Pigeonhole Sieve Extprograms (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.

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.

Settings

sieve_<extension>_bin_dir

Default	[None]
Value	string

Points to a directory where the plugin looks for programs (shell scripts) to execute directly and pipe messages to.

"<extension>" in the setting name is replaced by either pipe, filter or execute depending on which extension is being configured.

sieve_<extension>_exec_timeout

Default	10s
Value	time

Configures the maximum execution time after which the program is forcibly terminated.

"<extension>" in the setting name is replaced by either pipe, filter or execute depending on which extension is being configured.

sieve_<extension>_input_eol

Default	crlf
Value	string
Allowed Values	crlflf

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.

"<extension>" in the setting name is replaced by either pipe, filter or execute depending on which extension is being configured.

sieve_<extension>_socket_dir

Default	[None]
Value	string

Points to a directory relative to the base_dir where the plugin looks for script service sockets.

"<extension>" in the setting name is replaced by either pipe, filter or execute depending on which extension is being configured.

Specification

Read the specification for detailed information on how to use the new language extensions.

Configuration Examples

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 {
  }
}

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
}

Test Incoming Message

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

dovecot.conf

plugin {
  sieve_extensions = +vnd.dovecot.execute

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

Sieve Script

require "vnd.dovecot.execute";

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

hasfrop.sh

# 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

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.

Query/Update MySQL

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 Sieve duplicate extension.

dovecot.conf

plugin {
  sieve_extensions = +vnd.dovecot.execute

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

Sieve Script

require ["variables", "copy", "envelope", "vnd.dovecot.execute"];

# put the envelope-from address in a variable
if envelope :matches "from" "*" { set "from" "${1}"; }

# execute the vacationcheck.sh program and redirect the message based on
# its exit code
if execute :output "vacation_message" "vacationcheck.sh" ["${from}","300"]
{
    redirect :copy "foo@bar.net";
}

vacationcheck.sh

USER=postfixadmin
PASS=pass
DATABASE=postfixadmin

# DB STRUCTURE
#CREATE TABLE `sieve_count` (
#  `from_address` varchar(254) NOT NULL,
#  `date` datetime NOT NULL
#) ENGINE=InnoDB DEFAULT CHARSET=latin1;
#
#ALTER TABLE `sieve_count`
#  ADD KEY `from_address` (`from_address`);

MAILS=$(mysql -u$USER -p$PASS $DATABASE --batch --silent -e "SELECT count(*) as ile FROM sieve_count WHERE from_address='$1' AND DATE_SUB(now(),INTERVAL $2 SECOND) < date;")
ADDRESULT=$(mysql -u$USER -p$PASS $DATABASE --batch --silent -e "INSERT INTO sieve_count (from_address, date) VALUES ('$1', NOW());")

# uncomment below to debug
# echo User $1 sent $MAILS in last $2 s >> /usr/lib/dovecot/sieve-pipe/output.txt
# echo Add result : $ADDRESULT >> /usr/lib/dovecot/sieve-pipe/output.txt
# echo $MAILS

if [ "$MAILS" = "0" ]
then
    exit 0
fi

exit 1

At the location /usr/lib/dovecot/sieve-execute, create the executable script vacationcheck.sh.

In this example, the vacationcheck.sh script needs two parameters: the sender address and a time interval specified in seconds. The time interval is used to specify the minimum amount of time that needs to have passed since the sender was last seen. If the script returns exit code 0, then message is redirected in the Sieve script shown above.