Tonight I read the FILTER_README file from Postfix.

Introduction
============

This is a very first implementation of Postfix content filtering.
A Postfix content filter receives unfiltered mail from Postfix and
does one of the following:

- re-injects the mail back into Postfix, perhaps after changing content
- rejects the mail (by sending a suitable status code back to
  Postfix) so that it is returned to sender.
- sends the mail somewhere else

This document describes two approaches to content filtering: simple
and advanced. Both filter all the mail by default.

At the end are examples that show how to filter only mail from
users, about using different filters for different domains that
you provide MX service for, and about selective filtering on the
basis of message envelope and/or header/body patterns.

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<--+

Mail is filtered by a /some/where/filter program. This 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). Postfix will
  try again after some delay.

- If the content filter program finds a problem, the mail is bounced
  by terminating with exit status 69 (EX_UNAVAILABLE).  Postfix
  will return the message to the sender as undeliverable.

- If the content is OK, it is given as input to the Postfix sendmail
  command, and the exit status of the filter command is whatever
  exit status the Postfix sendmail command produces. Postfix will
  deliver the message as usual.

I suggest that you run this script by hand 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:dummy" to the master.cf entry that defines
the Postfix SMTP server:

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

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:dummy" 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. The reason is that 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 defined in the
file /usr/include/sysexits.h.  Instead of going to the deferred
queue, mail will bounce.  The same lack of robustness can happen
when the content filtering software itself runs into a resource
problem.

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

The second example is 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.

Some Anti-virus software is built to receive and deliver mail via
SMTP and is ready to use as an advanced Postfix content filter.
For non-SMTP capable content filtering software, Bennett Todd's
SMTP proxy implements a nice PERL/SMTP content filtering framework.
See: http://bent.latency.net/smtpprox/

The example given here filters all mail, including mail that arrives
via SMTP and mail that is locally submitted via the Postfix sendmail
command.

You can expect to lose about a factor of two in Postfix performance
for transit mail that arrives and leaves via SMTP, provided that
the content filter creates no temporary files. Each temporary file
created by the content filter 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 = scan:localhost:10025

This causes Postfix to add one extra content filtering record to
each incoming mail message, with content scan:localhost:10025.
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.

In this example, "scan" is an instance of the Postfix SMTP client
with slightly different configuration parameters. This is how
one would set up the service in the Postfix master.cf file:

    /etc/postfix/master.cf:
        scan      unix  -       -       n       -       10       smtp

Instead of a limit of 10 concurrent processes, use whatever process
limit is feasible for your machine.  Content inspection software
can gobble up a lot of system resources, so you don't want to have
too much of it running at the same time.

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...".  Postfix aborts delivery when it
connects to an SMTP server that uses the same hostname as Postfix
("host <servername> greeted me with my own hostname"), because that
normally means you have a mail delivery loop problem.

The example here assumes that the /some/where/filter command is 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.

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 relay_recipient_maps=
            -o myhostname=localhost.domain.tld
            -o smtpd_helo_restrictions=
            -o smtpd_client_restrictions=
            -o smtpd_sender_restrictions=
            -o smtpd_recipient_restrictions=permit_mynetworks,reject
            -o mynetworks=127.0.0.0/8

Warning for Postfix version 2 users: in this SMTP server after the
content filter, do not override main.cf settings for virtual_alias_maps
or virtual_alias_domains. That would cause mail to be rejected with
"User unknown".

This SMTP server has the same process limit as the "filter" master.cf
entry.

The "-o content_filter=" requests no content filtering for incoming
mail.

The "-o local_recipient_maps=" and "-o relay_recipient_maps=" avoid
unnecessary table lookups.

The "-o myhostname=localhost.domain.tld" avoids false alarms ("host
<servername> greeted me with my own hostname") if your content
filter is based on a proxy that simply relays SMTP commands.

The "-o smtpd_xxx_restrictions" and "-o mynetworks=127.0.0.0/8"
turn off UCE controls that would only waste time here.

Squeezing out more performance
==============================

Many refinements are possible, such as running a specially-configured
smtp delivery agent for feeding mail into the content filter, and
turning off address rewriting before content filtering.

As the example below shows, things quickly become very complex,
because a lot of main.cf like information gets listed in the
master.cf file. This makes the system hard to understand.

Even worse, details change as Postfix evolves and different
configuration parameters are implemented by different programs.

If you need to squeeze out more performance, it is probably simpler
to run multiple Postfix instances, one before and one after the
content filter. That way, each instance can have simple main.cf
and master.cf files, each instance can have its own mail queue,
and the system will be easier to understand.

As before, 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 \                            :
      :         -pre-cleanup-\       /local---->
   ---->pickup /              -queue-       :
      :             -cleanup-/   |   \smtp----->
      :     bounces/    ^        v          :
      : and locally     |        v          :
      :   forwarded   smtpd     scan        :
      :    messages   10026      |          :
      ...........................|...........
                        ^        |
                        |        v
                    ....|.............
                    :   |      10025 :
                    :   filter       :
                    :                :
                    ..................

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

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

/etc/postfix/master.cf:
#
# These are the usual input "smtpd" and local "pickup" servers already
# present in master.cf. We add an option to select a non-default
# cleanup service (defined further below).
#
smtp      inet  n       -       n       -       -       smtpd
    -o cleanup_service_name=pre-cleanup
pickup    fifo  n       -       n       60      1       pickup
    -o cleanup_service_name=pre-cleanup
#
# ------------------------------------------------------------------
#
# This is the cleanup daemon that handles messages in front of
# the content filter. It does header_checks and body_checks (if
# any), but does no virtual alias or canonical address mapping,
# so that mail passes through your content filter with the original
# recipient addresses mostly intact.
#
# Virtual alias or canonical address mapping happens in the second
# cleanup phase after the content filter. This gives the content_filter
# access to *largely* unmodified addresses for maximum flexibility.
#
# Some sites may specifically want to perform canonical or virtual
# address mapping in front of the content_filter.  In that case you
# still have to enable address rewriting in the after-filter cleanup
# instance, in order to correctly process forwarded mail or bounced
# mail.
#
pre-cleanup         unix  n      -      n      -       0      cleanup
        -o canonical_maps=
        -o sender_canonical_maps=
        -o recipient_canonical_maps=
        -o masquerade_domains=
        -o virtual_alias_maps=
#
# ------------------------------------------------------------------
#
# This is the delivery agent that injects mail into the content
# filter.  It is tuned for low concurrency, because most content
# filters burn CPU and use lots of memory.  The process limit of 10
# re-enforces the effect of $default_destination_concurrency_limit.
# Even without an explicit process limit, the concurrency is bounded
# because all messages heading into the content filter have the same
# destination.
#
scan                unix  -      -      n      -      10      smtp
#
# ------------------------------------------------------------------
#
# This is the SMTP listener that receives filtered messages from
# the content filter. It *MUST* clear the content_filter
# parameter to avoid loops, and use a different hostname to avoid
# triggering the Postfix SMTP loop detection code.
#
# This "smtpd" uses the normal cleanup service which is also used
# for bounces and for internally forwarded mail.
#
# The parameters from mynetworks onward disable all access
# control other than insisting on connections from one of the IP
# addresses of the host. This is typically overkill, but can
# reduce resource usage, if the default restrictions use lots of
# tables.
#
localhost:10026     inet  n      -      n      -       -      smtpd
    -o content_filter=
    -o myhostname=localhost.domain.tld
    -o local_recipient_maps=
    -o relay_recipient_maps=
    -o mynetworks=127.0.0.0/8
    -o mynetworks_style=host
    -o smtpd_restriction_classes=
    -o smtpd_client_restrictions=
    -o smtpd_helo_restrictions=
    -o smtpd_sender_restrictions=
    -o smtpd_recipient_restrictions=permit_mynetworks,reject
#
# Do not override main.cf settings here for virtual_alias_maps or
# virtual_mailbox_maps. This causes mail to be rejected with "User
# unknown in virtual (alias|mailbox) recipient table".
#
# ------------------------------------------------------------------
#
# This is the normal cleanup daemon for use after content filtering.
# No header or body checks, because those have already been taken
# care of by the pre-cleanup service before the content filter.
#
# The normal cleanup instance does all the virtual alias and canonical
# address mapping that was disabled in the pre-cleanup instance before
# the content filter. This rewriting must be done even when you didn't
# disable address rewriting in the pre-cleanup instance, in order to
# correctly process bounces and locally forwarded mail.
#
cleanup            unix  n      -      n      -       0      cleanup
    -o header_checks=
    -o mime_header_checks=
    -o nested_header_checks=
    -o body_checks=
#
# ------------------------------------------------------------------
#
# The normal "smtp" delivery agent for contrast with "scan".
#
smtp                unix  -      -      n      -      -       smtp

The above example causes Postfix to add one content filtering record
to each incoming mail message, with content scan: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.

The "scan" transport is a dedicated instance of the "smtp" delivery
agent for injecting messages into the SMTP content filter. Using
a dedicated "smtp" transport allows one to tune it for the specific
task of delivering mail to a local content filter (low latency,
low concurrency, throughput dependent on predictably low latency).

See the previous example for setting up the content filter with
the Postfix spawn service; you can of course use any server that
can be run stand-alone outside the Postfix environment.

Filtering mail from outside users only
======================================

The easiest approach is to configure ONE Postfix instance with TWO
SMTP server addresses in master.cf:

- One SMTP server address for inside users only that never invokes
  content filtering.

- One SMTP server address for outside users that always invokes
  content filtering.

/etc/postfix.master.cf:
    # SMTP service for internal users only, no content filtering.
    1.2.3.4:smtp        inet  n       -       n       -       -       smtpd
        -o smtpd_client_restrictions=permit_mynetworks,reject
    127.0.0.1:smtp      inet  n       -       n       -       -       smtpd
        -o smtpd_client_restrictions=permit_mynetworks,reject

    # SMTP service for external users, with content filtering.
    1.2.3.5:smtp        inet  n       -       n       -       -       smtpd
        -o content_filter=foo:bar

Getting really nasty
====================

The above filtering configurations are static. Mail that follows
a given path is either always filtered or it is never filtered. As
of Postfix 2.0 you can also turn on content filtering on the fly.
The Postfix UCE features allow you to specify a filtering action
on the fly:

    FILTER foo:bar

You can do this in smtpd access maps as well as the cleanup server's
header/body_checks.  This feature must be used with great care:
you must disable all the UCE features in the after-filter smtpd
and cleanup daemons or else you will have a content filtering loop.

Limitations:

- There can be only one content filter action per message.

- FILTER actions from smtpd access maps and header/body_checks take
  precedence over filters specified with the main.cf content_filter
  parameter.

- Only the last FILTER action from smtpd access maps or from
  header/body_checks takes effect.

- The same content filter is applied to all the recipients of a
  given message.