MARID Working Group E. Allman Internet Draft Sendmail, Inc Document: draft-ietf-marid-submitter-03.txt H. Katz Microsoft Corp Expires: February 2005 August 2004 SMTP Service Extension for Indicating the Responsible Submitter of an E-mail Message Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. [STD] Internet-Drafts are working documents of 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This memo defines an extension to the Simple Mail Transfer Protocol (SMTP) service, which allows an SMTP client to specify the responsible submitter of an e-mail message. The responsible submitter is the e-mail address of the entity most recently responsible for introducing a message into the transport stream. This extension helps receiving e-mail servers efficiently determine whether the SMTP client is authorized to transmit mail on behalf of the responsible submitter's domain. Allman, Katz Expires - February 2005 [Page 1] SMTP Responsible Submitter Extension August 2004 Conventions Used in This Document In examples, "C:" and "S:" indicate lines sent by the client and server respectively. 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 [KEYWORDS]. Table of Contents 1. Introduction...................................................2 2. The SUBMITTER Service Extension................................4 3. The SUBMITTER Keyword of the EHLO Command......................4 4. The SUBMITTER Parameter of the MAIL Command....................4 4.1 Setting the SUBMITTER Parameter Value......................5 4.2 Processing the SUBMITTER Parameter.........................5 4.3 Transmitting to a Non-SUBMITTER Aware SMTP Server..........6 5. Examples.......................................................6 5.1 Mail Submission............................................6 5.2 Mail Forwarding............................................6 5.3 Mobile User................................................8 5.4 Guest E-mail Service.......................................9 5.5 SUBMITTER Used on a Non-Delivery Report...................10 6. Security Considerations.......................................10 7. IANA Considerations...........................................11 8. References....................................................11 8.1 Normative References......................................11 8.2 Informative References....................................11 9. Acknowledgments...............................................12 10. Authors' Addresses...........................................12 11. Change History...............................................13 12. Full Copyright Statement.....................................14 1. Introduction The practice of falsifying the identity of the sender of an e-mail message, commonly called "spoofing", is a prevalent tactic used by senders of unsolicited commercial e-mail or "spam". This form of abuse has highlighted the need to improve identification of the "responsible submitter" of an e-mail message. In this specification, the responsible submitter is the entity most recently responsible for injecting a message into the e-mail transport stream. The e-mail address of the responsible submitter will be referred to as the "purported responsible address" (PRA) of Allman, Katz Expires - February 2005 [Page 2] SMTP Responsible Submitter Extension August 2004 the message. The "purported responsible domain" (PRD) is the domain portion of that address. This specification codifies rules for encoding the purported responsible address into the SMTP transport protocol. This will permit receiving SMTP servers to efficiently validate whether or not the SMTP client is authorized to transmit mail on behalf of the responsible submitter's domain. Broadly speaking, there are two possible approaches for determining the purported responsible address; either from RFC 2821 [SMTP] protocol data or from RFC 2822 [MSG-FORMAT] message headers. Each approach has certain advantages and disadvantages. Deriving the purported responsible domain from RFC 2821 data has the advantage that validation can be performed before the SMTP client has transmitted the message body. If spoofing is detected, then the SMTP server has the opportunity, depending upon local policy, to reject the message before it is ever transmitted. The disadvantage of this approach is the risk of false positives, that is, incorrectly concluding that the sender's e-mail address has been spoofed. There are today legitimate reasons why the Internet domain names used in RFC 2821 commands may be different from that of the sender of an e- mail message. Deriving the purported responsible domain from RFC 2822 headers has the advantage that validation can usually be based on an identity that is displayed to recipients by existing MUAs as the sender's identity. This aids in detection of a particularly noxious form of spoofing known as "phishing" in which a malicious sender attempts to fool a recipient into believing that a message originates from an entity well known to the recipient. This approach carries a lower risk of false positives since there are fewer legitimate reasons for RFC 2822 headers to differ from the true sender of the message. The disadvantage of this approach is that it does require parsing and analysis of message headers. In practice, much if not all the message body is also transmitted since the SMTP protocol described in RFC 2821 provides no mechanism to interrupt message transmission after the DATA command has been issued. It is desirable to unify these two approaches in a way that combines the benefits of both while minimizing their respective disadvantages. This specification describes just such a unified approach. It uses the mechanism described in [SMTP] to describe an extension to the SMTP protocol. Using this extension, an SMTP client can specify the e-mail address of the entity most recently responsible for submitting the message to the SMTP client in a new SUBMITTER parameter of the SMTP MAIL command. SMTP servers can use this information to validate Allman, Katz Expires - February 2005 [Page 3] SMTP Responsible Submitter Extension August 2004 that the SMTP client is authorized to transmit e-mail on behalf of the Internet domain contained in the SUBMITTER parameter. 2. The SUBMITTER Service Extension The following SMTP service extension is hereby defined: (1) The name of this SMTP service extension is "Responsible Submitter"; (2) The EHLO keyword value associated with this extension is "SUBMITTER"; (3) The SUBMITTER keyword has no parameters; (4) No additional SMTP verbs are defined by this extension; (5) An optional parameter is added to the MAIL command using the esmtp-keyword "SUBMITTER", and is used to specify the e-mail address of the entity responsible for submitting the message for delivery; (6) This extension is appropriate for the submission protocol [SUBMIT]. 3. The SUBMITTER Keyword of the EHLO Command An SMTP server includes the SUBMITTER keyword in its EHLO response to tell the SMTP client that the SUBMITTER service extension is supported. The SUBMITTER keyword has no parameters. 4. The SUBMITTER Parameter of the MAIL Command The syntax of the SUBMITTER parameter is: "SUBMITTER=" Mailbox where Mailbox is the ABNF [ABNF] production defined in Section 4.1.2 of [SMTP]. Characters such as SP, "+" and "=" which may occur in Mailbox but are not permitted in ESMTP parameter values MUST be encoded as "xtext" as described in section 4 of [DSN]. Allman, Katz Expires - February 2005 [Page 4] SMTP Responsible Submitter Extension August 2004 4.1 Setting the SUBMITTER Parameter Value The purpose of the SUBMITTER parameter is to allow the SMTP client to indicate to the server the purported responsible address of the message directly in the RFC 2821 protocol. Therefore, SMTP clients that support the Responsible Submitter extension MUST include the SUBMITTER parameter on all messages. This includes messages containing a null reverse-path in the MAIL command. SMTP clients MUST set the SUMBITTER parameter value to the purported responsible address of the message as defined in [PRA]. This also applies to messages containing a null reverse-path. In some circumstances, described in section 7 of [SENDER-ID], SMTP clients may need to add RFC 2822 headers to the message in order to ensure that the correct SUBMITTER parameter value can be set. 4.2 Processing the SUBMITTER Parameter Receivers of e-mail messages sent with the SUBMITTER parameter SHOULD select the domain part of the SUBMITTER address value as the purported responsible domain of the message, and SHOULD perform such tests, including those defined in [SENDER-ID], as are deemed necessary to determine whether the connecting SMTP client is authorized to transmit e-mail messages on behalf of that domain. If these tests indicate that the connecting SMTP client is not authorized to transmit e-mail messages on behalf of the SUBMITTER domain, the receiving SMTP server SHOULD reject the message and when rejecting MUST use "550 5.7.1 Submitter not allowed." If the receiving SMTP server allows the connecting SMTP client to transmit message data, then the server SHOULD determine the purported responsible address of the message by examining the RFC 2822 message headers as described in [PRA]. If this purported responsible address does not match the address appearing in the SUBMITTER parameter, the receiving SMTP server MUST reject the message and when rejecting MUST use "550 5.7.1 Submitter does not match header." If no purported responsible address is found according to the procedure defined in [PRA], the SMTP server SHOULD reject the message and when rejecting MUST use "554 5.7.7 Cannot verify submitter address." Verifying MTAs are strongly urged to validate the SUBMITTER parameter against the RFC 2822 headers; otherwise, an attacker can trivially defeat the algorithm. Allman, Katz Expires - February 2005 [Page 5] SMTP Responsible Submitter Extension August 2004 Note that the presence of the SUBMITTER parameter on the MAIL command MUST NOT change the effective reverse-path of a message. Any delivery status notifications must be sent to the reverse-path, if one exists, as per section 3.7 of [SMTP] regardless of the presence of a SUBMITTER parameter. If the reverse-path is null, delivery status notifications MUST NOT be sent to the SUBMITTER address. 4.3 Transmitting to a Non-SUBMITTER Aware SMTP Server Notwithstanding the provisions of section 4.1 above, when an MTA transmits a message to another MTA that does not support the SUBMITTER extension, the forwarding MTA MUST transmit the message without the SUBMITTER parameter. This should involve no information loss, since the SUBMITTER parameter is required to contain information derived from the message headers. 5. Examples This section provides examples of how the SUBMITTER parameter would be used. The following dramatis personae appear in the examples: alice@example.com: the original sender of each e-mail message. bob@company.com.example: the final recipient of each e-mail. bob@almamater.edu.example: an email address used by Bob which he has configured to forward mail to his office account at bob@company.com.example. alice@mobile.net.example: an e-mail account provided to Alice by her mobile e-mail network carrier. 5.1 Mail Submission Under normal circumstances, Alice would configure her MUA to submit her message to the mail system using the SUBMIT protocol [SUBMIT]. The MUA would transmit the message without the SUBMITTER parameter. The SUBMIT server would validate that the MUA is allowed to submit a message through some external scheme, perhaps SMTP Authentication [SMTPAUTH]. Under most circumstances this would look like a normal, authenticated SMTP transaction. The SUBMIT server would extract her name from the RFC 2822 headers for use in the SUBMITTER parameters of subsequent transmissions of the message. 5.2 Mail Forwarding When Alice sends a message to Bob at his almamater.edu.example account, the SMTP session from her SUBMIT server might look something like this: Allman, Katz Expires - February 2005 [Page 6] SMTP Responsible Submitter Extension August 2004 S: 220 almamater.edu.example ESMTP server ready C: EHLO example.com S: 250-almamater.edu.example S: 250-DSN S: 250-AUTH S: 250-SUBMITTER S: 250 SIZE C: MAIL FROM: SUBMITTER=alice@example.com S: 250 sender ok C: RCPT TO: S: 250 recipient ok C: DATA S: 354 okay, send message C: (message body goes here) C: . S: 250 message accepted C: QUIT S: 221 goodbye The almamater.edu.example MTA must now forward this message to bob@company.com.example. Although the original sender of the message is alice@example.com, Alice is not responsible for this most recent retransmission of the message. That role is filled by bob@almamater.edu.example who established the forwarding of mail to bob@company.com.example. Therefore, the almamater.edu.example MTA determines a new purported responsible address for the message, namely bob@almamater.edu.example, and sets the SUBMITTER parameter accordingly. The forwarding MTA also inserts a Resent-From header in the message body to ensure the purported responsible address derived from the RFC 2822 headers matches the SUBMITTER address. S: 220 company.com.example ESMTP server ready C: EHLO almamater.edu.example S: 250-company.com.example S: 250-DSN S: 250-AUTH S: 250-SUBMITTER S: 250 SIZE C: MAIL FROM: SUBMITTER=bob@almamater.edu.example S: 250 sender ok C: RCPT TO: S: 250 recipient ok C: DATA S: 354 okay, send message C: Resent-From: bob@almamater.edu.example C: Received By: ... C: (message body goes here) Allman, Katz Expires - February 2005 [Page 7] SMTP Responsible Submitter Extension August 2004 C: . S: 250 message accepted C: QUIT S: 221 goodbye 5.3 Mobile User Alice is at the airport and uses her mobile e-mail device to send a message to Bob. The message travels through the carrier network provided by mobile.net.example, but Alice uses her example.com address on the From line of all her messages so that replies go to her office mailbox. Here is an example of the SMTP session between the MTAs at consolidatedmessanger.net and almamater.edu.example. S: 220 almamater.edu.example ESMTP server ready C: EHLO mobile.net.example S: 250-almamater.edu.example S: 250-DSN S: 250-AUTH S: 250-SUBMITTER S: 250 SIZE C: MAIL FROM: SUBMITTER=alice@mobile.net.example S: 250 sender ok C: RCPT TO: S: 250 recipient ok C: DATA S: 354 okay, send message C: Sender: alice@mobile.net.example C: Received By: ... C: (message body goes here) C: . S: 250 message accepted C: QUIT S: 221 goodbye Note that mobile.net.example uses the SUBMITTER parameter to designate alice@mobile.net.example as the responsible submitter for this message. Further this MTA also inserts a Sender header to ensure the purported responsible address derived from the RFC 2822 headers matches the SUBMITTER address. Likewise, conventional ISPs may also choose to use the SUBMITTER parameter to designate as the responsible submitter the user's address on the ISP's network if that address is different from the MAIL FROM address. Allman, Katz Expires - February 2005 [Page 8] SMTP Responsible Submitter Extension August 2004 When the message is subsequently forwarded by the almamater.edu.example MTA, that MTA will replace the SUBMITTER parameter with bob@almamater.edu.example as in section 5.2 and add its own Resent-From header. 5.4 Guest E-mail Service While on a business trip, Alice uses the broadband access facilities provided by the Exemplar Hotel to connect to the Internet and send e- mail. The hotel routes all outbound e-mail through its own SMTP server, email.hotel.com.example. The SMTP session for Alice's message to Bob from the Exemplar Hotel would look like this: S: 220 almamater.edu.example ESMTP server ready C: EHLO email.hotel.com.example S: 250-almamater.edu.example S: 250-DSN S: 250-AUTH S: 250-SUBMITTER S: 250 SIZE C: MAIL FROM: SUBMITTER=guest.services@email.hotel.com.example S: 250 sender ok C: RCPT TO: S: 250 recipient ok C: DATA S: 354 okay, send message C: Resent-From: guest.services@email.hotel.com.example C: Received By: ... C: (message body goes here) C: . S: 250 message accepted C: QUIT S: 221 goodbye Note that email.hotel.com.example uses the SUBMITTER parameter to designate a generic account guest.services@email.hotel.com.example as the responsible submitter address for this message. A generic account is used since Alice herself does not have an account at that domain. Further this client also inserts a Resent-From header to ensure the purported responsible address derived from the RFC 2822 headers with the SUBMITTER address. As before, when the message is subsequently forwarded by the almamater.edu.example MTA, that MTA will replace the SUBMITTER parameter with bob@almamater.edu.example as in section 5.2 and add its own Resent-From header. Allman, Katz Expires - February 2005 [Page 9] SMTP Responsible Submitter Extension August 2004 5.5 SUBMITTER Used on a Non-Delivery Report Alice sends an incorrectly addressed e-mail message and receives a non-delivery report from a SUBMITTER-compliant server. S: 220 example.com ESMTP server ready C: EHLO almamater.edu.example S: 250-example.com S: 250-DSN S: 250-AUTH S: 250-SUBMITTER S: 250 SIZE C: MAIL FROM:<> SUBMITTER=mailer-daemon@almamater.edu.example S: 250 OK C: RCPT TO: S: 250 OK C: DATA S: 354 OK, send message C: (message body goes here) C: . S: 250 message accepted C: QUIT S: 221 goodbye 6. Security Considerations This extension provides an optimization to allow an SMTP client to identify the responsible submitter of an e-mail message in the SMTP protocol, and to enable SMTP servers to perform efficient validation of that identity before the message contents are transmitted. It is, however, quite possible for an attacker to forge the value of the SUBMITTER parameter. Furthermore, it is possible for an attacker to transmit an e-mail message whose SUBMITTER parameter does not match the purported responsible address of the message as derived from the RFC 2822 headers. Therefore the presence of the SUBMITTER parameter provides, by itself, no assurance of the authenticity of the message or the responsible submitter. Rather, the SUBMITTER parameter is intended to provide additional information to receiving e-mail systems to enable then to efficiently determine the validity of the responsible submitter, and specifically, whether the SMTP client is authorized to transmit e-mail on behalf of the purported responsible submitter's domain. Section 4.2 describes how receiving e-mail systems should process the SUBMITTER parameter. Allman, Katz Expires - February 2005 [Page 10] SMTP Responsible Submitter Extension August 2004 7. IANA Considerations IANA is hereby requested to register the SUBMITTER SMTP service extension. 8. References 8.1 Normative References [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [DSN] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)", RFC 3461, January 2003. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [MSG-FORMAT] Resnick, P., Ed., "Internet Message Format", RFC 2822, April 2001. [PRA] Lyon, J., "Purported Responsible Address in E-mail Messages", draft-ietf-marid-pra-00, August 2004. Work in progress. [SENDER-ID] Lyon, J. and Meng Weng Wong, "Sender ID: Authenticating E-mail", draft-ietf-marid-core-03, August 2004. Work in progress. [SUBMIT] Gellens, R. and J. Klensin, "Message Submission", RFC 2476, December 1998. [STD] Bradner, S., "Intellectual Property Rights in IETF Technology", BCP 79, RFC 3668, February 2004. [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [SMTPAUTH] Meyers, J., "SMTP Service Extension for Authentication", RFC 2554, March 1999. 8.2 Informative References None. Allman, Katz Expires - February 2005 [Page 11] SMTP Responsible Submitter Extension August 2004 9. Acknowledgments The authors would like to thank the participants of the MARID working group and the following individuals for their comments and suggestions, which greatly improved this document: Robert Atkinson, Simon Attwell, Roy Badami, Greg Connor, Dave Crocker, Matthew Elvey, Tony Finch, Ned Freed, Mark Lentczner, Jim Lyon, Bruce McMillan, Sam Neely, Daryl Odnert, Margaret Olson, Pete Resnick, Hector Santos, Nick Shelness, Rand Wacker, Meng Weng Wong 10. Authors' Addresses Eric Allman Sendmail, Inc. 6425 Christie Ave, Suite 400 Emeryville, CA 94608 USA E-mail: eric@sendmail.com Harry Katz Microsoft Corp. 1 Microsoft Way Redmond, WA 98052 USA E-mail: hkatz@microsoft.com Allman, Katz Expires - February 2005 [Page 12] SMTP Responsible Submitter Extension August 2004 11. Change History The following changes were made to this document in the -03 revision: - in 1, amended wording about the advantages of basing validation on RFC 2822 headers. - in 4.1, amended wording to use "null reverse-path" to conform with RFC 2821 terminology. - in 4.1, made SUBMITTER a MUST on all messages for conformance. - in 4.2, changed SHOULD reject to MUST reject when SUBMITTER value does not match PRA value derived from headers. - in 4.2, added a paragraph noting that the SUBMITTER parameter is not to be used as a reverse-path address. - added 5.5, example of SUBMITTER usage when reverse path is null. - changed several references from [SENDER-ID] to [PRA] to reflect creation of separate [PRA] document. - minor wording changes and corrections throughout. The following changes were made to this document in the -02 revision: - on title page, updated the intellectual property declaration to be consistent with RFC 3668. - in 1, reworked text removing references to various anti-spoofing proposals and clarifying the definition of several terms used herein. - in 4, removed redundant text from the first paragraph - in 4.1, strengthened the conformance requirements and added the recommendation for inclusion of the SUBMITTER parameter even when the MAIL FROM address is identical to the purported responsible address. - in 4.1, removed wording about making the SUBMITTER parameter mandatory at some future time. - in 4.1, moved the procedural descriptions for initial message submission and subsequent message retransmission to the non- normative Examples section. - in 4.2, removed the wording about procedures to be used at some future time when the SUBMITTER parameter becomes mandatory - in 4.2, significant rewording to simplify and clarify the verification process and error messages. - in 4.3, clarified the wording to include all cases of message transmission to a non-SUBMITTER aware server. - in 5, changed example addresses to be compliant with RFC 2606 - in 6, rewording and focus on security considerations specific to this proposal - added 7, IANA Considerations - in 8, removed unreferenced informative references - minor wording changes throughout. Allman, Katz Expires - February 2005 [Page 13] SMTP Responsible Submitter Extension August 2004 12. Full Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78 and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Allman, Katz Expires - February 2005 [Page 14]