Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1
RFC 7208
| Document | Type |
RFC
- Proposed Standard
(April 2014)
Errata
IPR
Obsoletes RFC 4408
|
|
|---|---|---|---|
| Author | Scott Kitterman | ||
| Last updated | 2020-01-21 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| IESG | Responsible AD | Pete Resnick | |
| Send notices to | (None) |
RFC 7208
lt;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]