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
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Copyright Notice
Copyright (C) The Internet Society 2003. All Rights Reserved.
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Internet Draft SIEVE Spamtest and Virustest Extensions October 2003
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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Internet Draft SIEVE Spamtest and Virustest Extensions October 2003
[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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