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rfc:rfc5235

Network Working Group C. Daboo Request for Comments: 5235 January 2008 Obsoletes: 3685 Category: Standards Track

      Sieve Email Filtering: Spamtest and Virustest Extensions

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 The Sieve email filtering language "spamtest", "spamtestplus", and
 "virustest" extensions permit users to use simple, portable commands
 for spam and virus tests on email messages.  Each extension provides
 a new test using matches against numeric "scores".  It is the
 responsibility of the underlying Sieve implementation to do the
 actual checks that result in proper input to the tests.

Table of Contents

 1. Introduction and Overview .......................................2
 2. Conventions Used in This Document ...............................2
 3. Sieve Extensions ................................................3
    3.1. General Considerations .....................................3
    3.2. Test spamtest ..............................................3
         3.2.1. spamtest without :percent Argument ..................4
         3.2.2. spamtest with :percent Argument .....................5
    3.3. Test virustest .............................................7
 4. Security Considerations .........................................9
 5. IANA Considerations .............................................9
    5.1. spamtest Registration ......................................9
    5.2. virustest Registration ....................................10
    5.3. spamtestplus Registration .................................10
 6. References .....................................................10
    6.1. Normative References ......................................10
    6.2. Informative References ....................................11
 Appendix A. Acknowledgments .......................................12
 Appendix B. Important Changes since RFC 3685 ......................12

Daboo Standards Track [Page 1] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

1. Introduction and Overview

 Sieve scripts are frequently being used to do spam and virus
 filtering either based on implicit script tests (e.g., tests for
 "black-listed" senders directly encoded in the Sieve script), or via
 testing messages modified by some external spam or virus checker that
 handled the message prior to Sieve.  The use of third-party spam and
 virus checker tools poses a problem since each tool has its own way
 of indicating the result of its checks.  These usually take the form
 of a header added to the message, the content of which indicates the
 status using some syntax defined by the particular tool.  Each user
 has to then create their own Sieve scripts to match the contents of
 these headers to do filtering.  This requires the script to stay in
 synchronization with the third-party tool as it gets updated or
 perhaps replaced with another.  Thus, scripts become tied to specific
 environments and lose portability.
 The purpose of this document is to introduce two Sieve tests that can
 be used to implement "generic" tests for spam and viruses in messages
 processed via Sieve scripts.  The spam and virus checks themselves
 are handled by the underlying Sieve implementation in whatever manner
 is appropriate, so that the Sieve spam and virus test commands can be
 used in a portable way.
 In order to do numeric comparisons against the returned strings,
 server implementations MUST also support the Sieve relational
 [RFC5231] extension, in addition to the extensions described here.
 All examples below assume the relational extension is present.

2. Conventions Used in This Document

 Conventions for notations are as in [RFC5228] Section 1.1.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 The term "spam" is used in this document to refer to unsolicited or
 unwanted email messages.  This document does not attempt to define
 what exactly constitutes spam, or how it should be identified, or
 what actions should be taken when detected.
 The term "virus" is used in this document to refer to any type of
 message whose content can cause malicious damage.  This document does
 not attempt to define what exactly constitutes a virus, or how it
 should be identified, or what actions should be taken when detected.

Daboo Standards Track [Page 2] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

3. Sieve Extensions

3.1. General Considerations

 The "spamtest" and "virustest" tests described below evaluate the
 results of implementation-specific spam and virus checks in a
 portable way.  The implementation may, for example, check for third-
 party spam tool headers and determine how those map into the way the
 test commands are used.  To do this, the underlying Sieve
 implementation provides a normalized result string as one of the
 inputs to each test command.  The normalized result string is
 considered to be the value on the left-hand side of the test, and the
 comparison values given in the test command are considered to be on
 the right-hand side.
 The normalized result starts with a digit string, with its numeric
 value within the range of values used by the specific test,
 indicating the severity of spam or viruses in a message or whether
 any tests were done at all.  This may optionally be followed by a
 space (%x20) character and arbitrary text, or in one specific case a
 single keyword is returned.  The numeric value can be compared to
 specific values using the Sieve relational [RFC5231] extension in
 conjunction with the "i;ascii-numeric" comparator [RFC4790], which
 will test for the presence of a numeric value at the start of the
 string, ignoring any additional text in the string.  The optional
 text can be used to carry implementation-specific details about the
 tests and descriptive comments about the result.  Tests can be done
 using standard string comparators against this text if it helps to
 refine behavior; however, this will break portability of the script
 as the text will likely be specific to a particular implementation.
 In addition, the Sieve relational [RFC5231] ":count" match type can
 be used to determine if the underlying implementation actually did a
 test.  If the underlying spam or virus test was done, the ":count" of
 the normalized result will return the numeric value "1", whilst if
 the test was not done, or the Sieve implementation could not
 determine if a test was done or not done, the ":count" value will be
 "0" (zero).

3.2. Test spamtest

         Usage:    spamtest [":percent"] [COMPARATOR] [MATCH-TYPE]
                   <value: string>
 Sieve implementations that implement the "spamtest" test use an
 identifier of either "spamtest" or "spamtestplus" for use with the
 capability mechanism.

Daboo Standards Track [Page 3] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

 If the ":percent" argument is not used with any spamtest test, then
 one or both of "spamtest" or "spamtestplus" capability identifiers
 MUST be present.
 If the ":percent" argument is used with any spamtest test, then the
 "spamtestplus" capability identifier MUST be present.  Sieve
 implementations MUST return an error if the ":percent" argument is
 used and "spamtestplus" is not specified.
 In the interests of brevity and clarity, scripts SHOULD NOT specify
 both "spamtestplus" and "spamtest" capability identifiers together.
 The "spamtest" test evaluates to true if the normalized spamtest
 result matches the value.  The type of match is specified by the
 optional match argument, which defaults to ":is" if not specified.

3.2.1. spamtest without :percent Argument

 When the ":percent" argument is not present in the "spamtest" test,
 the normalized result string provided for the left-hand side of the
 test starts with a numeric value in the range "0" (zero) through
 "10", with meanings summarized below:
 +----------+--------------------------------------------------------+
 | spamtest | interpretation                                         |
 | value    |                                                        |
 +----------+--------------------------------------------------------+
 | 0        | message was not tested for spam, or Sieve could not    |
 |          | determine whether any test was done                    |
 |          |                                                        |
 | 1        | message was tested and is clear of spam                |
 |          |                                                        |
 | 2 - 9    | message was tested and may contain spam; a higher      |
 |          | number indicates a greater likelihood of spam          |
 |          |                                                        |
 | 10       | message was tested and definitely contains spam        |
 +----------+--------------------------------------------------------+
 The underlying Sieve implementation will map whatever spam check is
 done into this numeric range, as appropriate.
 Examples:
         require ["spamtest", "fileinto", "relational", "comparator-
                  i;ascii-numeric"];

Daboo Standards Track [Page 4] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

         if spamtest :value "eq" :comparator "i;ascii-numeric" "0"
         {
             fileinto "INBOX.unclassified";
         }
         elsif spamtest :value "ge" :comparator "i;ascii-numeric" "3"
         {
             fileinto "INBOX.spam-trap";
         }
 In this example, any message that has not passed through a spam check
 tool will be filed into the mailbox "INBOX.unclassified".  Any
 message with a normalized result value greater than or equal to "3"
 is filed into a mailbox called "INBOX.spam-trap" in the user's
 mailstore.

3.2.2. spamtest with :percent Argument

 When the ":percent" argument is present in the "spamtest" test, the
 normalized result string provided for the left-hand side of the test
 starts with a numeric value in the range "0" (zero) through "100",
 with meanings summarized below:
 +----------+-------------------------------------------------------+
 | spamtest | interpretation                                        |
 | value    |                                                       |
 +----------+-------------------------------------------------------+
 | 0        | message was tested and is clear of spam, or was not   |
 |          | tested for spam, or Sieve could not determine whether |
 |          | any test was done                                     |
 |          |                                                       |
 | 1 - 99   | message was tested and may contain spam; a higher     |
 |          | percentage indicates a greater likelihood of spam     |
 |          |                                                       |
 | 100      | message was tested and definitely contains spam       |
 +----------+-------------------------------------------------------+
 The underlying Sieve implementation will map whatever spam check is
 done into the numeric range, as appropriate.
 To determine whether or not the message was tested for spam, two
 options can be used:
 a.  a test with or without the ":percent" argument and ":count" match
     type, testing for the value "0" as described in Section 3.1.
 b.  a test without the ":percent" argument using the ":value" match
     type, testing for the normalized result value "0" as described in
     Section 3.2.1.

Daboo Standards Track [Page 5] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

 Examples:
         require ["spamtestplus", "fileinto", "relational",
                  "comparator-i;ascii-numeric"];
         if spamtest :value "eq"
                     :comparator "i;ascii-numeric" "0"
         {
             fileinto "INBOX.unclassified";
         }
         elsif spamtest :percent :value "eq"
                        :comparator "i;ascii-numeric" "0"
         {
             fileinto "INBOX.not-spam";
         }
         elsif spamtest :percent :value "lt"
                        :comparator "i;ascii-numeric" "37"
         {
             fileinto "INBOX.spam-trap";
         }
         else
         {
             discard;
         }
 In this example, any message that has not passed through a spam check
 tool will be filed into the mailbox "INBOX.unclassified".  Any
 message that is classified as definitely not containing spam
 (normalized result value "0") will be filed into the mailbox
 "INBOX.not-spam".  Any message with a normalized result value less
 than "37" is filed into a mailbox called "INBOX.spam-trap" in the
 user's mailstore.  Any other normalized result value will result in
 the message being discarded.
 Alternatively, the Sieve relational [RFC5231] ":count" match type can
 be used:
 Examples:
         if spamtest :percent :count "eq"
                     :comparator "i;ascii-numeric" "0"
         {
             fileinto "INBOX.unclassified";
         }

Daboo Standards Track [Page 6] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

         elsif spamtest :percent :value "eq"
                        :comparator "i;ascii-numeric" "0"
         {
             fileinto "INBOX.not-spam";
         }
         elsif spamtest :percent :value "lt"
                        :comparator "i;ascii-numeric" "37"
         {
             fileinto "INBOX.spam-trap";
         }
         else
         {
             discard;
         }
 This example will result in exactly the same behavior as the previous
 one.

3.3. Test virustest

         Usage:    virustest [COMPARATOR] [MATCH-TYPE]
                   <value: string>
 Sieve implementations that implement the "virustest" test have an
 identifier of "virustest" for use with the capability mechanism.
 The "virustest" test evaluates to true if the normalized result
 string matches the value.  The type of match is specified by the
 optional match argument, which defaults to ":is" if not specified.
 The normalized result string provided for the left side of the test
 starts with a numeric value in the range "0" (zero) through "5", with
 meanings summarized below:

Daboo Standards Track [Page 7] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

 +-----------+-------------------------------------------------------+
 | virustest | interpretation                                        |
 | value     |                                                       |
 +-----------+-------------------------------------------------------+
 | 0         | message was not tested for viruses, or Sieve could    |
 |           | not determine whether any test was done               |
 |           |                                                       |
 | 1         | message was tested and contains no known viruses      |
 |           |                                                       |
 | 2         | message was tested and contained a known virus that   |
 |           | was replaced with harmless content                    |
 |           |                                                       |
 | 3         | message was tested and contained a known virus that   |
 |           | was "cured" such that it is now harmless              |
 |           |                                                       |
 | 4         | message was tested and possibly contains a known      |
 |           | virus                                                 |
 |           |                                                       |
 | 5         | message was tested and definitely contains a known    |
 |           | virus                                                 |
 +-----------+-------------------------------------------------------+
 The underlying Sieve implementation will map whatever virus checks
 are done into this numeric range, as appropriate.  If the message has
 not been categorized by any virus checking tools, then the virustest
 result is "0".
 Example:
         require ["virustest", "fileinto", "relational", "comparator-
                  i;ascii-numeric"];
         if virustest :value "eq" :comparator "i;ascii-numeric" "0"
         {
             fileinto "INBOX.unclassified";
         }
         if virustest :value "eq" :comparator "i;ascii-numeric" "4"
         {
             fileinto "INBOX.quarantine";
         }
         elsif virustest :value "eq" :comparator "i;ascii-numeric" "5"
         {
             discard;
         }
 In this example, any message that has not passed through a virus
 check tool will be filed into the mailbox "INBOX.unclassified".  Any
 message with a normalized result value equal to "4" is filed into a

Daboo Standards Track [Page 8] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

 mailbox called "INBOX.quarantine" in the user's mailstore.  Any
 message with a normalized result value equal to "5" is discarded
 (removed) and not delivered to the user's mailstore.

4. Security Considerations

 Sieve implementations SHOULD ensure that "spamtest" and "virustest"
 tests only report spam and virus test results for messages that
 actually have gone through a legitimate spam or virus check process.
 In particular, if such checks rely on the addition and subsequent
 checking of private header fields, it is the responsibility of the
 implementation to ensure that such headers cannot be spoofed by the
 sender or intermediary and thereby prevent the implementation from
 being tricked into returning the wrong result for the test.
 Server administrators must ensure that the virus checking tools are
 kept up to date, to provide reasonable protection for users using the
 "virustest" test.  Users should be made aware of the fact that the
 "virustest" test does not provide a 100% reliable way to remove all
 viruses, and they should continue to exercise caution when dealing
 with messages of unknown content and origin.
 Beyond that, the "spamtest" and "virustest" extensions do not raise
 any security considerations that are not present in the base
 [RFC5228] protocol, and these issues are discussed in [RFC5228].

5. IANA Considerations

 The following templates specify the IANA registration of the Sieve
 extensions specified in this document.  The registrations for
 "spamtest" and "virustest" replace those from [RFC3685]:

5.1. spamtest Registration

    To: iana@iana.org
    Subject: Registration of new Sieve extension
    Capability name: spamtest
    Description:     Provides a test to check for varying likelihood of
                     an email message being spam.
    RFC number:      RFC 5235
    Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
 This information has been added to the list of Sieve extensions given
 on http://www.iana.org/assignments/sieve-extensions.

Daboo Standards Track [Page 9] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

5.2. virustest Registration

    To: iana@iana.org
    Subject: Registration of new Sieve extension
    Capability name: virustest
    Description:     Provides a test to check for varying likelihood of
                     there being malicious content in an email message.
    RFC number:      RFC 5235
    Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
 This information has been added to the list of Sieve extensions given
 on http://www.iana.org/assignments/sieve-extensions.

5.3. spamtestplus Registration

    To: iana@iana.org
    Subject: Registration of new Sieve extension
    Capability name: spamtestplus
    Description:     Provides a test to check for varying likelihood of
                     an email message being spam, possibly using a
                     percentage range.
    RFC number:      RFC 5235
    Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
 This information has been added to the list of Sieve extensions given
 on http://www.iana.org/assignments/sieve-extensions.

6. References

6.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
            Application Protocol Collation Registry", RFC 4790, March
            2007.
 [RFC5228]  Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An Email
            Filtering Language", RFC 5228, January 2008.
 [RFC5231]  Segmuller, W. and B. Leiba, "Sieve Email Filtering:
            Relational Extension", RFC 5231, January 2008.

Daboo Standards Track [Page 10] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

6.2. Informative References

 [RFC3685]  Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest
            Extensions", RFC 3685, February 2004.

Daboo Standards Track [Page 11] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

Appendix A. Acknowledgments

 Thanks to Mark E. Mallett, Tony Hansen, Jutta Degener, Ned Freed,
 Ashish Gawarikar, Alexey Melnikov, Nigel Swinson, and Aaron Stone for
 comments and corrections.

Appendix B. Important Changes since RFC 3685

 Listed below are some of the major changes from the previous
 specification [RFC3685], which this one supersedes.
 1. A ":percent" argument has been added to the "spamtest" test adding
    a new 0-100 numerical range for test results.
 2. A "spamtestplus" requires item has been added to indicate the
    presence of this extension in scripts.
 3. The "count" match type from [RFC5231] can now be used to determine
    whether or not a message was tested.
 4. Clarified that "test not done" also means "Sieve system could not
    determine if a test was done".

Author's Address

 Cyrus Daboo
 EMail: cyrus@daboo.name

Daboo Standards Track [Page 12] RFC 5235 Sieve: Spamtest and Virustest Extensions January 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 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, THE IETF TRUST 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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Daboo Standards Track [Page 13]

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