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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 June 25, 2006 4 Internet-Draft 5 Expires: December 27, 2006 7 SIEVE Email Filtering: Spamtest and Virustest Extensions 8 draft-ietf-sieve-spamtestbis-04 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 December 27, 2006. 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 -03: 50 1. Clarified that there are two possible ways to test for not-spam. 51 2. Clarified that 'not tested for xxx' also implies 'SIEVE could not 52 determine whether a test was done or not'. 54 Changes from -02: 55 1. Changed formatting of tables. 56 2. Fixed missing 2119 definitions. 57 3. Moved reference to previous extension to informative. 58 4. Minor text improvements. 59 5. Fixed some single/double quote issues. 60 6. Reworded abstract, introduction and overview to use better SIEVE 61 terminology when describing tests, commands and results. 62 7. Remove "untested" string result from ":percent" test. 63 8. Allow ":count" match type to be used for tested/untested checks. 65 Changes from -01: 66 1. Changed ACAP reference to i18n-comparators draft. 67 2. Changed MUST in security section for virus checker updates to 68 plain must. 69 3. Return string "untested" when :percent is used and no test has 70 been done. 71 4. Remove MUST NOT for having both spamtestplus and spamtest 72 capabilities present, and instead make it a SHOULD NOT. 73 5. Add text to state that implementations MUST return an error if 74 spamtestplus is not present when :percent is used. 75 6. Tweak first para of security considerations to better reflect 76 reality of testing. 77 7. Syntax -> Usage. 78 8. Updated references to 3028bis and 3431bis. 80 Changes from -00: 81 1. Added description of how to check for untested when using 82 :percent. 83 2. Changed requires item to "spamtestplus". 84 3. Changed text describing which requires item needs to be present. 86 Changes from RFC3685: 87 1. Added ":percent" argument to spamtest. 89 Table of Contents 91 1. Introduction and Overview . . . . . . . . . . . . . . . . . . 4 92 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 93 3. SIEVE Extensions . . . . . . . . . . . . . . . . . . . . . . . 5 94 3.1. General Considerations . . . . . . . . . . . . . . . . . . 5 95 3.2. Test spamtest . . . . . . . . . . . . . . . . . . . . . . 5 96 3.2.1. spamtest without :percent argument . . . . . . . . . . 6 97 3.2.2. spamtest with :percent argument . . . . . . . . . . . 7 98 3.3. Test virustest . . . . . . . . . . . . . . . . . . . . . . 9 99 4. Security Considerations . . . . . . . . . . . . . . . . . . . 11 100 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 101 5.1. spamtest registration . . . . . . . . . . . . . . . . . . 12 102 5.2. virustest registration . . . . . . . . . . . . . . . . . . 12 103 5.3. spamtestplus registration . . . . . . . . . . . . . . . . 13 104 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 105 6.1. Normative References . . . . . . . . . . . . . . . . . . . 13 106 6.2. Informative References . . . . . . . . . . . . . . . . . . 13 107 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14 108 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 109 Intellectual Property and Copyright Statements . . . . . . . . . . 16 111 1. Introduction and Overview 113 SIEVE scripts are frequently being used to do spam and virus 114 filtering based on either implicit script tests (e.g. tests for 115 "black-listed" senders directly encoded in the SIEVE script), or via 116 testing messages modified by some external spam or virus checker that 117 handled the message prior to SIEVE. The use of third-party spam and 118 virus checker tools poses a problem since each tool has its own way 119 of indicating the result of its checks. These usually take the form 120 of a header added to the message, the content of which indicates the 121 status using some syntax defined by the particular tool. Each user 122 has to then create their own SIEVE scripts to match the contents of 123 these headers to do filtering. This requires the script to stay in 124 synchronization with the third party tool as it gets updated or 125 perhaps replaced with another. Thus scripts become tied to specific 126 environments, and lose portability. 128 The purpose of this document is to introduce two SIEVE tests that can 129 be used to implement "generic" tests for spam and viruses in messages 130 processed via SIEVE scripts. The spam and virus checks themselves 131 are handled by the underlying SIEVE implementation in whatever manner 132 is appropriate, so that the SIEVE spam and virus test commands can be 133 used in a portable way. 135 In order to do numeric comparisons against the returned strings, 136 server implementations MUST also support the SIEVE relational 137 [I-D.ietf-sieve-3431bis] extension, in addition to the extensions 138 described here. All examples below assume the relational extension 139 is present. 141 2. Conventions Used in This Document 143 Conventions for notations are as in [I-D.ietf-sieve-3028bis] section 144 1.1. 146 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 147 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 148 document are to be interpreted as described in [RFC2119]. 150 The term "spam" is used in this document to refer to unsolicited or 151 unwanted email messages. This document does not attempt to define 152 what exactly constitutes spam, or how it should be identified, or 153 what actions should be taken when detected. 155 The term "virus" is used in this document to refer to any type of 156 message whose content can cause malicious damage. This document does 157 not attempt to define what exactly constitutes a virus, or how it 158 should be identified, or what actions should be taken when detected. 160 3. SIEVE Extensions 162 3.1. General Considerations 164 The "spamtest" and "virustest" tests described below evaluate the 165 results of implementation-specific spam and virus checks in a 166 portable way. The implementation may, for example, check for third- 167 party spam tool headers and determine how those map into the way the 168 test commands are used. To do this, the underlying SIEVE 169 implementation provides a normalized result string as one of the 170 inputs to each test command. The normalized result string is 171 considered to be the value on the left hand side of the test, and the 172 comparison values given in the test command are considered to be on 173 the right hand side. 175 The normalized result starts with a digit string, with its numeric 176 value within the range of values used by the specific test, 177 indicating the severity of spam or viruses in a message or whether 178 any tests were done at all. This may optionally be followed by a 179 space (%x20) character and arbitrary text, or in one specific case a 180 single keyword is returned. The numeric value can be compared to 181 specific values using the SIEVE relational [I-D.ietf-sieve-3431bis] 182 extension in conjunction with the "i;ascii-numeric" comparator 183 [I-D.newman-i18n-comparator], which will test for the presence of a 184 numeric value at the start of the string, ignoring any additional 185 text in the string. The optional text can be used to carry 186 implementation specific details about the tests and descriptive 187 comments about the result. Tests can be done using standard string 188 comparators against this text if it helps to refine behavior, however 189 this will break portability of the script as the text will likely be 190 specific to a particular implementation. 192 In addition, the SIEVE relational [I-D.ietf-sieve-3431bis] ":count" 193 match type can be used to determine if the underlying implementation 194 actually did a test. If the underlying spam or virus test was done, 195 the ":count" of the normalized result will return the numeric value 196 "1", whilst if the test was not done, or the SIEVE implementation 197 could not determine if a test was done or not done, the ":count" 198 value will be "0" (zero). 200 3.2. Test spamtest 202 Usage: spamtest [":percent"] [COMPARATOR] [MATCH-TYPE] 203 205 SIEVE implementations that implement the "spamtest" test use an 206 identifier of either "spamtest" or "spamtestplus" for use with the 207 capability mechanism. 209 If the ":percent" argument is not used with any spamtest test, then 210 one of either the "spamtest" or "spamtestplus" capability identifiers 211 MUST be present. 213 If the ":percent" argument is used with any spamtest test, then the 214 "spamtestplus" capability identifier MUST be present. SIEVE 215 implementations MUST return an error if the ":percent" argument is 216 used and "spamtestplus" is not specified. 218 In the interests of brevity and clarity, scripts SHOULD NOT specify 219 both "spamtestplus" and "spamtest" capability identifiers together. 221 The "spamtest" test evaluates to true if the normalized spamtest 222 result matches the value. The type of match is specified by the 223 optional match argument, which defaults to ":is" if not specified. 225 3.2.1. spamtest without :percent argument 227 When the ":percent" argument is not present in the "spamtest" test, 228 the normalized result string provided for the left hand side of the 229 test starts with a numeric value in the range "0" (zero) through 230 "10", with meanings summarized below: 232 +----------+--------------------------------------------------------+ 233 | spamtest | interpretation | 234 | value | | 235 +----------+--------------------------------------------------------+ 236 | 0 | message was not tested for spam, or SIEVE could not | 237 | | determine whether any test was done | 238 | | | 239 | 1 | message was tested and is clear of spam | 240 | | | 241 | 2 - 9 | message was tested and has a varying likelihood of | 242 | | containing spam in increasing order | 243 | | | 244 | 10 | message was tested and definitely contains spam | 245 +----------+--------------------------------------------------------+ 247 The underlying SIEVE implementation will map whatever spam check is 248 done into this numeric range, as appropriate. 250 Examples: 252 require ["spamtest", "fileinto", 253 "relational", "comparator-i;ascii-numeric"]; 255 if spamtest :value "eq" :comparator "i;ascii-numeric" "0" 256 { 257 fileinto "INBOX.unclassified"; 258 } 259 elsif spamtest :value "ge" :comparator "i;ascii-numeric" "3" 260 { 261 fileinto "INBOX.spam-trap"; 262 } 264 In this example, any message that has not passed through a spam check 265 tool will be filed into the mailbox "INBOX.unclassified". Any 266 message with a normalized result value greater than or equal to "3" 267 is filed into a mailbox called "INBOX.spam-trap" in the user's 268 mailstore. 270 3.2.2. spamtest with :percent argument 272 When the ":percent" argument is present in the "spamtest" test, the 273 normalized result string provided for the left hand side of the test 274 starts with a numeric value in the range "0" (zero) through "100", 275 with meanings summarized below: 277 +----------+--------------------------------------------------------+ 278 | spamtest | interpretation | 279 | value | | 280 +----------+--------------------------------------------------------+ 281 | 0 | message was tested and is clear of spam, or was not | 282 | | tested for spam, or SIEVE could not determine whether | 283 | | any test was done | 284 | | | 285 | 1 - 99 | message was tested and has a varying likelihood of | 286 | | containing spam in increasing order based on the | 287 | | spamtest value | 288 | | | 289 | 100 | message was tested and definitely contains spam | 290 +----------+--------------------------------------------------------+ 292 The underlying SIEVE implementation will map whatever spam check is 293 done into the numeric range, as appropriate. 295 To determine whether the message was tested for spam or not, two 296 options can be used: 298 a. a test with or without the ":percent" argument and ":count" match 299 type, testing for the value "0" as described in Section 3.1. 301 b. a test without the ":percent" argument using the ":value" match 302 type, testing for the normalized result value "0" as described in 303 Section 3.2.1. 305 Examples: 307 require ["spamtestplus", "fileinto", 308 "relational", "comparator-i;ascii-numeric"]; 310 if spamtest :value "eq" 311 :comparator "i;ascii-numeric" "0" 312 { 313 fileinto "INBOX.unclassified"; 314 } 315 elsif spamtest :percent :value "eq" 316 :comparator "i;ascii-numeric" "0" 317 { 318 fileinto "INBOX.not-spam"; 319 } 320 elsif spamtest :percent :value "lt" 321 :comparator "i;ascii-numeric" "37" 322 { 323 fileinto "INBOX.spam-trap"; 324 } 325 else 326 { 327 discard; 328 } 330 In this example, any message that has not passed through a spam check 331 tool will be filed into the mailbox "INBOX.unclassified". Any 332 message that is classified as definitely not containing spam 333 (normalized result value "0") will be filed into the mailbox 334 "INBOX.not-spam". Any message with a normalized result value less 335 than "37" is filed into a mailbox called "INBOX.spam-trap" in the 336 user's mailstore. Any other normalized result value will result in 337 the message being discarded. 339 Alternatively, the SIEVE relational [I-D.ietf-sieve-3431bis] ":count" 340 match type can be used: 342 Examples: 344 if spamtest :percent :count "eq" 345 :comparator "i;ascii-numeric" "0" 346 { 347 fileinto "INBOX.unclassified"; 348 } 349 elsif spamtest :percent :value "eq" 350 :comparator "i;ascii-numeric" "0" 351 { 352 fileinto "INBOX.not-spam"; 353 } 354 elsif spamtest :percent :value "lt" 355 :comparator "i;ascii-numeric" "37" 356 { 357 fileinto "INBOX.spam-trap"; 358 } 359 else 360 { 361 discard; 362 } 364 This example will result in exactly the same behavior as the previous 365 one. 367 3.3. Test virustest 369 Usage: virustest [COMPARATOR] [MATCH-TYPE] 370 372 SIEVE implementations that implement the "virustest" test have an 373 identifier of "virustest" for use with the capability mechanism. 375 The "virustest" test evaluates to true if the normalized result 376 string matches the value. The type of match is specified by the 377 optional match argument, which defaults to ":is" if not specified. 379 The normalized result string provided for the left side of the test 380 starts with a numeric value in the range "0" (zero) through "5", with 381 meanings summarized below: 383 +-----------+-------------------------------------------------------+ 384 | virustest | interpretation | 385 | value | | 386 +-----------+-------------------------------------------------------+ 387 | 0 | message was not tested for viruses, or SIEVE could | 388 | | not determine whether any test was done | 389 | | | 390 | 1 | message was tested and contains no known viruses | 391 | | | 392 | 2 | message was tested and contained a known virus which | 393 | | was replaced with harmless content | 394 | | | 395 | 3 | message was tested and contained a known virus which | 396 | | was "cured" such that it is now harmless | 397 | | | 398 | 4 | message was tested and possibly contains a known | 399 | | virus | 400 | | | 401 | 5 | message was tested and definitely contains a known | 402 | | virus | 403 +-----------+-------------------------------------------------------+ 405 The underlying SIEVE implementation will map whatever virus checks 406 are done into this numeric range, as appropriate. If the message has 407 not been categorized by any virus checking tools, then the virustest 408 result is "0". 410 Example: 412 require ["virustest", "fileinto", 413 "relational", "comparator-i;ascii-numeric"]; 415 if virustest :value "eq" :comparator "i;ascii-numeric" "0" 416 { 417 fileinto "INBOX.unclassified"; 418 } 419 if virustest :value "eq" :comparator "i;ascii-numeric" "4" 420 { 421 fileinto "INBOX.quarantine"; 422 } 423 elsif virustest :value "eq" :comparator "i;ascii-numeric" "5" 424 { 425 discard; 426 } 428 In this example, any message that has not passed through a virus 429 check tool will be filed into the mailbox "INBOX.unclassified". Any 430 message with a normalized result value equal to "4" is filed into a 431 mailbox called "INBOX.quarantine" in the user's mailstore. Any 432 message with a normalized result value equal to "5" is discarded 433 (removed) and not delivered to the user's mailstore. 435 4. Security Considerations 437 SIEVE implementations SHOULD ensure that "spamtest" and "virustest" 438 tests only report spam and virus test results for messages that 439 actually have gone through a legitimate spam or virus check process. 440 In particular, if such checks rely on the addition and subsequent 441 checking of private header fields, it is the responsibility of the 442 implementation to ensure that such headers cannot be spoofed by the 443 sender or intermediary and thereby prevent the implementation from 444 being tricked into returning the wrong result for the test. 446 Server administrators must ensure that the virus checking tools are 447 kept up to date, to provide reasonable protection for users using the 448 "virustest" test. Users should be made aware of the fact that the 449 "virustest" test does not provide a 100% reliable way to remove all 450 viruses, and they should continue to exercise caution when dealing 451 with messages of unknown content and origin. 453 Beyond that, the "spamtest" and "virustest" extensions do not raise 454 any security considerations that are not present in the base 455 [I-D.ietf-sieve-3028bis] protocol, and these issues are discussed in 456 [I-D.ietf-sieve-3028bis]. 458 5. IANA Considerations 460 The following templates specify the IANA registration of the Sieve 461 extensions specified in this document. The registrations for 462 "spamtest" and "virustest" replace those from in [RFC3685]: 464 5.1. spamtest registration 466 To: iana@iana.org 467 Subject: Registration of new Sieve extension 469 Capability name: spamtest 470 Capability keyword: spamtest 471 Capability arguments: N/A 472 Standards Track/IESG-approved experimental RFC number: this RFC 473 Person and email address to contact for further information: 475 Cyrus Daboo 477 479 This information should be added to the list of sieve extensions 480 given on http://www.iana.org/assignments/sieve-extensions. 482 5.2. virustest registration 484 To: iana@iana.org 485 Subject: Registration of new Sieve extension 487 Capability name: virustest 488 Capability keyword: virustest 489 Capability arguments: N/A 490 Standards Track/IESG-approved experimental RFC number: this RFC 491 Person and email address to contact for further information: 493 Cyrus Daboo 495 497 This information should be added to the list of sieve extensions 498 given on http://www.iana.org/assignments/sieve-extensions. 500 5.3. spamtestplus registration 502 To: iana@iana.org 503 Subject: Registration of new Sieve extension 505 Capability name: spamtestplus 506 Capability keyword: spamtestplus 507 Capability arguments: :percent 508 Standards Track/IESG-approved experimental RFC number: this RFC 509 Person and email address to contact for further information: 511 Cyrus Daboo 513 515 This information should be added to the list of sieve extensions 516 given on http://www.iana.org/assignments/sieve-extensions. 518 6. References 520 6.1. Normative References 522 [I-D.ietf-sieve-3028bis] 523 Showalter, T. and P. Guenther, "Sieve: An Email Filtering 524 Language", draft-ietf-sieve-3028bis-06 (work in progress), 525 March 2006. 527 [I-D.ietf-sieve-3431bis] 528 Segmuller, W. and B. Leiba, "Sieve Extension: Relational 529 Tests", draft-ietf-sieve-3431bis-04 (work in progress), 530 December 2005. 532 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 533 Requirement Levels", BCP 14, RFC 2119, March 1997. 535 6.2. Informative References 537 [I-D.newman-i18n-comparator] 538 Newman, C., "Internet Application Protocol Collation 539 Registry", draft-newman-i18n-comparator-12 (work in 540 progress), June 2006. 542 [RFC3685] Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest 543 Extensions", RFC 3685, February 2004. 545 Appendix A. Acknowledgments 547 Thanks to Mark E. Mallett, Tony Hansen, Jutta Degener, Ned Freed, 548 Ashish Gawarikar, Alexey Melnikov and Nigel Swinson for comments and 549 corrections. 551 Author's Address 553 Cyrus Daboo 555 Email: cyrus@daboo.name 557 Intellectual Property Statement 559 The IETF takes no position regarding the validity or scope of any 560 Intellectual Property Rights or other rights that might be claimed to 561 pertain to the implementation or use of the technology described in 562 this document or the extent to which any license under such rights 563 might or might not be available; nor does it represent that it has 564 made any independent effort to identify any such rights. 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