 1582 A "temperror" result means the SPF verifier encountered a transient 1583 (generally DNS) error while performing the check. Checking software 1584 can choose to accept or temporarily reject the message. If the 1585 message is rejected during the SMTP transaction for this reason, the 1586 software SHOULD use an SMTP reply code of 451 and, if supported, the 1587 4.4.3 enhanced status code (see [RFC3463], Section 3.5). These 1588 errors can be caused by problems in either the sender's or receiver's 1589 DNS software. See Appendix H.4 for considerations on developing 1590 local policy. 1592 8.7. Permerror 1594 A "permerror" result means the domain's published records could not 1595 be correctly interpreted. This signals an error condition that 1596 definitely requires operator intervention to be resolved. If the 1597 message is rejected during the SMTP transaction for this reason, the 1598 software SHOULD use an SMTP reply code of 550 and, if supported, the 1599 5.5.2 enhanced status code (see [RFC3463], Section 3.6). Be aware 1600 that if the ADMD uses macros (Section 7), it is possible that this 1601 result is due to the checked identities having an unexpected format. 1602 It is also possible that this result is generated by certain SPF 1603 verifiers due to the input arguments having an unexpected format; see 1604 Section 4.8. See Appendix H.3 for considerations on developing local 1605 policy. 1607 9. Recording the Result 1609 To provide downstream agents, such as MUAs, with the information they 1610 might need in terms of evaluating or representing the apparent safety 1611 of the message content, it is RECOMMENDED that SMTP receivers record 1612 the result of SPF processing in the message header. For operators 1613 that choose to record SPF results in the header of the message for 1614 processing by internal filters or MUAs, two methods are presented. 1615 Section 9.1 defines the Received-SPF field, which is the results 1616 field originally defined for SPF use. Section 9.2 discusses 1617 Authentication-Results [RFC5451] which was specified more recently 1618 and is designed for use by SPF and other authentication methods. 1620 Both are in common use, and hence both are included here. However, 1621 it is important to note that they were designed to serve slightly 1622 different purposes. Received-SPF is intended to include enough 1623 information to enable reconstruction of the SPF evaluation of the 1624 message, while Authentication-Results is designed only to relay the 1625 result itself and related output details of likely use to end users 1626 (e.g., what property of the message was actually authenticated and 1627 what it contained), leaving reconstructive work to the purview of 1628 system logs and the Received field contents. Also, Received-SPF 1629 relies on compliance of agents within the receiving ADMD to adhere to 1630 the header field ordering rules of [RFC5321] and [RFC5322], while 1631 Authentication-Results includes some provisions to protect against 1632 non-compliant implementations. 1634 An operator could choose to use both to serve different downstream 1635 agents. In such cases, care needs to be taken to ensure both fields 1636 are conveying the same details, or unexpected results can occur. 1638 9.1. The Received-SPF Header Field 1640 The Received-SPF header field is a trace field (see [RFC5322] Section 1641 3.6.7) and SHOULD be prepended to the existing header, above the 1642 Received: field that is generated by the SMTP receiver. It MUST 1643 appear above all other Received-SPF fields in the message. The 1644 header field has the following format: 1646 header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] 1647 [ key-value-list ] CRLF 1649 result = "pass" / "fail" / "softfail" / "neutral" / 1650 "none" / "temperror" / "permerror" 1652 key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) 1653 [";"] 1655 key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) 1657 key = "client-ip" / "envelope-from" / "helo" / 1658 "problem" / "receiver" / "identity" / 1659 "mechanism" / name 1661 identity = "mailfrom" ; for the "MAIL FROM" identity 1662 / "helo" ; for the "HELO" identity 1663 / name ; other identities 1665 dot-atom = 1666 quoted-string = 1667 comment = 1668 CFWS = 1669 FWS = 1670 CRLF = 1672 The header field SHOULD include a "(...)" style comment after the 1673 result, conveying supporting information for the result, such as 1674 , , and . 1676 The following key-value pairs are designed for later machine parsing. 1677 SPF verifiers SHOULD give enough information so that the SPF results 1678 can be verified. That is, at least "client-ip", "helo", and, if the 1679 "MAIL FROM" identity was checked, "envelope-from". 1681 client-ip the IP address of the SMTP client 1683 envelope-from the envelope sender mailbox 1685 helo the host name given in the HELO or EHLO command 1687 mechanism the mechanism that matched (if no mechanisms matched, 1688 substitute the word "default") 1690 problem if an error was returned, details about the error 1691 receiver the host name of the SPF verifier 1693 identity the identity that was checked; see the ABNF 1694 rule 1696 Other keys MAY be defined by SPF verifiers. 1698 SPF verifiers MUST make sure that the Received-SPF header field does 1699 not contain invalid characters, is not excessively long (See 1700 [RFC5322] Section 2.1.1), and does not contain malicious data that 1701 has been provided by the sender. 1703 Examples of various header field styles that could be generated are 1704 the following: 1706 Received-SPF: pass (mybox.example.org: domain of 1707 myname@example.com designates 192.0.2.1 as permitted sender) 1708 receiver=mybox.example.org; client-ip=192.0.2.1; 1709 envelope-from="myname@example.com"; helo=foo.example.com; 1711 Received-SPF: fail (mybox.example.org: domain of 1712 myname@example.com does not designate 1713 192.0.2.1 as permitted sender) 1714 identity=mailfrom; client-ip=192.0.2.1; 1715 envelope-from="myname@example.com"; 1717 Received-SPF: pass (mybox.example.org: domain of 1718 myname@example.com designates 192.0.2.1 as permitted sender) 1719 receiver=mybox.example.org; client-ip=192.0.2.1; 1720 mechanism=ip4:192.0.2.1; envelope-from="myname@example.com"; 1721 helo=foo.example.com; 1723 9.2. SPF Results in the Authentication-Results Header Field 1725 As mentioned in Section 9, the Authentication-Results header field is 1726 designed to communicate lists of tests a border MTA did and their 1727 results. The specified elements of the field provide less 1728 information than the Received-SPF field: 1730 Authentication-Results: myhost.example.org; spf=pass 1731 smtp.mailfrom=example.net 1733 Received-SPF: pass (myhost.example.org: domain of 1734 myname@example.com designates 192.0.2.1 as permitted sender) 1735 receiver=mybox.example.org; client-ip=192.0.2.1; 1736 envelope-from="myname@example.com"; helo=foo.example.com; 1738 It is, however, possible to add CFWS in the "reason" part of an 1739 Authentication-Results header field and provide the equivalent 1740 information, if desired. 1742 As an example, an expanded Authentication-Results header field might 1743 look like (for a "MAIL FROM" check in this example): 1745 Authentication-Results: myhost.example.org; spf=pass 1746 reason="client-ip=192.0.2.1; smtp.helo=foo.example.com" 1747 smtp.mailfrom=user@example.net 1749 10. Effects on Infrastructure 1751 This section outlines the major implications that adoption of this 1752 protocol will have on various entities involved in Internet email. 1753 It is intended to make clear to the reader where this protocol 1754 knowingly affects the operation of such entities. This section is 1755 not a "how-to" manual, or a "best practices" document, and it is not 1756 a comprehensive list of what such entities ought do in light of this 1757 specification. 1759 This section provides operational advice and instruction only. It is 1760 non-normative. 1762 [RFC5598] describes the Internet email architecture. This section is 1763 organized based on the different segments of the architecture. 1765 10.1. Sending Domains 1767 Originating ADMDs (ADministrative Management Domains - [RFC5598] 1768 Section 2.2.1 and Section 2.3) that wish to be compliant with this 1769 specification will need to determine the list of relays ([RFC5598] 1770 Section 2.2.2) that they allow to use their domain name in the "HELO" 1771 and "MAIL FROM" identities when relaying to other ADMDs. It is 1772 recognized that forming such a list is not just a simple technical 1773 exercise, but involves policy decisions with both technical and 1774 administrative considerations. 1776 10.1.1. DNS Resource Considerations 1778 Minimizing the DNS resources needed for SPF lookups can be done by 1779 choosing directives that require less DNS information and by placing 1780 lower-cost mechanisms earlier in the SPF record. 1782 Section 4.6.4 specifies the limits receivers have to use. It is 1783 essential to publish records that do not exceed these requirements. 1784 It is also required to carefully weigh the cost and the 1785 maintainability of licit solutions. 1787 For example, consider a domain set up as follows: 1789 example.com. IN MX 10 mx.example.com. 1790 IN MX 20 mx2.example.com. 1791 mx.example.com. IN A 192.0.2.1 1792 mx2.example.com. IN A 192.0.2.129 1794 Assume the administrative point is to authorize (pass) mx and mx2 1795 while failing every other host. Compare the following solutions: 1797 Best record: 1798 example.com. IN TXT "v=spf1 ip4:192.0.2.1 ip4:192.0.2.129 -all" 1800 Good record: 1801 $ORIGIN example.com. 1802 @ IN TXT "v=spf1 a:authorized-spf.example.com -all" 1803 authorized-spf IN A 192.0.2.1 1804 IN A 192.0.2.129 1806 Expensive record: 1807 example.com. IN TXT "v=spf1 mx:example.com -all" 1809 Wasteful, bad record: 1810 example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 mx -all" 1812 10.1.2. Administrator's Considerations 1814 There might be administrative considerations: using "a" over "ip4" or 1815 "ip6" allows hosts to be renumbered easily at the cost of a DNS query 1816 per receiver. Using "mx" over "a" allows the set of mail hosts to be 1817 changed easily. Unless such changes are common, it is better to use 1818 the less resource intensive mechanisms like "ip4" and "ip6" over "a" 1819 or "a" over "mx". 1821 In some specific cases, standard advice on record content is 1822 appropriate. Publishing SPF records for domains that send no mail is 1823 a well established best practice. The record for a domain that sends 1824 no mail is: 1826 www.example.com. IN TXT "v=spf1 -all" 1828 Publishing SPF records for individual hosts is also best practice. 1829 The hostname is generally the identity used in the 5321.HELO/.EHLO 1830 command. In the case of messages with a null 5321.MailFrom, this is 1831 used as the domain for 5321.MailFrom SPF checks, in addition to being 1832 used in 5321.HELO/.EHLO based SPF checks. The standard SPF record 1833 for an individual host that is involved in mail processing is: 1835 relay.example.com. IN TXT "v=spf1 a -all" 1837 Validating correct deployment is difficult. [RFC6652] describes one 1838 mechanism for soliciting feedback on SPF failures. Another 1839 suggestion can be found in Appendix D. 1841 Regardless of the method used, understanding the ADMD's outbound mail 1842 architecture is essential to effective deployment. 1844 10.1.3. Bounces 1846 As explained in Section 1.1.3, [RFC5321] allows the MAIL FROM to be 1847 null, which is typical of some Delivery Status Notification 1848 [RFC3464], commonly called email bounces. In this case the only 1849 entity available for performing an SPF check is the "HELO" identity 1850 defined in Section 1.1.4. SPF functionality is enhanced by 1851 administrators ensuring this identity is set correctly and has an 1852 appropriate SPF record. It is normal to have the HELO identity set 1853 to the hostname instead of the domain. Zone file generation for 1854 significant numbers of hosts can be consolidated using the redirect 1855 modifier and scripted for initial deployment. Specific deployment 1856 advice is given above in Section 10.1.2. 1858 10.2. Receivers 1860 SPF results can be used in combination with other methods to 1861 determine the final local disposition (either positive or negative) 1862 of a message. It can also be considered dispositive on its own. 1864 An attempt to have one organization (sender) direct the email 1865 handling policies of another (receiver) is inherently challenging and 1866 often controversial. As stated elsewhere in this document, there is 1867 no comprehensive normative requirement for specific handling of a 1868 message based on SPF results. The information presented in Section 8 1869 and in Appendix H is offered for receiver consideration when forming 1870 local handling policies. 1872 The primary considerations are that SPF might return "pass" for mail 1873 that is ultimately harmful (e.g., spammers that arrange for SPF to 1874 pass using disposable domain names, or virus or spam outbreaks from 1875 within trusted sources), and might also return "fail" for mail that 1876 is ultimately legitimate (e.g., legitimate mail that has traversed a 1877 mail alias). It is important take both of these cases under 1878 consideration when establishing local handling policy. 1880 10.3. Mediators 1882 Mediators are a type of User actor [RFC5598]. That is, a mediator 1883 takes 'delivery' of a message and posts a 'submission' of a new 1884 message. The mediator can make the newly-posted message be as 1885 similar or as different from the original message as they wish. 1886 Examples include mailing lists (see [RFC5598] Section 5.3) and 1887 ReSenders ([RFC5598] Section 5.2). This is discussed in [RFC5321], 1888 Section 3.9. For the operation of SPF, the essential concern is the 1889 email address in the 5321.MailFrom command for the new message. 1891 Because SPF evaluation is based on the IP address of the "last" 1892 sending SMTP server, the address of the mediator will be used, rather 1893 than the address of the SMTP server that sent the message to the 1894 mediator. Some mediators retain the email address from the original 1895 message, while some use a new address. 1897 If the address is the same as for the original message, and the 1898 original message had an associated SPF record, then the SPF 1899 evaluation will fail unless mitigations such as those described in 1900 Appendix E are used. 1902 11. Security Considerations 1904 11.1. Processing Limits 1906 As with most aspects of email, there are a number of ways that 1907 malicious parties could use the protocol as an avenue for a 1908 Denial-of-Service (DoS) attack. The processing limits outlined in 1909 Section 4.6.4 are designed to prevent attacks such as the following: 1911 o A malicious party could create an SPF record with many references 1912 to a victim's domain and send many emails to different SPF 1913 verifiers; those SPF verifiers would then create a DoS attack. In 1914 effect, the SPF verifiers are being used to amplify the attacker's 1915 bandwidth by using fewer octets in the SMTP session than are used 1916 by the DNS queries. Using SPF verifiers also allows the attacker 1917 to hide the true source of the attack. This potential attack is 1918 based on large volumes of mail being transmitted. 1920 o Whereas implementations of check_host() are supposed to limit the 1921 number of DNS lookups, malicious domains could publish records 1922 that exceed these limits in an attempt to waste computation effort 1923 at their targets when they send them mail. Malicious domains 1924 could also design SPF records that cause particular 1925 implementations to use excessive memory or CPU usage, or to 1926 trigger bugs. If a receiver is configured to accept mail with an 1927 SPF result of "temperror", such an attack might result in mail 1928 that would otherwise have been rejected due to an SPF "fail" 1929 result being accepted. This potential attack is based on 1930 specially crafted SPF records being used to exhaust DNS resources 1931 of the victim. 1933 o Malicious parties could send a large volume of mail purporting to 1934 come from the intended target to a wide variety of legitimate mail 1935 hosts. These legitimate machines would then present a DNS load on 1936 the target as they fetched the relevant records. 1938 o Malicious parties could, in theory, use SPF records as a vehicle 1939 for DNS lookup amplification for a denial-of-service-attack. In 1940 this scenario, the attacker publishes an SPF record in its own DNS 1941 that uses "a" and "mx" mechanisms directed toward the intended 1942 victim, e.g. "a:example.com a:foo.example.com a:bar.example.com 1943 ..." and then distributes mail with a MAIL FROM value including 1944 its own domain in large volume to a wide variety of destinations. 1945 Any such destination operating an SPF verifier will begin querying 1946 all of the names associated with the "a" mechanisms in that 1947 record. The names used in the record needn't exist for the attack 1948 to be effective. Operational experience since publication of 1949 [RFC4408] suggests that mitigation of this class of attack can be 1950 accomplished with minimal impact on the deployed base by having 1951 the verifier abort processing and return "permerror" 1952 (Section 2.6.7) once more than two "void lookups" have been 1953 encountered (defined in Section 4.6.4). 1955 Of these, the case of a third party referenced in the SPF record is 1956 the easiest for a DoS attack to effectively exploit. As a result, 1957 limits that might seem reasonable for an individual mail server can 1958 still allow an unreasonable amount of bandwidth amplification. 1959 Therefore, the processing limits need to be quite low. 1961 11.2. SPF-Authorized Email May Contain Other False Identities 1963 Do not construe the "MAIL FROM" and "HELO" identity authorizations to 1964 provide more assurance than they do. It is entirely possible for a 1965 malicious sender to inject a message using his own domain in the 1966 identities used by SPF, to have that domain's SPF record authorize 1967 the sending host, and yet the message can easily list other 1968 identities in its header. Unless the user or the MUA takes care to 1969 note that the authorized identity does not match the other more 1970 commonly-presented identities (such as the From: header field), the 1971 user might be lulled into a false sense of security. 1973 11.3. Spoofed DNS and IP Data 1975 There are two aspects of this protocol that malicious parties could 1976 exploit to undermine the validity of the check_host() function: 1978 o The evaluation of check_host() relies heavily on DNS. A malicious 1979 attacker could attack the DNS infrastructure and cause 1980 check_host() to see spoofed DNS data, and then return incorrect 1981 results. This could include returning "pass" for an value 1982 where the actual domain's record would evaluate to "fail". See 1983 [RFC3833] for a description of DNS weaknesses. 1985 o The client IP address, , is assumed to be correct. In a 1986 modern, correctly configured system the risk of this not being 1987 true is nil. 1989 11.4. Cross-User Forgery 1991 By definition, SPF policies just map domain names to sets of 1992 authorized MTAs, not whole email addresses to sets of authorized 1993 users. Although the "l" macro (Section 7) provides a limited way to 1994 define individual sets of authorized MTAs for specific email 1995 addresses, it is generally impossible to verify, through SPF, the use 1996 of specific email addresses by individual users of the same MTA. 1998 It is up to mail services and their MTAs to directly prevent 1999 cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be 2000 restricted to using only those email addresses that are actually 2001 under their control (see [RFC6409], Section 6.1). Another means to 2002 verify the identity of individual users is message cryptography such 2003 as PGP ([RFC4880]) or S/MIME ([RFC5751]). 2005 11.5. Untrusted Information Sources 2007 An SPF compliant receiver gathers information from the SMTP commands 2008 it receives and from the published DNS records of the sending domain 2009 holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the 2010 envelope, and SPF DNS records published by the domain holder). These 2011 parameters are not validated in the SMTP process. 2013 All of these pieces of information are generated by actors outside of 2014 the authority of the receiver, and thus are not guaranteed to be 2015 accurate or legitimate. 2017 11.5.1. Recorded Results 2019 This information, passed to the receiver in the Received-SPF: or 2020 Authentication-Results: trace fields, can be returned to the client 2021 MTA as an SMTP rejection message. If such an SMTP rejection message 2022 is generated, the information from the trace fields has to be checked 2023 for such problems as invalid characters and excessively long lines. 2025 11.5.2. External Explanations 2027 When the authorization check fails, an explanation string could be 2028 included in the reject response. Both the sender and the rejecting 2029 receiver need to be aware that the explanation was determined by the 2030 publisher of the SPF record checked and, in general, not the 2031 receiver. The explanation can contain malicious URLs, or it might be 2032 offensive or misleading. 2034 Explanations returned to sender domains due to "exp" modifiers 2035 (Section 6.2) were generated by the sender policy published by the 2036 domain holders themselves. As long as messages are only returned 2037 with non-delivery notification ([RFC3464]) to domains publishing the 2038 explanation strings from their own DNS SPF records, the only affected 2039 parties are the original publishers of the domain's SPF records. 2041 In practice, such non-delivery notifications can be misdirected, such 2042 as when an MTA accepts an email and only later generates the 2043 notification to a forged address, or when an email forwarder does not 2044 direct the bounce back to the original sender. 2046 11.5.3. Macro Expansion 2048 Macros (Section 7) allow senders to inject arbitrary text (any non- 2049 null [US-ASCII] character) into receiver DNS queries. It is 2050 necessary to be prepared for hostile or unexpected content. 2052 11.6. Privacy Exposure 2054 Checking SPF records causes DNS queries to be sent to the domain 2055 owner. These DNS queries, especially if they are caused by the 2056 "exists" mechanism, can contain information about who is sending 2057 email and likely to which MTA the email is being sent. This can 2058 introduce some privacy concerns, which are more or less of an issue 2059 depending on local laws and the relationship between the ADMD and the 2060 person sending the email. 2062 11.7. Delivering Mail Producing a 'Fail' Result 2064 Operators that choose to deliver mail for which SPF produces a "fail" 2065 result need to understand that they are admitting content that is 2066 explicitly not authorized by the purported sender. While there are 2067 known failure modes that can be considered "false negatives", the 2068 distinct choice to admit those messages increases end-user exposure 2069 to likely harm. This is especially true for domains belonging to 2070 known good actors that are typically well-behaved; unauthorized mail 2071 from those sources might well be subjected to much higher skepticism 2072 and content analysis. 2074 SPF does not, however, include the capacity for identifying good 2075 actors from bad ones, nor does it handle the concept of known actors 2076 versus unknown ones. Those notions are out of scope for this 2077 specification. 2079 12. Contributors and Acknowledgements 2081 This document is largely based on the work of Meng Weng Wong, Mark 2082 Lentczner, and Wayne Schlitt. Although, as this section 2083 acknowledges, many people have contributed to this document, a very 2084 large portion of the writing and editing are due to Meng, Mark, and 2085 Wayne. 2087 This design owes a debt of parentage to [RMX] by Hadmut Danisch and 2088 to [DMP] by Gordon Fecyk. The idea of using a DNS record to check 2089 the legitimacy of an email address traces its ancestry further back 2090 through messages on the namedroppers mailing list by Paul Vixie 2091 [Vixie] (based on suggestion by Jim Miller) and by David Green 2092 [Green]. 2094 Philip Gladstone contributed the concept of macros to the 2095 specification, multiplying the expressiveness of the language and 2096 making per-user and per-IP lookups possible. 2098 The authors of both this document and [RFC4408] would also like to 2099 thank the literally hundreds of individuals who have participated in 2100 the development of this design. They are far too numerous to name, 2101 but they include the following: 2103 The participants in the SPFbis working group. 2104 The folks on the spf-discuss mailing list. 2105 The folks on the SPAM-L mailing list. 2106 The folks on the IRTF ASRG mailing list. 2107 The folks on the IETF MARID mailing list. 2108 The folks on #perl. 2110 13. IANA Considerations 2112 13.1. The SPF DNS Record Type 2114 Per [RFC4408], the IANA assigned the Resource Record Type and Qtype 2115 from the DNS Parameters Registry for the SPF RR type with code 99. 2116 The format of this type is identical to the TXT RR [RFC1035]. The 2117 character content of the record is encoded as [US-ASCII]. 2119 Studies have shown that RRTYPE 99 has not seen any substantial use, 2120 and in fact its existence and mechanism defined in [RFC4408] has led 2121 to some interoperability issues. Accordingly, its use is now 2122 obsolete, and new implementations are not to use it. 2124 IANA is requested to update the Resource Record (RR) TYPEs registry 2125 to indicate that this document is the reference document for that 2126 RRTYPE. 2128 [NOTE TO RFC EDITOR: (to be changed to " ... has updated ..." upon 2129 publication)] 2131 13.2. The Received-SPF Mail Header Field 2133 Per [RFC3864], the "Received-SPF:" header field is added to the IANA 2134 Permanent Message Header Field Registry. The following is the 2135 registration template: 2137 Header field name: Received-SPF 2138 Applicable protocol: mail ([RFC5322]) 2139 Status: standard 2140 Author/Change controller: IETF 2141 Specification document(s): RFC XXXX 2142 [NOTE TO RFC EDITOR: (this document)] 2144 13.3. SPF Modifier Registry 2146 IANA is requested to change the reference for the exp and redirect 2147 modifiers in the Modifier Names registry, under Sender Policy 2148 Framework Parameters, from [RFC4408] to this document. Their status 2149 is unchanged. 2151 14. References 2153 14.1. Normative References 2155 [RFC1035] Mockapetris, P., "Domain names - implementation and 2156 specification", STD 13, RFC 1035, November 1987. 2158 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 2159 and Support", STD 3, RFC 1123, October 1989. 2161 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2162 Requirement Levels", BCP 14, RFC 2119, March 1997. 2164 [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", 2165 RFC 3463, January 2003. 2167 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 2168 Procedures for Message Header Fields", BCP 90, RFC 3864, 2169 September 2004. 2171 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2172 Resource Identifier (URI): Generic Syntax", STD 66, 2173 RFC 3986, January 2005. 2175 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 2176 Architecture", RFC 4291, February 2006. 2178 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 2179 Specifications: ABNF", STD 68, RFC 5234, January 2008. 2181 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 2182 October 2008. 2184 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 2185 October 2008. 2187 [RFC5451] Kucherawy, M., "Message Header Field for Indicating 2188 Message Authentication Status", RFC 5451, April 2009. 2190 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 2191 July 2009. 2193 [RFC5890] Klensin, J., "Internationalized Domain Names for 2194 Applications (IDNA): Definitions and Document Framework", 2195 RFC 5890, August 2010. 2197 [US-ASCII] 2198 American National Standards Institute (formerly United 2199 States of America Standards Institute), "USA Code for 2200 Information Interchange, X3.4", 1968. 2202 ANSI X3.4-1968 has been replaced by newer versions with 2203 slight modifications, but the 1968 version remains 2204 definitive for the Internet. 2206 14.2. Informative References 2208 [DMP] Fecyk, G., "Designated Mailers Protocol". 2210 Work In Progress 2212 [Green] Green, D., "Domain-Authorized SMTP Mail", 2002. 2214 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 2215 STD 13, RFC 1034, November 1987. 2217 [RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983, 2218 August 1996. 2220 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", 2221 RFC 2671, August 1999. 2223 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 2224 specifying the location of services (DNS SRV)", RFC 2782, 2225 February 2000. 2227 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format 2228 for Delivery Status Notifications", RFC 3464, 2229 January 2003. 2231 [RFC3696] Klensin, J., "Application Techniques for Checking and 2232 Transformation of Names", RFC 3696, February 2004. 2234 [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain 2235 Name System (DNS)", RFC 3833, August 2004. 2237 [RFC3834] Moore, K., "Recommendations for Automatic Responses to 2238 Electronic Mail", RFC 3834, August 2004. 2240 [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 2241 for Authorizing Use of Domains in E-Mail, Version 1", 2242 RFC 4408, April 2006. 2244 [RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing 2245 (CIDR): The Internet Address Assignment and Aggregation 2246 Plan", BCP 122, RFC 4632, August 2006. 2248 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 2249 Thayer, "OpenPGP Message Format", RFC 4880, November 2007. 2251 [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension 2252 for Authentication", RFC 4954, July 2007. 2254 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 2255 Mail Extensions (S/MIME) Version 3.2 Message 2256 Specification", RFC 5751, January 2010. 2258 [RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782, 2259 February 2010. 2261 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 2262 STD 72, RFC 6409, November 2011. 2264 [RFC6647] Kucherawy, M. and D. Crocker, "Email Greylisting: An 2265 Applicability Statement for SMTP", RFC 6647, June 2012. 2267 [RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham, 2268 "Deprecating the "X-" Prefix and Similar Constructs in 2269 Application Protocols", BCP 178, RFC 6648, June 2012. 2271 [RFC6652] Kitterman, S., "Sender Policy Framework (SPF) 2272 Authentication Failure Reporting Using the Abuse Reporting 2273 Format", RFC 6652, June 2012. 2275 [RFC6686] Kucherawy, M., "Resolution of the Sender Policy Framework 2276 (SPF) and Sender ID Experiments", RFC 6686, July 2012. 2278 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 2279 for DNS (EDNS(0))", STD 75, RFC 6891, April 2013. 2281 [RMX] Danisch, H., "The RMX DNS RR Type for light weight sender 2282 authentication". 2284 Work In Progress 2286 [Vixie] Vixie, P., "Repudiating MAIL FROM", 2002. 2288 Appendix A. Collected ABNF 2290 This section is normative and any discrepancies with the ABNF 2291 fragments in the preceding text are to be resolved in favor of this 2292 grammar. 2294 See [RFC5234] for ABNF notation. Please note that as per this ABNF 2295 definition, literal text strings (those in quotes) are case- 2296 insensitive. Hence, "mx" matches "mx", "MX", "mX", and "Mx". 2298 record = version terms *SP 2299 version = "v=spf1" 2301 terms = *( 1*SP ( directive / modifier ) ) 2303 directive = [ qualifier ] mechanism 2304 qualifier = "+" / "-" / "?" / "~" 2305 mechanism = ( all / include 2306 / a / mx / ptr / ip4 / ip6 / exists ) 2308 all = "all" 2309 include = "include" ":" domain-spec 2310 a = "a" [ ":" domain-spec ] [ dual-cidr-length ] 2311 mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ] 2312 ptr = "ptr" [ ":" domain-spec ] 2313 ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] 2314 ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ] 2315 exists = "exists" ":" domain-spec 2317 modifier = redirect / explanation / unknown-modifier 2318 redirect = "redirect" "=" domain-spec 2319 explanation = "exp" "=" domain-spec 2320 unknown-modifier = name "=" macro-string 2321 ; where name is not any known modifier 2323 ip4-cidr-length = "/" 1*DIGIT 2324 ip6-cidr-length = "/" 1*DIGIT 2325 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] 2327 ip4-network = qnum "." qnum "." qnum "." qnum 2328 qnum = DIGIT ; 0-9 2329 / %x31-39 DIGIT ; 10-99 2330 / "1" 2DIGIT ; 100-199 2331 / "2" %x30-34 DIGIT ; 200-249 2332 / "25" %x30-35 ; 250-255 2333 ; conventional dotted quad notation. e.g., 192.0.2.0 2334 ip6-network = 2335 ; e.g., 2001:DB8::CD30 2337 domain-spec = macro-string domain-end 2338 domain-end = ( "." toplabel [ "." ] ) / macro-expand 2340 toplabel = ( *alphanum ALPHA *alphanum ) / 2341 ( 1*alphanum "-" *( alphanum / "-" ) alphanum ) 2342 ; LDH rule plus additional TLD restrictions 2343 ; (see [RFC3696], Section 2 for background) 2344 alphanum = ALPHA / DIGIT 2346 explain-string = *( macro-string / SP ) 2348 macro-string = *( macro-expand / macro-literal ) 2349 macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) 2350 / "%%" / "%_" / "%-" 2351 macro-literal = %x21-24 / %x26-7E 2352 ; visible characters except "%" 2353 macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / 2354 "c" / "r" / "t" / "v" 2355 transformers = *DIGIT [ "r" ] 2356 delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" 2358 name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) 2360 header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] 2361 [ key-value-list ] CRLF 2363 result = "pass" / "fail" / "softfail" / "neutral" / 2364 "none" / "temperror" / "permerror" 2366 key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) 2367 [";"] 2369 key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) 2371 key = "client-ip" / "envelope-from" / "helo" / 2372 "problem" / "receiver" / "identity" / 2373 "mechanism" / name 2375 identity = "mailfrom" ; for the "MAIL FROM" identity 2376 / "helo" ; for the "HELO" identity 2377 / name ; other identities 2379 ALPHA = 2380 DIGIT = <0-9 as per [RFC5234]> 2381 SP = 2382 domain = 2383 dot-atom = 2384 quoted-string = 2385 comment = 2386 CFWS = 2387 FWS = 2388 CRLF = 2390 Appendix B. Extended Examples 2392 These examples are based on the following DNS setup: 2394 ; A domain with two mail servers, two hosts 2395 ; and two servers at the domain name 2396 $ORIGIN example.com. 2397 @ MX 10 mail-a 2398 MX 20 mail-b 2399 A 192.0.2.10 2400 A 192.0.2.11 2401 amy A 192.0.2.65 2402 bob A 192.0.2.66 2403 mail-a A 192.0.2.129 2404 mail-b A 192.0.2.130 2405 www CNAME example.com. 2407 ; A related domain 2408 $ORIGIN example.org. 2409 @ MX 10 mail-c 2410 mail-c A 192.0.2.140 2412 ; The reverse IP for those addresses 2413 $ORIGIN 2.0.192.in-addr.arpa. 2414 10 PTR example.com. 2415 11 PTR example.com. 2416 65 PTR amy.example.com. 2417 66 PTR bob.example.com. 2418 129 PTR mail-a.example.com. 2419 130 PTR mail-b.example.com. 2420 140 PTR mail-c.example.org. 2422 ; A rogue reverse IP domain that claims to be 2423 ; something it's not 2424 $ORIGIN 0.0.10.in-addr.arpa. 2425 4 PTR bob.example.com. 2427 B.1. Simple Examples 2429 These examples show various possible published records for 2430 example.com and which values if would cause check_host() to 2431 return "pass". Note that is "example.com". 2433 v=spf1 +all 2434 -- any passes 2436 v=spf1 a -all 2437 -- hosts 192.0.2.10 and 192.0.2.11 pass 2439 v=spf1 a:example.org -all 2440 -- no sending hosts pass since example.org has no A records 2442 v=spf1 mx -all 2443 -- sending hosts 192.0.2.129 and 192.0.2.130 pass 2445 v=spf1 mx:example.org -all 2446 -- sending host 192.0.2.140 passes 2448 v=spf1 mx mx:example.org -all 2449 -- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass 2451 v=spf1 mx/30 mx:example.org/30 -all 2452 -- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes 2454 v=spf1 ptr -all 2455 -- sending host 192.0.2.65 passes (reverse DNS is valid and is in 2456 example.com) 2457 -- sending host 192.0.2.140 fails (reverse DNS is valid, but not 2458 in example.com) 2459 -- sending host 10.0.0.4 fails (reverse IP is not valid) 2461 v=spf1 ip4:192.0.2.128/28 -all 2462 -- sending host 192.0.2.65 fails 2463 -- sending host 192.0.2.129 passes 2465 B.2. Multiple Domain Example 2467 These examples show the effect of related records: 2469 example.org: "v=spf1 include:example.com include:example.net -all" 2471 This record would be used if mail from example.org actually came 2472 through servers at example.com and example.net. Example.org's 2473 designated servers are the union of example.com's and example.net's 2474 designated servers. 2476 la.example.org: "v=spf1 redirect=example.org" 2477 ny.example.org: "v=spf1 redirect=example.org" 2478 sf.example.org: "v=spf1 redirect=example.org" 2480 These records allow a set of domains that all use the same mail 2481 system to make use of that mail system's record. In this way, only 2482 the mail system's record needs to be updated when the mail setup 2483 changes. These domains' records never have to change. 2485 B.3. DNSBL Style Example 2487 Imagine that, in addition to the domain records listed above, there 2488 are these (see [RFC5782]): 2490 $ORIGIN _spf.example.com. 2491 mary.mobile-users A 127.0.0.2 2492 fred.mobile-users A 127.0.0.2 2493 15.15.168.192.joel.remote-users A 127.0.0.2 2494 16.15.168.192.joel.remote-users A 127.0.0.2 2496 The following records describe users at example.com who mail from 2497 arbitrary servers, or who mail from personal servers. 2499 example.com: 2501 v=spf1 mx 2502 include:mobile-users._spf.%{d} 2503 include:remote-users._spf.%{d} 2504 -all 2506 mobile-users._spf.example.com: 2508 v=spf1 exists:%{l1r+}.%{d} 2510 remote-users._spf.example.com: 2512 v=spf1 exists:%{ir}.%{l1r+}.%{d} 2514 B.4. Multiple Requirements Example 2516 Say that your sender policy requires both that the IP address is 2517 within a certain range and that the reverse DNS for the IP matches. 2518 This can be done several ways, including the following: 2520 example.com. SPF ( "v=spf1 " 2521 "-include:ip4._spf.%{d} " 2522 "-include:ptr._spf.%{d} " 2523 "+all" ) 2524 ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all" 2525 ptr._spf.example.com. SPF "v=spf1 -ptr +all" 2527 This example shows how the "-include" mechanism can be useful, how an 2528 SPF record that ends in "+all" can be very restrictive, and the use 2529 of De Morgan's Law. 2531 Appendix C. Changes in implementation requirements from RFC 4408 2533 The modifications to implementation requirements from [RFC4408] are 2534 all either (a) corrections to errors in [RFC4408], or (b) additional 2535 documentation based on consensus of operational experience acquired 2536 since publication of [RFC4408]. 2538 o Use of DNS RR type SPF (99) has been removed from the protocol, 2539 see [RFC6686] for background. 2541 o A new DNS related processing limit based on "void lookups" has 2542 been added (Section 4.6.4). 2544 o Use of the ptr mechanism and the %p macro have been strongly 2545 discouraged Section 5.5 and Section 7.2. They remain part of the 2546 protocol because they were found to be in use, but records ought 2547 to be updated to avoid them. 2549 o Use of the "Authentication-Results" header field [RFC5451] as a 2550 possible alternative to use of the "Received-SPF" header field is 2551 discussed (Section 9.2). 2553 o There have been a number of minor corrections to the ABNF to make 2554 it more clear and correct Appendix A. SPF library implementers 2555 should give the revised ABNF a careful review to determine if 2556 implementation changes are needed. 2558 o Use of X- fields in the ABNF has been removed see [RFC6648] for 2559 background. 2561 o Ambiguity about how to deal with invalid domain-spec after macro 2562 expansion has been documented. Depending on one specific behavior 2563 has to be avoided (Section 4.8). 2565 o General operational information has been updated and expanded 2566 based on eight years of post [RFC4408] operations experience. See 2567 Section 10 and Appendices D - H below. 2569 o Security considerations have been reviewed and updated 2570 (Section 11). 2572 Appendix D. Further Testing Advice 2574 Another approach that can be helpful to publish records that include 2575 a "tracking exists:" mechanism. By looking at the name server logs, 2576 a rough list can then be generated. For example: 2578 v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all 2580 Appendix E. SPF/Mediator Interactions 2582 There are three places that techniques can be used to ameliorate 2583 unintended SPF failures with mediators. 2585 E.1. Originating ADMDs 2587 The beginning, when email is first sent: 2589 o "Neutral" results could be given for IP addresses that might be 2590 forwarders, instead of "fail" results based on a list of known 2591 reliable forwarders. For example: 2593 "v=spf1 mx ?exists:%{ir}.whitlist.example.org -all" 2595 This would cause a lookup on an DNS white list (DNSWL) and cause a 2596 result of "fail" only for email not either coming from the 2597 domain's mx host(s) (SPF pass) or white listed sources (SPF 2598 neutral). This, in effect, outsources an element of sender policy 2599 to the maintainer of the whitelist. 2601 o The "MAIL FROM" identity could have additional information in the 2602 local-part that cryptographically identifies the mail as coming 2603 from an authorized source. In this case, such an SPF record could 2604 be used: 2606 "v=spf1 mx exists:%{l}._spf_verify.%{d} -all" 2608 Then, a specialized DNS server can be set up to serve the 2609 _spf_verify subdomain that validates the local-part. Although 2610 this requires an extra DNS lookup, this happens only when the 2611 email would otherwise be rejected as not coming from a known good 2612 source. 2613 Note that due to the 63-character limit for domain labels, this 2614 approach only works reliably if the local-part signature scheme is 2615 guaranteed either to only produce local-parts with a maximum of 63 2616 characters or to gracefully handle truncated local-parts. 2618 o Similarly, a specialized DNS server could be set up that will 2619 rate-limit the email coming from unexpected IP addresses. 2621 "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all" 2623 o SPF allows the creation of per-user policies for special cases. 2624 For example, the following SPF record and appropriate wildcard DNS 2625 records can be used: 2627 "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}" 2629 E.2. Mediators 2631 The middle, when email is forwarded:. 2633 o Mediators can solve the problem by rewriting the "MAIL FROM" to be 2634 in their own domain. This means mail rejected from the external 2635 mailbox will have to be forwarded back to the original sender by 2636 the forwarding service. Various schemes to do this exist though 2637 they vary widely in complexity and resource requirements on the 2638 part of the mediator. 2640 o Several popular MTAs can be forced from "alias" semantics to 2641 "mailing list" semantics by configuring an additional alias with 2642 "owner-" prepended to the original alias name (e.g., an alias of 2643 "friends: george@example.com, fred@example.org" would need another 2644 alias of the form "owner-friends: localowner"). 2646 o Mediators could reject mail that would "fail" SPF if forwarded 2647 using an SMTP reply code of 551, User not local, (see [RFC5321] 2648 section 3.4) to communicate the correct target address to resend 2649 the mail to. 2651 E.3. Receving ADMDs 2653 The end, when email is received: 2655 o If the owner of the external mailbox wishes to trust the mediator, 2656 he can direct the external mailbox's MTA to skip SPF tests when 2657 the client host belongs to the mediator. 2659 o Tests against other identities, such as the "HELO" identity, can 2660 be used to override a failed test against the "MAIL FROM" 2661 identity. 2663 o For larger domains, it might not be possible to have a complete or 2664 accurate list of forwarding services used by the owners of the 2665 domain's mailboxes. In such cases, whitelists of generally- 2666 recognized forwarding services could be employed. 2668 Appendix F. Mail Services 2670 MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail 2671 services to third-party domains, such as sending of bulk mail, might 2672 want to adjust their configurations in light of the authorization 2673 check described in this document. If the domain part of the "MAIL 2674 FROM" identity used for such email uses the domain of one of the MSPs 2675 domain, then the provider needs only to ensure that its sending host 2676 is authorized by its own SPF record, if any. 2678 If the "MAIL FROM" identity does not use the MSP's domain, then extra 2679 care has to be taken. The SPF record format has several options for 2680 the third-party domain to authorize the service provider's MTAs to 2681 send mail on its behalf. For MSPs, such as ISPs, that have a wide 2682 variety of customers using the same MTA, steps are required to 2683 mitiate the risk of cross-customer forgery (see Section 11.4). 2685 Appendix G. MTA Relays 2687 Relays are described in [RFC5598] Section 2.2.2. The authorization 2688 check generally precludes the use of arbitrary MTA relays between 2689 sender and receiver of an email message. 2691 Within an organization, MTA relays can be effectively deployed. 2692 However, for purposes of this document, such relays are effectively 2693 transparent. The SPF authorization check is a check between border 2694 MTAs of different ADMDs. 2696 For mail senders, this means that published SPF records have to 2697 authorize any MTAs that actually send across the Internet. Usually, 2698 these are just the border MTAs as internal MTAs simply forward mail 2699 to these MTAs for relaying. 2701 The receiving ADMD will generally want to perform the authorization 2702 check at the boundary MTAs, including all secondary MXs. Internal 2703 MTAs (including MTAs that might serve both as boundary MTAs and 2704 internal relays from secondary MXs when they are processing the 2705 relayed mail stream) then do not perform the authorization test. To 2706 perform the authorization test other than at the boundary, the host 2707 that first transferred the message to the receiving ADMD have to be 2708 determined, which can be difficult to extract from the message header 2709 because (a) header fields can be forged or malformed, and (b) there's 2710 no standard way to encode that information such that it can be 2711 reliably extracted. Testing other than at the boundary is likely to 2712 produce unreliable results. This is described further in Appendix C 2713 of [RFC5451]. 2715 Appendix H. Local Policy Considerations 2717 SPF results can be used in combination with other methods to 2718 determine the final local disposition (either positive or negative of 2719 a message. It can also be considered dispositive on its own. 2721 H.1. Policy For SPF Pass 2723 SPF pass results can be used in combination with "white lists" of 2724 known "good" domains to bypass some or all additional pre-delivery 2725 email checks. Exactly which checks and how to determine appropriate 2726 white list entries has to be based on local conditions and 2727 requirements. 2729 H.2. Policy For SPF Fail 2731 SPF fail results can be used to reject messages during the SMTP 2732 transaction based on either "MAIL FROM" or "HELO" identity results. 2733 This reduces resource requirements for various content filtering 2734 methods and conserves bandwidth since rejection can be done before 2735 the SMTP content is transferred. It also gives immediate feedback to 2736 the sender who might then be able to resolve the issue. Due to some 2737 of the issues described above in this section (Section 10), SPF based 2738 rejection does present some risk of rejecting legitimate email when 2739 rejecting based on "MAIL FROM" results. 2741 SPF fail results can alternately be used as one input into a larger 2742 set of evaluations which might, based on a combination with other 2743 evaluation techniques, result in the email being marked negatively in 2744 some way (this might be via delivery to a special spam folder, 2745 modifying subject lines, or other locally determined means). 2746 Developing the details of such an approach have to be based on local 2747 conditions and requirements. Using SPF results in this way does not 2748 have the advantages of resource conservation and immediate feedback 2749 to the sender associated with SMTP rejection, but could produce fewer 2750 undesirable rejections in a well designed system. Such an approach 2751 might result in email that was not authorized by the sending ADMD 2752 being unknowingly delivered to end users. 2754 Either general approach can be used as they both leave a clear 2755 disposition of emails. They are either delivered in some manner or 2756 the sender is notified of the failure. Other dispositions such as 2757 "dropping" or deleting email after acceptance are inappropriate 2758 because they leave uncertainty and reduce the overall reliabilility 2759 and utility of email across the Internet. 2761 H.3. Policy For SPF Permerror 2763 The "permerror" result (see Section 2.6.7) indicates the SPF 2764 processing module at the receiver determined that the retrieved SPF 2765 policy record could not be interpreted. This gives no true 2766 indication about the authorized use of the data found in the 2767 envelope. 2769 As with all results, implementers have a choice to make regarding 2770 what to do with a message that yields this result. SMTP allows only 2771 a few basic options. 2773 Rejection of the message is an option, in that it is the one thing a 2774 receiver can do to draw attention to the difficulty encountered while 2775 protecting itself from messages that do not have a definite SPF 2776 result of some kind. However, if the SPF implementation is defective 2777 and returns spurious "permerror" results, only the sender is actively 2778 notified of the defect (in the form of rejected mail), and not the 2779 receiver making use of SPF. 2781 The less intrusive handling choice is to deliver the message, perhaps 2782 with some kind of annotation of the difficulty encountered and/or 2783 logging of a similar nature. However, this will not be desirable to 2784 operators that wish to implement SPF checking as strictly as 2785 possible, nor is this sort of passive problem reporting typically 2786 effective. 2788 There is of course the option placing this choice in the hands of the 2789 operator rather than the implementer since this kind of choice is 2790 often a matter of local policy rather than a condition with a 2791 universal solution, but this adds one more piece of complexity to an 2792 already non-trivial environment. 2794 Both implementers and operators need to be cautious of all choices 2795 and outcomes when handling SPF results. 2797 H.4. Policy For SPF Temperror 2799 The "temperror" result (see Section 2.6.6) indicates the SPF 2800 processing module at the receiver could not retrieve and SPF policy 2801 record due to a (probably) transient condition. This gives no true 2802 indication about the authorized use of the data found in the 2803 envelope. 2805 As with all results, implementers have a choice to make regarding 2806 what to do with a message that yields this result. SMTP allows only 2807 a few basic options. 2809 Deferring the message is an option, in that it is the one thing a 2810 receiver can do to draw attention to the difficulty encountered while 2811 protecting itself from messages that do not have a definite SPF 2812 result of some kind. However, if the SPF implementation is defective 2813 and returns spurious "temperror" results, only the sender is actively 2814 notified of the defect (in the form of mail rejected after it times 2815 out of the sending queue), and not the receiver making use of SPF. 2817 Because of long queue lifetimes, it is possible that mail will be 2818 repeatedly deferred for several days and so any awareness by the 2819 sender of a problem could be quite delayed. If "temperrors" persist 2820 for multiple delivery attempts, it might be perferable to treat the 2821 error as permanent and reduce the amount of time the message is in 2822 transit. 2824 The less intrusive handling choice is to deliver the message, perhaps 2825 with some kind of annotation of the difficulty encountered and/or 2826 logging of a similar nature. However, this will not be desirable to 2827 operators that wish to implement SPF checking as strictly as 2828 possible, nor is this sort of passive problem reporting typically 2829 effective. 2831 There is of course the option placing this choice in the hands of the 2832 operator rather than the implementer since this kind of choice is 2833 often a matter of local policy rather than a condition with a 2834 universal solution, but this adds one more piece of complexity to an 2835 already non-trivial environment. 2837 Both implementers and operators need to be cautious of all choices 2838 and outcomes when handling SPF results. 2840 Appendix I. Protocol Status 2842 NOTE TO RFC EDITOR: To be removed prior to publication. 2844 SPF has been in development since the summer of 2003 and has seen 2845 deployment beyond the developers beginning in December 2003. The 2846 design of SPF slowly evolved until the spring of 2004 and has since 2847 stabilized. There have been quite a number of forms of SPF, some 2848 written up as documents, some submitted as Internet Drafts, and many 2849 discussed and debated in development forums. The protocol was 2850 originally defined in [RFC4408], which this document replaces. 2852 [RFC4408] was designed to clearly document the protocol defined by 2853 earlier draft specifications of SPF as used in existing 2854 implementations. This updated specification is intended to clarify 2855 identified ambiguities in [RFC4408], resolve technical issues 2856 identified in post-RFC 4408 deployment experience, and document 2857 widely deployed extensions to SPF that have been developed since 2858 [RFC4408] was published. 2860 This document updates and replaces RFC 4408 that was part of a group 2861 of simultaneously published Experimental RFCs (RFC 4405, RFC 4406, 2862 RFC 4407, and RFC 4408) in 2006. At that time the IESG requested the 2863 community observe the success or failure of the two approaches 2864 documented in these RFCs during the two years following publication, 2865 in order that a community consensus could be reached in the future. 2867 SPF is widely deployed by large and small email providers alike. 2868 There are multiple, interoperable implementations. 2870 For SPF (as documented in RFC 4408) a careful effort was made to 2871 collect and document lessons learned and errata during the two year 2872 period. The errata list has been stable (no new submissions) and 2873 only minor protocol lessons learned were identified. Resolution of 2874 the IESG's experiment is documented in [RFC6686]. 2876 Appendix J. Change History 2878 NOTE TO RFC EDITOR: Changes since RFC 4408 (to be removed prior to 2879 publication) 2881 Moved to standards track 2883 Authors updated 2885 IESG Note regarding experimental use replaced with discussion of 2886 results 2888 Process errata: 2890 Resolved Section 2.5.7 PermError on invalid domains after macro 2891 expansion errata in favor of documenting that different verifiers 2892 produce different results. 2894 Add %v macro to ABNF grammar 2896 Replace "uric" by "unreserved" 2898 Recommend an SMTP reply code for optional permerror rejections 2900 Correct syntax in Received-SPF examples 2902 Fix unknown-modifier clause is too greedy in ABNF 2904 Correct use of empty domain-spec on exp modifier 2906 Fix minor typo errata 2908 Convert to spfbis working group draft, 2909 draft-ietf-spfbis-4408bis-00 2911 Clarified text about IPv4 mapped addresses to resolve test suite 2912 ambiguity 2914 Clarified ambiguity about result when more than 10 "mx" or "ptr" 2915 records are returned for lookup to specify permerror. This 2916 resolves one of the test suite ambiguities 2918 Made all references to result codes lower case per issue #7 2920 Adjusted section 2.2 Requirement to check mail from per issue #15 2922 Added missing "v" element in macro-letter in the collected ABNF 2923 per issue #16 - section 8.1 was already fixed in the pre-WG draft 2924 Marked ptr and "p" macro SHOULD NOT use per issue #27 2926 Expunged lower case may from the draft per issue #8 2928 Expunged "x-" name as an obsolete concept 2930 Updated obslete references: RFC2821 to RFC5321, RFC2822 to 2931 RFC5322, and RFC4234 to RFC5234 2933 Refer to RFC6647 to describe greylisting instead of trying to 2934 describe it directly. 2936 Updated informative references to the current versions. 2938 Start to rework section 9 with some RFC5598 terms. 2940 Added mention of RFC 6552 feedback reports in section 9. 2942 Added draft-ietf-spfbis-experiment as an informational reference. 2944 Drop Type SPF. 2946 Try and clarify informational nature of RFC3696 2948 Fix ABNF nits and add missing definitions per Bill's ABNF checker. 2950 Make DNS lookup time limit SHOULD instead of MAY. 2952 Reorganize and clarify processing limits. Move hard limits to new 2953 section 4.6.4, Evaluation Limits. Move advice to non-normative 2954 section 10. 2956 Removed paragraph in section 11.1 about limiting total data 2957 volumes as it is unused (and removable per the charter) and serves 2958 no purpose (it isn't something that actually can be implemented in 2959 any reasonable way). 2961 Added text from Alessandro Vesely in section 10.1 to better 2962 explain DNS resource limits. 2964 Multiple editorial fixes from Murray Kucherawy's review. 2966 Also based on Murray's review, reworked SMTP identity definitions 2967 and made RFC 5598 a normative reference instead of informative. 2968 This is a downref that will have to be mentioned in the last call. 2970 Added RFC 3834 as an informative reference about backscatter. 2972 Added IDN requirements and normative reference to RFC 5890 to deal 2973 with the question "like DKIM did it.: 2975 Added informative reference to RFC 4632 for CIDR and use CIDR 2976 prefix length instead of CIDR-length to match its terminology. 2978 Simplified the exists description. 2980 Added text on creating a Authentication-Results header field that 2981 matches the Received-SPF header field information and added a 2982 normative reference to RFC 5451. 2984 Added informative reference to RFC 2782 due to SRV mention. 2986 Added informative reference to RFC 3464 due to DSN mention. 2988 Added informative reference to RFC 5617 for its DNS wildcard use. 2990 Clarified the intended match/no-match method for exists. 2992 Added new sections on Receiver policy for SPF pass, fail, and 2993 permerror. 2995 Added new section 10 discussion on treatment of bounces and the 2996 significance of HELO records. 2998 Added request to IANA to update the SPF modifier registry. 3000 Substantially reorganized the document for improved readability 3001 for new users based on WG consensus. 3003 Added new DNS "void lookup" processing limit to mitigate potential 3004 future risk of SPF being used as a DDoS vector. 3006 Author's Address 3008 Scott Kitterman 3009 Kitterman Technical Services 3010 3611 Scheel Dr 3011 Ellicott City, MD 21042 3012 United States of America 3014 Email: scott@kitterman.com