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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SIEVE Email Filtering Working C. Daboo 3 Group July 13, 2006 4 Internet-Draft 5 Expires: January 14, 2007 7 SIEVE Email Filtering: Spamtest and Virustest Extensions 8 draft-ietf-sieve-spamtestbis-05 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on January 14, 2007. 35 Copyright Notice 37 Copyright (C) The Internet Society (2006). 39 Abstract 41 The SIEVE email filtering language "spamtest", "spamtestplus" and 42 "virustest" extensions permit users to use simple, portable commands 43 for spam and virus tests on email messages. Each extension provides 44 a new test using matches against numeric "scores". It is the 45 responsibility of the underlying SIEVE implementation to do the 46 actual checks that result in proper input to the tests. 48 Change History (to be removed prior to publication as an RFC) 49 Changes from -04: 50 1. Relaxed capability requirement so that both spamtest and 51 spamtestplus can be present. 52 2. Tweaked text describing spam numeric value ranges. 54 Changes from -03: 55 1. Clarified that there are two possible ways to test for not-spam. 56 2. Clarified that 'not tested for xxx' also implies 'SIEVE could not 57 determine whether a test was done or not'. 59 Changes from -02: 60 1. Changed formatting of tables. 61 2. Fixed missing 2119 definitions. 62 3. Moved reference to previous extension to informative. 63 4. Minor text improvements. 64 5. Fixed some single/double quote issues. 65 6. Reworded abstract, introduction and overview to use better SIEVE 66 terminology when describing tests, commands and results. 67 7. Remove "untested" string result from ":percent" test. 68 8. Allow ":count" match type to be used for tested/untested checks. 70 Changes from -01: 71 1. Changed ACAP reference to i18n-comparators draft. 72 2. Changed MUST in security section for virus checker updates to 73 plain must. 74 3. Return string "untested" when :percent is used and no test has 75 been done. 76 4. Remove MUST NOT for having both spamtestplus and spamtest 77 capabilities present, and instead make it a SHOULD NOT. 78 5. Add text to state that implementations MUST return an error if 79 spamtestplus is not present when :percent is used. 80 6. Tweak first para of security considerations to better reflect 81 reality of testing. 82 7. Syntax -> Usage. 83 8. Updated references to 3028bis and 3431bis. 85 Changes from -00: 86 1. Added description of how to check for untested when using 87 :percent. 88 2. Changed requires item to "spamtestplus". 89 3. Changed text describing which requires item needs to be present. 91 Changes from RFC3685: 92 1. Added ":percent" argument to spamtest. 94 Table of Contents 96 1. Introduction and Overview . . . . . . . . . . . . . . . . . . 4 97 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 98 3. SIEVE Extensions . . . . . . . . . . . . . . . . . . . . . . . 5 99 3.1. General Considerations . . . . . . . . . . . . . . . . . . 5 100 3.2. Test spamtest . . . . . . . . . . . . . . . . . . . . . . 5 101 3.2.1. spamtest without :percent argument . . . . . . . . . . 6 102 3.2.2. spamtest with :percent argument . . . . . . . . . . . 7 103 3.3. Test virustest . . . . . . . . . . . . . . . . . . . . . . 9 104 4. Security Considerations . . . . . . . . . . . . . . . . . . . 11 105 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 106 5.1. spamtest registration . . . . . . . . . . . . . . . . . . 12 107 5.2. virustest registration . . . . . . . . . . . . . . . . . . 12 108 5.3. spamtestplus registration . . . . . . . . . . . . . . . . 13 109 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 110 6.1. Normative References . . . . . . . . . . . . . . . . . . . 13 111 6.2. Informative References . . . . . . . . . . . . . . . . . . 13 112 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14 113 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 114 Intellectual Property and Copyright Statements . . . . . . . . . . 16 116 1. Introduction and Overview 118 SIEVE scripts are frequently being used to do spam and virus 119 filtering based on either implicit script tests (e.g. tests for 120 "black-listed" senders directly encoded in the SIEVE script), or via 121 testing messages modified by some external spam or virus checker that 122 handled the message prior to SIEVE. The use of third-party spam and 123 virus checker tools poses a problem since each tool has its own way 124 of indicating the result of its checks. These usually take the form 125 of a header added to the message, the content of which indicates the 126 status using some syntax defined by the particular tool. Each user 127 has to then create their own SIEVE scripts to match the contents of 128 these headers to do filtering. This requires the script to stay in 129 synchronization with the third party tool as it gets updated or 130 perhaps replaced with another. Thus scripts become tied to specific 131 environments, and lose portability. 133 The purpose of this document is to introduce two SIEVE tests that can 134 be used to implement "generic" tests for spam and viruses in messages 135 processed via SIEVE scripts. The spam and virus checks themselves 136 are handled by the underlying SIEVE implementation in whatever manner 137 is appropriate, so that the SIEVE spam and virus test commands can be 138 used in a portable way. 140 In order to do numeric comparisons against the returned strings, 141 server implementations MUST also support the SIEVE relational 142 [I-D.ietf-sieve-3431bis] extension, in addition to the extensions 143 described here. All examples below assume the relational extension 144 is present. 146 2. Conventions Used in This Document 148 Conventions for notations are as in [I-D.ietf-sieve-3028bis] section 149 1.1. 151 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 152 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 153 document are to be interpreted as described in [RFC2119]. 155 The term "spam" is used in this document to refer to unsolicited or 156 unwanted email messages. This document does not attempt to define 157 what exactly constitutes spam, or how it should be identified, or 158 what actions should be taken when detected. 160 The term "virus" is used in this document to refer to any type of 161 message whose content can cause malicious damage. This document does 162 not attempt to define what exactly constitutes a virus, or how it 163 should be identified, or what actions should be taken when detected. 165 3. SIEVE Extensions 167 3.1. General Considerations 169 The "spamtest" and "virustest" tests described below evaluate the 170 results of implementation-specific spam and virus checks in a 171 portable way. The implementation may, for example, check for third- 172 party spam tool headers and determine how those map into the way the 173 test commands are used. To do this, the underlying SIEVE 174 implementation provides a normalized result string as one of the 175 inputs to each test command. The normalized result string is 176 considered to be the value on the left hand side of the test, and the 177 comparison values given in the test command are considered to be on 178 the right hand side. 180 The normalized result starts with a digit string, with its numeric 181 value within the range of values used by the specific test, 182 indicating the severity of spam or viruses in a message or whether 183 any tests were done at all. This may optionally be followed by a 184 space (%x20) character and arbitrary text, or in one specific case a 185 single keyword is returned. The numeric value can be compared to 186 specific values using the SIEVE relational [I-D.ietf-sieve-3431bis] 187 extension in conjunction with the "i;ascii-numeric" comparator 188 [I-D.newman-i18n-comparator], which will test for the presence of a 189 numeric value at the start of the string, ignoring any additional 190 text in the string. The optional text can be used to carry 191 implementation specific details about the tests and descriptive 192 comments about the result. Tests can be done using standard string 193 comparators against this text if it helps to refine behavior, however 194 this will break portability of the script as the text will likely be 195 specific to a particular implementation. 197 In addition, the SIEVE relational [I-D.ietf-sieve-3431bis] ":count" 198 match type can be used to determine if the underlying implementation 199 actually did a test. If the underlying spam or virus test was done, 200 the ":count" of the normalized result will return the numeric value 201 "1", whilst if the test was not done, or the SIEVE implementation 202 could not determine if a test was done or not done, the ":count" 203 value will be "0" (zero). 205 3.2. Test spamtest 207 Usage: spamtest [":percent"] [COMPARATOR] [MATCH-TYPE] 208 210 SIEVE implementations that implement the "spamtest" test use an 211 identifier of either "spamtest" or "spamtestplus" for use with the 212 capability mechanism. 214 If the ":percent" argument is not used with any spamtest test, then 215 one or both of "spamtest" or "spamtestplus" capability identifiers 216 MUST be present. 218 If the ":percent" argument is used with any spamtest test, then the 219 "spamtestplus" capability identifier MUST be present. SIEVE 220 implementations MUST return an error if the ":percent" argument is 221 used and "spamtestplus" is not specified. 223 In the interests of brevity and clarity, scripts SHOULD NOT specify 224 both "spamtestplus" and "spamtest" capability identifiers together. 226 The "spamtest" test evaluates to true if the normalized spamtest 227 result matches the value. The type of match is specified by the 228 optional match argument, which defaults to ":is" if not specified. 230 3.2.1. spamtest without :percent argument 232 When the ":percent" argument is not present in the "spamtest" test, 233 the normalized result string provided for the left hand side of the 234 test starts with a numeric value in the range "0" (zero) through 235 "10", with meanings summarized below: 237 +----------+--------------------------------------------------------+ 238 | spamtest | interpretation | 239 | value | | 240 +----------+--------------------------------------------------------+ 241 | 0 | message was not tested for spam, or SIEVE could not | 242 | | determine whether any test was done | 243 | | | 244 | 1 | message was tested and is clear of spam | 245 | | | 246 | 2 - 9 | message was tested and may contain spam; a higher | 247 | | number indicates a greater likelihood of spam | 248 | | | 249 | 10 | message was tested and definitely contains spam | 250 +----------+--------------------------------------------------------+ 252 The underlying SIEVE implementation will map whatever spam check is 253 done into this numeric range, as appropriate. 255 Examples: 257 require ["spamtest", "fileinto", 258 "relational", "comparator-i;ascii-numeric"]; 260 if spamtest :value "eq" :comparator "i;ascii-numeric" "0" 261 { 262 fileinto "INBOX.unclassified"; 263 } 264 elsif spamtest :value "ge" :comparator "i;ascii-numeric" "3" 265 { 266 fileinto "INBOX.spam-trap"; 267 } 269 In this example, any message that has not passed through a spam check 270 tool will be filed into the mailbox "INBOX.unclassified". Any 271 message with a normalized result value greater than or equal to "3" 272 is filed into a mailbox called "INBOX.spam-trap" in the user's 273 mailstore. 275 3.2.2. spamtest with :percent argument 277 When the ":percent" argument is present in the "spamtest" test, the 278 normalized result string provided for the left hand side of the test 279 starts with a numeric value in the range "0" (zero) through "100", 280 with meanings summarized below: 282 +----------+-------------------------------------------------------+ 283 | spamtest | interpretation | 284 | value | | 285 +----------+-------------------------------------------------------+ 286 | 0 | message was tested and is clear of spam, or was not | 287 | | tested for spam, or SIEVE could not determine whether | 288 | | any test was done | 289 | | | 290 | 1 - 99 | message was tested and may contain spam; a higher | 291 | | percentage indicates a greater likelihood of spam | 292 | | | 293 | 100 | message was tested and definitely contains spam | 294 +----------+-------------------------------------------------------+ 296 The underlying SIEVE implementation will map whatever spam check is 297 done into the numeric range, as appropriate. 299 To determine whether the message was tested for spam or not, two 300 options can be used: 302 a. a test with or without the ":percent" argument and ":count" match 303 type, testing for the value "0" as described in Section 3.1. 305 b. a test without the ":percent" argument using the ":value" match 306 type, testing for the normalized result value "0" as described in 307 Section 3.2.1. 309 Examples: 311 require ["spamtestplus", "fileinto", 312 "relational", "comparator-i;ascii-numeric"]; 314 if spamtest :value "eq" 315 :comparator "i;ascii-numeric" "0" 316 { 317 fileinto "INBOX.unclassified"; 318 } 319 elsif spamtest :percent :value "eq" 320 :comparator "i;ascii-numeric" "0" 321 { 322 fileinto "INBOX.not-spam"; 323 } 324 elsif spamtest :percent :value "lt" 325 :comparator "i;ascii-numeric" "37" 326 { 327 fileinto "INBOX.spam-trap"; 328 } 329 else 330 { 331 discard; 332 } 334 In this example, any message that has not passed through a spam check 335 tool will be filed into the mailbox "INBOX.unclassified". Any 336 message that is classified as definitely not containing spam 337 (normalized result value "0") will be filed into the mailbox 338 "INBOX.not-spam". Any message with a normalized result value less 339 than "37" is filed into a mailbox called "INBOX.spam-trap" in the 340 user's mailstore. Any other normalized result value will result in 341 the message being discarded. 343 Alternatively, the SIEVE relational [I-D.ietf-sieve-3431bis] ":count" 344 match type can be used: 346 Examples: 348 if spamtest :percent :count "eq" 349 :comparator "i;ascii-numeric" "0" 350 { 351 fileinto "INBOX.unclassified"; 352 } 353 elsif spamtest :percent :value "eq" 354 :comparator "i;ascii-numeric" "0" 355 { 356 fileinto "INBOX.not-spam"; 357 } 358 elsif spamtest :percent :value "lt" 359 :comparator "i;ascii-numeric" "37" 360 { 361 fileinto "INBOX.spam-trap"; 362 } 363 else 364 { 365 discard; 366 } 368 This example will result in exactly the same behavior as the previous 369 one. 371 3.3. Test virustest 373 Usage: virustest [COMPARATOR] [MATCH-TYPE] 374 376 SIEVE implementations that implement the "virustest" test have an 377 identifier of "virustest" for use with the capability mechanism. 379 The "virustest" test evaluates to true if the normalized result 380 string matches the value. The type of match is specified by the 381 optional match argument, which defaults to ":is" if not specified. 383 The normalized result string provided for the left side of the test 384 starts with a numeric value in the range "0" (zero) through "5", with 385 meanings summarized below: 387 +-----------+-------------------------------------------------------+ 388 | virustest | interpretation | 389 | value | | 390 +-----------+-------------------------------------------------------+ 391 | 0 | message was not tested for viruses, or SIEVE could | 392 | | not determine whether any test was done | 393 | | | 394 | 1 | message was tested and contains no known viruses | 395 | | | 396 | 2 | message was tested and contained a known virus which | 397 | | was replaced with harmless content | 398 | | | 399 | 3 | message was tested and contained a known virus which | 400 | | was "cured" such that it is now harmless | 401 | | | 402 | 4 | message was tested and possibly contains a known | 403 | | virus | 404 | | | 405 | 5 | message was tested and definitely contains a known | 406 | | virus | 407 +-----------+-------------------------------------------------------+ 409 The underlying SIEVE implementation will map whatever virus checks 410 are done into this numeric range, as appropriate. If the message has 411 not been categorized by any virus checking tools, then the virustest 412 result is "0". 414 Example: 416 require ["virustest", "fileinto", 417 "relational", "comparator-i;ascii-numeric"]; 419 if virustest :value "eq" :comparator "i;ascii-numeric" "0" 420 { 421 fileinto "INBOX.unclassified"; 422 } 423 if virustest :value "eq" :comparator "i;ascii-numeric" "4" 424 { 425 fileinto "INBOX.quarantine"; 426 } 427 elsif virustest :value "eq" :comparator "i;ascii-numeric" "5" 428 { 429 discard; 430 } 432 In this example, any message that has not passed through a virus 433 check tool will be filed into the mailbox "INBOX.unclassified". Any 434 message with a normalized result value equal to "4" is filed into a 435 mailbox called "INBOX.quarantine" in the user's mailstore. Any 436 message with a normalized result value equal to "5" is discarded 437 (removed) and not delivered to the user's mailstore. 439 4. Security Considerations 441 SIEVE implementations SHOULD ensure that "spamtest" and "virustest" 442 tests only report spam and virus test results for messages that 443 actually have gone through a legitimate spam or virus check process. 444 In particular, if such checks rely on the addition and subsequent 445 checking of private header fields, it is the responsibility of the 446 implementation to ensure that such headers cannot be spoofed by the 447 sender or intermediary and thereby prevent the implementation from 448 being tricked into returning the wrong result for the test. 450 Server administrators must ensure that the virus checking tools are 451 kept up to date, to provide reasonable protection for users using the 452 "virustest" test. Users should be made aware of the fact that the 453 "virustest" test does not provide a 100% reliable way to remove all 454 viruses, and they should continue to exercise caution when dealing 455 with messages of unknown content and origin. 457 Beyond that, the "spamtest" and "virustest" extensions do not raise 458 any security considerations that are not present in the base 459 [I-D.ietf-sieve-3028bis] protocol, and these issues are discussed in 460 [I-D.ietf-sieve-3028bis]. 462 5. IANA Considerations 464 The following templates specify the IANA registration of the Sieve 465 extensions specified in this document. The registrations for 466 "spamtest" and "virustest" replace those from in [RFC3685]: 468 5.1. spamtest registration 470 To: iana@iana.org 471 Subject: Registration of new Sieve extension 473 Capability name: spamtest 474 Capability keyword: spamtest 475 Capability arguments: N/A 476 Standards Track/IESG-approved experimental RFC number: this RFC 477 Person and email address to contact for further information: 479 Cyrus Daboo 481 483 This information should be added to the list of sieve extensions 484 given on http://www.iana.org/assignments/sieve-extensions. 486 5.2. virustest registration 488 To: iana@iana.org 489 Subject: Registration of new Sieve extension 491 Capability name: virustest 492 Capability keyword: virustest 493 Capability arguments: N/A 494 Standards Track/IESG-approved experimental RFC number: this RFC 495 Person and email address to contact for further information: 497 Cyrus Daboo 499 501 This information should be added to the list of sieve extensions 502 given on http://www.iana.org/assignments/sieve-extensions. 504 5.3. spamtestplus registration 506 To: iana@iana.org 507 Subject: Registration of new Sieve extension 509 Capability name: spamtestplus 510 Capability keyword: spamtestplus 511 Capability arguments: :percent 512 Standards Track/IESG-approved experimental RFC number: this RFC 513 Person and email address to contact for further information: 515 Cyrus Daboo 517 519 This information should be added to the list of sieve extensions 520 given on http://www.iana.org/assignments/sieve-extensions. 522 6. References 524 6.1. Normative References 526 [I-D.ietf-sieve-3028bis] 527 Showalter, T. and P. Guenther, "Sieve: An Email Filtering 528 Language", draft-ietf-sieve-3028bis-07 (work in progress), 529 June 2006. 531 [I-D.ietf-sieve-3431bis] 532 Segmuller, W. and B. Leiba, "Sieve Extension: Relational 533 Tests", draft-ietf-sieve-3431bis-04 (work in progress), 534 December 2005. 536 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 537 Requirement Levels", BCP 14, RFC 2119, March 1997. 539 6.2. Informative References 541 [I-D.newman-i18n-comparator] 542 Newman, C., "Internet Application Protocol Collation 543 Registry", draft-newman-i18n-comparator-12 (work in 544 progress), June 2006. 546 [RFC3685] Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest 547 Extensions", RFC 3685, February 2004. 549 Appendix A. Acknowledgments 551 Thanks to Mark E. Mallett, Tony Hansen, Jutta Degener, Ned Freed, 552 Ashish Gawarikar, Alexey Melnikov, Nigel Swinson and Aaron Stone for 553 comments and corrections. 555 Author's Address 557 Cyrus Daboo 559 Email: cyrus@daboo.name 561 Intellectual Property Statement 563 The IETF takes no position regarding the validity or scope of any 564 Intellectual Property Rights or other rights that might be claimed to 565 pertain to the implementation or use of the technology described in 566 this document or the extent to which any license under such rights 567 might or might not be available; nor does it represent that it has 568 made any independent effort to identify any such rights. 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