











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





    - 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]



