This is a very first implementation of Postfix content filtering.
A Postfix content filter receives unfiltered mail from Postfix and
either bounces the mail or re-injects filtered mail back into Postfix.

This document describes two approaches to content filtering.

Simple content filtering example
================================

The first example is simple to set up.  It uses a shell script that
receives unfiltered mail from the Postfix pipe delivery agent, and
that feeds filtered mail back into the Postfix sendmail command.
Only mail arriving via SMTP will be content filtered.

                  ..................................
                  :            Postfix             :
Unfiltered mail----->smtpd \                /local---->Filtered mail
                  :         -cleanup->queue-       :
               ---->pickup /                \smtp----->Filtered mail
               ^  :                        |       :
               |  :                         \pipe-----+
               |  ..................................  |
               |                                      |
               |                                      |
               +-Postfix sendmail<----filter script<--+

The /some/where/filter program can be a simple shell script like this:

    #!/bin/sh

    # Localize these.
    INSPECT_DIR=/var/spool/filter
    SENDMAIL="/usr/sbin/sendmail -i"

    # Exit codes from <sysexits.h>
    EX_TEMPFAIL=75
    EX_UNAVAILABLE=69

    # Clean up when done or when aborting.
    trap "rm -f in.$$" 0 1 2 3 15

    # Start processing.
    cd $INSPECT_DIR || { echo $INSPECT_DIR does not exist; exit $EX_TEMPFAIL; }

    cat >in.$$ || { echo Cannot save mail to file; exit $EX_TEMPFAIL; }

    # filter <in.$$ || { echo Message content rejected; exit $EX_UNAVAILABLE; }

    $SENDMAIL "$@" <in.$$

    exit $?

The idea is to first capture the message to file and then run the
content through a third-party content filter program.  If the
mail cannot be captured to file, mail delivery is deferred by
terminating with exit status 75 (EX_TEMPFAIL).  If the content
filter program finds a problem, the mail is bounced by terminating
the shell script with exit status 69 (EX_UNAVAILABLE).  If the
content is OK, it is given as input to Postfix sendmail, and the
exit status of the filter command is whatever exit status Postfix
sendmail produces.

I suggest that you play with this script for a while until you are
satisfied with the results. Run it with a real message (headers+body)
as input:

    % /some/where/filter -f sender recipient... <message-file

Once you're satisfied with the content filtering script:

1 - Create a dedicated local user account called "filter".  This
    user handles all potentially dangerous mail content - that is
    why it should be a separate account. Do not use "nobody", and
    most certainly do not use "root" or "postfix".  The user will
    never log in, and can be given a "*" password and non-existent
    shell and home directory.

2 - Create a directory /var/spool/filter that is accessible only
    to the "filter" user. This is where the content filtering script
    is supposed to store its temporary files.

3 - Define the content filter in the Postfix master file:

    /etc/postfix/master.cf:
      filter    unix  -       n       n       -       -       pipe
        flags=Rq user=filter argv=/somewhere/filter -f ${sender} -- ${recipient}

To turn on content filtering for mail arriving via SMTP only, append
"-o content_filter=filter:" to the master.cf entry that defines
the Postfix SMTP server:

    /etc/postfix/master.cf:
	smtp      inet     ...stuff...      smtpd
	    -o content_filter=filter:

Note the ":" at the end!!  The content_filter configuration parameter
accepts the same syntax as the right-hand side in a Postfix transport
table. Execute "postfix reload" to complete the change.

To turn off content filtering, edit the master.cf file, remove the
"-o content_filter=filter:" text from the entry that defines the
Postfix SMTP server, and execute another "postfix reload".

With the shell script as shown above you will lose a factor of four
in Postfix performance for transit mail that arrives and leaves
via SMTP. You will lose another factor in transit performance for
each additional temporary file that is created and deleted in the
process of content filtering.  The performance impact is less for
mail that is submitted or delivered locally, because such deliveries
are already slower than SMTP transit mail.

Simple content filter limitations
=================================

The problem with content filters like the one above is that they
are not very robust, because the software does not talk a well-defined
protocol with Postfix. If the filter shell script aborts because
the shell runs into some memory allocation problem, the script will
not produce a nice exit status as per /usr/include/sysexits.h and
mail will probably bounce. The same lack of robustness is possible
when the content filtering software itself runs into a resource
problem.

Advanced content filtering example
===================================

The second example is considerably more complex, but can give much
better performance, and is less likely to bounce mail when the
machine runs into a resource problem.  This approach uses content
filtering software that can receive and deliver mail via SMTP.
You can expect to lose about a factor of two in Postfix performance
for transit mail that arrives and leaves via SMTP, provided that
you create no temporary files. Each temporary file adds another
factor to the performance loss.

We will set up a content filtering program that receives SMTP mail
via localhost port 10025, and that submits SMTP mail back into
Postfix via localhost port 10026.

      ..................................
      :            Postfix             :
   ----->smtpd \                /local---->
      :         -cleanup->queue-       :
   ---->pickup /    ^       |   \smtp----->
      :             |       v          :
      :           smtpd    smtp        :
      :           10026     |          :
      ......................|...........
                    ^       |
                    |       v
                ....|............
                :   |     10025 :
                :   filter      :
                :               :
                .................

To enable content filtering in this manner, specify in main.cf a
new parameter:

    /etc/postfix/main.cf:
	content_filter = smtp:localhost:10025

This causes Postfix to add one extra content filtering record to
each incoming mail message, with content smtp:localhost:10025.
You can use the same syntax as in the right-hand side of a Postfix
transport table.  The content filtering records are added by the
smtpd and pickup servers.

When a queue file has content filtering information, the queue
manager will deliver the mail to the specified content filter
regardless of its final destination.

The content filter can be set up with the Postfix spawn service,
which is the Postfix equivalent of inetd. For example, to instantiate
up to 10 content filtering processes on demand:

    /etc/postfix/master.cf:
	localhost:10025     inet  n      n      n      -      10     spawn
	    user=filter argv=/some/where/filter localhost 10026

"filter" is a dedicated local user account.  The user will never
log in, and can be given a "*" password and non-existent shell and
home directory.  This user handles all potentially dangerous mail
content - that is why it should be a separate account.

In the above example, Postfix listens on port localhost:10025.  If
you want to have your filter listening on port localhost:10025
instead of Postfix, then you must run your filter as a stand-alone
program.

Note: the localhost port 10025 SMTP server filter should announce
itself as "220 localhost...", to silence warnings in the log.

The /some/where/filter command is most likely a PERL script. PERL
has modules that make talking SMTP easy. The command-line specifies
that mail should be sent back into Postfix via localhost port 10026.

For now, it is left up to the Postfix users to come up with a
PERL/SMTP framework for Postfix content filtering. If done well,
it can be used with other mailers too, which is a nice spin-off.

The simplest content filter just copies SMTP commands and data
between its inputs and outputs. If it has a problem, all it has to
do is to reply to an input of `.' with `550 content rejected', and
to disconnect without sending `.' on the connection that injects
mail back into Postfix.

The job of the content filter is to either bounce mail with a
suitable diagnostic, or to feed the mail back into Postfix through
a dedicated listener on port localhost 10026:

    /etc/postfix/master.cf:
        localhost:10026     inet  n      -      n      -      10      smtpd
	    -o content_filter= 
	    -o local_recipient_maps=
	    -o myhostname=localhost.domain.name

This is just another SMTP server. It is configured NOT to request
content filtering for incoming mail. The server has the same process
limit as the filter master.cf entry.

The "-o local_recipient_maps=" is a safety in case you have specified
local_recipient_maps in the main.cf file. That setting could
interfere with content filtering.

The SMTP server is configured to use a different hostname in the
greeting message (this is necessary for testing when I simply use
no filtering program and let the SMTP content filtering interfaces
talk directly to each other).