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RFC 7208

Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1

Pages: 64
Proposed Standard
Errata
Obsoletes:  4408
Updated by:  737285538616
Part 1 of 4 – Pages 1 to 14
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Top   ToC   RFC7208 - Page 1
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.
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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
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   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
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   Without obtaining an adequate license from the person(s) controlling
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   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
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           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
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   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
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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].
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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.
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   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.
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   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.
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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.
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   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.
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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
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   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.
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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"
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   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.



(page 14 continued on part 2)

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