Pigeonhole Sieve: Extprograms Plugin¶

The “sieve_extprograms” plugin provides an extension to the Sieve filtering language adding new action commands for invoking a predefined set of external programs. Messages can be piped to or filtered through those programs and string data can be input to and retrieved from those programs. To mitigate the security concerns, the external programs cannot be chosen arbitrarily; the available programs are restricted through administrator configuration.

This plugin is only available for Pigeonhole v0.3 and higher (available for Dovecot v2.1). For Pigeonhole v0.4 this plugin is part of the release. This is an evolution of the pipe plugin for Pigeonhole v0.2 and now provides the filter and execute commands (and corresponding extensions) in addition to the pipe command that was provided earlier by the Pipe plugin.

Configuration¶

The plugin is activated by adding it to the sieve_plugins setting:

sieve_plugins = sieve_extprograms

This plugin registers the vnd.dovecot.pipe, vnd.dovecot.filter and vnd.dovecot.execute extensions with the Sieve interpreter. However, these extensions are not enabled by default and thus need to be enabled explicitly. It is recommended to restrict the use of these extensions to global context by adding these to the sieve_global_extensions setting. If personal user scripts also need to directly access external programs, the extensions need to be added to the sieve_extensions setting.

The commands introduced by the Sieve language extensions in this plugin can directly pipe a message or string data to an external program (typically a shell script) by forking a new process. Alternatively, these can connect to a unix socket behind which a Dovecot script service is listening to start the external program, e.g. to execute as a different user or for added security.

The program name specified for the new Sieve pipe, filter and execute commands is used to find the program or socket in a configured directory. Separate directories are specified for the sockets and the directly executed binaries. The socket directory is searched first. Since the use of “/” in program names is prohibited, it is not possible to build a hierarchical structure.

Directly forked programs are executed with a limited set of environment variables: HOME, USER, SENDER, RECIPIENT and ORIG_RECIPIENT. Programs executed through the script-pipe socket service currently have no environment set at all.

If a shell script is expected to read a message or string data, it must fully read the provided input until the data ends with EOF, otherwise the Sieve action invoking the program will fail. The action will also fail when the shell script returns a nonzero exit code. Standard output is available for returning a message (for the filter command) or string data (for the execute command) to the Sieve interpreter. Standard error is written to the LDA log file.

The three extensions introduced by this plugin - vnd.dovecot.pipe, vnd.dovecot.filter and vnd.dovecot.execute - each have separate but similar configuration. The following configuration settings are used, for which “<extension>” in the setting name is replaced by either pipe, filter or execute depending on which extension is being configured:

sieve_extension_socket_dir =: Points to a directory relative to the Dovecot base_dir where the plugin looks for script service sockets.
sieve_extension_bin_dir =: Points to a directory where the plugin looks for programs (shell scripts) to execute directly and pipe messages to.
sieve_extension_exec_timeout = 10s: Configures the maximum execution time after which the program is forcibly terminated.
sieve_extension_input_eol = crlf: Determines the end-of-line character sequence used for the data piped to external programs. The default is currently “crlf”, which represents a sequence of the carriage return (CR) and line feed (LF) characters. This matches the Internet Message Format (RFC 5322) and what Sieve itself uses as a line ending. Set this setting to “lf” to use a single LF character instead.

Configuration Example 1: socket service for “pipe” and “execute”¶

plugin {
  sieve = ~/.dovecot.sieve

  sieve_plugins = sieve_extprograms
  sieve_global_extensions = +vnd.dovecot.pipe +vnd.dovecot.execute

  # pipe sockets in /var/run/dovecot/sieve-pipe
  sieve_pipe_socket_dir = sieve-pipe

  # execute sockets in /var/run/dovecot/sieve-execute
  sieve_execute_socket_dir = sieve-execute
}

service sieve-pipe-script {
  # This script is executed for each service connection
  executable = script /usr/lib/dovecot/sieve-extprograms/sieve-pipe-action.sh

  # use some unprivileged user for execution
  user = dovenull

  # socket name is program-name in Sieve (without sieve-pipe/ prefix)
  unix_listener sieve-pipe/sieve-pipe-script {
  }
}

service sieve-execute-action {
  # This script is executed for each service connection
  executable = script /usr/lib/dovecot/sieve-extprograms/sieve-execute-action.sh

  # use some unprivileged user for execution
  user = dovenull

  # socket name is program-name in Sieve (without sieve-execute/ prefix)
  unix_listener sieve-execute/sieve-execute-action {
  }
}

Configuration Example 2: direct execution for “pipe” and “filter”¶

plugin {
  sieve = ~/.dovecot.sieve

  sieve_plugins = sieve_extprograms
  sieve_global_extensions = +vnd.dovecot.pipe +vnd.dovecot.filter

  # This directory contains the scripts that are available for the pipe command.
  sieve_pipe_bin_dir = /usr/lib/dovecot/sieve-pipe

  # This directory contains the scripts that are available for the filter
  # command.
  sieve_filter_bin_dir = /usr/lib/dovecot/sieve-filter
}

Usage¶

Read the specification (v0.3 plugin/v0.4+) for detailed information on how to use the new language extensions.

Full Examples¶

Example 1¶

This simple example shows how to use the “vnd.dovecot.execute” extension to perform some sort of test on the incoming message.

Relevant configuration:

plugin {
 sieve_extensions = +vnd.dovecot.execute

 sieve_plugins = sieve_extprograms
 sieve_execute_bin_dir = /usr/lib/dovecot/sieve-execute
}

The sieve script:

require "vnd.dovecot.execute";

if not execute :pipe "hasfrop.sh" {
        discard;
        stop;
}

At the location /usr/lib/dovecot/sieve-execute, create the executable script hasfrop.sh. In this example, the hasfrop.sh checks whether the message contains the literal text “FROP” anywhere in the message. The Sieve script shown above discards the message if this scripts ends with an exit code other than 0, which happens when “FROP” was found.

# Something that reads the whole message and inspects it for some
# property. Not that the whole message needs to be read from input!
N=`cat | grep -i "FROP"` # Check it for the undesirable text "FROP"
if [ ! -z "$N" ]; then
        # Result: deny
        exit 1;
fi

# Result: accept
exit 0

Example 2¶

This example shows how to use the vnd.dovecot.execute extension for querying/updating a MySQL database. This is used to redirect messages only once every 300s for a particular sender. Note that this particular use case could also be implemented using the Sieve “duplicate” extension