Sieve Email Filtering: Include Extension
draft-ietf-sieve-include-03

Status of this Memo

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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:

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.

e.

Explicitly state that an error occurs when a missing script is included.

Table of Contents

1.  Introduction and Overview
2.  Conventions Used in This Document
3.  Include Extension
    3.1.  General Considerations
    3.2.  Control Structure include
    3.3.  Control Structure return
    3.4.  Interaction with Variables
        3.4.1.  Control Structure global
        3.4.2.  Variables Namespace global
4.  Security Considerations
5.  IANA Considerations
    5.1.  "include" Extension Registration
6.  References
    6.1.  Normative References
    6.2.  Informative References
Appendix A.  Acknowledgments
§  Authors' Addresses

1.  Introduction and Overview

It's convenient to be able to break SIEVE (Guenther, P. and T. Showalter, “Sieve: An Email Filtering Language,” January 2008.) [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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

Conventions for notations are as in SIEVE (Guenther, P. and T. Showalter, “Sieve: An Email Filtering Language,” January 2008.) [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", "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] (Homme, K., “Sieve Email Filtering: Variables Extension,” January 2008.) 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 (Interaction with Variables) 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 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] (Melnikov, A. and T. Martin, “A Protocol for Remotely Managing Sieve Scripts,” January 2009.) 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 (Homme, K., “Sieve Email Filtering: Variables Extension,” January 2008.) [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 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;
   }

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";
   }

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 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 (Homme, K., “Sieve Email Filtering: Variables Extension,” January 2008.) [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.

   require ["variables", "include"];
   global ["test", "test-mailbox"];

   if header :contains "Subject" "${test}"
   {
       set "test-mailbox" "spam-${test};
   }

3.4.2.  Variables Namespace global

In addition to the "global" command, this document defines the variables namespace "global", as specified in VARIABLES (Homme, K., “Sieve Email Filtering: Variables Extension,” January 2008.) [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 (Guenther, P. and T. Showalter, “Sieve: An Email Filtering Language,” January 2008.) [RFC5228] document and the VARIABLES (Homme, K., “Sieve Email Filtering: Variables Extension,” January 2008.) [RFC5229] extension.

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

6.2. Informative References

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.

Authors' Addresses