Network Working Group C. Daboo
Internet Draft: SIEVE Spamtest and Virustest Extensions
Document: draft-daboo-sieve-spamtest-03.txt April 2003
SIEVE Spamtest and Virustest Extensions
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society 2003. All Rights Reserved.
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Table of Contents
1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Conventions Used in This Document . . . . . . . . . . . . . . 2
3 Change History . . . . . . . . . . . . . . . . . . . . . . . 2
4 Introduction and Overview . . . . . . . . . . . . . . . . . . 3
5 SIEVE Extensions . . . . . . . . . . . . . . . . . . . . . . 3
5.1 General Considerations . . . . . . . . . . . . . . . . . . 3
5.2 Test spamtest . . . . . . . . . . . . . . . . . . . . . . 4
5.3 Test virustest . . . . . . . . . . . . . . . . . . . . . . 5
6 Security Considerations . . . . . . . . . . . . . . . . . . . 5
7 IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7.1 spamtest registration . . . . . . . . . . . . . . . . . . 6
7.2 virustest registration . . . . . . . . . . . . . . . . . . 6
8 References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1 Normative References . . . . . . . . . . . . . . . . . . . 7
8.2 Non-Normative References . . . . . . . . . . . . . . . . 7
9 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
10 Author's Address . . . . . . . . . . . . . . . . . . . . . . 7
11 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 7
1 Abstract
The SIEVE [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.
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.
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 Change History
Changes from -02 to -03:
1 Changed test values to be arbitrary strings with numeric value
as first token.
2 Changed 'virii' to 'viruses'.
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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.
4 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.
5 SIEVE Extensions
5.1 General Considerations
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The "spamtest" and "virustest" tests described below both return a
string which has one of two formats:
1 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.
2 a string containing "NIL" only.
5.2 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 "0" meaning the message is
definitely clear of spam, and "10" meaning the message is definitely
spam. The underlying SIEVE implementation will map whatever spam
check is done into this numeric range, as appropriate. If the
message has not been categorised by any spam checking tools, then
the spamtest result is "NIL".
Examples:
require ["spamtest", "fileinto",
"relational", "comparator-i;ascii-numeric"];
if spamtest :value "ge" :comparator "i;ascii-numeric" "3"
{
fileinto "INBOX.spam-trap";
}
elsif spamtest :is "NIL"
{
fileinto "INBOX.unclassified";
}
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In this example, 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. Any message that has not passed
through a spam check tool will be filed into the mailbox
"INBOX.unclassified".
5.3 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 "2", with "0" meaning the message is
definitely clear of viruses, "1" meaning the message may contain
viruses (looks suspicious) and "2" meaning the message definitely
contains a virus. The underlying SIEVE implementation will map
whatever virus check is done into this numeric range, as
appropriate. If the message has not been categorised by any virus
checking tools, then the virustest result is "NIL".
Example:
require ["virustest", "fileinto",
"relational", "comparator-i;ascii-numeric"];
if virustest :value "eq" :comparator "i;ascii-numeric" "1"
{
fileinto "INBOX.quarantine";
}
elsif virustest :value "eq" :comparator "i;ascii-numeric" "2"
{
discard;
}
elsif virustest :is "NIL"
{
fileinto "INBOX.unclassified";
}
In this example, any message with a virustest value equal to "1"
is filed into a mailbox called "INBOX.quarantine" in the user's
mailstore. Any message with a virustest value equal to "2" is
discarded (removed) and not delivered to the user's mailstore.
Any message that has not passed through a virus check tool will
be filed into the mailbox "INBOX.unclassified".
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6 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.
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].
7 IANA Considerations
The following templates specify the IANA registration of the Sieve
extensions specified in this document:
7.1 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.
Suite 780, 5001 Baum Blvd.
Pittsburgh, PA 15213
U.S.A.
Email: daboo@cyrusoft.com
This information should be added to the list of sieve extensions
given on http://www.iana.org/assignments/sieve-extensions.
7.2 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:
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Cyrus Daboo
Cyrusoft International, Inc.
Suite 780, 5001 Baum Blvd.
Pittsburgh, PA 15213
U.S.A.
Email: daboo@cyrusoft.com
This information should be added to the list of sieve extensions
given on http://www.iana.org/assignments/sieve-extensions.
8 References
8.1 Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RELATIONAL] Segmuller, W. "Sieve Extension: Relational Tests", RFC
3431, December 2002.
[SIEVE] Showalter, "Sieve: A Mail Filtering Language", RFC 3028,
January 2001.
8.2 Non-Normative References
[ACAP] Newman, C. and J. G. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November 1997.
9 Acknowledgments
Thanks to Tony Hansen, Jutta Degener and Ned Freed for comments on
the original draft. Thanks to Ashish Gawarikar for catching some
example syntax errors.
10 Author's Address
Cyrus Daboo
Cyrusoft International, Inc.
Suite 780, 5001 Baum Blvd.
Pittsburgh, PA 15213
U.S.A.
Email: daboo@cyrusoft.com
11 Full Copyright Statement
Copyright (C) The Internet Society 2003. All Rights Reserved.
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