draft-ietf-uta-smtp-require-tls-06.txt		draft-ietf-uta-smtp-require-tls-07.txt
	Internet Engineering Task Force J. Fenton		Internet Engineering Task Force J. Fenton
	Internet-Draft Altmode Networks		Internet-Draft Altmode Networks
	Intended status: Standards Track December 4, 2018		Intended status: Standards Track January 22, 2019
	Expires: June 7, 2019		Expires: July 26, 2019
	SMTP Require TLS Option		SMTP Require TLS Option
	draft-ietf-uta-smtp-require-tls-06		draft-ietf-uta-smtp-require-tls-07
	Abstract		Abstract
	The SMTP STARTTLS option, used in negotiating transport-level		The SMTP STARTTLS option, used in negotiating transport-level
	encryption of SMTP connections, is not as useful from a security		encryption of SMTP connections, is not as useful from a security
	standpoint as it might be because of its opportunistic nature;		standpoint as it might be because of its opportunistic nature;
	message delivery is, by default, prioritized over security. This		message delivery is, by default, prioritized over security. This
	document describes an SMTP service extension, REQUIRETLS, and message		document describes an SMTP service extension, REQUIRETLS, and message
	header field, RequireTLS. If the REQUIRETLS option or RequireTLS		header field, RequireTLS. If the REQUIRETLS option or RequireTLS
	message header field is used when sending a message, it asserts a		message header field is used when sending a message, it asserts a
	skipping to change at page 1, line 41 ¶		skipping to change at page 1, line 41 ¶
	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 https://datatracker.ietf.org/drafts/current/.		Drafts is at https://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 June 7, 2019.		This Internet-Draft will expire on July 26, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2019 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
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://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
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3		1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
	2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 4		2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 4
	3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 4		3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 5
	4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5		4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 6
	4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5		4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 6
	4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 5		4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 6
	4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 5		4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 6
	4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 6		4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 7
	4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7		4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 8
	4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7		4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 8
	5. Non-delivery message handling . . . . . . . . . . . . . . . . 7		5. Non-delivery message handling . . . . . . . . . . . . . . . . 8
	6. Mailing list considerations . . . . . . . . . . . . . . . . . 8		6. Mailing list considerations . . . . . . . . . . . . . . . . . 9
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 9		8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
	8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10		8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 11
	8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10		8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 11
	8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10		8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 11
	9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11		9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
	10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11		10. Revision History . . . . . . . . . . . . . . . . . . . . . . 12
	10.1. Changes since -05 Draft . . . . . . . . . . . . . . . . 11		10.1. Changes since -06 Draft . . . . . . . . . . . . . . . . 12
	10.2. Changes since -04 Draft . . . . . . . . . . . . . . . . 11		10.2. Changes since -05 Draft . . . . . . . . . . . . . . . . 12
	10.3. Changes since -03 Draft . . . . . . . . . . . . . . . . 11		10.3. Changes since -04 Draft . . . . . . . . . . . . . . . . 12
	10.4. Changes since -02 Draft . . . . . . . . . . . . . . . . 11		10.4. Changes since -03 Draft . . . . . . . . . . . . . . . . 13
	10.5. Changes since -01 Draft . . . . . . . . . . . . . . . . 12		10.5. Changes since -02 Draft . . . . . . . . . . . . . . . . 13
	10.6. Changes since -00 Draft . . . . . . . . . . . . . . . . 12		10.6. Changes since -01 Draft . . . . . . . . . . . . . . . . 13
	10.7. Changes since fenton-03 Draft . . . . . . . . . . . . . 12		10.7. Changes since -00 Draft . . . . . . . . . . . . . . . . 13
	10.8. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12		10.8. Changes since fenton-03 Draft . . . . . . . . . . . . . 13
	10.9. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12		10.9. Changes Since -02 Draft . . . . . . . . . . . . . . . . 14
	10.10. Changes Since -00 Draft . . . . . . . . . . . . . . . . 13		10.10. Changes Since -01 Draft . . . . . . . . . . . . . . . . 14
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13		10.11. Changes Since -00 Draft . . . . . . . . . . . . . . . . 14
	11.1. Normative References . . . . . . . . . . . . . . . . . . 13		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
	11.2. Informative References . . . . . . . . . . . . . . . . . 15		11.1. Normative References . . . . . . . . . . . . . . . . . . 14
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15		11.2. Informative References . . . . . . . . . . . . . . . . . 16
			Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 17
			A.1. REQUIRETLS SMTP Option . . . . . . . . . . . . . . . . . 17
			A.2. RequireTLS Header Field . . . . . . . . . . . . . . . . . 18
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
	1. Introduction		1. Introduction
	The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a		The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a
	means by which an SMTP server and client can establish a Transport		means by which an SMTP server and client can establish a Transport
	Layer Security (TLS) protected session for the transmission of email		Layer Security (TLS) protected session for the transmission of email
	messages. By default, TLS is used only upon mutual agreement		messages. By default, TLS is used only upon mutual agreement
	(successful negotiation) of STARTTLS between the client and server;		(successful negotiation) of STARTTLS between the client and server;
	if this is not possible, the message is sent without transport		if this is not possible, the message is sent without transport
	encryption. Furthermore, it is common practice for the client to		encryption. Furthermore, it is common practice for the client to
	skipping to change at page 4, line 7 ¶		skipping to change at page 4, line 7 ¶
	to transit through one or more MTAs that do not support REQUIRETLS.		to transit through one or more MTAs that do not support REQUIRETLS.
	1.1. Requirements Language		1.1. Requirements Language
	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 BCP		"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [RFC2119] [RFC8174] when, and only when, they appear in all		14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
			The formal syntax uses the Augmented Backus-Naur Form (ABNF)
			[RFC5234] including the core rules defined in Appendix B of that
			document.
	2. The REQUIRETLS Service Extension		2. The REQUIRETLS Service Extension
	1. The textual name of the extension is "Require TLS".		1. The textual name of the extension is "Require TLS".
	2. The EHLO keyword value associated with this extension is		2. The EHLO keyword value associated with this extension is
	"REQUIRETLS".		"REQUIRETLS".
	3. One MAIL FROM option is defined by this extension.		3. No additional SMTP verbs are defined by this extension.
	4. One new SMTP status code is defined by this extension to convey		4. One optional parameter ("REQUIRETLS") is added to the MAIL FROM
			command by this extension. No value is associated with this
			parameter.
			5. The maximum length of a MAIL FROM command line is increased by 11
			octets by the possible addition of a space and the REQUIRETLS
			keyword.
			6. One new SMTP status code is defined by this extension to convey
	an error condition resulting from failure of the client to send		an error condition resulting from failure of the client to send
	to a server not also supporting the REQUIRETLS extension.		to a server not also supporting the REQUIRETLS extension.
			7. The REQUIRETLS extension is valid for message relay [RFC5321],
			submission [RFC6409], and the Local Mail Transfer Protocol (LMTP)
			[RFC2033]
			8. The ABNF syntax for the MAIL FROM parameter is as follows:
			requiretls-param = "REQUIRETLS"
			; where requiretls-param is an instance of an
			; esmtp-param used in Mail-parameters in
			; RFC 5321 Section 4.1.2. There is no esmtp-value
			; associated with requiretls-param.
	In order to specify REQUIRETLS treatment for a given message, the		In order to specify REQUIRETLS treatment for a given message, the
	REQUIRETLS option is specified on the MAIL FROM command when that		REQUIRETLS option is specified on the MAIL FROM command when that
	message is transmitted. This option MUST only be specified in the		message is transmitted. This option MUST only be specified in the
	context of an SMTP session meeting the security requirements that		context of an SMTP session meeting the security requirements of
	have been specified:		REQUIRETLS:
	o The session itself MUST employ TLS transmission.		o The session itself MUST employ TLS transmission.
	o If the SMTP server to which the message is being transmitted is		o If the SMTP server to which the message is being transmitted is
	identified through an MX record lookup, its name MUST be validated		identified through an MX record lookup, its name MUST be validated
	via a DNSSEC signature on the recipient domain's MX record, or the		via a DNSSEC signature on the recipient domain's MX record, or the
	MX hostname MUST be validated by an MTA-STS policy as described in		MX hostname MUST be validated by an MTA-STS policy as described in
	Section 4.1 of RFC 8461 [RFC8461]. DNSSEC is defined in RFC 4033		Section 4.1 of RFC 8461 [RFC8461]. DNSSEC is defined in RFC 4033
	[RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035].		[RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035].
	skipping to change at page 4, line 48 ¶		skipping to change at page 5, line 25 ¶
	specified in RFC 7672 [RFC7672]. For trust chains, the choice of		specified in RFC 7672 [RFC7672]. For trust chains, the choice of
	trusted (root) certificates is at the discretion of the SMTP		trusted (root) certificates is at the discretion of the SMTP
	client.		client.
	o Following the negotiation of STARTTLS, the SMTP server MUST		o Following the negotiation of STARTTLS, the SMTP server MUST
	advertise in the subsequent EHLO response that it supports		advertise in the subsequent EHLO response that it supports
	REQUIRETLS.		REQUIRETLS.
	3. The RequireTLS Header Field		3. The RequireTLS Header Field
	One new message header field, RequireTLS, is defined by this		One new message header field [RFC5322], RequireTLS, is defined by
	specification. It is used for messages requesting that recipient TLS		this specification. It is used for messages for which the originator
	policy (including MTA-STS [RFC8461] and DANE [RFC7672]) be ignored.		requests that recipient TLS policy (including MTA-STS [RFC8461] and
			DANE [RFC7672]) be ignored. This might be done, for example, to
			report a misconfigured mail server, such as an expired TLS
			certificate.
	The RequireTLS header field has a single REQUIRED parameter:		The RequireTLS header field has a single REQUIRED parameter:
	o NO - The SMTP client SHOULD attempt to send the message regardless		o NO - The SMTP client SHOULD attempt to send the message regardless
	of its ability to negotiate STARTTLS with the SMTP server,		of its ability to negotiate STARTTLS with the SMTP server,
	ignoring policy-based mechanisms (including MTA-STS and DANE), if		ignoring policy-based mechanisms (including MTA-STS and DANE), if
	any, asserted by the recipient domain. Nevertheless, the client		any, asserted by the recipient domain. Nevertheless, the client
	SHOULD negotiate STARTTLS with the server if available.		SHOULD negotiate STARTTLS with the server if available.
	More than one instance of the RequireTLS header field MUST NOT appear		More than one instance of the RequireTLS header field MUST NOT appear
	in a given message.		in a given message.
			The ABNF syntax for the RequireTLS header field is as follows:
			requiretls-field = "RequireTLS:" [FWS] "No" CRLF
			; where requiretls-field in an instance of an
			; optional-field defined in RFC 5322 Section
			; 3.6.8.
			FWS = <as defined in RFC 5322>
			CRLF = <as defined in RFC 5322>
	4. REQUIRETLS Semantics		4. REQUIRETLS Semantics
	4.1. REQUIRETLS Receipt Requirements		4.1. REQUIRETLS Receipt Requirements
	Upon receipt of the REQUIRETLS option on a MAIL FROM command during		Upon receipt of the REQUIRETLS option on a MAIL FROM command during
	the receipt of a message, an SMTP server MUST tag that message as		the receipt of a message for which the return-path is not empty
	needing REQUIRETLS handling.		(indicating a bounce message), an SMTP server MUST tag that message
			as needing REQUIRETLS handling.
	Upon receipt of a message not specifying the REQUIRETLS option on its		Upon receipt of a message not specifying the REQUIRETLS option on its
	MAIL FROM command but containing the RequireTLS header field in its		MAIL FROM command but containing the RequireTLS header field in its
	message header, an SMTP server implementing this specification MUST		message header, an SMTP server implementing this specification MUST
	tag that message with the option specified in the RequireTLS header		tag that message with the option specified in the RequireTLS header
	field. If the REQUIRETLS MAIL FROM parameter is specified, the		field. If the REQUIRETLS MAIL FROM parameter is specified, the
	RequireTLS header field MUST be ignored but MAY be included in onward		RequireTLS header field MUST be ignored but MAY be included in onward
	relay of the message.		relay of the message.
	The manner in which the above tagging takes place is implementation-		The manner in which the above tagging takes place is implementation-
	dependent. If the message is being locally aliased and redistributed		dependent. If the message is being locally aliased and redistributed
	to multiple addresses, all instances of the message MUST be tagged in		to multiple addresses, all instances of the message MUST be tagged in
	the same manner.		the same manner.
	4.2. REQUIRETLS Sender Requirements		4.2. REQUIRETLS Sender Requirements
	4.2.1. Sending with TLS Required		4.2.1. Sending with TLS Required
	When sending a message tagged as requiring TLS, the sending (client)		When sending a message tagged as requiring TLS for which the MAIL
	MTA MUST:		FROM return-path is not empty (an empty MAIL FROM return-path
			indicating a bounce message), the sending (client) MTA MUST:
	1. Look up the SMTP server to which the message is to be sent as		1. Look up the SMTP server to which the message is to be sent as
	described in [RFC5321] Section 5.1.		described in [RFC5321] Section 5.1.
	2. If the server lookup is accomplished via the recipient domain's		2. If the server lookup is accomplished via the recipient domain's
	MX record (the usual case) and is not accompanied by a valid		MX record (the usual case) and is not accompanied by a valid
	DNSSEC signature, the client MUST also validate the SMTP server		DNSSEC signature, the client MUST also validate the SMTP server
	name using MTA-STS as described in Section 4.1 of RFC 8461		name using MTA-STS as described in RFC 8461 [RFC8461]
	[RFC8461].		Section 4.1.
	3. Open an SMTP session with the peer SMTP server using the EHLO		3. Open an SMTP session with the peer SMTP server using the EHLO
	verb.		verb.
	4. Establish a TLS-protected SMTP session with its peer SMTP server		4. Establish a TLS-protected SMTP session with its peer SMTP server
	and authenticate the server's certificate as specified in		and authenticate the server's certificate as specified in
	[RFC6125] or [RFC7672] as applicable.		[RFC6125] or [RFC7672] as applicable.
	5. Ensure that the response to the subsequent EHLO following		5. Ensure that the response to the subsequent EHLO following
	establishment of the TLS protection advertises the REQUIRETLS		establishment of the TLS protection advertises the REQUIRETLS
	capability.		capability.
	6. The SMTP client SHOULD also require that meaningfully secure		The SMTP client SHOULD follow the recommendations in [RFC7525] or its
	cipher algorithms and key lengths be negotiated with the server.		successor with respect to negotiation of the TLS session.
	The choices of key lengths and algorithms change over time, so a
	specific requirement is not presented here.
	If any of the above steps fail, the client MUST issue a QUIT to the		If any of the above steps fail, the client MUST issue a QUIT to the
	server and repeat steps 2-5 with each host on the recipient domain's		server and repeat steps 2-5 with each host on the recipient domain's
	list of MX hosts in an attempt to find a mail path that meets the		list of MX hosts in an attempt to find a mail path that meets the
	sender's requirements. The client MAY send other, unprotected,		sender's requirements. The client MAY send other, unprotected,
	messages to that server if it has any prior to issuing the QUIT. If		messages to that server if it has any prior to issuing the QUIT. If
	there are no more MX hosts, the client MUST NOT transmit the message		there are no more MX hosts, the client MUST NOT transmit the message
	to the domain.		to the domain.
	Following such a failure, the SMTP client MUST send a non-delivery		Following such a failure, the SMTP client MUST send a non-delivery
	notification to the reverse-path of the failed message as described		notification to the reverse-path of the failed message as described
	in section 3.6 of [RFC5321]. The following status codes [RFC5248]		in section 3.6 of [RFC5321]. The following status codes [RFC5248]
	SHOULD be used:		SHOULD be used:
	o REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed		o REQUIRETLS not supported by server: 5.7.YYY REQUIRETLS needed
	o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption		o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption
	needed		needed
	Refer to Section 5 for further requirements regarding non-delivery		Refer to Section 5 for further requirements regarding non-delivery
	messages.		messages.
	If all REQUIRETLS requirements have been met, transmit the message,		If all REQUIRETLS requirements have been met, transmit the message,
	issuing the REQUIRETLS option on the MAIL FROM command with the		issuing the REQUIRETLS option on the MAIL FROM command with the
	required option(s), if any.		required option(s), if any.
	skipping to change at page 8, line 17 ¶		skipping to change at page 9, line 9 ¶
	The path from the origination of an error bounce message back to the		The path from the origination of an error bounce message back to the
	MAIL FROM address may not share the same REQUIRETLS support as the		MAIL FROM address may not share the same REQUIRETLS support as the
	forward path. Therefore, users requiring TLS are advised to make		forward path. Therefore, users requiring TLS are advised to make
	sure that they are capable of receiving mail using REQUIRETLS as		sure that they are capable of receiving mail using REQUIRETLS as
	well. Otherwise, such non-delivery messages will be lost.		well. Otherwise, such non-delivery messages will be lost.
	If a REQUIRETLS message is bounced, the server MUST behave as if		If a REQUIRETLS message is bounced, the server MUST behave as if
	RET=HDRS was present as described in [RFC3461]. If both RET=FULL and		RET=HDRS was present as described in [RFC3461]. If both RET=FULL and
	REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be		REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be
	transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS		transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS
	bounce message MUST use an empty MAIL FROM return-path as required by		bounce message uses an empty MAIL FROM return-path as required by
	[RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS		[RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS
	parameter SHOULD NOT cause a bounce message to be discarded even if		parameter SHOULD NOT cause a bounce message to be discarded even if
	the next-hop relay does not advertise REQUIRETLS.		the next-hop relay does not advertise REQUIRETLS.
	Senders of messages requiring TLS are advised to consider the		Senders of messages requiring TLS are advised to consider the
	possibility that bounce messages will be lost as a result of		possibility that bounce messages will be lost as a result of
	REQUIRETLS return path failure, and that some information could be		REQUIRETLS return path failure, and that some information could be
	leaked if a bounce message is not able to be transmitted with		leaked if a bounce message is not able to be transmitted with
	REQUIRETLS.		REQUIRETLS.
	skipping to change at page 9, line 47 ¶		skipping to change at page 10, line 47 ¶
	Header field name: RequireTLS		Header field name: RequireTLS
	Applicable protocol: mail		Applicable protocol: mail
	Status: standard		Status: standard
	Author/change controller: IETF		Author/change controller: IETF
	Specification document: (this document)		Specification document: (this document)
	This section is to be updated for publication by the RFC Editor.		This section is to be updated for publication by the RFC Editor.
	8. Security Considerations		8. Security Considerations
	The purpose of REQUIRETLS is to improve communications security for		The purpose of REQUIRETLS is to give the originator of a message
	email by giving the originator of a message an expectation that it		control over the security of email they send, either by conveying an
	will be transmitted in an encrypted form "over the wire". When used,		expectation that it will be transmitted in an encrypted form "over
	REQUIRETLS changes the traditional behavior of email transmission,		the wire" or explicitly that transport encryption is not required if
	which favors delivery over the ability to send email messages using		it cannot be successfully negotiated.
	transport-layer security, to one in which requested security takes
	precedence over delivery and domain-level policy.
	The following considerations apply to the REQUIRETLS service		The following considerations apply to the REQUIRETLS service
	extension but not the RequireTLS header field, since messages		extension but not the RequireTLS header field, since messages
	specifying the header field are less concerned with transport		specifying the header field are less concerned with transport
	security.		security.
	8.1. Passive attacks		8.1. Passive attacks
	REQUIRETLS is generally effective against passive attackers who are		REQUIRETLS is generally effective against passive attackers who are
	merely trying to eavesdrop on an SMTP exchange between an SMTP client		merely trying to eavesdrop on an SMTP exchange between an SMTP client
	skipping to change at page 11, line 25 ¶		skipping to change at page 12, line 23 ¶
	The author would like to acknowledge many helpful suggestions on the		The author would like to acknowledge many helpful suggestions on the
	ietf-smtp and uta mailing lists, in particular those of Viktor		ietf-smtp and uta mailing lists, in particular those of Viktor
	Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock,		Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock,
	John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.		John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.
	10. Revision History		10. Revision History
	To be removed by RFC Editor upon publication as an RFC.		To be removed by RFC Editor upon publication as an RFC.
	10.1. Changes since -05 Draft		10.1. Changes since -06 Draft
			Various changes in response to AD review:
			o Reference RFC 7525 for TLS negotiation recommendations.
			o Make reference to requested 5.7.YYY error code consistent.
			o Clarify applicability to LMTP and submission.
			o Provide ABNF for syntax of SMTP option and header field and
			examples in Appendix A.
			o Correct use of normative language in Section 5.
			o Clarify case where REQUIRETLS option is used on bounce messages.
			o Improve Security Requirements wording to be incusive of both SMTP
			option and header field.
			10.2. Changes since -05 Draft
	Corrected IANA Permanent Message Header Fields Registry request.		Corrected IANA Permanent Message Header Fields Registry request.
	10.2. Changes since -04 Draft		10.3. Changes since -04 Draft
	Require validation of SMTP server hostname via DNSSEC or MTA-STS		Require validation of SMTP server hostname via DNSSEC or MTA-STS
	policy when TLS is required.		policy when TLS is required.
	10.3. Changes since -03 Draft		10.4. Changes since -03 Draft
	Working Group Last Call changes, including:		Working Group Last Call changes, including:
	o Correct reference for SMTP DANE		o Correct reference for SMTP DANE
	o Clarify that RequireTLS: NO applies to both MTA-STS and DANE		o Clarify that RequireTLS: NO applies to both MTA-STS and DANE
	policies		policies
	o Correct newly-defined status codes		o Correct newly-defined status codes
	o Update MTA-STS references to RFC		o Update MTA-STS references to RFC
	10.4. Changes since -02 Draft		10.5. Changes since -02 Draft
	o More complete documentation for IANA registration requests.		o More complete documentation for IANA registration requests.
	o Changed bounce handling to use RET parameters of RFC 3461, along		o Changed bounce handling to use RET parameters of RFC 3461, along
	with slightly more liberal transmission of bounces even if		with slightly more liberal transmission of bounces even if
	REQUIRETLS can't be negotiated.		REQUIRETLS can't be negotiated.
	10.5. Changes since -01 Draft		10.6. Changes since -01 Draft
	o Converted DEEP references to RFC 8314.		o Converted DEEP references to RFC 8314.
	o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC.		o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC.
	o Editorial corrections, notably making the header field name		o Editorial corrections, notably making the header field name
	consistent (RequireTLS rather than Require-TLS).		consistent (RequireTLS rather than Require-TLS).
	10.6. Changes since -00 Draft		10.7. Changes since -00 Draft
	o Created new header field, Require-TLS, for use by "NO" option.		o Created new header field, Require-TLS, for use by "NO" option.
	o Removed "NO" option from SMTP service extension.		o Removed "NO" option from SMTP service extension.
	o Recommend DEEP requirements for delivery of messages requiring		o Recommend DEEP requirements for delivery of messages requiring
	TLS.		TLS.
	o Assorted copy edits		o Assorted copy edits
	10.7. Changes since fenton-03 Draft		10.8. Changes since fenton-03 Draft
	o Wording improvements from Rolf Sonneveld review 22 July 2017		o Wording improvements from Rolf Sonneveld review 22 July 2017
	o A few copy edits		o A few copy edits
	o Conversion from individual to UTA WG draft		o Conversion from individual to UTA WG draft
	10.8. Changes Since -02 Draft		10.9. Changes Since -02 Draft
	o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per		o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per
	suggestion on UTA WG mailing list.		suggestion on UTA WG mailing list.
	o Additional guidance on bounce messages		o Additional guidance on bounce messages
	10.9. Changes Since -01 Draft		10.10. Changes Since -01 Draft
	o Specified retries when multiple MX hosts exist for a given domain.		o Specified retries when multiple MX hosts exist for a given domain.
	o Clarified generation of non-delivery messages		o Clarified generation of non-delivery messages
	o Specified requirements for application of REQUIRETLS to mail		o Specified requirements for application of REQUIRETLS to mail
	forwarders and mailing lists.		forwarders and mailing lists.
	o Clarified DNSSEC requirements to include MX lookup only.		o Clarified DNSSEC requirements to include MX lookup only.
	o Corrected terminology regarding message retrieval vs. delivery.		o Corrected terminology regarding message retrieval vs. delivery.
	o Changed category to standards track.		o Changed category to standards track.
	10.10. Changes Since -00 Draft		10.11. Changes Since -00 Draft
	o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM		o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
	parameter to better associate REQUIRETLS requirements with		parameter to better associate REQUIRETLS requirements with
	transmission of individual messages.		transmission of individual messages.
	o Addition of an option to require DNSSEC lookup of the remote mail		o Addition of an option to require DNSSEC lookup of the remote mail
	server, since this affects the common name of the certificate that		server, since this affects the common name of the certificate that
	is presented.		is presented.
	o Clarified the wording to more clearly state that TLS sessions must		o Clarified the wording to more clearly state that TLS sessions must
	skipping to change at page 14, line 25 ¶		skipping to change at page 15, line 40 ¶
	[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.		[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
	Rose, "Resource Records for the DNS Security Extensions",		Rose, "Resource Records for the DNS Security Extensions",
	RFC 4034, DOI 10.17487/RFC4034, March 2005,		RFC 4034, DOI 10.17487/RFC4034, March 2005,
	<https://www.rfc-editor.org/info/rfc4034>.		<https://www.rfc-editor.org/info/rfc4034>.
	[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.		[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
	Rose, "Protocol Modifications for the DNS Security		Rose, "Protocol Modifications for the DNS Security
	Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,		Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
	<https://www.rfc-editor.org/info/rfc4035>.		<https://www.rfc-editor.org/info/rfc4035>.
			[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
			Specifications: ABNF", STD 68, RFC 5234,
			DOI 10.17487/RFC5234, January 2008,
			<https://www.rfc-editor.org/info/rfc5234>.
	[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced		[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
	Mail System Status Codes", BCP 138, RFC 5248,		Mail System Status Codes", BCP 138, RFC 5248,
	DOI 10.17487/RFC5248, June 2008,		DOI 10.17487/RFC5248, June 2008,
	<https://www.rfc-editor.org/info/rfc5248>.		<https://www.rfc-editor.org/info/rfc5248>.
	[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,		[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
	DOI 10.17487/RFC5321, October 2008,		DOI 10.17487/RFC5321, October 2008,
	<https://www.rfc-editor.org/info/rfc5321>.		<https://www.rfc-editor.org/info/rfc5321>.
			[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
			DOI 10.17487/RFC5322, October 2008,
			<https://www.rfc-editor.org/info/rfc5322>.
	[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and		[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
	Verification of Domain-Based Application Service Identity		Verification of Domain-Based Application Service Identity
	within Internet Public Key Infrastructure Using X.509		within Internet Public Key Infrastructure Using X.509
	(PKIX) Certificates in the Context of Transport Layer		(PKIX) Certificates in the Context of Transport Layer
	Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March		Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
	2011, <https://www.rfc-editor.org/info/rfc6125>.		2011, <https://www.rfc-editor.org/info/rfc6125>.
			[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
			"Recommendations for Secure Use of Transport Layer
			Security (TLS) and Datagram Transport Layer Security
			(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
			2015, <https://www.rfc-editor.org/info/rfc7525>.
			[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
			Opportunistic DNS-Based Authentication of Named Entities
			(DANE) Transport Layer Security (TLS)", RFC 7672,
			DOI 10.17487/RFC7672, October 2015,
			<https://www.rfc-editor.org/info/rfc7672>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:		[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:
	Use of Transport Layer Security (TLS) for Email Submission		Use of Transport Layer Security (TLS) for Email Submission
	and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,		and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
	<https://www.rfc-editor.org/info/rfc8314>.		<https://www.rfc-editor.org/info/rfc8314>.
	[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,		[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
	skipping to change at page 15, line 22 ¶		skipping to change at page 17, line 5 ¶
	Transfer Protocol (SMTP) Enhanced Status Codes Registry",		Transfer Protocol (SMTP) Enhanced Status Codes Registry",
	2008, <http://www.iana.org/assignments/		2008, <http://www.iana.org/assignments/
	smtp-enhanced-status-codes>.		smtp-enhanced-status-codes>.
	11.2. Informative References		11.2. Informative References
	[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",		[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
	STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,		STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
	<https://www.rfc-editor.org/info/rfc1939>.		<https://www.rfc-editor.org/info/rfc1939>.
			[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
			DOI 10.17487/RFC2033, October 1996,
			<https://www.rfc-editor.org/info/rfc2033>.
	[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION		[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
	4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,		4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
	<https://www.rfc-editor.org/info/rfc3501>.		<https://www.rfc-editor.org/info/rfc3501>.
	[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.		[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
	Thayer, "OpenPGP Message Format", RFC 4880,		Thayer, "OpenPGP Message Format", RFC 4880,
	DOI 10.17487/RFC4880, November 2007,		DOI 10.17487/RFC4880, November 2007,
	<https://www.rfc-editor.org/info/rfc4880>.		<https://www.rfc-editor.org/info/rfc4880>.
	[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet		[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
	Mail Extensions (S/MIME) Version 3.2 Message		Mail Extensions (S/MIME) Version 3.2 Message
	Specification", RFC 5751, DOI 10.17487/RFC5751, January		Specification", RFC 5751, DOI 10.17487/RFC5751, January
	2010, <https://www.rfc-editor.org/info/rfc5751>.		2010, <https://www.rfc-editor.org/info/rfc5751>.
	[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via		[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
	Opportunistic DNS-Based Authentication of Named Entities		STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
	(DANE) Transport Layer Security (TLS)", RFC 7672,		<https://www.rfc-editor.org/info/rfc6409>.
	DOI 10.17487/RFC7672, October 2015,
	<https://www.rfc-editor.org/info/rfc7672>.		Appendix A. Examples
			This section is informative.
			A.1. REQUIRETLS SMTP Option
			The RequireTLS SMTP option is used to express the intent of the
			sender that the associated message be relayed using TLS. In the
			following example, lines beginning with C: are transmitted from the
			SMTP client to the server, and lines beginning with S: are
			transmitted in the opposite direction.
			S: 220 mail.example.net ESMTP
			C: EHLO mail.example.org
			S: 250-mail.example.net Hello example.org [192.0.2.1]
			S: 250-SIZE 52428800
			S: 250-8BITMIME
			S: 250-PIPELINING
			S: 250-STARTTLS
			S: 250 HELP
			C: STARTTLS
			S: TLS go ahead
			(at this point TLS negotiation takes place. The remainder of this
			session occurs within TLS.)
			S: 220 mail.example.net ESMTP
			C: EHLO mail.example.org
			S: 250-mail.example.net Hello example.org [192.0.2.1]
			S: 250-SIZE 52428800
			S: 250-8BITMIME
			S: 250-PIPELINING
			S: 250-REQUIRETLS
			S: 250 HELP
			C: MAIL FROM:<roger@example.org> REQUIRETLS
			S: 250 OK
			C: RCPT TO:<editor@example.net>
			S: 250 Accepted
			C: DATA
			S: 354 Enter message, ending with "." on a line by itself
			(message follows)
			C: .
			S: 250 OK
			C: QUIT
			A.2. RequireTLS Header Field
			The RequireTLS header field is used when the sender of the message
			wants to override the default policy of the recipient domain to
			require TLS. It might be used, for example, to allow problems with
			the recipient domain's TLS certificate to be reported:
			From: Roger Reporter <roger@example.org>
			To: Andy Admin <admin@example.com>
			Subject: Certificate problem?
			RequireTLS: NO
			Date: Fri, 18 Jan 2019 10:26:55 -0800
			Message-ID: <5c421a6f79c0e_d153ff8286d45c468473@mail.example.org>
			Andy, there seems to be a problem with the TLS certificate
			on your mail server. Are you aware of this?
			Roger
	Author's Address		Author's Address
	Jim Fenton		Jim Fenton
	Altmode Networks		Altmode Networks
	Los Altos, California 94024		Los Altos, California 94024
	USA		USA
	Email: fenton@bluepopcorn.net		Email: fenton@bluepopcorn.net
End of changes. 34 change blocks.
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