Internet Draft M. Elvey Document: draft-ietf-sieve-refuse-reject-00 The Elvey Partnership,LLC Expires: November 2005 A. Melnikov Isode Ltd May 2005 The SIEVE mail filtering language - reject and refuse extensions draft-ietf-sieve-refuse-reject Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 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. A revised version of this draft document will be submitted to the RFC editor as a Proposed Standard for the Internet Community. Discussion and suggestions for improvement are requested. Distribution of this draft is unlimited. Abstract This memo defines the SIEVE mail filtering language [SIEVE] "reject" and "refuse" extensions. A Joe-job is a spam run forged to appear as though it came from an innocent party, who is then generally flooded by the bounces, MDNs and messages with complaints. With the Sieve "reject" action, MDNs contribute to the flood of Joe-job spam to victims of Joe-jobs; SMTP level refusals usually don't. With "refuse", Sieve gains the ability to simply not accept an email during the SMTP transaction (instead of accepting it and then sending an MDN [MDN] back to the alleged sender using "reject"). Table of Contents 1. Introduction 4 2. Conventions Used in this Document 4 3. Discussion of finer points 4 4. SIEVE "reject" extension 5 4.1 Action reject 5 4.2 "reject" compatibility with other actions 6 5. SIEVE "refuse" extension 6 5.1 Action refuse 6 5.2 "refuse" compatibility with other actions 7 5.3 Explicit accomodation for servers that support Enhanced Error Codes [ENHANCED-CODES] 7 6. Security Considerations 8 7. IANA Considerations 8 7.1 reject extension registration 8 7.2 refuse extension registration 8 8. References 9 8.1 Normative References 9 8.2 Informative References 9 9. Acknowledgments 9 10. Author's Addresses 10 11. Intellectual Property Rights Statement 10 12. Full Copyright Statement 11 13. Changes from RFC 3028 11 14. Change Log 11 1. Introduction The SIEVE mail filtering language [SIEVE] "reject" action allows users to refuse delivery of a message by sending an [MDN]. This action was originally defined in RFC 3028 [SIEVE]. The "refuse" extension, if supported, permits users to handle unwanted email in a way that is sometimes preferable to the existing 'discard' and 'reject' capabilities. When a spam- detection system suspects a message is spam, but isn't certain, discarding the email is considered too risky for some users, for example, those who receive sales leads by email. They are willing to use the reject command. Users are willing to reject but not discard because the sender of an email incorrectly marked as spam will receive a notification that the email was refused, and will likely try again to contact the intended recipient, perhaps via another method of communication. Unfortunately, this usage is problematic, because in the usual case, the email is indeed spam, and the alleged sender to whom the MDN caused by the reject will be sent will often be an innocent Joe-job victim. "Refuse" is intended to be superior to "reject" because it will be less likely to result in email to an innocent victim. "Refuse" refuses to accept an email for delivery instead of accepting it and then sending an MDN. Much spam is sent through open proxies, so "refuse" reduces Joe-job bounces resulting from usage of reject. "Refuse" will also reduce Joe-jobs caused by virus self-propagation via emails with false sender information. "Refuse" may conserve bandwidth, by reducing the number of MDNs sent. Further discussion highlighting the risks of "reject" and the benefits of "refuse" can be found in [Joe-DoS]. 2. Conventions Used in this Document Conventions for notations are as in [SIEVE] section 1.1, including use of [KEYWORDS]. This document does not attempt to define what exactly constitutes a spam or virus containing email or how it should be identified, or what actions should be taken when detected. 3. Discussion of finer points The "refuse" action MUST refuse to accept an email for delivery at the SMTP/LMTP level by returning a 5XX reply code, instead of sending an MDN as required by the "reject" action, other than for the two exceptions specified below. A SIEVE implementation that cannot do so MUST NOT claim to support the refuse extension. There is an exception when a message has multiple valid recipients, and at least one but not all of them are refusing delivery (whether the refusal is caused by execution of a Sieve "refuse" or for another reason). In this case, the server MUST accept the message and generate DSNs for all recipients that are refusing it. Note that this exception only applies to SMTP, as LMTP is able to reject messages on a per-recipient basis. If a "refuse" implementation performs a return-path verification and it clearly indicates that the message has a forged return-path, the implementation need not refuse to accept the mail, but rather MAY accept and discard it. The "reject" action is defined so that it can be used by implementations unable to implement "refuse" (i.e. by MUAs) or for backwards compatibility with scripts based on RFC3028. 4. SIEVE "reject" extension SIEVE implementations that implement the "reject" action must use the "reject" capability string. 4.1 Action reject Syntax: reject The "reject" action refuses delivery of a message by sending back an [MDN] to the sender. The "reject" action also cancels the implicit keep. It resends the message to the sender, wrapping it in a "reject" form, noting that it was rejected by the recipient. In the following script, a message is rejected and returned to the sender. Example: require ["reject"] if header :contains "from" "coyote@desert.example.org" { reject "I am not taking mail from you, and I don't want your birdseed, either!"; } A reject message MUST take the form of a failure MDN as specified by [MDN]. The human-readable portion of the message, the first component of the MDN, contains the human readable message describing the error, and it SHOULD contain additional text alerting the original sender that mail was refused by a filter. This part of the MDN might appear as follows: ------------------------------------------------------------ The message was refused by the recipient's mail filtering program. The reason given was as follows: I am not taking mail from you, and I don't want your birdseed, either! ------------------------------------------------------------ The MDN action-value field as defined in the MDN specification MUST be "deleted" and MUST have the MDN-sent-automatically and automatic- action modes set. 4.2 "reject" compatibility with other actions A "reject" action cancels the implicit keep. Implementations MUST prohibit more than one reject in a SIEVE script. "Reject" is also incompatible with the "refuse" and "vacation" [VACATION] extensions. Implementations SHOULD prohibit reject when used with other actions. 5. SIEVE "refuse" extension SIEVE implementations that implement the "refuse" action must use the "refuse" capability string. 5.1 Action refuse Syntax: refuse The "refuse" action refuses delivery of a message by sending back the 550 SMTP response code to an SMTP client. This extension can be only supported by a Sieve implementation running in an MTA. Note that SMTP [SMTP] doesn't allow for non-ASCII characters in SMTP response text. It is an error for non-ASCII characters to appear in the "reason" string (unless the client and the server use an SMTP extension that allows for transmission of non-ASCII reply text; such an extension is not known to the authors). If the "reason" string is multiline, than the reason text MUST be returned as a multiline SMTP/LMTP response, per [SMTP], section 4.2.1. In the following script (which assumes support for the spamtest extension), messages that test highly positive for spam are refused. Example: require ["refuse", "spamtest"] if spamtest :value "ge" :comparator "i;ascii-numeric" "6" { refuse text: SpamAssassin thinks the message is spam. It is therefore being refused. Please call 1-900-PAY-US if you want to reach us. . ; elsif spamtest :value "ge" :comparator "i;ascii-numeric" "4" { fileinto "Suspect"; } The following excerpt from an SMTP session shows it in action. C: DATA S: 354 Send message, ending in CRLF.CRLF. ... C: . S: 550-SpamAssassin thinks the message is spam. S: 550-It is therefore being refused. S: 550 Please call 1-900-PAY-US if you want to reach us. 5.2 "refuse" compatibility with other actions "Refuse" cancels the implicit keep, and is incompatible with "reject" and "discard". "Refuse" is also incompatible with the "vacation" [VACATION] action. Any action that would modify the message body will necessarily have no effect on the body of any message refused by "refuse" using the 550 SMTP response code. If a script attempts to "refuse" the same message more than once, the implementation may ignore the later attempts or consider it an error." 5.3 Explicit accomodation for servers that support Enhanced Error Codes [ENHANCED-CODES] This section only concerns implementations that support Enhanced Error Codes. If the server supports RFC 2034 [ENHANCED-CODES] it MUST select an appropriate Enhanced Error Code (e.g. 5.7.1 or a more generic 5.7.0) and prepend it to the "reason" text. I.e. on such an implementation, the example in section 4.1 would show up in SMTP as: 550-5.7.1 SpamAssassin thinks the message is spam. 550-5.7.1 It is therefore being refused. 550 5.7.1 Please call 1-900-PAY-US if you want to reach us. if the server selected "5.7.1" as appropriate. 6. Security Considerations The "refuse" extension does not raise any security considerations that are not present in the base [SIEVE] protocol, and these issues are discussed in [SIEVE]. 7. IANA Considerations The following section provides the IANA registrations for the Sieve extensions specified in this document: 7.1 reject extension registration IANA is requested to update the registration for the SIEVE "reject" and "refuse" extensions to point to this document. <> 7.2 refuse extension registration To: iana@iana.org Subject: Registration of new Sieve extension Capability name: refuse Capability keyword: refuse Capability arguments: N/A Standards Track/IESG-approved experimental RFC number: this RFC Person and email address to contact for further information: Matthew Elvey The Elvey Partnership, LLC 3042 Sacramento-ietf St Ste 04 San Francisco, CA U.S.A. 8. References 8.1 Normative References [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [SIEVE] Showalter, "Sieve: A Mail Filtering Language", RFC 3028, January 2001. Guenther, P., "Sieve: An Email Filtering Language", Work-in- progress, draft-ietf-sieve-3028bis-XX.txt [SMTP] Klensin, J. (Editor), "Simple Mail Transfer Protocol", AT&T Laboratories, RFC 2821, April 2001. [LMTP] Myers, J., "Local Mail Transfer Protocol", Carnegie-Mellon University, RFC 2033, October 1996. [DSN] Moore , K., Vaudreuil, G., "An Extensible Message Format for Delivery Status Notifications", University of Tennessee, Lucent Technologies, RFC 3464, January 2003. [MDN] Fajman, R., "An Extensible Message Format for Message Disposition Notifications", National Institutes of Health, RFC 2298, March 1998. [ENHANCED-CODES] Freed, N., "SMTP Service Extension for Returning Enhanced Error Codes", Innosoft, RFC 2034, October 1996. 8.2 Informative References [Joe-DoS] Stefan Frei, Ivo Silvestri, Gunter Ollmann, "Mail Non Delivery Message DDoS Attacks", 5 April 2004", . 9. Acknowledgments Thanks to Ned Freed, Cyrus Daboo, Arnt Gulbrandsen, Kristin Hubner, Mark E. Mallett and Philip Guenther for comments and corrections. The authors gratefully acknowledge the extensive work of Tim Showalter as the author of the RFC 3028, which originally defined "reject". 10. Author's Addresses Matthew Elvey The Elvey Partnership, LLC 3042 Sacramento-ietf St Ste 04 San Francisco, CA U.S.A. Email: sieve3@matthew.elvey.com Alexey Melnikov Isode Limited 5 Castle Business Village 36 Station Road Hampton, Middlesex, TW12 2BX UK Email: Alexey.Melnikov@isode.com 11. Intellectual Property Rights Statement 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. 12. Full Copyright Statement Copyright (C) The Internet Society (2005). 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. 13. Changes from RFC 3028 Clarified that the "reject" action cancels the implicit keep. 14. Change Log <> 00 First formal draft. 01 Explicit RFC 2034 support, disallow "refuse" in MUAs, typos corrected, clarifications, etc. 02 Many insubstantial editorial changes (mostly rewording text for readability). Added text regarding non-ASCII characters in the refuse "reason" string. Added an exception allowing return-path forgery to justify discarding a message. 03 (Renamed to be SIEVE WG 00) - Updated boilerplate, added reject action from the base spec, acknowledged Tim as the author of "reject".