en-US/about_SPF.help.txt
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TOPIC about_SPF Internet Engineering Task Force (IETF) S. Kitterman Request for Comments: 7208 Kitterman Technical Services Obsoletes: 4408 April 2014 Category: Standards Track ISSN: 2070-1721 Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1 Abstract Email on the Internet can be forged in a number of ways. In particular, existing protocols place no restriction on what a sending host can use as the "MAIL FROM" of a message or the domain given on the SMTP HELO/EHLO commands. This document describes version 1 of the Sender Policy Framework (SPF) protocol, whereby ADministrative Management Domains (ADMDs) can explicitly authorize the hosts that are allowed to use their domain names, and a receiving host can check such authorization. This document obsoletes RFC 4408. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7208. Kitterman Standards Track [Page 1] RFC 7208 Sender Policy Framework (SPF) April 2014 Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction ....................................................5 1.1. Terminology ................................................5 1.1.1. Key Words ...........................................5 1.1.2. Imported Definitions ................................5 1.1.3. MAIL FROM Definition ................................6 1.1.4. HELO Definition .....................................6 1.2. check_host() ...............................................6 2. Operational Overview ............................................6 2.1. Publishing Authorization ...................................6 2.2. Checking Authorization .....................................7 2.3. The "HELO" Identity ........................................8 2.4. The "MAIL FROM" Identity ...................................9 2.5. Location of Checks .........................................9 2.6. Results of Evaluation ......................................9 2.6.1. None ...............................................10 2.6.2. Neutral ............................................10 2.6.3. Pass ...............................................10 2.6.4. Fail ...............................................10 Kitterman Standards Track [Page 2] RFC 7208 Sender Policy Framework (SPF) April 2014 2.6.5. Softfail ...........................................10 2.6.6. Temperror ..........................................10 2.6.7. Permerror ..........................................10 3. SPF Records ....................................................11 3.1. DNS Resource Records ......................................11 3.2. Multiple DNS Records ......................................12 3.3. Multiple Strings in a Single DNS Record ...................12 3.4. Record Size ...............................................13 3.5. Wildcard Records ..........................................13 4. The check_host() Function ......................................14 4.1. Arguments .................................................14 4.2. Results ...................................................15 4.3. Initial Processing ........................................15 4.4. Record Lookup .............................................15 4.5. Selecting Records .........................................15 4.6. Record Evaluation .........................................16 4.6.1. Term Evaluation ....................................16 4.6.2. Mechanisms .........................................16 4.6.3. Modifiers ..........................................17 4.6.4. DNS Lookup Limits ..................................17 4.7. Default Result ............................................18 4.8. Domain Specification ......................................19 5. Mechanism Definitions ..........................................20 5.1. "all" .....................................................21 5.2. "include" .................................................21 5.3. "a" .......................................................23 5.4. "mx" ......................................................23 5.5. "ptr" (do not use) ........................................23 5.6. "ip4" and "ip6" ...........................................25 5.7. "exists" ..................................................25 6. Modifier Definitions ...........................................26 6.1. redirect: Redirected Query ................................26 6.2. exp: Explanation ..........................................27 7. Macros .........................................................28 7.1. Formal Specification ......................................29 7.2. Macro Definitions .........................................29 7.3. Macro Processing Details ..................................30 7.4. Expansion Examples ........................................32 8. Result Handling ................................................33 8.1. None ......................................................34 8.2. Neutral ...................................................34 8.3. Pass ......................................................34 8.4. Fail ......................................................35 8.5. Softfail ..................................................35 8.6. Temperror .................................................36 8.7. Permerror .................................................36 Kitterman Standards Track [Page 3] RFC 7208 Sender Policy Framework (SPF) April 2014 9. Recording the Result ...........................................36 9.1. The Received-SPF Header Field .............................37 9.2. SPF Results in the Authentication-Results Header Field ....39 10. Effects on Infrastructure .....................................39 10.1. Sending Domains ..........................................40 10.1.1. DNS Resource Considerations .......................40 10.1.2. Administrator's Considerations ....................41 10.1.3. Bounces ...........................................41 10.2. Receivers ................................................42 10.3. Mediators ................................................42 11. Security Considerations .......................................43 11.1. Processing Limits ........................................43 11.2. SPF-Authorized Email May Contain Other False Identities ..44 11.3. Spoofed DNS and IP Data ..................................44 11.4. Cross-User Forgery .......................................44 11.5. Untrusted Information Sources ............................45 11.5.1. Recorded Results ..................................45 11.5.2. External Explanations .............................45 11.5.3. Macro Expansion ...................................46 11.6. Privacy Exposure .........................................46 11.7. Delivering Mail Producing a "Fail" Result ................46 12. Collected ABNF ................................................46 13. Contributors and Acknowledgements .............................48 14. IANA Considerations ...........................................49 14.1. The SPF DNS Record Type ..................................49 14.2. The Received-SPF Mail Header Field .......................50 14.3. SPF Modifier Registry ....................................50 15. References ....................................................50 15.1. Normative References .....................................50 15.2. Informative References ...................................51 Appendix A. Extended Examples .....................................54 A.1. Simple Examples ............................................55 A.2. Multiple Domain Example ....................................56 A.3. DNS Blacklist (DNSBL) Style Example ........................56 A.4. Multiple Requirements Example ..............................57 Appendix B. Changes in Implementation Requirements from RFC 4408 ..57 Appendix C. Further Testing Advice ................................58 Appendix D. SPF/Mediator Interactions .............................59 D.1. Originating ADMDs ..........................................59 D.2. Mediators ..................................................60 D.3. Receiving ADMDs ............................................60 Appendix E. Mail Services .........................................61 Appendix F. MTA Relays ............................................61 Appendix G. Local Policy Considerations ...........................62 G.1. Policy for SPF Pass ........................................62 G.2. Policy for SPF Fail ........................................62 G.3. Policy for SPF Permerror ...................................63 G.4. Policy for SPF Temperror ...................................63 Kitterman Standards Track [Page 4] RFC 7208 Sender Policy Framework (SPF) April 2014 1. Introduction The current email infrastructure has the property that any host injecting mail into the system can use any DNS domain name it wants in each of the various identifiers specified by [RFC5321] and [RFC5322]. Although this feature is desirable in some circumstances, it is a major obstacle to reducing Unsolicited Bulk Email (UBE, aka spam). Furthermore, ADMDs (as described in [RFC5598]) are understandably concerned about the ease with which other entities can make use of their domain names, often with malicious intent. This document defines a protocol by which ADMDs can authorize hosts to use their domain names in the "MAIL FROM" or "HELO" identities. Compliant ADMDs publish Sender Policy Framework (SPF) records in the DNS specifying which hosts are permitted to use their names, and compliant mail receivers use the published SPF records to test the authorization of sending Mail Transfer Agents (MTAs) using a given "HELO" or "MAIL FROM" identity during a mail transaction. An additional benefit to mail receivers is that after the use of an identity is verified, local policy decisions about the mail can be made based on the sender's domain, rather than the host's IP address. This is advantageous because reputation of domain names is likely to be more accurate than reputation of host IP addresses since domains are likely to be more stable over a longer period. Furthermore, if a claimed identity fails verification, local policy can take stronger action against such email, such as rejecting it. 1.1. Terminology 1.1.1. Key Words The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.1.2. Imported Definitions ABNF (Augmented Backus-Naur Form) ABNF is defined in [RFC5234], as are the tokens "ALPHA", "DIGIT", and "SP" (space). The tokens "Local-part", "Domain", and "Mailbox" are defined in [RFC5321]. "dot-atom", "quoted-string", "comment", "CFWS" (comment folded white space), "FWS" (folded white space), and "CRLF" (carriage-return/ line-feed) are defined in [RFC5322]. Kitterman Standards Track [Page 5] RFC 7208 Sender Policy Framework (SPF) April 2014 1.1.3. MAIL FROM Definition This document is concerned with the identity of the sender of a mail message, as referred to in [RFC5321]: The transaction starts with a MAIL command that gives the sender identification. Since there are many other names for this identity, it is important to choose a name that is: 1. commonly used 2. well defined As such, throughout this document the term "MAIL FROM" will be used, which is defined as the RFC5321.MailFrom (reverse-path) identity described in [RFC5598]. 1.1.4. HELO Definition This document also makes use of the HELO/EHLO identity. The "HELO" identity derives from either the SMTP HELO or EHLO command (see [RFC5321]). Since HELO and EHLO can, in many cases, be used interchangeably, they are identified commonly as "HELO" in this document. This means RFC5321.HELO/.EHLO as defined in [RFC5598]. These commands supply the identity of the SMTP client (sending host) for the SMTP session. 1.2. check_host() Section 4 introduces an algorithm to evaluate an SPF policy against an arriving email transaction. In an early implementation, this algorithm was encoded in a function called check_host(). That name is used in this document as symbolic of the SPF evaluation algorithm, but of course implementers are not required to use this name. 2. Operational Overview 2.1. Publishing Authorization An SPF-compliant domain publishes valid SPF records as described in Section 3. These records authorize the use of the relevant domain names in the "HELO" and "MAIL FROM" identities by the MTAs specified therein. Kitterman Standards Track [Page 6] RFC 7208 Sender Policy Framework (SPF) April 2014 SPF results can be used to make both positive (source is authorized) and negative (source is not authorized) determinations. If ADMDs choose to publish SPF records and want to support receivers making negative authorization determinations, it is necessary for them to publish records that end in "-all", or redirect to other records that do; otherwise, no definitive determination of authorization can be made. Potential issues and mitigations associated with negative determinations are discussed in Section 10. ADMDs that wish to declare that no hosts are authorized to use their DNS domain names in the HELO or MAIL FROM commands during SMTP sessions can publish SPF records that say so for domain names that are neither used in the domain part of email addresses nor expected to originate mail. When changing SPF records, care has to be taken to ensure that there is a transition period so that the old policy remains valid until all legitimate email can reasonably expect to have been checked. [RFC5321], Section 4.5.4.1 discusses how long a message might be in transit. While offline checks are possible, the closer to the original transmission time checks are performed, the more likely they are to get an SPF result that matches the sending ADMD intent at the time the message was sent. 2.2. Checking Authorization A mail receiver can perform a set of SPF checks for each mail message it receives. An SPF check tests the authorization of a client host to emit mail with a given identity. Typically, such checks are done by a receiving MTA, but can be performed elsewhere in the mail processing chain so long as the required information is available and reliable. The "MAIL FROM" and "HELO" identities are checked as described in Sections 2.4 and 2.3, respectively. Without explicit approval of the publishing ADMD, checking other identities against SPF version 1 records is NOT RECOMMENDED because there are cases that are known to give incorrect results. For example, almost all mailing lists rewrite the "MAIL FROM" identity (see Section 10.3), but some do not change any other identities in the message. Documents that define other identities will have to define the method for explicit approval. It is possible that mail receivers will use the SPF check as part of a larger set of tests on incoming mail. The results of other tests might influence whether or not a particular SPF check is performed. For example, finding the sending host's IP address on a local whitelist might cause all other tests to be skipped and all mail from that host to be accepted. Kitterman Standards Track [Page 7] RFC 7208 Sender Policy Framework (SPF) April 2014 When a mail receiver decides to perform an SPF check, it has to use a correctly implemented check_host() function (Section 4) evaluated with the correct parameters. Although the test as a whole is optional, once it has been decided to perform a test it has to be performed as specified so that the correct semantics are preserved between publisher and receiver. To make the test, the mail receiver MUST evaluate the check_host() function with the arguments described in Section 4.1. Although invalid, malformed, or non-existent domains cause SPF checks to return "none" because no SPF record can be found, it has long been the policy of many MTAs to reject email from such domains, especially in the case of invalid "MAIL FROM". Rejecting email will prevent one method of circumventing of SPF records. Implementations have to take care to correctly extract the <domain> from the data given with the SMTP MAIL FROM command as many MTAs will still accept such things as source routes (see Appendix C of [RFC5321]), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]). These archaic features have been maliciously used to bypass security systems. 2.3. The "HELO" Identity It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM" identity but also separately check the "HELO" identity by applying the check_host() function (Section 4) to the "HELO" identity as the <sender>. Checking "HELO" promotes consistency of results and can reduce DNS resource usage. If a conclusive determination about the message can be made based on a check of "HELO", then the use of DNS resources to process the typically more complex "MAIL FROM" can be avoided. Additionally, since SPF records published for "HELO" identities refer to a single host, when available, they are a very reliable source of host authorization status. Checking "HELO" before "MAIL FROM" is the RECOMMENDED sequence if both are checked. Note that requirements for the domain presented in the EHLO or HELO command are not always clear to the sending party, and SPF verifiers have to be prepared for the identity to be an IP address literal (see [RFC5321], Section 4.1.3) or simply be malformed. This SPF check can only be performed when the "HELO" string is a valid, multi-label domain name. Kitterman Standards Track [Page 8] RFC 7208 Sender Policy Framework (SPF) April 2014 2.4. The "MAIL FROM" Identity SPF verifiers MUST check the "MAIL FROM" identity if a "HELO" check either has not been performed or has not reached a definitive policy result by applying the check_host() function to the "MAIL FROM" identity as the <sender>. [RFC5321] allows the reverse-path to be null (see Section 4.5.5 in [RFC5321]). In this case, there is no explicit sender mailbox, and such a message can be assumed to be a notification message from the mail system itself. When the reverse-path is null, this document defines the "MAIL FROM" identity to be the mailbox composed of the local-part "postmaster" and the "HELO" identity (which might or might not have been checked separately before). 2.5. Location of Checks The authorization check SHOULD be performed during the processing of the SMTP transaction that receives the mail. This reduces the complexity of determining the correct IP address to use as an input to check_host() and allows errors to be returned directly to the sending MTA by way of SMTP replies. Appendix D of [RFC7001] provides a more thorough discussion of this topic. The authorization check is performed during the SMTP transaction at the time of the MAIL command, and uses the MAIL FROM value and the client IP address. Performing the check at later times or with other input can cause problems such as the following: o It might be difficult to accurately extract the required information from potentially deceptive headers. o Legitimate email might fail the authorization check because the sender's policy has since changed. Generating non-delivery notifications to forged identities that have failed the authorization check often constitutes backscatter, i.e., nuisance rejection notices that are not actionable. Operators are strongly advised to avoid such practices. Section 2 of [RFC3834] describes backscatter and the problems it causes. 2.6. Results of Evaluation Section 4 defines check_host(), a model function definition that uses the inputs defined above and the sender's policy published in the DNS to reach a conclusion about client authorization. An SPF verifier implements something semantically equivalent to the function defined there. Kitterman Standards Track [Page 9] RFC 7208 Sender Policy Framework (SPF) April 2014 This section enumerates and briefly defines the possible outputs of that function. Note, however, that the protocol establishes no normative requirements for handling any particular result. Discussion of handling options for each result can be found in Section 8. 2.6.1. None A result of "none" means either (a) no syntactically valid DNS domain name was extracted from the SMTP session that could be used as the one to be authorized, or (b) no SPF records were retrieved from the DNS. 2.6.2. Neutral A "neutral" result means the ADMD has explicitly stated that it is not asserting whether the IP address is authorized. 2.6.3. Pass A "pass" result is an explicit statement that the client is authorized to inject mail with the given identity. 2.6.4. Fail A "fail" result is an explicit statement that the client is not authorized to use the domain in the given identity. 2.6.5. Softfail A "softfail" result is a weak statement by the publishing ADMD that the host is probably not authorized. It has not published a stronger, more definitive policy that results in a "fail". 2.6.6. Temperror A "temperror" result means the SPF verifier encountered a transient (generally DNS) error while performing the check. A later retry may succeed without further DNS operator action. 2.6.7. Permerror A "permerror" result means the domain's published records could not be correctly interpreted. This signals an error condition that definitely requires DNS operator intervention to be resolved. Kitterman Standards Track [Page 10] RFC 7208 Sender Policy Framework (SPF) April 2014 3. SPF Records An SPF record is a DNS record that declares which hosts are, and are not, authorized to use a domain name for the "HELO" and "MAIL FROM" identities. Loosely, the record partitions hosts into permitted and not-permitted sets (though some hosts might fall into neither category). The SPF record is expressed as a single string of text found in the RDATA of a single DNS TXT resource record; multiple SPF records are not permitted for the same owner name. The record format and the process for selecting records are described below in Section 4. An example record is the following: v=spf1 +mx a:colo.example.com/28 -all This record has a version of "spf1" and three directives: "+mx", "a:colo.example.com/28" (the "+" is implied), and "-all". Each SPF record is placed in the DNS tree at the owner name it pertains to, not in a subdomain under the owner name. This is similar to how SRV records [RFC2782] are done. The example in this section might be published via these lines in a domain zone file: example.com. TXT "v=spf1 +mx a:colo.example.com/28 -all" Since TXT records have multiple uses, beware of other TXT records published there for other purposes. They might cause problems with size limits (see Section 3.4), and care has to be taken to ensure that only SPF records are used for SPF processing. ADMDs publishing SPF records ought to keep the amount of DNS information needed to evaluate a record to a minimum. Sections 4.6.4 and 10.1.1 provide some suggestions about "include" mechanisms and chained "redirect" modifiers. 3.1. DNS Resource Records SPF records MUST be published as a DNS TXT (type 16) Resource Record (RR) [RFC1035] only. The character content of the record is encoded as [US-ASCII]. Use of alternative DNS RR types was supported in SPF's experimental phase but has been discontinued. In 2003, when SPF was first being developed, the requirements for assignment of a new DNS RR type were considerably more stringent than they are now. Additionally, support for easy deployment of new DNS Kitterman Standards Track [Page 11] RFC 7208 Sender Policy Framework (SPF) April 2014 RR types was not widely deployed in DNS servers and provisioning systems. As a result, developers of SPF found it easier and more practical to use the TXT RR type for SPF records. In its review of [RFC4408], the SPFbis working group concluded that its dual RR type transition model was fundamentally flawed since it contained no common RR type that implementers were required to serve and required to check. Many alternatives were considered to resolve this issue, but ultimately the working group concluded that significant migration to the SPF RR type in the foreseeable future was very unlikely and that the best solution for resolving this interoperability issue was to drop support for the SPF RR type from SPF version 1. See Appendix A of [RFC6686] for further information. The circumstances surrounding SPF's initial deployment a decade ago are unique. If a future update to SPF were developed that did not reuse existing SPF records, it could use the SPF RR type. SPF's use of the TXT RR type for structured data should in no way be taken as precedent for future protocol designers. Further discussion of design considerations when using new DNS RR types can be found in [RFC5507]. 3.2. Multiple DNS Records A domain name MUST NOT have multiple records that would cause an authorization check to select more than one record. See Section 4.5 for the selection rules. 3.3. Multiple Strings in a Single DNS Record As defined in [RFC1035], Sections 3.3 and 3.3.14, a single text DNS record can be composed of more than one string. If a published record contains multiple character-strings, then the record MUST be treated as if those strings are concatenated together without adding spaces. For example: IN TXT "v=spf1 .... first" "second string..." is equivalent to: IN TXT "v=spf1 .... firstsecond string..." TXT records containing multiple strings are useful in constructing records that would exceed the 255-octet maximum length of a character-string within a single TXT record. Kitterman Standards Track [Page 12] RFC 7208 Sender Policy Framework (SPF) April 2014 3.4. Record Size The published SPF record for a given domain name SHOULD remain small enough that the results of a query for it will fit within 512 octets. Otherwise, there is a possibility of exceeding a DNS protocol limit. This UDP limit is defined in [RFC1035], Section 2.3.4, although it was raised by [RFC2671]. Staying below 512 octets ought to prevent older DNS implementations from failing over to TCP and will work with UDP in the absence of EDNS0 [RFC6891] support. Since the answer size is dependent on many things outside the scope of this document, it is only possible to give this guideline: If the size of the DNS message, the combined length of the DNS name and the text of all the records of a given type is under 450 octets, then DNS answers ought to fit in UDP packets. Records that are too long to fit in a single UDP packet could be silently ignored by SPF verifiers due to firewall and other issues that interfere with the operation of DNS over TCP or using ENDS0. Note that when computing the sizes for replies to queries of the TXT format, one has to take into account any other TXT records published at the domain name. Similarly, the sizes for replies to all queries related to SPF have to be evaluated to fit in a single 512-octet UDP packet (i.e., DNS message size limited to 450 octets). 3.5. Wildcard Records Use of wildcard records for publishing is discouraged, and care has to be taken if they are used. If a zone includes wildcard MX records, it might want to publish wildcard declarations, subject to the same requirements and problems. In particular, the declaration MUST be repeated for any host that has any RR records at all, and for subdomains thereof. Consider the example in [RFC1034], Section 4.3.3. Based on that, we can do the following: EXAMPLE.COM. MX 10 A.EXAMPLE.COM EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" *.EXAMPLE.COM. MX 10 A.EXAMPLE.COM *.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" A.EXAMPLE.COM. A 203.0.113.1 A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" *.A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM *.A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all" Kitterman Standards Track [Page 13] RFC 7208 Sender Policy Framework (SPF) April 2014 SPF records have to be listed twice for every name within the zone: once for the name, and once with a wildcard to cover the tree under the name, in order to cover all domains in use in outgoing mail. 4. The check_host() Function This description is not an application programming interface definition, but rather a function description used to illustrate the algorithm. A compliant SPF implementation MUST produce results semantically equivalent to this description. The check_host() function fetches SPF records, parses them, and evaluates them to determine whether a particular host is or is not permitted to send mail with a given identity. Receiving ADMDs that perform this check MUST correctly evaluate the check_host() function as described here. Implementations MAY use a different algorithm than the canonical algorithm defined here, so long as the results are the same in all cases. 4.1. Arguments The check_host() function takes these arguments: <ip> - the IP address of the SMTP client that is emitting the mail, either IPv4 or IPv6. <domain> - the domain that provides the sought-after authorization information; initially, the domain portion of the "MAIL FROM" or "HELO" identity. <sender> - the "MAIL FROM" or "HELO" identity. For recursive evaluations, the domain portion of <sender> might not be the same as the <domain> argument when check_host() is initially evaluated. In most other cases it will be the same (see Section 5.2 below). The overall DNS lookup limit for SPF terms described below in Section 4.6.4 must be tracked as a single global limit for all evaluations, not just for a single instance of a recursive evaluation. Note that the <domain> argument might not be a well-formed domain name. For example, if the reverse-path was null, then the EHLO/HELO domain is used, with its associated problems (see Section 2.3). In these cases, check_host() is defined in Section 4.3 to return a "none" result. Kitterman Standards Track [Page 14] RFC 7208 Sender Policy Framework (SPF) April 2014 4.2. Results The check_host() function can return one of several results described in Section 2.6. Based on the result, the action to be taken is determined by the local policies of the receiver. This is discussed in Section 8. 4.3. Initial Processing If the <domain> is malformed (e.g., label longer than 63 characters, zero-length label not at the end, etc.) or is not a multi-label domain name, or if the DNS lookup returns "Name Error" (RCODE 3, also known as "NXDOMAIN" [RFC2308]), check_host() immediately returns the result "none". DNS RCODEs are defined in [RFC1035]. Properly formed domains are fully qualified domains as defined in [RFC1983]. That is, in the DNS they are implicitly qualified relative to the root (see Section 3.1 of [RFC1034]). Internationalized domain names MUST be encoded as A-labels, as described in Section 2.3 of [RFC5890]. If the <sender> has no local-part, substitute the string "postmaster" for the local-part. 4.4. Record Lookup In accordance with how the records are published (see Section 3 above), a DNS query needs to be made for the <domain> name, querying for type TXT only. If the DNS lookup returns a server failure (RCODE 2) or some other error (RCODE other than 0 or 3), or if the lookup times out, then check_host() terminates immediately with the result "temperror". 4.5. Selecting Records Records begin with a version section: record = version terms *SP version = "v=spf1" Starting with the set of records that were returned by the lookup, discard records that do not begin with a version section of exactly "v=spf1". Note that the version section is terminated by either an SP character or the end of the record. As an example, a record with a version section of "v=spf10" does not match and is discarded. If the resultant record set includes no records, check_host() produces the "none" result. If the resultant record set includes more than one record, check_host() produces the "permerror" result. Kitterman Standards Track [Page 15] RFC 7208 Sender Policy Framework (SPF) April 2014 4.6. Record Evaluation The check_host() function parses and interprets the SPF record to find a result for the current test. The syntax of the record is validated first, and if there are any syntax errors anywhere in the record, check_host() returns immediately with the result "permerror", without further interpretation or evaluation. 4.6.1. Term Evaluation There are two types of terms: mechanisms (defined in Section 5) and modifiers (defined in Section 6). A record contains an ordered list of these as specified in the following Augmented Backus-Naur Form (ABNF). terms = *( 1*SP ( directive / modifier ) ) directive = [ qualifier ] mechanism qualifier = "+" / "-" / "?" / "~" mechanism = ( all / include / a / mx / ptr / ip4 / ip6 / exists ) modifier = redirect / explanation / unknown-modifier unknown-modifier = name "=" macro-string ; where name is not any known modifier name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) Most mechanisms allow a ":" or "/" character after the name. Modifiers always contain an equals ('=') character immediately after the name, and before any ":" or "/" characters that might be part of the macro-string. Terms that do not contain any of "=", ":", or "/" are mechanisms, as defined in Section 5. As per the definition of the ABNF notation in [RFC5234], mechanism and modifier names are case-insensitive. 4.6.2. Mechanisms Each mechanism is considered in turn from left to right. If there are no more mechanisms, the result is the default result as described in Section 4.7. When a mechanism is evaluated, one of three things can happen: it can match, not match, or return an exception. Kitterman Standards Track [Page 16] RFC 7208 Sender Policy Framework (SPF) April 2014 If it matches, processing ends and the qualifier value is returned as the result of that record. If it does not match, processing continues with the next mechanism. If it returns an exception, mechanism processing ends and the exception value is returned. The possible qualifiers, and the results they cause check_host() to return, are as follows: "+" pass "-" fail "~" softfail "?" neutral The qualifier is optional and defaults to "+". When a mechanism matches and the qualifier is "-", then a "fail" result is returned and the explanation string is computed as described in Section 6.2. The specific mechanisms are described in Section 5. 4.6.3. Modifiers Modifiers are not mechanisms. They do not return match or not-match. Instead, they provide additional information. Although modifiers do not directly affect the evaluation of the record, the "redirect" modifier has an effect after all the mechanisms have been evaluated. 4.6.4. DNS Lookup Limits Some mechanisms and modifiers (collectively, "terms") cause DNS queries at the time of evaluation, and some do not. The following terms cause DNS queries: the "include", "a", "mx", "ptr", and "exists" mechanisms, and the "redirect" modifier. SPF implementations MUST limit the total number of those terms to 10 during SPF evaluation, to avoid unreasonable load on the DNS. If this limit is exceeded, the implementation MUST return "permerror". The other terms -- the "all", "ip4", and "ip6" mechanisms, and the "exp" modifier -- do not cause DNS queries at the time of SPF evaluation (the "exp" modifier only causes a lookup at a later time), and their use is not subject to this limit. When evaluating the "mx" mechanism, the number of "MX" resource records queried is included in the overall limit of 10 mechanisms/ modifiers that cause DNS lookups as described above. In addition to that limit, the evaluation of each "MX" record MUST NOT result in Kitterman Standards Track [Page 17] RFC 7208 Sender Policy Framework (SPF) April 2014 querying more than 10 address records -- either "A" or "AAAA" resource records. If this limit is exceeded, the "mx" mechanism MUST produce a "permerror" result. When evaluating the "ptr" mechanism or the %{p} macro, the number of "PTR" resource records queried is included in the overall limit of 10 mechanisms/modifiers that cause DNS lookups as described above. In addition to that limit, the evaluation of each "PTR" record MUST NOT result in querying more than 10 address records -- either "A" or "AAAA" resource records. If this limit is exceeded, all records other than the first 10 MUST be ignored. The reason for the disparity is that the set of and contents of the MX record are under control of the publishing ADMD, while the set of and contents of PTR records are under control of the owner of the IP address actually making the connection. These limits are per mechanism or macro in the record, and are in addition to the lookup limits specified above. MTAs or other processors SHOULD impose a limit on the maximum amount of elapsed time to evaluate check_host(). Such a limit SHOULD allow at least 20 seconds. If such a limit is exceeded, the result of authorization SHOULD be "temperror". As described at the end of Section 11.1, there may be cases where it is useful to limit the number of "terms" for which DNS queries return either a positive answer (RCODE 0) with an answer count of 0, or a "Name Error" (RCODE 3) answer. These are sometimes collectively referred to as "void lookups". SPF implementations SHOULD limit "void lookups" to two. An implementation MAY choose to make such a limit configurable. In this case, a default of two is RECOMMENDED. Exceeding the limit produces a "permerror" result. 4.7. Default Result If none of the mechanisms match and there is no "redirect" modifier, then the check_host() returns a result of "neutral", just as if "?all" were specified as the last directive. If there is a "redirect" modifier, check_host() proceeds as defined in Section 6.1. It is better to use either a "redirect" modifier or an "all" mechanism to explicitly terminate processing. Although there is an implicit "?all" at the end of every record that is not explicitly terminated, it aids debugging efforts when it is explicitly provided. Kitterman Standards Track [Page 18] RFC 7208 Sender Policy Framework (SPF) April 2014 For example: v=spf1 +mx -all or v=spf1 +mx redirect=_spf.example.com 4.8. Domain Specification Several of these mechanisms and modifiers have a <domain-spec> section. The <domain-spec> string is subject to macro expansion (see Section 7). The resulting string is the common presentation form of a fully qualified DNS name: a series of labels separated by periods. This domain is called the <target-name> in the rest of this document. Note: The result of the macro expansion is not subject to any further escaping. Hence, this facility cannot produce all characters that are legal in a DNS label (e.g., the control characters). However, this facility is powerful enough to express legal host names and common utility labels (such as "_spf") that are used in DNS. For several mechanisms, the <domain-spec> is optional. If it is not provided, the <domain> from the check_host() arguments (see Section 4.1) is used as the <target-name>. "domain" and <domain-spec> are syntactically identical after macro expansion. "domain" is an input value for check_host(), while <domain-spec> is computed by check_host(). The result of evaluating check_host() with a syntactically invalid domain is undefined. Note: This document and its predecessors make no provisions for defining correct handling of a syntactically invalid <domain-spec> (which might be the result of macro expansion), per [RFC1035]. Examples include names with empty labels, such as "foo..example.com", and labels that are longer than 63 characters. Some implementations choose to treat such errors as not-match and therefore ignore such names, while others return a "permerror" exception. Kitterman Standards Track [Page 19] RFC 7208 Sender Policy Framework (SPF) April 2014 5. Mechanism Definitions This section defines two types of mechanisms: basic language framework mechanisms and designated sender mechanisms. Basic mechanisms contribute to the language framework. They do not specify a particular type of authorization scheme. The basic mechanisms are as follows: all include Designated sender mechanisms are used to identify a set of <ip> addresses as being permitted or not permitted to use the <domain> for sending mail. The designated sender mechanisms are as follows: a mx ptr (do not use) ip4 ip6 exists The following conventions apply to all mechanisms that perform a comparison between <ip> and an IP address at any point: If no CIDR prefix length is given in the directive, then <ip> and the IP address are compared for equality. (Here, CIDR is Classless Inter-Domain Routing, described in [RFC4632].) If a CIDR prefix length is specified, then only the specified number of high-order bits of <ip> and the IP address are compared for equality. When any mechanism fetches host addresses to compare with <ip>, when <ip> is an IPv4, "A" records are fetched; when <ip> is an IPv6 address, "AAAA" records are fetched. SPF implementations on IPv6 servers need to handle both "AAAA" and "A" records, for clients on IPv4-mapped IPv6 addresses [RFC4291]. IPv4 <ip> addresses are only listed in an SPF record using the "ip4" mechanism. Several mechanisms rely on information fetched from the DNS. For these DNS queries, except where noted, if the DNS server returns an error (RCODE other than 0 or 3) or the query times out, the mechanism stops and the topmost check_host() returns "temperror". If the server returns "Name Error" (RCODE 3), then evaluation of the mechanism continues as if the server returned no error (RCODE 0) and zero answer records. Kitterman Standards Track [Page 20] RFC 7208 Sender Policy Framework (SPF) April 2014 5.1. "all" all = "all" The "all" mechanism is a test that always matches. It is used as the rightmost mechanism in a record to provide an explicit default. For example: v=spf1 a mx -all Mechanisms after "all" will never be tested. Mechanisms listed after "all" MUST be ignored. Any "redirect" modifier (Section 6.1) MUST be ignored when there is an "all" mechanism in the record, regardless of the relative ordering of the terms. 5.2. "include" include = "include" ":" domain-spec The "include" mechanism triggers a recursive evaluation of check_host(). 1. The <domain-spec> is expanded as per Section 7. 2. check_host() is evaluated with the resulting string as the <domain>. The <ip> and <sender> arguments remain the same as in the current evaluation of check_host(). 3. The recursive evaluation returns match, not-match, or an error. 4. If it returns match, then the appropriate result for the "include" mechanism is used (e.g., include or +include produces a "pass" result and -include produces "fail"). 5. If it returns not-match or an error, the parent check_host() resumes processing as per the table below, with the previous value of <domain> restored. In hindsight, the name "include" was poorly chosen. Only the evaluated result of the referenced SPF record is used, rather than literally including the mechanisms of the referenced record in the first. For example, evaluating a "-all" directive in the referenced record does not terminate the overall processing and does not necessarily result in an overall "fail". (Better names for this mechanism would have been "if-match", "on-match", etc.) Kitterman Standards Track [Page 21] RFC 7208 Sender Policy Framework (SPF) April 2014 The "include" mechanism makes it possible for one domain to designate multiple administratively independent domains. For example, a vanity domain "example.net" might send mail using the servers of administratively independent domains example.com and example.org. Example.net could say IN TXT "v=spf1 include:example.com include:example.org -all" This would direct check_host() to, in effect, check the records of example.com and example.org for a "pass" result. Only if the host were not permitted for either of those domains would the result be "fail". Whether this mechanism matches, does not match, or returns an exception depends on the result of the recursive evaluation of check_host(): +---------------------------------+---------------------------------+ | A recursive check_host() result | Causes the "include" mechanism | | of: | to: | +---------------------------------+---------------------------------+ | pass | match | | | | | fail | not match | | | | | softfail | not match | | | | | neutral | not match | | | | | temperror | return temperror | | | | | permerror | return permerror | | | | | none | return permerror | +---------------------------------+---------------------------------+ The "include" mechanism is intended for crossing administrative boundaries. When remaining within one administrative authority, "include" is usually not the best choice. For example, if example.com and example.org were managed by the same entity, and if the permitted set of hosts for both domains was "mx:example.com", it would be possible for example.org to specify "include:example.com", but it would be preferable to specify "redirect=example.com" or even "mx:example.com". Kitterman Standards Track [Page 22] RFC 7208 Sender Policy Framework (SPF) April 2014 With the "include" mechanism, an administratively external set of hosts can be authorized, but determination of sender policy is still a function of the original domain's SPF record (as determined by the "all" mechanism in that record). The "redirect" modifier is more suitable for consolidating both authorizations and policy into a common set to be shared within an ADMD. Redirect is much more like a common code element to be shared among records in a single ADMD. It is possible to control both authorized hosts and policy for an arbitrary number of domains from a single record. 5.3. "a" This mechanism matches if <ip> is one of the <target-name>'s IP addresses. For clarity, this means the "a" mechanism also matches AAAA records. a = "a" [ ":" domain-spec ] [ dual-cidr-length ] An address lookup is done on the <target-name> using the type of lookup (A or AAAA) appropriate for the connection type (IPv4 or IPv6). The <ip> is compared to the returned address(es). If any address matches, the mechanism matches. 5.4. "mx" This mechanism matches if <ip> is one of the MX hosts for a domain name. mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ] check_host() first performs an MX lookup on the <target-name>. Then it performs an address lookup on each MX name returned. The <ip> is compared to each returned IP address. To prevent denial-of-service (DoS) attacks, the processing limits defined in Section 4.6.4 MUST be followed. If the MX lookup limit is exceeded, then "permerror" is returned and the evaluation is terminated. If any address matches, the mechanism matches. Note regarding implicit MXes: If the <target-name> has no MX record, check_host() MUST NOT apply the implicit MX rules of [RFC5321] by querying for an A or AAAA record for the same name. 5.5. "ptr" (do not use) This mechanism tests whether the DNS reverse-mapping for <ip> exists and correctly points to a domain name within a particular domain. This mechanism SHOULD NOT be published. See the note at the end of this section for more information. Kitterman Standards Track [Page 23] RFC 7208 Sender Policy Framework (SPF) April 2014 ptr = "ptr" [ ":" domain-spec ] The <ip>'s name is looked up using this procedure: o Perform a DNS reverse-mapping for <ip>: Look up the corresponding PTR record in "in-addr.arpa." if the address is an IPv4 address and in "ip6.arpa." if it is an IPv6 address. o For each record returned, validate the domain name by looking up its IP addresses. To prevent DoS attacks, the PTR processing limits defined in Section 4.6.4 MUST be applied. If they are exceeded, processing is terminated and the mechanism does not match. o If <ip> is among the returned IP addresses, then that domain name is validated. Check all validated domain names to see if they either match the <target-name> domain or are a subdomain of the <target-name> domain. If any do, this mechanism matches. If no validated domain name can be found, or if none of the validated domain names match or are a subdomain of the <target-name>, this mechanism fails to match. If a DNS error occurs while doing the PTR RR lookup, then this mechanism fails to match. If a DNS error occurs while doing an A RR lookup, then that domain name is skipped and the search continues. This mechanism matches if o the <target-name> is a subdomain of a validated domain name, or o the <target-name> and a validated domain name are the same. For example, "mail.example.com" is within the domain "example.com", but "mail.bad-example.com" is not. Note: This mechanism is slow, it is not as reliable as other mechanisms in cases of DNS errors, and it places a large burden on the .arpa name servers. If used, proper PTR records have to be in place for the domain's hosts and the "ptr" mechanism SHOULD be one of the last mechanisms checked. After many years of SPF deployment experience, it has been concluded that it is unnecessary and more reliable alternatives should be used instead. It is, however, still in use as part of the SPF protocol, so compliant check_host() implementations MUST support it. Kitterman Standards Track [Page 24] RFC 7208 Sender Policy Framework (SPF) April 2014 5.6. "ip4" and "ip6" These mechanisms test whether <ip> is contained within a given IP network. ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ] ip4-cidr-length = "/" ("0" / %x31-39 0*1DIGIT) ; value range 0-32 ip6-cidr-length = "/" ("0" / %x31-39 0*2DIGIT) ; value range 0-128 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] ip4-network = qnum "." qnum "." qnum "." qnum qnum = DIGIT ; 0-9 / %x31-39 DIGIT ; 10-99 / "1" 2DIGIT ; 100-199 / "2" %x30-34 DIGIT ; 200-249 / "25" %x30-35 ; 250-255 ; as per conventional dotted-quad notation, e.g., 192.0.2.0 ip6-network = <as per Section 2.2 of [RFC4291]> ; e.g., 2001:db8::cd30 The <ip> is compared to the given network. If CIDR prefix length high-order bits match, the mechanism matches. If ip4-cidr-length is omitted, it is taken to be "/32". If ip6-cidr-length is omitted, it is taken to be "/128". It is not permitted to omit parts of the IP address instead of using CIDR notations. That is, use 192.0.2.0/24 instead of 192.0.2. 5.7. "exists" This mechanism is used to construct an arbitrary domain name that is used for a DNS A record query. It allows for complicated schemes involving arbitrary parts of the mail envelope to determine what is permitted. exists = "exists" ":" domain-spec The <domain-spec> is expanded as per Section 7. The resulting domain name is used for a DNS A RR lookup (even when the connection type is IPv6). If any A record is returned, this mechanism matches. Domains can use this mechanism to specify arbitrarily complex queries. For example, suppose example.com publishes the record: v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all Kitterman Standards Track [Page 25] RFC 7208 Sender Policy Framework (SPF) April 2014 The <target-name> might expand to "1.2.0.192.someuser._spf.example.com". This makes fine-grained decisions possible at the level of the user and client IP address. 6. Modifier Definitions Modifiers are name/value pairs that provide additional information. Modifiers always have an "=" separating the name and the value. The modifiers defined in this document ("redirect" and "exp") SHOULD appear at the end of the record, after all mechanisms, though syntactically they can appear anywhere in the record. Ordering of these two modifiers does not matter. These two modifiers MUST NOT appear in a record more than once each. If they do, then check_host() exits with a result of "permerror". Unrecognized modifiers MUST be ignored no matter where, or how often, they appear in a record. This allows implementations conforming to this document to gracefully handle records with modifiers that are defined in other specifications. 6.1. redirect: Redirected Query The "redirect" modifier is intended for consolidating both authorizations and policy into a common set to be shared within a single ADMD. It is possible to control both authorized hosts and policy for an arbitrary number of domains from a single record. redirect = "redirect" "=" domain-spec If all mechanisms fail to match, and a "redirect" modifier is present, then processing proceeds as follows: The <domain-spec> portion of the redirect section is expanded as per the macro rules in Section 7. Then check_host() is evaluated with the resulting string as the <domain>. The <ip> and <sender> arguments remain the same as in the current evaluation of check_host(). The result of this new evaluation of check_host() is then considered the result of the current evaluation with the exception that if no SPF record is found, or if the <target-name> is malformed, the result is a "permerror" rather than "none". Note that the newly queried domain can itself specify redirect processing. Kitterman Standards Track [Page 26] RFC 7208 Sender Policy Framework (SPF) April 2014 This facility is intended for use by organizations that wish to apply the same record to multiple domains. For example: la.example.com. TXT "v=spf1 redirect=_spf.example.com" ny.example.com. TXT "v=spf1 redirect=_spf.example.com" sf.example.com. TXT "v=spf1 redirect=_spf.example.com" _spf.example.com. TXT "v=spf1 mx:example.com -all" In this example, mail from any of the three domains is described by the same record. This can be an administrative advantage. Note: In general, the domain "A" cannot reliably use a redirect to another domain "B" not under the same administrative control. Since the <sender> stays the same, there is no guarantee that the record at domain "B" will correctly work for mailboxes in domain "A", especially if domain "B" uses mechanisms involving local-parts. An "include" directive will generally be more appropriate. For clarity, any "redirect" modifier SHOULD appear as the very last term in a record. Any "redirect" modifier MUST be ignored if there is an "all" mechanism anywhere in the record. 6.2. exp: Explanation explanation = "exp" "=" domain-spec If check_host() results in a "fail" due to a mechanism match (such as "-all"), and the "exp" modifier is present, then the explanation string returned is computed as described below. If no "exp" modifier is present, then either a default explanation string or an empty explanation string MUST be returned to the calling application. The <domain-spec> is macro expanded (see Section 7) and becomes the <target-name>. The DNS TXT RRset for the <target-name> is fetched. If there are any DNS processing errors (any RCODE other than 0), or if no records are returned, or if more than one record is returned, or if there are syntax errors in the explanation string, then proceed as if no "exp" modifier was given. The fetched TXT record's strings are concatenated with no spaces, and then treated as an explain-string, which is macro-expanded. This final result is the explanation string. Implementations MAY limit the length of the resulting explanation string to allow for other protocol constraints and/or reasonable processing limits. Since the explanation string is intended for an SMTP response and Section 2.4 of [RFC5321] says that responses are in [US-ASCII], the explanation string MUST be limited to [US-ASCII]. Kitterman Standards Track [Page 27] RFC 7208 Sender Policy Framework (SPF) April 2014 Software evaluating check_host() can use this string to communicate information from the publishing domain in the form of a short message or URL. Software SHOULD make it clear that the explanation string comes from a third party. For example, it can prepend the macro string "%{o} explains: " to the explanation, as shown in the example in Section 8.4. Suppose example.com has this record: v=spf1 mx -all exp=explain._spf.%{d} Here are some examples of possible explanation TXT records at explain._spf.example.com: "Mail from example.com should only be sent by its own servers." -- a simple, constant message "%{i} is not one of %{d}'s designated mail servers." -- a message with a little more information, including the IP address that failed the check "See http://%{d}/why.html?s=%{S}&i=%{I}" -- a complicated example that constructs a URL with the arguments to check_host() so that a web page can be generated with detailed, custom instructions Note: During recursion into an "include" mechanism, an "exp" modifier from the <target-name> MUST NOT be used. In contrast, when executing a "redirect" modifier, an "exp" modifier from the original domain MUST NOT be used. This is because "include" is meant to cross administrative boundaries and the explanation provided should be the one from the receiving ADMD, while "redirect" is meant to operate as a tool to consolidate policy records within an ADMD so the redirected explanation is the one that ought to have priority. 7. Macros When evaluating an SPF policy record, certain character sequences are intended to be replaced by parameters of the message or of the connection. These character sequences are referred to as "macros". Kitterman Standards Track [Page 28] RFC 7208 Sender Policy Framework (SPF) April 2014 7.1. Formal Specification The ABNF description for a macro is as follows: domain-spec = macro-string domain-end domain-end = ( "." toplabel [ "." ] ) / macro-expand toplabel = ( *alphanum ALPHA *alphanum ) / ( 1*alphanum "-" *( alphanum / "-" ) alphanum ) alphanum = ALPHA / DIGIT explain-string = *( macro-string / SP ) macro-string = *( macro-expand / macro-literal ) macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) / "%%" / "%_" / "%-" macro-literal = %x21-24 / %x26-7E ; visible characters except "%" macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / "c" / "r" / "t" / "v" transformers = *DIGIT [ "r" ] delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" The "toplabel" construction is subject to the letter-digit-hyphen (LDH) rule plus additional top-level domain (TLD) restrictions. See Section 2 of [RFC3696] for background. Some special cases: o A literal "%" is expressed by "%%". o "%_" expands to a single " " space. o "%-" expands to a URL-encoded space, viz., "%20". 7.2. Macro Definitions The following macro letters are expanded in term arguments: s = <sender> l = local-part of <sender> o = domain of <sender> d = <domain> i = <ip> p = the validated domain name of <ip> (do not use) v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6 h = HELO/EHLO domain Kitterman Standards Track [Page 29] RFC 7208 Sender Policy Framework (SPF) April 2014 <domain>, <sender>, and <ip> are defined in Section 4.1. The following macro letters are allowed only in "exp" text: c = SMTP client IP (easily readable format) r = domain name of host performing the check t = current timestamp 7.3. Macro Processing Details A '%' character not followed by a '{', '%', '-', or '_' character is a syntax error. So: -exists:%(ir).sbl.example.org is incorrect and will cause check_host() to yield a "permerror". Instead, the following is legal: -exists:%{ir}.sbl.example.org Optional transformers are the following: *DIGIT = zero or more digits 'r' = reverse value, splitting on dots by default If transformers or delimiters are provided, the replacement value for a macro letter is split into parts separated by one or more of the specified delimiter characters. After performing any reversal operation and/or removal of left-hand parts, the parts are rejoined using "." and not the original splitting characters. By default, strings are split on "." (dots). Note that no special treatment is given to leading, trailing, or consecutive delimiters in input strings, and so the list of parts might contain empty strings. Some older implementations of SPF prohibit trailing dots in domain names, so trailing dots SHOULD NOT be published, although they MUST be accepted by implementations conforming to this document. Macros can specify delimiter characters that are used instead of ".". The "r" transformer indicates a reversal operation: if the client IP address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1" and the macro %{ir} would expand to "1.2.0.192". The DIGIT transformer indicates the number of right-hand parts to use, after optional reversal. If a DIGIT is specified, the value MUST be nonzero. If no DIGITs are specified, or if the value specifies more parts than are available, all the available parts are Kitterman Standards Track [Page 30] RFC 7208 Sender Policy Framework (SPF) April 2014 used. If the DIGIT was 5, and only 3 parts were available, the macro interpreter would pretend the DIGIT was 3. Implementations MUST support at least a value of 127, as that is the maximum number of labels in a domain name (less the zero-length label at the end). The "s" macro expands to the <sender> argument. It is an email address with a local-part, an "@" character, and a domain. The "l" macro expands to just the local-part. The "o" macro expands to just the domain part. Note that these values remain the same during recursive and chained evaluations due to "include" and/or "redirect". Note also that if the original <sender> had no local-part, the local-part was set to "postmaster" in initial processing (see Section 4.3). For IPv4 addresses, both the "i" and "c" macros expand to the standard dotted-quad format. For IPv6 addresses, the "i" macro expands to a dot-format address; it is intended for use in %{ir}. The "c" macro can expand to any of the hexadecimal colon-format addresses specified in Section 2.2 of [RFC4291]. It is intended for humans to read. The "p" macro expands to the validated domain name of <ip>. The procedure for finding the validated domain name is defined in Section 5.5. If the <domain> is present in the list of validated domains, it SHOULD be used. Otherwise, if a subdomain of the <domain> is present, it SHOULD be used. Otherwise, any name from the list can be used. If there are no validated domain names or if a DNS error occurs, the string "unknown" is used. This macro SHOULD NOT be published (see Section 5.5 for the discussion). The "h" macro expands to the parameter that was provided to the SMTP server via the HELO or EHLO SMTP verb. For sessions where that verb was provided more than once, the most recent instance is used. The "r" macro expands to the name of the receiving MTA. This SHOULD be a fully qualified domain name, but if one does not exist (as when the checking is done by a Mail User Agent (MUA)) or if policy restrictions dictate otherwise, the word "unknown" SHOULD be substituted. The domain name can be different from the name found in the MX record that the client MTA used to locate the receiving MTA. Kitterman Standards Track [Page 31] RFC 7208 Sender Policy Framework (SPF) April 2014 The "t" macro expands to the decimal representation of the approximate number of seconds since the Epoch (Midnight, January 1, 1970, UTC) at the time of the evaluation. This is the same value as the value that is returned by the Portable Operating System Interface (POSIX) time() function in most standards-compliant libraries. When the result of macro expansion is used in a domain name query, if the expanded domain name exceeds 253 characters (the maximum length of a domain name in this format), the left side is truncated to fit, by removing successive domain labels (and their following dots) until the total length does not exceed 253 characters. Uppercase macros expand exactly as their lowercase equivalents, and are then URL escaped. URL escaping MUST be performed for characters not in the "unreserved" set, which is defined in [RFC3986]. Care has to be taken by the sending ADMD so that macro expansion for legitimate email does not exceed the 63-character limit on DNS labels. The local-part of email addresses, in particular, can have more than 63 characters between dots. To minimize DNS lookup resource requirements, it is better if sending ADMDs avoid using the "s", "l", "o", or "h" macros in conjunction with any mechanism directive. Although these macros are powerful and allow per-user records to be published, they severely limit the ability of implementations to cache results of check_host() and they reduce the effectiveness of DNS caches. If no directive processed during the evaluation of check_host() contains an "s", "l", "o", or "h" macro, then the results of the evaluation can be cached on the basis of <domain> and <ip> alone for as long as the DNS record involved with the shortest Time to Live (TTL) has not expired. 7.4. Expansion Examples The <sender> is strong-bad@email.example.com. The IPv4 SMTP client IP is 192.0.2.3. The IPv6 SMTP client IP is 2001:db8::cb01. The PTR domain name of the client IP is mx.example.org. Kitterman Standards Track [Page 32] RFC 7208 Sender Policy Framework (SPF) April 2014 macro expansion ------- ---------------------------- %{s} strong-bad@email.example.com %{o} email.example.com %{d} email.example.com %{d4} email.example.com %{d3} email.example.com %{d2} example.com %{d1} com %{dr} com.example.email %{d2r} example.email %{l} strong-bad %{l-} strong.bad %{lr} strong-bad %{lr-} bad.strong %{l1r-} strong macro-string expansion -------------------------------------------------------------------- %{ir}.%{v}._spf.%{d2} 3.2.0.192.in-addr._spf.example.com %{lr-}.lp._spf.%{d2} bad.strong.lp._spf.example.com %{lr-}.lp.%{ir}.%{v}._spf.%{d2} bad.strong.lp.3.2.0.192.in-addr._spf.example.com %{ir}.%{v}.%{l1r-}.lp._spf.%{d2} 3.2.0.192.in-addr.strong.lp._spf.example.com %{d2}.trusted-domains.example.net example.com.trusted-domains.example.net IPv6: %{ir}.%{v}._spf.%{d2} 1.0.b.c.0.0.0.0. 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 8. Result Handling This section provides guidance for SPF verifier operators in response to the various possible outputs of check_host() on a message. Definitions of SPF results are presented in Section 2.6; this section provides more detail on each for use in developing local policy for message handling. Every operating environment is different. There are some receivers for whom strict adherence to SPF is appropriate, and definitive treatment of messages that are evaluated to be explicitly unauthorized ("fail" and sometimes "softfail") is the norm. There are others for which the "false negative" cases are more of a Kitterman Standards Track [Page 33] RFC 7208 Sender Policy Framework (SPF) April 2014 concern. This concern is typically handled by merely recording the result in the header and allowing the message to pass on for additional processing. There are still others where SPF is one of several inputs to the message-handling decision. As such, there is no comprehensive normative requirement for message handling in response to any particular result. This section is provided to present a complete picture of the likely cause of each result and, where available, the experience gained during experimental deployment. There are essentially two classes of handling choices: o Handling within the SMTP session that attempted to deliver the message, such as by returning a permanent SMTP error (rejection) or temporary SMTP error ("try again later"); o Permitting the message to pass (a successful SMTP reply code) and adding an additional header field that indicates the result returned by check_host() and other salient details; this is discussed in more detail in Section 9. 8.1. None With a "none" result, the SPF verifier has no information at all about the authorization or lack thereof of the client to use the checked identity or identities. The check_host() function completed without errors but was not able to reach any conclusion. 8.2. Neutral A "neutral" result indicates that although a policy for the identity was discovered, there is no definite assertion (positive or negative) about the client. A "neutral" result MUST be treated exactly like the "none" result; the distinction exists only for informational purposes. Treating "neutral" more harshly than "none" would discourage ADMDs from testing the use of SPF records (see Section 10.1). 8.3. Pass A "pass" result means the client is authorized to inject mail with the given identity. The domain can now, in the sense of reputation, be considered responsible for sending the message. Further policy checks can now proceed with confidence in the legitimate use of the identity. This is further discussed in Appendix G.1. Kitterman Standards Track [Page 34] RFC 7208 Sender Policy Framework (SPF) April 2014 8.4. Fail A "fail" result is an explicit statement that the client is not authorized to use the domain in the given identity. Disposition of SPF fail messages is a matter of local policy. See Appendix G.2 for considerations on developing local policy. If the checking software chooses to reject the mail during the SMTP transaction, then it SHOULD use an SMTP reply code of 550 (see [RFC5321]) and, if supported, the 5.7.1 enhanced status code (see [RFC3463], Section 3.8), in addition to an appropriate reply text. The check_host() function will return either a default explanation string or one from the domain that published the SPF records (see Section 6.2). If the information does not originate with the checking software, it is good to make it clear that the text is provided by the sender's domain. For example: 550 5.7.1 SPF MAIL FROM check failed: 550 5.7.1 The domain example.com explains: 550 5.7.1 Please see http://www.example.com/mailpolicy.html If the checking software chooses not to reject the mail during the SMTP transaction, then it SHOULD add a Received-SPF or Authentication-Results header field (see Section 9) to communicate this result to downstream message processors. While this is true for all SPF results, it is of particular importance for "fail" results since the message is explicitly not authorized by the ADMD. 8.5. Softfail A "softfail" result ought to be treated as somewhere between "fail" and "neutral"/"none". The ADMD believes the host is not authorized but is not willing to make a strong policy statement. Receiving software SHOULD NOT reject the message based solely on this result, but MAY subject the message to closer scrutiny than normal. The ADMD wants to discourage the use of this host and thus desires limited feedback when a "softfail" result occurs. For example, the recipient's MUA could highlight the "softfail" status, or the receiving MTA could give the sender a message using greylisting [RFC6647], with a note the first time the message is received, but accept it on a later attempt based on receiver policy. Kitterman Standards Track [Page 35] RFC 7208 Sender Policy Framework (SPF) April 2014 8.6. Temperror A "temperror" result means the SPF verifier encountered a transient (generally DNS) error while performing the check. Checking software can choose to accept or temporarily reject the message. If the message is rejected during the SMTP transaction for this reason, the software SHOULD use an SMTP reply code of 451 and, if supported, the 4.4.3 enhanced status code (see Section 3.5 of [RFC3463]). These errors can be caused by problems in either the sender's or receiver's DNS software. See Appendix G.4 for considerations on developing local policy. 8.7. Permerror A "permerror" result means the domain's published records could not be correctly interpreted. This signals an error condition that definitely requires DNS operator intervention to be resolved. If the message is rejected during the SMTP transaction for this reason, the software SHOULD use an SMTP reply code of 550 and, if supported, the 5.5.2 enhanced status code (see [RFC3463], Section 3.6). Be aware that if the ADMD uses macros (Section 7), it is possible that this result is due to the checked identities having an unexpected format. It is also possible that this result is generated by certain SPF verifiers due to the input arguments having an unexpected format; see Section 4.8. See Appendix G.3 for considerations on developing local policy. 9. Recording the Result To provide downstream agents, such as MUAs, with the information they might need in terms of evaluating or representing the apparent safety of the message content, it is RECOMMENDED that SMTP receivers record the result of SPF processing in the message header. For SPF verifier operators that choose to record SPF results in the header of the message for processing by internal filters or MUAs, two methods are presented: Section 9.1 defines the Received-SPF field, which is the results field originally defined for SPF use. Section 9.2 discusses the Authentication-Results header field [RFC7001], which was specified more recently and is designed for use by SPF and other authentication methods. Both are in common use, and hence both are included here. However, it is important to note that they were designed to serve slightly different purposes. Received-SPF is intended to include enough information to enable reconstruction of the SPF evaluation of the message, while Authentication-Results is designed only to relay the result itself and related output details of likely use to end users (e.g., what property of the message was actually authenticated and Kitterman Standards Track [Page 36] RFC 7208 Sender Policy Framework (SPF) April 2014 what it contained), leaving reconstructive work to the purview of system logs and the Received field contents. Also, Received-SPF relies on compliance of agents within the receiving ADMD to adhere to the header field ordering rules of [RFC5321] and [RFC5322], while Authentication-Results includes some provisions to protect against non-compliant implementations. An SPF verifier operator could choose to use both to serve different downstream agents. In such cases, care needs to be taken to ensure that both fields are conveying the same details, or unexpected results can occur. 9.1. The Received-SPF Header Field The Received-SPF header field is a trace field (see [RFC5322], Section 3.6.7) and SHOULD be prepended to the existing header, above the Received: field that is generated by the SMTP receiver. It MUST appear above all other Received-SPF fields in the message. The header field has the following format: header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] [ key-value-list ] CRLF result = "pass" / "fail" / "softfail" / "neutral" / "none" / "temperror" / "permerror" key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) [";"] key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) key = "client-ip" / "envelope-from" / "helo" / "problem" / "receiver" / "identity" / "mechanism" / name identity = "mailfrom" ; for the "MAIL FROM" identity / "helo" ; for the "HELO" identity / name ; other identities dot-atom = <unquoted word as per [RFC5322]> quoted-string = <quoted string as per [RFC5322]> comment = <comment string as per [RFC5322]> CFWS = <comment or folding white space as per [RFC5322]> FWS = <folding white space as per [RFC5322]> CRLF = <standard end-of-line token as per [RFC5322]> Kitterman Standards Track [Page 37] RFC 7208 Sender Policy Framework (SPF) April 2014 The header field SHOULD include a "(...)" style comment after the result, conveying supporting information for the result, such as <ip>, <sender>, and <domain>. The following key-value pairs are designed for later machine parsing. SPF verifiers SHOULD give enough information so that the SPF results can be verified -- that is, at least "client-ip", "helo", and, if the "MAIL FROM" identity was checked, "envelope-from". client-ip the IP address of the SMTP client envelope-from the envelope sender mailbox helo the host name given in the HELO or EHLO command mechanism the mechanism that matched (if no mechanisms matched, substitute the word "default") problem if an error was returned, details about the error receiver the host name of the SPF verifier identity the identity that was checked; see the <identity> ABNF rule Other keys MAY be defined by SPF verifiers. SPF verifiers MUST make sure that the Received-SPF header field does not contain invalid characters, is not excessively long (see [RFC5322], Section 2.1.1), and does not contain malicious data that has been provided by the sender. Examples of various header field styles that could be generated are the following: Received-SPF: pass (mybox.example.org: domain of myname@example.com designates 192.0.2.1 as permitted sender) receiver=mybox.example.org; client-ip=192.0.2.1; envelope-from="myname@example.com"; helo=foo.example.com; Received-SPF: fail (mybox.example.org: domain of myname@example.com does not designate 192.0.2.1 as permitted sender) identity=mailfrom; client-ip=192.0.2.1; envelope-from="myname@example.com"; Kitterman Standards Track [Page 38] RFC 7208 Sender Policy Framework (SPF) April 2014 Received-SPF: pass (mybox.example.org: domain of myname@example.com designates 192.0.2.1 as permitted sender) receiver=mybox.example.org; client-ip=192.0.2.1; mechanism=ip4:192.0.2.1; envelope-from="myname@example.com"; helo=foo.example.com; 9.2. SPF Results in the Authentication-Results Header Field As mentioned in Section 9, the Authentication-Results header field is designed to communicate lists of tests a border MTA did and their results. The specified elements of the field provide less information than the Received-SPF field: Authentication-Results: myhost.example.org; spf=pass smtp.mailfrom=example.net Received-SPF: pass (myhost.example.org: domain of myname@example.com designates 192.0.2.1 as permitted sender) receiver=mybox.example.org; client-ip=192.0.2.1; envelope-from="myname@example.com"; helo=foo.example.com; It is, however, possible to add CFWS in the "reason" part of an Authentication-Results header field and provide the equivalent information, if desired. As an example, an expanded Authentication-Results header field might look like (for a "MAIL FROM" check in this example): Authentication-Results: myhost.example.org; spf=pass reason="client-ip=192.0.2.1; smtp.helo=foo.example.com" smtp.mailfrom=user@example.net 10. Effects on Infrastructure This section outlines the major implications that adoption of this protocol will have on various entities involved in Internet email. It is intended to make clear to the reader where this protocol knowingly affects the operation of such entities. This section is not a "how-to" manual, or a "best practices" document, and it is not a comprehensive list of what such entities ought to do in light of this specification. This section provides operational advice and instruction only. It is non-normative. [RFC5598] describes the Internet email architecture. This section is organized based on the different segments of the architecture. Kitterman Standards Track [Page 39] RFC 7208 Sender Policy Framework (SPF) April 2014 10.1. Sending Domains Originating ADMDs (ADministrative Management Domains -- Sections 2.2.1 and 2.3 of [RFC5598]) that wish to be compliant with this specification will need to determine the list of relays ([RFC5598], Section 2.2.2) that they allow to use their domain name in the "HELO" and "MAIL FROM" identities when relaying to other ADMDs. It is recognized that forming such a list is not just a simple technical exercise, but involves policy decisions with both technical and administrative considerations. 10.1.1. DNS Resource Considerations Minimizing the DNS resources needed for SPF lookups can be done by choosing directives that require less DNS information and by placing lower-cost mechanisms earlier in the SPF record. Section 4.6.4 specifies the limits receivers have to use. It is essential to publish records that do not exceed these requirements. It is also required to carefully weigh the cost and the maintainability of licit solutions. For example, consider a domain set up as follows: example.com. IN MX 10 mx.example.com. IN MX 20 mx2.example.com. mx.example.com. IN A 192.0.2.1 mx2.example.com. IN A 192.0.2.129 Assume the administrative point is to authorize (pass) mx and mx2 while failing every other host. Compare the following solutions: Best record: example.com. IN TXT "v=spf1 ip4:192.0.2.1 ip4:192.0.2.129 -all" Good record: $ORIGIN example.com. @ IN TXT "v=spf1 a:authorized-spf.example.com -all" authorized-spf IN A 192.0.2.1 IN A 192.0.2.129 Expensive record: example.com. IN TXT "v=spf1 mx:example.com -all" Wasteful, bad record: example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 mx -all" Kitterman Standards Track [Page 40] RFC 7208 Sender Policy Framework (SPF) April 2014 10.1.2. Administrator's Considerations There might be administrative considerations: using "a" over "ip4" or "ip6" allows hosts to be renumbered easily at the cost of a DNS query per receiver. Using "mx" over "a" allows the set of mail hosts to be changed easily. Unless such changes are common, it is better to use the less resource-intensive mechanisms like "ip4" and "ip6" over "a" or "a" over "mx". In some specific cases, standard advice on record content is appropriate. Publishing SPF records for domains that send no mail is a well-established best practice. The record for a domain that sends no mail is: www.example.com. IN TXT "v=spf1 -all" Publishing SPF records for individual hosts is also best practice. The host name is generally the identity used in the 5321.HELO/.EHLO command. In the case of messages with a null 5321.MailFrom, this is used as the domain for 5321.MailFrom SPF checks, in addition to being used in 5321.HELO/.EHLO-based SPF checks. The standard SPF record for an individual host that is involved in mail processing is: relay.example.com. IN TXT "v=spf1 a -all" Validating correct deployment is difficult. [RFC6652] describes one mechanism for soliciting feedback on SPF failures. Another suggestion can be found in Appendix C. Regardless of the method used, understanding the ADMD's outbound mail architecture is essential to effective deployment. 10.1.3. Bounces As explained in Section 2.4, [RFC5321] allows the MAIL FROM to be null, which is typical of some Delivery Status Notifications [RFC3464], commonly called email bounces. In this case, the only entity available for performing an SPF check is the "HELO" identity defined in Section 1.1.4. SPF functionality is enhanced by administrators ensuring this identity is set correctly and has an appropriate SPF record. It is normal to have the "HELO" identity set to the host name instead of the domain. Zone file generation for significant numbers of hosts can be consolidated using the "redirect" modifier and scripted for initial deployment. Specific deployment advice is given above in Section 10.1.2. Kitterman Standards Track [Page 41] RFC 7208 Sender Policy Framework (SPF) April 2014 10.2. Receivers SPF results can be used in combination with other methods to determine the final local disposition (either positive or negative) of a message. It can also be considered dispositive on its own. An attempt to have one organization (sender) direct the email- handling policies of another (receiver) is inherently challenging and often controversial. As stated elsewhere in this document, there is no comprehensive normative requirement for specific handling of a message based on SPF results. The information presented in Section 8 and in Appendix G is offered for receiver consideration when forming local handling policies. The primary considerations are that SPF might return "pass" for mail that is ultimately harmful (e.g., spammers that arrange for SPF to pass using disposable domain names, or virus or spam outbreaks from within trusted sources), and might also return "fail" for mail that is ultimately legitimate (e.g., legitimate mail that has traversed a mail alias). It is important to take both of these cases under consideration when establishing local handling policy. 10.3. Mediators Mediators are a type of User Actor [RFC5598]. That is, a mediator takes 'delivery' of a message and posts a 'submission' of a new message. The mediator can make the newly posted message be as similar to or as different from the original message as they wish. Examples include mailing lists (see Section 5.3 of [RFC5598]) and ReSenders (Section 5.2 of [RFC5598]). This is discussed in [RFC5321], Section 3.9. For the operation of SPF, the essential concern is the email address in the 5321.MailFrom command for the new message. Because SPF evaluation is based on the IP address of the "last" sending SMTP server, the address of the mediator will be used, rather than the address of the SMTP server that sent the message to the mediator. Some mediators retain the email address from the original message, while some use a new address. If the address is the same as for the original message, and the original message had an associated SPF record, then the SPF evaluation will fail unless mitigations such as those described in Appendix D are used. Kitterman Standards Track [Page 42] RFC 7208 Sender Policy Framework (SPF) April 2014 11. Security Considerations 11.1. Processing Limits As with most aspects of email, there are a number of ways that malicious parties could use the protocol as an avenue for a DoS attack. The processing limits outlined in Section 4.6.4 are designed to prevent attacks such as the following: o A malicious party could create an SPF record with many references to a victim's domain and send many emails to different SPF verifiers; those SPF verifiers would then create a DoS attack. In effect, the SPF verifiers are being used to amplify the attacker's bandwidth by using fewer octets in the SMTP session than are used by the DNS queries. Using SPF verifiers also allows the attacker to hide the true source of the attack. This potential attack is based on large volumes of mail being transmitted. o Whereas implementations of check_host() are supposed to limit the number of DNS lookups, malicious domains could publish records that exceed these limits in an attempt to waste computation effort at their targets when they send them mail. Malicious domains could also design SPF records that cause particular implementations to use excessive memory or CPU or to trigger bugs. If a receiver is configured to accept mail with an SPF result of "temperror", such an attack might result in mail that would otherwise have been rejected due to an SPF "fail" result being accepted. This potential attack is based on specially crafted SPF records being used to exhaust DNS resources of the victim. o Malicious parties could send a large volume of mail purporting to come from the intended target to a wide variety of legitimate mail hosts. These legitimate machines would then present a DNS load on the target as they fetched the relevant records. o Malicious parties could, in theory, use SPF records as a vehicle for DNS lookup amplification for a DoS attack. In this scenario, the attacker publishes an SPF record in its own DNS that uses "a" and "mx" mechanisms directed toward the intended victim, e.g., "a:example.com a:foo.example.com a:bar.example.com ..." and then distributes mail with a MAIL FROM value including its own domain in large volume to a wide variety of destinations. Any such destination operating an SPF verifier will begin querying all of the names associated with the "a" mechanisms in that record. The names used in the record needn't exist for the attack to be effective. Operational experience since the publication of [RFC4408] suggests that mitigation of this class of attack can be accomplished with minimal impact on the deployed base by having Kitterman Standards Track [Page 43] RFC 7208 Sender Policy Framework (SPF) April 2014 the verifier abort processing and return "permerror" (Section 2.6.7) as soon as more than two "void lookups" have been encountered (defined in Section 4.6.4). Of these, the case of a third party referenced in the SPF record is the easiest for a DoS attack to effectively exploit. As a result, limits that might seem reasonable for an individual mail server can still allow an unreasonable amount of bandwidth amplification. Therefore, the processing limits need to be quite low. 11.2. SPF-Authorized Email May Contain Other False Identities The "MAIL FROM" and "HELO" identity authorizations do not provide assurance about the authorization/authenticity of other identities used in the message. It is entirely possible for a malicious sender to inject a message using his own domain in the identities used by SPF and have that domain's SPF record authorize the sending host, and yet the message can easily list other identities in its header. Unless the user or the MUA takes care to note that the authorized identity does not match the other more commonly presented identities (such as the From: header field), the user might be lulled into a false sense of security. 11.3. Spoofed DNS and IP Data There are two aspects of this protocol that malicious parties could exploit to undermine the validity of the check_host() function: o The evaluation of check_host() relies heavily on DNS. A malicious attacker could attack the DNS infrastructure and cause check_host() to see spoofed DNS data, and then return incorrect results. This could include returning "pass" for an <ip> value where the actual domain's record would evaluate to "fail". See [RFC3833] for a description of DNS weaknesses, and see [RFC4033] for a countermeasure. o The client IP address, <ip>, is assumed to be correct. In a modern, correctly configured system, the risk of this not being true is nil. 11.4. Cross-User Forgery By definition, SPF policies just map domain names to sets of authorized MTAs, not whole email addresses to sets of authorized users. Although the "l" macro (Section 7) provides a limited way to define individual sets of authorized MTAs for specific email addresses, it is generally impossible to verify, through SPF, the use of specific email addresses by individual users of the same MTA. Kitterman Standards Track [Page 44] RFC 7208 Sender Policy Framework (SPF) April 2014 It is up to mail services and their MTAs to directly prevent cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be restricted to using only those email addresses that are actually under their control (see Section 6.1 of [RFC6409]). Another means to verify the identity of individual users is message cryptography, such as Pretty Good Privacy (PGP) ([RFC4880]) or S/MIME ([RFC5751]). 11.5. Untrusted Information Sources An SPF-compliant receiver gathers information from the SMTP commands it receives and from the published DNS records of the sending domain holder (e.g., "HELO" domain name, the "MAIL FROM" address from the envelope, and SPF DNS records published by the domain holder). These parameters are not validated in the SMTP process. All of these pieces of information are generated by actors outside of the authority of the receiver, and thus are not guaranteed to be accurate or legitimate. 11.5.1. Recorded Results This information, passed to the receiver in the Received-SPF: or Authentication-Results: trace fields, can be returned to the client MTA as an SMTP rejection message. If such an SMTP rejection message is generated, the information from the trace fields has to be checked for such problems as invalid characters and excessively long lines. 11.5.2. External Explanations When the authorization check fails, an explanation string could be included in the reject response. Both the sender and the rejecting receiver need to be aware that the explanation was determined by the publisher of the SPF record checked and, in general, not the receiver. The explanation can contain malicious URLs, or it might be offensive or misleading. Explanations returned to sender domains due to "exp" modifiers (Section 6.2) were generated by the sender policy published by the domain holders themselves. As long as messages are only returned with non-delivery notifications ([RFC3464]) to domains publishing the explanation strings from their own DNS SPF records, the only affected parties are the original publishers of the domain's SPF records. In practice, such non-delivery notifications can be misdirected, such as when an MTA accepts an email and only later generates the notification to a forged address, or when an email forwarder does not direct the bounce back to the original sender. Kitterman Standards Track [Page 45] RFC 7208 Sender Policy Framework (SPF) April 2014 11.5.3. Macro Expansion Macros (Section 7) allow senders to inject arbitrary text (any non-null [US-ASCII] character) into receiver DNS queries. It is necessary to be prepared for hostile or unexpected content. 11.6. Privacy Exposure Checking SPF records causes DNS queries to be sent to the domain owner. These DNS queries, especially if they are caused by the "exists" mechanism, can contain information about who is sending email and likely to which MTA the email is being sent. This can introduce some privacy concerns, which are more or less of an issue depending on local laws and the relationship between the ADMD and the person sending the email. 11.7. Delivering Mail Producing a "Fail" Result Operators that choose to deliver mail for which SPF produces a "fail" result need to understand that they are admitting content that is explicitly not authorized by the purported sender. While there are known failure modes that can be considered "false negatives", the distinct choice to admit those messages increases end-user exposure to likely harm. This is especially true for domains belonging to known good actors that are typically well-behaved; unauthorized mail from those sources might well be subjected to much higher skepticism and content analysis. SPF does not, however, include the capacity to distinguish good actors from bad ones, nor does it handle the concept of known actors versus unknown ones. Those notions are out of scope for this specification. 12. Collected ABNF This section is normative, and any discrepancies with the ABNF fragments in the preceding text are to be resolved in favor of this grammar. See [RFC5234] for ABNF notation. Please note that as per this ABNF definition, literal text strings (those in quotes) are case- insensitive. Hence, "mx" matches "mx", "MX", "mX", and "Mx". record = version terms *SP version = "v=spf1" terms = *( 1*SP ( directive / modifier ) ) Kitterman Standards Track [Page 46] RFC 7208 Sender Policy Framework (SPF) April 2014 directive = [ qualifier ] mechanism qualifier = "+" / "-" / "?" / "~" mechanism = ( all / include / a / mx / ptr / ip4 / ip6 / exists ) all = "all" include = "include" ":" domain-spec a = "a" [ ":" domain-spec ] [ dual-cidr-length ] mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ] ptr = "ptr" [ ":" domain-spec ] ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ] exists = "exists" ":" domain-spec modifier = redirect / explanation / unknown-modifier redirect = "redirect" "=" domain-spec explanation = "exp" "=" domain-spec unknown-modifier = name "=" macro-string ; where name is not any known modifier ip4-cidr-length = "/" ("0" / %x31-39 0*1DIGIT) ; value range 0-32 ip6-cidr-length = "/" ("0" / %x31-39 0*2DIGIT) ; value range 0-128 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] ip4-network = qnum "." qnum "." qnum "." qnum qnum = DIGIT ; 0-9 / %x31-39 DIGIT ; 10-99 / "1" 2DIGIT ; 100-199 / "2" %x30-34 DIGIT ; 200-249 / "25" %x30-35 ; 250-255 ; conventional dotted-quad notation, e.g., 192.0.2.0 ip6-network = <as per Section 2.2 of [RFC4291]> ; e.g., 2001:db8::cd30 domain-spec = macro-string domain-end domain-end = ( "." toplabel [ "." ] ) / macro-expand toplabel = ( *alphanum ALPHA *alphanum ) / ( 1*alphanum "-" *( alphanum / "-" ) alphanum ) ; LDH rule plus additional TLD restrictions ; (see Section 2 of [RFC3696] for background) alphanum = ALPHA / DIGIT explain-string = *( macro-string / SP ) macro-string = *( macro-expand / macro-literal ) macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) / "%%" / "%_" / "%-" Kitterman Standards Track [Page 47] RFC 7208 Sender Policy Framework (SPF) April 2014 macro-literal = %x21-24 / %x26-7E ; visible characters except "%" macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / "c" / "r" / "t" / "v" transformers = *DIGIT [ "r" ] delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] [ key-value-list ] CRLF result = "pass" / "fail" / "softfail" / "neutral" / "none" / "temperror" / "permerror" key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) [";"] key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) key = "client-ip" / "envelope-from" / "helo" / "problem" / "receiver" / "identity" / "mechanism" / name identity = "mailfrom" ; for the "MAIL FROM" identity / "helo" ; for the "HELO" identity / name ; other identities sender = Mailbox ip = ip4-network / ip6-network ALPHA = <A-Z / a-z as per [RFC5234]> DIGIT = <0-9 as per [RFC5234]> SP = <space character as per [RFC5234]> dot-atom = <unquoted word as per [RFC5322]> quoted-string = <quoted string as per [RFC5322]> comment = <comment string as per [RFC5322]> CFWS = <comment or folding white space as per [RFC5322]> FWS = <folding white space as per [RFC5322]> CRLF = <standard end-of-line token as per [RFC5322]> 13. Contributors and Acknowledgements This document is largely based on the work of Meng Weng Wong, Mark Lentczner, and Wayne Schlitt. Although, as this section acknowledges, many people have contributed to this document, a very large portion of the writing and editing is due to Meng, Mark, and Wayne. Kitterman Standards Track [Page 48] RFC 7208 Sender Policy Framework (SPF) April 2014 This design owes a debt of parentage to [RMX] by Hadmut Danisch and to [DMP] by Gordon Fecyk. The idea of using a DNS record to check the legitimacy of an email address traces its ancestry further back through messages on the namedroppers mailing list by Paul Vixie [Vixie] (based on suggestion by Jim Miller) and by David Green [Green]. Philip Gladstone contributed the concept of macros to the specification, multiplying the expressiveness of the language and making per-user and per-IP lookups possible. The authors of both this document and [RFC4408] would also like to thank the literally hundreds of individuals who have participated in the development of this design. They are far too numerous to name, but they include the following: The participants in the SPFbis working group. The folks on the spf-discuss mailing list. The folks on the SPAM-L mailing list. The folks on the IRTF ASRG mailing list. The folks on the IETF MARID mailing list. The folks on #perl. 14. IANA Considerations 14.1. The SPF DNS Record Type Per [RFC4408], the IANA assigned the Resource Record Type and Qtype from the "Domain Name System (DNS) Parameters" registry for the SPF RR type with code 99. The format of this type is identical to the TXT RR [RFC1035]. The character content of the record is encoded as [US-ASCII]. Studies have shown that RRTYPE 99 has not seen any substantial use, and in fact its existence and mechanism defined in [RFC4408] have led to some interoperability issues. Accordingly, its use is no longer appropriate for SPF version 1; implementations are not to use it. IANA has updated the "Resource Record (RR) TYPEs" registry to indicate that this document is the reference document for that RRTYPE. Kitterman Standards Track [Page 49] RFC 7208 Sender Policy Framework (SPF) April 2014 14.2. The Received-SPF Mail Header Field Per [RFC3864], the "Received-SPF:" header field is added to the IANA "Permanent Message Header Field Names" registry. The following is the registration template: Header field name: Received-SPF Applicable protocol: mail ([RFC5322]) Status: standard Author/Change controller: IETF Specification document(s): RFC 7208 14.3. SPF Modifier Registry IANA has changed the reference for the "exp" and "redirect" modifiers in the "Modifier Names" registry, under Sender Policy Framework Parameters, from [RFC4408] to this document. Their status is unchanged. 15. References 15.1. Normative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463, January 2003. [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. Kitterman Standards Track [Page 50] RFC 7208 Sender Policy Framework (SPF) April 2014 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010. [RFC7001] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 7001, September 2013. [US-ASCII] American National Standards Institute (formerly United States of America Standards Institute), "USA Code for Information Interchange, X3.4", 1968. ANSI X3.4-1968 has been replaced by newer versions with slight modifications, but the 1968 version remains definitive for the Internet. 15.2. Informative References [BATV] Levine, J., Crocker, D., Silberman, S., and T. Finch, "Bounce Address Tag Validation (BATV)", Work in Progress, May 2008. [DMP] Fecyk, G., "Designated Mailers Protocol", Work in Progress, May 2004. [Green] Green, D., "Domain-Authorized SMTP Mail", June 2002, <http://www.mhonarc.org/archive/html/ietf-asrg/2003-03/ msg01525.html>. [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996. [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, March 1998. Kitterman Standards Track [Page 51] RFC 7208 Sender Policy Framework (SPF) April 2014 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications", RFC 3464, January 2003. [RFC3696] Klensin, J., "Application Techniques for Checking and Transformation of Names", RFC 3696, February 2004. [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain Name System (DNS)", RFC 3833, August 2004. [RFC3834] Moore, K., "Recommendations for Automatic Responses to Electronic Mail", RFC 3834, August 2004. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006. [RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan", BCP 122, RFC 4632, August 2006. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", RFC 4880, November 2007. [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension for Authentication", RFC 4954, July 2007. [RFC5507] IAB, Faltstrom, P., Austein, R., and P. Koch, "Design Choices When Expanding the DNS", RFC 5507, April 2009. [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, January 2010. [RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782, February 2010. Kitterman Standards Track [Page 52] RFC 7208 Sender Policy Framework (SPF) April 2014 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", STD 72, RFC 6409, November 2011. [RFC6647] Kucherawy, M. and D. Crocker, "Email Greylisting: An Applicability Statement for SMTP", RFC 6647, June 2012. [RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham, "Deprecating the "X-" Prefix and Similar Constructs in Application Protocols", BCP 178, RFC 6648, June 2012. [RFC6652] Kitterman, S., "Sender Policy Framework (SPF) Authentication Failure Reporting Using the Abuse Reporting Format", RFC 6652, June 2012. [RFC6686] Kucherawy, M., "Resolution of the Sender Policy Framework (SPF) and Sender ID Experiments", RFC 6686, July 2012. [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, April 2013. [RMX] Danisch, H., "The RMX DNS RR and method for lightweight SMTP sender authorization", Work in Progress, May 2004. [Vixie] Vixie, P., "Repudiating MAIL FROM", 2002, <http://marc.info/?l=namedroppers&m=102298170127004&w=4>. Kitterman Standards Track [Page 53] RFC 7208 Sender Policy Framework (SPF) April 2014 Appendix A. Extended Examples These examples are based on the following DNS setup: ; A domain with two mail servers, two hosts, and two servers ; at the domain name $ORIGIN example.com. @ MX 10 mail-a MX 20 mail-b A 192.0.2.10 A 192.0.2.11 amy A 192.0.2.65 bob A 192.0.2.66 mail-a A 192.0.2.129 mail-b A 192.0.2.130 www CNAME example.com. ; A related domain $ORIGIN example.org. @ MX 10 mail-c mail-c A 192.0.2.140 ; The reverse IP for those addresses $ORIGIN 2.0.192.in-addr.arpa. 10 PTR example.com. 11 PTR example.com. 65 PTR amy.example.com. 66 PTR bob.example.com. 129 PTR mail-a.example.com. 130 PTR mail-b.example.com. 140 PTR mail-c.example.org. ; A rogue reverse IP domain that claims to be ; something it's not $ORIGIN 0.0.10.in-addr.arpa. 4 PTR bob.example.com. Kitterman Standards Track [Page 54] RFC 7208 Sender Policy Framework (SPF) April 2014 A.1. Simple Examples These examples show various possible published records for example.com and which values of <ip> would cause check_host() to return "pass". Note that <domain> is "example.com". v=spf1 +all -- any <ip> passes v=spf1 a -all -- hosts 192.0.2.10 and 192.0.2.11 pass v=spf1 a:example.org -all -- no sending hosts pass since example.org has no A records v=spf1 mx -all -- sending hosts 192.0.2.129 and 192.0.2.130 pass v=spf1 mx:example.org -all -- sending host 192.0.2.140 passes v=spf1 mx mx:example.org -all -- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass v=spf1 mx/30 mx:example.org/30 -all -- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes v=spf1 ptr -all -- sending host 192.0.2.65 passes (reverse DNS is valid and is in example.com) -- sending host 192.0.2.140 fails (reverse DNS is valid, but not in example.com) -- sending host 10.0.0.4 fails (reverse IP is not valid) Kitterman Standards Track [Page 55] RFC 7208 Sender Policy Framework (SPF) April 2014 v=spf1 ip4:192.0.2.128/28 -all -- sending host 192.0.2.65 fails -- sending host 192.0.2.129 passes A.2. Multiple Domain Example These examples show the effect of related records: example.org: "v=spf1 include:example.com include:example.net -all" This record would be used if mail from example.org actually came through servers at example.com and example.net. Example.org's designated servers are the union of example.com's and example.net's designated servers. la.example.org: "v=spf1 redirect=example.org" ny.example.org: "v=spf1 redirect=example.org" sf.example.org: "v=spf1 redirect=example.org" These records allow a set of domains that all use the same mail system to make use of that mail system's record. In this way, only the mail system's record needs to be updated when the mail setup changes. These domains' records never have to change. A.3. DNS Blacklist (DNSBL) Style Example Imagine that, in addition to the domain records listed above, there are these (see [RFC5782]): $ORIGIN _spf.example.com. mary.mobile-users A 127.0.0.2 fred.mobile-users A 127.0.0.2 15.15.168.192.joel.remote-users A 127.0.0.2 16.15.168.192.joel.remote-users A 127.0.0.2 Kitterman Standards Track [Page 56] RFC 7208 Sender Policy Framework (SPF) April 2014 The following records describe users at example.com who mail from arbitrary servers, or who mail from personal servers. example.com: v=spf1 mx include:mobile-users._spf.%{d} include:remote-users._spf.%{d} -all mobile-users._spf.example.com: v=spf1 exists:%{l1r+}.%{d} remote-users._spf.example.com: v=spf1 exists:%{ir}.%{l1r+}.%{d} A.4. Multiple Requirements Example Say that your sender policy requires both that the IP address is within a certain range and that the reverse DNS for the IP matches. This can be done several ways, including the following: example.com. SPF ( "v=spf1 " "-include:ip4._spf.%{d} " "-include:ptr._spf.%{d} " "+all" ) ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all" ptr._spf.example.com. SPF "v=spf1 -ptr +all" This example shows how the "-include" mechanism can be useful, how an SPF record that ends in "+all" can be very restrictive, and the use of De Morgan's Law. Appendix B. Changes in Implementation Requirements from RFC 4408 The modifications to implementation requirements from [RFC4408] are all either (a) corrections to errors in [RFC4408] or (b) additional documentation based on consensus of operational experience acquired since the publication of [RFC4408]. o Use of DNS RR type SPF (99) has been removed from the protocol; see [RFC6686] for background. o A new DNS-related processing limit based on "void lookups" has been added (Section 4.6.4). Kitterman Standards Track [Page 57] RFC 7208 Sender Policy Framework (SPF) April 2014 o Use of the ptr mechanism and the %p macro has been strongly discouraged (Sections 5.5 and 7.2). The ptr mechanism and the %p macro remain part of the protocol because they were found to be in use, but records ought to be updated to avoid them. o Use of the "Authentication-Results" header field [RFC7001] as a possible alternative to use of the "Received-SPF" header field is discussed (Section 9.2). o There have been a number of minor corrections to the ABNF to make it more clear and correct (Section 12). SPF library implementers should give the revised ABNF a careful review to determine if implementation changes are needed. o Use of X- fields in the ABNF has been removed; see [RFC6648] for background. o Ambiguity about how to deal with invalid <domain-spec> after macro expansion has been documented. Depending on one specific behavior has to be avoided (Section 4.8). o General operational information has been updated and expanded based on eight years of post-[RFC4408] operations experience. See Section 10 and Appendices D through G below. o Security considerations have been reviewed and updated (Section 11). Appendix C. Further Testing Advice Another approach that can be helpful is to publish records that include a "tracking exists:" mechanism. By looking at the name server logs, a rough list can then be generated. For example: v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all This associated macro expansion would cause the sending HELO domain, local-part of the sending email address, domain part of the sending email address, and the IP address from which the connection was received to be embedded in an SPF query and logged in the sender's DNS logs. This approach, which has been used since very early in the SPF project, allows senders to unilaterally collect data to evaluate the correctness of their SPF records. Unlike newer feedback mechanisms, it does not require any special cooperation from SPF verifiers. A similar example, one of the earliest SPF records published, can still be found as of this writing at altavista.net. Kitterman Standards Track [Page 58] RFC 7208 Sender Policy Framework (SPF) April 2014 Appendix D. SPF/Mediator Interactions There are three places that techniques can be used to ameliorate unintended SPF failures with mediators. D.1. Originating ADMDs The beginning, when email is first sent: o "Neutral" results could be given for IP addresses that might be forwarders, instead of "fail" results based on a list of known reliable forwarders. For example: "v=spf1 mx ?exists:%{ir}.whitelist.example.org -all" This would cause a lookup on a DNS White List (DNSWL) and cause a result of "fail" only for email not coming from either the domain's mx host(s) (SPF pass) or whitelisted sources (SPF neutral). This, in effect, outsources an element of sender policy to the maintainer of the whitelist. o The "MAIL FROM" identity could have additional information in the local-part that cryptographically identifies the mail as coming from an authorized source. In this case, an SPF record such as the following could be used: "v=spf1 mx exists:%{l}._spf_verify.%{d} -all" Then, a specialized DNS server can be set up to serve the _spf_verify subdomain that validates the local-part. Although this requires an extra DNS lookup, this happens only when the email would otherwise be rejected as not coming from a known good source. Note that due to the 63-character limit for domain labels, this approach only works reliably if the local-part signature scheme is guaranteed to either only produce local-parts with a maximum of 63 characters or gracefully handle truncated local-parts. The method used to secure the local-part is a local implementation issue; it need not be standard. An example of one way to do it can be found in [BATV]. o Similarly, a specialized DNS server could be set up that will rate-limit the email coming from unexpected IP addresses. "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all" Kitterman Standards Track [Page 59] RFC 7208 Sender Policy Framework (SPF) April 2014 o SPF allows the creation of per-user policies for special cases. For example, the following SPF record and appropriate wildcard DNS records can be used: "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}" D.2. Mediators The middle, when email is forwarded: o Mediators can solve the problem by rewriting the "MAIL FROM" to be in their own domain. This means mail rejected from the external mailbox will have to be forwarded back to the original sender by the forwarding service. Various schemes to do this exist, though they vary widely in complexity and resource requirements on the part of the mediator. o Several popular MTAs can be forced from "alias" semantics to "mailing list" semantics by configuring an additional alias with "owner-" prepended to the original alias name (e.g., an alias of "friends: george@example.com, fred@example.org" would need another alias of the form "owner-friends: localowner"). o Mediators could reject mail that would "fail" SPF if forwarded using an SMTP reply code of 551, User not local (see Section 3.4 of [RFC5321]) to communicate the correct target address to resend the mail to. D.3. Receiving ADMDs The end, when email is received: o If the owner of the external mailbox wishes to trust the mediator, he can direct the external mailbox's MTA to skip SPF tests when the client host belongs to the mediator. o Tests against other identities, such as the "HELO" identity, can be used to override a failed test against the "MAIL FROM" identity. o For larger domains, it might not be possible to have a complete or accurate list of forwarding services used by the owners of the domain's mailboxes. In such cases, whitelists of generally recognized forwarding services could be employed. Kitterman Standards Track [Page 60] RFC 7208 Sender Policy Framework (SPF) April 2014 Appendix E. Mail Services MSPs (Mail Service Providers -- Section 2.3 of [RFC5598]) that offer mail services to third-party domains, such as the sending of bulk mail, might want to adjust their configurations in light of the authorization check described in this document. If the domain part of the "MAIL FROM" identity used for such email uses one of the MSP's domains, then the provider needs only to ensure that its sending host is authorized by its own SPF record, if any. If the "MAIL FROM" identity does not use the MSP's domain, then extra care has to be taken. The SPF record format has several options for the third-party domain to authorize the service provider's MTAs to send mail on its behalf. For MSPs, such as ISPs, that have a wide variety of customers using the same MTA, steps are required to mitigate the risk of cross-customer forgery (see Section 11.4). Appendix F. MTA Relays Relays are described in [RFC5598], Section 2.2.2. The authorization check generally precludes the use of arbitrary MTA relays between the sender and receiver of an email message. Within an organization, MTA relays can be effectively deployed. However, for the purposes of this document, such relays are effectively transparent. The SPF authorization check is a check between border MTAs of different ADMDs. For mail senders, this means published SPF records have to authorize any MTAs that actually send across the Internet. Usually, these are just the border MTAs as internal MTAs simply forward mail to these MTAs for relaying. The receiving ADMD will generally want to perform the authorization check at the boundary MTAs, including all secondary MXs. Internal MTAs (including MTAs that might serve as both boundary MTAs and internal relays from secondary MXs when they are processing the relayed mail stream) then do not perform the authorization test. To perform the authorization test other than at the boundary, the host that first transferred the message to the receiving ADMD has to be determined, which can be difficult to extract from the message header because (a) header fields can be forged or malformed, and (b) there's no standard way to encode that information such that it can be reliably extracted. Testing other than at the boundary is likely to produce unreliable results. This is described further in Appendix D of [RFC7001]. Kitterman Standards Track [Page 61] RFC 7208 Sender Policy Framework (SPF) April 2014 Appendix G. Local Policy Considerations SPF results can be used in combination with other methods to determine the final local disposition (either positive or negative) of a message. It can also be considered dispositive on its own. G.1. Policy for SPF Pass SPF "pass" results can be used in combination with "whitelists" of known "good" domains to bypass some or all additional pre-delivery email checks. Exactly which checks and how to determine appropriate whitelist entries have to be based on local conditions and requirements. G.2. Policy for SPF Fail SPF "fail" results can be used to reject messages during the SMTP transaction based on either "MAIL FROM" or "HELO" identity results. This reduces resource requirements for various content-filtering methods and conserves bandwidth since rejection can be done before the SMTP content is transferred. It also gives immediate feedback to the sender, who might then be able to resolve the issue. Due to some of the issues described in this section (Appendix G), SPF-based rejection does present some risk of rejecting legitimate email when rejecting email based on "MAIL FROM" results. SPF "fail" results can alternately be used as one input into a larger set of evaluations that might, based on a combination of SPF "fail" results with other evaluation techniques, result in the email being marked negatively in some way (this might be via delivery to a special spam folder, modifying subject lines, or other locally determined means). Developing the details of such an approach has to be based on local conditions and requirements. Using SPF results in this way does not have the advantages of resource conservation and immediate feedback to the sender associated with SMTP rejection, but could produce fewer undesirable rejections in a well-designed system. Such an approach might result in email that was not authorized by the sending ADMD being unknowingly delivered to end users. Either general approach can be used, as they both leave a clear disposition of emails; either they are delivered in some manner or the sender is notified of the failure. Other dispositions such as "dropping" or deleting email after acceptance are inappropriate because they leave uncertainty and reduce the overall reliability and utility of email across the Internet. Kitterman Standards Track [Page 62] RFC 7208 Sender Policy Framework (SPF) April 2014 G.3. Policy for SPF Permerror The "permerror" result (see Section 2.6.7) indicates that the SPF processing module at the receiver determined that the retrieved SPF policy record could not be interpreted. This gives no true indication about the authorized use of the data found in the envelope. As with all results, implementers have a choice to make regarding what to do with a message that yields this result. SMTP allows only a few basic options. Rejection of the message is an option, in that it is the one thing a receiver can do to draw attention to the difficulty encountered while protecting itself from messages that do not have a definite SPF result of some kind. However, if the SPF implementation is defective and returns spurious "permerror" results, only the sender is actively notified of the defect (in the form of rejected mail), and not the receiver making use of SPF. The less intrusive handling choice is to deliver the message, perhaps with some kind of annotation of the difficulty encountered and/or logging of a similar nature. However, this will not be desirable to SPF verifier operators that wish to implement SPF checking as strictly as possible, nor is this sort of passive reporting of problems typically effective. There is of course the option of placing this choice in the hands of the SPF verifier operator rather than the implementer since this kind of choice is often a matter of local policy rather than a condition with a universal solution, but this adds one more piece of complexity to an already non-trivial environment. Both implementers and SPF verifier operators need to be cautious of all choices and outcomes when handling SPF results. G.4. Policy for SPF Temperror The "temperror" result (see Section 2.6.6) indicates that the SPF processing module at the receiver could not retrieve an SPF policy record due to a (probably) transient condition. This gives no true indication about the authorized use of the data found in the envelope. As with all results, implementers have a choice to make regarding what to do with a message that yields this result. SMTP allows only a few basic options. Kitterman Standards Track [Page 63] RFC 7208 Sender Policy Framework (SPF) April 2014 Deferring the message is an option, in that it is the one thing a receiver can do to draw attention to the difficulty encountered while protecting itself from messages that do not have a definite SPF result of some kind. However, if the SPF implementation is defective and returns spurious "temperror" results, only the sender is actively notified of the defect (in the form of mail rejected after it times out of the sending queue), and not the receiver making use of SPF. Because of long queue lifetimes, it is possible that mail will be repeatedly deferred for several days, and so any awareness that the sender may have regarding a problem could be quite delayed. If "temperrors" persist for multiple delivery attempts, it might be preferable to treat the error as permanent and reduce the amount of time the message is in transit. The less intrusive handling choice is to deliver the message, perhaps with some kind of annotation of the difficulty encountered and/or logging of a similar nature. However, this will not be desirable to SPF verifier operators that wish to implement SPF checking as strictly as possible, nor is this sort of passive reporting of problems typically effective. There is of course the option of placing this choice in the hands of the SPF verifier operator rather than the implementer since this kind of choice is often a matter of local policy rather than a condition with a universal solution, but this adds one more piece of complexity to an already non-trivial environment. Both implementers and SPF verifier operators need to be cautious of all choices and outcomes when handling SPF results. Author's Address Scott Kitterman Kitterman Technical Services 3611 Scheel Dr. Ellicott City, MD 21042 United States of America EMail: scott@kitterman.com Kitterman Standards Track [Page 64] |