< draft-hoffman-rfc2487bis-05.txt		draft-hoffman-rfc2487bis-06.txt >
	Internet Draft Paul Hoffman		A new Request for Comments is now available in online RFC libraries.
	draft-hoffman-rfc2487bis-05.txt Internet Mail Consortium
	February 24, 2001
	Expires in six months
	SMTP Service Extension for Secure SMTP over TLS
	Status of this Memo
	This document is an Internet-Draft and is in full conformance with all
	provisions of Section 10 of RFC2026.
	Internet-Drafts are working documents of the Internet Engineering Task
	Force (IETF), its areas, and its working groups. Note that other
	groups may also distribute working documents as Internet-Drafts.
	Internet-Drafts are draft documents valid for a maximum of six months
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	Copyright (C) The Internet Society (1999). All Rights Reserved.
	1. Abstract
	This document describes an extension to the SMTP service that allows
	an SMTP server and client to use transport-layer security to provide
	private, authenticated communication over the Internet. This gives
	SMTP agents the ability to protect some or all of their
	communications from eavesdroppers and attackers.
	This document updates RFC 2487, as described in Appendix B.
	2. Introduction
	SMTP [RFC-821] servers and clients normally communicate in the clear
	over the Internet. In many cases, this communication goes through one
	or more router that is not controlled or trusted by either entity.
	Such an untrusted router might allow a third party to monitor or
	alter the communications between the server and client.
	Further, there is often a desire for two SMTP agents to be able to
	authenticate each others' identities. For example, a secure SMTP
	server might only allow communications from other SMTP agents it
	knows, or it might act differently for messages received from an
	agent it knows than from one it doesn't know.
	TLS [TLS], more commonly known as SSL, is a popular mechanism for
	enhancing TCP communications with privacy and authentication. TLS is
	in wide use with the HTTP protocol, and is also being used for adding
	security to many other common protocols that run over TCP.
	2.1 Terminology
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC-2119].
	3. STARTTLS Extension
	The STARTTLS extension to SMTP is laid out as follows:
	(1) the name of the SMTP service defined here is STARTTLS;
	(2) the EHLO keyword value associated with the extension is STARTTLS;
	(3) the STARTTLS keyword has no parameters;
	(4) a new SMTP verb, "STARTTLS", is defined;
	(5) no additional parameters are added to any SMTP command.
	4. The STARTTLS Keyword
	The STARTTLS keyword is used to tell the SMTP client that the SMTP
	server is currently able to negotiate the use of TLS. It takes no
	parameters.
	5. The STARTTLS Command
	The format for the STARTTLS command is:
	STARTTLS
	with no parameters.
	After the client gives the STARTTLS command, the server responds with
	one of the following reply codes:
	220 Ready to start TLS
	501 Syntax error (no parameters allowed)
	454 TLS not available due to temporary reason
	If the client receives the 454 response, the client must decide
	whether or not to continue the SMTP session. Such a decision is
	based on local policy. For instance, if TLS was being used for
	client authentication, the client might try to continue the
	session, in case the server allows it even with no authentication.
	However, if TLS was being negotiated for encryption, a client
	that gets a 454 response needs to decide whether to send the
	message anyway with no TLS encryption, whether to wait and try
	again later, or whether to give up and notify the sender of the
	error.
	A publicly-referenced SMTP server MUST NOT require use of the
	STARTTLS extension in order to deliver mail locally. This rule
	prevents the STARTTLS extension from damaging the interoperability of
	the Internet's SMTP infrastructure. A publicly-referenced SMTP server
	is an SMTP server which runs on port 25 of an Internet host listed in
	the MX record (or A record if an MX record is not present) for the
	domain name on the right hand side of an Internet mail address.
	Any SMTP server may refuse to accept messages for relay based on
	authentication supplied during the TLS negotiation. An SMTP server
	that is not publicly referenced may refuse to accept any messages for
	relay or local delivery based on authentication supplied during the
	TLS negotiation.
	A SMTP server that is not publicly referenced may choose to require
	that the client perform a TLS negotiation before accepting any
	commands. In this case, the server SHOULD return the reply code:
	530 Must issue a STARTTLS command first
	to every command other than NOOP, EHLO, STARTTLS, or QUIT. If the
	client and server are using the ENHANCEDSTATUSCODES ESMTP extension
	[RFC-2034], the status code to be returned SHOULD be 5.7.0.
	After receiving a 220 response to a STARTTLS command, the client MUST
	start the TLS negotiation before giving any other SMTP commands. If,
	after having issued the STARTTLS command, the client finds out that
	some failure prevents it from actually starting a TLS handshake, then
	it SHOULD abort the connection.
	If the SMTP client is using pipelining as defined in RFC 1854, the
	STARTTLS command must be the last command in a group.
	Servers MUST be able to understand backwards compatible SSL Client
	Hello messages (provided that client_version is TLS 1.0 or later),
	and clients MAY use backwards compatible Client Hellos messages.
	Neither clients or servers are required to actually offer Client
	Hello messages for anything other than TLS 1.0.
	5.1 Processing After the STARTTLS Command
	After the TLS handshake has been completed, both parties MUST
	immediately decide whether or not to continue based on the
	authentication and privacy achieved. The SMTP client and server may
	decide to move ahead even if the TLS negotiation ended with no
	authentication and/or no privacy because most SMTP services are
	performed with no authentication and no privacy, but some SMTP
	clients or servers may want to continue only if a particular level of
	authentication and/or privacy was achieved.
	If the SMTP client decides that the level of authentication or
	privacy is not high enough for it to continue, it SHOULD issue an
	SMTP QUIT command immediately after the TLS negotiation is complete.
	If the SMTP server decides that the level of authentication or
	privacy is not high enough for it to continue, it SHOULD reply to
	every SMTP command from the client (other than a QUIT command) with
	the 554 reply code (with a possible text string such as "Command
	refused due to lack of security").
	The decision of whether or not to believe the authenticity of the
	other party in a TLS negotiation is a local matter. However, some
	general rules for the decisions are:
	- A SMTP client would probably only want to authenticate an SMTP
	server whose server certificate has a domain name that is the
	domain name that the client thought it was connecting to.
	- A publicly-referenced SMTP server would probably want to accept
	any verifiable certificate from an SMTP client, and would
	possibly want to put distinguishing information about the
	certificate in the Received header of messages that were relayed
	or submitted from the client.
	5.2 Result of the STARTTLS Command
	Upon completion of the TLS handshake, the SMTP protocol is reset to
	the initial state (the state in SMTP after a server issues a 220
	service ready greeting). The server MUST discard any knowledge
	obtained from the client, such as the argument to the EHLO command,
	which was not obtained from the TLS negotiation itself. The client
	MUST discard any knowledge obtained from the server, such as the list
	of SMTP service extensions, which was not obtained from the TLS
	negotiation itself. The client SHOULD send an EHLO command as the
	first command after a successful TLS negotiation.
	The list of SMTP service extensions returned in response to an EHLO
	command received after the TLS handshake MAY be different than the
	list returned before the TLS handshake. For example, an SMTP server
	might not want to advertise support for a particular SASL mechanism
	[SASL] unless a client has sent an appropriate client certificate
	during a TLS handshake.
	Both the client and the server MUST know if there is a TLS session
	active. A client MUST NOT attempt to start a TLS session if a TLS
	session is already active. A server MUST NOT return the STARTTLS
	extension in response to an EHLO command received after a TLS
	handshake has completed.
	5.3 STARTTLS on the Submission Port
	STARTTLS is a valid ESMTP extension when used on the Submission
	port, as defined in [RFC-2476]. In fact, since the submission port
	is by definition not a publicly referenced SMTP server, the STARTTLS
	extension can be particularly useful by providing security and
	authentication for this service.
	6. Usage Example
	The following dialog illustrates how a client and server can start a
	TLS session:
	S: <waits for connection on TCP port 25>
	C: <opens connection>
	S: 220 mail.imc.org SMTP service ready
	C: EHLO mail.ietf.org
	S: 250-mail.imc.org offers a warm hug of welcome
	S: 250-8BITMIME
	S: 250-STARTTLS
	S: 250 DSN
	C: STARTTLS
	S: 220 Go ahead
	C: <starts TLS negotiation>
	C & S: <negotiate a TLS session>
	C & S: <check result of negotiation>
	C: EHLO mail.ietf.org
	S: 250-mail.imc.org touches your hand gently for a moment
	S: 250-8BITMIME
	S: 250 DSN
	. . .
	7. Security Considerations
	It should be noted that SMTP is not an end-to-end mechanism. Thus, if
	an SMTP client/server pair decide to add TLS privacy, they are not
	securing the transport from the originating mail user agent to the
	recipient. Further, because delivery of a single piece of mail may
	go between more than two SMTP servers, adding TLS privacy to one pair
	of servers does not mean that the entire SMTP chain has been made
	private. Further, just because an SMTP server can authenticate an
	SMTP client, it does not mean that the mail from the SMTP client was
	authenticated by the SMTP client when the client received it.
	Both the SMTP client and server must check the result of the TLS
	negotiation to see whether an acceptable degree of authentication
	and privacy was achieved. Ignoring this step completely invalidates
	using TLS for security. The decision about whether acceptable
	authentication or privacy was achieved is made locally, is
	implementation-dependant, and is beyond the scope of this document.
	The SMTP client and server should note carefully the result of the
	TLS negotiation. If the negotiation results in no privacy, or if it
	results in privacy using algorithms or key lengths that are deemed
	not strong enough, or if the authentication is not good enough for
	either party, the client may choose to end the SMTP session with an
	immediate QUIT command, or the server may choose to not accept any
	more SMTP commands.
	A man-in-the-middle attack can be launched by deleting the "250
	STARTTLS" response from the server. This would cause the client not
	to try to start a TLS session. Another man-in-the-middle attack is
	to allow the server to announce its STARTTLS capability, but to
	alter the client's request to start TLS and the server's response.
	An SMTP client can partially protect against these attacks by
	recording the fact that a particular SMTP server offers TLS during
	one session and generating an alarm if it does not appear in the
	EHLO response for a later session. The lack of TLS during a session
	SHOULD NOT result in the bouncing of email, although it could result
	in delayed processing.
	If the TLS negotiation fails or if the client receives a 454
	response, the client has to decide what to do next. There are three
	main choices: go ahead with the rest of the SMTP session, retry TLS
	at a later time, or give up and return the mail to the sender. If a
	failure or error occurs, the client can assume that the server may
	be able to negotiate TLS in the future, and should try negotiate TLS
	in a later session, until some locally-chosen timeout occurs, at
	which point, the client should return the mail to the sender.
	However, if the client and server were only using TLS for
	authentication, the client may want to proceed with the SMTP
	session, in case some of the operations the client wanted to perform
	are accepted by the server even if the client is unauthenticated.
	Before the TLS handshake has begun, any protocol interactions are
	performed in the clear and may be modified by an active attacker. For
	this reason, clients and servers MUST discard any knowledge obtained
	prior to the start of the TLS handshake upon completion of the TLS
	handshake.
	The STARTTLS extension is not suitable for authenticating the author
	of an email message unless every hop in the delivery chain, including
	the submission to the first SMTP server, is authenticated. Another
	proposal [SMTP-AUTH] can be used to authenticate delivery and MIME
	security multiparts [MIME-SEC] can be used to authenticate the author
	of an email message. In addition, the [SMTP-AUTH] proposal offers
	simpler and more flexible options to authenticate an SMTP client and
	the SASL EXTERNAL mechanism [SASL] MAY be used in conjunction with
	the STARTTLS command to provide an authorization identity.
	A. References
	[RFC-821] Postel, J., "Simple Mail Transfer Protocol", RFC 821,
	August 1982.
	[RFC-1869] Klensin, J., Freed, N, Rose, M, Stefferud, E. and D.
	Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
	November 1995.
	[RFC-2034] Freed, N., "SMTP Service Extension for Returning Enhanced		RFC 3207
	Error Codes", RFC 2034, October 1996.
	[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate		Title: SMTP Service Extension for
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Secure SMTP over Transport Layer Security
			Author(s): P. Hoffman
			Status: Standards Track
			Date: February 2002
			Mailbox: phoffman@imc.org
			Pages: 9
			Characters: 18679
			Obsoletes: 2487
	[RFC-2476] Gellens, R. and Klensin, J., "Message Submission",		I-D Tag: draft-hoffman-rfc2487bis-06.txt
	RFC 2476, December 1998.
	[SASL] Myers, J., "Simple Authentication and Security Layer		URL: ftp://ftp.rfc-editor.org/in-notes/rfc3207.txt
	(SASL)", RFC 2222, October 1997.
	[SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication",		This document describes an extension to the SMTP (Simple Mail Transfer
	RFC 2554, March 1999.		Protocol) service that allows an SMTP server and client to use TLS
			(Transport Layer Security) to provide private, authenticated
			communication over the Internet. This gives SMTP agents the ability
			to protect some or all of their communications from eavesdroppers and
			attackers.
	[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",		This is now a Proposed Standard Protocol.
	RFC 2246, January 1999.
	B. Changes from RFC 2487		This document specifies an Internet standards track protocol for
			the Internet community, and requests discussion and suggestions
			for improvements. Please refer to the current edition of the
			"Internet Official Protocol Standards" (STD 1) for the
			standardization state and status of this protocol. Distribution
			of this memo is unlimited.
	This document is a revision of RFC 2487, which is a Proposed		This announcement is sent to the IETF list and the RFC-DIST list.
	Standard. The changes from that document are:		Requests to be added to or deleted from the IETF distribution list
			should be sent to IETF-REQUEST@IETF.ORG. Requests to be
			added to or deleted from the RFC-DIST distribution list should
			be sent to RFC-DIST-REQUEST@RFC-EDITOR.ORG.
	- Section 5 and 7: More discussion of the man-in-the-middle attacks		Details on obtaining RFCs via FTP or EMAIL may be obtained by sending
	- Section 5: Additional discussion of when a server should and should		an EMAIL message to rfc-info@RFC-EDITOR.ORG with the message body
	not advertise the STARTTLS extension		help: ways_to_get_rfcs. For example:
	- Section 5: Changed the requirements on SMTP clients after receiving
	a 220 response.
	- Section 5: Added a requirement to understand Client Hello messages
	for earlier versions of SSL.
	- Section 5.1: Clarified description of verifying certificates.
	- Section 5.3: Added the section on "STARTTLS on the Submission Port"
	- Section 6: Bug fix in the example to indicate that the client needs
	to issue a new EHLO command, as already is described in section 5.2.
	- Section 7: Clarification of the paragraph on acceptable degree of
	privacy.
	C. Author's Address		To: rfc-info@RFC-EDITOR.ORG
			Subject: getting rfcs
	Paul Hoffman		help: ways_to_get_rfcs
	Internet Mail Consortium
	127 Segre Place
	Santa Cruz, CA 95060
	Phone: (831) 426-9827		Requests for special distribution should be addressed to either the
	EMail: phoffman@imc.org		author of the RFC in question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
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			RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
			Authors, for further information.
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