Internet Draft Philip Guenther Expires: November 2005 Sendmail, Inc. May 2005 Sieve Email Filtering: Editheader Extension draft-ietf-sieve-editheader-01.txt 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/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document defines two new actions for the "Sieve" email filtering language that add and delete email header fields. 1. Introduction Email header fields are a flexible and easy to understand means of communication between email processors. This extension enables sieve scripts to interact with other components that consume or produce header fields by allowing the script to delete and add header fields. 2. Conventions used. Conventions for notations are as in [SIEVE] section 1.1, including use of [KEYWORDS] and the "Syntax:" label for the definition of action and tagged arguments syntax. The term "header field" is used here as in [IMAIL] to mean a logical line of an email message header. The capability string associated with the extension defined in this document is "editheader". 3. Action addheader Syntax: "addheader" [":last"] The addheader action adds a header field to the existing message header. The field-name MUST be a valid 7-bit US-ASCII header field name as described by the [IMAIL] "field-name" nonterminal syntax element. The addheader action does not affect Sieve's implicit keep. If the specified field value does not match the RFC 2822 "unstructured" nonterminal syntax element or exceeds a length limit set by the implementation, the implementation MUST either flag an error or encode the field using folding white space and the encodings described in [RFC2047] or [RFC2231] to be compliant with RFC 2822. An implementation MAY impose a length limit onto the size of the encoded header field; such a limit MUST NOT be less than 998 characters, not including the terminating CRLF supplied by the implementation. By default, the header field is inserted at the beginning of the existing message header. If the optional flag ":last" is specified, it is appended at the end. Example: /* Don't redirect if we already redirected */ if not header :contains "X-Sieve-Filtered" ["", ""] { addheader "X-Sieve-Filtered" ""; redirect "kim@home.example.com"; } 4. Action deleteheader Syntax: "deleteheader" [":index" [":last"]] [COMPARATOR] [MATCH-TYPE] [] By default, the deleteheader action deletes all occurrences of the named header field. The deleteheader action does not affect Sieve's implicit keep. The field-name is mandatory and always matched as a case-insensitive US-ASCII string. The value-patterns, if specified, are matched according to the match type and comparator. If none are specified, all values match. The field-name MUST be a valid 7-bit header field name as described by the [IMAIL] "field-name" nonterminal syntax element. If :index is specified, the attempts to match a value are limited to the header field (beginning at 1, the first named header field). If :last is specified, the count is backwards; 1 denotes the last named header field, 2 the second to last, and so on. The counting happens before the match, if any. For example: deleteheader :index 2 :contains "Received" "via carrier-pigeon" deletes the second "Received:" header field if it contains the string "via carrier-pigeon" (not the second Received: field that contains "via carrier-pigeon"). It is not an error if no header fields match the conditions in the deleteheader action or if the :index argument is greater than the number of named header fields. 5. Interaction with Other Sieve Extensions Tests and actions such as "exists", "header", or "vacation" [VACATION] that examine header fields MUST examine the current state of a header as modified by any actions that have taken place so far. As an example, the "header" test in the following fragment will always evaluate to true, regardless of whether the incoming message contained an "X-Hello" header field or not: addheader "X-Hello" "World"; if header :contains "X-Hello" "World" { fileinto "international"; } However, if the presence or value of a header field affects how the implementation parses or decodes other parts of the message, then for the purposes of that parsing or decoding the implementation MAY ignore some or all changes made to those header fields. For example, in an implementation that supports the [BODY] extension, "body" tests may be unaffected by deleting or adding Content-Type or Content-Transfer-Encoding header fields. This does not rescind the requirement that changes to those header fields affect direct tests; only the semantic side effects of changes to the fields may be ignored. Actions that store or send the message MUST do so with the current set of header fields. For the purpose of weeding out duplicates, a message modified by addheader or deleteheader MUST be considered the same as the original message. For example, in an implementation that obeys the constraint in [SIEVE] section 2.10.3 and does not deliver the same message to a folder more than once, the following code fragment keep; addheader "X-Flavor" "vanilla"; keep; MUST only file one message. It is up to the implementation to pick which of the redundant "fileinto" or "keep" actions is executed, and which ones are ignored. The "implicit keep" is thought to be executed at the end of the script, after the headers have been modified. (However, a canceled "implicit keep" remains canceled.) 6. IANA Considerations The following template specifies the IANA registration of the Sieve extension specified in this document: To: iana@iana.org Subject: Registration of new Sieve extension Capability name: editheader Capability keyword: editheader Capability arguments: N/A Standards Track/IESG-approved experimental RFC number: this RFC Person and email address to contact for further information: Jutta Degener jutta@pobox.com This information should be added to the list of sieve extensions given on http://www.iana.org/assignments/sieve-extensions. 7. Security Considerations Someone with write access to a user's script storage may use this extension to generate headers that a user would otherwise be shielded from (by a gateway MTA that removes them). A sieve filter that removes header fields may unwisely destroy evidence about the path a message has taken. Any change in a message content may interfere with digital signature mechanisms that include the header in the signed material. Since normal message delivery adds "Received:" header fields to the beginning of a message, many such schemas are impervious to headers prefixed to a message, and will work with "addheader" unless :last is used. Any decision mechanism in a user's filter that is based on headers is vulnerable to header spoofing. For example, if the user adds an APPROVED header or tag, a malicious sender may add that tag or header themselves. One way to guard against this is to delete or rename any such headers or stamps prior to processing the message. Modifying the header of a message and then using the "reject" action may let a sender 'probe' the logic of the sieve filter. 8. Acknowledgments Thanks to Eric Allman, Cyrus Daboo, Matthew Elvey, Ned Freed, Arnt Gulbrandsen, Simon Josefsson, Will Lee, William Leibzon, Mark E. Mallett, Chris Markle, Randall Schwartz, Nigel Swinson, Kjetil Torgrim Homme, and Rand Wacker for extensive corrections and suggestions. 9. Authors' Addresses Jutta Degener 5245 College Ave, Suite #127 Oakland, CA 94618 Email: jutta@pobox.com Philip Guenther Sendmail, Inc. 6425 Christie Ave, 4th Floor Emeryville, CA 94608 Email: guenther@sendmail.com 10. Discussion This section will be removed when this document leaves the Internet-Draft stage. This draft is intended as an extension to the Sieve mail filtering language. Sieve extensions are discussed on the MTA Filters mailing list at . Subscription requests can be sent to (send an email message with the word "subscribe" in the body). More information on the mailing list along with a WWW archive of back messages is available at . 10.1 Changes from draft-ietf-sieve-editheader-00.txt Updated IPR boilerplate to RFC 3978/3979. Many corrections in response to WGLC comments. Of particular note: - correct a number of spelling and grammar errors - document that neither addheader nor deleteheader affects the implicit keep - add normative references to RFC 2047 and RFC 2231 - it is not an error for deleteheader to affect nothing - change "foo.tld" to "foo.example.com" - add an informative reference to [VACATION], citing it as an example of an action that examines header fields - add weasel words about changes to fields that have secondary effects - add security consideration for combination of header changes and "reject" 10.2 Changes from draft-degener-sieve-editheader-03.txt Renamed to draft-ietf-sieve-editheader-00.txt; tweaked the title and abstract. Added Philip Guenther as co-author. Updated IPR boilerplate. 10.3 Changes from draft-degener-sieve-editheader-02.txt Changed the duplicate restrictions from "messages with different headers MUST be considered different" to their direct opposite, "messages with different headers MUST be considered the same," as requested by workgroup members on the mailing list. Expanded mention of header signature schemes to Security Considerations. Added IANA Considerations section. Appendices Appendix A. Normative References [IMAIL] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations", RFC 2231, November 1997. [SIEVE] Showalter, T., "Sieve: A Mail Filtering Language", RFC 3028, January 2001. Appendix B. Informative References [VACATION] Showalter, T. and N. Freed, "Sieve Email Filtering: Vacation Extension", draft-ietf-sieve-vacation-02, April 2005 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. 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