idnits 2.17.1 draft-ietf-spfbis-4408bis-17.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 1 instance of lines with non-RFC2606-compliant FQDNs in the document. == There are 6 instances of lines with non-RFC6890-compliant IPv4 addresses in the document. If these are example addresses, they should be changed. == There are 1 instance of lines with private range IPv4 addresses in the document. If these are generic example addresses, they should be changed to use any of the ranges defined in RFC 6890 (or successor): 192.0.2.x, 198.51.100.x or 203.0.113.x. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == Line 1682 has weird spacing: '...pe-from the e...' == The document seems to contain a disclaimer for pre-RFC5378 work, but was first submitted on or after 10 November 2008. The disclaimer is usually necessary only for documents that revise or obsolete older RFCs, and that take significant amounts of text from those RFCs. If you can contact all authors of the source material and they are willing to grant the BCP78 rights to the IETF Trust, you can and should remove the disclaimer. Otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (July 14, 2013) is 3939 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 2365, but not defined ** Downref: Normative reference to an Informational RFC: RFC 1983 ** Obsolete normative reference: RFC 5451 (Obsoleted by RFC 7001) ** Downref: Normative reference to an Informational RFC: RFC 5598 ** Downref: Normative reference to an Informational RFC: RFC 5782 -- 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: 4 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) July 14, 2013 5 Intended status: Standards Track 6 Expires: January 15, 2014 8 Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, 9 Version 1 10 draft-ietf-spfbis-4408bis-17 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 January 15, 2014. 42 Copyright Notice 44 Copyright (c) 2013 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 This document may contain material from IETF Documents or IETF 58 Contributions published or made publicly available before November 59 10, 2008. The person(s) controlling the copyright in some of this 60 material may not have granted the IETF Trust the right to allow 61 modifications of such material outside the IETF Standards Process. 62 Without obtaining an adequate license from the person(s) controlling 63 the copyright in such materials, this document may not be modified 64 outside the IETF Standards Process, and derivative works of it may 65 not be created outside the IETF Standards Process, except to format 66 it for publication as an RFC or to translate it into languages other 67 than English. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6 72 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 73 1.1.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . 6 74 1.1.2. Imported Definitions . . . . . . . . . . . . . . . . . 6 75 1.1.3. MAIL FROM Definition . . . . . . . . . . . . . . . . . 7 76 1.1.4. HELO Definition . . . . . . . . . . . . . . . . . . . 7 77 1.2. check_host() . . . . . . . . . . . . . . . . . . . . . . . 7 78 2. Operational Overview . . . . . . . . . . . . . . . . . . . . . 8 79 2.1. Publishing Authorization . . . . . . . . . . . . . . . . . 8 80 2.2. Checking Authorization . . . . . . . . . . . . . . . . . . 8 81 2.3. The "HELO" Identity . . . . . . . . . . . . . . . . . . . 9 82 2.4. The "MAIL FROM" Identity . . . . . . . . . . . . . . . . . 10 83 2.5. Location of Checks . . . . . . . . . . . . . . . . . . . . 10 84 2.6. Results of Evaluation . . . . . . . . . . . . . . . . . . 10 85 2.6.1. None . . . . . . . . . . . . . . . . . . . . . . . . . 11 86 2.6.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . 11 87 2.6.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . 11 88 2.6.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . 11 89 2.6.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . 11 90 2.6.6. Temperror . . . . . . . . . . . . . . . . . . . . . . 11 91 2.6.7. Permerror . . . . . . . . . . . . . . . . . . . . . . 11 92 3. SPF Records . . . . . . . . . . . . . . . . . . . . . . . . . 12 93 3.1. DNS Resource Records . . . . . . . . . . . . . . . . . . . 12 94 3.2. Multiple DNS Records . . . . . . . . . . . . . . . . . . . 13 95 3.3. Multiple Strings in a Single DNS record . . . . . . . . . 13 96 3.4. Record Size . . . . . . . . . . . . . . . . . . . . . . . 13 97 3.5. Wildcard Records . . . . . . . . . . . . . . . . . . . . . 14 98 4. The check_host() Function . . . . . . . . . . . . . . . . . . 15 99 4.1. Arguments . . . . . . . . . . . . . . . . . . . . . . . . 15 100 4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . 15 101 4.3. Initial Processing . . . . . . . . . . . . . . . . . . . . 16 102 4.4. Record Lookup . . . . . . . . . . . . . . . . . . . . . . 16 103 4.5. Selecting Records . . . . . . . . . . . . . . . . . . . . 16 104 4.6. Record Evaluation . . . . . . . . . . . . . . . . . . . . 16 105 4.6.1. Term Evaluation . . . . . . . . . . . . . . . . . . . 17 106 4.6.2. Mechanisms . . . . . . . . . . . . . . . . . . . . . . 17 107 4.6.3. Modifiers . . . . . . . . . . . . . . . . . . . . . . 18 108 4.6.4. DNS Lookup Limits . . . . . . . . . . . . . . . . . . 18 109 4.7. Default Result . . . . . . . . . . . . . . . . . . . . . . 19 110 4.8. Domain Specification . . . . . . . . . . . . . . . . . . . 19 111 5. Mechanism Definitions . . . . . . . . . . . . . . . . . . . . 21 112 5.1. "all" . . . . . . . . . . . . . . . . . . . . . . . . . . 22 113 5.2. "include" . . . . . . . . . . . . . . . . . . . . . . . . 22 114 5.3. "a" . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 115 5.4. "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 116 5.5. "ptr" (do not use) . . . . . . . . . . . . . . . . . . . . 24 117 5.6. "ip4" and "ip6" . . . . . . . . . . . . . . . . . . . . . 26 118 5.7. "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 26 119 6. Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 28 120 6.1. redirect: Redirected Query . . . . . . . . . . . . . . . . 28 121 6.2. exp: Explanation . . . . . . . . . . . . . . . . . . . . . 29 122 7. Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 123 7.1. Formal Specification . . . . . . . . . . . . . . . . . . . 31 124 7.2. Macro Definitions . . . . . . . . . . . . . . . . . . . . 31 125 7.3. Macro Processing Details . . . . . . . . . . . . . . . . . 32 126 7.4. Expansion Examples . . . . . . . . . . . . . . . . . . . . 34 127 8. Result Handling . . . . . . . . . . . . . . . . . . . . . . . 36 128 8.1. None . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 129 8.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . . . 36 130 8.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 131 8.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 132 8.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . . . 37 133 8.6. Temperror . . . . . . . . . . . . . . . . . . . . . . . . 38 134 8.7. Permerror . . . . . . . . . . . . . . . . . . . . . . . . 38 135 9. Recording the Result . . . . . . . . . . . . . . . . . . . . . 39 136 9.1. The Received-SPF Header Field . . . . . . . . . . . . . . 39 137 9.2. SPF Results in the Authentication-Results Header Field . . 41 138 10. Effects on Infrastructure . . . . . . . . . . . . . . . . . . 43 139 10.1. Sending Domains . . . . . . . . . . . . . . . . . . . . . 43 140 10.1.1. DNS Resource Considerations . . . . . . . . . . . . . 43 141 10.1.2. Administrator's Considerations . . . . . . . . . . . . 44 142 10.1.3. Bounces . . . . . . . . . . . . . . . . . . . . . . . 45 143 10.2. Receivers . . . . . . . . . . . . . . . . . . . . . . . . 45 144 10.3. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 45 145 11. Security Considerations . . . . . . . . . . . . . . . . . . . 47 146 11.1. Processing Limits . . . . . . . . . . . . . . . . . . . . 47 147 11.2. SPF-Authorized Email May Contain Other False Identities . 48 148 11.3. Spoofed DNS and IP Data . . . . . . . . . . . . . . . . . 48 149 11.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 48 150 11.5. Untrusted Information Sources . . . . . . . . . . . . . . 49 151 11.5.1. Recorded Results . . . . . . . . . . . . . . . . . . . 49 152 11.5.2. External Explanations . . . . . . . . . . . . . . . . 49 153 11.5.3. Macro Expansion . . . . . . . . . . . . . . . . . . . 50 154 11.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 50 155 11.7. Delivering Mail Producing a 'Fail' Result . . . . . . . . 50 156 12. Contributors and Acknowledgements . . . . . . . . . . . . . . 51 157 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52 158 13.1. The SPF DNS Record Type . . . . . . . . . . . . . . . . . 52 159 13.2. The Received-SPF Mail Header Field . . . . . . . . . . . . 52 160 13.3. SPF Modifier Registry . . . . . . . . . . . . . . . . . . 52 161 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53 162 14.1. Normative References . . . . . . . . . . . . . . . . . . . 53 163 14.2. Informative References . . . . . . . . . . . . . . . . . . 54 164 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 56 165 Appendix B. Extended Examples . . . . . . . . . . . . . . . . . . 59 166 B.1. Simple Examples . . . . . . . . . . . . . . . . . . . . . 59 167 B.2. Multiple Domain Example . . . . . . . . . . . . . . . . . 60 168 B.3. DNSBL Style Example . . . . . . . . . . . . . . . . . . . 61 169 B.4. Multiple Requirements Example . . . . . . . . . . . . . . 61 170 Appendix C. Changes in implementation requirements from RFC 171 4408 . . . . . . . . . . . . . . . . . . . . . . . . 62 172 Appendix D. Further Testing Advice . . . . . . . . . . . . . . . 63 173 Appendix E. SPF/Mediator Interactions . . . . . . . . . . . . . . 64 174 E.1. Originating ADMDs . . . . . . . . . . . . . . . . . . . . 64 175 E.2. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 65 176 E.3. Receving ADMDs . . . . . . . . . . . . . . . . . . . . . . 65 177 Appendix F. Mail Services . . . . . . . . . . . . . . . . . . . . 66 178 Appendix G. MTA Relays . . . . . . . . . . . . . . . . . . . . . 67 179 Appendix H. Local Policy Considerations . . . . . . . . . . . . . 68 180 H.1. Policy For SPF Pass . . . . . . . . . . . . . . . . . . . 68 181 H.2. Policy For SPF Fail . . . . . . . . . . . . . . . . . . . 68 182 H.3. Policy For SPF Permerror . . . . . . . . . . . . . . . . . 69 183 H.4. Policy For SPF Temperror . . . . . . . . . . . . . . . . . 69 184 Appendix I. Protocol Status . . . . . . . . . . . . . . . . . . . 71 185 Appendix J. Change History . . . . . . . . . . . . . . . . . . . 72 186 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 75 188 1. Introduction 190 The current email infrastructure has the property that any host 191 injecting mail into the system can use any DNS domain name it wants 192 in each of the various identifiers specified by [RFC5321] and 193 [RFC5322]. Although this feature is desirable in some circumstances, 194 it is a major obstacle to reducing Unsolicited Bulk Email (UBE, aka 195 spam). Furthermore, many domain owning ADMDs (as described in 196 [RFC5598]) are understandably concerned about the ease with which 197 other entities can make use of their domain names, often with 198 malicious intent. 200 This document defines a protocol by which ADMDs can authorize hosts 201 to use their domain names in the "MAIL FROM" or "HELO" identities. 202 Compliant ADMDs publish Sender Policy Framework (SPF) records in the 203 DNS specifying which hosts are permitted to use their names, and 204 compliant mail receivers use the published SPF records to test the 205 authorization of sending Mail Transfer Agents (MTAs) using a given 206 "HELO" or "MAIL FROM" identity during a mail transaction. 208 An additional benefit to mail receivers is that after the use of an 209 identity is verified, local policy decisions about the mail can be 210 made based on the sender's domain, rather than the host's IP address. 211 This is advantageous because reputation of domain names is likely to 212 be more accurate than reputation of host IP addresses since domains 213 are likely to be more stable over a longer period. Furthermore, if a 214 claimed identity fails verification, local policy can take stronger 215 action against such email, such as rejecting it. 217 1.1. Terminology 219 1.1.1. Keywords 221 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 222 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 223 "OPTIONAL" in this document are to be interpreted as described in 224 [RFC2119]. 226 1.1.2. Imported Definitions 228 ABNF (Augmented Backus-Naur Form) ABNF is defined in [RFC5234], as 229 are the tokens "ALPHA", "DIGIT", and "SP" (space). 231 The token "local-part" is defined in [RFC5321]. 233 "dot-atom", "quoted-string", "comment", "CFWS" (comment folded white 234 space), "FWS" (folded white space), and "CRLF" (carriage-return/ 235 line-feed) are defined in [RFC5322]. 237 1.1.3. MAIL FROM Definition 239 This document is concerned with the portion of a mail message 240 commonly called "envelope sender", "return path", "reverse path", 241 "bounce address", "5321 FROM", "MAIL FROM", or RFC5321.MailFrom. 242 Since these terms are either not well defined or often used casually, 243 this document uses "MAIL FROM" for consistency. This means the 244 RFC5321.MailFrom as defined in [RFC5598]. Note that other terms that 245 might superficially look like the common terms, such as 'reverse- 246 path', are used only as they are specified in their defining 247 documents. 249 1.1.4. HELO Definition 251 This document also makes use of the HELO/EHLO identity. The "HELO" 252 identity derives from either the SMTP HELO or EHLO command (see 253 [RFC5321]). Since HELO and EHLO can, in many cases, be used 254 interchangeably, they are identified commonly as "HELO" in this 255 document. This means RFC5321.HELO/.EHLO as defined in [RFC5598]. 256 These commands supply the identity of the SMTP client (sending host) 257 for the SMTP session. 259 1.2. check_host() 261 Section 4 introduces an algorithm to evaluate an SPF policy against 262 an arriving email transaction. In an early implementation, this 263 algorithm was encoded in a function called check_host(). That name 264 is used in this document as symbolic of the SPF evaluation algorithm, 265 but of course implementers are not required to use this name. 267 2. Operational Overview 269 2.1. Publishing Authorization 271 An SPF-compliant domain publishes valid SPF records as described in 272 Section 3. These records authorize the use of the relevant domain 273 names in the "HELO" and "MAIL FROM" identities by the MTAs specified 274 therein. 276 SPF results can be used to make both positive (source is authorized) 277 and negative (source is not authorized) determinations. If ADMDs 278 choose to publish SPF records and want to support receivers making 279 negative authorization determinations, it is necessary for them to 280 publish records that end in "-all", or redirect to other records that 281 do, otherwise, no definitive determination of authorization can be 282 made. Potential issues and mitigations associated with negative 283 determinations are discussed in Section 10. 285 ADMDs that wish to declare that no hosts are authorized to use their 286 DNS domain names in the HELO or MAIL FROM commands during SMTP 287 sessions can publish SPF records that say so for domain names that 288 are neither used in the domain part of email addresses nor expected 289 to originate mail. 291 When changing SPF records, care has to be taken to ensure that there 292 is a transition period so that the old policy remains valid until all 293 legitimate email can reasonably expect to have been checked. 294 [RFC5321] Section 4.5.4.1 discusses how long a message might be in 295 transit. While offline checks are possible, the closer to the 296 original transmission time checks are performed, the more likely they 297 are to get an SPF result that matches the sending ADMD intent at the 298 time the message was sent. 300 2.2. Checking Authorization 302 A mail receiver can perform a set of SPF checks for each mail message 303 it receives. An SPF check tests the authorization of a client host 304 to emit mail with a given identity. Typically, such checks are done 305 by a receiving MTA, but can be performed elsewhere in the mail 306 processing chain so long as the required information is available and 307 reliable. The "MAIL FROM" and "HELO" identities are checked as 308 described in Section 2.4 and Section 2.3 respectively. 310 Without explicit approval of the publishing ADMD, checking other 311 identities against SPF version 1 records is NOT RECOMMENDED because 312 there are cases that are known to give incorrect results. For 313 example, almost all mailing lists rewrite the "MAIL FROM" identity 314 (see Section 10.3), but some do not change any other identities in 315 the message. Documents that define other identities will have to 316 define the method for explicit approval. 318 It is possible that mail receivers will use the SPF check as part of 319 a larger set of tests on incoming mail. The results of other tests 320 might influence whether or not a particular SPF check is performed. 321 For example, finding the sending host's IP address on a local white 322 list might cause all other tests to be skipped and all mail from that 323 host to be accepted. 325 When a mail receiver decides to perform an SPF check, it has to use a 326 correctly-implemented check_host() function (Section 4) evaluated 327 with the correct parameters. Although the test as a whole is 328 optional, once it has been decided to perform a test it has to be 329 performed as specified so that the correct semantics are preserved 330 between publisher and receiver. 332 To make the test, the mail receiver MUST evaluate the check_host() 333 function with the arguments described in Section 4.1. 335 Although invalid, malformed, or non-existent domains cause SPF checks 336 to return "none" because no SPF record can be found, it has long been 337 the policy of many MTAs to reject email from such domains, especially 338 in the case of invalid "MAIL FROM". Rejecting email will prevent one 339 method of circumventing of SPF records. 341 Implementations have to take care to correctly extract the 342 from the data given with the SMTP MAIL FROM command as many MTAs will 343 still accept such things as source routes (see [RFC5321], Appendix 344 C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]). 345 These archaic features have been maliciously used to bypass security 346 systems. 348 2.3. The "HELO" Identity 350 It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM" 351 identity, but also separately check the "HELO" identity by applying 352 the check_host() function (Section 4) to the "HELO" identity as the 353 . Checking "HELO" promotes consistency of results and can 354 reduce DNS resource usage. If a conclusive determination about the 355 message can be made based on a check of "HELO", then the use of DNS 356 resources to process the typically more complex "MAIL FROM" can be 357 avoided. Additionally, since SPF records published for "HELO" 358 identities refer to a single host, when available, they are a very 359 reliable source of host authorization status. Checking "HELO" before 360 "MAIL FROM" is the RECOMMENDED sequence if both are checked. 362 Note that requirements for the domain presented in the EHLO or HELO 363 command are not always clear to the sending party, and SPF verifiers 364 have to be prepared for the identity to be an IP address literal (see 365 [RFC5321] section 4.1.3), or simply be malformed. This SPF check can 366 only be performed when the "HELO" string is a valid, multi-label 367 domain name. 369 2.4. The "MAIL FROM" Identity 371 SPF verifiers MUST check the "MAIL FROM" identity if a "HELO" check 372 has either not been performed or has not reached a definitive policy 373 result by applying the check_host() function to the "MAIL FROM" 374 identity as the . 376 [RFC5321] allows the reverse-path to be null (see Section 4.5.5 in 377 [RFC5321]). In this case, there is no explicit sender mailbox, and 378 such a message can be assumed to be a notification message from the 379 mail system itself. When the reverse-path is null, this document 380 defines the "MAIL FROM" identity to be the mailbox composed of the 381 local-part "postmaster" and the "HELO" identity (which might or might 382 not have been checked separately before). 384 2.5. Location of Checks 386 The authorization check SHOULD be performed during the processing of 387 the SMTP transaction that recieves the mail. This reduces the 388 complexity of determining the correct IP address to use as an input 389 to check_host() and allows errors to be returned directly to the 390 sending MTA by way of SMTP replies. Appendix C of [RFC5451] provides 391 a more thorough discussion of this topic. 393 Performing the authorization check other than using the MAIL FROM and 394 client address at the time of the MAIL command during the SMTP 395 transaction can cause problems, such as the following: (1) It might 396 be difficult to accurately extract the required information from 397 potentially deceptive headers; (2) legitimate email might fail 398 because the sender's policy had since changed. 400 Generating non-delivery notifications to forged identities that have 401 failed the authorization check often constitutes backscatter, i.e., 402 inactionable, nuisance rejection notices. Operators are strongly 403 advised to avoid such practices. Section 2 of [RFC3834] describes 404 backscatter and the problems it causes. 406 2.6. Results of Evaluation 408 Section 4 defines check_host(), a model function definition that uses 409 the inputs defined above and the sender's policy published in the DNS 410 to reach a conclusion about client authorization. An SPF verifier 411 implements something semantically equivalent to the function defined 412 there. 414 This section enumerates and briefly defines the possible outputs of 415 that function. Note, however, that the protocol establishes no 416 normative requirements for handling any particular result. 417 Discussion of handling options for each result can be found in 418 Section 8. 420 2.6.1. None 422 A result of "none" means either (a) no syntactically valid DNS domain 423 name was extracted from the SMTP session that could be used as the 424 one to be authorized, or (b) no TXT records were retrieved from the 425 DNS that appeared to be intended for use by SPF verifiers. 427 2.6.2. Neutral 429 The ADMD has explicitly stated that it is not asserting whether the 430 IP address is authorized. 432 2.6.3. Pass 434 A "pass" result means that the client is authorized to inject mail 435 with the given identity. 437 2.6.4. Fail 439 A "fail" result is an explicit statement that the client is not 440 authorized to use the domain in the given identity. 442 2.6.5. Softfail 444 The ADMD has published a weak statement that the host is probably not 445 authorized. It has not published a stronger, more definitive policy 446 that results in a "fail". 448 2.6.6. Temperror 450 A "temperror" result means the SPF verifier encountered a transient 451 (generally DNS) error while performing the check. A later retry may 452 succeed without further operator action. 454 2.6.7. Permerror 456 A "permerror" result means the domain's published records could not 457 be correctly interpreted. This signals an error condition that 458 definitely requires operator intervention to be resolved. 460 3. SPF Records 462 An SPF record is a DNS record that declares which hosts are, and are 463 not, authorized to use a domain name for the "HELO" and "MAIL FROM" 464 identities. Loosely, the record partitions hosts into permitted and 465 not-permitted sets (though some hosts might fall into neither 466 category). 468 The SPF record is expressed as a single string of text found in the 469 RDATA of a single DNS TXT resource record; multiple SPF records are 470 not permitted for the same owner name. The record format and the 471 process for selecting records is described below in Section 4. An 472 example record is the following: 474 v=spf1 +mx a:colo.example.com/28 -all 476 This record has a version of "spf1" and three directives: "+mx", 477 "a:colo.example.com/28" (the + is implied), and "-all". 479 Each SPF record is placed in the DNS tree at the owner name it 480 pertains to, not a subdomain under it, such as is done with SRV 481 records [RFC2782]. 483 The example in this section might be published via these lines in a 484 domain zone file: 486 example.com. TXT "v=spf1 +mx a:colo.example.com/28 -all" 487 smtp-out.example.com. TXT "v=spf1 a -all" 489 Since TXT records have multiple uses, beware of other TXT records 490 published there for other purposes. They might cause problems with 491 size limits (see Section 3.4) and care has to be taken to ensure only 492 SPF records are used for SPF processing. 494 ADMDs publishing SPF records ought to keep the amount of DNS 495 information needed to evaluate a record to a minimum. Section 4.6.4 496 and Section 10.1.1 provide some suggestions about "include" 497 mechanisms and chained "redirect" modifiers. 499 3.1. DNS Resource Records 501 SPF records MUST be published as a DNS TXT (type 16) Resource Record 502 (RR) [RFC1035] only. The character content of the record is encoded 503 as [US-ASCII]. Use of alternative DNS RR types was supported in 504 SPF's experimental phase, but has been discontinued. See Appendix A 505 of [RFC6686] for further information. 507 3.2. Multiple DNS Records 509 A domain name MUST NOT have multiple records that would cause an 510 authorization check to select more than one record. See Section 4.5 511 for the selection rules. 513 3.3. Multiple Strings in a Single DNS record 515 As defined in [RFC1035] sections 3.3 and 3.3.14, a single text DNS 516 record can be composed of more than one string. If a published 517 record contains multiple character-strings, then the record MUST be 518 treated as if those strings are concatenated together without adding 519 spaces. For example: 521 IN TXT "v=spf1 .... first" "second string..." 523 is equivalent to: 525 IN TXT "v=spf1 .... firstsecond string..." 527 TXT records containing multiple strings are useful in constructing 528 records that would exceed the 255-octet maximum length of a 529 character-string within a single TXT record. 531 3.4. Record Size 533 The published SPF record for a given domain name SHOULD remain small 534 enough that the results of a query for it will fit within 512 octets. 535 This UDP limit is defined in [RFC1035] section 2.3.4, although it was 536 raised by [RFC2671]. Staying below 512 octets ought to prevent older 537 DNS implementations from falling over to TCP,and will work with UDP 538 in the absence of EDNS0 [RFC6891] support. Since the answer size is 539 dependent on many things outside the scope of this document, it is 540 only possible to give this guideline: If the combined length of the 541 DNS name and the text of all the records of a given type is under 450 542 octets, then DNS answers ought to fit in UDP packets. Records that 543 are too long to fit in a single UDP packet could be silently ignored 544 by SPF verifiers due to firewall and other issues that interfere with 545 the operation of DNS over TCP or using ENDS0. 547 Note that when computing the sizes for replies to queries of the TXT 548 format, one has to take into account any other TXT records published 549 at the domain name. Similarly, the sizes for replies to all queries 550 related to SPF have to be evaluated to fit in a single 512 octet UDP 551 packet. 553 3.5. Wildcard Records 555 Use of wildcard records for publishing is discouraged and care has to 556 be taken if they are used. If a zone includes wildcard MX records, 557 it might want to publish wildcard declarations, subject to the same 558 requirements and problems. In particular, the declaration MUST be 559 repeated for any host that has any RR records at all, and for 560 subdomains thereof. Consider the example in [RFC1034], Section 561 4.3.3. Based on that, we can do the following: 563 EXAMPLE.COM. MX 10 A.EXAMPLE.COM 564 EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 566 *.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 567 *.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 569 A.EXAMPLE.COM. A 203.0.113.1 570 A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 571 A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 573 *.A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM 574 *.A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" 576 SPF records have to be listed twice for every name within the zone: 577 once for the name, and once with a wildcard to cover the tree under 578 the name, in order to cover all domains in use in outgoing mail. 580 4. The check_host() Function 582 This description is not an API (Application Program Interface) 583 definition, but rather a function description used to illustrate the 584 algorithm. A compliant SPF implementation MUST produce results 585 semantically equivalent to this description. 587 The check_host() function fetches SPF records, parses them, and 588 evaluates them to determine whether a particular host is or is not 589 permitted to send mail with a given identity. Receiving ADMDs that 590 perform this check MUST correctly evaluate the check_host() function 591 as described here. 593 Implementations MAY use a different algorithm than the canonical 594 algorithm defined here, so long as the results are the same in all 595 cases. 597 4.1. Arguments 599 The check_host() function takes these arguments: 601 - the IP address of the SMTP client that is emitting the 602 mail, either IPv4 or IPv6. 604 - the domain that provides the sought-after authorization 605 information; initially, the domain portion of the "MAIL 606 FROM" or "HELO" identity. 608 - the "MAIL FROM" or "HELO" identity. 610 For recursive evaluations, the domain portion of might not 611 be the same as the argument when check_host() is initially 612 evaluated. In most other cases it will be the same. (See 613 Section 5.2 below). 615 Note that the argument might not be a well-formed domain 616 name. For example, if the reverse-path was null, then the EHLO/HELO 617 domain is used, with its associated problems (see Section 2.3). In 618 these cases, check_host() is defined in Section 4.3 to return a 619 "none" result. 621 4.2. Results 623 The function check_host() can return one of several results described 624 in Section 2.6. Based on the result, the action to be taken is 625 determined by the local policies of the receiver. This is discussed 626 in Section 8. 628 4.3. Initial Processing 630 If the is malformed (e.g. label longer than 63 characters, 631 zero-length label not at the end, etc.) or is not a multi-label 632 domain name, or if the DNS lookup returns "domain does not exist" 633 (RCODE 3), check_host() immediately returns the result "none". DNS 634 RCODES are defined in [RFC1035]. Properly formed domains are fully 635 qualified domains as defined in [RFC1983]. That is, in the DNS they 636 are implicitly qualified relative to the root (see section 3.1 of 637 [RFC1034]). Internationalized domain names MUST be encoded as 638 A-labels, as described in Section 2.3 of [RFC5890]. 640 If the has no local-part, substitute the string "postmaster" 641 for the local-part. 643 4.4. Record Lookup 645 In accordance with how the records are published (see Section 3 646 above), a DNS query needs to be made for the name, querying 647 for type TXT only. 649 If the DNS lookup returns a server failure (RCODE 2), or other error 650 (RCODE other than 0 or 3), or time out, then check_host() terminates 651 immediately with the result "temperror". 653 4.5. Selecting Records 655 Records begin with a version section: 657 record = version terms *SP 658 version = "v=spf1" 660 Starting with the set of records that were returned by the lookup, 661 discard records that do not begin with a version section of exactly 662 "v=spf1". Note that the version section is terminated either by an 663 SP character or the end of the record. A record with a version 664 section of "v=spf10" does not match and is discarded. 666 If the resultant record set includes no records, check_host() 667 produces the "none" result. If the resultant record set includes 668 more than one record, check_host() produces the "permerror" result. 670 4.6. Record Evaluation 672 The check_host() function parses and interprets the SPF record to 673 find a result for the current test. If there are any syntax errors 674 anywhere in the record, check_host() returns immediately with the 675 result "permerror", without further interpretation. 677 4.6.1. Term Evaluation 679 There are two types of terms: mechanisms and modifiers. A record 680 contains an ordered list of these as specified in the following 681 Augmented Backus-Naur Form (ABNF). 683 terms = *( 1*SP ( directive / modifier ) ) 685 directive = [ qualifier ] mechanism 686 qualifier = "+" / "-" / "?" / "~" 687 mechanism = ( all / include 688 / a / mx / ptr / ip4 / ip6 / exists ) 689 modifier = redirect / explanation / unknown-modifier 690 unknown-modifier = name "=" macro-string 691 ; where name is not any known modifier 693 name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) 695 Most mechanisms allow a ":" or "/" character after the name. 697 Modifiers always contain an equals ('=') character immediately after 698 the name, and before any ":" or "/" characters that might be part of 699 the macro-string. 701 Terms that do not contain any of "=", ":", or "/" are mechanisms, as 702 defined in Section 5. 704 As per the definition of the ABNF notation in [RFC5234], mechanism 705 and modifier names are case-insensitive. 707 4.6.2. Mechanisms 709 Each mechanism is considered in turn from left to right. If there 710 are no more mechanisms, the result is the default result as described 711 in Section 4.7. 713 When a mechanism is evaluated, one of three things can happen: it can 714 match, not match, or return an exception. 716 If it matches, processing ends and the qualifier value is returned as 717 the result of that record. If it does not match, processing 718 continues with the next mechanism. If it returns an exception, 719 mechanism processing ends and the exception value is returned. 721 The possible qualifiers, and the results they cause check_host() to 722 return are as follows: 724 "+" pass 725 "-" fail 726 "~" softfail 727 "?" neutral 729 The qualifier is optional and defaults to "+". 731 When a mechanism matches and the qualifier is "-", then a "fail" 732 result is returned and the explanation string is computed as 733 described in Section 6.2. 735 The specific mechanisms are described in Section 5. 737 4.6.3. Modifiers 739 Modifiers are not mechanisms. They do not return match or not-match. 740 Instead, they provide additional information. Although modifiers do 741 not directly affect the evaluation of the record, the "redirect" 742 modifier has an effect after all the mechanisms have been evaluated. 744 4.6.4. DNS Lookup Limits 746 SPF implementations MUST limit the total number of mechanisms and 747 modifiers ("terms") that cause any DNS query to 10 during SPF 748 evaluation. Specifically, the "include", "a", "mx", "ptr", and 749 "exists" mechanisms as well as the "redirect" modifier count against 750 this collective limit. The "all", "ip4", and "ip6" mechanisms do not 751 count against this limit. If this number is exceeded during a check, 752 a "permerror" MUST be returned. The "exp" modifier does not count 753 against this limit because the DNS lookup to fetch the explanation 754 string occurs after the SPF record evaluation has been completed. 756 When evaluating the "mx" mechanism, the number of "MX" resource 757 records queried is included in the overall limit of 10 mechanisms/ 758 modifiers that cause DNS lookups described above. The evaluation of 759 each "MX" record MUST NOT result in querying more than 10 address 760 records, either "A" or "AAAA" resource records. If this limit is 761 exceeded, the "mx" mechanism MUST produce a "permerror" result. 763 When evaluating the "ptr" mechanism or the %{p} macro, the number of 764 "PTR" resource records queried is included in the overall limit of 10 765 mechanisms/modifiers that cause DNS lookups described above. The 766 evaluation of each "PTR" record MUST NOT result in querying more than 767 10 address records, either "A" or "AAAA" resource records. If this 768 limit is exceeded, all records other than the first 10 MUST be 769 ignored. 771 The reason for the disparity is that the set of and contents of the 772 MX record are under control of the publishing ADMD, while the set of 773 and contents of PTR records are under control of the owner of the IP 774 address actually making the connection. 776 These limits are per mechanism or macro in the record, and are in 777 addition to the lookup limits specified above. 779 MTAs or other processors SHOULD impose a limit on the maximum amount 780 of elapsed time to evaluate check_host(). Such a limit SHOULD allow 781 at least 20 seconds. If such a limit is exceeded, the result of 782 authorization SHOULD be "temperror". 784 As described at the end of Section 11.1, there may be cases where it 785 is useful to limit the number of "terms" for which DNS queries return 786 either a positive answer (RCODE 0) with an answer count of 0, or a no 787 such record (RCODE 3) answer. These are sometimes collectively 788 referred to as "void lookups". SPF implementations SHOULD limit 789 "void lookups" to two. An implementation MAY choose to make such a 790 limit configurable. In this case, a default of two is RECOMMENDED. 792 4.7. Default Result 794 If none of the mechanisms match and there is no "redirect" modifier, 795 then the check_host() returns a result of "neutral", just as if 796 "?all" were specified as the last directive. If there is a 797 "redirect" modifier, check_host() proceeds as defined in Section 6.1. 799 It is better to use either a "redirect" modifier or an "all" 800 mechanism to explicitly terminate processing. Although the latter 801 has a default (specifically "?all"), it aids debugging efforts if it 802 is explicitly provided. 804 For example: 806 v=spf1 +mx -all 807 or 808 v=spf1 +mx redirect=_spf.example.com 810 4.8. Domain Specification 812 Several of these mechanisms and modifiers have a domain-spec section. 813 The domain-spec string is subject to macro expansion (see Section 7). 814 The resulting string is the common presentation form of a fully- 815 qualified DNS name: a series of labels separated by periods. This 816 domain is called the in the rest of this document. 818 Note: The result of the macro expansion is not subject to any further 819 escaping. Hence, this facility cannot produce all characters that 820 are legal in a DNS label (e.g., the control characters). However, 821 this facility is powerful enough to express legal host names and 822 common utility labels (such as "_spf") that are used in DNS. 824 For several mechanisms, the domain-spec is optional. If it is not 825 provided, the from the check_host() arguments (see 826 Section 4.1) is used as the . "domain" and domain-spec 827 are syntactically identical after macro expansion. "domain" is an 828 input value for check_host() while domain-spec is computed by 829 check_host(). 831 The result of evaluating check_host() with a syntactically invalid 832 domain is undefined. 834 5. Mechanism Definitions 836 This section defines two types of mechanisms: basic language 837 framework mechanisms and designated sender mechanisms. 839 Basic mechanisms contribute to the language framework. They do not 840 specify a particular type of authorization scheme. 842 all 843 include 845 Designated sender mechanisms are used to identify a set of 846 addresses as being permitted or not permitted to use the for 847 sending mail. 849 a 850 mx 851 ptr (do not publish) 852 ip4 853 ip6 854 exists 856 The following conventions apply to all mechanisms that perform a 857 comparison between and an IP address at any point: 859 If no CIDR prefix length is given in the directive, then and the 860 IP address are compared for equality. (Here, CIDR is Classless 861 Inter-Domain Routing, described in [RFC4632].) 863 If a CIDR prefix length is specified, then only the specified number 864 of high-order bits of and the IP address are compared for 865 equality. 867 When any mechanism fetches host addresses to compare with , when 868 is an IPv4, "A" records are fetched; when is an IPv6 869 address, "AAAA" records are fetched. SPF implementations on IPv6 870 servers need to handle both "AAAA" and "A" records, for clients on 871 IPv4 mapped IPv6 addresses [RFC4291]. IPv4 addresses are only 872 listed in an SPF record using the "ip4" mechanism. 874 Several mechanisms rely on information fetched from the DNS. For 875 these DNS queries, except where noted, if the DNS server returns an 876 error (RCODE other than 0 or 3) or the query times out, the mechanism 877 stops and the topmost check_host() returns "temperror". If the 878 server returns "domain does not exist" (RCODE 3), then evaluation of 879 the mechanism continues as if the server returned no error (RCODE 0) 880 and zero answer records. 882 5.1. "all" 884 all = "all" 886 The "all" mechanism is a test that always matches. It is used as the 887 rightmost mechanism in a record to provide an explicit default. 889 For example: 891 v=spf1 a mx -all 893 Mechanisms after "all" will never be tested. Mechanisms listed after 894 "all" MUST be ignored. Any "redirect" modifier (Section 6.1) MUST be 895 ignored when there is an "all" mechanism in the record. 897 5.2. "include" 899 include = "include" ":" domain-spec 901 The "include" mechanism triggers a recursive evaluation of 902 check_host(). 904 1. The domain-spec is expanded as per Section 7. 906 2. check_host() is evaluated with the resulting string as the 907 . The and arguments remain the same as in 908 the current evaluation of check_host(). 910 3. The recursive evaluation returns either match, not match, or an 911 error. If it matches, then the appropriate result for the 912 include: mechanism is used (e.g. include or +include produces a 913 "pass" result and -include produces "fail"). 915 4. If there is no match, the parent check_host() resumes processing 916 as per the table below, with the previous value of 917 restored. 919 In hindsight, the name "include" was poorly chosen. Only the 920 evaluated result of the referenced SPF record is used, rather than 921 literally including the mechanisms of the referenced record in the 922 first. For example, evaluating a "-all" directive in the referenced 923 record does not terminate the overall processing and does not 924 necessarily result in an overall "fail". (Better names for this 925 mechanism would have been "if-match", "on-match", etc.) 927 The "include" mechanism makes it possible for one domain to designate 928 multiple administratively-independent domains. For example, a vanity 929 domain "example.net" might send mail using the servers of 930 administratively-independent domains example.com and example.org. 932 Example.net could say 934 IN TXT "v=spf1 include:example.com include:example.org -all" 936 This would direct check_host() to, in effect, check the records of 937 example.com and example.org for a "pass" result. Only if the host 938 were not permitted for either of those domains would the result be 939 "fail". 941 Whether this mechanism matches, does not match, or returns an 942 exception depends on the result of the recursive evaluation of 943 check_host(): 945 +---------------------------------+---------------------------------+ 946 | A recursive check_host() result | Causes the "include" mechanism | 947 | of: | to: | 948 +---------------------------------+---------------------------------+ 949 | pass | match | 950 | | | 951 | fail | not match | 952 | | | 953 | softfail | not match | 954 | | | 955 | neutral | not match | 956 | | | 957 | temperror | return temperror | 958 | | | 959 | permerror | return permerror | 960 | | | 961 | none | return permerror | 962 +---------------------------------+---------------------------------+ 964 The "include" mechanism is intended for crossing administrative 965 boundaries. For example, if example.com and example.org were managed 966 by the same entity, and if the permitted set of hosts for both 967 domains was "mx:example.com", it would be possible for example.org to 968 specify "include:example.com", but it would be preferable to specify 969 "redirect=example.com" or even "mx:example.com". 971 With the "include" mechanism an administratively external set of 972 hosts can be authorized, but determination of sender policy is still 973 a function of the original domain's SPF record (as determined by the 974 "all" mechanism in that record). The redirect modifier is more 975 suitable for consolidating both authorizations and policy into a 976 common set to be shared within an ADMD. Redirect is much more like a 977 common code element to be shared among records in a single ADMD. It 978 is possible to control both authorized hosts and policy for an 979 arbitrary number of domains from a single record. 981 5.3. "a" 983 This mechanism matches if is one of the 's IP 984 addresses. For clarity, this means the "a" mechanism also matches 985 AAAA records. 987 a = "a" [ ":" domain-spec ] [ dual-cidr-length ] 989 An address lookup is done on the using the type of 990 lookup (A or AAAA) appropriate for the connection type (IPv4 or 991 IPv6). The is compared to the returned address(es). If any 992 address matches, the mechanism matches. 994 5.4. "mx" 996 This mechanism matches if is one of the MX hosts for a domain 997 name. 999 mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ] 1001 check_host() first performs an MX lookup on the . Then 1002 it performs an address lookup on each MX name returned. The is 1003 compared to each returned IP address. To prevent Denial of Service 1004 (DoS) attacks, the processing limits defined in Section 4.6.4 MUST be 1005 followed. If the MX lookup limit is exceeded, then "permerror" is 1006 returned and the evaluation is terminated. If any address matches, 1007 the mechanism matches. 1009 Note regarding implicit MXes: If the has no MX record, 1010 check_host() MUST NOT apply the implicit MX rules of[RFC5321] by 1011 querying for an A or AAAA record for the same name. 1013 5.5. "ptr" (do not use) 1015 This mechanism tests whether the DNS reverse-mapping for exists 1016 and correctly points to a domain name within a particular domain. 1017 This mechanism SHOULD NOT be published. See below for discussion. 1019 ptr = "ptr" [ ":" domain-spec ] 1021 The 's name is looked up using this procedure: 1023 o Perform a DNS reverse-mapping for : Look up the corresponding 1024 PTR record in "in-addr.arpa." if the address is an IPv4 one and in 1025 "ip6.arpa." if it is an IPv6 address. 1027 o For each record returned, validate the domain name by looking up 1028 its IP addresses. To prevent DoS attacks, the PTR processing 1029 limits defined in Section 4.6.4 MUST be applied. If they are 1030 exceeded, processing is terminated and the mechanism does not 1031 match. 1033 o If is among the returned IP addresses, then that domain name 1034 is validated. 1036 Check all validated domain names to see if they either match the 1037 domain or are a subdomain of the domain. 1038 If any do, this mechanism matches. If no validated domain name can 1039 be found, or if none of the validated domain names match or are a 1040 subdomain of the , this mechanism fails to match. If a 1041 DNS error occurs while doing the PTR RR lookup, then this mechanism 1042 fails to match. If a DNS error occurs while doing an A RR lookup, 1043 then that domain name is skipped and the search continues. 1045 Pseudocode: 1047 sending-domain_names := ptr_lookup(sending-host_IP); 1048 if more than 10 sending-domain_names are found, use at most 10. 1049 for each name in (sending-domain_names) { 1050 IP_addresses := a_lookup(name); 1051 if the sending-domain_IP is one of the IP_addresses { 1052 validated-sending-domain_names += name; 1053 } 1054 } 1056 for each name in (validated-sending-domain_names) { 1057 if name ends in , return match. 1058 if name is , return match. 1059 } 1060 return no-match. 1062 This mechanism matches if the is either a subdomain of 1063 a validated domain name or if the and a validated 1064 domain name are the same. For example: "mail.example.com" is within 1065 the domain "example.com", but "mail.bad-example.com" is not. 1067 Note: This mechanism is slow, it is not as reliable as other 1068 mechanisms in cases of DNS errors, and it places a large burden on 1069 the .arpa name servers. If used, proper PTR records have to be in 1070 place for the domain's hosts and the "ptr" mechanism SHOULD be one of 1071 the last mechanisms checked. After many years of SPF deployment 1072 experience, it has been concluded it is unnecessary and more reliable 1073 alternatives should be used instead. It is, however, still in use as 1074 part of the SPF protocol, so compliant check_host() implementations 1075 MUST support it. 1077 5.6. "ip4" and "ip6" 1079 These mechanisms test whether is contained within a given IP 1080 network. 1082 ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] 1083 ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ] 1085 ip4-cidr-length = "/" 1*DIGIT 1086 ip6-cidr-length = "/" 1*DIGIT 1087 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] 1089 ip4-network = qnum "." qnum "." qnum "." qnum 1090 qnum = DIGIT ; 0-9 1091 / %x31-39 DIGIT ; 10-99 1092 / "1" 2DIGIT ; 100-199 1093 / "2" %x30-34 DIGIT ; 200-249 1094 / "25" %x30-35 ; 250-255 1095 ; as per conventional dotted quad notation. e.g., 192.0.2.0 1096 ip6-network = 1097 ; e.g., 2001:DB8::CD30 1099 The is compared to the given network. If CIDR prefix length 1100 high-order bits match, the mechanism matches. 1102 If ip4-cidr-length is omitted, it is taken to be "/32". If 1103 ip6-cidr-length is omitted, it is taken to be "/128". It is not 1104 permitted to omit parts of the IP address instead of using CIDR 1105 notations. That is, use 192.0.2.0/24 instead of 192.0.2. 1107 5.7. "exists" 1109 This mechanism is used to construct an arbitrary domain name that is 1110 used for a DNS A record query. It allows for complicated schemes 1111 involving arbitrary parts of the mail envelope to determine what is 1112 permitted. 1114 exists = "exists" ":" domain-spec 1116 The domain-spec is expanded as per Section 7. The resulting domain 1117 name is used for a DNS A RR lookup (even when the connection type is 1118 IPv6). If any A record is returned, this mechanism matches. 1120 Domains can use this mechanism to specify arbitrarily complex 1121 queries. For example, suppose example.com publishes the record: 1123 v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all 1125 The might expand to 1126 "1.2.0.192.someuser._spf.example.com". This makes fine-grained 1127 decisions possible at the level of the user and client IP address. 1129 6. Modifier Definitions 1131 Modifiers are name/value pairs that provide additional information. 1132 Modifiers always have an "=" separating the name and the value. 1134 The modifiers defined in this document ("redirect" and "exp") MAY 1135 appear anywhere in the record, but SHOULD appear at the end, after 1136 all mechanisms. Ordering of these two modifiers does not matter. 1137 These two modifiers MUST NOT appear in a record more than once each. 1138 If they do, then check_host() exits with a result of "permerror". 1140 Unrecognized modifiers MUST be ignored no matter where in a record, 1141 or how often. This allows implementations of this document to 1142 gracefully handle records with modifiers that are defined in other 1143 specifications. 1145 6.1. redirect: Redirected Query 1147 The redirect modifier is intended for consolidating both 1148 authorizations and policy into a common set to be shared within a 1149 single ADMD. It is possible to control both authorized hosts and 1150 policy for an arbitrary number of domains from a single record. 1152 redirect = "redirect" "=" domain-spec 1154 If all mechanisms fail to match, and a "redirect" modifier is 1155 present, then processing proceeds as follows: 1157 The domain-spec portion of the redirect section is expanded as per 1158 the macro rules in Section 7. Then check_host() is evaluated with 1159 the resulting string as the . The and 1160 arguments remain the same as in the current evaluation of 1161 check_host(). 1163 The result of this new evaluation of check_host() is then considered 1164 the result of the current evaluation with the exception that if no 1165 SPF record is found, or if the is malformed, the result 1166 is a "permerror" rather than "none". 1168 Note that the newly-queried domain can itself specify redirect 1169 processing. 1171 This facility is intended for use by organizations that wish to apply 1172 the same record to multiple domains. For example: 1174 la.example.com. TXT "v=spf1 redirect=_spf.example.com" 1175 ny.example.com. TXT "v=spf1 redirect=_spf.example.com" 1176 sf.example.com. TXT "v=spf1 redirect=_spf.example.com" 1178 _spf.example.com. TXT "v=spf1 mx:example.com -all" 1180 In this example, mail from any of the three domains is described by 1181 the same record. This can be an administrative advantage. 1183 Note: In general, the domain "A" cannot reliably use a redirect to 1184 another domain "B" not under the same administrative control. Since 1185 the stays the same, there is no guarantee that the record at 1186 domain "B" will correctly work for mailboxes in domain "A", 1187 especially if domain "B" uses mechanisms involving local-parts. An 1188 "include" directive will generally be more appropriate. 1190 For clarity, any "redirect" modifier SHOULD appear as the very last 1191 term in a record. 1193 6.2. exp: Explanation 1195 explanation = "exp" "=" domain-spec 1197 If check_host() results in a "fail" due to a mechanism match (such as 1198 "-all"), and the "exp" modifier is present, then the explanation 1199 string returned is computed as described below. If no "exp" modifier 1200 is present, then either a default explanation string or an empty 1201 explanation string MUST be returned to the calling application. 1203 The domain-spec is macro expanded (see Section 7) and becomes the 1204 . The DNS TXT RRset for the is fetched. 1206 If there are any DNS processing errors (any RCODE other than 0), or 1207 if no records are returned, or if more than one record is returned, 1208 or if there are syntax errors in the explanation string, then proceed 1209 as if no "exp" modifier was given. 1211 The fetched TXT record's strings are concatenated with no spaces, and 1212 then treated as an explain-string, which is macro-expanded. This 1213 final result is the explanation string. Implementations MAY limit 1214 the length of the resulting explanation string to allow for other 1215 protocol constraints and/or reasonable processing limits. Since the 1216 explanation string is intended for an SMTP response and [RFC5321] 1217 Section 2.4 says that responses are in [US-ASCII], the explanation 1218 string MUST be limited to [US-ASCII]. 1220 Software evaluating check_host() can use this string to communicate 1221 information from the publishing domain in the form of a short message 1222 or URL. Software SHOULD make it clear that the explanation string 1223 comes from a third party. For example, it can prepend the macro 1224 string "%{o} explains: " to the explanation, such as shown in 1225 Section 8.4. 1227 Suppose example.com has this record: 1229 v=spf1 mx -all exp=explain._spf.%{d} 1231 Here are some examples of possible explanation TXT records at 1232 explain._spf.example.com: 1234 "Mail from example.com should only be sent by its own servers." 1235 -- a simple, constant message 1237 "%{i} is not one of %{d}'s designated mail servers." 1238 -- a message with a little more information, including the IP 1239 address that failed the check 1241 "See http://%{d}/why.html?s=%{S}&i=%{I}" 1242 -- a complicated example that constructs a URL with the 1243 arguments to check_host() so that a web page can be 1244 generated with detailed, custom instructions 1246 Note: During recursion into an "include" mechanism, an "exp" modifier 1247 from the MUST NOT be used. In contrast, when executing 1248 a "redirect" modifier, an "exp" modifier from the original domain 1249 MUST NOT be used. This is because "include" is meant to cross 1250 administrative boundaries and the explanation provided should be the 1251 one from the receiving ADMD, while "redirect" is meant to operate as 1252 a tool to consolidate policy records within an ADMD an so the 1253 redirected explanation is the one that ought to have priority. 1255 7. Macros 1257 When evaluating an SPF policy record, certain character sequences are 1258 intended to be replaced by parameters of the message or of the 1259 connection. These character sequences are referred to as "macros". 1261 7.1. Formal Specification 1263 The ABNF description for a macro is as follows: 1265 domain-spec = macro-string domain-end 1266 domain-end = ( "." toplabel [ "." ] ) / macro-expand 1268 toplabel = ( *alphanum ALPHA *alphanum ) / 1269 ( 1*alphanum "-" *( alphanum / "-" ) alphanum ) 1270 alphanum = ALPHA / DIGIT 1272 explain-string = *( macro-string / SP ) 1274 macro-string = *( macro-expand / macro-literal ) 1275 macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) 1276 / "%%" / "%_" / "%-" 1277 macro-literal = %x21-24 / %x26-7E 1278 ; visible characters except "%" 1279 macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / 1280 "c" / "r" / "t" / "v" 1281 transformers = *DIGIT [ "r" ] 1282 delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" 1284 The "toplabel" construction is subject to the LDH rule plus 1285 additional top-level domain (TLD) restrictions. See Section 2 of 1286 [RFC3696] for background. 1288 Some special cases: 1290 o A literal "%" is expressed by "%%". 1292 o "%_" expands to a single " " space. 1294 o "%-" expands to a URL-encoded space, viz., "%20". 1296 7.2. Macro Definitions 1298 The following macro letters are expanded in term arguments: 1300 s = 1301 l = local-part of 1302 o = domain of 1303 d = 1304 i = 1305 p = the validated domain name of (do not use) 1306 v = the string "in-addr" if is ipv4, or "ip6" if is ipv6 1307 h = HELO/EHLO domain 1309 , , and are defined in Section 2.2. 1311 The following macro letters are allowed only in "exp" text: 1313 c = SMTP client IP (easily readable format) 1314 r = domain name of host performing the check 1315 t = current timestamp 1317 7.3. Macro Processing Details 1319 A '%' character not followed by a '{', '%', '-', or '_' character is 1320 a syntax error. So: 1322 -exists:%(ir).sbl.example.org 1324 is incorrect and will cause check_host() to yield a "permerror". 1325 Instead, the following is legal: 1327 -exists:%{ir}.sbl.example.org 1329 Optional transformers are the following: 1331 *DIGIT = zero or more digits 1332 r = reverse value, splitting on dots by default 1334 If transformers or delimiters are provided, the replacement value for 1335 a macro letter is split into parts separated by one or more of the 1336 specified delimiter characters. After performing any reversal 1337 operation and/or removal of left-hand parts, the parts are rejoined 1338 using "." and not the original splitting characters. 1340 By default, strings are split on "." (dots). Note that no special 1341 treatment is given to leading, trailing, or consecutive delimiters in 1342 input strings, and so the list of parts might contain empty strings. 1343 Some older implementations of SPF prohibit trailing dots in domain 1344 names, so trailing dots SHOULD NOT be published, although they MUST 1345 be accepted by implementations conforming to this document. Macros 1346 can specify delimiter characters that are used instead of ".". 1348 The "r" transformer indicates a reversal operation: if the client IP 1349 address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1" 1350 and the macro %{ir} would expand to "1.2.0.192". 1352 The DIGIT transformer indicates the number of right-hand parts to 1353 use, after optional reversal. If a DIGIT is specified, the value 1354 MUST be nonzero. If no DIGITs are specified, or if the value 1355 specifies more parts than are available, all the available parts are 1356 used. If the DIGIT was 5, and only 3 parts were available, the macro 1357 interpreter would pretend the DIGIT was 3. Implementations MUST 1358 support at least a value of 127, as that is the maximum number of 1359 labels in a domain name (less the zero-length label at the end). 1361 The "s" macro expands to the argument. It is an email 1362 address with a local-part, an "@" character, and a domain. The "l" 1363 macro expands to just the local-part. The "o" macro expands to just 1364 the domain part. Note that these values remain the same during 1365 recursive and chained evaluations due to "include" and/or "redirect". 1366 Note also that if the original had no local-part, the local- 1367 part was set to "postmaster" in initial processing (see Section 4.3). 1369 For IPv4 addresses, both the "i" and "c" macros expand to the 1370 standard dotted-quad format. 1372 For IPv6 addresses, the "i" macro expands to a dot-format address; it 1373 is intended for use in %{ir}. The "c" macro can expand to any of the 1374 hexadecimal colon-format addresses specified in [RFC4291], Section 1375 2.2. It is intended for humans to read. 1377 The "p" macro expands to the validated domain name of . The 1378 procedure for finding the validated domain name is defined in 1379 Section 5.5. If the is present in the list of validated 1380 domains, it SHOULD be used. Otherwise, if a subdomain of the 1381 is present, it SHOULD be used. Otherwise, any name from the 1382 list can be used. If there are no validated domain names or if a DNS 1383 error occurs, the string "unknown" is used. 1385 This macro SHOULD NOT be published (see Section 5.5 for the 1386 discussion). 1388 The "h" macro expands to the parameter that was provided to the SMTP 1389 server via the HELO or EHLO SMTP verb. For sessions where that verb 1390 was provided more than once, the most recent instance is used. 1392 The "r" macro expands to the name of the receiving MTA. This SHOULD 1393 be a fully qualified domain name, but if one does not exist (as when 1394 the checking is done by a MUA) or if policy restrictions dictate 1395 otherwise, the word "unknown" SHOULD be substituted. The domain name 1396 can be different from the name found in the MX record that the client 1397 MTA used to locate the receiving MTA. 1399 The "t" macro expands to the decimal representation of the 1400 approximate number of seconds since the Epoch (Midnight, January 1, 1401 1970, UTC) at the time of the evaluation. This is the same value as 1402 is returned by the POSIX time() function in most standards-compliant 1403 libraries. 1405 When the result of macro expansion is used in a domain name query, if 1406 the expanded domain name exceeds 253 characters (the maximum length 1407 of a domain name in this format), the left side is truncated to fit, 1408 by removing successive domain labels (and their following dots) until 1409 the total length does not exceed 253 characters. 1411 Uppercased macros expand exactly as their lowercased equivalents, and 1412 are then URL escaped. URL escaping MUST be performed for characters 1413 not in the "unreserved" set, which is defined in [RFC3986]. 1415 Care has to be taken by the sending ADMD so that macro expansion for 1416 legitimate email does not exceed the 63-character limit on DNS 1417 labels. The local-part of email addresses, in particular, can have 1418 more than 63 characters between dots. 1420 To minimize DNS lookup resource requirements, it is better if sending 1421 ADMDs avoid using the "s", "l", "o", or "h" macros in conjunction 1422 with any mechanism directive. Although these macros are powerful and 1423 allow per-user records to be published, they severely limit the 1424 ability of implementations to cache results of check_host() and they 1425 reduce the effectiveness of DNS caches. 1427 If no directive processed during the evaluation of check_host() 1428 contains an "s", "l", "o", or "h" macro, then the results of the 1429 evaluation can be cached on the basis of and alone for 1430 as long as the DNS record involved with the shortest TTL has not 1431 expired. 1433 7.4. Expansion Examples 1435 The is strong-bad@email.example.com. 1436 The IPv4 SMTP client IP is 192.0.2.3. 1437 The IPv6 SMTP client IP is 2001:DB8::CB01. 1438 The PTR domain name of the client IP is mx.example.org. 1440 macro expansion 1441 ------- ---------------------------- 1442 %{s} strong-bad@email.example.com 1443 %{o} email.example.com 1444 %{d} email.example.com 1445 %{d4} email.example.com 1446 %{d3} email.example.com 1447 %{d2} example.com 1448 %{d1} com 1449 %{dr} com.example.email 1450 %{d2r} example.email 1451 %{l} strong-bad 1452 %{l-} strong.bad 1453 %{lr} strong-bad 1454 %{lr-} bad.strong 1455 %{l1r-} strong 1457 macro-string expansion 1458 -------------------------------------------------------------------- 1459 %{ir}.%{v}._spf.%{d2} 3.2.0.192.in-addr._spf.example.com 1460 %{lr-}.lp._spf.%{d2} bad.strong.lp._spf.example.com 1462 %{lr-}.lp.%{ir}.%{v}._spf.%{d2} 1463 bad.strong.lp.3.2.0.192.in-addr._spf.example.com 1465 %{ir}.%{v}.%{l1r-}.lp._spf.%{d2} 1466 3.2.0.192.in-addr.strong.lp._spf.example.com 1468 %{d2}.trusted-domains.example.net 1469 example.com.trusted-domains.example.net 1471 IPv6: 1472 %{ir}.%{v}._spf.%{d2} 1.0.B.C.0.0.0.0. 1473 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 1475 8. Result Handling 1477 This section provides guidance for operators in response to the 1478 various possible outputs of check_host() on a message. Definitions 1479 of SPF results are presented in Section 2.6; this section provides 1480 more detail on each for use in developing local policy for message 1481 handling. 1483 Every operating environment is different. There are some receivers 1484 for whom strict adherence to SPF is appropriate, and definitive 1485 treatment of messages that are evaluated to be explicitly 1486 unauthorized ("fail" and sometimes "softfail") is the norm. There 1487 are others for which the "false negative" cases are more of a 1488 concern. This concern is typically handled by merely recording the 1489 result in the header and allowing the message to pass on for 1490 additional processing. There are still others where SPF is one of 1491 several inputs to the message handling decision. As such, there is 1492 no comprehensive normative requirement for message handling in 1493 response to any particular result. This section is provided to 1494 present a complete picture of the likely cause of each result and, 1495 where available, the experience gained during experimental 1496 deployment. 1498 There are essentially two classes of handling choices: 1500 o Handling within the SMTP session that attempted to deliver the 1501 message, such as by returning a permanent SMTP error (rejection) 1502 or temporary SMTP error ("try again later"); 1504 o Permitting the message to pass (a successful SMTP reply code) and 1505 adding an additional header field that indicates the result 1506 returned by check_host() and other salient details; this is 1507 discussed in more detail in Section 9. 1509 8.1. None 1511 With a "none" result, the SPF verifier has no information at all 1512 about the authorization or lack thereof of the client to use the 1513 checked identity or identities. The check_host() function completed 1514 without errors but was not able to reach any conclusion. 1516 8.2. Neutral 1518 A "neutral" result indicates that although a policy for the identity 1519 was discovered, there is no definite assertion (positive or negative) 1520 about the client. 1522 A "neutral" result MUST be treated exactly like the "none" result; 1523 the distinction exists only for informational purposes. Treating 1524 "neutral" more harshly than "none" would discourage ADMDs from 1525 testing the use of SPF records (see Section 10.1). 1527 8.3. Pass 1529 A "pass" result means that the client is authorized to inject mail 1530 with the given identity. The domain can now, in the sense of 1531 reputation, be considered responsible for sending the message. 1532 Further policy checks can now proceed with confidence in the 1533 legitimate use of the identity. This is further discussed in 1534 Appendix H.1. 1536 8.4. Fail 1538 A "fail" result is an explicit statement that the client is not 1539 authorized to use the domain in the given identity. Disposition of 1540 SPF fail messages is a matter of local policy. See Appendix H.2 for 1541 considerations on developing local policy. 1543 If the checking software chooses to reject the mail during the SMTP 1544 transaction, then it SHOULD use an SMTP reply code of 550 (see 1545 [RFC5321]) and, if supported, the 5.7.1 enhanced status code (see 1546 [RFC3463], Section 3.8), in addition to an appropriate reply text. 1547 The check_host() function will return either a default explanation 1548 string or one from the domain that published the SPF records (see 1549 Section 6.2). If the information does not originate with the 1550 checking software, it is good to make it clear that the text is 1551 provided by the sender's domain. For example: 1553 550-5.7.1 SPF MAIL FROM check failed: 1554 550-5.7.1 The domain example.com explains: 1555 550 5.7.1 Please see http://www.example.com/mailpolicy.html 1557 If the checking software chooses not to reject the mail during the 1558 SMTP transaction, then it SHOULD add a Received-SPF or 1559 Authentication-Results header field (see Section 9) to communicate 1560 this result to downstream message processors. While this is true for 1561 all SPF results, it is of particular importance for "fail" results 1562 since the message is explicitly not authorized by the ADMD. 1564 8.5. Softfail 1566 A "softfail" result ought to be treated as somewhere between "fail" 1567 and "neutral"/"none". The ADMD believes the host is not authorized 1568 but is not willing to make a strong policy statement. Receiving 1569 software SHOULD NOT reject the message based solely on this result, 1570 but MAY subject the message to closer scrutiny than normal. 1572 The ADMD wants to discourage the use of this host and thus desires 1573 limited feedback when a "softfail" result occurs. For example, the 1574 recipient's Mail User Agent (MUA) could highlight the "softfail" 1575 status, or the receiving MTA could give the sender a message using 1576 greylisting, [RFC6647], with a note the first time the message is 1577 received, but accept it on a later attempt based on receiver policy. 1579 8.6. Temperror 1581 A "temperror" result means the SPF verifier encountered a transient 1582 (generally DNS) error while performing the check. Checking software 1583 can choose to accept or temporarily reject the message. If the 1584 message is rejected during the SMTP transaction for this reason, the 1585 software SHOULD use an SMTP reply code of 451 and, if supported, the 1586 4.4.3 enhanced status code (see [RFC3463], Section 3.5). These 1587 errors can be caused by problems in either the sender's or receiver's 1588 DNS software. See Appendix H.4 for considerations on developing 1589 local policy. 1591 8.7. Permerror 1593 A "permerror" result means the domain's published records could not 1594 be correctly interpreted. This signals an error condition that 1595 definitely requires operator intervention to be resolved. If the 1596 message is rejected during the SMTP transaction for this reason, the 1597 software SHOULD use an SMTP reply code of 550 and, if supported, the 1598 5.5.2 enhanced status code (see [RFC3463], Section 3.6). Be aware 1599 that if the ADMD uses macros (Section 7), it is possible that this 1600 result is due to the checked identities having an unexpected format. 1601 It is also possible that this result is generated by certain SPF 1602 verifiers due to the input arguments having an unexpected format; see 1603 Section 4.8. See Appendix H.3 for considerations on developing local 1604 policy. 1606 9. Recording the Result 1608 To provide downstream agents, such as MUAs, with the information they 1609 might need in terms of evaluating or representing the apparent safety 1610 of the message content, it is RECOMMENDED that SMTP receivers record 1611 the result of SPF processing in the message header. For operators 1612 that choose to record SPF results in the header of the message for 1613 processing by internal filters or MUAs, two methods are presented. 1614 Section 9.1 defines the Received-SPF field, which is the results 1615 field originally defined for SPF use. Section 9.2 discusses 1616 Authentication-Results [RFC5451] which was specified more recently 1617 and is designed for use by SPF and other authentication methods. 1619 Both are in common use, and hence both are included here. However, 1620 it is important to note that they were designed to serve slightly 1621 different purposes. Received-SPF is intended to include enough 1622 forensic information to enable reconstruction of the SPF evaluation 1623 of the message, while Authentication-Results is designed only to 1624 relay the result itself and related output details of likely use to 1625 end users (e.g., what property of the message was actually 1626 authenticated and what it contained), leaving forensic work to the 1627 purview of system logs and the Received field contents. Also, 1628 Received-SPF relies on compliance of agents within the receiving ADMD 1629 to adhere to the header field ordering rules of [RFC5321] and 1630 [RFC5322], while Authentication-Results includes some provisions to 1631 protect against non-compliant implementations. 1633 An operator could choose to use both to serve different downstream 1634 agents. In such cases, care needs to be taken to ensure both fields 1635 are conveying the same details, or unexpected results can occur. 1637 9.1. The Received-SPF Header Field 1639 The Received-SPF header field is a trace field (see [RFC5322] Section 1640 3.6.7) and SHOULD be prepended to the existing header, above the 1641 Received: field that is generated by the SMTP receiver. It MUST 1642 appear above all other Received-SPF fields in the message. The 1643 header field has the following format: 1645 header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] 1646 [ key-value-list ] CRLF 1648 result = "pass" / "fail" / "softfail" / "neutral" / 1649 "none" / "temperror" / "permerror" 1651 key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) 1652 [";"] 1654 key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) 1656 key = "client-ip" / "envelope-from" / "helo" / 1657 "problem" / "receiver" / "identity" / 1658 "mechanism" / name 1660 identity = "mailfrom" ; for the "MAIL FROM" identity 1661 / "helo" ; for the "HELO" identity 1662 / name ; other identities 1664 dot-atom = 1665 quoted-string = 1666 comment = 1667 CFWS = 1668 FWS = 1669 CRLF = 1671 The header field SHOULD include a "(...)" style comment after the 1672 result, conveying supporting information for the result, such as 1673 , , and . 1675 The following key-value pairs are designed for later machine parsing. 1676 SPF verifiers SHOULD give enough information so that the SPF results 1677 can be verified. That is, at least "client-ip", "helo", and, if the 1678 "MAIL FROM" identity was checked, "envelope-from". 1680 client-ip the IP address of the SMTP client 1682 envelope-from the envelope sender mailbox 1684 helo the host name given in the HELO or EHLO command 1686 mechanism the mechanism that matched (if no mechanisms matched, 1687 substitute the word "default") 1689 problem if an error was returned, details about the error 1690 receiver the host name of the SPF verifier 1692 identity the identity that was checked; see the ABNF 1693 rule 1695 Other keys MAY be defined by SPF verifiers. 1697 SPF verifiers MUST make sure that the Received-SPF header field does 1698 not contain invalid characters, is not excessively long (See 1699 [RFC5322] Section 2.1.1), and does not contain malicious data that 1700 has been provided by the sender. 1702 Examples of various header field styles that could be generated are 1703 the following: 1705 Received-SPF: pass (mybox.example.org: domain of 1706 myname@example.com designates 192.0.2.1 as permitted sender) 1707 receiver=mybox.example.org; client-ip=192.0.2.1; 1708 envelope-from="myname@example.com"; helo=foo.example.com; 1710 Received-SPF: fail (mybox.example.org: domain of 1711 myname@example.com does not designate 1712 192.0.2.1 as permitted sender) 1713 identity=mailfrom; client-ip=192.0.2.1; 1714 envelope-from="myname@example.com"; 1716 Received-SPF: pass (mybox.example.org: domain of 1717 myname@example.com designates 192.0.2.1 as permitted sender) 1718 receiver=mybox.example.org; client-ip=192.0.2.1; 1719 mechanism=ip4:192.0.2.1; envelope-from="myname@example.com"; 1720 helo=foo.example.com; 1722 9.2. SPF Results in the Authentication-Results Header Field 1724 As mentioned in Section 9, the Authentication-Results header field is 1725 designed to communicate lists of tests a border MTA did and their 1726 results. The specified elements of the field provide less 1727 information than the Received-SPF field: 1729 Authentication-Results: myhost.example.org; spf=pass 1730 smtp.mailfrom=example.net 1732 Received-SPF: pass (myhost.example.org: domain of 1733 myname@example.com designates 192.0.2.1 as permitted sender) 1734 receiver=mybox.example.org; client-ip=192.0.2.1; 1735 envelope-from="myname@example.com"; helo=foo.example.com; 1737 It is, however, possible to add CFWS in the "reason" part of an 1738 Authentication-Results header field and provide the equivalent 1739 information, if desired. 1741 As an example, an expanded Authentication-Results header field might 1742 look like (for a "MAIL FROM" check in this example): 1744 Authentication-Results: myhost.example.org; spf=pass 1745 reason="client-ip=192.0.2.1; smtp.helo=foo.example.com" 1746 smtp.mailfrom=user@example.net 1748 10. Effects on Infrastructure 1750 This section outlines the major implications that adoption of this 1751 protocol will have on various entities involved in Internet email. 1752 It is intended to make clear to the reader where this protocol 1753 knowingly affects the operation of such entities. This section is 1754 not a "how-to" manual, or a "best practices" document, and it is not 1755 a comprehensive list of what such entities ought do in light of this 1756 specification. 1758 This section provides operational advice and instruction only. It is 1759 non-normative. 1761 [RFC5598] describes the Internet email architecture. This section is 1762 organized based on the different segments of the architecture. 1764 10.1. Sending Domains 1766 Originating ADMDs (ADministrative Management Domains - [RFC5598] 1767 Section 2.2.1 and Section 2.3) that wish to be compliant with this 1768 specification will need to determine the list of relays ([RFC5598] 1769 Section 2.2.2) that they allow to use their domain name in the "HELO" 1770 and "MAIL FROM" identities when relaying to other ADMDs. It is 1771 recognized that forming such a list is not just a simple technical 1772 exercise, but involves policy decisions with both technical and 1773 administrative considerations. 1775 10.1.1. DNS Resource Considerations 1777 Minimizing the DNS resources needed for SPF lookups can be done by 1778 choosing directives that require less DNS information and by placing 1779 lower-cost mechanisms earlier in the SPF record. 1781 Section 4.6.4 specifies the limits receivers have to use. It is 1782 essential to publish records that do not exceed these requirements. 1783 It is also required to carefully weigh the cost and the 1784 maintainability of licit solutions. 1786 For example, consider a domain set up as follows: 1788 example.com. IN MX 10 mx.example.com. 1789 IN MX 20 mx2.example.com. 1790 mx.example.com. IN A 192.0.2.1 1791 mx2.example.com. IN A 192.0.2.129 1793 Assume the administrative point is to authorize (pass) mx and mx2 1794 while failing every other host. Compare the following solutions: 1796 Best record: 1797 example.com. IN TXT "v=spf1 ip4:192.0.2.1 ip4:192.0.2.129 -all" 1799 Good record: 1800 $ORIGIN example.com. 1801 @ IN TXT "v=spf1 a:authorized-spf.example.com -all" 1802 authorized-spf IN A 192.0.2.1 1803 IN A 192.0.2.129 1805 Expensive record: 1806 example.com. IN TXT "v=spf1 mx:example.com -all" 1808 Wasteful, bad record: 1809 example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 mx -all" 1811 10.1.2. Administrator's Considerations 1813 There might be administrative considerations: using "a" over "ip4" or 1814 "ip6" allows hosts to be renumbered easily at the cost of a DNS query 1815 per receiver. Using "mx" over "a" allows the set of mail hosts to be 1816 changed easily. Unless such changes are common, it is better to use 1817 the less resource intensive mechanisms like "ip4" and "ip6" over "a" 1818 or "a" over "mx". 1820 In some specific cases, standard advice on record content is 1821 appropriate. Publishing SPF records for domains that send no mail is 1822 a well established best practice. The record for a domain that sends 1823 no mail is: 1825 www.example.com. IN TXT "v=spf1 -all" 1827 Publishing SPF records for individual hosts is also best practice. 1828 The hostname is generally the identity used in the 5321.HELO/.EHLO 1829 command. In the case of messages with a null 5321.MailFrom, this is 1830 used as the domain for 5321.MailFrom SPF checks, in addition to being 1831 used in 5321.HELO/.EHLO based SPF checks. The standard SPF record 1832 for an individual host that is involved in mail processing is: 1834 relay.example.com. IN TXT "v=spf1 a -all" 1836 Validating correct deployment is difficult. [RFC6652] describes one 1837 mechanism for soliciting feedback on SPF failures. Another 1838 suggestion can be found in Appendix D. 1840 Regardless of the method used, understanding the ADMD's outbound mail 1841 architecture is essential to effective deployment. 1843 10.1.3. Bounces 1845 As explained in Section 1.1.3, [RFC5321] allows the MAIL FROM to be 1846 null, which is typical of some Delivery Status Notification 1847 [RFC3464], commonly called email bounces. In this case the only 1848 entity available for performing an SPF check is the "HELO" identity 1849 defined in Section 1.1.4. SPF functionality is enhanced by 1850 administrators ensuring this identity is set correctly and has an 1851 appropriate SPF record. It is normal to have the HELO identity set 1852 to the hostname instead of the domain. Zone file generation for 1853 significant numbers of hosts can be consolidated using the redirect 1854 modifier and scripted for initial deployment. Specific deployment 1855 advice is given above in Section 10.1.2. 1857 10.2. Receivers 1859 SPF results can be used in combination with other methods to 1860 determine the final local disposition (either positive or negative) 1861 of a message. It can also be considered dispositive on its own. 1863 An attempt to have one organization (sender) direct the email 1864 handling policies of another (receiver) is inherently challenging and 1865 often controversial. As stated elsewhere in this document, there is 1866 no comprehensive normative requirement for specific handling of a 1867 message based on SPF results. The information presented in Section 8 1868 and in Appendix H is offered for receiver consideration when forming 1869 local handling policies. 1871 The primary considerations are that SPF might return "pass" for mail 1872 that is ultimately harmful (e.g., spammers that arrange for SPF to 1873 pass using disposable domain names, or virus or spam outbreaks from 1874 within trusted sources), and might also return "fail" for mail that 1875 is ultimately legitimate (e.g., legitimate mail that has traversed a 1876 mail alias). It is important take both of these cases under 1877 consideration when establishing local handling policy. 1879 10.3. Mediators 1881 Mediators are a type of User actor [RFC5598]. That is, a mediator 1882 takes 'delivery' of a message and posts a 'submission' of a new 1883 message. The mediator can make the newly-posted message be as 1884 similar or as different from the original message as they wish. 1885 Examples include mailing lists (see [RFC5598] Section 5.3) and 1886 ReSenders ([RFC5598] Section 5.2). This is discussed in [RFC5321], 1887 Section 3.9. For the operation of SPF, the essential concern is the 1888 email address in the 5321.MailFrom command for the new message. 1890 Because SPF evaluation is based on the IP address of the "last" 1891 sending SMTP server, the address of the mediator will be used, rather 1892 than the address of the SMTP server that sent the message to the 1893 mediator. Some mediators retain the email address from the original 1894 message, while some use a new address. 1896 If the address is the same as for the original message, and the 1897 original message had an associated SPF record, then the SPF 1898 evaluation will fail unless mitigations such as those described in 1899 Appendix E are used. 1901 11. Security Considerations 1903 11.1. Processing Limits 1905 As with most aspects of email, there are a number of ways that 1906 malicious parties could use the protocol as an avenue for a 1907 Denial-of-Service (DoS) attack. The processing limits outlined in 1908 Section 4.6.4 are designed to prevent attacks such as the following: 1910 o A malicious party could create an SPF record with many references 1911 to a victim's domain and send many emails to different SPF 1912 verifiers; those SPF verifiers would then create a DoS attack. In 1913 effect, the SPF verifiers are being used to amplify the attacker's 1914 bandwidth by using fewer octets in the SMTP session than are used 1915 by the DNS queries. Using SPF verifiers also allows the attacker 1916 to hide the true source of the attack. This potential attack is 1917 based on large volumes of mail being transmitted. 1919 o Whereas implementations of check_host() are supposed to limit the 1920 number of DNS lookups, malicious domains could publish records 1921 that exceed these limits in an attempt to waste computation effort 1922 at their targets when they send them mail. Malicious domains 1923 could also design SPF records that cause particular 1924 implementations to use excessive memory or CPU usage, or to 1925 trigger bugs. If a receiver is configured to accept mail with an 1926 SPF result of "temperror", such an attack might result in mail 1927 that would otherwise have been rejected due to an SPF "fail" 1928 result being accepted. This potential attack is based on 1929 specially crafted SPF records being used to exhaust DNS resources 1930 of the victim. 1932 o Malicious parties could send a large volume of mail purporting to 1933 come from the intended target to a wide variety of legitimate mail 1934 hosts. These legitimate machines would then present a DNS load on 1935 the target as they fetched the relevant records. 1937 o Malicious parties could, in theory, use SPF records as a vehicle 1938 for DNS lookup amplification for a denial-of-service-attack. In 1939 this scenario, the attacker publishes an SPF record in its own DNS 1940 that uses "a" and "mx" mechanisms directed toward the intended 1941 victim, e.g. "a:example.com a:foo.example.com a:bar.example.com 1942 ..." and then distributes mail with a MAIL FROM value including 1943 its own domain in large volume to a wide variety of destinations. 1944 Any such destination operating an SPF verifier will begin querying 1945 all of the names associated with the "a" mechanisms in that 1946 record. The names used in the record needn't exist for the attack 1947 to be effective. Operational experience since publication of 1948 [RFC4408] suggests that mitigation of this class of attack can be 1949 accomplished with minimal impact on the deployed base by having 1950 the verifier abort processing and return "permerror" 1951 (Section 2.6.7) once more than two "void lookups" have been 1952 encountered (defined in Section 4.6.4). 1954 Of these, the case of a third party referenced in the SPF record is 1955 the easiest for a DoS attack to effectively exploit. As a result, 1956 limits that might seem reasonable for an individual mail server can 1957 still allow an unreasonable amount of bandwidth amplification. 1958 Therefore, the processing limits need to be quite low. 1960 11.2. SPF-Authorized Email May Contain Other False Identities 1962 Do not construe the "MAIL FROM" and "HELO" identity authorizations to 1963 provide more assurance than they do. It is entirely possible for a 1964 malicious sender to inject a message using his own domain in the 1965 identities used by SPF, to have that domain's SPF record authorize 1966 the sending host, and yet the message can easily list other 1967 identities in its header. Unless the user or the MUA takes care to 1968 note that the authorized identity does not match the other more 1969 commonly-presented identities (such as the From: header field), the 1970 user might be lulled into a false sense of security. 1972 11.3. Spoofed DNS and IP Data 1974 There are two aspects of this protocol that malicious parties could 1975 exploit to undermine the validity of the check_host() function: 1977 o The evaluation of check_host() relies heavily on DNS. A malicious 1978 attacker could attack the DNS infrastructure and cause 1979 check_host() to see spoofed DNS data, and then return incorrect 1980 results. This could include returning "pass" for an value 1981 where the actual domain's record would evaluate to "fail". See 1982 [RFC3833] for a description of DNS weaknesses. 1984 o The client IP address, , is assumed to be correct. In a 1985 modern, correctly configured system the risk of this not being 1986 true is nil. 1988 11.4. Cross-User Forgery 1990 By definition, SPF policies just map domain names to sets of 1991 authorized MTAs, not whole email addresses to sets of authorized 1992 users. Although the "l" macro (Section 7) provides a limited way to 1993 define individual sets of authorized MTAs for specific email 1994 addresses, it is generally impossible to verify, through SPF, the use 1995 of specific email addresses by individual users of the same MTA. 1997 It is up to mail services and their MTAs to directly prevent 1998 cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be 1999 restricted to using only those email addresses that are actually 2000 under their control (see [RFC6409], Section 6.1). Another means to 2001 verify the identity of individual users is message cryptography such 2002 as PGP ([RFC4880]) or S/MIME ([RFC5751]). 2004 11.5. Untrusted Information Sources 2006 An SPF compliant receiver gathers information from the SMTP commands 2007 it receives and from the published DNS records of the sending domain 2008 holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the 2009 envelope, and SPF DNS records published by the domain holder). These 2010 parameters are not validated in the SMTP process. 2012 All of these pieces of information are generated by actors outside of 2013 the authority of the receiver, and thus are not guaranteed to be 2014 accurate or legitimate. 2016 11.5.1. Recorded Results 2018 This information, passed to the receiver in the Received-SPF: or 2019 Authentication-Results: trace fields, can be returned to the client 2020 MTA as an SMTP rejection message. If such an SMTP rejection message 2021 is generated, the information from the trace fields has to be checked 2022 for such problems as invalid characters and excessively long lines. 2024 11.5.2. External Explanations 2026 When the authorization check fails, an explanation string could be 2027 included in the reject response. Both the sender and the rejecting 2028 receiver need to be aware that the explanation was determined by the 2029 publisher of the SPF record checked and, in general, not the 2030 receiver. The explanation can contain malicious URLs, or it might be 2031 offensive or misleading. 2033 Explanations returned to sender domains due to "exp" modifiers 2034 (Section 6.2) were generated by the sender policy published by the 2035 domain holders themselves. As long as messages are only returned 2036 with non-delivery notification ([RFC3464]) to domains publishing the 2037 explanation strings from their own DNS SPF records, the only affected 2038 parties are the original publishers of the domain's SPF records. 2040 In practice, such non-delivery notifications can be misdirected, such 2041 as when an MTA accepts an email and only later generates the 2042 notification to a forged address, or when an email forwarder does not 2043 direct the bounce back to the original sender. 2045 11.5.3. Macro Expansion 2047 Macros (Section 7) allow senders to inject arbitrary text (any non- 2048 null [US-ASCII] character) into receiver DNS queries. It is 2049 necessary to be prepared for hostile or unexpected content. 2051 11.6. Privacy Exposure 2053 Checking SPF records causes DNS queries to be sent to the domain 2054 owner. These DNS queries, especially if they are caused by the 2055 "exists" mechanism, can contain information about who is sending 2056 email and likely to which MTA the email is being sent. This can 2057 introduce some privacy concerns, which are more or less of an issue 2058 depending on local laws and the relationship between the ADMD and the 2059 person sending the email. 2061 11.7. Delivering Mail Producing a 'Fail' Result 2063 Operators that choose to deliver mail for which SPF produces a "fail" 2064 result need to understand that they are admitting content that is 2065 explicitly not authorized by the purported sender. While there are 2066 known failure modes that can be considered "false negatives", the 2067 distinct choice to admit those messages increases end-user exposure 2068 to likely harm. This is especially true for domains belonging to 2069 known good actors that are typically well-behaved; unauthorized mail 2070 from those sources might well be subjected to much higher skepticism 2071 and content analysis. 2073 SPF does not, however, include the capacity for identifying good 2074 actors from bad ones, nor does it handle the concept of known actors 2075 versus unknown ones. Those notions are out of scope for this 2076 specification. 2078 12. Contributors and Acknowledgements 2080 This document is largely based on the work of Meng Weng Wong, Mark 2081 Lentczner, and Wayne Schlitt. Although, as this section 2082 acknowledges, many people have contributed to this document, a very 2083 large portion of the writing and editing are due to Meng, Mark, and 2084 Wayne. 2086 This design owes a debt of parentage to [RMX] by Hadmut Danisch and 2087 to [DMP] by Gordon Fecyk. The idea of using a DNS record to check 2088 the legitimacy of an email address traces its ancestry further back 2089 through messages on the namedroppers mailing list by Paul Vixie 2090 [Vixie] (based on suggestion by Jim Miller) and by David Green 2091 [Green]. 2093 Philip Gladstone contributed the concept of macros to the 2094 specification, multiplying the expressiveness of the language and 2095 making per-user and per-IP lookups possible. 2097 The authors of both this document and [RFC4408] would also like to 2098 thank the literally hundreds of individuals who have participated in 2099 the development of this design. They are far too numerous to name, 2100 but they include the following: 2102 The participants in the SPFbis working group. 2103 The folks on the spf-discuss mailing list. 2104 The folks on the SPAM-L mailing list. 2105 The folks on the IRTF ASRG mailing list. 2106 The folks on the IETF MARID mailing list. 2107 The folks on #perl. 2109 13. IANA Considerations 2111 13.1. The SPF DNS Record Type 2113 Per [RFC4408], the IANA assigned the Resource Record Type and Qtype 2114 from the DNS Parameters Registry for the SPF RR type with code 99. 2115 The format of this type is identical to the TXT RR [RFC1035]. The 2116 character content of the record is encoded as [US-ASCII]. 2118 Studies have shown that RRTYPE 99 has not seen any substantial use, 2119 and in fact its existence and mechanism defined in [RFC4408] has led 2120 to some interoperability issues. Accordingly, its use is now 2121 obsolete, and new implementations are not to use it. 2123 IANA is requested to update the Resource Record (RR) TYPEs registry 2124 to indicate that this document is the reference document for that 2125 RRTYPE. 2127 [NOTE TO RFC EDITOR: (to be changed to " ... has updated ..." upon 2128 publication)] 2130 13.2. The Received-SPF Mail Header Field 2132 Per [RFC3864], the "Received-SPF:" header field is added to the IANA 2133 Permanent Message Header Field Registry. The following is the 2134 registration template: 2136 Header field name: Received-SPF 2137 Applicable protocol: mail ([RFC5322]) 2138 Status: standard 2139 Author/Change controller: IETF 2140 Specification document(s): RFC XXXX 2141 [NOTE TO RFC EDITOR: (this document)] 2143 13.3. SPF Modifier Registry 2145 IANA is requested to change the reference for the exp and redirect 2146 modifiers in the Modifier Names registry, under Sender Policy 2147 Framework Parameters, from [RFC4408] to this document. Their status 2148 is unchanged. 2150 14. References 2152 14.1. Normative References 2154 [RFC1035] Mockapetris, P., "Domain names - implementation and 2155 specification", STD 13, RFC 1035, November 1987. 2157 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 2158 and Support", STD 3, RFC 1123, October 1989. 2160 [RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983, 2161 August 1996. 2163 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2164 Requirement Levels", BCP 14, RFC 2119, March 1997. 2166 [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", 2167 RFC 3463, January 2003. 2169 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 2170 Procedures for Message Header Fields", BCP 90, RFC 3864, 2171 September 2004. 2173 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2174 Resource Identifier (URI): Generic Syntax", STD 66, 2175 RFC 3986, January 2005. 2177 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 2178 Architecture", RFC 4291, February 2006. 2180 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 2181 Specifications: ABNF", STD 68, RFC 5234, January 2008. 2183 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 2184 October 2008. 2186 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 2187 October 2008. 2189 [RFC5451] Kucherawy, M., "Message Header Field for Indicating 2190 Message Authentication Status", RFC 5451, April 2009. 2192 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, 2193 July 2009. 2195 [RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782, 2196 February 2010. 2198 [RFC5890] Klensin, J., "Internationalized Domain Names for 2199 Applications (IDNA): Definitions and Document Framework", 2200 RFC 5890, August 2010. 2202 [US-ASCII] 2203 American National Standards Institute (formerly United 2204 States of America Standards Institute), "USA Code for 2205 Information Interchange, X3.4", 1968. 2207 ANSI X3.4-1968 has been replaced by newer versions with 2208 slight modifications, but the 1968 version remains 2209 definitive for the Internet. 2211 14.2. Informative References 2213 [DMP] Fecyk, G., "Designated Mailers Protocol". 2215 Work In Progress 2217 [Green] Green, D., "Domain-Authorized SMTP Mail", 2002. 2219 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 2220 STD 13, RFC 1034, November 1987. 2222 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", 2223 RFC 2671, August 1999. 2225 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 2226 specifying the location of services (DNS SRV)", RFC 2782, 2227 February 2000. 2229 [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format 2230 for Delivery Status Notifications", RFC 3464, 2231 January 2003. 2233 [RFC3696] Klensin, J., "Application Techniques for Checking and 2234 Transformation of Names", RFC 3696, February 2004. 2236 [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain 2237 Name System (DNS)", RFC 3833, August 2004. 2239 [RFC3834] Moore, K., "Recommendations for Automatic Responses to 2240 Electronic Mail", RFC 3834, August 2004. 2242 [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 2243 for Authorizing Use of Domains in E-Mail, Version 1", 2244 RFC 4408, April 2006. 2246 [RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing 2247 (CIDR): The Internet Address Assignment and Aggregation 2248 Plan", BCP 122, RFC 4632, August 2006. 2250 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 2251 Thayer, "OpenPGP Message Format", RFC 4880, November 2007. 2253 [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension 2254 for Authentication", RFC 4954, July 2007. 2256 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 2257 Mail Extensions (S/MIME) Version 3.2 Message 2258 Specification", RFC 5751, January 2010. 2260 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 2261 STD 72, RFC 6409, November 2011. 2263 [RFC6647] Kucherawy, M. and D. Crocker, "Email Greylisting: An 2264 Applicability Statement for SMTP", RFC 6647, June 2012. 2266 [RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham, 2267 "Deprecating the "X-" Prefix and Similar Constructs in 2268 Application Protocols", BCP 178, RFC 6648, June 2012. 2270 [RFC6652] Kitterman, S., "Sender Policy Framework (SPF) 2271 Authentication Failure Reporting Using the Abuse Reporting 2272 Format", RFC 6652, June 2012. 2274 [RFC6686] Kucherawy, M., "Resolution of the Sender Policy Framework 2275 (SPF) and Sender ID Experiments", RFC 6686, July 2012. 2277 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 2278 for DNS (EDNS(0))", STD 75, RFC 6891, April 2013. 2280 [RMX] Danisch, H., "The RMX DNS RR Type for light weight sender 2281 authentication". 2283 Work In Progress 2285 [Vixie] Vixie, P., "Repudiating MAIL FROM", 2002. 2287 Appendix A. Collected ABNF 2289 This section is normative and any discrepancies with the ABNF 2290 fragments in the preceding text are to be resolved in favor of this 2291 grammar. 2293 See [RFC5234] for ABNF notation. Please note that as per this ABNF 2294 definition, literal text strings (those in quotes) are case- 2295 insensitive. Hence, "mx" matches "mx", "MX", "mX", and "Mx". 2297 record = version terms *SP 2298 version = "v=spf1" 2300 terms = *( 1*SP ( directive / modifier ) ) 2302 directive = [ qualifier ] mechanism 2303 qualifier = "+" / "-" / "?" / "~" 2304 mechanism = ( all / include 2305 / a / mx / ptr / ip4 / ip6 / exists ) 2307 all = "all" 2308 include = "include" ":" domain-spec 2309 a = "a" [ ":" domain-spec ] [ dual-cidr-length ] 2310 mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ] 2311 ptr = "ptr" [ ":" domain-spec ] 2312 ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] 2313 ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ] 2314 exists = "exists" ":" domain-spec 2316 modifier = redirect / explanation / unknown-modifier 2317 redirect = "redirect" "=" domain-spec 2318 explanation = "exp" "=" domain-spec 2319 unknown-modifier = name "=" macro-string 2320 ; where name is not any known modifier 2322 ip4-cidr-length = "/" 1*DIGIT 2323 ip6-cidr-length = "/" 1*DIGIT 2324 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] 2326 ip4-network = qnum "." qnum "." qnum "." qnum 2327 qnum = DIGIT ; 0-9 2328 / %x31-39 DIGIT ; 10-99 2329 / "1" 2DIGIT ; 100-199 2330 / "2" %x30-34 DIGIT ; 200-249 2331 / "25" %x30-35 ; 250-255 2332 ; conventional dotted quad notation. e.g., 192.0.2.0 2333 ip6-network = 2334 ; e.g., 2001:DB8::CD30 2336 domain-spec = macro-string domain-end 2337 domain-end = ( "." toplabel [ "." ] ) / macro-expand 2339 toplabel = ( *alphanum ALPHA *alphanum ) / 2340 ( 1*alphanum "-" *( alphanum / "-" ) alphanum ) 2341 ; LDH rule plus additional TLD restrictions 2342 ; (see [RFC3696], Section 2 for background) 2343 alphanum = ALPHA / DIGIT 2345 explain-string = *( macro-string / SP ) 2347 macro-string = *( macro-expand / macro-literal ) 2348 macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) 2349 / "%%" / "%_" / "%-" 2350 macro-literal = %x21-24 / %x26-7E 2351 ; visible characters except "%" 2352 macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / 2353 "c" / "r" / "t" / "v" 2354 transformers = *DIGIT [ "r" ] 2355 delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" 2357 name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) 2359 header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] 2360 [ key-value-list ] CRLF 2362 result = "pass" / "fail" / "softfail" / "neutral" / 2363 "none" / "temperror" / "permerror" 2365 key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) 2366 [";"] 2368 key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) 2370 key = "client-ip" / "envelope-from" / "helo" / 2371 "problem" / "receiver" / "identity" / 2372 "mechanism" / name 2374 identity = "mailfrom" ; for the "MAIL FROM" identity 2375 / "helo" ; for the "HELO" identity 2376 / name ; other identities 2378 ALPHA = 2379 DIGIT = <0-9 as per [RFC5234]> 2380 SP = 2381 domain = 2382 dot-atom = 2383 quoted-string = 2384 comment = 2385 CFWS = 2386 FWS = 2387 CRLF = 2389 Appendix B. Extended Examples 2391 These examples are based on the following DNS setup: 2393 ; A domain with two mail servers, two hosts 2394 ; and two servers at the domain name 2395 $ORIGIN example.com. 2396 @ MX 10 mail-a 2397 MX 20 mail-b 2398 A 192.0.2.10 2399 A 192.0.2.11 2400 amy A 192.0.2.65 2401 bob A 192.0.2.66 2402 mail-a A 192.0.2.129 2403 mail-b A 192.0.2.130 2404 www CNAME example.com. 2406 ; A related domain 2407 $ORIGIN example.org. 2408 @ MX 10 mail-c 2409 mail-c A 192.0.2.140 2411 ; The reverse IP for those addresses 2412 $ORIGIN 2.0.192.in-addr.arpa. 2413 10 PTR example.com. 2414 11 PTR example.com. 2415 65 PTR amy.example.com. 2416 66 PTR bob.example.com. 2417 129 PTR mail-a.example.com. 2418 130 PTR mail-b.example.com. 2419 140 PTR mail-c.example.org. 2421 ; A rogue reverse IP domain that claims to be 2422 ; something it's not 2423 $ORIGIN 0.0.10.in-addr.arpa. 2424 4 PTR bob.example.com. 2426 B.1. Simple Examples 2428 These examples show various possible published records for 2429 example.com and which values if would cause check_host() to 2430 return "pass". Note that is "example.com". 2432 v=spf1 +all 2433 -- any passes 2435 v=spf1 a -all 2436 -- hosts 192.0.2.10 and 192.0.2.11 pass 2438 v=spf1 a:example.org -all 2439 -- no sending hosts pass since example.org has no A records 2441 v=spf1 mx -all 2442 -- sending hosts 192.0.2.129 and 192.0.2.130 pass 2444 v=spf1 mx:example.org -all 2445 -- sending host 192.0.2.140 passes 2447 v=spf1 mx mx:example.org -all 2448 -- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass 2450 v=spf1 mx/30 mx:example.org/30 -all 2451 -- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes 2453 v=spf1 ptr -all 2454 -- sending host 192.0.2.65 passes (reverse DNS is valid and is in 2455 example.com) 2456 -- sending host 192.0.2.140 fails (reverse DNS is valid, but not 2457 in example.com) 2458 -- sending host 10.0.0.4 fails (reverse IP is not valid) 2460 v=spf1 ip4:192.0.2.128/28 -all 2461 -- sending host 192.0.2.65 fails 2462 -- sending host 192.0.2.129 passes 2464 B.2. Multiple Domain Example 2466 These examples show the effect of related records: 2468 example.org: "v=spf1 include:example.com include:example.net -all" 2470 This record would be used if mail from example.org actually came 2471 through servers at example.com and example.net. Example.org's 2472 designated servers are the union of example.com's and example.net's 2473 designated servers. 2475 la.example.org: "v=spf1 redirect=example.org" 2476 ny.example.org: "v=spf1 redirect=example.org" 2477 sf.example.org: "v=spf1 redirect=example.org" 2479 These records allow a set of domains that all use the same mail 2480 system to make use of that mail system's record. In this way, only 2481 the mail system's record needs to be updated when the mail setup 2482 changes. These domains' records never have to change. 2484 B.3. DNSBL Style Example 2486 Imagine that, in addition to the domain records listed above, there 2487 are these (see [RFC5782]): 2489 $ORIGIN _spf.example.com. 2490 mary.mobile-users A 127.0.0.2 2491 fred.mobile-users A 127.0.0.2 2492 15.15.168.192.joel.remote-users A 127.0.0.2 2493 16.15.168.192.joel.remote-users A 127.0.0.2 2495 The following records describe users at example.com who mail from 2496 arbitrary servers, or who mail from personal servers. 2498 example.com: 2500 v=spf1 mx 2501 include:mobile-users._spf.%{d} 2502 include:remote-users._spf.%{d} 2503 -all 2505 mobile-users._spf.example.com: 2507 v=spf1 exists:%{l1r+}.%{d} 2509 remote-users._spf.example.com: 2511 v=spf1 exists:%{ir}.%{l1r+}.%{d} 2513 B.4. Multiple Requirements Example 2515 Say that your sender policy requires both that the IP address is 2516 within a certain range and that the reverse DNS for the IP matches. 2517 This can be done several ways, including the following: 2519 example.com. SPF ( "v=spf1 " 2520 "-include:ip4._spf.%{d} " 2521 "-include:ptr._spf.%{d} " 2522 "+all" ) 2523 ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all" 2524 ptr._spf.example.com. SPF "v=spf1 -ptr +all" 2526 This example shows how the "-include" mechanism can be useful, how an 2527 SPF record that ends in "+all" can be very restrictive, and the use 2528 of De Morgan's Law. 2530 Appendix C. Changes in implementation requirements from RFC 4408 2532 The modifications to implementation requirements from [RFC4408] are 2533 all either (a) corrections to errors in [RFC4408], or (b) additional 2534 documentation based on consensus of operational experience acquired 2535 since publication of [RFC4408]. 2537 o Use of DNS RR type SPF (99) has been removed from the protocol, 2538 see [RFC6686] for background. 2540 o A new DNS related processing limit based on "void lookups" has 2541 been added (Section 4.6.4). 2543 o Use of the ptr mechanism and the %p macro have been strongly 2544 discouraged Section 5.5 and Section 7.2. They remain part of the 2545 protocol because they were found to be in use, but records ought 2546 to be updated to avoid them. 2548 o Use of the "Authentication-Results" header field [RFC5451] as a 2549 possible alternative to use of the "Received-SPF" header field is 2550 discussed (Section 9.2). 2552 o There have been a number of minor corrections to the ABNF to make 2553 it more clear and correct Appendix A. SPF library implementers 2554 should give the revised ABNF a careful review to determine if 2555 implementation changes are needed. 2557 o Use of X- fields in the ABNF has been removed see [RFC6648] for 2558 background. 2560 o Ambiguity about how to deal with invalid domain-spec after macro 2561 expansion has been documented. Depending on one specific behavior 2562 has to be avoided (Section 4.8). 2564 o General operational information has been updated and expanded 2565 based on eight years of post [RFC4408] operations experience. See 2566 Section 10 and Appendices D - H below. 2568 o Security considerations have been reviewed and updated 2569 (Section 11). 2571 Appendix D. Further Testing Advice 2573 Another approach that can be helpful to publish records that include 2574 a "tracking exists:" mechanism. By looking at the name server logs, 2575 a rough list can then be generated. For example: 2577 v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all 2579 Appendix E. SPF/Mediator Interactions 2581 There are three places that techniques can be used to ameliorate 2582 unintended SPF failures with mediators. 2584 E.1. Originating ADMDs 2586 The beginning, when email is first sent: 2588 o "Neutral" results could be given for IP addresses that might be 2589 forwarders, instead of "fail" results based on a list of known 2590 reliable forwarders. For example: 2592 "v=spf1 mx ?exists:%{ir}.whitlist.example.org -all" 2594 This would cause a lookup on an DNS white list (DNSWL) and cause a 2595 result of "fail" only for email not either coming from the 2596 domain's mx host(s) (SPF pass) or white listed sources (SPF 2597 neutral). This, in effect, outsources an element of sender policy 2598 to the maintainer of the whitelist. 2600 o The "MAIL FROM" identity could have additional information in the 2601 local-part that cryptographically identifies the mail as coming 2602 from an authorized source. In this case, such an SPF record could 2603 be used: 2605 "v=spf1 mx exists:%{l}._spf_verify.%{d} -all" 2607 Then, a specialized DNS server can be set up to serve the 2608 _spf_verify subdomain that validates the local-part. Although 2609 this requires an extra DNS lookup, this happens only when the 2610 email would otherwise be rejected as not coming from a known good 2611 source. 2612 Note that due to the 63-character limit for domain labels, this 2613 approach only works reliably if the local-part signature scheme is 2614 guaranteed either to only produce local-parts with a maximum of 63 2615 characters or to gracefully handle truncated local-parts. 2617 o Similarly, a specialized DNS server could be set up that will 2618 rate-limit the email coming from unexpected IP addresses. 2620 "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all" 2622 o SPF allows the creation of per-user policies for special cases. 2623 For example, the following SPF record and appropriate wildcard DNS 2624 records can be used: 2626 "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}" 2628 E.2. Mediators 2630 The middle, when email is forwarded:. 2632 o Mediators can solve the problem by rewriting the "MAIL FROM" to be 2633 in their own domain. This means mail rejected from the external 2634 mailbox will have to be forwarded back to the original sender by 2635 the forwarding service. Various schemes to do this exist though 2636 they vary widely in complexity and resource requirements on the 2637 part of the mediator. 2639 o Several popular MTAs can be forced from "alias" semantics to 2640 "mailing list" semantics by configuring an additional alias with 2641 "owner-" prepended to the original alias name (e.g., an alias of 2642 "friends: george@example.com, fred@example.org" would need another 2643 alias of the form "owner-friends: localowner"). 2645 o Mediators could reject mail that would "fail" SPF if forwarded 2646 using an SMTP reply code of 551, User not local, (see [RFC5321] 2647 section 3.4) to communicate the correct target address to resend 2648 the mail to. 2650 E.3. Receving ADMDs 2652 The end, when email is received: 2654 o If the owner of the external mailbox wishes to trust the mediator, 2655 he can direct the external mailbox's MTA to skip SPF tests when 2656 the client host belongs to the mediator. 2658 o Tests against other identities, such as the "HELO" identity, MAY 2659 be used to override a failed test against the "MAIL FROM" 2660 identity. 2662 o For larger domains, it might not be possible to have a complete or 2663 accurate list of forwarding services used by the owners of the 2664 domain's mailboxes. In such cases, whitelists of generally- 2665 recognized forwarding services could be employed. 2667 Appendix F. Mail Services 2669 MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail 2670 services to third-party domains, such as sending of bulk mail, might 2671 want to adjust their configurations in light of the authorization 2672 check described in this document. If the domain part of the "MAIL 2673 FROM" identity used for such email uses the domain of one of the MSPs 2674 domain, then the provider needs only to ensure that its sending host 2675 is authorized by its own SPF record, if any. 2677 If the "MAIL FROM" identity does not use the MSP's domain, then extra 2678 care has to be taken. The SPF record format has several options for 2679 the third-party domain to authorize the service provider's MTAs to 2680 send mail on its behalf. For MSPs, such as ISPs, that have a wide 2681 variety of customers using the same MTA, steps are required to 2682 mitiate the risk of cross-customer forgery (see Section 11.4). 2684 Appendix G. MTA Relays 2686 Relays are described in [RFC5598] Section 2.2.2. The authorization 2687 check generally precludes the use of arbitrary MTA relays between 2688 sender and receiver of an email message. 2690 Within an organization, MTA relays can be effectively deployed. 2691 However, for purposes of this document, such relays are effectively 2692 transparent. The SPF authorization check is a check between border 2693 MTAs of different ADMDs. 2695 For mail senders, this means that published SPF records have to 2696 authorize any MTAs that actually send across the Internet. Usually, 2697 these are just the border MTAs as internal MTAs simply forward mail 2698 to these MTAs for relaying. 2700 The receiving ADMD will generally want to perform the authorization 2701 check at the boundary MTAs, including all secondary MXs. Internal 2702 MTAs (including MTAs that might serve both as boundary MTAs and 2703 internal relays from secondary MXs when they are processing the 2704 relayed mail stream) then do not perform the authorization test. To 2705 perform the authorization test other than at the boundary, the host 2706 that first transferred the message to the receiving ADMD have to be 2707 determined, which can be difficult to extract from the message header 2708 because (a) header fields can be forged or malformed, and (b) there's 2709 no standard way to encode that information such that it can be 2710 reliably extracted. Testing other than at the boundary is likely to 2711 produce unreliable results. This is described further in Appendix C 2712 of [RFC5451]. 2714 Appendix H. Local Policy Considerations 2716 SPF results can be used in combination with other methods to 2717 determine the final local disposition (either positive or negative of 2718 a message. It can also be considered dispositive on its own. 2720 H.1. Policy For SPF Pass 2722 SPF pass results can be used in combination with "white lists" of 2723 known "good" domains to bypass some or all additional pre-delivery 2724 email checks. Exactly which checks and how to determine appropriate 2725 white list entries has to be based on local conditions and 2726 requirements. 2728 H.2. Policy For SPF Fail 2730 SPF fail results can be used to reject messages during the SMTP 2731 transaction based on either "MAIL FROM" or "HELO" identity results. 2732 This reduces resource requirements for various content filtering 2733 methods and conserves bandwidth since rejection can be done before 2734 the SMTP content is transferred. It also gives immediate feedback to 2735 the sender who might then be able to resolve the issue. Due to some 2736 of the issues described above in this section (Section 10), SPF based 2737 rejection does present some risk of rejecting legitimate email when 2738 rejecting based on "MAIL FROM" results. 2740 SPF fail results can alternately be used as one input into a larger 2741 set of evaluations which might, based on a combination with other 2742 evaluation techniques, result in the email being marked negatively in 2743 some way (this might be via delivery to a special spam folder, 2744 modifying subject lines, or other locally determined means). 2745 Developing the details of such an approach have to be based on local 2746 conditions and requirements. Using SPF results in this way does not 2747 have the advantages of resource conservation and immediate feedback 2748 to the sender associated with SMTP rejection, but could produce fewer 2749 undesirable rejections in a well designed system. Such an approach 2750 might result in email that was not authorized by the sending ADMD 2751 being unknowingly delivered to end users. 2753 Either general approach can be used as they both leave a clear 2754 disposition of emails. They are either delivered in some manner or 2755 the sender is notified of the failure. Other dispositions such as 2756 "dropping" or deleting email after acceptance are inappropriate 2757 because they leave uncertainty and reduce the overall reliabilility 2758 and utility of email across the Internet. 2760 H.3. Policy For SPF Permerror 2762 The "permerror" result (see Section 2.6.7) indicates the SPF 2763 processing module at the receiver determined that the retrieved SPF 2764 policy record could not be interpreted. This gives no true 2765 indication about the authorized use of the data found in the 2766 envelope. 2768 As with all results, implementers have a choice to make regarding 2769 what to do with a message that yields this result. SMTP allows only 2770 a few basic options. 2772 Rejection of the message is an option, in that it is the one thing a 2773 receiver can do to draw attention to the difficulty encountered while 2774 protecting itself from messages that do not have a definite SPF 2775 result of some kind. However, if the SPF implementation is defective 2776 and returns spurious "permerror" results, only the sender is actively 2777 notified of the defect (in the form of rejected mail), and not the 2778 receiver making use of SPF. 2780 The less intrusive handling choice is to deliver the message, perhaps 2781 with some kind of annotation of the difficulty encountered and/or 2782 logging of a similar nature. However, this will not be desirable to 2783 operators that wish to implement SPF checking as strictly as 2784 possible, nor is this sort of passive problem reporting typically 2785 effective. 2787 There is of course the option placing this choice in the hands of the 2788 operator rather than the implementer since this kind of choice is 2789 often a matter of local policy rather than a condition with a 2790 universal solution, but this adds one more piece of complexity to an 2791 already non-trivial environment. 2793 Both implementers and operators need to be cautious of all choices 2794 and outcomes when handling SPF results. 2796 H.4. Policy For SPF Temperror 2798 The "temperror" result (see Section 2.6.6) indicates the SPF 2799 processing module at the receiver could not retrieve and SPF policy 2800 record due to a (probably) transient condition. This gives no true 2801 indication about the authorized use of the data found in the 2802 envelope. 2804 As with all results, implementers have a choice to make regarding 2805 what to do with a message that yields this result. SMTP allows only 2806 a few basic options. 2808 Deferring the message is an option, in that it is the one thing a 2809 receiver can do to draw attention to the difficulty encountered while 2810 protecting itself from messages that do not have a definite SPF 2811 result of some kind. However, if the SPF implementation is defective 2812 and returns spurious "temperror" results, only the sender is actively 2813 notified of the defect (in the form of mail rejected after it times 2814 out of the sending queue), and not the receiver making use of SPF. 2816 Because of long queue lifetimes, it is possible that mail will be 2817 repeatedly deferred for several days and so any awareness by the 2818 sender of a problem could be quite delayed. If "temperrors" persist 2819 for multiple delivery attempts, it might be perferable to treat the 2820 error as permanent and reduce the amount of time the message is in 2821 transit. 2823 The less intrusive handling choice is to deliver the message, perhaps 2824 with some kind of annotation of the difficulty encountered and/or 2825 logging of a similar nature. However, this will not be desirable to 2826 operators that wish to implement SPF checking as strictly as 2827 possible, nor is this sort of passive problem reporting typically 2828 effective. 2830 There is of course the option placing this choice in the hands of the 2831 operator rather than the implementer since this kind of choice is 2832 often a matter of local policy rather than a condition with a 2833 universal solution, but this adds one more piece of complexity to an 2834 already non-trivial environment. 2836 Both implementers and operators need to be cautious of all choices 2837 and outcomes when handling SPF results. 2839 Appendix I. Protocol Status 2841 NOTE TO RFC EDITOR: To be removed prior to publication. 2843 SPF has been in development since the summer of 2003 and has seen 2844 deployment beyond the developers beginning in December 2003. The 2845 design of SPF slowly evolved until the spring of 2004 and has since 2846 stabilized. There have been quite a number of forms of SPF, some 2847 written up as documents, some submitted as Internet Drafts, and many 2848 discussed and debated in development forums. The protocol was 2849 originally defined in [RFC4408], which this document replaces. 2851 [RFC4408] was designed to clearly document the protocol defined by 2852 earlier draft specifications of SPF as used in existing 2853 implementations. This updated specification is intended to clarify 2854 identified ambiguities in [RFC4408], resolve technical issues 2855 identified in post-RFC 4408 deployment experience, and document 2856 widely deployed extensions to SPF that have been developed since 2857 [RFC4408] was published. 2859 This document updates and replaces RFC 4408 that was part of a group 2860 of simultaneously published Experimental RFCs (RFC 4405, RFC 4406, 2861 RFC 4407, and RFC 4408) in 2006. At that time the IESG requested the 2862 community observe the success or failure of the two approaches 2863 documented in these RFCs during the two years following publication, 2864 in order that a community consensus could be reached in the future. 2866 SPF is widely deployed by large and small email providers alike. 2867 There are multiple, interoperable implementations. 2869 For SPF (as documented in RFC 4408) a careful effort was made to 2870 collect and document lessons learned and errata during the two year 2871 period. The errata list has been stable (no new submissions) and 2872 only minor protocol lessons learned were identified. Resolution of 2873 the IESG's experiment is documented in [RFC6686]. 2875 Appendix J. Change History 2877 NOTE TO RFC EDITOR: Changes since RFC 4408 (to be removed prior to 2878 publication) 2880 Moved to standards track 2882 Authors updated 2884 IESG Note regarding experimental use replaced with discussion of 2885 results 2887 Process errata: 2889 Resolved Section 2.5.7 PermError on invalid domains after macro 2890 expansion errata in favor of documenting that different verifiers 2891 produce different results. 2893 Add %v macro to ABNF grammar 2895 Replace "uric" by "unreserved" 2897 Recommend an SMTP reply code for optional permerror rejections 2899 Correct syntax in Received-SPF examples 2901 Fix unknown-modifier clause is too greedy in ABNF 2903 Correct use of empty domain-spec on exp modifier 2905 Fix minor typo errata 2907 Convert to spfbis working group draft, 2908 draft-ietf-spfbis-4408bis-00 2910 Clarified text about IPv4 mapped addresses to resolve test suite 2911 ambiguity 2913 Clarified ambiguity about result when more than 10 "mx" or "ptr" 2914 records are returned for lookup to specify permerror. This 2915 resolves one of the test suite ambiguities 2917 Made all references to result codes lower case per issue #7 2919 Adjusted section 2.2 Requirement to check mail from per issue #15 2921 Added missing "v" element in macro-letter in the collected ABNF 2922 per issue #16 - section 8.1 was already fixed in the pre-WG draft 2923 Marked ptr and "p" macro SHOULD NOT use per issue #27 2925 Expunged lower case may from the draft per issue #8 2927 Expunged "x-" name as an obsolete concept 2929 Updated obslete references: RFC2821 to RFC5321, RFC2822 to 2930 RFC5322, and RFC4234 to RFC5234 2932 Refer to RFC6647 to describe greylisting instead of trying to 2933 describe it directly. 2935 Updated informative references to the current versions. 2937 Start to rework section 9 with some RFC5598 terms. 2939 Added mention of RFC 6552 feedback reports in section 9. 2941 Added draft-ietf-spfbis-experiment as an informational reference. 2943 Drop Type SPF. 2945 Try and clarify informational nature of RFC3696 2947 Fix ABNF nits and add missing definitions per Bill's ABNF checker. 2949 Make DNS lookup time limit SHOULD instead of MAY. 2951 Reorganize and clarify processing limits. Move hard limits to new 2952 section 4.6.4, Evaluation Limits. Move advice to non-normative 2953 section 10. 2955 Removed paragraph in section 11.1 about limiting total data 2956 volumes as it is unused (and removable per the charter) and serves 2957 no purpose (it isn't something that actually can be implemented in 2958 any reasonable way). 2960 Added text from Alessandro Vesely in section 10.1 to better 2961 explain DNS resource limits. 2963 Multiple editorial fixes from Murray Kucherawy's review. 2965 Also based on Murray's review, reworked SMTP identity definitions 2966 and made RFC 5598 a normative reference instead of informative. 2967 This is a downref that will have to be mentioned in the last call. 2969 Added RFC 3834 as an informative reference about backscatter. 2971 Added IDN requirements and normative reference to RFC 5890 to deal 2972 with the question "like DKIM did it.: 2974 Added informative reference to RFC 4632 for CIDR and use CIDR 2975 prefix length instead of CIDR-length to match its terminology. 2977 Simplified the exists description. 2979 Added text on creating a Authentication-Results header field that 2980 matches the Received-SPF header field information and added a 2981 normative reference to RFC 5451. 2983 Added informative reference to RFC 2782 due to SRV mention. 2985 Added informative reference to RFC 3464 due to DSN mention. 2987 Added informative reference to RFC 5617 for its DNS wildcard use. 2989 Clarified the intended match/no-match method for exists. 2991 Added new sections on Receiver policy for SPF pass, fail, and 2992 permerror. 2994 Added new section 10 discussion on treatment of bounces and the 2995 significance of HELO records. 2997 Added request to IANA to update the SPF modifier registry. 2999 Substantially reorganized the document for improved readability 3000 for new users based on WG consensus. 3002 Added new DNS "void lookup" processing limit to mitigate potential 3003 future risk of SPF being used as a DDoS vector. 3005 Author's Address 3007 Scott Kitterman 3008 Kitterman Technical Services 3009 3611 Scheel Dr 3010 Ellicott City, MD 21042 3011 United States of America 3013 Email: scott@kitterman.com