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       courierfilter - Courier mail filters


       courierfilter [[start] | [stop] | [restart]]

       filterctl [[start] | [stop]] [filter]


       The filterctl commands install or uninstall global mail filters. Global
       mail filters are used to selectively block unwanted mail. More than one
       mail filter can be enabled at the same time. Two filters -
       dupfilter(8)[1] and courierperlfilter(8)[2] - are provided as examples
       for writing mail filters.

       courierfilter start runs all mail filters that have been installed by
       filterctl.  courierfilter stop shuts down all running mail filters.
       After courierfilter start, any filterctl commands take effect
       immediately. After courierfilter stop any filterctl commands will take
       effect at the next courierfilter start.

       courierfilter restart signals the running courierfilter to reread its
       configuration files. This is normally done automatically, by filterctl.

       If any mail filter is installed, the mail filter must be running in
       order for any mail to be processed. Mail filters are assumed to be
       empowered to enforce system-wide mail policies, so if an installed mail
       filter is not running then mail will not be accepted by the system.
       Note that mail will not be rejected, if possible. Every attempt will be
       made to send a temporary error code to an external mail system, asking
       it to try again later.

       For this reason, you should modify your system boot script to run
       courierfilter start as soon as possible, and run courierfilter stop
       during the final portion of your system shutdown script. It is not
       necessary to run courierfilter if you do not install a mail filter with


       This section explains how mail filters are implemented, and how to
       write a new global mail filter.

       Available mail filter binaries are located in the directory
       /usr/lib/courier/filters. The filterctl script looks in this directory
       to see which mail filters are available to be installed. Installing a
       mail filter consists of simply creating a soft link from the directory
       /etc/courier/filters/active to its corresponding binary in
       /usr/lib/courier/filters. The courierfilter start command simply reads
       /etc/courier/filters/active and runs every program in this directory.

       The filterctl script sends a HUP signal to courierfilter after
       installing or uninstalling a filter.  courierfilter will reread the
       contents of /etc/courier/filters/active then start or stop individual
       mail filters.

       After starting, an individual mail filter must create a filesystem
       domain socket in one of two directories: /var/lib/courier/filters or
       /var/lib/courier/allfilters. The name of the socket should be the same
       as a name of the filter, and the mail filter must make sure to remove
       any socket by the same name in the other directory. For various silly
       reasons, the recommended implementation is to create
       /var/lib/courier/filters/.NAME or /var/lib/courier/allfilters/.NAME
       (after making sure that it doesn´t exist) then rename .NAME to NAME.

       After initializing the socket, the mail filter must then close its file
       descriptor #3. File descriptor 3 is inherited by every mail filter
       that´s executed by the courierfilter start command. The mail filter´s
       file descriptor 3 is connected to the write end of a pipe, which may be
       relevant to certain ways of implementing the closing of the file
       descriptor, for instance in Perl where you may be forced to pseudo-open
       the descriptor (in write mode) before closing it. The courierfilter
       start command will not exit until every started mail filter closes its
       file descriptor 3. This allows for all mail filters to orderly
       initialize themselves before courierfilter start command returns.

       All mail filters also inherit a pipe on standard input, and must
       terminate when the pipe is closed. Mail filters must simultaneously
       listen for new connections on the mail filter socket, and for their
       standard input to close.

       The mail filter receives a new connection on its socket for every
       message that needs to be filtered. After establishing a connection, the
       mail filter will immediately read the following information from the
       new socket:

       A pathname to a file containing the contents of the message.

       One or more pathnames to control files for this message.

       Each pathname is terminated by a single newline character. The last
       pathname is followed by a second newline character. The pathnames may
       either be relative pathnames to /usr or absolute pathnames, depending
       on the system configuration.

       The mail filter is free to judge the message´s worthiness by reading
       its contents and/or control file(s) as soon as a second consecutive
       newline character is received. The final verdict is rendered by writing
       back a result code on the same socket. The result code follows the same
       format as regular SMTP replies (even though the message may not have
       been received via SMTP), and can be used to communicate acceptance,
       temporary failure, or a permanent failure. If it´s a failure, then the
       text portion of the result code will be used, if possible. The result
       code may be a multiline response, just like a regular SMTP reply. The
       mail filter must immediately close the connection after writing the
       result code. After closing the socket the mail filter must then proceed
       to wait for another connection request on the original listening

       The mail filter can be multithreaded or multitasked, and can accept
       multiple connections simultaneously. When its standard input is closed
       the mail filter should stop accepting new connections and wait for any
       existing connections to be closed, prior to exiting.

       Global mail filters must be EXTREMELY resilient to runtime failures.
       Since mail will not be processed if an installed mail filter is not
       running, if a mail filter crashes it will effectively shut down the
       mail server. Currently courierfilter does not attempt to restart mail
       filters which crash.


       The system administrator defines what mail gets filtered by editing the
       contents of the enablefiltering configuration file in /etc/courier.
       This configuration file contains a list of mail sources that should be
       filtered, like esmtp or local. See courier(8)[3] for more information.
       A default /etc/courier/enablefiltering file is installed that specifies
       only the esmtp mail source as subject to filtering.

       A message is not subject to filtering if its source is not listed in
       /etc/courier/enablefiltering. Otherwise the following rules apply.

       Certain mail destinations have the ability to selectively whitelist
       arbitrary messages. For example, local mail recipients have the ability
       to selectively whitelist individual messages, provided that a local
       mail filter (independent of any global mail filter) is installed that
       implements the maildrop filtering API[4].

       New messages are filtered by connecting to every socket in
       /var/lib/courier/filters and/or /var/lib/courier/allfilters, one at a
       time. All mail filters must accept the message, for it to be accepted
       by Courier. If a socket exists but a connection cannot be established
       then the message is not accepted, and a temporary failure indication is
       returned. That´s why no mail will be accepted unless all installed mail
       filters are running.

       Mail recipients that did not whitelist the sender, via the maildrop
       API, will have their mail filtered against everything in
       /var/lib/courier/filters and /var/lib/courier/allfilters. Mail to
       recipients that whitelisted the sender, or mail to destinations that do
       not use a maildrop API-compatible filter, will be filtered only against
       the contents of /var/lib/courier/allfilters.

       This gives system administrators a choice whether to install selective,
       or mandatory mail filters, or a combination of both.


       Many filesystem domain socket implementation are buggy.

       Handling of crashed mail filters could be improved.


           Available mail filters.

           Miscellaneous configuration files.

           Installed mail filters.

           Which mail sources to filter.

           Mandatory filters.

           Optional filters.


       localmailfilter(7)[4], courier(8)[3], dupfilter(8)[1],


        1. dupfilter(8)
           [set $man.base.url.for.relative.links]/dupfilter.html

        2. courierperlfilter(8)
           [set $man.base.url.for.relative.links]/courierperlfilter.html

        3. courier(8)
           [set $man.base.url.for.relative.links]/courier.html

        4. maildrop filtering API
           [set $man.base.url.for.relative.links]/localmailfilter.html