SIEVE Email Filtering: Spamtest and VirusTest Extensions
draft-daboo-sieve-spamtest-04

Network Working Group                                          C. Daboo
Internet Draft: SIEVE Spamtest and Virustest Extensions
Document: draft-daboo-sieve-spamtest-04.txt                October 2003

                SIEVE Spamtest and Virustest Extensions
    
Status of this Memo
    
    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.
    
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Copyright Notice
    
     Copyright (C) The Internet Society 2003. All Rights Reserved.

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Abstract
    
    The SIEVE mail filtering language [SIEVE] "spamtest" 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 values returned by the tests.
    
Change History (to be removed prior to publication as an RFC)
    
    Changes from -03 to -04:
    1   Added IPR boiler plate.
    2   Re-ordered sections at start to conform to RFC style.
    3   Eliminated the use of "NIL" for untested value.
    4   Expanded virus test ranges to indicate additional state.
    5   Added security consideration requiring admins to keep virus
        tools up to date
        
    Changes from -02 to -03:
    1   Changed test values to be arbitrary strings with numeric value
        as first token.
    2   Changed 'virii' to 'viruses'.
        
    Changes from -01 to -02:
    1   Fixed syntax in examples.
    2   Updated references section to normative/informative.
        
    Changes from -00 to -01:
    1   Changed so that tests use standard SIEVE syntax.
    2   Added requirement for relation extension for numeric
        comparisons.
    3   Changed spamtest numeric range to 0->10.
        
        
Table of Contents
        
     1  Introduction and Overview . . . . . . . . . . . . . . . . . .  3
     2  Conventions Used in This Document  . . . . . . . . . . . . . . 3
     3  SIEVE Extensions  . . . . . . . . . . . . . . . . . . . . . .  4
       3.1  General Considerations . . . . . . . . . . . . . . . . . . 4
       3.2  Test spamtest . . . . . . . . . . . . . . . . . . . . . .  4
       3.3  Test virustest . . . . . . . . . . . . . . . . . . . . . . 5
     4  Security Considerations . . . . . . . . . . . . . . . . . . .  6
     5  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 6
       5.1  spamtest registration . . . . . . . . . . . . . . . . . .  7
       5.2  virustest registration . . . . . . . . . . . . . . . . . . 7
     6  References  . . . . . . . . . . . . . . . . . . . . . . . . .  7
       6.1  Normative References . . . . . . . . . . . . . . . . . . . 7
       6.2  Non-Normative References  . . . . . . . . . . . . . . . .  8
     7  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 8
     8  Author's Address  . . . . . . . . . . . . . . . . . . . . . .  8

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     9  Intellectual Property Rights Statement . . . . . . . . . . . . 8
    10  Full Copyright Statement  . . . . . . . . . . . . . . . . . .  8
        
        
1 Introduction and Overview
    
    SIEVE scripts are frequently being used to do spam and virus
    filtering based on either 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 synchronisation 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.  These tests return a string
    containing a range of numeric values that indicate the severity of
    spam or viruses in a message, or a string that indicates the message
    has not passed through any spam or virus checking tools.  The spam
    and virus checks themselves are handled by the underlying SIEVE
    implementation in whatever manner is appropriate, and the
    implementation maps the results of these checks into the numeric
    ranges defined by the new tests.  Thus a SIEVE implementation can
    have a spam test that implicitly checks for third-party spam tool
    headers and determines how those map into the spamtest numeric
    range.
    
    In order to do numeric comparisons against the returned strings,
    server implementations MUST also support the SIEVE relational
    [RELATIONAL] 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 [SIEVE] section 1.1, including
    use of [KEYWORDS].
    
    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.
    

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    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.
    
    
3 SIEVE Extensions
    
3.1 General Considerations
    
    The "spamtest" and "virustest" tests described below both return a
    string that starts with a numeric value, followed by an optional
    space (%x20) character and optional arbitrary text.  The numeric
    value can be compared to specific values using the SIEVE relational
    [RELATIONAL] extension in conjunction with the "i;ascii-numeric"
    comparator [ACAP], which will test for the presence of a numeric
    value at the start of the string, ignoring any additional text in
    the string.  The additional text can be used to carry implementation
    specific details about the tests performed and descriptive comments
    about the result.  Tests can be done using standard string
    comparators against this text if it helps to refine behaviour,
    however this will break portability of the script as the text will
    likely be specific to a particular implementation.
    
3.2 Test spamtest
    
        Syntax: spamtest [COMPARATOR] [MATCH-TYPE] <value: string>
    
    SIEVE implementations that implement the "spamtest" test have an
    identifier of "spamtest" for use with the capability mechanism.
    
    The "spamtest" test evaluates to true if the spamtest result matches
    the value.  The type of match is specified by the optional match
    argument, which defaults to ":is" if not specified.
    
    The spamtest result is a string starting with a numeric value in the
    range "0" (zero) through "10", with meanings summarised below:
    
        spamtest    interpretation
        value

                 0          message was not tested for spam
                 1          message was tested and is clear of spam
                 2 - 9      message was tested and has a varying likelihood of
                            containing spam in increasing order
                 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.
    

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    Examples:
    
        require ["spamtest", "fileinto",
                 "relational", "comparator-i;ascii-numeric"];

        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 spamtest value greater than or equal to "3" is filed
    into a mailbox called "INBOX.spam-trap" in the user's mailstore.
    
3.3 Test virustest
    
        Syntax: 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 virustest result
    matches the value.  The type of match is specified by the optional
    match argument, which defaults to ":is" if not specified.
    
    The virustest result is a string starting with a numeric value in
    the range "0" (zero) through "5", with meanings summarised below:
    
        virustest   interpretation
        value

                 0          message was not tested for viruses
                 1          message was tested and contains no known viruses
                 2          message was tested and contained a known virus which
                            was replaced with harmless content
                 3          message was tested and contained a known virus which
                            was "cured" such that it is now harmless
                 4          message was tested and possibly contains a known virus
                 5          message was tested and definately 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 categorised by any virus checking tools, then the
    virustest result is "0".
    

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    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 virustest value equal to "4" is filed into a mailbox
    called "INBOX.quarantine" in the user's mailstore.  Any message with
    a virustest 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 can only occur for messages that have gone through a
    legitimate spam or virus check process.  If such checks rely on the
    addition of special headers to messages, it is the responsibility of
    the implementation to ensure that such headers cannot be spoofed by
    the sender, to 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 [SIEVE]
    protocol, and these issues are discussed in [SIEVE].
    
    
5 IANA Considerations
    
    The following templates specify the IANA registration of the Sieve
    extensions specified in this document:

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5.1 spamtest registration
    
    To: iana@iana.org
    Subject: Registration of new Sieve extension

    Capability name: spamtest
    Capability keyword: spamtest
    Capability arguments: N/A
    Standards Track/IESG-approved experimental RFC number: this RFC
    Person and email address to contact for further information:

      Cyrus Daboo
      Cyrusoft International, Inc.
      5001 Baum Blvd., Suite 780, 
      Pittsburgh, PA 15213
      U.S.A.

      <mailto:daboo@cyrusoft.com>

    This information should be added to the list of sieve extensions
    given on http://www.iana.org/assignments/sieve-extensions.
    
5.2 virustest registration
    
    To: iana@iana.org
    Subject: Registration of new Sieve extension

    Capability name: virustest
    Capability keyword: virustest
    Capability arguments: N/A
    Standards Track/IESG-approved experimental RFC number: this RFC
    Person and email address to contact for further information:

      Cyrus Daboo
      Cyrusoft International, Inc.
      5001 Baum Blvd., Suite 780, 
      Pittsburgh, PA 15213
      U.S.A.

      <mailto:daboo@cyrusoft.com>

    This information should be added to the list of sieve extensions
    given on http://www.iana.org/assignments/sieve-extensions.
    
    
6 References
    
6.1 Normative References
    
    [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
    Requirement Levels", RFC 2119, March 1997.
    

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    [RELATIONAL] Segmuller, W. "Sieve Extension:  Relational Tests", RFC
    3431, December 2002.
    
    [SIEVE] Showalter, "Sieve:  A Mail Filtering Language", RFC 3028,
    January 2001.
    
6.2 Non-Normative References
    
    [ACAP] Newman, C. and J. G. Myers, "ACAP -- Application
    Configuration Access Protocol", RFC 2244, November 1997.
    
    
7 Acknowledgments
    
    Thanks to Tony Hansen, Jutta Degener, Ned Freed, Ashish Gawarikar
    and Nigel Swinson for comments and corrections.
    
    
8 Author's Address
    
    Cyrus Daboo
    Cyrusoft International, Inc.
    5001 Baum Blvd., Suite 780,
    Pittsburgh, PA 15213
    U.S.A.

    <mailto:daboo@cyrusoft.com>
    
    
9 Intellectual Property Rights Statement
    
    The IETF takes no position regarding the validity or scope of any
    intellectual property or other rights that might be claimed to
    pertain to the implementation or use of the technology described in
    this document or the extent to which any license under such rights
    might or might not be available; neither does it represent that it
    has made any effort to identify any such rights.  Information on the
    IETF's procedures with respect to rights in standards-track and
    standards-related documentation can be found in BCP-11.  Copies of
    claims of rights made available for publication and any assurances
    of licenses to be made available, or the result of an attempt made
    to obtain a general license or permission for the use of such
    proprietary rights by implementors or users of this specification
    can be obtained from the IETF Secretariat.
    
    
10 Full Copyright Statement
    
    Copyright (C) The Internet Society 2003.  All Rights Reserved.
    
    This document and translations of it may be copied and furnished to
    others, and derivative works that comment on or otherwise explain it

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    or assist in its implementation may be prepared, copied, published
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