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) and courierperlfilter(8) - 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
MAIL FILTER IMPLEMENTATION
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
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
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.
MAIL FILTER INVOCATION
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) 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.
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.
localmailfilter(7), courier(8), dupfilter(8),
4. maildrop filtering API