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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (August 17, 2013) is 3895 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 2367, 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 17, 2013 5 Intended status: Standards Track 6 Expires: February 18, 2014 8 Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, 9 Version 1 10 draft-ietf-spfbis-4408bis-19 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 18, 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 tokens "Local-part", "Domain", and "Mailbox are defined in 232 [RFC5321]. 234 "dot-atom", "quoted-string", "comment", "CFWS" (comment folded white 235 space), "FWS" (folded white space), and "CRLF" (carriage-return/ 236 line-feed) are defined in [RFC5322]. 238 1.1.3. MAIL FROM Definition 240 This document is concerned with the portion of a mail message 241 commonly called "envelope sender", "return path", "reverse path", 242 "bounce address", "5321 FROM", "MAIL FROM", or RFC5321.MailFrom. 243 Since these terms are either not well defined or often used casually, 244 this document uses "MAIL FROM" for consistency. This means the 245 RFC5321.MailFrom as defined in [RFC5598]. Note that other terms that 246 might superficially look like the common terms, such as 'reverse- 247 path', are used only as they are specified in their defining 248 documents. 250 1.1.4. HELO Definition 252 This document also makes use of the HELO/EHLO identity. The "HELO" 253 identity derives from either the SMTP HELO or EHLO command (see 254 [RFC5321]). Since HELO and EHLO can, in many cases, be used 255 interchangeably, they are identified commonly as "HELO" in this 256 document. This means RFC5321.HELO/.EHLO as defined in [RFC5598]. 257 These commands supply the identity of the SMTP client (sending host) 258 for the SMTP session. 260 1.2. check_host() 262 Section 4 introduces an algorithm to evaluate an SPF policy against 263 an arriving email transaction. In an early implementation, this 264 algorithm was encoded in a function called check_host(). That name 265 is used in this document as symbolic of the SPF evaluation algorithm, 266 but of course implementers are not required to use this name. 268 2. Operational Overview 270 2.1. Publishing Authorization 272 An SPF-compliant domain publishes valid SPF records as described in 273 Section 3. These records authorize the use of the relevant domain 274 names in the "HELO" and "MAIL FROM" identities by the MTAs specified 275 therein. 277 SPF results can be used to make both positive (source is authorized) 278 and negative (source is not authorized) determinations. If ADMDs 279 choose to publish SPF records and want to support receivers making 280 negative authorization determinations, it is necessary for them to 281 publish records that end in "-all", or redirect to other records that 282 do, otherwise, no definitive determination of authorization can be 283 made. Potential issues and mitigations associated with negative 284 determinations are discussed in Section 10. 286 ADMDs that wish to declare that no hosts are authorized to use their 287 DNS domain names in the HELO or MAIL FROM commands during SMTP 288 sessions can publish SPF records that say so for domain names that 289 are neither used in the domain part of email addresses nor expected 290 to originate mail. 292 When changing SPF records, care has to be taken to ensure that there 293 is a transition period so that the old policy remains valid until all 294 legitimate email can reasonably expect to have been checked. 295 [RFC5321] Section 4.5.4.1 discusses how long a message might be in 296 transit. While offline checks are possible, the closer to the 297 original transmission time checks are performed, the more likely they 298 are to get an SPF result that matches the sending ADMD intent at the 299 time the message was sent. 301 2.2. Checking Authorization 303 A mail receiver can perform a set of SPF checks for each mail message 304 it receives. An SPF check tests the authorization of a client host 305 to emit mail with a given identity. Typically, such checks are done 306 by a receiving MTA, but can be performed elsewhere in the mail 307 processing chain so long as the required information is available and 308 reliable. The "MAIL FROM" and "HELO" identities are checked as 309 described in Section 2.4 and Section 2.3 respectively. 311 Without explicit approval of the publishing ADMD, checking other 312 identities against SPF version 1 records is NOT RECOMMENDED because 313 there are cases that are known to give incorrect results. For 314 example, almost all mailing lists rewrite the "MAIL FROM" identity 315 (see Section 10.3), but some do not change any other identities in 316 the message. Documents that define other identities will have to 317 define the method for explicit approval. 319 It is possible that mail receivers will use the SPF check as part of 320 a larger set of tests on incoming mail. The results of other tests 321 might influence whether or not a particular SPF check is performed. 322 For example, finding the sending host's IP address on a local white 323 list might cause all other tests to be skipped and all mail from that 324 host to be accepted. 326 When a mail receiver decides to perform an SPF check, it has to use a 327 correctly-implemented check_host() function (Section 4) evaluated 328 with the correct parameters. Although the test as a whole is 329 optional, once it has been decided to perform a test it has to be 330 performed as specified so that the correct semantics are preserved 331 between publisher and receiver. 333 To make the test, the mail receiver MUST evaluate the check_host() 334 function with the arguments described in Section 4.1. 336 Although invalid, malformed, or non-existent domains cause SPF checks 337 to return "none" because no SPF record can be found, it has long been 338 the policy of many MTAs to reject email from such domains, especially 339 in the case of invalid "MAIL FROM". Rejecting email will prevent one 340 method of circumventing of SPF records. 342 Implementations have to take care to correctly extract the 343 from the data given with the SMTP MAIL FROM command as many MTAs will 344 still accept such things as source routes (see [RFC5321], Appendix 345 C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]). 346 These archaic features have been maliciously used to bypass security 347 systems. 349 2.3. The "HELO" Identity 351 It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM" 352 identity, but also separately check the "HELO" identity by applying 353 the check_host() function (Section 4) to the "HELO" identity as the 354 . Checking "HELO" promotes consistency of results and can 355 reduce DNS resource usage. If a conclusive determination about the 356 message can be made based on a check of "HELO", then the use of DNS 357 resources to process the typically more complex "MAIL FROM" can be 358 avoided. Additionally, since SPF records published for "HELO" 359 identities refer to a single host, when available, they are a very 360 reliable source of host authorization status. Checking "HELO" before 361 "MAIL FROM" is the RECOMMENDED sequence if both are checked. 363 Note that requirements for the domain presented in the EHLO or HELO 364 command are not always clear to the sending party, and SPF verifiers 365 have to be prepared for the identity to be an IP address literal (see 366 [RFC5321] section 4.1.3), or simply be malformed. This SPF check can 367 only be performed when the "HELO" string is a valid, multi-label 368 domain name. 370 2.4. The "MAIL FROM" Identity 372 SPF verifiers MUST check the "MAIL FROM" identity if a "HELO" check 373 has either not been performed or has not reached a definitive policy 374 result by applying the check_host() function to the "MAIL FROM" 375 identity as the . 377 [RFC5321] allows the reverse-path to be null (see Section 4.5.5 in 378 [RFC5321]). In this case, there is no explicit sender mailbox, and 379 such a message can be assumed to be a notification message from the 380 mail system itself. When the reverse-path is null, this document 381 defines the "MAIL FROM" identity to be the mailbox composed of the 382 local-part "postmaster" and the "HELO" identity (which might or might 383 not have been checked separately before). 385 2.5. Location of Checks 387 The authorization check SHOULD be performed during the processing of 388 the SMTP transaction that receives the mail. This reduces the 389 complexity of determining the correct IP address to use as an input 390 to check_host() and allows errors to be returned directly to the 391 sending MTA by way of SMTP replies. Appendix C of [RFC5451] provides 392 a more thorough discussion of this topic. 394 Performing the authorization check other than using the MAIL FROM and 395 client address at the time of the MAIL command during the SMTP 396 transaction can cause problems, such as the following: (1) It might 397 be difficult to accurately extract the required information from 398 potentially deceptive headers; (2) legitimate email might fail 399 because the sender's policy had since changed. 401 Generating non-delivery notifications to forged identities that have 402 failed the authorization check often constitutes backscatter, i.e., 403 inactionable, nuisance rejection notices. Operators are strongly 404 advised to avoid such practices. Section 2 of [RFC3834] describes 405 backscatter and the problems it causes. 407 2.6. Results of Evaluation 409 Section 4 defines check_host(), a model function definition that uses 410 the inputs defined above and the sender's policy published in the DNS 411 to reach a conclusion about client authorization. An SPF verifier 412 implements something semantically equivalent to the function defined 413 there. 415 This section enumerates and briefly defines the possible outputs of 416 that function. Note, however, that the protocol establishes no 417 normative requirements for handling any particular result. 418 Discussion of handling options for each result can be found in 419 Section 8. 421 2.6.1. None 423 A result of "none" means either (a) no syntactically valid DNS domain 424 name was extracted from the SMTP session that could be used as the 425 one to be authorized, or (b) no SPF records were retrieved from the 426 DNS. 428 2.6.2. Neutral 430 The ADMD has explicitly stated that it is not asserting whether the 431 IP address is authorized. 433 2.6.3. Pass 435 A "pass" result means that the client is authorized to inject mail 436 with the given identity. 438 2.6.4. Fail 440 A "fail" result is an explicit statement that the client is not 441 authorized to use the domain in the given identity. 443 2.6.5. Softfail 445 The ADMD has published a weak statement that the host is probably not 446 authorized. It has not published a stronger, more definitive policy 447 that results in a "fail". 449 2.6.6. Temperror 451 A "temperror" result means the SPF verifier encountered a transient 452 (generally DNS) error while performing the check. A later retry may 453 succeed without further operator action. 455 2.6.7. Permerror 457 A "permerror" result means the domain's published records could not 458 be correctly interpreted. This signals an error condition that 459 definitely requires operator intervention to be resolved. 461 3. SPF Records 463 An SPF record is a DNS record that declares which hosts are, and are 464 not, authorized to use a domain name for the "HELO" and "MAIL FROM" 465 identities. Loosely, the record partitions hosts into permitted and 466 not-permitted sets (though some hosts might fall into neither 467 category). 469 The SPF record is expressed as a single string of text found in the 470 RDATA of a single DNS TXT resource record; multiple SPF records are 471 not permitted for the same owner name. The record format and the 472 process for selecting records is described below in Section 4. An 473 example record is the following: 475 v=spf1 +mx a:colo.example.com/28 -all 477 This record has a version of "spf1" and three directives: "+mx", 478 "a:colo.example.com/28" (the + is implied), and "-all". 480 Each SPF record is placed in the DNS tree at the owner name it 481 pertains to, not a subdomain under it, such as is done with SRV 482 records [RFC2782]. 484 The example in this section might be published via these lines in a 485 domain zone file: 487 example.com. TXT "v=spf1 +mx a:colo.example.com/28 -all" 488 smtp-out.example.com. TXT "v=spf1 a -all" 490 Since TXT records have multiple uses, beware of other TXT records 491 published there for other purposes. They might cause problems with 492 size limits (see Section 3.4) and care has to be taken to ensure only 493 SPF records are used for SPF processing. 495 ADMDs publishing SPF records ought to keep the amount of DNS 496 information needed to evaluate a record to a minimum. Section 4.6.4 497 and Section 10.1.1 provide some suggestions about "include" 498 mechanisms and chained "redirect" modifiers. 500 3.1. DNS Resource Records 502 SPF records MUST be published as a DNS TXT (type 16) Resource Record 503 (RR) [RFC1035] only. The character content of the record is encoded 504 as [US-ASCII]. Use of alternative DNS RR types was supported in 505 SPF's experimental phase, but has been discontinued. See Appendix A 506 of [RFC6686] for further information. 508 3.2. Multiple DNS Records 510 A domain name MUST NOT have multiple records that would cause an 511 authorization check to select more than one record. See Section 4.5 512 for the selection rules. 514 3.3. Multiple Strings in a Single DNS record 516 As defined in [RFC1035] sections 3.3 and 3.3.14, a single text DNS 517 record can be composed of more than one string. If a published 518 record contains multiple character-strings, then the record MUST be 519 treated as if those strings are concatenated together without adding 520 spaces. For example: 522 IN TXT "v=spf1 .... first" "second string..." 524 is equivalent to: 526 IN TXT "v=spf1 .... firstsecond string..." 528 TXT records containing multiple strings are useful in constructing 529 records that would exceed the 255-octet maximum length of a 530 character-string within a single TXT record. 532 3.4. Record Size 534 The published SPF record for a given domain name SHOULD remain small 535 enough that the results of a query for it will fit within 512 octets. 536 This UDP limit is defined in [RFC1035] section 2.3.4, although it was 537 raised by [RFC2671]. Staying below 512 octets ought to prevent older 538 DNS implementations from failing over to TCP,and will work with UDP 539 in the absence of EDNS0 [RFC6891] support. Since the answer size is 540 dependent on many things outside the scope of this document, it is 541 only possible to give this guideline: If the combined length of the 542 DNS name and the text of all the records of a given type is under 450 543 octets, then DNS answers ought to fit in UDP packets. Records that 544 are too long to fit in a single UDP packet could be silently ignored 545 by SPF verifiers due to firewall and other issues that interfere with 546 the operation of DNS over TCP or using ENDS0. 548 Note that when computing the sizes for replies to queries of the TXT 549 format, one has to take into account any other TXT records published 550 at the domain name. Similarly, the sizes for replies to all queries 551 related to SPF have to be evaluated to fit in a single 512 octet UDP 552 packet. 554 3.5. Wildcard Records 556 Use of wildcard records for publishing is discouraged and care has to 557 be taken if they are used. If a zone includes wildcard MX records, 558 it might want to publish wildcard declarations, subject to the same 559 requirements and problems. In particular, the declaration MUST be 560 repeated for any host that has any RR records at all, and for 561 subdomains thereof. Consider the example in [RFC1034], Section 562 4.3.3. Based on that, we can do the following: 564 EXAMPLE.COM. MX 10 A.EXAMPLE.COM 565 EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 567 *.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 568 *.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 570 A.EXAMPLE.COM. A 203.0.113.1 571 A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 572 A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 574 *.A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 575 *.A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 577 SPF records have to be listed twice for every name within the zone: 578 once for the name, and once with a wildcard to cover the tree under 579 the name, in order to cover all domains in use in outgoing mail. 581 4. The check_host() Function 583 This description is not an API (Application Program Interface) 584 definition, but rather a function description used to illustrate the 585 algorithm. A compliant SPF implementation MUST produce results 586 semantically equivalent to this description. 588 The check_host() function fetches SPF records, parses them, and 589 evaluates them to determine whether a particular host is or is not 590 permitted to send mail with a given identity. Receiving ADMDs that 591 perform this check MUST correctly evaluate the check_host() function 592 as described here. 594 Implementations MAY use a different algorithm than the canonical 595 algorithm defined here, so long as the results are the same in all 596 cases. 598 4.1. Arguments 600 The check_host() function takes these arguments: 602 - the IP address of the SMTP client that is emitting the 603 mail, either IPv4 or IPv6. 605 - the domain that provides the sought-after authorization 606 information; initially, the domain portion of the "MAIL 607 FROM" or "HELO" identity. 609 - the "MAIL FROM" or "HELO" identity. 611 For recursive evaluations, the domain portion of might not 612 be the same as the argument when check_host() is initially 613 evaluated. In most other cases it will be the same. (See 614 Section 5.2 below). 616 Note that the argument might not be a well-formed domain 617 name. For example, if the reverse-path was null, then the EHLO/HELO 618 domain is used, with its associated problems (see Section 2.3). In 619 these cases, check_host() is defined in Section 4.3 to return a 620 "none" result. 622 4.2. Results 624 The function check_host() can return one of several results described 625 in Section 2.6. Based on the result, the action to be taken is 626 determined by the local policies of the receiver. This is discussed 627 in Section 8. 629 4.3. Initial Processing 631 If the is malformed (e.g. label longer than 63 characters, 632 zero-length label not at the end, etc.) or is not a multi-label 633 domain name, or if the DNS lookup returns "domain does not exist" 634 (RCODE 3), check_host() immediately returns the result "none". DNS 635 RCODES are defined in [RFC1035]. Properly formed domains are fully 636 qualified domains as defined in [RFC1983]. That is, in the DNS they 637 are implicitly qualified relative to the root (see section 3.1 of 638 [RFC1034]). Internationalized domain names MUST be encoded as 639 A-labels, as described in Section 2.3 of [RFC5890]. 641 If the has no local-part, substitute the string "postmaster" 642 for the local-part. 644 4.4. Record Lookup 646 In accordance with how the records are published (see Section 3 647 above), a DNS query needs to be made for the name, querying 648 for type TXT only. 650 If the DNS lookup returns a server failure (RCODE 2), or other error 651 (RCODE other than 0 or 3), or time out, then check_host() terminates 652 immediately with the result "temperror". 654 4.5. Selecting Records 656 Records begin with a version section: 658 record = version terms *SP 659 version = "v=spf1" 661 Starting with the set of records that were returned by the lookup, 662 discard records that do not begin with a version section of exactly 663 "v=spf1". Note that the version section is terminated either by an 664 SP character or the end of the record. A record with a version 665 section of "v=spf10" does not match and is discarded. 667 If the resultant record set includes no records, check_host() 668 produces the "none" result. If the resultant record set includes 669 more than one record, check_host() produces the "permerror" result. 671 4.6. Record Evaluation 673 The check_host() function parses and interprets the SPF record to 674 find a result for the current test. If there are any syntax errors 675 anywhere in the record, check_host() returns immediately with the 676 result "permerror", without further interpretation. 678 4.6.1. Term Evaluation 680 There are two types of terms: mechanisms (defined in Section 5) and 681 modifiers (defined in Section 6). A record contains an ordered list 682 of these as specified in the following Augmented Backus-Naur Form 683 (ABNF). 685 terms = *( 1*SP ( directive / modifier ) ) 687 directive = [ qualifier ] mechanism 688 qualifier = "+" / "-" / "?" / "~" 689 mechanism = ( all / include 690 / a / mx / ptr / ip4 / ip6 / exists ) 691 modifier = redirect / explanation / unknown-modifier 692 unknown-modifier = name "=" macro-string 693 ; where name is not any known modifier 695 name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) 697 Most mechanisms allow a ":" or "/" character after the name. 699 Modifiers always contain an equals ('=') character immediately after 700 the name, and before any ":" or "/" characters that might be part of 701 the macro-string. 703 Terms that do not contain any of "=", ":", or "/" are mechanisms, as 704 defined in Section 5. 706 As per the definition of the ABNF notation in [RFC5234], mechanism 707 and modifier names are case-insensitive. 709 4.6.2. Mechanisms 711 Each mechanism is considered in turn from left to right. If there 712 are no more mechanisms, the result is the default result as described 713 in Section 4.7. 715 When a mechanism is evaluated, one of three things can happen: it can 716 match, not match, or return an exception. 718 If it matches, processing ends and the qualifier value is returned as 719 the result of that record. If it does not match, processing 720 continues with the next mechanism. If it returns an exception, 721 mechanism processing ends and the exception value is returned. 723 The possible qualifiers, and the results they cause check_host() to 724 return are as follows: 726 "+" pass 727 "-" fail 728 "~" softfail 729 "?" neutral 731 The qualifier is optional and defaults to "+". 733 When a mechanism matches and the qualifier is "-", then a "fail" 734 result is returned and the explanation string is computed as 735 described in Section 6.2. 737 The specific mechanisms are described in Section 5. 739 4.6.3. Modifiers 741 Modifiers are not mechanisms. They do not return match or not-match. 742 Instead, they provide additional information. Although modifiers do 743 not directly affect the evaluation of the record, the "redirect" 744 modifier has an effect after all the mechanisms have been evaluated. 746 4.6.4. DNS Lookup Limits 748 SPF implementations MUST limit the total number of mechanisms and 749 modifiers ("terms") that cause any DNS query to 10 during SPF 750 evaluation. Specifically, the "include", "a", "mx", "ptr", and 751 "exists" mechanisms as well as the "redirect" modifier count against 752 this collective limit. The "all", "ip4", and "ip6" mechanisms do not 753 count against this limit. If this number is exceeded during a check, 754 a "permerror" MUST be returned. The "exp" modifier does not count 755 against this limit because the DNS lookup to fetch the explanation 756 string occurs after the SPF record evaluation has been completed. 758 When evaluating the "mx" mechanism, the number of "MX" resource 759 records queried is included in the overall limit of 10 mechanisms/ 760 modifiers that cause DNS lookups described above. The evaluation of 761 each "MX" record MUST NOT result in querying more than 10 address 762 records, either "A" or "AAAA" resource records. If this limit is 763 exceeded, the "mx" mechanism MUST produce a "permerror" result. 765 When evaluating the "ptr" mechanism or the %{p} macro, the number of 766 "PTR" resource records queried is included in the overall limit of 10 767 mechanisms/modifiers that cause DNS lookups described above. The 768 evaluation of each "PTR" record MUST NOT result in querying more than 769 10 address records, either "A" or "AAAA" resource records. If this 770 limit is exceeded, all records other than the first 10 MUST be 771 ignored. 773 The reason for the disparity is that the set of and contents of the 774 MX record are under control of the publishing ADMD, while the set of 775 and contents of PTR records are under control of the owner of the IP 776 address actually making the connection. 778 These limits are per mechanism or macro in the record, and are in 779 addition to the lookup limits specified above. 781 MTAs or other processors SHOULD impose a limit on the maximum amount 782 of elapsed time to evaluate check_host(). Such a limit SHOULD allow 783 at least 20 seconds. If such a limit is exceeded, the result of 784 authorization SHOULD be "temperror". 786 As described at the end of Section 11.1, there may be cases where it 787 is useful to limit the number of "terms" for which DNS queries return 788 either a positive answer (RCODE 0) with an answer count of 0, or a no 789 such record (RCODE 3) answer. These are sometimes collectively 790 referred to as "void lookups". SPF implementations SHOULD limit 791 "void lookups" to two. An implementation MAY choose to make such a 792 limit configurable. In this case, a default of two is RECOMMENDED. 794 4.7. Default Result 796 If none of the mechanisms match and there is no "redirect" modifier, 797 then the check_host() returns a result of "neutral", just as if 798 "?all" were specified as the last directive. If there is a 799 "redirect" modifier, check_host() proceeds as defined in Section 6.1. 801 It is better to use either a "redirect" modifier or an "all" 802 mechanism to explicitly terminate processing. Although the latter 803 has a default (specifically "?all"), it aids debugging efforts if it 804 is explicitly provided. 806 For example: 808 v=spf1 +mx -all 809 or 810 v=spf1 +mx redirect=_spf.example.com 812 4.8. Domain Specification 814 Several of these mechanisms and modifiers have a domain-spec section. 815 The domain-spec string is subject to macro expansion (see Section 7). 816 The resulting string is the common presentation form of a fully- 817 qualified DNS name: a series of labels separated by periods. This 818 domain is called the in the rest of this document. 820 Note: The result of the macro expansion is not subject to any further 821 escaping. Hence, this facility cannot produce all characters that 822 are legal in a DNS label (e.g., the control characters). However, 823 this facility is powerful enough to express legal host names and 824 common utility labels (such as "_spf") that are used in DNS. 826 For several mechanisms, the domain-spec is optional. If it is not 827 provided, the from the check_host() arguments (see 828 Section 4.1) is used as the . "domain" and domain-spec 829 are syntactically identical after macro expansion. "domain" is an 830 input value for check_host() while domain-spec is computed by 831 check_host(). 833 The result of evaluating check_host() with a syntactically invalid 834 domain is undefined. 836 5. Mechanism Definitions 838 This section defines two types of mechanisms: basic language 839 framework mechanisms and designated sender mechanisms. 841 Basic mechanisms contribute to the language framework. They do not 842 specify a particular type of authorization scheme. 844 all 845 include 847 Designated sender mechanisms are used to identify a set of 848 addresses as being permitted or not permitted to use the for 849 sending mail. 851 a 852 mx 853 ptr (do not publish) 854 ip4 855 ip6 856 exists 858 The following conventions apply to all mechanisms that perform a 859 comparison between and an IP address at any point: 861 If no CIDR prefix length is given in the directive, then and the 862 IP address are compared for equality. (Here, CIDR is Classless 863 Inter-Domain Routing, described in [RFC4632].) 865 If a CIDR prefix length is specified, then only the specified number 866 of high-order bits of and the IP address are compared for 867 equality. 869 When any mechanism fetches host addresses to compare with , when 870 is an IPv4, "A" records are fetched; when is an IPv6 871 address, "AAAA" records are fetched. SPF implementations on IPv6 872 servers need to handle both "AAAA" and "A" records, for clients on 873 IPv4 mapped IPv6 addresses [RFC4291]. IPv4 addresses are only 874 listed in an SPF record using the "ip4" mechanism. 876 Several mechanisms rely on information fetched from the DNS. For 877 these DNS queries, except where noted, if the DNS server returns an 878 error (RCODE other than 0 or 3) or the query times out, the mechanism 879 stops and the topmost check_host() returns "temperror". If the 880 server returns "domain does not exist" (RCODE 3), then evaluation of 881 the mechanism continues as if the server returned no error (RCODE 0) 882 and zero answer records. 884 5.1. "all" 886 all = "all" 888 The "all" mechanism is a test that always matches. It is used as the 889 rightmost mechanism in a record to provide an explicit default. 891 For example: 893 v=spf1 a mx -all 895 Mechanisms after "all" will never be tested. Mechanisms listed after 896 "all" MUST be ignored. Any "redirect" modifier (Section 6.1) MUST be 897 ignored when there is an "all" mechanism in the record. 899 5.2. "include" 901 include = "include" ":" domain-spec 903 The "include" mechanism triggers a recursive evaluation of 904 check_host(). 906 1. The domain-spec is expanded as per Section 7. 908 2. check_host() is evaluated with the resulting string as the 909 . The and arguments remain the same as in 910 the current evaluation of check_host(). 912 3. The recursive evaluation returns either match, not match, or an 913 error. If it matches, then the appropriate result for the 914 include: mechanism is used (e.g. include or +include produces a 915 "pass" result and -include produces "fail"). 917 4. If there is no match, the parent check_host() resumes processing 918 as per the table below, with the previous value of 919 restored. 921 In hindsight, the name "include" was poorly chosen. Only the 922 evaluated result of the referenced SPF record is used, rather than 923 literally including the mechanisms of the referenced record in the 924 first. For example, evaluating a "-all" directive in the referenced 925 record does not terminate the overall processing and does not 926 necessarily result in an overall "fail". (Better names for this 927 mechanism would have been "if-match", "on-match", etc.) 929 The "include" mechanism makes it possible for one domain to designate 930 multiple administratively-independent domains. For example, a vanity 931 domain "example.net" might send mail using the servers of 932 administratively-independent domains example.com and example.org. 934 Example.net could say 936 IN TXT "v=spf1 include:example.com include:example.org -all" 938 This would direct check_host() to, in effect, check the records of 939 example.com and example.org for a "pass" result. Only if the host 940 were not permitted for either of those domains would the result be 941 "fail". 943 Whether this mechanism matches, does not match, or returns an 944 exception depends on the result of the recursive evaluation of 945 check_host(): 947 +---------------------------------+---------------------------------+ 948 | A recursive check_host() result | Causes the "include" mechanism | 949 | of: | to: | 950 +---------------------------------+---------------------------------+ 951 | pass | match | 952 | | | 953 | fail | not match | 954 | | | 955 | softfail | not match | 956 | | | 957 | neutral | not match | 958 | | | 959 | temperror | return temperror | 960 | | | 961 | permerror | return permerror | 962 | | | 963 | none | return permerror | 964 +---------------------------------+---------------------------------+ 966 The "include" mechanism is intended for crossing administrative 967 boundaries. For example, if example.com and example.org were managed 968 by the same entity, and if the permitted set of hosts for both 969 domains was "mx:example.com", it would be possible for example.org to 970 specify "include:example.com", but it would be preferable to specify 971 "redirect=example.com" or even "mx:example.com". 973 With the "include" mechanism an administratively external set of 974 hosts can be authorized, but determination of sender policy is still 975 a function of the original domain's SPF record (as determined by the 976 "all" mechanism in that record). The redirect modifier is more 977 suitable for consolidating both authorizations and policy into a 978 common set to be shared within an ADMD. Redirect is much more like a 979 common code element to be shared among records in a single ADMD. It 980 is possible to control both authorized hosts and policy for an 981 arbitrary number of domains from a single record. 983 5.3. "a" 985 This mechanism matches if is one of the 's IP 986 addresses. For clarity, this means the "a" mechanism also matches 987 AAAA records. 989 a = "a" [ ":" domain-spec ] [ dual-cidr-length ] 991 An address lookup is done on the using the type of 992 lookup (A or AAAA) appropriate for the connection type (IPv4 or 993 IPv6). The is compared to the returned address(es). If any 994 address matches, the mechanism matches. 996 5.4. "mx" 998 This mechanism matches if is one of the MX hosts for a domain 999 name. 1001 mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ] 1003 check_host() first performs an MX lookup on the . Then 1004 it performs an address lookup on each MX name returned. The is 1005 compared to each returned IP address. To prevent Denial of Service 1006 (DoS) attacks, the processing limits defined in Section 4.6.4 MUST be 1007 followed. If the MX lookup limit is exceeded, then "permerror" is 1008 returned and the evaluation is terminated. If any address matches, 1009 the mechanism matches. 1011 Note regarding implicit MXes: If the has no MX record, 1012 check_host() MUST NOT apply the implicit MX rules of[RFC5321] by 1013 querying for an A or AAAA record for the same name. 1015 5.5. "ptr" (do not use) 1017 This mechanism tests whether the DNS reverse-mapping for exists 1018 and correctly points to a domain name within a particular domain. 1019 This mechanism SHOULD NOT be published. See below for discussion. 1021 ptr = "ptr" [ ":" domain-spec ] 1023 The 's name is looked up using this procedure: 1025 o Perform a DNS reverse-mapping for : Look up the corresponding 1026 PTR record in "in-addr.arpa." if the address is an IPv4 one and in 1027 "ip6.arpa." if it is an IPv6 address. 1029 o For each record returned, validate the domain name by looking up 1030 its IP addresses. To prevent DoS attacks, the PTR processing 1031 limits defined in Section 4.6.4 MUST be applied. If they are 1032 exceeded, processing is terminated and the mechanism does not 1033 match. 1035 o If is among the returned IP addresses, then that domain name 1036 is validated. 1038 Check all validated domain names to see if they either match the 1039 domain or are a subdomain of the domain. 1040 If any do, this mechanism matches. If no validated domain name can 1041 be found, or if none of the validated domain names match or are a 1042 subdomain of the , this mechanism fails to match. If a 1043 DNS error occurs while doing the PTR RR lookup, then this mechanism 1044 fails to match. If a DNS error occurs while doing an A RR lookup, 1045 then that domain name is skipped and the search continues. 1047 Pseudocode: 1049 sending-domain_names := ptr_lookup(sending-host_IP); 1050 if more than 10 sending-domain_names are found, use at most 10. 1051 for each name in (sending-domain_names) { 1052 IP_addresses := a_lookup(name); 1053 if the sending-domain_IP is one of the IP_addresses { 1054 validated-sending-domain_names += name; 1055 } 1056 } 1058 for each name in (validated-sending-domain_names) { 1059 if name ends in , return match. 1060 if name is , return match. 1061 } 1062 return no-match. 1064 This mechanism matches if the is either a subdomain of 1065 a validated domain name or if the and a validated 1066 domain name are the same. For example: "mail.example.com" is within 1067 the domain "example.com", but "mail.bad-example.com" is not. 1069 Note: This mechanism is slow, it is not as reliable as other 1070 mechanisms in cases of DNS errors, and it places a large burden on 1071 the .arpa name servers. If used, proper PTR records have to be in 1072 place for the domain's hosts and the "ptr" mechanism SHOULD be one of 1073 the last mechanisms checked. After many years of SPF deployment 1074 experience, it has been concluded it is unnecessary and more reliable 1075 alternatives should be used instead. It is, however, still in use as 1076 part of the SPF protocol, so compliant check_host() implementations 1077 MUST support it. 1079 5.6. "ip4" and "ip6" 1081 These mechanisms test whether is contained within a given IP 1082 network. 1084 ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] 1085 ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ] 1087 ip4-cidr-length = "/" 1*DIGIT 1088 ip6-cidr-length = "/" 1*DIGIT 1089 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] 1091 ip4-network = qnum "." qnum "." qnum "." qnum 1092 qnum = DIGIT ; 0-9 1093 / %x31-39 DIGIT ; 10-99 1094 / "1" 2DIGIT ; 100-199 1095 / "2" %x30-34 DIGIT ; 200-249 1096 / "25" %x30-35 ; 250-255 1097 ; as per conventional dotted quad notation. e.g., 192.0.2.0 1098 ip6-network = 1099 ; e.g., 2001:DB8::CD30 1101 The is compared to the given network. If CIDR prefix length 1102 high-order bits match, the mechanism matches. 1104 If ip4-cidr-length is omitted, it is taken to be "/32". If 1105 ip6-cidr-length is omitted, it is taken to be "/128". It is not 1106 permitted to omit parts of the IP address instead of using CIDR 1107 notations. That is, use 192.0.2.0/24 instead of 192.0.2. 1109 5.7. "exists" 1111 This mechanism is used to construct an arbitrary domain name that is 1112 used for a DNS A record query. It allows for complicated schemes 1113 involving arbitrary parts of the mail envelope to determine what is 1114 permitted. 1116 exists = "exists" ":" domain-spec 1118 The domain-spec is expanded as per Section 7. The resulting domain 1119 name is used for a DNS A RR lookup (even when the connection type is 1120 IPv6). If any A record is returned, this mechanism matches. 1122 Domains can use this mechanism to specify arbitrarily complex 1123 queries. For example, suppose example.com publishes the record: 1125 v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all 1127 The might expand to 1128 "1.2.0.192.someuser._spf.example.com". This makes fine-grained 1129 decisions possible at the level of the user and client IP address. 1131 6. Modifier Definitions 1133 Modifiers are name/value pairs that provide additional information. 1134 Modifiers always have an "=" separating the name and the value. 1136 The modifiers defined in this document ("redirect" and "exp") SHOULD 1137 appear at the end of the record, after all mechanisms, though 1138 syntactically they can appear anywhere in the record. Ordering of 1139 these two modifiers does not matter. These two modifiers MUST NOT 1140 appear in a record more than once each. If they do, then 1141 check_host() exits with a result of "permerror". 1143 Unrecognized modifiers MUST be ignored no matter where in a record, 1144 or how often. This allows implementations of this document to 1145 gracefully handle records with modifiers that are defined in other 1146 specifications. 1148 6.1. redirect: Redirected Query 1150 The redirect modifier is intended for consolidating both 1151 authorizations and policy into a common set to be shared within a 1152 single ADMD. It is possible to control both authorized hosts and 1153 policy for an arbitrary number of domains from a single record. 1155 redirect = "redirect" "=" domain-spec 1157 If all mechanisms fail to match, and a "redirect" modifier is 1158 present, then processing proceeds as follows: 1160 The domain-spec portion of the redirect section is expanded as per 1161 the macro rules in Section 7. Then check_host() is evaluated with 1162 the resulting string as the . The and 1163 arguments remain the same as in the current evaluation of 1164 check_host(). 1166 The result of this new evaluation of check_host() is then considered 1167 the result of the current evaluation with the exception that if no 1168 SPF record is found, or if the is malformed, the result 1169 is a "permerror" rather than "none". 1171 Note that the newly-queried domain can itself specify redirect 1172 processing. 1174 This facility is intended for use by organizations that wish to apply 1175 the same record to multiple domains. For example: 1177 la.example.com. TXT "v=spf1 redirect=_spf.example.com" 1178 ny.example.com. TXT "v=spf1 redirect=_spf.example.com" 1179 sf.example.com. TXT "v=spf1 redirect=_spf.example.com" 1180 _spf.example.com. TXT "v=spf1 mx:example.com -all" 1182 In this example, mail from any of the three domains is described by 1183 the same record. This can be an administrative advantage. 1185 Note: In general, the domain "A" cannot reliably use a redirect to 1186 another domain "B" not under the same administrative control. Since 1187 the stays the same, there is no guarantee that the record at 1188 domain "B" will correctly work for mailboxes in domain "A", 1189 especially if domain "B" uses mechanisms involving local-parts. An 1190 "include" directive will generally be more appropriate. 1192 For clarity, any "redirect" modifier SHOULD appear as the very last 1193 term in a record. 1195 6.2. exp: Explanation 1197 explanation = "exp" "=" domain-spec 1199 If check_host() results in a "fail" due to a mechanism match (such as 1200 "-all"), and the "exp" modifier is present, then the explanation 1201 string returned is computed as described below. If no "exp" modifier 1202 is present, then either a default explanation string or an empty 1203 explanation string MUST be returned to the calling application. 1205 The domain-spec is macro expanded (see Section 7) and becomes the 1206 . The DNS TXT RRset for the is fetched. 1208 If there are any DNS processing errors (any RCODE other than 0), or 1209 if no records are returned, or if more than one record is returned, 1210 or if there are syntax errors in the explanation string, then proceed 1211 as if no "exp" modifier was given. 1213 The fetched TXT record's strings are concatenated with no spaces, and 1214 then treated as an explain-string, which is macro-expanded. This 1215 final result is the explanation string. Implementations MAY limit 1216 the length of the resulting explanation string to allow for other 1217 protocol constraints and/or reasonable processing limits. Since the 1218 explanation string is intended for an SMTP response and [RFC5321] 1219 Section 2.4 says that responses are in [US-ASCII], the explanation 1220 string MUST be limited to [US-ASCII]. 1222 Software evaluating check_host() can use this string to communicate 1223 information from the publishing domain in the form of a short message 1224 or URL. Software SHOULD make it clear that the explanation string 1225 comes from a third party. For example, it can prepend the macro 1226 string "%{o} explains: " to the explanation, such as shown in 1227 Section 8.4. 1229 Suppose example.com has this record: 1231 v=spf1 mx -all exp=explain._spf.%{d} 1233 Here are some examples of possible explanation TXT records at 1234 explain._spf.example.com: 1236 "Mail from example.com should only be sent by its own servers." 1237 -- a simple, constant message 1239 "%{i} is not one of %{d}'s designated mail servers." 1240 -- a message with a little more information, including the IP 1241 address that failed the check 1243 "See http://%{d}/why.html?s=%{S}&i=%{I}" 1244 -- a complicated example that constructs a URL with the 1245 arguments to check_host() so that a web page can be 1246 generated with detailed, custom instructions 1248 Note: During recursion into an "include" mechanism, an "exp" modifier 1249 from the MUST NOT be used. In contrast, when executing 1250 a "redirect" modifier, an "exp" modifier from the original domain 1251 MUST NOT be used. This is because "include" is meant to cross 1252 administrative boundaries and the explanation provided should be the 1253 one from the receiving ADMD, while "redirect" is meant to operate as 1254 a tool to consolidate policy records within an ADMD an so the 1255 redirected explanation is the one that ought to have priority. 1257 7. Macros 1259 When evaluating an SPF policy record, certain character sequences are 1260 intended to be replaced by parameters of the message or of the 1261 connection. These character sequences are referred to as "macros". 1263 7.1. Formal Specification 1265 The ABNF description for a macro is as follows: 1267 domain-spec = macro-string domain-end 1268 domain-end = ( "." toplabel [ "." ] ) / macro-expand 1270 toplabel = ( *alphanum ALPHA *alphanum ) / 1271 ( 1*alphanum "-" *( alphanum / "-" ) alphanum ) 1272 alphanum = ALPHA / DIGIT 1274 explain-string = *( macro-string / SP ) 1276 macro-string = *( macro-expand / macro-literal ) 1277 macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) 1278 / "%%" / "%_" / "%-" 1279 macro-literal = %x21-24 / %x26-7E 1280 ; visible characters except "%" 1281 macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / 1282 "c" / "r" / "t" / "v" 1283 transformers = *DIGIT [ "r" ] 1284 delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" 1286 The "toplabel" construction is subject to the LDH rule plus 1287 additional top-level domain (TLD) restrictions. See Section 2 of 1288 [RFC3696] for background. 1290 Some special cases: 1292 o A literal "%" is expressed by "%%". 1294 o "%_" expands to a single " " space. 1296 o "%-" expands to a URL-encoded space, viz., "%20". 1298 7.2. Macro Definitions 1300 The following macro letters are expanded in term arguments: 1302 s = 1303 l = local-part of 1304 o = domain of 1305 d = 1306 i = 1307 p = the validated domain name of (do not use) 1308 v = the string "in-addr" if is ipv4, or "ip6" if is ipv6 1309 h = HELO/EHLO domain 1311 , , and are defined in Section 2.2. 1313 The following macro letters are allowed only in "exp" text: 1315 c = SMTP client IP (easily readable format) 1316 r = domain name of host performing the check 1317 t = current timestamp 1319 7.3. Macro Processing Details 1321 A '%' character not followed by a '{', '%', '-', or '_' character is 1322 a syntax error. So: 1324 -exists:%(ir).sbl.example.org 1326 is incorrect and will cause check_host() to yield a "permerror". 1327 Instead, the following is legal: 1329 -exists:%{ir}.sbl.example.org 1331 Optional transformers are the following: 1333 *DIGIT = zero or more digits 1334 'r' = reverse value, splitting on dots by default 1336 If transformers or delimiters are provided, the replacement value for 1337 a macro letter is split into parts separated by one or more of the 1338 specified delimiter characters. After performing any reversal 1339 operation and/or removal of left-hand parts, the parts are rejoined 1340 using "." and not the original splitting characters. 1342 By default, strings are split on "." (dots). Note that no special 1343 treatment is given to leading, trailing, or consecutive delimiters in 1344 input strings, and so the list of parts might contain empty strings. 1345 Some older implementations of SPF prohibit trailing dots in domain 1346 names, so trailing dots SHOULD NOT be published, although they MUST 1347 be accepted by implementations conforming to this document. Macros 1348 can specify delimiter characters that are used instead of ".". 1350 The "r" transformer indicates a reversal operation: if the client IP 1351 address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1" 1352 and the macro %{ir} would expand to "1.2.0.192". 1354 The DIGIT transformer indicates the number of right-hand parts to 1355 use, after optional reversal. If a DIGIT is specified, the value 1356 MUST be nonzero. If no DIGITs are specified, or if the value 1357 specifies more parts than are available, all the available parts are 1358 used. If the DIGIT was 5, and only 3 parts were available, the macro 1359 interpreter would pretend the DIGIT was 3. Implementations MUST 1360 support at least a value of 127, as that is the maximum number of 1361 labels in a domain name (less the zero-length label at the end). 1363 The "s" macro expands to the argument. It is an email 1364 address with a local-part, an "@" character, and a domain. The "l" 1365 macro expands to just the local-part. The "o" macro expands to just 1366 the domain part. Note that these values remain the same during 1367 recursive and chained evaluations due to "include" and/or "redirect". 1368 Note also that if the original had no local-part, the local- 1369 part was set to "postmaster" in initial processing (see Section 4.3). 1371 For IPv4 addresses, both the "i" and "c" macros expand to the 1372 standard dotted-quad format. 1374 For IPv6 addresses, the "i" macro expands to a dot-format address; it 1375 is intended for use in %{ir}. The "c" macro can expand to any of the 1376 hexadecimal colon-format addresses specified in [RFC4291], Section 1377 2.2. It is intended for humans to read. 1379 The "p" macro expands to the validated domain name of . The 1380 procedure for finding the validated domain name is defined in 1381 Section 5.5. If the is present in the list of validated 1382 domains, it SHOULD be used. Otherwise, if a subdomain of the 1383 is present, it SHOULD be used. Otherwise, any name from the 1384 list can be used. If there are no validated domain names or if a DNS 1385 error occurs, the string "unknown" is used. 1387 This macro SHOULD NOT be published (see Section 5.5 for the 1388 discussion). 1390 The "h" macro expands to the parameter that was provided to the SMTP 1391 server via the HELO or EHLO SMTP verb. For sessions where that verb 1392 was provided more than once, the most recent instance is used. 1394 The "r" macro expands to the name of the receiving MTA. This SHOULD 1395 be a fully qualified domain name, but if one does not exist (as when 1396 the checking is done by a MUA) or if policy restrictions dictate 1397 otherwise, the word "unknown" SHOULD be substituted. The domain name 1398 can be different from the name found in the MX record that the client 1399 MTA used to locate the receiving MTA. 1401 The "t" macro expands to the decimal representation of the 1402 approximate number of seconds since the Epoch (Midnight, January 1, 1403 1970, UTC) at the time of the evaluation. This is the same value as 1404 is returned by the POSIX time() function in most standards-compliant 1405 libraries. 1407 When the result of macro expansion is used in a domain name query, if 1408 the expanded domain name exceeds 253 characters (the maximum length 1409 of a domain name in this format), the left side is truncated to fit, 1410 by removing successive domain labels (and their following dots) until 1411 the total length does not exceed 253 characters. 1413 Uppercase macros expand exactly as their lowercase equivalents, and 1414 are then URL escaped. URL escaping MUST be performed for characters 1415 not in the "unreserved" set, which is defined in [RFC3986]. 1417 Care has to be taken by the sending ADMD so that macro expansion for 1418 legitimate email does not exceed the 63-character limit on DNS 1419 labels. The local-part of email addresses, in particular, can have 1420 more than 63 characters between dots. 1422 To minimize DNS lookup resource requirements, it is better if sending 1423 ADMDs avoid using the "s", "l", "o", or "h" macros in conjunction 1424 with any mechanism directive. Although these macros are powerful and 1425 allow per-user records to be published, they severely limit the 1426 ability of implementations to cache results of check_host() and they 1427 reduce the effectiveness of DNS caches. 1429 If no directive processed during the evaluation of check_host() 1430 contains an "s", "l", "o", or "h" macro, then the results of the 1431 evaluation can be cached on the basis of and alone for 1432 as long as the DNS record involved with the shortest TTL has not 1433 expired. 1435 7.4. Expansion Examples 1437 The is strong-bad@email.example.com. 1438 The IPv4 SMTP client IP is 192.0.2.3. 1439 The IPv6 SMTP client IP is 2001:DB8::CB01. 1440 The PTR domain name of the client IP is mx.example.org. 1442 macro expansion 1443 ------- ---------------------------- 1444 %{s} strong-bad@email.example.com 1445 %{o} email.example.com 1446 %{d} email.example.com 1447 %{d4} email.example.com 1448 %{d3} email.example.com 1449 %{d2} example.com 1450 %{d1} com 1451 %{dr} com.example.email 1452 %{d2r} example.email 1453 %{l} strong-bad 1454 %{l-} strong.bad 1455 %{lr} strong-bad 1456 %{lr-} bad.strong 1457 %{l1r-} strong 1459 macro-string expansion 1460 -------------------------------------------------------------------- 1461 %{ir}.%{v}._spf.%{d2} 3.2.0.192.in-addr._spf.example.com 1462 %{lr-}.lp._spf.%{d2} bad.strong.lp._spf.example.com 1464 %{lr-}.lp.%{ir}.%{v}._spf.%{d2} 1465 bad.strong.lp.3.2.0.192.in-addr._spf.example.com 1467 %{ir}.%{v}.%{l1r-}.lp._spf.%{d2} 1468 3.2.0.192.in-addr.strong.lp._spf.example.com 1470 %{d2}.trusted-domains.example.net 1471 example.com.trusted-domains.example.net 1473 IPv6: 1474 %{ir}.%{v}._spf.%{d2} 1.0.B.C.0.0.0.0. 1475 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 1477 8. Result Handling 1479 This section provides guidance for operators in response to the 1480 various possible outputs of check_host() on a message. Definitions 1481 of SPF results are presented in Section 2.6; this section provides 1482 more detail on each for use in developing local policy for message 1483 handling. 1485 Every operating environment is different. There are some receivers 1486 for whom strict adherence to SPF is appropriate, and definitive 1487 treatment of messages that are evaluated to be explicitly 1488 unauthorized ("fail" and sometimes "softfail") is the norm. There 1489 are others for which the "false negative" cases are more of a 1490 concern. This concern is typically handled by merely recording the 1491 result in the header and allowing the message to pass on for 1492 additional processing. There are still others where SPF is one of 1493 several inputs to the message handling decision. As such, there is 1494 no comprehensive normative requirement for message handling in 1495 response to any particular result. This section is provided to 1496 present a complete picture of the likely cause of each result and, 1497 where available, the experience gained during experimental 1498 deployment. 1500 There are essentially two classes of handling choices: 1502 o Handling within the SMTP session that attempted to deliver the 1503 message, such as by returning a permanent SMTP error (rejection) 1504 or temporary SMTP error ("try again later"); 1506 o Permitting the message to pass (a successful SMTP reply code) and 1507 adding an additional header field that indicates the result 1508 returned by check_host() and other salient details; this is 1509 discussed in more detail in Section 9. 1511 8.1. None 1513 With a "none" result, the SPF verifier has no information at all 1514 about the authorization or lack thereof of the client to use the 1515 checked identity or identities. The check_host() function completed 1516 without errors but was not able to reach any conclusion. 1518 8.2. Neutral 1520 A "neutral" result indicates that although a policy for the identity 1521 was discovered, there is no definite assertion (positive or negative) 1522 about the client. 1524 A "neutral" result MUST be treated exactly like the "none" result; 1525 the distinction exists only for informational purposes. Treating 1526 "neutral" more harshly than "none" would discourage ADMDs from 1527 testing the use of SPF records (see Section 10.1). 1529 8.3. Pass 1531 A "pass" result means that the client is authorized to inject mail 1532 with the given identity. The domain can now, in the sense of 1533 reputation, be considered responsible for sending the message. 1534 Further policy checks can now proceed with confidence in the 1535 legitimate use of the identity. This is further discussed in 1536 Appendix H.1. 1538 8.4. Fail 1540 A "fail" result is an explicit statement that the client is not 1541 authorized to use the domain in the given identity. Disposition of 1542 SPF fail messages is a matter of local policy. See Appendix H.2 for 1543 considerations on developing local policy. 1545 If the checking software chooses to reject the mail during the SMTP 1546 transaction, then it SHOULD use an SMTP reply code of 550 (see 1547 [RFC5321]) and, if supported, the 5.7.1 enhanced status code (see 1548 [RFC3463], Section 3.8), in addition to an appropriate reply text. 1549 The check_host() function will return either a default explanation 1550 string or one from the domain that published the SPF records (see 1551 Section 6.2). If the information does not originate with the 1552 checking software, it is good to make it clear that the text is 1553 provided by the sender's domain. For example: 1555 550-5.7.1 SPF MAIL FROM check failed: 1556 550-5.7.1 The domain example.com explains: 1557 550 5.7.1 Please see http://www.example.com/mailpolicy.html 1559 If the checking software chooses not to reject the mail during the 1560 SMTP transaction, then it SHOULD add a Received-SPF or 1561 Authentication-Results header field (see Section 9) to communicate 1562 this result to downstream message processors. While this is true for 1563 all SPF results, it is of particular importance for "fail" results 1564 since the message is explicitly not authorized by the ADMD. 1566 8.5. Softfail 1568 A "softfail" result ought to be treated as somewhere between "fail" 1569 and "neutral"/"none". The ADMD believes the host is not authorized 1570 but is not willing to make a strong policy statement. Receiving 1571 software SHOULD NOT reject the message based solely on this result, 1572 but MAY subject the message to closer scrutiny than normal. 1574 The ADMD wants to discourage the use of this host and thus desires 1575 limited feedback when a "softfail" result occurs. For example, the 1576 recipient's Mail User Agent (MUA) could highlight the "softfail" 1577 status, or the receiving MTA could give the sender a message using 1578 greylisting, [RFC6647], with a note the first time the message is 1579 received, but accept it on a later attempt based on receiver policy. 1581 8.6. Temperror 1583 A "temperror" result means the SPF verifier encountered a transient 1584 (generally DNS) error while performing the check. Checking software 1585 can choose to accept or temporarily reject the message. If the 1586 message is rejected during the SMTP transaction for this reason, the 1587 software SHOULD use an SMTP reply code of 451 and, if supported, the 1588 4.4.3 enhanced status code (see [RFC3463], Section 3.5). These 1589 errors can be caused by problems in either the sender's or receiver's 1590 DNS software. See Appendix H.4 for considerations on developing 1591 local policy. 1593 8.7. Permerror 1595 A "permerror" result means the domain's published records could not 1596 be correctly interpreted. This signals an error condition that 1597 definitely requires operator intervention to be resolved. If the 1598 message is rejected during the SMTP transaction for this reason, the 1599 software SHOULD use an SMTP reply code of 550 and, if supported, the 1600 5.5.2 enhanced status code (see [RFC3463], Section 3.6). Be aware 1601 that if the ADMD uses macros (Section 7), it is possible that this 1602 result is due to the checked identities having an unexpected format. 1603 It is also possible that this result is generated by certain SPF 1604 verifiers due to the input arguments having an unexpected format; see 1605 Section 4.8. See Appendix H.3 for considerations on developing local 1606 policy. 1608 9. Recording the Result 1610 To provide downstream agents, such as MUAs, with the information they 1611 might need in terms of evaluating or representing the apparent safety 1612 of the message content, it is RECOMMENDED that SMTP receivers record 1613 the result of SPF processing in the message header. For operators 1614 that choose to record SPF results in the header of the message for 1615 processing by internal filters or MUAs, two methods are presented. 1616 Section 9.1 defines the Received-SPF field, which is the results 1617 field originally defined for SPF use. Section 9.2 discusses 1618 Authentication-Results [RFC5451] which was specified more recently 1619 and is designed for use by SPF and other authentication methods. 1621 Both are in common use, and hence both are included here. However, 1622 it is important to note that they were designed to serve slightly 1623 different purposes. Received-SPF is intended to include enough 1624 information to enable reconstruction of the SPF evaluation of the 1625 message, while Authentication-Results is designed only to relay the 1626 result itself and related output details of likely use to end users 1627 (e.g., what property of the message was actually authenticated and 1628 what it contained), leaving reconstructive work to the purview of 1629 system logs and the Received field contents. Also, Received-SPF 1630 relies on compliance of agents within the receiving ADMD to adhere to 1631 the header field ordering rules of [RFC5321] and [RFC5322], while 1632 Authentication-Results includes some provisions to protect against 1633 non-compliant implementations. 1635 An operator could choose to use both to serve different downstream 1636 agents. In such cases, care needs to be taken to ensure both fields 1637 are conveying the same details, or unexpected results can occur. 1639 9.1. The Received-SPF Header Field 1641 The Received-SPF header field is a trace field (see [RFC5322] Section 1642 3.6.7) and SHOULD be prepended to the existing header, above the 1643 Received: field that is generated by the SMTP receiver. It MUST 1644 appear above all other Received-SPF fields in the message. The 1645 header field has the following format: 1647 header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] 1648 [ key-value-list ] CRLF 1650 result = "pass" / "fail" / "softfail" / "neutral" / 1651 "none" / "temperror" / "permerror" 1653 key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) 1654 [";"] 1656 key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) 1658 key = "client-ip" / "envelope-from" / "helo" / 1659 "problem" / "receiver" / "identity" / 1660 "mechanism" / name 1662 identity = "mailfrom" ; for the "MAIL FROM" identity 1663 / "helo" ; for the "HELO" identity 1664 / name ; other identities 1666 dot-atom = 1667 quoted-string = 1668 comment = 1669 CFWS = 1670 FWS = 1671 CRLF = 1673 The header field SHOULD include a "(...)" style comment after the 1674 result, conveying supporting information for the result, such as 1675 , , and . 1677 The following key-value pairs are designed for later machine parsing. 1678 SPF verifiers SHOULD give enough information so that the SPF results 1679 can be verified. That is, at least "client-ip", "helo", and, if the 1680 "MAIL FROM" identity was checked, "envelope-from". 1682 client-ip the IP address of the SMTP client 1684 envelope-from the envelope sender mailbox 1686 helo the host name given in the HELO or EHLO command 1688 mechanism the mechanism that matched (if no mechanisms matched, 1689 substitute the word "default") 1691 problem if an error was returned, details about the error 1692 receiver the host name of the SPF verifier 1694 identity the identity that was checked; see the ABNF 1695 rule 1697 Other keys MAY be defined by SPF verifiers. 1699 SPF verifiers MUST make sure that the Received-SPF header field does 1700 not contain invalid characters, is not excessively long (See 1701 [RFC5322] Section 2.1.1), and does not contain malicious data that 1702 has been provided by the sender. 1704 Examples of various header field styles that could be generated are 1705 the following: 1707 Received-SPF: pass (mybox.example.org: domain of 1708 myname@example.com designates 192.0.2.1 as permitted sender) 1709 receiver=mybox.example.org; client-ip=192.0.2.1; 1710 envelope-from="myname@example.com"; helo=foo.example.com; 1712 Received-SPF: fail (mybox.example.org: domain of 1713 myname@example.com does not designate 1714 192.0.2.1 as permitted sender) 1715 identity=mailfrom; client-ip=192.0.2.1; 1716 envelope-from="myname@example.com"; 1718 Received-SPF: pass (mybox.example.org: domain of 1719 myname@example.com designates 192.0.2.1 as permitted sender) 1720 receiver=mybox.example.org; client-ip=192.0.2.1; 1721 mechanism=ip4:192.0.2.1; envelope-from="myname@example.com"; 1722 helo=foo.example.com; 1724 9.2. SPF Results in the Authentication-Results Header Field 1726 As mentioned in Section 9, the Authentication-Results header field is 1727 designed to communicate lists of tests a border MTA did and their 1728 results. The specified elements of the field provide less 1729 information than the Received-SPF field: 1731 Authentication-Results: myhost.example.org; spf=pass 1732 smtp.mailfrom=example.net 1734 Received-SPF: pass (myhost.example.org: domain of 1735 myname@example.com designates 192.0.2.1 as permitted sender) 1736 receiver=mybox.example.org; client-ip=192.0.2.1; 1737 envelope-from="myname@example.com"; helo=foo.example.com; 1739 It is, however, possible to add CFWS in the "reason" part of an 1740 Authentication-Results header field and provide the equivalent 1741 information, if desired. 1743 As an example, an expanded Authentication-Results header field might 1744 look like (for a "MAIL FROM" check in this example): 1746 Authentication-Results: myhost.example.org; spf=pass 1747 reason="client-ip=192.0.2.1; smtp.helo=foo.example.com" 1748 smtp.mailfrom=user@example.net 1750 10. Effects on Infrastructure 1752 This section outlines the major implications that adoption of this 1753 protocol will have on various entities involved in Internet email. 1754 It is intended to make clear to the reader where this protocol 1755 knowingly affects the operation of such entities. This section is 1756 not a "how-to" manual, or a "best practices" document, and it is not 1757 a comprehensive list of what such entities ought do in light of this 1758 specification. 1760 This section provides operational advice and instruction only. It is 1761 non-normative. 1763 [RFC5598] describes the Internet email architecture. This section is 1764 organized based on the different segments of the architecture. 1766 10.1. Sending Domains 1768 Originating ADMDs (ADministrative Management Domains - [RFC5598] 1769 Section 2.2.1 and Section 2.3) that wish to be compliant with this 1770 specification will need to determine the list of relays ([RFC5598] 1771 Section 2.2.2) that they allow to use their domain name in the "HELO" 1772 and "MAIL FROM" identities when relaying to other ADMDs. It is 1773 recognized that forming such a list is not just a simple technical 1774 exercise, but involves policy decisions with both technical and 1775 administrative considerations. 1777 10.1.1. DNS Resource Considerations 1779 Minimizing the DNS resources needed for SPF lookups can be done by 1780 choosing directives that require less DNS information and by placing 1781 lower-cost mechanisms earlier in the SPF record. 1783 Section 4.6.4 specifies the limits receivers have to use. It is 1784 essential to publish records that do not exceed these requirements. 1785 It is also required to carefully weigh the cost and the 1786 maintainability of licit solutions. 1788 For example, consider a domain set up as follows: 1790 example.com. IN MX 10 mx.example.com. 1791 IN MX 20 mx2.example.com. 1792 mx.example.com. IN A 192.0.2.1 1793 mx2.example.com. IN A 192.0.2.129 1795 Assume the administrative point is to authorize (pass) mx and mx2 1796 while failing every other host. Compare the following solutions: 1798 Best record: 1799 example.com. IN TXT "v=spf1 ip4:192.0.2.1 ip4:192.0.2.129 -all" 1801 Good record: 1802 $ORIGIN example.com. 1803 @ IN TXT "v=spf1 a:authorized-spf.example.com -all" 1804 authorized-spf IN A 192.0.2.1 1805 IN A 192.0.2.129 1807 Expensive record: 1808 example.com. IN TXT "v=spf1 mx:example.com -all" 1810 Wasteful, bad record: 1811 example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 mx -all" 1813 10.1.2. Administrator's Considerations 1815 There might be administrative considerations: using "a" over "ip4" or 1816 "ip6" allows hosts to be renumbered easily at the cost of a DNS query 1817 per receiver. Using "mx" over "a" allows the set of mail hosts to be 1818 changed easily. Unless such changes are common, it is better to use 1819 the less resource intensive mechanisms like "ip4" and "ip6" over "a" 1820 or "a" over "mx". 1822 In some specific cases, standard advice on record content is 1823 appropriate. Publishing SPF records for domains that send no mail is 1824 a well established best practice. The record for a domain that sends 1825 no mail is: 1827 www.example.com. IN TXT "v=spf1 -all" 1829 Publishing SPF records for individual hosts is also best practice. 1830 The hostname is generally the identity used in the 5321.HELO/.EHLO 1831 command. In the case of messages with a null 5321.MailFrom, this is 1832 used as the domain for 5321.MailFrom SPF checks, in addition to being 1833 used in 5321.HELO/.EHLO based SPF checks. The standard SPF record 1834 for an individual host that is involved in mail processing is: 1836 relay.example.com. IN TXT "v=spf1 a -all" 1838 Validating correct deployment is difficult. [RFC6652] describes one 1839 mechanism for soliciting feedback on SPF failures. Another 1840 suggestion can be found in Appendix D. 1842 Regardless of the method used, understanding the ADMD's outbound mail 1843 architecture is essential to effective deployment. 1845 10.1.3. Bounces 1847 As explained in Section 1.1.3, [RFC5321] allows the MAIL FROM to be 1848 null, which is typical of some Delivery Status Notification 1849 [RFC3464], commonly called email bounces. In this case the only 1850 entity available for performing an SPF check is the "HELO" identity 1851 defined in Section 1.1.4. SPF functionality is enhanced by 1852 administrators ensuring this identity is set correctly and has an 1853 appropriate SPF record. It is normal to have the HELO identity set 1854 to the hostname instead of the domain. Zone file generation for 1855 significant numbers of hosts can be consolidated using the redirect 1856 modifier and scripted for initial deployment. Specific deployment 1857 advice is given above in Section 10.1.2. 1859 10.2. Receivers 1861 SPF results can be used in combination with other methods to 1862 determine the final local disposition (either positive or negative) 1863 of a message. It can also be considered dispositive on its own. 1865 An attempt to have one organization (sender) direct the email 1866 handling policies of another (receiver) is inherently challenging and 1867 often controversial. As stated elsewhere in this document, there is 1868 no comprehensive normative requirement for specific handling of a 1869 message based on SPF results. The information presented in Section 8 1870 and in Appendix H is offered for receiver consideration when forming 1871 local handling policies. 1873 The primary considerations are that SPF might return "pass" for mail 1874 that is ultimately harmful (e.g., spammers that arrange for SPF to 1875 pass using disposable domain names, or virus or spam outbreaks from 1876 within trusted sources), and might also return "fail" for mail that 1877 is ultimately legitimate (e.g., legitimate mail that has traversed a 1878 mail alias). It is important take both of these cases under 1879 consideration when establishing local handling policy. 1881 10.3. Mediators 1883 Mediators are a type of User actor [RFC5598]. That is, a mediator 1884 takes 'delivery' of a message and posts a 'submission' of a new 1885 message. The mediator can make the newly-posted message be as 1886 similar or as different from the original message as they wish. 1887 Examples include mailing lists (see [RFC5598] Section 5.3) and 1888 ReSenders ([RFC5598] Section 5.2). This is discussed in [RFC5321], 1889 Section 3.9. For the operation of SPF, the essential concern is the 1890 email address in the 5321.MailFrom command for the new message. 1892 Because SPF evaluation is based on the IP address of the "last" 1893 sending SMTP server, the address of the mediator will be used, rather 1894 than the address of the SMTP server that sent the message to the 1895 mediator. Some mediators retain the email address from the original 1896 message, while some use a new address. 1898 If the address is the same as for the original message, and the 1899 original message had an associated SPF record, then the SPF 1900 evaluation will fail unless mitigations such as those described in 1901 Appendix E are used. 1903 11. Security Considerations 1905 11.1. Processing Limits 1907 As with most aspects of email, there are a number of ways that 1908 malicious parties could use the protocol as an avenue for a 1909 Denial-of-Service (DoS) attack. The processing limits outlined in 1910 Section 4.6.4 are designed to prevent attacks such as the following: 1912 o A malicious party could create an SPF record with many references 1913 to a victim's domain and send many emails to different SPF 1914 verifiers; those SPF verifiers would then create a DoS attack. In 1915 effect, the SPF verifiers are being used to amplify the attacker's 1916 bandwidth by using fewer octets in the SMTP session than are used 1917 by the DNS queries. Using SPF verifiers also allows the attacker 1918 to hide the true source of the attack. This potential attack is 1919 based on large volumes of mail being transmitted. 1921 o Whereas implementations of check_host() are supposed to limit the 1922 number of DNS lookups, malicious domains could publish records 1923 that exceed these limits in an attempt to waste computation effort 1924 at their targets when they send them mail. Malicious domains 1925 could also design SPF records that cause particular 1926 implementations to use excessive memory or CPU usage, or to 1927 trigger bugs. If a receiver is configured to accept mail with an 1928 SPF result of "temperror", such an attack might result in mail 1929 that would otherwise have been rejected due to an SPF "fail" 1930 result being accepted. This potential attack is based on 1931 specially crafted SPF records being used to exhaust DNS resources 1932 of the victim. 1934 o Malicious parties could send a large volume of mail purporting to 1935 come from the intended target to a wide variety of legitimate mail 1936 hosts. These legitimate machines would then present a DNS load on 1937 the target as they fetched the relevant records. 1939 o Malicious parties could, in theory, use SPF records as a vehicle 1940 for DNS lookup amplification for a denial-of-service-attack. In 1941 this scenario, the attacker publishes an SPF record in its own DNS 1942 that uses "a" and "mx" mechanisms directed toward the intended 1943 victim, e.g. "a:example.com a:foo.example.com a:bar.example.com 1944 ..." and then distributes mail with a MAIL FROM value including 1945 its own domain in large volume to a wide variety of destinations. 1946 Any such destination operating an SPF verifier will begin querying 1947 all of the names associated with the "a" mechanisms in that 1948 record. The names used in the record needn't exist for the attack 1949 to be effective. Operational experience since publication of 1950 [RFC4408] suggests that mitigation of this class of attack can be 1951 accomplished with minimal impact on the deployed base by having 1952 the verifier abort processing and return "permerror" 1953 (Section 2.6.7) once more than two "void lookups" have been 1954 encountered (defined in Section 4.6.4). 1956 Of these, the case of a third party referenced in the SPF record is 1957 the easiest for a DoS attack to effectively exploit. As a result, 1958 limits that might seem reasonable for an individual mail server can 1959 still allow an unreasonable amount of bandwidth amplification. 1960 Therefore, the processing limits need to be quite low. 1962 11.2. SPF-Authorized Email May Contain Other False Identities 1964 Do not construe the "MAIL FROM" and "HELO" identity authorizations to 1965 provide more assurance than they do. It is entirely possible for a 1966 malicious sender to inject a message using his own domain in the 1967 identities used by SPF, to have that domain's SPF record authorize 1968 the sending host, and yet the message can easily list other 1969 identities in its header. Unless the user or the MUA takes care to 1970 note that the authorized identity does not match the other more 1971 commonly-presented identities (such as the From: header field), the 1972 user might be lulled into a false sense of security. 1974 11.3. Spoofed DNS and IP Data 1976 There are two aspects of this protocol that malicious parties could 1977 exploit to undermine the validity of the check_host() function: 1979 o The evaluation of check_host() relies heavily on DNS. A malicious 1980 attacker could attack the DNS infrastructure and cause 1981 check_host() to see spoofed DNS data, and then return incorrect 1982 results. This could include returning "pass" for an value 1983 where the actual domain's record would evaluate to "fail". See 1984 [RFC3833] for a description of DNS weaknesses. 1986 o The client IP address, , is assumed to be correct. In a 1987 modern, correctly configured system the risk of this not being 1988 true is nil. 1990 11.4. Cross-User Forgery 1992 By definition, SPF policies just map domain names to sets of 1993 authorized MTAs, not whole email addresses to sets of authorized 1994 users. Although the "l" macro (Section 7) provides a limited way to 1995 define individual sets of authorized MTAs for specific email 1996 addresses, it is generally impossible to verify, through SPF, the use 1997 of specific email addresses by individual users of the same MTA. 1999 It is up to mail services and their MTAs to directly prevent 2000 cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be 2001 restricted to using only those email addresses that are actually 2002 under their control (see [RFC6409], Section 6.1). Another means to 2003 verify the identity of individual users is message cryptography such 2004 as PGP ([RFC4880]) or S/MIME ([RFC5751]). 2006 11.5. Untrusted Information Sources 2008 An SPF compliant receiver gathers information from the SMTP commands 2009 it receives and from the published DNS records of the sending domain 2010 holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the 2011 envelope, and SPF DNS records published by the domain holder). These 2012 parameters are not validated in the SMTP process. 2014 All of these pieces of information are generated by actors outside of 2015 the authority of the receiver, and thus are not guaranteed to be 2016 accurate or legitimate. 2018 11.5.1. Recorded Results 2020 This information, passed to the receiver in the Received-SPF: or 2021 Authentication-Results: trace fields, can be returned to the client 2022 MTA as an SMTP rejection message. If such an SMTP rejection message 2023 is generated, the information from the trace fields has to be checked 2024 for such problems as invalid characters and excessively long lines. 2026 11.5.2. External Explanations 2028 When the authorization check fails, an explanation string could be 2029 included in the reject response. Both the sender and the rejecting 2030 receiver need to be aware that the explanation was determined by the 2031 publisher of the SPF record checked and, in general, not the 2032 receiver. The explanation can contain malicious URLs, or it might be 2033 offensive or misleading. 2035 Explanations returned to sender domains due to "exp" modifiers 2036 (Section 6.2) were generated by the sender policy published by the 2037 domain holders themselves. As long as messages are only returned 2038 with non-delivery notification ([RFC3464]) to domains publishing the 2039 explanation strings from their own DNS SPF records, the only affected 2040 parties are the original publishers of the domain's SPF records. 2042 In practice, such non-delivery notifications can be misdirected, such 2043 as when an MTA accepts an email and only later generates the 2044 notification to a forged address, or when an email forwarder does not 2045 direct the bounce back to the original sender. 2047 11.5.3. Macro Expansion 2049 Macros (Section 7) allow senders to inject arbitrary text (any non- 2050 null [US-ASCII] character) into receiver DNS queries. It is 2051 necessary to be prepared for hostile or unexpected content. 2053 11.6. Privacy Exposure 2055 Checking SPF records causes DNS queries to be sent to the domain 2056 owner. These DNS queries, especially if they are caused by the 2057 "exists" mechanism, can contain information about who is sending 2058 email and likely to which MTA the email is being sent. This can 2059 introduce some privacy concerns, which are more or less of an issue 2060 depending on local laws and the relationship between the ADMD and the 2061 person sending the email. 2063 11.7. Delivering Mail Producing a 'Fail' Result 2065 Operators that choose to deliver mail for which SPF produces a "fail" 2066 result need to understand that they are admitting content that is 2067 explicitly not authorized by the purported sender. While there are 2068 known failure modes that can be considered "false negatives", the 2069 distinct choice to admit those messages increases end-user exposure 2070 to likely harm. This is especially true for domains belonging to 2071 known good actors that are typically well-behaved; unauthorized mail 2072 from those sources might well be subjected to much higher skepticism 2073 and content analysis. 2075 SPF does not, however, include the capacity for identifying good 2076 actors from bad ones, nor does it handle the concept of known actors 2077 versus unknown ones. Those notions are out of scope for this 2078 specification. 2080 12. Contributors and Acknowledgements 2082 This document is largely based on the work of Meng Weng Wong, Mark 2083 Lentczner, and Wayne Schlitt. Although, as this section 2084 acknowledges, many people have contributed to this document, a very 2085 large portion of the writing and editing are due to Meng, Mark, and 2086 Wayne. 2088 This design owes a debt of parentage to [RMX] by Hadmut Danisch and 2089 to [DMP] by Gordon Fecyk. The idea of using a DNS record to check 2090 the legitimacy of an email address traces its ancestry further back 2091 through messages on the namedroppers mailing list by Paul Vixie 2092 [Vixie] (based on suggestion by Jim Miller) and by David Green 2093 [Green]. 2095 Philip Gladstone contributed the concept of macros to the 2096 specification, multiplying the expressiveness of the language and 2097 making per-user and per-IP lookups possible. 2099 The authors of both this document and [RFC4408] would also like to 2100 thank the literally hundreds of individuals who have participated in 2101 the development of this design. They are far too numerous to name, 2102 but they include the following: 2104 The participants in the SPFbis working group. 2105 The folks on the spf-discuss mailing list. 2106 The folks on the SPAM-L mailing list. 2107 The folks on the IRTF ASRG mailing list. 2108 The folks on the IETF MARID mailing list. 2109 The folks on #perl. 2111 13. IANA Considerations 2113 13.1. The SPF DNS Record Type 2115 Per [RFC4408], the IANA assigned the Resource Record Type and Qtype 2116 from the DNS Parameters Registry for the SPF RR type with code 99. 2117 The format of this type is identical to the TXT RR [RFC1035]. The 2118 character content of the record is encoded as [US-ASCII]. 2120 Studies have shown that RRTYPE 99 has not seen any substantial use, 2121 and in fact its existence and mechanism defined in [RFC4408] has led 2122 to some interoperability issues. Accordingly, its use is now 2123 obsolete, and new implementations are not to use it. 2125 IANA is requested to update the Resource Record (RR) TYPEs registry 2126 to indicate that this document is the reference document for that 2127 RRTYPE. 2129 [NOTE TO RFC EDITOR: (to be changed to " ... has updated ..." upon 2130 publication)] 2132 13.2. The Received-SPF Mail Header Field 2134 Per [RFC3864], the "Received-SPF:" header field is added to the IANA 2135 Permanent Message Header Field Registry. The following is the 2136 registration template: 2138 Header field name: Received-SPF 2139 Applicable protocol: mail ([RFC5322]) 2140 Status: standard 2141 Author/Change controller: IETF 2142 Specification document(s): RFC XXXX 2143 [NOTE TO RFC EDITOR: (this document)] 2145 13.3. SPF Modifier Registry 2147 IANA is requested to change the reference for the exp and redirect 2148 modifiers in the Modifier Names registry, under Sender Policy 2149 Framework Parameters, from [RFC4408] to this document. Their status 2150 is unchanged. 2152 14. References 2154 14.1. Normative References 2156 [RFC1035] Mockapetris, P., "Domain names - implementation and 2157 specification", STD 13, RFC 1035, November 1987. 2159 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 2160 and Support", STD 3, RFC 1123, October 1989. 2162 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2163 Requirement Levels", BCP 14, RFC 2119, March 1997. 2165 [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", 2166 RFC 3463, January 2003. 2168 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 2169 Procedures for Message Header Fields", BCP 90, RFC 3864, 2170 September 2004. 2172 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2173 Resource Identifier (URI): Generic Syntax", STD 66, 2174 RFC 3986, January 2005. 2176 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 2177 Architecture", RFC 4291, February 2006. 2179 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 2180 Specifications: ABNF", STD 68, RFC 5234, January 2008. 2182 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 2183 October 2008. 2185 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 2186 October 2008. 2188 [RFC5451] Kucherawy, M., "Message Header Field for Indicating 2189 Message Authentication Status", RFC 5451, April 2009. 2191 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 2192 July 2009. 2194 [RFC5890] Klensin, J., "Internationalized Domain Names for 2195 Applications (IDNA): Definitions and Document Framework", 2196 RFC 5890, August 2010. 2198 [US-ASCII] 2199 American National Standards Institute (formerly United 2200 States of America Standards Institute), "USA Code for 2201 Information Interchange, X3.4", 1968. 2203 ANSI X3.4-1968 has been replaced by newer versions with 2204 slight modifications, but the 1968 version remains 2205 definitive for the Internet. 2207 14.2. Informative References 2209 [DMP] Fecyk, G., "Designated Mailers Protocol". 2211 Work In Progress 2213 [Green] Green, D., "Domain-Authorized SMTP Mail", 2002. 2215 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 2216 STD 13, RFC 1034, November 1987. 2218 [RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983, 2219 August 1996. 2221 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", 2222 RFC 2671, August 1999. 2224 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 2225 specifying the location of services (DNS SRV)", RFC 2782, 2226 February 2000. 2228 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format 2229 for Delivery Status Notifications", RFC 3464, 2230 January 2003. 2232 [RFC3696] Klensin, J., "Application Techniques for Checking and 2233 Transformation of Names", RFC 3696, February 2004. 2235 [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain 2236 Name System (DNS)", RFC 3833, August 2004. 2238 [RFC3834] Moore, K., "Recommendations for Automatic Responses to 2239 Electronic Mail", RFC 3834, August 2004. 2241 [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 2242 for Authorizing Use of Domains in E-Mail, Version 1", 2243 RFC 4408, April 2006. 2245 [RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing 2246 (CIDR): The Internet Address Assignment and Aggregation 2247 Plan", BCP 122, RFC 4632, August 2006. 2249 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 2250 Thayer, "OpenPGP Message Format", RFC 4880, November 2007. 2252 [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension 2253 for Authentication", RFC 4954, July 2007. 2255 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 2256 Mail Extensions (S/MIME) Version 3.2 Message 2257 Specification", RFC 5751, January 2010. 2259 [RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782, 2260 February 2010. 2262 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 2263 STD 72, RFC 6409, November 2011. 2265 [RFC6647] Kucherawy, M. and D. Crocker, "Email Greylisting: An 2266 Applicability Statement for SMTP", RFC 6647, June 2012. 2268 [RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham, 2269 "Deprecating the "X-" Prefix and Similar Constructs in 2270 Application Protocols", BCP 178, RFC 6648, June 2012. 2272 [RFC6652] Kitterman, S., "Sender Policy Framework (SPF) 2273 Authentication Failure Reporting Using the Abuse Reporting 2274 Format", RFC 6652, June 2012. 2276 [RFC6686] Kucherawy, M., "Resolution of the Sender Policy Framework 2277 (SPF) and Sender ID Experiments", RFC 6686, July 2012. 2279 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 2280 for DNS (EDNS(0))", STD 75, RFC 6891, April 2013. 2282 [RMX] Danisch, H., "The RMX DNS RR Type for light weight sender 2283 authentication". 2285 Work In Progress 2287 [Vixie] Vixie, P., "Repudiating MAIL FROM", 2002. 2289 Appendix A. Collected ABNF 2291 This section is normative and any discrepancies with the ABNF 2292 fragments in the preceding text are to be resolved in favor of this 2293 grammar. 2295 See [RFC5234] for ABNF notation. Please note that as per this ABNF 2296 definition, literal text strings (those in quotes) are case- 2297 insensitive. Hence, "mx" matches "mx", "MX", "mX", and "Mx". 2299 record = version terms *SP 2300 version = "v=spf1" 2302 terms = *( 1*SP ( directive / modifier ) ) 2304 directive = [ qualifier ] mechanism 2305 qualifier = "+" / "-" / "?" / "~" 2306 mechanism = ( all / include 2307 / a / mx / ptr / ip4 / ip6 / exists ) 2309 all = "all" 2310 include = "include" ":" domain-spec 2311 a = "a" [ ":" domain-spec ] [ dual-cidr-length ] 2312 mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ] 2313 ptr = "ptr" [ ":" domain-spec ] 2314 ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] 2315 ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ] 2316 exists = "exists" ":" domain-spec 2318 modifier = redirect / explanation / unknown-modifier 2319 redirect = "redirect" "=" domain-spec 2320 explanation = "exp" "=" domain-spec 2321 unknown-modifier = name "=" macro-string 2322 ; where name is not any known modifier 2324 ip4-cidr-length = "/" 1*DIGIT 2325 ip6-cidr-length = "/" 1*DIGIT 2326 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] 2328 ip4-network = qnum "." qnum "." qnum "." qnum 2329 qnum = DIGIT ; 0-9 2330 / %x31-39 DIGIT ; 10-99 2331 / "1" 2DIGIT ; 100-199 2332 / "2" %x30-34 DIGIT ; 200-249 2333 / "25" %x30-35 ; 250-255 2334 ; conventional dotted quad notation. e.g., 192.0.2.0 2335 ip6-network = 2336 ; e.g., 2001:DB8::CD30 2338 domain-spec = macro-string domain-end 2339 domain-end = ( "." toplabel [ "." ] ) / macro-expand 2341 toplabel = ( *alphanum ALPHA *alphanum ) / 2342 ( 1*alphanum "-" *( alphanum / "-" ) alphanum ) 2343 ; LDH rule plus additional TLD restrictions 2344 ; (see [RFC3696], Section 2 for background) 2345 alphanum = ALPHA / DIGIT 2347 explain-string = *( macro-string / SP ) 2349 macro-string = *( macro-expand / macro-literal ) 2350 macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) 2351 / "%%" / "%_" / "%-" 2352 macro-literal = %x21-24 / %x26-7E 2353 ; visible characters except "%" 2354 macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / 2355 "c" / "r" / "t" / "v" 2356 transformers = *DIGIT [ "r" ] 2357 delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" 2359 name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) 2361 header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] 2362 [ key-value-list ] CRLF 2364 result = "pass" / "fail" / "softfail" / "neutral" / 2365 "none" / "temperror" / "permerror" 2367 key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) 2368 [";"] 2370 key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) 2372 key = "client-ip" / "envelope-from" / "helo" / 2373 "problem" / "receiver" / "identity" / 2374 "mechanism" / name 2376 identity = "mailfrom" ; for the "MAIL FROM" identity 2377 / "helo" ; for the "HELO" identity 2378 / name ; other identities 2380 sender = Mailbox 2381 ip = ip4-network / ip6-network 2382 ALPHA = 2383 DIGIT = <0-9 as per [RFC5234]> 2384 SP = 2385 dot-atom = 2386 quoted-string = 2387 comment = 2388 CFWS = 2389 FWS = 2390 CRLF = 2392 Appendix B. Extended Examples 2394 These examples are based on the following DNS setup: 2396 ; A domain with two mail servers, two hosts 2397 ; and two servers at the domain name 2398 $ORIGIN example.com. 2399 @ MX 10 mail-a 2400 MX 20 mail-b 2401 A 192.0.2.10 2402 A 192.0.2.11 2403 amy A 192.0.2.65 2404 bob A 192.0.2.66 2405 mail-a A 192.0.2.129 2406 mail-b A 192.0.2.130 2407 www CNAME example.com. 2409 ; A related domain 2410 $ORIGIN example.org. 2411 @ MX 10 mail-c 2412 mail-c A 192.0.2.140 2414 ; The reverse IP for those addresses 2415 $ORIGIN 2.0.192.in-addr.arpa. 2416 10 PTR example.com. 2417 11 PTR example.com. 2418 65 PTR amy.example.com. 2419 66 PTR bob.example.com. 2420 129 PTR mail-a.example.com. 2421 130 PTR mail-b.example.com. 2422 140 PTR mail-c.example.org. 2424 ; A rogue reverse IP domain that claims to be 2425 ; something it's not 2426 $ORIGIN 0.0.10.in-addr.arpa. 2427 4 PTR bob.example.com. 2429 B.1. Simple Examples 2431 These examples show various possible published records for 2432 example.com and which values if would cause check_host() to 2433 return "pass". Note that is "example.com". 2435 v=spf1 +all 2436 -- any passes 2438 v=spf1 a -all 2439 -- hosts 192.0.2.10 and 192.0.2.11 pass 2441 v=spf1 a:example.org -all 2442 -- no sending hosts pass since example.org has no A records 2444 v=spf1 mx -all 2445 -- sending hosts 192.0.2.129 and 192.0.2.130 pass 2447 v=spf1 mx:example.org -all 2448 -- sending host 192.0.2.140 passes 2450 v=spf1 mx mx:example.org -all 2451 -- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass 2453 v=spf1 mx/30 mx:example.org/30 -all 2454 -- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes 2456 v=spf1 ptr -all 2457 -- sending host 192.0.2.65 passes (reverse DNS is valid and is in 2458 example.com) 2459 -- sending host 192.0.2.140 fails (reverse DNS is valid, but not 2460 in example.com) 2461 -- sending host 10.0.0.4 fails (reverse IP is not valid) 2463 v=spf1 ip4:192.0.2.128/28 -all 2464 -- sending host 192.0.2.65 fails 2465 -- sending host 192.0.2.129 passes 2467 B.2. Multiple Domain Example 2469 These examples show the effect of related records: 2471 example.org: "v=spf1 include:example.com include:example.net -all" 2473 This record would be used if mail from example.org actually came 2474 through servers at example.com and example.net. Example.org's 2475 designated servers are the union of example.com's and example.net's 2476 designated servers. 2478 la.example.org: "v=spf1 redirect=example.org" 2479 ny.example.org: "v=spf1 redirect=example.org" 2480 sf.example.org: "v=spf1 redirect=example.org" 2482 These records allow a set of domains that all use the same mail 2483 system to make use of that mail system's record. In this way, only 2484 the mail system's record needs to be updated when the mail setup 2485 changes. These domains' records never have to change. 2487 B.3. DNSBL Style Example 2489 Imagine that, in addition to the domain records listed above, there 2490 are these (see [RFC5782]): 2492 $ORIGIN _spf.example.com. 2493 mary.mobile-users A 127.0.0.2 2494 fred.mobile-users A 127.0.0.2 2495 15.15.168.192.joel.remote-users A 127.0.0.2 2496 16.15.168.192.joel.remote-users A 127.0.0.2 2498 The following records describe users at example.com who mail from 2499 arbitrary servers, or who mail from personal servers. 2501 example.com: 2503 v=spf1 mx 2504 include:mobile-users._spf.%{d} 2505 include:remote-users._spf.%{d} 2506 -all 2508 mobile-users._spf.example.com: 2510 v=spf1 exists:%{l1r+}.%{d} 2512 remote-users._spf.example.com: 2514 v=spf1 exists:%{ir}.%{l1r+}.%{d} 2516 B.4. Multiple Requirements Example 2518 Say that your sender policy requires both that the IP address is 2519 within a certain range and that the reverse DNS for the IP matches. 2520 This can be done several ways, including the following: 2522 example.com. SPF ( "v=spf1 " 2523 "-include:ip4._spf.%{d} " 2524 "-include:ptr._spf.%{d} " 2525 "+all" ) 2526 ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all" 2527 ptr._spf.example.com. SPF "v=spf1 -ptr +all" 2529 This example shows how the "-include" mechanism can be useful, how an 2530 SPF record that ends in "+all" can be very restrictive, and the use 2531 of De Morgan's Law. 2533 Appendix C. Changes in implementation requirements from RFC 4408 2535 The modifications to implementation requirements from [RFC4408] are 2536 all either (a) corrections to errors in [RFC4408], or (b) additional 2537 documentation based on consensus of operational experience acquired 2538 since publication of [RFC4408]. 2540 o Use of DNS RR type SPF (99) has been removed from the protocol, 2541 see [RFC6686] for background. 2543 o A new DNS related processing limit based on "void lookups" has 2544 been added (Section 4.6.4). 2546 o Use of the ptr mechanism and the %p macro have been strongly 2547 discouraged Section 5.5 and Section 7.2. They remain part of the 2548 protocol because they were found to be in use, but records ought 2549 to be updated to avoid them. 2551 o Use of the "Authentication-Results" header field [RFC5451] as a 2552 possible alternative to use of the "Received-SPF" header field is 2553 discussed (Section 9.2). 2555 o There have been a number of minor corrections to the ABNF to make 2556 it more clear and correct Appendix A. SPF library implementers 2557 should give the revised ABNF a careful review to determine if 2558 implementation changes are needed. 2560 o Use of X- fields in the ABNF has been removed see [RFC6648] for 2561 background. 2563 o Ambiguity about how to deal with invalid domain-spec after macro 2564 expansion has been documented. Depending on one specific behavior 2565 has to be avoided (Section 4.8). 2567 o General operational information has been updated and expanded 2568 based on eight years of post [RFC4408] operations experience. See 2569 Section 10 and Appendices D - H below. 2571 o Security considerations have been reviewed and updated 2572 (Section 11). 2574 Appendix D. Further Testing Advice 2576 Another approach that can be helpful to publish records that include 2577 a "tracking exists:" mechanism. By looking at the name server logs, 2578 a rough list can then be generated. For example: 2580 v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all 2582 Appendix E. SPF/Mediator Interactions 2584 There are three places that techniques can be used to ameliorate 2585 unintended SPF failures with mediators. 2587 E.1. Originating ADMDs 2589 The beginning, when email is first sent: 2591 o "Neutral" results could be given for IP addresses that might be 2592 forwarders, instead of "fail" results based on a list of known 2593 reliable forwarders. For example: 2595 "v=spf1 mx ?exists:%{ir}.whitlist.example.org -all" 2597 This would cause a lookup on an DNS white list (DNSWL) and cause a 2598 result of "fail" only for email not either coming from the 2599 domain's mx host(s) (SPF pass) or white listed sources (SPF 2600 neutral). This, in effect, outsources an element of sender policy 2601 to the maintainer of the whitelist. 2603 o The "MAIL FROM" identity could have additional information in the 2604 local-part that cryptographically identifies the mail as coming 2605 from an authorized source. In this case, such an SPF record could 2606 be used: 2608 "v=spf1 mx exists:%{l}._spf_verify.%{d} -all" 2610 Then, a specialized DNS server can be set up to serve the 2611 _spf_verify subdomain that validates the local-part. Although 2612 this requires an extra DNS lookup, this happens only when the 2613 email would otherwise be rejected as not coming from a known good 2614 source. 2615 Note that due to the 63-character limit for domain labels, this 2616 approach only works reliably if the local-part signature scheme is 2617 guaranteed either to only produce local-parts with a maximum of 63 2618 characters or to gracefully handle truncated local-parts. 2620 o Similarly, a specialized DNS server could be set up that will 2621 rate-limit the email coming from unexpected IP addresses. 2623 "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all" 2625 o SPF allows the creation of per-user policies for special cases. 2626 For example, the following SPF record and appropriate wildcard DNS 2627 records can be used: 2629 "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}" 2631 E.2. Mediators 2633 The middle, when email is forwarded:. 2635 o Mediators can solve the problem by rewriting the "MAIL FROM" to be 2636 in their own domain. This means mail rejected from the external 2637 mailbox will have to be forwarded back to the original sender by 2638 the forwarding service. Various schemes to do this exist though 2639 they vary widely in complexity and resource requirements on the 2640 part of the mediator. 2642 o Several popular MTAs can be forced from "alias" semantics to 2643 "mailing list" semantics by configuring an additional alias with 2644 "owner-" prepended to the original alias name (e.g., an alias of 2645 "friends: george@example.com, fred@example.org" would need another 2646 alias of the form "owner-friends: localowner"). 2648 o Mediators could reject mail that would "fail" SPF if forwarded 2649 using an SMTP reply code of 551, User not local, (see [RFC5321] 2650 section 3.4) to communicate the correct target address to resend 2651 the mail to. 2653 E.3. Receving ADMDs 2655 The end, when email is received: 2657 o If the owner of the external mailbox wishes to trust the mediator, 2658 he can direct the external mailbox's MTA to skip SPF tests when 2659 the client host belongs to the mediator. 2661 o Tests against other identities, such as the "HELO" identity, can 2662 be used to override a failed test against the "MAIL FROM" 2663 identity. 2665 o For larger domains, it might not be possible to have a complete or 2666 accurate list of forwarding services used by the owners of the 2667 domain's mailboxes. In such cases, whitelists of generally- 2668 recognized forwarding services could be employed. 2670 Appendix F. Mail Services 2672 MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail 2673 services to third-party domains, such as sending of bulk mail, might 2674 want to adjust their configurations in light of the authorization 2675 check described in this document. If the domain part of the "MAIL 2676 FROM" identity used for such email uses the domain of one of the MSPs 2677 domain, then the provider needs only to ensure that its sending host 2678 is authorized by its own SPF record, if any. 2680 If the "MAIL FROM" identity does not use the MSP's domain, then extra 2681 care has to be taken. The SPF record format has several options for 2682 the third-party domain to authorize the service provider's MTAs to 2683 send mail on its behalf. For MSPs, such as ISPs, that have a wide 2684 variety of customers using the same MTA, steps are required to 2685 mitigate the risk of cross-customer forgery (see Section 11.4). 2687 Appendix G. MTA Relays 2689 Relays are described in [RFC5598] Section 2.2.2. The authorization 2690 check generally precludes the use of arbitrary MTA relays between 2691 sender and receiver of an email message. 2693 Within an organization, MTA relays can be effectively deployed. 2694 However, for purposes of this document, such relays are effectively 2695 transparent. The SPF authorization check is a check between border 2696 MTAs of different ADMDs. 2698 For mail senders, this means that published SPF records have to 2699 authorize any MTAs that actually send across the Internet. Usually, 2700 these are just the border MTAs as internal MTAs simply forward mail 2701 to these MTAs for relaying. 2703 The receiving ADMD will generally want to perform the authorization 2704 check at the boundary MTAs, including all secondary MXs. Internal 2705 MTAs (including MTAs that might serve both as boundary MTAs and 2706 internal relays from secondary MXs when they are processing the 2707 relayed mail stream) then do not perform the authorization test. To 2708 perform the authorization test other than at the boundary, the host 2709 that first transferred the message to the receiving ADMD have to be 2710 determined, which can be difficult to extract from the message header 2711 because (a) header fields can be forged or malformed, and (b) there's 2712 no standard way to encode that information such that it can be 2713 reliably extracted. Testing other than at the boundary is likely to 2714 produce unreliable results. This is described further in Appendix C 2715 of [RFC5451]. 2717 Appendix H. Local Policy Considerations 2719 SPF results can be used in combination with other methods to 2720 determine the final local disposition (either positive or negative of 2721 a message. It can also be considered dispositive on its own. 2723 H.1. Policy For SPF Pass 2725 SPF pass results can be used in combination with "white lists" of 2726 known "good" domains to bypass some or all additional pre-delivery 2727 email checks. Exactly which checks and how to determine appropriate 2728 white list entries has to be based on local conditions and 2729 requirements. 2731 H.2. Policy For SPF Fail 2733 SPF fail results can be used to reject messages during the SMTP 2734 transaction based on either "MAIL FROM" or "HELO" identity results. 2735 This reduces resource requirements for various content filtering 2736 methods and conserves bandwidth since rejection can be done before 2737 the SMTP content is transferred. It also gives immediate feedback to 2738 the sender who might then be able to resolve the issue. Due to some 2739 of the issues described above in this section (Section 10), SPF based 2740 rejection does present some risk of rejecting legitimate email when 2741 rejecting based on "MAIL FROM" results. 2743 SPF fail results can alternately be used as one input into a larger 2744 set of evaluations which might, based on a combination with other 2745 evaluation techniques, result in the email being marked negatively in 2746 some way (this might be via delivery to a special spam folder, 2747 modifying subject lines, or other locally determined means). 2748 Developing the details of such an approach have to be based on local 2749 conditions and requirements. Using SPF results in this way does not 2750 have the advantages of resource conservation and immediate feedback 2751 to the sender associated with SMTP rejection, but could produce fewer 2752 undesirable rejections in a well designed system. Such an approach 2753 might result in email that was not authorized by the sending ADMD 2754 being unknowingly delivered to end users. 2756 Either general approach can be used as they both leave a clear 2757 disposition of emails. They are either delivered in some manner or 2758 the sender is notified of the failure. Other dispositions such as 2759 "dropping" or deleting email after acceptance are inappropriate 2760 because they leave uncertainty and reduce the overall reliability and 2761 utility of email across the Internet. 2763 H.3. Policy For SPF Permerror 2765 The "permerror" result (see Section 2.6.7) indicates the SPF 2766 processing module at the receiver determined that the retrieved SPF 2767 policy record could not be interpreted. This gives no true 2768 indication about the authorized use of the data found in the 2769 envelope. 2771 As with all results, implementers have a choice to make regarding 2772 what to do with a message that yields this result. SMTP allows only 2773 a few basic options. 2775 Rejection of the message is an option, in that it is the one thing a 2776 receiver can do to draw attention to the difficulty encountered while 2777 protecting itself from messages that do not have a definite SPF 2778 result of some kind. However, if the SPF implementation is defective 2779 and returns spurious "permerror" results, only the sender is actively 2780 notified of the defect (in the form of rejected mail), and not the 2781 receiver making use of SPF. 2783 The less intrusive handling choice is to deliver the message, perhaps 2784 with some kind of annotation of the difficulty encountered and/or 2785 logging of a similar nature. However, this will not be desirable to 2786 operators that wish to implement SPF checking as strictly as 2787 possible, nor is this sort of passive problem reporting typically 2788 effective. 2790 There is of course the option placing this choice in the hands of the 2791 operator rather than the implementer since this kind of choice is 2792 often a matter of local policy rather than a condition with a 2793 universal solution, but this adds one more piece of complexity to an 2794 already non-trivial environment. 2796 Both implementers and operators need to be cautious of all choices 2797 and outcomes when handling SPF results. 2799 H.4. Policy For SPF Temperror 2801 The "temperror" result (see Section 2.6.6) indicates the SPF 2802 processing module at the receiver could not retrieve and SPF policy 2803 record due to a (probably) transient condition. This gives no true 2804 indication about the authorized use of the data found in the 2805 envelope. 2807 As with all results, implementers have a choice to make regarding 2808 what to do with a message that yields this result. SMTP allows only 2809 a few basic options. 2811 Deferring the message is an option, in that it is the one thing a 2812 receiver can do to draw attention to the difficulty encountered while 2813 protecting itself from messages that do not have a definite SPF 2814 result of some kind. However, if the SPF implementation is defective 2815 and returns spurious "temperror" results, only the sender is actively 2816 notified of the defect (in the form of mail rejected after it times 2817 out of the sending queue), and not the receiver making use of SPF. 2819 Because of long queue lifetimes, it is possible that mail will be 2820 repeatedly deferred for several days and so any awareness by the 2821 sender of a problem could be quite delayed. If "temperrors" persist 2822 for multiple delivery attempts, it might be preferable to treat the 2823 error as permanent and reduce the amount of time the message is in 2824 transit. 2826 The less intrusive handling choice is to deliver the message, perhaps 2827 with some kind of annotation of the difficulty encountered and/or 2828 logging of a similar nature. However, this will not be desirable to 2829 operators that wish to implement SPF checking as strictly as 2830 possible, nor is this sort of passive problem reporting typically 2831 effective. 2833 There is of course the option placing this choice in the hands of the 2834 operator rather than the implementer since this kind of choice is 2835 often a matter of local policy rather than a condition with a 2836 universal solution, but this adds one more piece of complexity to an 2837 already non-trivial environment. 2839 Both implementers and operators need to be cautious of all choices 2840 and outcomes when handling SPF results. 2842 Appendix I. Protocol Status 2844 NOTE TO RFC EDITOR: To be removed prior to publication. 2846 SPF has been in development since the summer of 2003 and has seen 2847 deployment beyond the developers beginning in December 2003. The 2848 design of SPF slowly evolved until the spring of 2004 and has since 2849 stabilized. There have been quite a number of forms of SPF, some 2850 written up as documents, some submitted as Internet Drafts, and many 2851 discussed and debated in development forums. The protocol was 2852 originally defined in [RFC4408], which this document replaces. 2854 [RFC4408] was designed to clearly document the protocol defined by 2855 earlier draft specifications of SPF as used in existing 2856 implementations. This updated specification is intended to clarify 2857 identified ambiguities in [RFC4408], resolve technical issues 2858 identified in post-RFC 4408 deployment experience, and document 2859 widely deployed extensions to SPF that have been developed since 2860 [RFC4408] was published. 2862 This document updates and replaces RFC 4408 that was part of a group 2863 of simultaneously published Experimental RFCs (RFC 4405, RFC 4406, 2864 RFC 4407, and RFC 4408) in 2006. At that time the IESG requested the 2865 community observe the success or failure of the two approaches 2866 documented in these RFCs during the two years following publication, 2867 in order that a community consensus could be reached in the future. 2869 SPF is widely deployed by large and small email providers alike. 2870 There are multiple, interoperable implementations. 2872 For SPF (as documented in RFC 4408) a careful effort was made to 2873 collect and document lessons learned and errata during the two year 2874 period. The errata list has been stable (no new submissions) and 2875 only minor protocol lessons learned were identified. Resolution of 2876 the IESG's experiment is documented in [RFC6686]. 2878 Appendix J. Change History 2880 NOTE TO RFC EDITOR: Changes since RFC 4408 (to be removed prior to 2881 publication) 2883 Moved to standards track 2885 Authors updated 2887 IESG Note regarding experimental use replaced with discussion of 2888 results 2890 Process errata: 2892 Resolved Section 2.5.7 PermError on invalid domains after macro 2893 expansion errata in favor of documenting that different verifiers 2894 produce different results. 2896 Add %v macro to ABNF grammar 2898 Replace "uric" by "unreserved" 2900 Recommend an SMTP reply code for optional permerror rejections 2902 Correct syntax in Received-SPF examples 2904 Fix unknown-modifier clause is too greedy in ABNF 2906 Correct use of empty domain-spec on exp modifier 2908 Fix minor typo errata 2910 Convert to spfbis working group draft, 2911 draft-ietf-spfbis-4408bis-00 2913 Clarified text about IPv4 mapped addresses to resolve test suite 2914 ambiguity 2916 Clarified ambiguity about result when more than 10 "mx" or "ptr" 2917 records are returned for lookup to specify permerror. This 2918 resolves one of the test suite ambiguities 2920 Made all references to result codes lower case per issue #7 2922 Adjusted section 2.2 Requirement to check mail from per issue #15 2924 Added missing "v" element in macro-letter in the collected ABNF 2925 per issue #16 - section 8.1 was already fixed in the pre-WG draft 2926 Marked ptr and "p" macro SHOULD NOT use per issue #27 2928 Expunged lower case may from the draft per issue #8 2930 Expunged "x-" name as an obsolete concept 2932 Updated obslete references: RFC2821 to RFC5321, RFC2822 to 2933 RFC5322, and RFC4234 to RFC5234 2935 Refer to RFC6647 to describe greylisting instead of trying to 2936 describe it directly. 2938 Updated informative references to the current versions. 2940 Start to rework section 9 with some RFC5598 terms. 2942 Added mention of RFC 6552 feedback reports in section 9. 2944 Added draft-ietf-spfbis-experiment as an informational reference. 2946 Drop Type SPF. 2948 Try and clarify informational nature of RFC3696 2950 Fix ABNF nits and add missing definitions per Bill's ABNF checker. 2952 Make DNS lookup time limit SHOULD instead of MAY. 2954 Reorganize and clarify processing limits. Move hard limits to new 2955 section 4.6.4, Evaluation Limits. Move advice to non-normative 2956 section 10. 2958 Removed paragraph in section 11.1 about limiting total data 2959 volumes as it is unused (and removable per the charter) and serves 2960 no purpose (it isn't something that actually can be implemented in 2961 any reasonable way). 2963 Added text from Alessandro Vesely in section 10.1 to better 2964 explain DNS resource limits. 2966 Multiple editorial fixes from Murray Kucherawy's review. 2968 Also based on Murray's review, reworked SMTP identity definitions 2969 and made RFC 5598 a normative reference instead of informative. 2970 This is a downref that will have to be mentioned in the last call. 2972 Added RFC 3834 as an informative reference about backscatter. 2974 Added IDN requirements and normative reference to RFC 5890 to deal 2975 with the question "like DKIM did it.: 2977 Added informative reference to RFC 4632 for CIDR and use CIDR 2978 prefix length instead of CIDR-length to match its terminology. 2980 Simplified the exists description. 2982 Added text on creating a Authentication-Results header field that 2983 matches the Received-SPF header field information and added a 2984 normative reference to RFC 5451. 2986 Added informative reference to RFC 2782 due to SRV mention. 2988 Added informative reference to RFC 3464 due to DSN mention. 2990 Added informative reference to RFC 5617 for its DNS wildcard use. 2992 Clarified the intended match/no-match method for exists. 2994 Added new sections on Receiver policy for SPF pass, fail, and 2995 permerror. 2997 Added new section 10 discussion on treatment of bounces and the 2998 significance of HELO records. 3000 Added request to IANA to update the SPF modifier registry. 3002 Substantially reorganized the document for improved readability 3003 for new users based on WG consensus. 3005 Added new DNS "void lookup" processing limit to mitigate potential 3006 future risk of SPF being used as a DDoS vector. 3008 Author's Address 3010 Scott Kitterman 3011 Kitterman Technical Services 3012 3611 Scheel Dr 3013 Ellicott City, MD 21042 3014 United States of America 3016 Email: scott@kitterman.com