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rfc:rfc2487

Network Working Group P. Hoffman Request for Comments: 2487 Internet Mail Consortium Category: Standards Track January 1999

          SMTP Service Extension for Secure SMTP over TLS

Status of this Memo

 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.

Copyright Notice

 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.

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.

Hoffman Standards Track [Page 1] RFC 2487 SMTP Service Extension January 1999

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 allows 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
 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.

Hoffman Standards Track [Page 2] RFC 2487 SMTP Service Extension January 1999

 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
 SHOULD start the TLS negotiation before giving any other SMTP
 commands.
 If the SMTP client is using pipelining as defined in RFC 1854, the
 STARTTLS command must be the last command in a group.

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:

Hoffman Standards Track [Page 3] RFC 2487 SMTP Service Extension January 1999

  1. 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 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 TLS extension
 in response to an EHLO command received after a TLS handshake has
 completed.

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 STARTTLS
 C: STARTTLS
 S: 220 Go ahead
 C: <starts TLS negotiation>

Hoffman Standards Track [Page 4] RFC 2487 SMTP Service Extension January 1999

 C & S: <negotiate a TLS session>
 C & S: <check result of negotiation>
 C: <continues by sending an SMTP command>
 . . .

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 STMP client and server must check the result of the TLS
 negotiation to see whether acceptable authentication or 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 server announcing in an EHLO response that it uses a particular TLS
 protocol should not pose any security issues, since any use of TLS
 will be at least as secure as no use of TLS.
 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. An SMTP client can protect against
 this attack 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.

Hoffman Standards Track [Page 5] RFC 2487 SMTP Service Extension January 1999

 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.

Hoffman Standards Track [Page 6] RFC 2487 SMTP Service Extension January 1999

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
             Error Codes", RFC 2034, October 1996.
 [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [SASL]      Myers, J., "Simple Authentication and Security Layer
             (SASL)", RFC 2222, October 1997.
 [SMTP-AUTH] "SMTP Service Extension for Authentication", Work in
             Progress.
 [TLS]       Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
             RFC 2246, January 1999.

B. Author's Address

 Paul Hoffman
 Internet Mail Consortium
 127 Segre Place
 Santa Cruz, CA  95060
 Phone: (831) 426-9827
 EMail: phoffman@imc.org

Hoffman Standards Track [Page 7] RFC 2487 SMTP Service Extension January 1999

C. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Hoffman Standards Track [Page 8]

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