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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft Philip Guenther 2 Expires: November 2005 Sendmail, Inc. 3 May 2005 5 Sieve Email Filtering: Editheader Extension 6 draft-ietf-sieve-editheader-01.txt 8 Status of this memo 10 By submitting this Internet-Draft, each author represents that any 11 applicable patent or other IPR claims of which he or she is aware 12 have been or will be disclosed, and any of which he or she becomes 13 aware will be disclosed, in accordance with Section 6 of BCP 79. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as 18 Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six 21 months and may be updated, replaced, or obsoleted by other 22 documents at any time. It is inappropriate to use Internet- 23 Drafts as reference material or to cite them other than as 24 "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/1id-abstracts.html 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html 32 Copyright Notice 34 Copyright (C) The Internet Society (2005). 36 Abstract 38 This document defines two new actions for the "Sieve" email 39 filtering language that add and delete email header fields. 41 1. Introduction 43 Email header fields are a flexible and easy to understand means 44 of communication between email processors. 45 This extension enables sieve scripts to interact with other 46 components that consume or produce header fields by allowing 47 the script to delete and add header fields. 49 2. Conventions used. 51 Conventions for notations are as in [SIEVE] section 1.1, including 52 use of [KEYWORDS] and the "Syntax:" label for the definition of 53 action and tagged arguments syntax. 55 The term "header field" is used here as in [IMAIL] to mean a 56 logical line of an email message header. 58 The capability string associated with the extension defined in 59 this document is "editheader". 61 3. Action addheader 63 Syntax: "addheader" [":last"] 65 The addheader action adds a header field to the existing message 66 header. The field-name MUST be a valid 7-bit US-ASCII header 67 field name as described by the [IMAIL] "field-name" nonterminal 68 syntax element. The addheader action does not affect Sieve's 69 implicit keep. 71 If the specified field value does not match the RFC 2822 72 "unstructured" nonterminal syntax element or exceeds a length 73 limit set by the implementation, the implementation MUST either 74 flag an error or encode the field using folding white space and 75 the encodings described in [RFC2047] or [RFC2231] to be compliant 76 with RFC 2822. 78 An implementation MAY impose a length limit onto the size of 79 the encoded header field; such a limit MUST NOT be less 80 than 998 characters, not including the terminating CRLF 81 supplied by the implementation. 83 By default, the header field is inserted at the beginning of the 84 existing message header. If the optional flag ":last" is 85 specified, it is appended at the end. 87 Example: 88 /* Don't redirect if we already redirected */ 89 if not header :contains "X-Sieve-Filtered" 90 ["", ""] 91 { 92 addheader "X-Sieve-Filtered" ""; 93 redirect "kim@home.example.com"; 94 } 96 4. Action deleteheader 98 Syntax: "deleteheader" [":index" [":last"]] 99 [COMPARATOR] [MATCH-TYPE] 100 101 [] 103 By default, the deleteheader action deletes all occurrences of 104 the named header field. The deleteheader action does not affect 105 Sieve's implicit keep. 107 The field-name is mandatory and always matched as a 108 case-insensitive US-ASCII string. The value-patterns, 109 if specified, are matched according to the match type and 110 comparator. If none are specified, all values match. 112 The field-name MUST be a valid 7-bit header field name as described 113 by the [IMAIL] "field-name" nonterminal syntax element. 115 If :index is specified, the attempts to match 116 a value are limited to the header field (beginning 117 at 1, the first named header field). If :last is specified, 118 the count is backwards; 1 denotes the last named header field, 119 2 the second to last, and so on. The counting happens 120 before the match, if any. For example: 122 deleteheader :index 2 :contains "Received" "via carrier-pigeon" 124 deletes the second "Received:" header field if it contains 125 the string "via carrier-pigeon" (not the second Received: field 126 that contains "via carrier-pigeon"). 128 It is not an error if no header fields match the conditions in 129 the deleteheader action or if the :index argument is greater 130 than the number of named header fields. 132 5. Interaction with Other Sieve Extensions 134 Tests and actions such as "exists", "header", or "vacation" 135 [VACATION] that examine header fields MUST examine the current 136 state of a header as modified by any actions that have taken 137 place so far. 139 As an example, the "header" test in the following fragment will 140 always evaluate to true, regardless of whether the incoming 141 message contained an "X-Hello" header field or not: 143 addheader "X-Hello" "World"; 144 if header :contains "X-Hello" "World" 145 { 146 fileinto "international"; 147 } 149 However, if the presence or value of a header field affects how 150 the implementation parses or decodes other parts of the message, 151 then for the purposes of that parsing or decoding the implementation 152 MAY ignore some or all changes made to those header fields. For 153 example, in an implementation that supports the [BODY] extension, 154 "body" tests may be unaffected by deleting or adding Content-Type 155 or Content-Transfer-Encoding header fields. This does not rescind 156 the requirement that changes to those header fields affect direct 157 tests; only the semantic side effects of changes to the fields 158 may be ignored. 160 Actions that store or send the message MUST do so with the current 161 set of header fields. 163 For the purpose of weeding out duplicates, a message modified 164 by addheader or deleteheader MUST be considered the same as 165 the original message. For example, in an implementation that 166 obeys the constraint in [SIEVE] section 2.10.3 and does not deliver 167 the same message to a folder more than once, the following 168 code fragment 170 keep; 171 addheader "X-Flavor" "vanilla"; 172 keep; 174 MUST only file one message. It is up to the implementation 175 to pick which of the redundant "fileinto" or "keep" actions is 176 executed, and which ones are ignored. 178 The "implicit keep" is thought to be executed at the end of 179 the script, after the headers have been modified. (However, 180 a canceled "implicit keep" remains canceled.) 182 6. IANA Considerations 184 The following template specifies the IANA registration of the Sieve 185 extension specified in this document: 187 To: iana@iana.org 188 Subject: Registration of new Sieve extension 190 Capability name: editheader 191 Capability keyword: editheader 192 Capability arguments: N/A 193 Standards Track/IESG-approved experimental RFC number: this RFC 194 Person and email address to contact for further information: 196 Jutta Degener 197 jutta@pobox.com 199 This information should be added to the list of sieve extensions 200 given on http://www.iana.org/assignments/sieve-extensions. 202 7. Security Considerations 204 Someone with write access to a user's script storage may use this 205 extension to generate headers that a user would otherwise be 206 shielded from (by a gateway MTA that removes them). 208 A sieve filter that removes header fields may unwisely destroy 209 evidence about the path a message has taken. 211 Any change in a message content may interfere with digital 212 signature mechanisms that include the header in the signed 213 material. Since normal message delivery adds "Received:" 214 header fields to the beginning of a message, many such schemas 215 are impervious to headers prefixed to a message, and will 216 work with "addheader" unless :last is used. 218 Any decision mechanism in a user's filter that is based 219 on headers is vulnerable to header spoofing. For example, 220 if the user adds an APPROVED header or tag, a malicious sender 221 may add that tag or header themselves. One way to guard against 222 this is to delete or rename any such headers or stamps prior 223 to processing the message. 225 Modifying the header of a message and then using the "reject" 226 action may let a sender 'probe' the logic of the sieve filter. 228 8. Acknowledgments 230 Thanks to Eric Allman, Cyrus Daboo, Matthew Elvey, Ned Freed, 231 Arnt Gulbrandsen, Simon Josefsson, Will Lee, William Leibzon, 232 Mark E. Mallett, Chris Markle, Randall Schwartz, Nigel Swinson, 233 Kjetil Torgrim Homme, and Rand Wacker for extensive corrections 234 and suggestions. 236 9. Authors' Addresses 238 Jutta Degener 239 5245 College Ave, Suite #127 240 Oakland, CA 94618 242 Email: jutta@pobox.com 244 Philip Guenther 245 Sendmail, Inc. 246 6425 Christie Ave, 4th Floor 247 Emeryville, CA 94608 249 Email: guenther@sendmail.com 251 10. Discussion 253 This section will be removed when this document leaves the 254 Internet-Draft stage. 256 This draft is intended as an extension to the Sieve mail filtering 257 language. Sieve extensions are discussed on the MTA Filters mailing 258 list at . Subscription requests can 259 be sent to (send an email 260 message with the word "subscribe" in the body). 262 More information on the mailing list along with a WWW archive of 263 back messages is available at . 265 10.1 Changes from draft-ietf-sieve-editheader-00.txt 267 Updated IPR boilerplate to RFC 3978/3979. 269 Many corrections in response to WGLC comments. Of particular note: 270 - correct a number of spelling and grammar errors 271 - document that neither addheader nor deleteheader affects the 272 implicit keep 273 - add normative references to RFC 2047 and RFC 2231 274 - it is not an error for deleteheader to affect nothing 275 - change "foo.tld" to "foo.example.com" 276 - add an informative reference to [VACATION], citing it as an 277 example of an action that examines header fields 278 - add weasel words about changes to fields that have secondary 279 effects 280 - add security consideration for combination of header changes 281 and "reject" 283 10.2 Changes from draft-degener-sieve-editheader-03.txt 285 Renamed to draft-ietf-sieve-editheader-00.txt; 286 tweaked the title and abstract. 288 Added Philip Guenther as co-author. 290 Updated IPR boilerplate. 292 10.3 Changes from draft-degener-sieve-editheader-02.txt 294 Changed the duplicate restrictions from "messages with different 295 headers MUST be considered different" to their direct opposite, 296 "messages with different headers MUST be considered the same," 297 as requested by workgroup members on the mailing list. 299 Expanded mention of header signature schemes to Security 300 Considerations. 302 Added IANA Considerations section. 304 Appendices 306 Appendix A. Normative References 308 [IMAIL] Resnick, P., "Internet Message Format", RFC 2822, April 309 2001. 311 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate 312 Requirement Levels", RFC 2119, March 1997. 314 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail 315 Extensions) Part Three: Message Header Extensions for 316 Non-ASCII Text", RFC 2047, November 1996. 318 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and 319 Encoded Word Extensions: Character Sets, Languages, and 320 Continuations", RFC 2231, November 1997. 322 [SIEVE] Showalter, T., "Sieve: A Mail Filtering Language", RFC 3028, 323 January 2001. 325 Appendix B. Informative References 327 [VACATION] Showalter, T. and N. Freed, "Sieve Email Filtering: 328 Vacation Extension", draft-ietf-sieve-vacation-02, 329 April 2005 331 Copyright Statement 333 Copyright (C) The Internet Society (2005). 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