Sieve Email Filtering: Spamtest and Virustest Extensions
draft-ietf-sieve-spamtestbis-05
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 5235.
|
|
|---|---|---|---|
| Author | Cyrus Daboo | ||
| Last updated | 2015-10-14 (Latest revision 2006-07-13) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 5235 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Lisa M. Dusseault | ||
| Send notices to | (None) |
draft-ietf-sieve-spamtestbis-05
SIEVE Email Filtering Working C. Daboo
Group July 13, 2006
Internet-Draft
Expires: January 14, 2007
SIEVE Email Filtering: Spamtest and Virustest Extensions
draft-ietf-sieve-spamtestbis-05
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Copyright Notice
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 proper input to the tests.
Change History (to be removed prior to publication as an RFC)
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Changes from -04:
1. Relaxed capability requirement so that both spamtest and
spamtestplus can be present.
2. Tweaked text describing spam numeric value ranges.
Changes from -03:
1. Clarified that there are two possible ways to test for not-spam.
2. Clarified that 'not tested for xxx' also implies 'SIEVE could not
determine whether a test was done or not'.
Changes from -02:
1. Changed formatting of tables.
2. Fixed missing 2119 definitions.
3. Moved reference to previous extension to informative.
4. Minor text improvements.
5. Fixed some single/double quote issues.
6. Reworded abstract, introduction and overview to use better SIEVE
terminology when describing tests, commands and results.
7. Remove "untested" string result from ":percent" test.
8. Allow ":count" match type to be used for tested/untested checks.
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 . . . . . . . . . . . 7
3.3. Test virustest . . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5.1. spamtest registration . . . . . . . . . . . . . . . . . . 12
5.2. virustest registration . . . . . . . . . . . . . . . . . . 12
5.3. spamtestplus registration . . . . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . . 13
6.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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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
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
[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.
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
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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 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 [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 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 [I-D.ietf-sieve-3431bis] ":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>
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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 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:
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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 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 the message was tested for spam or not, 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.
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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.
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 [I-D.ietf-sieve-3431bis] ":count"
match type can be used:
Examples:
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if spamtest :percent :count "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;
}
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:
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+-----------+-------------------------------------------------------+
| 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 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 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
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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
[I-D.ietf-sieve-3028bis] protocol, and these issues are discussed in
[I-D.ietf-sieve-3028bis].
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 in [RFC3685]:
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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
<mailto:cyrus@daboo.name>
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
<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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5.3. spamtestplus registration
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: spamtestplus
Capability keyword: spamtestplus
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.
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-07 (work in progress),
June 2006.
[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.
6.2. Informative References
[I-D.newman-i18n-comparator]
Newman, C., "Internet Application Protocol Collation
Registry", draft-newman-i18n-comparator-12 (work in
progress), June 2006.
[RFC3685] Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest
Extensions", RFC 3685, February 2004.
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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.
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Author's Address
Cyrus Daboo
Email: cyrus@daboo.name
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