SIEVE Email Filtering Working C. Daboo
Group January 20, 2006
Internet-Draft
Expires: July 24, 2006
SIEVE Email Filtering: Spamtest and Virustest Extensions
draft-ietf-sieve-spamtestbis-02
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Copyright (C) The Internet Society (2006).
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 values returned by the tests.
Change History (to be removed prior to publication as an RFC)
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Changes from -01:
1. Changed ACAP reference to i18n-comparators draft.
2. Changed MUST in security section for virus checker updates to
plain must.
3. Return string "untested" when :percent is used and no test has
been done.
4. Remove MUST NOT for having both spamtestplus and spamtest
capabilities present, and instead make it a SHOULD NOT.
5. Add text to state that implementations MUST return an error if
spamtestplus is not present when :percent is used.
6. Tweak first para of security considerations to better reflect
reality of testing.
7. Syntax -> Usage.
8. Updated references to 3028bis and 3431bis.
Changes from -00:
1. Added description of how to check for untested when using
:percent.
2. Changed requires item to 'spamtestplus'.
3. Changed text describing which requires item needs to be present.
Changes from RFC3685:
1. Added ':percent' argument to spamtest.
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Table of Contents
1. Introduction and Overview . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. SIEVE Extensions . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. General Considerations . . . . . . . . . . . . . . . . . . 5
3.2. Test spamtest . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. spamtest without :percent argument . . . . . . . . . . 6
3.2.2. spamtest with :percent argument . . . . . . . . . . . 6
3.3. Test virustest . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5.1. spamtestplus registration . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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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, or provides a direct
indication of whether the message has been tested for spam or not.
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
[I-D.ietf-sieve-3431bis] 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 [I-D.ietf-sieve-3028bis] section
1.1, including use of [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.
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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 can 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
[I-D.ietf-sieve-3431bis] extension in conjunction with the "i;ascii-
numeric" comparator [I-D.newman-i18n-comparator], 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
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.
If the ":percent" argument is not used with any spamtest test, then
one of either the "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 spamtest result matches
the value. The type of match is specified by the optional match
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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 result of the test 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.
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.
3.2.2. spamtest with :percent argument
When the ":percent" argument is present in the "spamtest" test, the
result of the test is a string starting with a numeric value in the
range "0" (zero) through "100", with meanings summarised below, or
the string "untested" for the case where the message was not tested
for spam (corresponding to the "0" value returned when the ":percent"
argument is not used):
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spamtest interpretation
value
"untested" message was not tested for spam
0 message was tested and is clear of spam
1 - 99 message was tested and has a varying likelihood
of containing spam in increasing order
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 the message was tested for spam or not, the
preferred solution is to use the test without the ":percent"
argument, testing for the value "0" as described in Section 3.2.1.
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 "ge"
:comparator "i;ascii-numeric" "30"
{
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 percentage value greater than or equal to
"30" is filed into a mailbox called "INBOX.spam-trap" in the user's
mailstore.
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 virustest result
matches the value. The type of match is specified by the optional
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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".
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.
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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
[I-D.ietf-sieve-3028bis] protocol, and these issues are discussed in
[I-D.ietf-sieve-3028bis].
5. IANA Considerations
The following template specifies the IANA registration of the Sieve
extensions specified in this document, that are not already
registered in [RFC3685]:
5.1. spamtestplus registration
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: spamtestplus
Capability keyword: spamtest
Capability arguments: :percent
Standards Track/IESG-approved experimental RFC number: this RFC
Person and email address to contact for further information:
Cyrus Daboo
<mailto:cyrus@daboo.name>
This information should be added to the list of sieve extensions
given on http://www.iana.org/assignments/sieve-extensions.
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6. References
6.1. Normative References
[I-D.ietf-sieve-3028bis]
Showalter, T. and P. Guenther, "Sieve: An Email Filtering
Language", draft-ietf-sieve-3028bis-05 (work in progress),
November 2005.
[I-D.ietf-sieve-3431bis]
Segmuller, W. and B. Leiba, "Sieve Extension: Relational
Tests", draft-ietf-sieve-3431bis-04 (work in progress),
December 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3685] Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest
Extensions", RFC 3685, February 2004.
6.2. Informative References
[I-D.newman-i18n-comparator]
Newman, C., "Internet Application Protocol Collation
Registry", draft-newman-i18n-comparator-05 (work in
progress), September 2005.
Appendix A. Acknowledgments
Thanks to Tony Hansen, Jutta Degener, Ned Freed, Ashish Gawarikar,
Alexey Melnikov and Nigel Swinson for comments and corrections.
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Author's Address
Cyrus Daboo
Email: cyrus@daboo.name
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