Network Working Group C. Daboo
Internet-Draft A. Stone
Expires: January 30, 2010 July 29, 2009
Sieve Email Filtering: Include Extension
draft-ietf-sieve-include-03
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Abstract
The Sieve Email Filtering "include" extension permits users to
include one Sieve script inside another. This can make managing
large scripts or multiple sets of scripts much easier, and allows a
site and its users to build up libraries of scripts. Users are able
to include their own personal scripts or site-wide scripts.
Change History (to be removed prior to publication as an RFC)
Changes from ietf-02 to ietf-03:
a. Setting a variable then calling global on it is an error
(something like 'use strict').
b. Specify that the 'global' keyword is only available when
'variables' has also been required.
c. Uploading a script that includes a nonexistent script is not an
error at upload time.
Changes from ietf-01 to ietf-02:
a. Require that script names must be constant strings, not subject
to variable expansion.
b. Try the phrase immediate script instead of current script.
c. Clarify that "global 'varname'" and "global.varname" refer to the
same variable.
d. Drop the requirement the global keywords come after require and
before anything else.
Changes from ietf-00 to ietf-01:
a. Replaced import/export with global.
b. Added :once modifier to include.
c. Added global namespace to see if it holds water.
Changes from daboo-06 to ietf-00:
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a. None
Changes from -05 to -06:
a. Aaron Stone joins as author.
b. Removed | characters from the script examples.
c. Updated draft references to published RFCs.
Changes from -04 to -05:
a. Fixed examples.
b. Relaxed requirement that imported/exported variables be set
before being used.
Changes from -03 to -04:
a. Fixed missing 2119 definitions.
b. Defined interaction with variables through use of import and
export commands.
Changes from -02 to -03:
a. Refreshing expired draft (updated for nits).
b. Syntax -> Usage.
c. Updated to 3028bis reference.
Changes from -01 to -02:
a. Minor formatting changes only - refreshing expired draft.
Changes from -00 to -01:
a. Added IPR boiler plate.
b. Re-ordered sections at start to conform to RFC style.
c. Moved recursion comment into General Considerations section.
d. Switched to using optional parameter to indicate personal vs
global.
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e. Explicitly state that an error occurs when a missing script is
included.
Table of Contents
1. Introduction and Overview . . . . . . . . . . . . . . . . . . 5
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Include Extension . . . . . . . . . . . . . . . . . . . . . . 5
3.1. General Considerations . . . . . . . . . . . . . . . . . . 5
3.2. Control Structure include . . . . . . . . . . . . . . . . 6
3.3. Control Structure return . . . . . . . . . . . . . . . . . 10
3.4. Interaction with Variables . . . . . . . . . . . . . . . . 10
3.4.1. Control Structure global . . . . . . . . . . . . . . . 10
3.4.2. Variables Namespace global . . . . . . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
5.1. "include" Extension Registration . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . . 13
6.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction and Overview
It's convenient to be able to break SIEVE [RFC5228] scripts down into
smaller components which can be reused in a variety of different
circumstances. For example, users may want to have a default script
and a special 'vacation' script, the latter being activated when the
user goes on vacation. In that case the default actions should
continue to be run, but a vacation command should be executed first.
One option is to edit the default script to add or remove the
vacation command as needed. Another is to have a vacation script
that simply has a vacation command and then includes the default
script.
2. Conventions Used in This Document
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].
Conventions for notations are as in SIEVE [RFC5228] Section 1.1.
The following key phrases are used to describe scripts and script
execution:
script
a valid Sieve script.
script execution
an instance of a Sieve interpreter invoked for a given message
delivery, starting with the user's active script and continuing
through any included scripts until the message is delivered.
immediate script
the individual Sieve script file being executed.
including script
the individual Sieve script file that had an include statement
which included the immediate script.
3. Include Extension
3.1. General Considerations
Sieve implementations that implement the "include", "return", and
"global" commands described below have an identifier of "include" for
use with the capability mechanism. If any of the "include",
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"return", or "global" commands are used in a script, the "include"
capability MUST be listed in the "require" statement in that script.
Sieve implementations must track the use of actions in included
scripts so that implicit "keep" behavior can be properly determined
based on whether any actions have executed in any script.
Sieve implementations are allowed to limit the total number of nested
included scripts, but MUST provide for a total of at least three
levels of nested scripts including the top-level script. An error
MUST be generated either when the script is uploaded to the Sieve
repository, or when the script is executed, if any nesting limit is
exceeded. If such an error is detected whilst processing a Sieve
script, an implicit "keep" action MUST be executed to prevent loss of
any messages.
Sieve implementations MUST ensure that recursive includes are not
possible. For example, if script "A" includes script "B", and script
"B" includes script "A" an error MUST be generated either when the
script is uploaded to the Sieve repository, or when the script is
executed. If such an error is detected whilst processing a Sieve
script, an implicit "keep" action MUST be executed to prevent loss of
any messages.
Sieve implementations MUST generate an error at execution time if an
included script does not exist. Implementations MUST NOT generate
errors for scripts missing at upload time, as this would force an
upload ordering requirement upon script authors / generators.
If the Sieve "variables" extension [RFC5229] is present, an issue
arises with the "scope" of variables defined in scripts that may
include each other. For example, if a script defines the variable
"${status}" with one particular meaning or usage, and another defines
"${status}" with a different meaning, then if one script includes the
other there is an issue as to which "${status}" is being referenced.
To solve this problem, Sieve implementations MUST follow the scoping
rules defined in Section 3.4 and support the "global" command defined
there.
3.2. Control Structure include
Usage: include [LOCATION] [ONCE] <value: string>
LOCATION = ":personal" / ":global"
ONCE = ":once"
The "include" command takes an optional "location" parameter, an
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optional ":once" parameter, and a single string argument representing
the name of the script to include for processing at that point. It
is RECOMMENDED that implementations restrict script names according
to [I-D.ietf-sieve-managesieve] Section 1.7. Implementations MUST
NOT allow variables to be expanded into the names of Sieve scripts;
in other words, the value MUST be a constant string as defined in
VARIABLES [RFC5229], Section 3.
The "location" parameter MUST default to ":personal" if not
specified. The "location" has the following meanings:
:personal
Indicates that the named script is stored in the user's own
personal (private) Sieve repository.
:global
Indicates that the named script is stored in a site-wide Sieve
repository, accessible to all users of the Sieve system.
The ":once" parameter tells the interpreter only to include the named
script if it has not already been included at any other point during
script execution. If the script has already been included,
processing continues immediately following the include command.
Implementations MUST NOT generate an error if an "include :once"
command names a script whose inclusion would be recursive; in this
case, the script MUST be considered previously included and therefore
"include :once" will not include it again.
Note: It is RECOMMENDED that script authors / generators use this
parameter only when including a script that performs general duties
such as declaring global variables and making sanity checks of the
environment.
The included script MUST be a valid Sieve script, including having
necessary "require" statements for all optional capabilities used by
the script. The scope of a "require" statement in an included script
is for the immediate script only, not the including script. For
example, if script "A" includes script "B", and script "B" uses the
"fileinto" extension, script "B" must have a "require" statement for
"fileinto", irrespective of whether script "A" has one. In addition,
if script "A" does not have a "require" statement for "fileinto",
"fileinto" cannot be used anywhere in script "A", even after
inclusion of script "B".
A "stop" command in an included script MUST stop all script
processing, including the processing of the scripts that include the
immediate one. The "return" command (described below) stops
processing of the immediate script only, and allows the scripts that
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include it to continue.
Examples:
The user has four scripts stored in their personal repository:
"default"
This is the default active script that includes several others.
require ["include"];
include :personal "always_allow";
include :global "spam_tests";
include :personal "spam_tests";
include :personal "mailing_lists";
Personal script "always_allow"
This script special cases some correspondent email addresses and
makes sure any message containing those addresses are always kept.
if header :is "From" "boss@example.com"
{
keep;
}
elsif header :is "From" "ceo@example.com"
{
keep;
}
Personal script "spam_tests"
This script does some user-specific spam tests to catch spam
messages not caught by the site-wide spam tests.
require ["reject"];
if header :contains "Subject" "XXXX"
{
reject;
}
elsif header :is "From" "money@example.com"
{
reject;
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}
Personal script "mailing_lists"
This script looks for messages from different mailing lists and
files each into a mailbox specific to the mailing list.
require ["fileinto"];
if header :is "Sender" "owner-ietf-mta-filters@imc.org"
{
fileinto "lists.sieve";
}
elsif header :is "Sender" "owner-ietf-imapext@imc.org"
{
fileinto "lists.imapext";
}
There is one script stored in the global repository:
Site script "spam_tests"
This script does some site-wide spam tests which any user at the
site can include in their own scripts at a suitable point. The
script content is kept up to date by the site administrator.
require ["reject"];
if anyof (header :contains "Subject" "$$",
header :contains "Subject" "Make money")
{
reject;
}
The "include" command may appear anywhere in the script where a
control structure is legal.
Example:
require ["include"];
if anyof (header :contains "Subject" "$$",
header :contains "Subject" "Make money")
{
include "my_reject_script";
}
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3.3. Control Structure return
Usage: return
The "return" command stops processing of the immediately included
script only and returns processing control to the script which
includes it. If used in the main script (i.e. not in an included
script), it has the same effect as the "stop" command, including the
appropriate "keep" action if no other actions have been executed up
to that point.
3.4. Interaction with Variables
In order to avoid problems of variables in an included script
"overwriting" those from the script that includes it, this
specification requires that all variables defined in a script MUST be
kept "private" to the immediate script by default - i.e. they are not
"visible" to other scripts. This ensures that two script authors
cannot inadvertently cause problems by choosing the same name for a
variable.
However, sometimes there is a need to make a variable defined in one
script available to others. This specification defines the new
command "global" to declare that a variable is shared among scripts.
Effectively, two namespaces are defined: one local to the immediate
script, and another shared among all scripts. Implementations MUST
allow a non-global variable to have the same name as a global
variable but have no interaction between them.
3.4.1. Control Structure global
Usage: global <value: string-list>
The "global" command contains a string list argument that defines one
or more names of variables to be stored in the global variable space.
The "global" command is only available when the script has both
"include" and "variables" in its require line. If the "global"
command appears when only "include" or only "variables" has been
required, an error MUST be generated when the script is uploaded.
If a "global" command is given the name of a variable that has
previously been defined in the immediate script with "set", an error
MUST be generated either when the script is uploaded or at execution
time.
If a "global" command lists a variable that has not been defined in
the global namespace, the name of the variable is now marked as
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global, and any subsequent "set" command will set the value of the
variable in global scope.
Interpretation of a string containing a variable marked as global,
but without any value set, SHALL behave as any other access to an
unknown variable, as specified in VARIABLES [RFC5229], Section 3
(i.e., evaluates to an empty string).
Example:
require ["variables", "include"];
global "test";
global "test-mailbox";
# The included script may contain repetitive code that is
# effectively a subroutine that can be factored out.
set "test" "$$"
include "spam_filter_script";
set "test" "Make money"
include "spam_filter_script";
# Message will be filed according to the test that matched last.
if string :count "${test-mailbox}" "1"
{
fileinto "INBOX${test-mailbox}";
stop;
}
# If nothing matched, the message is implicitly kept.
Active script
require ["variables", "include"];
global ["test", "test-mailbox"];
if header :contains "Subject" "${test}"
{
set "test-mailbox" "spam-${test};
}
spam_filter_script
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3.4.2. Variables Namespace global
In addition to the "global" command, this document defines the
variables namespace "global", as specified in VARIABLES [RFC5229],
Section 3.
Example:
require ["variables", "include"];
set "global.i_am_on_vacation" "1";
Variables declared global and variables accessed via the global
namespace MUST be one and the same. In the following example script,
we see the variable "i_am_on_vacation" used in a "global" command,
and again with the "global." namespace. Consider these as two
syntaxes with identical meaning.
Example:
require ["variables", "include"];
global "i_am_on_vacation";
set "global.i_am_on_vacation" "1";
if string :is "${i_am_on_vacation}" "1"
{
vacation "It's true, I am on vacation."
}
4. Security Considerations
Sieve implementations MUST ensure adequate security for the global
script repository to prevent unauthorized changes to global scripts.
Sieve implementations MUST ensure that script names are checked for
validity and proper permissions prior to inclusion, in order to
prevent a malicious user from gaining acess to files accessible to
the mail server software that should not be accessible to the user.
Beyond these, the "include" extension does not raise any security
considerations that are not present in the base SIEVE [RFC5228]
document and the VARIABLES [RFC5229] extension.
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5. IANA Considerations
The following template specifies the IANA registration of the Sieve
extension specified in this document:
5.1. "include" Extension Registration
Capability name: include
Description: adds the "include" command to execute other Sieve
scripts, and the "global" command and "global" variables
namespace to access variables shared among included scripts.
RFC number: this RFC
Contact address: the Sieve discussion list <ietf-mta-filters@imc.org>
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
[RFC5229] Homme, K., "Sieve Email Filtering: Variables Extension",
RFC 5229, January 2008.
6.2. Informative References
[I-D.ietf-sieve-managesieve]
Martin, T. and A. Melnikov, "A Protocol for Remotely
Managing Sieve Scripts", draft-ietf-sieve-managesieve-09
(work in progress), January 2009.
Appendix A. Acknowledgments
Thanks to Ken Murchison, Rob Siemborski, Alexey Melnikov, Marc Mutz,
Kjetil Torgrim Homme, Stephan Bosch, Arnt Gulbrandsen, Barry Leiba,
and Jeffrey Hutzelman for comments and corrections.
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Authors' Addresses
Cyrus Daboo
Email: cyrus@daboo.name
Aaron Stone
Email: aaron@serendipity.cx
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