draft-ietf-spfbis-4408bis-08.txt		draft-ietf-spfbis-4408bis-09.txt
	Network Working Group S. Kitterman		Network Working Group S. Kitterman
	Internet-Draft Kitterman Technical Services		Internet-Draft Kitterman Technical Services
	Obsoletes: 4408 (if approved) October 22, 2012		Obsoletes: 4408 (if approved) January 10, 2013
	Intended status: Standards Track		Intended status: Standards Track
	Expires: April 25, 2013		Expires: July 14, 2013
	Sender Policy Framework (SPF) for Authorizing Use of Domains in Email,		Sender Policy Framework (SPF) for Authorizing Use of Domains in Email,
	Version 1		Version 1
	draft-ietf-spfbis-4408bis-08.txt		draft-ietf-spfbis-4408bis-09.txt
	Abstract		Abstract
	Email on the Internet can be forged in a number of ways. In		Email on the Internet can be forged in a number of ways. In
	particular, existing protocols place no restriction on what a sending		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		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 SMTP HELO/EHLO commands. This document describes version 1 of
	the Sender Policy Framework (SPF) protocol, whereby an ADMD can		the Sender Policy Framework (SPF) protocol, whereby an ADMD can
	explicitly authorize the hosts that are allowed to use its domain		explicitly authorize the hosts that are allowed to use its domain
	names, and a receiving host can check such authorization.		names, and a receiving host can check such authorization.
	skipping to change at page 1, line 40		skipping to change at page 1, line 40
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on April 25, 2013.		This Internet-Draft will expire on July 14, 2013.
	Copyright Notice		Copyright Notice
	Copyright (c) 2012 IETF Trust and the persons identified as the		Copyright (c) 2013 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	skipping to change at page 3, line 8		skipping to change at page 3, line 8
	Without obtaining an adequate license from the person(s) controlling		Without obtaining an adequate license from the person(s) controlling
	the copyright in such materials, this document may not be modified		the copyright in such materials, this document may not be modified
	outside the IETF Standards Process, and derivative works of it may		outside the IETF Standards Process, and derivative works of it may
	not be created outside the IETF Standards Process, except to format		not be created outside the IETF Standards Process, except to format
	it for publication as an RFC or to translate it into languages other		it for publication as an RFC or to translate it into languages other
	than English.		than English.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
	1.1. Protocol Status . . . . . . . . . . . . . . . . . . . . . 6		1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
	1.2. Experimental History . . . . . . . . . . . . . . . . . . . 7		1.1.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . 6
	1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7		1.1.2. Imported Definitions . . . . . . . . . . . . . . . . . 6
	1.3.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . 7		1.1.3. MAIL FROM Definition . . . . . . . . . . . . . . . . . 7
	1.3.2. Imported Definitions . . . . . . . . . . . . . . . . . 7		1.1.4. HELO Definition . . . . . . . . . . . . . . . . . . . 7
	1.3.3. Mail From Definition . . . . . . . . . . . . . . . . . 7		1.1.5. Deprecated . . . . . . . . . . . . . . . . . . . . . . 7
	1.3.4. HELO Definition . . . . . . . . . . . . . . . . . . . 8		2. Operational Overview . . . . . . . . . . . . . . . . . . . . . 8
	1.3.5. Deprecated . . . . . . . . . . . . . . . . . . . . . . 8		2.1. The "HELO" Identity . . . . . . . . . . . . . . . . . . . 8
	2. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 9		2.2. The "MAIL FROM" Identity . . . . . . . . . . . . . . . . . 8
	2.1. The "HELO" Identity . . . . . . . . . . . . . . . . . . . 9		2.3. Publishing Authorization . . . . . . . . . . . . . . . . . 8
	2.2. The "MAIL FROM" Identity . . . . . . . . . . . . . . . . . 9		2.4. Checking Authorization . . . . . . . . . . . . . . . . . . 9
	2.3. Publishing Authorization . . . . . . . . . . . . . . . . . 9		2.5. Location of Checks . . . . . . . . . . . . . . . . . . . . 10
	2.4. Checking Authorization . . . . . . . . . . . . . . . . . . 10		2.6. Results of Evaluation . . . . . . . . . . . . . . . . . . 10
	2.5. Interpreting the Result . . . . . . . . . . . . . . . . . 11		2.6.1. None . . . . . . . . . . . . . . . . . . . . . . . . . 11
	2.5.1. None . . . . . . . . . . . . . . . . . . . . . . . . . 12		2.6.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . 11
	2.5.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . 12		2.6.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . 11
	2.5.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . 12		2.6.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . 11
	2.5.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . 12		2.6.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . 11
	2.5.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . 13		2.6.6. Temperror . . . . . . . . . . . . . . . . . . . . . . 11
	2.5.6. Temperror . . . . . . . . . . . . . . . . . . . . . . 13		2.6.7. Permerror . . . . . . . . . . . . . . . . . . . . . . 11
	2.5.7. Permerror . . . . . . . . . . . . . . . . . . . . . . 13		3. SPF Records . . . . . . . . . . . . . . . . . . . . . . . . . 12
	3. SPF Records . . . . . . . . . . . . . . . . . . . . . . . . . 14		3.1. DNS Resource Records . . . . . . . . . . . . . . . . . . . 12
	3.1. DNS Resource Records . . . . . . . . . . . . . . . . . . . 14		3.2. Multiple DNS Records . . . . . . . . . . . . . . . . . . . 13
	3.2. Multiple DNS Records . . . . . . . . . . . . . . . . . . . 15		3.3. Multiple Strings in a Single DNS record . . . . . . . . . 13
	3.3. Multiple Strings in a Single DNS record . . . . . . . . . 15		3.4. Record Size . . . . . . . . . . . . . . . . . . . . . . . 13
	3.4. Record Size . . . . . . . . . . . . . . . . . . . . . . . 15		3.5. Wildcard Records . . . . . . . . . . . . . . . . . . . . . 13
	3.5. Wildcard Records . . . . . . . . . . . . . . . . . . . . . 15		4. The check_host() Function . . . . . . . . . . . . . . . . . . 15
	4. The check_host() Function . . . . . . . . . . . . . . . . . . 17		4.1. Arguments . . . . . . . . . . . . . . . . . . . . . . . . 15
	4.1. Arguments . . . . . . . . . . . . . . . . . . . . . . . . 17		4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . 15
	4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . 17		4.3. Initial Processing . . . . . . . . . . . . . . . . . . . . 16
	4.3. Initial Processing . . . . . . . . . . . . . . . . . . . . 17		4.4. Record Lookup . . . . . . . . . . . . . . . . . . . . . . 16
	4.4. Record Lookup . . . . . . . . . . . . . . . . . . . . . . 18		4.5. Selecting Records . . . . . . . . . . . . . . . . . . . . 16
	4.5. Selecting Records . . . . . . . . . . . . . . . . . . . . 18		4.6. Record Evaluation . . . . . . . . . . . . . . . . . . . . 17
	4.6. Record Evaluation . . . . . . . . . . . . . . . . . . . . 18		4.6.1. Term Evaluation . . . . . . . . . . . . . . . . . . . 17
	4.6.1. Term Evaluation . . . . . . . . . . . . . . . . . . . 19		4.6.2. Mechanisms . . . . . . . . . . . . . . . . . . . . . . 17
	4.6.2. Mechanisms . . . . . . . . . . . . . . . . . . . . . . 19		4.6.3. Modifiers . . . . . . . . . . . . . . . . . . . . . . 18
	4.6.3. Modifiers . . . . . . . . . . . . . . . . . . . . . . 20		4.6.4. DNS Lookup Limits . . . . . . . . . . . . . . . . . . 18
	4.6.4. DNS Lookup Limits . . . . . . . . . . . . . . . . . . 20		4.7. Default Result . . . . . . . . . . . . . . . . . . . . . . 19
	4.7. Default Result . . . . . . . . . . . . . . . . . . . . . . 21		4.8. Domain Specification . . . . . . . . . . . . . . . . . . . 19
	4.8. Domain Specification . . . . . . . . . . . . . . . . . . . 21		5. Mechanism Definitions . . . . . . . . . . . . . . . . . . . . 21
	5. Mechanism Definitions . . . . . . . . . . . . . . . . . . . . 22		5.1. "all" . . . . . . . . . . . . . . . . . . . . . . . . . . 22
	5.1. "all" . . . . . . . . . . . . . . . . . . . . . . . . . . 23		5.2. "include" . . . . . . . . . . . . . . . . . . . . . . . . 22
	5.2. "include" . . . . . . . . . . . . . . . . . . . . . . . . 23		5.3. "a" . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
	5.3. "a" . . . . . . . . . . . . . . . . . . . . . . . . . . . 25		5.4. "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
	5.4. "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 25		5.5. "ptr" (deprecated) . . . . . . . . . . . . . . . . . . . . 24
	5.5. "ptr" (deprecated) . . . . . . . . . . . . . . . . . . . . 25		5.6. "ip4" and "ip6" . . . . . . . . . . . . . . . . . . . . . 26
	5.6. "ip4" and "ip6" . . . . . . . . . . . . . . . . . . . . . 27		5.7. "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 26
	5.7. "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 27		6. Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 28
	6. Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 29		6.1. redirect: Redirected Query . . . . . . . . . . . . . . . . 28
	6.1. redirect: Redirected Query . . . . . . . . . . . . . . . . 29		6.2. exp: Explanation . . . . . . . . . . . . . . . . . . . . . 29
	6.2. exp: Explanation . . . . . . . . . . . . . . . . . . . . . 30		7. Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
	7. Recording The Result . . . . . . . . . . . . . . . . . . . . . 32		7.1. Macro Definitions . . . . . . . . . . . . . . . . . . . . 31
	7.1. The Received-SPF Header Field . . . . . . . . . . . . . . 32		7.2. Expansion Examples . . . . . . . . . . . . . . . . . . . . 34
	7.2. SPF Results in the Authentication-Results Header Field . . 34		8. Result Handling . . . . . . . . . . . . . . . . . . . . . . . 36
	8. Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36		8.1. None . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
	8.1. Macro Definitions . . . . . . . . . . . . . . . . . . . . 36		8.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . . . 36
	8.2. Expansion Examples . . . . . . . . . . . . . . . . . . . . 39		8.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
	9. Implications . . . . . . . . . . . . . . . . . . . . . . . . . 41		8.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
	9.1. Sending Domains . . . . . . . . . . . . . . . . . . . . . 41		8.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . . . 37
	9.1.1. DNS Resource Considerations . . . . . . . . . . . . . 41		8.6. Temperror . . . . . . . . . . . . . . . . . . . . . . . . 38
	9.1.2. Administrator's Considerations . . . . . . . . . . . . 42		8.7. Permerror . . . . . . . . . . . . . . . . . . . . . . . . 38
	9.1.3. Bounces . . . . . . . . . . . . . . . . . . . . . . . 43		9. Recording The Result . . . . . . . . . . . . . . . . . . . . . 39
	9.2. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 43		9.1. The Received-SPF Header Field . . . . . . . . . . . . . . 39
	9.2.1. Mailing Lists . . . . . . . . . . . . . . . . . . . . 43		9.2. SPF Results in the Authentication-Results Header Field . . 41
	9.2.2. Forwarding Services and Aliases . . . . . . . . . . . 44		10. Effects on Infrastructure . . . . . . . . . . . . . . . . . . 43
	9.2.3. Mail Services . . . . . . . . . . . . . . . . . . . . 46		10.1. Sending Domains . . . . . . . . . . . . . . . . . . . . . 43
	9.2.4. MTA Relays . . . . . . . . . . . . . . . . . . . . . . 46		10.1.1. DNS Resource Considerations . . . . . . . . . . . . . 43
	9.3. Receivers . . . . . . . . . . . . . . . . . . . . . . . . 47		10.1.2. Administrator's Considerations . . . . . . . . . . . . 44
	9.3.1. Policy For SPF Pass . . . . . . . . . . . . . . . . . 47		10.1.3. Bounces . . . . . . . . . . . . . . . . . . . . . . . 45
	9.3.2. Policy For SPF Fail . . . . . . . . . . . . . . . . . 47		10.2. Receivers . . . . . . . . . . . . . . . . . . . . . . . . 45
	9.3.3. Policy For SPF Permerror . . . . . . . . . . . . . . . 48		10.3. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 46
	10. Security Considerations . . . . . . . . . . . . . . . . . . . 49		10.3.1. Mailing Lists . . . . . . . . . . . . . . . . . . . . 46
	10.1. Processing Limits . . . . . . . . . . . . . . . . . . . . 49		10.3.2. Forwarding Services and Aliases . . . . . . . . . . . 46
	10.2. SPF-Authorized Email May Contain Other False Identities . 49		11. Security Considerations . . . . . . . . . . . . . . . . . . . 47
	10.3. Spoofed DNS and IP Data . . . . . . . . . . . . . . . . . 50		11.1. Processing Limits . . . . . . . . . . . . . . . . . . . . 47
	10.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 50		11.2. SPF-Authorized Email May Contain Other False Identities . 47
	10.5. Untrusted Information Sources . . . . . . . . . . . . . . 50		11.3. Spoofed DNS and IP Data . . . . . . . . . . . . . . . . . 48
	10.5.1. Recorded Results . . . . . . . . . . . . . . . . . . . 50		11.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 48
	10.5.2. External Explanations . . . . . . . . . . . . . . . . 51		11.5. Untrusted Information Sources . . . . . . . . . . . . . . 48
	10.5.3. Macro Expansion . . . . . . . . . . . . . . . . . . . 51		11.5.1. Recorded Results . . . . . . . . . . . . . . . . . . . 48
	10.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 51		11.5.2. External Explanations . . . . . . . . . . . . . . . . 49
	11. Contributors and Acknowledgements . . . . . . . . . . . . . . 52		11.5.3. Macro Expansion . . . . . . . . . . . . . . . . . . . 49
	12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53		11.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 49
	12.1. The SPF DNS Record Type . . . . . . . . . . . . . . . . . 53		12. Contributors and Acknowledgements . . . . . . . . . . . . . . 50
	12.2. The Received-SPF Mail Header Field . . . . . . . . . . . . 53		13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51
	12.3. SPF Modifier Registration . . . . . . . . . . . . . . . . 53		13.1. The SPF DNS Record Type . . . . . . . . . . . . . . . . . 51
	13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 54		13.2. The Received-SPF Mail Header Field . . . . . . . . . . . . 51
	13.1. Normative References . . . . . . . . . . . . . . . . . . . 54		13.3. SPF Modifier Registration . . . . . . . . . . . . . . . . 51
	13.2. Informative References . . . . . . . . . . . . . . . . . . 55		14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 52
	Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 57		14.1. Normative References . . . . . . . . . . . . . . . . . . . 52
	Appendix B. Extended Examples . . . . . . . . . . . . . . . . . . 60		14.2. Informative References . . . . . . . . . . . . . . . . . . 53
	B.1. Simple Examples . . . . . . . . . . . . . . . . . . . . . 60		Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 55
	B.2. Multiple Domain Example . . . . . . . . . . . . . . . . . 61		Appendix B. Extended Examples . . . . . . . . . . . . . . . . . . 58
	B.3. DNSBL Style Example . . . . . . . . . . . . . . . . . . . 62		B.1. Simple Examples . . . . . . . . . . . . . . . . . . . . . 58
	B.4. Multiple Requirements Example . . . . . . . . . . . . . . 62		B.2. Multiple Domain Example . . . . . . . . . . . . . . . . . 59
	Appendix C. Change History . . . . . . . . . . . . . . . . . . . 63		B.3. DNSBL Style Example . . . . . . . . . . . . . . . . . . . 60
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 66		B.4. Multiple Requirements Example . . . . . . . . . . . . . . 60
			Appendix C. Further Testing Advice . . . . . . . . . . . . . . . 61
			Appendix D. Updating Mail Forwarders . . . . . . . . . . . . . . 62
			Appendix E. Mail Services . . . . . . . . . . . . . . . . . . . . 64
			Appendix F. MTA Relays . . . . . . . . . . . . . . . . . . . . . 65
			Appendix G. Local Policy Considerations . . . . . . . . . . . . . 66
			G.1. Policy For SPF Pass . . . . . . . . . . . . . . . . . . . 66
			G.2. Policy For SPF Fail . . . . . . . . . . . . . . . . . . . 66
			G.3. Policy For SPF Permerror . . . . . . . . . . . . . . . . . 67
			Appendix H. Protocol Status . . . . . . . . . . . . . . . . . . . 68
			Appendix I. Experimental History . . . . . . . . . . . . . . . . 69
			Appendix J. Change History . . . . . . . . . . . . . . . . . . . 70
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 73
	1. Introduction		1. Introduction
	The current email infrastructure has the property that any host		The current email infrastructure has the property that any host
	injecting mail into the system can use any DNS domain name it wants		injecting mail into the system can use any DNS domain name it wants
	in each of the various identifiers specified by [RFC5321] and		in each of the various identifiers specified by [RFC5321] and
	[RFC5322]. Although this feature is desirable in some circumstances,		[RFC5322]. Although this feature is desirable in some circumstances,
	it is a major obstacle to reducing Unsolicited Bulk Email (UBE, aka		it is a major obstacle to reducing Unsolicited Bulk Email (UBE, aka
	spam). Furthermore, many domain owning ADMDs (ADministrative		spam). Furthermore, many domain owning ADMDs (ADministrative
	Management Domains, see [RFC5598]) are understandably concerned about		Management Domains, see [RFC5598]) are understandably concerned about
	skipping to change at page 6, line 33		skipping to change at page 6, line 33
	"HELO" or "MAIL FROM" identity during a mail transaction.		"HELO" or "MAIL FROM" identity during a mail transaction.
	An additional benefit to mail receivers is that after the use of an		An additional benefit to mail receivers is that after the use of an
	identity is verified, local policy decisions about the mail can be		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.		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		This is advantageous because reputation of domain names is likely to
	be more accurate than reputation of host IP addresses. Furthermore,		be more accurate than reputation of host IP addresses. Furthermore,
	if a claimed identity fails verification, local policy can take		if a claimed identity fails verification, local policy can take
	stronger action against such email, such as rejecting it.		stronger action against such email, such as rejecting it.
	1.1. Protocol Status		1.1. Terminology
	SPF has been in development since the summer of 2003 and has seen
	deployment beyond the developers beginning in December 2003. The
	design of SPF slowly evolved until the spring of 2004 and has since
	stabilized. There have been quite a number of forms of SPF, some
	written up as documents, some submitted as Internet Drafts, and many
	discussed and debated in development forums. The protocol was
	originally defined in [RFC4408], which this document replaces.
	[RFC4408] was designed to clearly document the protocol defined by
	earlier draft specifications of SPF as used in existing
	implementations. This updated specification is intended to clarify
	identified ambiguities in [RFC4408], resolve techincal issues
	identified in post-RFC 4408 deplyment experience, and document widely
	deployed extensions to SPF that have been developed since [RFC4408]
	was published.
	1.2. Experimental History
	This document updates and replaces RFC 4408 that was part of a group
	of simultaneously published Experimental RFCs (RFC 4405, RFC 4406,
	RFC 4407, and RFC 4408) in 2006. At that time the IESG requested the
	community observe the success or failure of the two approaches
	documented in these RFCs during the two years following publication,
	in order that a community consensus could be reached in the future.
	SPF is widely deployed by large and small email providers alike.
	There are multiple, interoperable implementations.
	For SPF (as documented in RFC 4408) a careful effort was made to
	collect and document lessons learned and errata during the two year
	period. The errata list has been stable (no new submissions) and
	only minor protocol lessons learned were identified. Resolution of
	the IESG's experiment is documented in [RFC6686].
	1.3. Terminology
	1.3.1. Keywords		1.1.1. Keywords
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in		"OPTIONAL" in this document are to be interpreted as described in
	[RFC2119].		[RFC2119].
	1.3.2. Imported Definitions		1.1.2. Imported Definitions
	The ABNF tokens "ALPHA", "DIGIT", and "SP" are defined in [RFC5234].		The ABNF tokens "ALPHA", "DIGIT", and "SP" are defined in [RFC5234].
	The token "local-part" is defined in [RFC5321].		The token "local-part" is defined in [RFC5321].
	"dot-atom", "quoted-string", "comment", "CFWS", "FWS", and "CRLF" are		"dot-atom", "quoted-string", "comment", "CFWS", "FWS", and "CRLF" are
	defined in [RFC5322].		defined in [RFC5322].
	1.3.3. Mail From Definition		1.1.3. MAIL FROM Definition
	This document is concerned with the portion of a mail message		This document is concerned with the portion of a mail message
	commonly called "envelope sender", "return path", "reverse path",		commonly called "envelope sender", "return path", "reverse path",
	"bounce address", "5321 FROM", "MAIL FROM", or RFC5321.MailFrom.		"bounce address", "5321 FROM", "MAIL FROM", or RFC5321.MailFrom.
	Since these terms are either not well defined or often used casually,		Since these terms are either not well defined or often used casually,
	this document uses "MAIL FROM" for consistency. This means the		this document uses "MAIL FROM" for consistency. This means the
	RFC5321.MailFrom as defined in [RFC5598]. Note that other terms that		RFC5321.MailFrom as defined in [RFC5598]. Note that other terms that
	might superficially look like the common terms, such as "reverse-		might superficially look like the common terms, such as "reverse-
	path", are used only with the defined meanings from normative		path", are used only with the defined meanings from normative
	documents.		documents.
	1.3.4. HELO Definition		1.1.4. HELO Definition
	This document also makes use of the HELO/EHLO identity. The "HELO"		This document also makes use of the HELO/EHLO identity. The "HELO"
	identity derives from either the SMTP HELO or EHLO command (see		identity derives from either the SMTP HELO or EHLO command (see
	[RFC5321]). Since HELO and EHLO can, in many cases, be used		[RFC5321]). Since HELO and EHLO can, in many cases, be used
	interchangeably, they are identified commonly as "HELO" in this		interchangeably, they are identified commonly as "HELO" in this
	document. This means RFC5321.HELO/.EHLO as defined in [RFC5598].		document. This means RFC5321.HELO/.EHLO as defined in [RFC5598].
	These commands supply the identity of the SMTP client (sending host)		These commands supply the identity of the SMTP client (sending host)
	for the SMTP session.		for the SMTP session.
	1.3.5. Deprecated		1.1.5. Deprecated
	There are [RFC4408] features that are marked "deprecated". In the		There are [RFC4408] features that are marked "deprecated". In the
	context of this document, deprecated means that senders SHOULD NOT		context of this document, deprecated means that senders SHOULD NOT
	publish SPF records that make use of such features because they might		publish SPF records that make use of such features because they might
	be removed entirely in future updates to the protocol. Such features		be removed entirely in future updates to the protocol. Such features
	do, however, remain part of the SPF protocol and receiving systems		do, however, remain part of the SPF protocol and receiving systems
	MUST support them unless this document explicitly says otherwise.		MUST support them unless this document explicitly says otherwise.
	2. Operation		[List of deprecated features to be added here]
			2. Operational Overview
	2.1. The "HELO" Identity		2.1. The "HELO" Identity
	It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM"		It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM"
	identity, but also separately check the "HELO" identity by applying		identity, but also separately check the "HELO" identity by applying
	the check_host() function (Section 4) to the "HELO" identity as the		the check_host() function (Section 4) to the "HELO" identity as the
	<sender>. Checking "HELO" promotes consistency of results and can		<sender>. Checking "HELO" promotes consistency of results and can
	reduce DNS resource usage. Additionally, since SPF records published		reduce DNS resource usage. Additionally, since SPF records published
	for "HELO" identities refer to a single host, when available, they		for "HELO" identities refer to a single host, when available, they
	are a very reliable source of host authorization status.		are a very reliable source of host authorization status.
	skipping to change at page 9, line 52		skipping to change at page 8, line 52
	names in the "HELO" and "MAIL FROM" identities by the MTAs specified		names in the "HELO" and "MAIL FROM" identities by the MTAs specified
	therein.		therein.
	SPF results can be used to make both positive (source is authorized)		SPF results can be used to make both positive (source is authorized)
	and negative (source is not authorized) determinations. If domain		and negative (source is not authorized) determinations. If domain
	owners choose to publish SPF records and want to support receivers		owners choose to publish SPF records and want to support receivers
	making negative authorization determinations, then they MUST publish		making negative authorization determinations, then they MUST publish
	records that end in "-all", or redirect to other records that do,		records that end in "-all", or redirect to other records that do,
	otherwise, no definitive determination of authorization can be made.		otherwise, no definitive determination of authorization can be made.
	Potential issues and mitigations associated with negative		Potential issues and mitigations associated with negative
	determinations are discussed in Section 9.		determinations are discussed in Section 10.
	ADMDs can publish SPF records that explicitly authorize no hosts for		ADMDs can publish SPF records that explicitly authorize no hosts for
	domain names that are neither used in the domain part of email		domain names that are neither used in the domain part of email
	addresses nor expected to originate mail.		addresses nor expected to originate mail.
	When changing SPF records, care has to be taken to ensure that there		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		is a transition period so that the old policy remains valid until all
	legitimate email can reasonably expect to have been checked. This		legitimate email can reasonably expect to have been checked. This
	can be as much as 30 days.		can be as much as 30 days.
	skipping to change at page 10, line 28		skipping to change at page 9, line 28
	to emit mail with a given identity. Typically, such checks are done		to emit mail with a given identity. Typically, such checks are done
	by a receiving MTA, but can be performed elsewhere in the mail		by a receiving MTA, but can be performed elsewhere in the mail
	processing chain so long as the required information is available and		processing chain so long as the required information is available and
	reliable. At least the "MAIL FROM" identity MUST be checked, but it		reliable. At least the "MAIL FROM" identity MUST be checked, but it
	is RECOMMENDED that the "HELO" identity also be checked beforehand.		is RECOMMENDED that the "HELO" identity also be checked beforehand.
	Without explicit approval of the domain owner, checking other		Without explicit approval of the domain owner, checking other
	identities against SPF version 1 records is NOT RECOMMENDED because		identities against SPF version 1 records is NOT RECOMMENDED because
	there are cases that are known to give incorrect results. For		there are cases that are known to give incorrect results. For
	example, almost all mailing lists rewrite the "MAIL FROM" identity		example, almost all mailing lists rewrite the "MAIL FROM" identity
	(see Section 9.2.1), but some do not change any other identities in		(see Section 10.3.1), but some do not change any other identities in
	the message. The scenario described in Section 9.2.2, sub-section		the message. The scenario described in Section 10.3.2, sub-section
	1.2, is another example. Documents that define other identities will		1.2, is another example. Documents that define other identities will
	have to define the method for explicit approval.		have to define the method for explicit approval.
	It is possible that mail receivers will use the SPF check as part of		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		a larger set of tests on incoming mail. The results of other tests
	might influence whether or not a particular SPF check is performed.		might influence whether or not a particular SPF check is performed.
	For example, finding the sending host's IP address on a local white		For example, finding the sending host's IP address on a local white
	list might cause all other tests to be skipped and all mail from that		list might cause all other tests to be skipped and all mail from that
	host to be accepted.		host to be accepted.
	skipping to change at page 11, line 12		skipping to change at page 10, line 12
	To make the test, the mail receiver MUST evaluate the check_host()		To make the test, the mail receiver MUST evaluate the check_host()
	function with the arguments set as follows:		function with the arguments set as follows:
	<ip> - the IP address of the SMTP client that is emitting the		<ip> - the IP address of the SMTP client that is emitting the
	mail, either IPv4 or IPv6.		mail, either IPv4 or IPv6.
	<domain> - the domain portion of the "MAIL FROM" or "HELO" identity.		<domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
	<sender> - the "MAIL FROM" or "HELO" identity.		<sender> - the "MAIL FROM" or "HELO" identity.
	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 2.1). In
	these cases, check_host() is defined in Section 4.3 to return a
	"none" result.
	Although invalid, malformed, or non-existent domains cause SPF checks		Although invalid, malformed, or non-existent domains cause SPF checks
	to return "none" because no SPF record can be found, it has long been		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		the policy of many MTAs to reject email from such domains, especially
	in the case of invalid "MAIL FROM". Rejecting email will prevent one		in the case of invalid "MAIL FROM". Rejecting email will prevent one
	method of circumventing of SPF records.		method of circumventing of SPF records.
	Implementations have to take care to correctly extract the <domain>		Implementations have to take care to correctly extract the <domain>
	from the data given with the SMTP MAIL FROM command as many MTAs will		from the data given with the SMTP MAIL FROM command as many MTAs will
	still accept such things as source routes (see [RFC5321], Appendix		still accept such things as source routes (see [RFC5321], Appendix
	C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).		C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).
	These archaic features have been maliciously used to bypass security		These archaic features have been maliciously used to bypass security
	systems.		systems.
	2.5. Interpreting the Result		2.5. Location of Checks
	This section describes how software that performs the authorization		The authorization check SHOULD be performed during the processing of
	interprets the results of the check_host() function. The		the SMTP transaction that sends the mail. This allows errors to be
	authorization check SHOULD be performed during the processing of the
	SMTP transaction that sends the mail. This allows errors to be
	returned directly to the sending MTA by way of SMTP replies.		returned directly to the sending MTA by way of SMTP replies.
	Performing the authorization other than using the return-path and		Performing the authorization other than using the return-path and
	client address at the time of the MAIL command during the SMTP		client address at the time of the MAIL command during the SMTP
	transaction can cause problems, such as the following: (1) It might		transaction can cause problems, such as the following: (1) It might
	be difficult to accurately extract the required information from		be difficult to accurately extract the required information from
	potentially deceptive headers; (2) legitimate email might fail		potentially deceptive headers; (2) legitimate email might fail
	because the sender's policy had since changed.		because the sender's policy had since changed.
	Generating non-delivery notifications to forged identities that have		Generating non-delivery notifications to forged identities that have
	failed the authorization check is a source of backscatter and SHOULD		failed the authorization check is a source of backscatter and SHOULD
	be avoided. [RFC3834] section 2 describes backscatter and the		be avoided. Section 2 of [RFC3834] describes backscatter and the
	problems it causes.		problems it causes.
	2.5.1. None		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 identical to the function defined
			there.
			This section enumerates and briefly defines the possible outputs of
			that function. Information about how to handle these outpus is in
			Section 8
			2.6.1. None
	A result of "none" means either (a) no syntactically valid DNS domain		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		name was extracted from the SMTP session that could be used as the
	one to be authorized, or (b) no TXT records were retrieved from the		one to be authorized, or (b) no TXT records were retrieved from the
	DNS that appeared to be intended for use by SPF verifiers.		DNS that appeared to be intended for use by SPF verifiers.
	2.5.2. Neutral		2.6.2. Neutral
	The domain owner has explicitly stated that they cannot or do not		The domain owner has explicitly stated that it is not asserting if
	want to assert whether the IP address is authorized or not. A		the IP address is authorized. This result MUST be treated exactly
	"neutral" result MUST be treated exactly like the "none" result; the		like the "none" result; the distinction exists only for informational
	distinction exists only for informational purposes. Treating		purposes.
	"neutral" more harshly than "none" would discourage domain owners
	from testing the use of SPF records (see Section 9.1).
	2.5.3. Pass		2.6.3. Pass
	A "pass" result means that the client is authorized to inject mail		A "pass" result means that the client is authorized to inject mail
	with the given identity. The domain can now, in the sense of		with the given identity. The domain can now, in the sense of
	reputation, be considered responsible for sending the message.		reputation, be considered responsible for sending the message.
	Further policy checks can now proceed with confidence in the		Further policy checks can now proceed with confidence in the
	legitimate use of the identity. This is further discussed in		legitimate use of the identity. This is further discussed in
	Section 9.3.1.		Appendix G.1.
	2.5.4. Fail		2.6.4. Fail
	A "fail" result is an explicit statement that the client is not		A "fail" result is an explicit statement that the client is not
	authorized to use the domain in the given identity. Disposition of		authorized to use the domain in the given identity.
	SPF fail messages is a matter of local policy. See Section 9.3.2 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]), 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 7) 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 domain owner.
	2.5.5. Softfail
	A "softfail" result ought to be treated as somewhere between "fail"		2.6.5. Softfail
	and "neutral"/"none". The domain owner 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 domain owner wants to discourage the use of this host and thus		The domain owner has published a weak statement that the host is
	desires limited feedback when a "softfail" result occurs. For		probably not authorized. It has not published a stronger, more
	example, the recipient's Mail User Agent (MUA) could highlight the		definitive policy that results in a "fail"
	"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.
	2.5.6. Temperror		2.6.6. Temperror
	A "temperror" result means the SPF verifier encountered a transient		A "temperror" result means the SPF verifier encountered a transient
	(generally DNS) error while performing the check. Checking software		(generally DNS) error while performing the check.
	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. These errors can be caused by problems
	in either the sender's or receiver's DNS software.
	2.5.7. Permerror		2.6.7. Permerror
	A "permerror" result means the domain's published records could not		A "permerror" result means the domain's published records could not
	be correctly interpreted. This signals an error condition that		be correctly interpreted. This signals an error condition that
	definitely requires manual intervention to be resolved. If the		definitely requires manual intervention to be resolved.
	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. Be aware that if the domain owner uses
	macros (Section 8), 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 clients due to the input
	arguments having an unexpected format; see Section 4.8.
	3. SPF Records		3. SPF Records
	An SPF record is a DNS record that declares which hosts are, and are		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"		not, authorized to use a domain name for the "HELO" and "MAIL FROM"
	identities. Loosely, the record partitions all hosts into permitted		identities. Loosely, the record partitions all hosts into permitted
	and not-permitted sets (though some hosts might fall into neither		and not-permitted sets (though some hosts might fall into neither
	category).		category).
	The SPF record is a single string of text. The record format is		The SPF record is a single string of text. The record format is
	skipping to change at page 14, line 39		skipping to change at page 12, line 39
	smtp-out.example.com. TXT "v=spf1 a -all"		smtp-out.example.com. TXT "v=spf1 a -all"
	Since TXT records have multiple uses, beware of other TXT records		Since TXT records have multiple uses, beware of other TXT records
	published there for other purposes. They might cause problems with		published there for other purposes. They might cause problems with
	size limits (see Section 3.4) and care has to be taken to ensure only		size limits (see Section 3.4) and care has to be taken to ensure only
	SPF records are used for SPF processing.		SPF records are used for SPF processing.
	ADMDs publishing SPF records SHOULD try to keep the number of		ADMDs publishing SPF records SHOULD try to keep the number of
	"include" mechanisms and chained "redirect" modifiers to a minimum.		"include" mechanisms and chained "redirect" modifiers to a minimum.
	ADMDs SHOULD also try to minimize the amount of other DNS information		ADMDs SHOULD also try to minimize the amount of other DNS information
	needed to evaluate a record. Section 4.6.4 and Section 9.1.1 provide		needed to evaluate a record. Section 4.6.4 and Section 10.1.1
	some suggestions on how to achieve this.		provide some suggestions on how to achieve this.
	3.1. DNS Resource Records		3.1. DNS Resource Records
	SPF records MUST be published as a DNS TXT (type 16) Resource Record		SPF records MUST be published as a DNS TXT (type 16) Resource Record
	(RR) [RFC1035] only. The character content of the record is encoded		(RR) [RFC1035] only. The character content of the record is encoded
	as [US-ASCII]. Use of alternate DNS RR types was supported in SPF's		as [US-ASCII]. Use of alternate DNS RR types was supported in SPF's
	experimental phase, but has been discontinued. See Appendix A of		experimental phase, but has been discontinued. See Appendix A of
	[RFC6686] for further information.		[RFC6686] for further information.
	3.2. Multiple DNS Records		3.2. Multiple DNS Records
	skipping to change at page 15, line 21		skipping to change at page 13, line 21
	3.3. Multiple Strings in a Single DNS record		3.3. Multiple Strings in a Single DNS record
	As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS		As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS
	record can be composed of more than one string. If a published		record can be composed of more than one string. If a published
	record contains multiple character-strings, then the record MUST be		record contains multiple character-strings, then the record MUST be
	treated as if those strings are concatenated together without adding		treated as if those strings are concatenated together without adding
	spaces. For example:		spaces. For example:
	IN TXT "v=spf1 .... first" "second string..."		IN TXT "v=spf1 .... first" "second string..."
	MUST be treated as equivalent to		MUST be treated as equivalent to:
	IN TXT "v=spf1 .... firstsecond string..."		IN TXT "v=spf1 .... firstsecond string..."
	TXT records containing multiple strings are useful in constructing		TXT records containing multiple strings are useful in constructing
	records that would exceed the 255-byte maximum length of a character-		records that would exceed the 255-byte maximum length of a character-
	string within a single TXT record.		string within a single TXT record.
	3.4. Record Size		3.4. Record Size
	The published SPF record for a given domain name SHOULD remain small		The published SPF record for a given domain name SHOULD remain small
	skipping to change at page 17, line 35		skipping to change at page 15, line 35
	<ip> - the IP address of the SMTP client that is emitting the		<ip> - the IP address of the SMTP client that is emitting the
	mail, either IPv4 or IPv6.		mail, either IPv4 or IPv6.
	<domain> - the domain that provides the sought-after authorization		<domain> - the domain that provides the sought-after authorization
	information; initially, the domain portion of the "MAIL		information; initially, the domain portion of the "MAIL
	FROM" or "HELO" identity.		FROM" or "HELO" identity.
	<sender> - the "MAIL FROM" or "HELO" identity.		<sender> - the "MAIL FROM" or "HELO" identity.
	The domain portion of <sender> will usually be the same as the		For recursive evaluations, the domain portion of <sender> might not
	<domain> argument when check_host() is initially evaluated. However,		be the same as the <domain> argument when check_host() is initially
	this will generally not be true for recursive evaluations (see		evaluated. In most other cases it will be the same. (See
	Section 5.2 below).		Section 5.2 below).
			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 2.1). In
			these cases, check_host() is defined in Section 4.3 to return a
			"none" result.
	4.2. Results		4.2. Results
	The function check_host() can return one of several results described		The function check_host() can return one of several results described
	in Section 2.5. Based on the result, the action to be taken is		in Section 2.6. Based on the result, the action to be taken is
	determined by the local policies of the receiver.		determined by the local policies of the receiver. This is discussed
			in Section 8
	4.3. Initial Processing		4.3. Initial Processing
	If the <domain> is malformed (e.g. label longer than 63 characters,		If the <domain> is malformed (e.g. label longer than 63 characters,
	zero-length label not at the end, etc.) or is not a fully qualified		zero-length label not at the end, etc.) or is not a fully qualified
	domain name, or if the DNS lookup returns "domain does not exist"		domain name, or if the DNS lookup returns "domain does not exist"
	(RCODE 3), check_host() immediately returns the result "none".		(RCODE 3), check_host() immediately returns the result "none".
	Properly formed domains are fully qualified email domains as		Properly formed domains are fully qualified email domains as
	described in [RFC5321] Section 2.3.5. Internationalized domain names		described in [RFC5321] Section 2.3.5. Internationalized domain names
	MUST be encoded as A-labels, as described in Section 2.3 of		MUST be encoded as A-labels, as described in Section 2.3 of
	skipping to change at page 21, line 26		skipping to change at page 19, line 32
	For example:		For example:
	v=spf1 +mx -all		v=spf1 +mx -all
	or		or
	v=spf1 +mx redirect=_spf.example.com		v=spf1 +mx redirect=_spf.example.com
	4.8. Domain Specification		4.8. Domain Specification
	Several of these mechanisms and modifiers have a domain-spec section.		Several of these mechanisms and modifiers have a domain-spec section.
	The domain-spec string is subject to macro expansion (see Section 8).		The domain-spec string is subject to macro expansion (see Section 7).
	The resulting string is the common presentation form of a fully-		The resulting string is the common presentation form of a fully-
	qualified DNS name: a series of labels separated by periods. This		qualified DNS name: a series of labels separated by periods. This
	domain is called the <target-name> in the rest of this document.		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		Note: The result of the macro expansion is not subject to any further
	escaping. Hence, this facility cannot produce all characters that		escaping. Hence, this facility cannot produce all characters that
	are legal in a DNS label (e.g., the control characters). However,		are legal in a DNS label (e.g., the control characters). However,
	this facility is powerful enough to express legal host names and		this facility is powerful enough to express legal host names and
	common utility labels (such as "_spf") that are used in DNS.		common utility labels (such as "_spf") that are used in DNS.
	skipping to change at page 23, line 27		skipping to change at page 22, line 27
	"all" MUST be ignored. Any "redirect" modifier (Section 6.1) MUST be		"all" MUST be ignored. Any "redirect" modifier (Section 6.1) MUST be
	ignored when there is an "all" mechanism in the record.		ignored when there is an "all" mechanism in the record.
	5.2. "include"		5.2. "include"
	include = "include" ":" domain-spec		include = "include" ":" domain-spec
	The "include" mechanism triggers a recursive evaluation of		The "include" mechanism triggers a recursive evaluation of
	check_host().		check_host().
	1. The domain-spec is expanded as per Section 8.		1. The domain-spec is expanded as per Section 7.
	2. Check_host() is evaluated with the resulting string as the		2. Check_host() is evaluated with the resulting string as the
	<domain>. The <ip> and <sender> arguments remain the same as in		<domain>. The <ip> and <sender> arguments remain the same as in
	the current evaluation of check_host().		the current evaluation of check_host().
	3. The recursive evaluation returns either match, not match, or an		3. The recursive evaluation returns either match, not match, or an
	error. If it matches, then the appropriate result for the		error. If it matches, then the appropriate result for the
	include: mechanism is used (e.g. include or +include gives a		include: mechanism is used (e.g. include or +include gives a
	"pass" result and -include gives "fail).		"pass" result and -include gives "fail).
	skipping to change at page 27, line 48		skipping to change at page 26, line 48
	5.7. "exists"		5.7. "exists"
	This mechanism is used to construct an arbitrary domain name that is		This mechanism is used to construct an arbitrary domain name that is
	used for a DNS A record query. It allows for complicated schemes		used for a DNS A record query. It allows for complicated schemes
	involving arbitrary parts of the mail envelope to determine what is		involving arbitrary parts of the mail envelope to determine what is
	permitted.		permitted.
	exists = "exists" ":" domain-spec		exists = "exists" ":" domain-spec
	The domain-spec is expanded as per Section 8. The resulting domain		The domain-spec is expanded as per Section 7. The resulting domain
	name is used for a DNS A RR lookup. If any A record is returned,		name is used for a DNS A RR lookup. If any A record is returned,
	this mechanism matches. The lookup type is A even when the		this mechanism matches. The lookup type is A even when the
	connection type is IPv6.		connection type is IPv6.
	Domains can use this mechanism to specify arbitrarily complex		Domains can use this mechanism to specify arbitrarily complex
	queries. For example, suppose example.com publishes the record:		queries. For example, suppose example.com publishes the record:
	v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all		v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all
	The <target-name> might expand to		The <target-name> might expand to
	skipping to change at page 29, line 36		skipping to change at page 28, line 36
	among records in a single ADMD. It is possible to control both		among records in a single ADMD. It is possible to control both
	authorized hosts and policy for an arbitrary number of domains from a		authorized hosts and policy for an arbitrary number of domains from a
	single record.		single record.
	redirect = "redirect" "=" domain-spec		redirect = "redirect" "=" domain-spec
	If all mechanisms fail to match, and a "redirect" modifier is		If all mechanisms fail to match, and a "redirect" modifier is
	present, then processing proceeds as follows:		present, then processing proceeds as follows:
	The domain-spec portion of the redirect section is expanded as per		The domain-spec portion of the redirect section is expanded as per
	the macro rules in Section 8. Then check_host() is evaluated with		the macro rules in Section 7. Then check_host() is evaluated with
	the resulting string as the <domain>. The <ip> and <sender>		the resulting string as the <domain>. The <ip> and <sender>
	arguments remain the same as in the current evaluation of		arguments remain the same as in the current evaluation of
	check_host().		check_host().
	The result of this new evaluation of check_host() is then considered		The result of this new evaluation of check_host() is then considered
	the result of the current evaluation with the exception that if no		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		SPF record is found, or if the target-name is malformed, the result
	is a "permerror" rather than "none".		is a "permerror" rather than "none".
	Note that the newly-queried domain can itself specify redirect		Note that the newly-queried domain can itself specify redirect
	skipping to change at page 30, line 33		skipping to change at page 29, line 33
	6.2. exp: Explanation		6.2. exp: Explanation
	explanation = "exp" "=" domain-spec		explanation = "exp" "=" domain-spec
	If check_host() results in a "fail" due to a mechanism match (such as		If check_host() results in a "fail" due to a mechanism match (such as
	"-all"), and the "exp" modifier is present, then the explanation		"-all"), and the "exp" modifier is present, then the explanation
	string returned is computed as described below. If no "exp" modifier		string returned is computed as described below. If no "exp" modifier
	is present, then either a default explanation string or an empty		is present, then either a default explanation string or an empty
	explanation string MUST be returned.		explanation string MUST be returned.
	The domain-spec is macro expanded (see Section 8) and becomes the		The domain-spec is macro expanded (see Section 7) and becomes the
	<target-name>. The DNS TXT record for the <target-name> is fetched.		<target-name>. The DNS TXT record for the <target-name> is fetched.
	If there are any DNS processing errors (any RCODE other than 0), or		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,		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		or if there are syntax errors in the explanation string, then proceed
	as if no exp modifier was given.		as if no exp modifier was given.
	The fetched TXT record's strings are concatenated with no spaces, and		The fetched TXT record's strings are concatenated with no spaces, and
	then treated as an explain-string, which is macro-expanded. This		then treated as an explain-string, which is macro-expanded. This
	final result is the explanation string. Implementations MAY limit		final result is the explanation string. Implementations MAY limit
	skipping to change at page 31, line 6		skipping to change at page 30, line 6
	protocol constraints and/or reasonable processing limits. Since the		protocol constraints and/or reasonable processing limits. Since the
	explanation string is intended for an SMTP response and [RFC5321]		explanation string is intended for an SMTP response and [RFC5321]
	Section 2.4 says that responses are in [US-ASCII], the explanation		Section 2.4 says that responses are in [US-ASCII], the explanation
	string MUST be limited to US-ASCII.		string MUST be limited to US-ASCII.
	Software evaluating check_host() can use this string to communicate		Software evaluating check_host() can use this string to communicate
	information from the publishing domain in the form of a short message		information from the publishing domain in the form of a short message
	or URL. Software SHOULD make it clear that the explanation string		or URL. Software SHOULD make it clear that the explanation string
	comes from a third party. For example, it can prepend the macro		comes from a third party. For example, it can prepend the macro
	string "%{o} explains: " to the explanation, such as shown in		string "%{o} explains: " to the explanation, such as shown in
	Section 2.5.4.		Section 2.6.4.
	Suppose example.com has this record:		Suppose example.com has this record:
	v=spf1 mx -all exp=explain._spf.%{d}		v=spf1 mx -all exp=explain._spf.%{d}
	Here are some examples of possible explanation TXT records at		Here are some examples of possible explanation TXT records at
	explain._spf.example.com:		explain._spf.example.com:
	"Mail from example.com should only be sent by its own servers."		"Mail from example.com should only be sent by its own servers."
	-- a simple, constant message		-- a simple, constant message
	skipping to change at page 32, line 5		skipping to change at page 31, line 5
	"See http://%{d}/why.html?s=%{S}&i=%{I}"		"See http://%{d}/why.html?s=%{S}&i=%{I}"
	-- a complicated example that constructs a URL with the		-- a complicated example that constructs a URL with the
	arguments to check_host() so that a web page can be		arguments to check_host() so that a web page can be
	generated with detailed, custom instructions		generated with detailed, custom instructions
	Note: During recursion into an "include" mechanism, an exp= modifier		Note: During recursion into an "include" mechanism, an exp= modifier
	from the <target-name> MUST NOT be used. In contrast, when executing		from the <target-name> MUST NOT be used. In contrast, when executing
	a "redirect" modifier, an exp= modifier from the original domain MUST		a "redirect" modifier, an exp= modifier from the original domain MUST
	NOT be used.		NOT be used.
	7. Recording The Result		7. Macros
	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 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 7.1 defines the Received-SPF field, which is the results
	field originally defined for SPF use. Section 7.2 discusses
	Authentication-Results [RFC5451] 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
	forensic 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 what it contained), leaving forensic 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 operator could choose to use both to serve different downstream
	agents. In such cases, care needs to be taken to ensure both fields
	are conveying the same details, or unexpected results can occur.
	7.1. 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 [RFC2532]>
	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";
	7.2. SPF Results in the Authentication-Results Header Field
	As mentioned in Section 7, 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 SPF-Received 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
	8. Macros
	8.1. Macro Definitions		7.1. Macro Definitions
	Many mechanisms and modifiers perform macro expansion on a term.		Many mechanisms and modifiers perform macro expansion on a term.
	domain-spec = macro-string domain-end		domain-spec = macro-string domain-end
	domain-end = ( "." toplabel [ "." ] ) / macro-expand		domain-end = ( "." toplabel [ "." ] ) / macro-expand
	toplabel = ( *alphanum ALPHA *alphanum ) /		toplabel = ( *alphanum ALPHA *alphanum ) /
	( 1*alphanum "-" *( alphanum / "-" ) alphanum )		( 1*alphanum "-" *( alphanum / "-" ) alphanum )
	; LDH rule plus additional TLD restrictions		; LDH rule plus additional TLD restrictions
	; (see [RFC3696], Section 2 for background)		; (see [RFC3696], Section 2 for background)
	skipping to change at page 39, line 12		skipping to change at page 34, line 12
	powerful and allow per-user records to be published, they severely		powerful and allow per-user records to be published, they severely
	limit the ability of implementations to cache results of check_host()		limit the ability of implementations to cache results of check_host()
	and they reduce the effectiveness of DNS caches.		and they reduce the effectiveness of DNS caches.
	Note: If no directive processed during the evaluation of check_host()		Note: If no directive processed during the evaluation of check_host()
	contains an "s", "l", "o", or "h" macro, then the results of the		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		evaluation can be cached on the basis of <domain> and <ip> alone for
	as long as the shortest Time To Live (TTL) of all the DNS records		as long as the shortest Time To Live (TTL) of all the DNS records
	involved.		involved.
	8.2. Expansion Examples		7.2. Expansion Examples
	The <sender> is strong-bad@email.example.com.		The <sender> is strong-bad@email.example.com.
	The IPv4 SMTP client IP is 192.0.2.3.		The IPv4 SMTP client IP is 192.0.2.3.
	The IPv6 SMTP client IP is 2001:DB8::CB01.		The IPv6 SMTP client IP is 2001:DB8::CB01.
	The PTR domain name of the client IP is mx.example.org.		The PTR domain name of the client IP is mx.example.org.
	macro expansion		macro expansion
	------- ----------------------------		------- ----------------------------
	%{s} strong-bad@email.example.com		%{s} strong-bad@email.example.com
	%{o} email.example.com		%{o} email.example.com
	skipping to change at page 41, line 5		skipping to change at page 36, line 5
	3.2.0.192.in-addr.strong.lp._spf.example.com		3.2.0.192.in-addr.strong.lp._spf.example.com
	%{d2}.trusted-domains.example.net		%{d2}.trusted-domains.example.net
	example.com.trusted-domains.example.net		example.com.trusted-domains.example.net
	IPv6:		IPv6:
	%{ir}.%{v}._spf.%{d2} 1.0.B.C.0.0.0.0.		%{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		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
	9. Implications		8. Result Handling
			This section provides guidance for operators in response to the
			various possible outputs of check_host() on a message. Terse
			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 explicity unauthorized
			("fail" and sometimes "softfail") is the norm. There are others for
			which the "false negative" cases are more of a concern. In these,
			SPF reports "fail" for legitimate mail. This concern is typically
			handled by merely recording the result in the header and allow the
			message to pass. There are still others where SPF is one of several
			inputs to the message handling decision. As such, there is no
			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 idenity 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 about the (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 domain owners
			from testing the use of SPF records (see Section 10.1).
			8.3. Pass
			A "pass" result means that 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.
			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]), 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 domain owner.
			8.5. Softfail
			A "softfail" result ought to be treated as somewhere between "fail"
			and "neutral"/"none". The domain owner 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 domain owner wants to discourage the use of this host and thus
			desires limited feedback when a "softfail" result occurs. For
			example, the recipient's Mail User Agent (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.
			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. These errors can be caused by problems
			in either the sender's or receiver's DNS software.
			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 manual 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. Be aware that if the domain owner 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 clients due to the input
			arguments having an unexpected format; see Section 4.8.
			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 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
			Authentication-Results [RFC5451] 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
			forensic 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 what it contained), leaving forensic 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 operator could choose to use both to serve different downstream
			agents. In such cases, care needs to be taken to ensure 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 [RFC2532]>
			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";
			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		This section outlines the major implications that adoption of this
	document will have on various entities involved in Internet email.		document will have on various entities involved in Internet email.
	It is intended to make clear to the reader where this document		It is intended to make clear to the reader where this document
	knowingly affects the operation of such entities. This section is		knowingly affects the operation of such entities. This section is
	not a "how-to" manual, or a "best practices" document, and it is not		not a "how-to" manual, or a "best practices" document, and it is not
	a comprehensive list of what such entities SHOULD do in light of this		a comprehensive list of what such entities SHOULD do in light of this
	document.		document.
	This section is non-normative. [RFC5598] describes the Internet		This section provides operational advice and instruction only. It is
	email architecture. This section is organized based on the different		non-normative.
	segments of the architecture.
	9.1. Sending Domains		[RFC5598] describes the Internet email architecture. This section is
			organized based on the different segments of the architecture.
			10.1. Sending Domains
	Originating ADMDs (ADministrative Management Domains - [RFC5598]		Originating ADMDs (ADministrative Management Domains - [RFC5598]
	Section 2.2.1 and Section 2.3) that wish to be compliant with this		Section 2.2.1 and Section 2.3) that wish to be compliant with this
	specification will need to determine the list of relays ([RFC5598]		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"		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		and "MAIL FROM" identities when relaying to other ADMDs. It is
	recognized that forming such a list is not just a simple technical		recognized that forming such a list is not just a simple technical
	exercise, but involves policy decisions with both technical and		exercise, but involves policy decisions with both technical and
	administrative considerations.		administrative considerations.
	9.1.1. DNS Resource Considerations		10.1.1. DNS Resource Considerations
	Minimizing the DNS resources required for SPF lookups can be done by		Minimizing the DNS resources required for SPF lookups can be done by
	choosing directives that require less DNS information and by placing		choosing directives that require less DNS information and by placing
	lower-cost mechanisms earlier in the SPF record.		lower-cost mechanisms earlier in the SPF record.
	+----------+--------+-----------------+		+----------+--------+-----------------+
	| term | cost | limit |		| term | cost | limit |
	+----------+--------+-----------------+		+----------+--------+-----------------+
	| ip4/ip6 | 0 | - |		| ip4/ip6 | 0 | - |
	| a | 1 | 10 |		| a | 1 | 10 |
	skipping to change at page 42, line 31		skipping to change at page 44, line 33
	@ IN TXT "v=spf1 a:authorized-spf.example.com -all"		@ IN TXT "v=spf1 a:authorized-spf.example.com -all"
	authorized-spf IN A 192.0.2.1		authorized-spf IN A 192.0.2.1
	IN A 192.0.2.129		IN A 192.0.2.129
	Expensive record:		Expensive record:
	example.com. IN TXT "v=spf1 mx:example.com -all"		example.com. IN TXT "v=spf1 mx:example.com -all"
	Wasteful, bad record:		Wasteful, bad record:
	example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 mx -all"		example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 mx -all"
	9.1.2. Administrator's Considerations		10.1.2. Administrator's Considerations
	There might be administrative considerations: using "a" over "ip4" or		There might be administrative considerations: using "a" over "ip4" or
	"ip6" allows hosts to be renumbered easily. Using "mx" over "a"		"ip6" allows hosts to be renumbered easily. Using "mx" over "a"
	allows the set of mail hosts to be changed easily. Unless such		allows the set of mail hosts to be changed easily. Unless such
	changes are common, it is better to use the less resource intensive		changes are common, it is better to use the less resource intensive
	mechanisms like "ip4" and "ip6" over "a" or "a" or "mx".		mechanisms like "ip4" and "ip6" over "a" or "a" or "mx".
	In some specific cases, standard advice on record content is		In some specific cases, standard advice on record content is
	appropriate. Publishing SPF records for domains that send no mail is		appropriate. Publishing SPF records for domains that send no mail is
	a well established best practice. The record for a domain that sends		a well established best practice. The record for a domain that sends
	skipping to change at page 42, line 47		skipping to change at page 45, line 4
	mechanisms like "ip4" and "ip6" over "a" or "a" or "mx".		mechanisms like "ip4" and "ip6" over "a" or "a" or "mx".
	In some specific cases, standard advice on record content is		In some specific cases, standard advice on record content is
	appropriate. Publishing SPF records for domains that send no mail is		appropriate. Publishing SPF records for domains that send no mail is
	a well established best practice. The record for a domain that sends		a well established best practice. The record for a domain that sends
	no mail is:		no mail is:
	www.example.com. IN TXT "v=spf1 -all"		www.example.com. IN TXT "v=spf1 -all"
	Publishing SPF records for individual hosts is also best practice.		Publishing SPF records for individual hosts is also best practice.
	The hostname is generally the identity used in the 5321.HELO/.EHLO		The hostname is generally the identity used in the 5321.HELO/.EHLO
	command. In the case of messages with a null 5321.MailFrom, this is		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 as the domain for 5321.MailFrom SPF checks, in addition to being
	used in 5321.HELO/.EHLO based SPF checks. The standard SPF record		used in 5321.HELO/.EHLO based SPF checks. The standard SPF record
	for an individual host that is involved in mail processing is:		for an individual host that is involved in mail processing is:
	relay.example.com. IN TXT "v=spf1 a -all"		relay.example.com. IN TXT "v=spf1 a -all"
	Validating correct deployment is difficult. [RFC6652] describes one		Validating correct deployment is difficult. [RFC6652] describes one
	mechanism for soliciting feedback on SPF failures. Another approach		mechanism for soliciting feedback on SPF failures. Another
	that can be helpful to publish records that include a "tracking		suggestion can be found in Appendix C.
	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
	Regardless of the method used, understanding the ADMD's outbound mail		Regardless of the method used, understanding the ADMD's outbound mail
	architecture is essential to effective deployment.		architecture is essential to effective deployment.
	9.1.3. Bounces		10.1.3. Bounces
	As explained in Section 1.3.3, [RFC5321] allows the reverse-path to		As explained in Section 1.1.3, [RFC5321] allows the reverse-path to
	be null, which is typical of some Delivery Status Notification		be null, which is typical of some Delivery Status Notification
	[RFC3464], commonly called email bounces. In this case the only		[RFC3464], commonly called email bounces. In this case the only
	entity available for performing an SPF check is the "HELO" identity		entity available for performing an SPF check is the "HELO" identity
	defined in Section 1.3.4. SPF functionality is enhanced by		defined in Section 1.1.4. SPF functionality is enhanced by
	administrators ensuring this identity is set correctly and has an		administrators ensuring this identity is set correctly and has an
	appropriate SPF record. It is normal to have the HELO identity set		appropriate SPF record. It is normal to have the HELO identity set
	to hostname instead of domain. Zone file generation for significant		to hostname instead of domain. Zone file generation for significant
	numbers of hosts can be consolidated using the redirect modifier and		numbers of hosts can be consolidated using the redirect modifier and
	scripted for initial deployment. Specific deployment advice is given		scripted for initial deployment. Specific deployment advice is given
	above in Section 9.1.2.		above in Section 10.1.2.
	9.2. Mediators		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 normative requirement for specific handling of a message based on
			any SPF result. 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 nonsense 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 take both of these cases under
			consideration when establishing local handling policy.
			10.3. Mediators
	Broadly speaking, there are two types of mediating ADMDs that can		Broadly speaking, there are two types of mediating ADMDs that can
	affect SPF deployment of other ADMDs: mailing lists (see [RFC5598]		affect SPF deployment of other ADMDs: mailing lists (see [RFC5598]
	Section 5.3) and ReSenders ([RFC5598] Section 5.2).		Section 5.3) and ReSenders ([RFC5598] Section 5.2).
	9.2.1. Mailing Lists		10.3.1. Mailing Lists
	Mailing lists have to be aware of how they re-inject mail that is		Mailing lists have to be aware of how they re-inject mail that is
	sent to the list. Mailing lists MUST comply with the requirements in		sent to the list. Mailing lists MUST comply with the requirements in
	[RFC5321], Section 3.10, and [RFC1123], Section 5.3.6, that say that		[RFC5321], Section 3.10, and [RFC1123], Section 5.3.6, that say that
	the reverse-path MUST be changed to be the mailbox of a person or		the reverse-path MUST be changed to be the mailbox of a person or
	other entity who administers the list. Whereas the reasons for		other entity who administers the list. Whereas the reasons for
	changing the reverse-path are many and long-standing, SPF adds		changing the reverse-path are many and long-standing, SPF adds
	enforcement to this requirement.		enforcement to this requirement.
	In practice, almost all mailing list software in use already complies		In practice, almost all mailing list software in use already complies
	with this requirement. Mailing lists that do not comply might		with this requirement. Mailing lists that do not comply might
	encounter problems depending on how access to the list is restricted.		encounter problems depending on how access to the list is restricted.
	Such lists that are entirely internal to a domain (only people in the		Such lists that are entirely internal to a domain (only people in the
	domain can send to or receive from the list) are not affected.		domain can send to or receive from the list) are not affected.
	9.2.2. Forwarding Services and Aliases		10.3.2. Forwarding Services and Aliases
	Forwarding services take mail that is received at a mailbox and		Forwarding services take mail that is received at a mailbox and
	direct it to some external mailbox. At the time of this writing, the		direct it to some external mailbox. At the time of this writing, the
	near-universal practice of such services is to use the original "MAIL		near-universal practice of such services is to use the original "MAIL
	FROM" of a message when re-injecting it for delivery to the external		FROM" of a message when re-injecting it for delivery to the external
	mailbox. [RFC1123] and [RFC5321] describe this action as an "alias"		mailbox. [RFC1123] and [RFC5321] describe this action as an "alias"
	rather than a "mail list". This means the external mailbox's MTA		rather than a "mail list". This means the external mailbox's MTA
	sees all such mail in a connection from a host of the forwarding		sees all such mail in a connection from a host of the forwarding
	service, and so the "MAIL FROM" identity will not, in general, pass		service, and so the "MAIL FROM" identity will not, in general, pass
	authorization.		authorization.
	There are three places that techniques can be used to ameliorate this		Appendix D provides some operational suggestions to adapt these
	problem.		services to an SPF-aware environment.
	1. The beginning, when email is first sent (Originating ADMDs).
	1. "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}.whitlist.example.org -all"
	This would cause a lookup on an DNS white list (DNSWL) and
	cause a result of "fail" only for email not either coming
	from the domain's mx host(s) (SPF pass) or white listed
	sources (SPF neutral). This, in effect, outsources an
	element of sender policy to the maintainer of the whitelist.
	2. 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, such an SPF
	record 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 either to only produce local-parts with
	a maximum of 63 characters or to gracefully handle truncated
	local-parts.
	3. 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"
	4. 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}"
	2. The middle, when email is forwarded (Mediating ADMDs).
	1. Forwarding services 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
	forwarding service.
	2. 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").
	3. Forwarding servers could reject mail that would "fail" SPF if
	forwarded using an SMTP reply code of 551, User not local,
	(see [RFC5321] section 3.4) to communicate the correct target
	address to resend the mail to.
	3. The end, when email is received (Receiving ADMDs).
	1. If the owner of the external mailbox wishes to trust the
	forwarding service, he can direct the external mailbox's MTA
	to skip SPF tests when the client host belongs to the
	forwarding service.
	2. Tests against other identities, such as the "HELO" identity,
	MAY be used to override a failed test against the "MAIL FROM"
	identity.
	3. 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.
	9.2.3. Mail Services
	MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail
	services to third-party domains, such as 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 the domain of one of the MSPs
	domain, 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
	mitiate the risk of cross-customer forgery (see Section 10.4).
	9.2.4. MTA Relays
	Relays are described in [RFC5598] Section 2.2.2. The authorization
	check generally precludes the use of arbitrary MTA relays between
	sender and receiver of an email message.
	Within an organization, MTA relays can be effectively deployed.
	However, for 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 that 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 both as 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 have 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.
	9.3. 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.
	9.3.1. Policy For SPF Pass
	SPF pass results can be used in combination with "white lists" of
	known "good" domains to bypass some or all additional pre-delivery
	email checks. Exactly which checks and how to determine appropriate
	white list entries has to be based on local conditions and
	requirements.
	9.3.2. 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 above in this section (Section 9), SPF based
	rejection does present some risk of rejecting legitimate email when
	rejecting based on "MAIL FROM" results.
	SPF fail results can alternately be used as one input into a larger
	set of evaluations which might, based on a combination 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 have 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. They are either 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 reliabilility
	and utility of email across the Internet.
	9.3.3. Policy For SPF Permerror
	The "permerror" result (see Section 2.5.7) indicates 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
	operators that wish to implement SPF checking as strictly as
	possible, nor is this sort of passive problem reporting typically
	effective.
	There is of course the option placing this choice in the hands of the
	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 operators need to be cautious of all choices
	and outcomes when handling SPF results.
	10. Security Considerations		11. Security Considerations
	10.1. Processing Limits		11.1. Processing Limits
	As with most aspects of email, there are a number of ways that		As with most aspects of email, there are a number of ways that
	malicious parties could use the protocol as an avenue for a		malicious parties could use the protocol as an avenue for a
	Denial-of-Service (DoS) attack. The processing limits outlined in		Denial-of-Service (DoS) attack. The processing limits outlined in
	Section 4.6.4 are designed to prevent attacks such as the following:		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		o A malicious party could create an SPF record with many references
	to a victim's domain and send many emails to different SPF		to a victim's domain and send many emails to different SPF
	verifiers; those SPF verifiers would then create a DoS attack. In		verifiers; those SPF verifiers would then create a DoS attack. In
	effect, the SPF verifiers are being used to amplify the attacker's		effect, the SPF verifiers are being used to amplify the attacker's
	skipping to change at page 49, line 41		skipping to change at page 47, line 41
	come from the intended target to a wide variety of legitimate mail		come from the intended target to a wide variety of legitimate mail
	hosts. These legitimate machines would then present a DNS load on		hosts. These legitimate machines would then present a DNS load on
	the target as they fetched the relevant records.		the target as they fetched the relevant records.
	Of these, the case of a third party referenced in the SPF record is		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,		the easiest for a DoS attack to effectively exploit. As a result,
	limits that might seem reasonable for an individual mail server can		limits that might seem reasonable for an individual mail server can
	still allow an unreasonable amount of bandwidth amplification.		still allow an unreasonable amount of bandwidth amplification.
	Therefore, the processing limits need to be quite low.		Therefore, the processing limits need to be quite low.
	10.2. SPF-Authorized Email May Contain Other False Identities		11.2. SPF-Authorized Email May Contain Other False Identities
	Do not construe the "MAIL FROM" and "HELO" identity authorizations to		Do not construe the "MAIL FROM" and "HELO" identity authorizations to
	provide more assurance than they do. It is entirely possible for a		provide more assurance than they do. It is entirely possible for a
	malicious sender to inject a message using his own domain in the		malicious sender to inject a message using his own domain in the
	identities used by SPF, to have that domain's SPF record authorize		identities used by SPF, to have that domain's SPF record authorize
	the sending host, and yet the message can easily list other		the sending host, and yet the message can easily list other
	identities in its header. Unless the user or the MUA takes care to		identities in its header. Unless the user or the MUA takes care to
	note that the authorized identity does not match the other more		note that the authorized identity does not match the other more
	commonly-presented identities (such as the From: header field), the		commonly-presented identities (such as the From: header field), the
	user might be lulled into a false sense of security.		user might be lulled into a false sense of security.
	10.3. Spoofed DNS and IP Data		11.3. Spoofed DNS and IP Data
	There are two aspects of this protocol that malicious parties could		There are two aspects of this protocol that malicious parties could
	exploit to undermine the validity of the check_host() function:		exploit to undermine the validity of the check_host() function:
	o The evaluation of check_host() relies heavily on DNS. A malicious		o The evaluation of check_host() relies heavily on DNS. A malicious
	attacker could attack the DNS infrastructure and cause		attacker could attack the DNS infrastructure and cause
	check_host() to see spoofed DNS data, and then return incorrect		check_host() to see spoofed DNS data, and then return incorrect
	results. This could include returning "pass" for an <ip> value		results. This could include returning "pass" for an <ip> value
	where the actual domain's record would evaluate to "fail". See		where the actual domain's record would evaluate to "fail". See
	[RFC3833] for a description of DNS weaknesses.		[RFC3833] for a description of DNS weaknesses.
	o The client IP address, <ip>, is assumed to be correct. In a		o The client IP address, <ip>, is assumed to be correct. In a
	modern, correctly configured system the risk of this not being		modern, correctly configured system the risk of this not being
	true is nil.		true is nil.
	10.4. Cross-User Forgery		11.4. Cross-User Forgery
	By definition, SPF policies just map domain names to sets of		By definition, SPF policies just map domain names to sets of
	authorized MTAs, not whole email addresses to sets of authorized		authorized MTAs, not whole email addresses to sets of authorized
	users. Although the "l" macro (Section 8) provides a limited way to		users. Although the "l" macro (Section 7) provides a limited way to
	define individual sets of authorized MTAs for specific email		define individual sets of authorized MTAs for specific email
	addresses, it is generally impossible to verify, through SPF, the use		addresses, it is generally impossible to verify, through SPF, the use
	of specific email addresses by individual users of the same MTA.		of specific email addresses by individual users of the same MTA.
	It is up to mail services and their MTAs to directly prevent		It is up to mail services and their MTAs to directly prevent
	cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be		cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be
	restricted to using only those email addresses that are actually		restricted to using only those email addresses that are actually
	under their control (see [RFC6409], Section 6.1). Another means to		under their control (see [RFC6409], Section 6.1). Another means to
	verify the identity of individual users is message cryptography such		verify the identity of individual users is message cryptography such
	as PGP ([RFC4880]) or S/MIME ([RFC5751]).		as PGP ([RFC4880]) or S/MIME ([RFC5751]).
	10.5. Untrusted Information Sources		11.5. Untrusted Information Sources
	An SPF compliant receiver gathers information from the SMTP commands		An SPF compliant receiver gathers information from the SMTP commands
	it receives and from the published DNS records of the sending domain		it receives and from the published DNS records of the sending domain
	holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the		holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the
	envelope, and SPF DNS records published by the domain holder).		envelope, and SPF DNS records published by the domain holder).
	10.5.1. Recorded Results		11.5.1. Recorded Results
	This information, passed to the receiver in the Received-SPF: or		This information, passed to the receiver in the Received-SPF: or
	Authentication-Results: trace fields, may be returned to the client		Authentication-Results: trace fields, may be returned to the client
	MTA as an SMTP rejection message. If such an SMTP rejection message		MTA as an SMTP rejection message. If such an SMTP rejection message
	is generated, the information from the trace fields has to be checked		is generated, the information from the trace fields has to be checked
	for such problems as invalid characters and excessively long lines.		for such problems as invalid characters and excessively long lines.
	10.5.2. External Explanations		11.5.2. External Explanations
	When the authorization check fails, an explanation string could be		When the authorization check fails, an explanation string could be
	included in the reject response. Both the sender and the rejecting		included in the reject response. Both the sender and the rejecting
	receiver need to be aware that the explanation was determined by the		receiver need to be aware that the explanation was determined by the
	publisher of the SPF record checked and, in general, not the		publisher of the SPF record checked and, in general, not the
	receiver. The explanation can contain malicious URLs, or it might be		receiver. The explanation can contain malicious URLs, or it might be
	offensive or misleading.		offensive or misleading.
	Explanations returned to sender domains due to "exp" modifiers,		Explanations returned to sender domains due to "exp" modifiers,
	(Section 6.2), were generated by the sender policy published by the		(Section 6.2), were generated by the sender policy published by the
	domain holders themselves. As long as messages are only returned		domain holders themselves. As long as messages are only returned
	with non-delivery notification ([RFC3464]) to domains publishing the		with non-delivery notification ([RFC3464]) to domains publishing the
	explanation strings from their own DNS SPF records, the only affected		explanation strings from their own DNS SPF records, the only affected
	parties are the original publishers of the domain's SPF records.		parties are the original publishers of the domain's SPF records.
	In practice, such non-delivery notifications can be misdirected, such		In practice, such non-delivery notifications can be misdirected, such
	as when an MTA accepts an email and only later generates the		as when an MTA accepts an email and only later generates the
	notification to a forged address, or when an email forwarder does not		notification to a forged address, or when an email forwarder does not
	direct the bounce back to the original sender.		direct the bounce back to the original sender.
	10.5.3. Macro Expansion		11.5.3. Macro Expansion
	Macros (Section 8) allow senders to inject arbitrary text (any non-		Macros (Section 7) allow senders to inject arbitrary text (any non-
	null [US-ASCII] character) into receiver DNS queries. It is necesary		null [US-ASCII] character) into receiver DNS queries. It is necesary
	to be prepared for hostile or unexpected content.		to be prepared for hostile or unexpected content.
	10.6. Privacy Exposure		11.6. Privacy Exposure
	Checking SPF records causes DNS queries to be sent to the domain		Checking SPF records causes DNS queries to be sent to the domain
	owner. These DNS queries, especially if they are caused by the		owner. These DNS queries, especially if they are caused by the
	"exists" mechanism, can contain information about who is sending		"exists" mechanism, can contain information about who is sending
	email and likely to which MTA the email is being sent. This can		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		introduce some privacy concerns, which are more or less of an issue
	depending on local laws and the relationship between the domain owner		depending on local laws and the relationship between the domain owner
	and the person sending the email.		and the person sending the email.
	11. Contributors and Acknowledgements		12. Contributors and Acknowledgements
	This document is largely based on the work of Meng Weng Wong, Mark		This document is largely based on the work of Meng Weng Wong, Mark
	Lentczner, and Wayne Schlitt. Although, as this section		Lentczner, and Wayne Schlitt. Although, as this section
	acknowledges, many people have contributed to this document, a very		acknowledges, many people have contributed to this document, a very
	large portion of the writing and editing are due to Meng, Mark, and		large portion of the writing and editing are due to Meng, Mark, and
	Wayne.		Wayne.
	This design owes a debt of parentage to [RMX] by Hadmut Danisch and		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		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		the legitimacy of an email address traces its ancestry further back
	skipping to change at page 53, line 5		skipping to change at page 51, line 5
	the development of this design. They are far too numerous to name,		the development of this design. They are far too numerous to name,
	but they include the following:		but they include the following:
	The participants in the SPFbis working group.		The participants in the SPFbis working group.
	The folks on the spf-discuss mailing list.		The folks on the spf-discuss mailing list.
	The folks on the SPAM-L mailing list.		The folks on the SPAM-L mailing list.
	The folks on the IRTF ASRG mailing list.		The folks on the IRTF ASRG mailing list.
	The folks on the IETF MARID mailing list.		The folks on the IETF MARID mailing list.
	The folks on #perl.		The folks on #perl.
	12. IANA Considerations		13. IANA Considerations
	12.1. The SPF DNS Record Type		13.1. The SPF DNS Record Type
	Per [RFC4408], the IANA assigned the Resource Record Type and Qtype		Per [RFC4408], the IANA assigned the Resource Record Type and Qtype
	from the DNS Parameters Registry for the SPF RR type with code 99.		from the DNS Parameters Registry for the SPF RR type with code 99.
	The format of this type is identical to the TXT RR [RFC1035]. The		The format of this type is identical to the TXT RR [RFC1035]. The
	character content of the record is encoded as [US-ASCII]. Use of		character content of the record is encoded as [US-ASCII]. Use of
	this record type is obsolete for SPF Version 1.		this record type is obsolete for SPF Version 1.
	IANA is requested to add an annotation to the SPF RRTYPE saying		IANA is requested to add an annotation to the SPF RRTYPE saying
	"(OBSOLETE - use TXT)" in the DNS Parameters registry.		"(OBSOLETE - use TXT)" in the DNS Parameters registry.
	[NOTE TO RFC EDITOR: (to be changed to " ... has added ..." upon		[NOTE TO RFC EDITOR: (to be changed to " ... has added ..." upon
	publication)]		publication)]
	12.2. The Received-SPF Mail Header Field		13.2. The Received-SPF Mail Header Field
	Per [RFC3864], the "Received-SPF:" header field is added to the IANA		Per [RFC3864], the "Received-SPF:" header field is added to the IANA
	Permanent Message Header Field Registry. The following is the		Permanent Message Header Field Registry. The following is the
	registration template:		registration template:
	Header field name: Received-SPF		Header field name: Received-SPF
	Applicable protocol: mail ([RFC5322])		Applicable protocol: mail ([RFC5322])
	Status: Standards Track		Status: Standards Track
	Author/Change controller: IETF		Author/Change controller: IETF
	Specification document(s): RFC XXXX		Specification document(s): RFC XXXX
	[NOTE TO RFC EDITOR: (this document)]		[NOTE TO RFC EDITOR: (this document)]
	12.3. SPF Modifier Registration		13.3. SPF Modifier Registration
	[RFC6652] created a new SPF Modifier Registration. IANA is requested		[RFC6652] created a new SPF Modifier Registration. IANA is requested
	to change the reference for the exp and redirect modifiers from		to change the reference for the exp and redirect modifiers from
	[RFC4408] to this document. Their status should not be changed.		[RFC4408] to this document. Their status should not be changed.
	13. References		14. References
	13.1. Normative References		14.1. Normative References
	[RFC1035] Mockapetris, P., "Domain names - implementation and		[RFC1035] Mockapetris, P., "Domain names - implementation and
	specification", STD 13, RFC 1035, November 1987.		specification", STD 13, RFC 1035, November 1987.
	[RFC1123] Braden, R., "Requirements for Internet Hosts - Application		[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
	and Support", STD 3, RFC 1123, October 1989.		and Support", STD 3, RFC 1123, October 1989.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	skipping to change at page 55, line 11		skipping to change at page 53, line 11
	[US-ASCII]		[US-ASCII]
	American National Standards Institute (formerly United		American National Standards Institute (formerly United
	States of America Standards Institute), "USA Code for		States of America Standards Institute), "USA Code for
	Information Interchange, X3.4", 1968.		Information Interchange, X3.4", 1968.
	ANSI X3.4-1968 has been replaced by newer versions with		ANSI X3.4-1968 has been replaced by newer versions with
	slight modifications, but the 1968 version remains		slight modifications, but the 1968 version remains
	definitive for the Internet.		definitive for the Internet.
	13.2. Informative References		14.2. Informative References
	[DMP] Fecyk, G., "Designated Mailers Protocol".		[DMP] Fecyk, G., "Designated Mailers Protocol".
	Work In Progress		Work In Progress
	[Green] Green, D., "Domain-Authorized SMTP Mail", 2002.		[Green] Green, D., "Domain-Authorized SMTP Mail", 2002.
	[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",		[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
	STD 13, RFC 1034, November 1987.		STD 13, RFC 1034, November 1987.
	skipping to change at page 63, line 5		skipping to change at page 61, line 5
	"-include:ip4._spf.%{d} "		"-include:ip4._spf.%{d} "
	"-include:ptr._spf.%{d} "		"-include:ptr._spf.%{d} "
	"+all" )		"+all" )
	ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all"		ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all"
	ptr._spf.example.com. SPF "v=spf1 -ptr +all"		ptr._spf.example.com. SPF "v=spf1 -ptr +all"
	This example shows how the "-include" mechanism can be useful, how an		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		SPF record that ends in "+all" can be very restrictive, and the use
	of De Morgan's Law.		of De Morgan's Law.
	Appendix C. Change History		Appendix C. Further Testing Advice
			Another approach that can be helpful 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
			Appendix D. Updating Mail Forwarders
			There are three places that techniques can be used to ameliorate this
			problem.
			1. The beginning, when email is first sent (Originating ADMDs).
			* "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}.whitlist.example.org -all"
			This would cause a lookup on an DNS white list (DNSWL) and
			cause a result of "fail" only for email not either coming from
			the domain's mx host(s) (SPF pass) or white listed sources
			(SPF neutral). This, in effect, outsources an element of
			sender policy to the maintainer of the whitelist.
			* 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, such an SPF
			record 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 either to only produce local-parts with a
			maximum of 63 characters or to gracefully handle truncated
			local-parts.
			* 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"
			* 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}"
			2. The middle, when email is forwarded (Mediating ADMDs).
			* Forwarding services 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
			forwarding service.
			* 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").
			* Forwarding servers could reject mail that would "fail" SPF if
			forwarded using an SMTP reply code of 551, User not local,
			(see [RFC5321] section 3.4) to communicate the correct target
			address to resend the mail to.
			3. The end, when email is received (Receiving ADMDs).
			* If the owner of the external mailbox wishes to trust the
			forwarding service, he can direct the external mailbox's MTA
			to skip SPF tests when the client host belongs to the
			forwarding service.
			* Tests against other identities, such as the "HELO" identity,
			MAY be used to override a failed test against the "MAIL FROM"
			identity.
			* 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.
			Appendix E. Mail Services
			MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail
			services to third-party domains, such as 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 the domain of one of the MSPs
			domain, 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
			mitiate 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
			sender and receiver of an email message.
			Within an organization, MTA relays can be effectively deployed.
			However, for 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 that 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 both as 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 have 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.
			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 "white lists" of
			known "good" domains to bypass some or all additional pre-delivery
			email checks. Exactly which checks and how to determine appropriate
			white list entries has 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 above in this section (Section 10), SPF based
			rejection does present some risk of rejecting legitimate email when
			rejecting based on "MAIL FROM" results.
			SPF fail results can alternately be used as one input into a larger
			set of evaluations which might, based on a combination 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 have 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. They are either 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 reliabilility
			and utility of email across the Internet.
			G.3. Policy For SPF Permerror
			The "permerror" result (see Section 2.6.7) indicates 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
			operators that wish to implement SPF checking as strictly as
			possible, nor is this sort of passive problem reporting typically
			effective.
			There is of course the option placing this choice in the hands of the
			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 operators need to be cautious of all choices
			and outcomes when handling SPF results.
			Appendix H. Protocol Status
			SPF has been in development since the summer of 2003 and has seen
			deployment beyond the developers beginning in December 2003. The
			design of SPF slowly evolved until the spring of 2004 and has since
			stabilized. There have been quite a number of forms of SPF, some
			written up as documents, some submitted as Internet Drafts, and many
			discussed and debated in development forums. The protocol was
			originally defined in [RFC4408], which this document replaces.
			[RFC4408] was designed to clearly document the protocol defined by
			earlier draft specifications of SPF as used in existing
			implementations. This updated specification is intended to clarify
			identified ambiguities in [RFC4408], resolve techincal issues
			identified in post-RFC 4408 deplyment experience, and document widely
			deployed extensions to SPF that have been developed since [RFC4408]
			was published.
			Appendix I. Experimental History
			This document updates and replaces RFC 4408 that was part of a group
			of simultaneously published Experimental RFCs (RFC 4405, RFC 4406,
			RFC 4407, and RFC 4408) in 2006. At that time the IESG requested the
			community observe the success or failure of the two approaches
			documented in these RFCs during the two years following publication,
			in order that a community consensus could be reached in the future.
			SPF is widely deployed by large and small email providers alike.
			There are multiple, interoperable implementations.
			For SPF (as documented in RFC 4408) a careful effort was made to
			collect and document lessons learned and errata during the two year
			period. The errata list has been stable (no new submissions) and
			only minor protocol lessons learned were identified. Resolution of
			the IESG's experiment is documented in [RFC6686].
			Appendix J. Change History
	Changes since RFC 4408 (to be removed prior to publication)		Changes since RFC 4408 (to be removed prior to publication)
	Moved to standards track		Moved to standards track
	Authors updated		Authors updated
	IESG Note regarding experimental use replaced with discussion of		IESG Note regarding experimental use replaced with discussion of
	results		results
End of changes. 82 change blocks.
	631 lines changed or deleted		767 lines changed or added
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