Postfix After-Queue Content Filter

Introduction

This document requires Postfix version 2.1 or later.

Normally, Postfix receives mail, stores it in the mail queue and then delivers it. With the external content filter described here, mail is filtered AFTER it is queued. This approach decouples mail receiving processes from mail filtering processes, and gives you maximal control over how many filtering processes you are willing to run in parallel.

The after-queue content filter is meant to be used as follows:

Network or
local users 		-> Postfix
queue 		-> Content
filter 		-> Postfix
queue 		-> Network or
local mailbox

This document describes implementations that use a single Postfix instance for everything: receiving, filtering and delivering mail. Applications that use two separate Postfix instances will be covered by a later version of this document.

The after-queue content filter is not to be confused with the approaches described in the SMTPD_PROXY_README or MILTER_README documents, where incoming SMTP mail is filtered BEFORE it is stored into the Postfix queue.

This document describes two approaches to content filter all email, as well as several options to filter mail selectively:

Principles of operation

An after-queue content filter receives unfiltered mail from Postfix (as described further below) and can do one of the following:

  1. Re-inject the mail back into Postfix, perhaps after changing content and/or destination.

  2. Discard or quarantine the mail.

  3. Reject the mail (by sending a suitable status code back to Postfix). Postfix will send the mail back to the sender address.

NOTE: in this time of mail worms and forged spam, it is a VERY BAD IDEA to send viruses back to the sender address, because the sender address is almost certainly not the originator. It is better to discard known viruses, and to quarantine material that is suspect so that a human can decide what to do with it.

Simple content filter example

The first example is simple to set up, but has major limitations that will be addressed in a second example. Postfix receives unfiltered mail from the network with the smtpd(8) server, and delivers unfiltered mail to a content filter with the Postfix pipe(8) delivery agent. The content filter injects filtered mail back into Postfix with the Postfix sendmail(1) command, so that Postfix can deliver it to the final destination.

This means that mail submitted via the Postfix sendmail(1) command cannot be content filtered.

In the figure below, names followed by a number represent Postfix commands or daemon programs. See the OVERVIEW document for an introduction to the Postfix architecture.

Unfiltered

 ->

 smtpd(8)

pickup(8) 		>- cleanup(8) -> qmgr(8)
Postfix
queue 		-< local(8)
smtp(8)
pipe(8) 		->
->
 Filtered
Filtered
^
| 		|
v
maildrop
queue 		<- Postfix
postdrop(1) 		<- Postfix
sendmail(1) 		<- Content
filter

The content filter can be a simple shell script like this: 

 1 #!/bin/sh
 2 
 3 # Simple shell-based filter. It is meant to be invoked as follows:
 4 #       /path/to/script -f sender recipients...
 5 
 6 # Localize these. The -G option does nothing before Postfix 2.3.
 7 INSPECT_DIR=/var/spool/filter
 8 SENDMAIL="/usr/sbin/sendmail -G -i" # NEVER NEVER NEVER use "-t" here.
 9 
10 # Exit codes from <sysexits.h>
11 EX_TEMPFAIL=75
12 EX_UNAVAILABLE=69
13 
14 # Clean up when done or when aborting.
15 trap "rm -f in.$$" 0 1 2 3 15
16 
17 # Start processing.
18 cd $INSPECT_DIR || {
19     echo $INSPECT_DIR does not exist; exit $EX_TEMPFAIL; }
20 
21 cat >in.$$ || { 
22     echo Cannot save mail to file; exit $EX_TEMPFAIL; }
23 
24 # Specify your content filter here.
25 # filter <in.$$ || {
26 #   echo Message content rejected; exit $EX_UNAVAILABLE; }
27 
28 $SENDMAIL "$@" <in.$$
29 
30 exit $?

Notes:

I suggest that you first run this script by hand until you are satisfied with the results. Run it with a real message (headers+body) as input: 

% /path/to/script -f sender -- recipient... <message-file

Once you're satisfied with the content filtering script:

Simple content filter performance

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.

The simple content filter method is not suitable for content filter actions that are invoked via header_checks or body_checks patterns. These patterns will be applied again after mail is re-injected with the Postfix sendmail command, resulting in a mail filtering loop. The advanced content filtering method (see below) makes it possible to turn off header_checks or body_checks patterns for filtered mail.

Turning off the simple content filter

To turn off "simple" content filtering:

Advanced content filter example

The second example is more complex, but can give better performance, and is less likely to bounce mail when the machine runs into some resource problem. This content filter receives unfiltered mail with SMTP on localhost port 10025, and sends filtered mail back into Postfix with SMTP on localhost port 10026.

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/.

In the figure below, names followed by a number represent Postfix commands or daemon programs. See the OVERVIEW document for an introduction to the Postfix architecture.

Unfiltered

Unfiltered		 ->

-> 		smtpd(8)

pickup(8) 		>- cleanup(8) -> qmgr(8)
Postfix
queue 		-< smtp(8)

local(8) 		->

-> 		Filtered

Filtered
^
| 		|
v
smtpd(8)
10026 		smtp(8)
^
| 		|
v
content filter 10025

The example given here filters all mail, including mail that arrives via SMTP and mail that is locally submitted via the Postfix sendmail command (local submissions enter Postfix via the pickup(8) server; to keep the figure simple we omit local submission details). See examples near the end of this document for how to exclude local users from filtering, or how to configure a destination dependent content filter.

You can expect to lose about a factor of two in Postfix performance for 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.

Advanced content filter: requesting that all mail is filtered

To enable the advanced content filter method for all mail, specify in main.cf: 

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

Advanced content filter: sending unfiltered mail to the content filter

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:
    # =============================================================
    # service type  private unpriv  chroot  wakeup  maxproc command
    #               (yes)   (yes)   (yes)   (never) (100)
    # =============================================================
    scan      unix  -       -       n       -       10      smtp
        -o smtp_send_xforward_command=yes
        -o disable_mime_output_conversion=yes
        -o smtp_generic_maps=

Advanced content filter: running the content filter

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 localhost port 10025: 

/etc/postfix/master.cf:
    # ===================================================================
    # service       type  private unpriv  chroot  wakeup  maxproc command
    #                     (yes)   (yes)   (yes)   (never) (100)
    # ===================================================================
    localhost:10025 inet  n       n       n       -       10      spawn
        user=filter argv=/path/to/filter localhost 10026

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, and must not use the Postfix spawn service.

Advanced filter: injecting 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.

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 `.' from Postfix with `550 content rejected', and to disconnect without sending `.' on the connection that injects mail back into Postfix. 

/etc/postfix/master.cf:
    # ===================================================================
    # service       type  private unpriv  chroot  wakeup  maxproc command
    #                     (yes)   (yes)   (yes)   (never) (100)
    # ===================================================================
    localhost:10026 inet  n       -       n       -       10      smtpd
        -o content_filter= 
        -o receive_override_options=no_unknown_recipient_checks,no_header_body_checks,no_milters
        -o smtpd_helo_restrictions=
        -o smtpd_client_restrictions=
        -o smtpd_sender_restrictions=
        # Postfix 2.10 and later: specify empty smtpd_relay_restrictions.
        -o smtpd_relay_restrictions=
        -o smtpd_recipient_restrictions=permit_mynetworks,reject
        -o mynetworks=127.0.0.0/8
        -o smtpd_authorized_xforward_hosts=127.0.0.0/8

Advanced content filter performance

With the "sandwich" approach to content filtering described here, it is important to match the filter concurrency to the available CPU, memory and I/O resources. Too few content filter processes and mail accumulates in the active queue even with low traffic volume; too much concurrency and Postfix ends up deferring mail destined for the content filter because processes fail due to insufficient resources.

Currently, content filter performance tuning is a process of trial and error; analysis is handicapped because filtered and unfiltered messages share the same queue. As mentioned in the introduction of this document, content filtering with multiple Postfix instances will be covered in a future version.

Turning off the advanced content filter

To turn off "advanced" content filtering:

Filtering mail from outside users only

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

After this, you can follow the same procedure as outlined in the "advanced" or "simple" content filtering examples above, except that you must not specify "content_filter" or "receive_override_options" in the main.cf file.

Different filters for different domains

If you are an MX service provider and want to apply different content filters for different domains, you can configure ONE Postfix instance with multiple SMTP server IP addresses in master.cf. Each address provides a different content filter service. 

/etc/postfix.master.cf:
    # =================================================================
    # service     type  private unpriv  chroot  wakeup  maxproc command
    #                   (yes)   (yes)   (yes)   (never) (100)
    # =================================================================
    # SMTP service for domains that are filtered with service1:dest1
    1.2.3.4:smtp  inet  n       -       n       -       -       smtpd
        -o content_filter=service1:dest1 
        -o receive_override_options=no_address_mappings

    # SMTP service for domains that are filtered with service2:dest2
    1.2.3.5:smtp  inet  n       -       n       -       -       smtpd
        -o content_filter=service2:dest2
        -o receive_override_options=no_address_mappings

After this, you can follow the same procedure as outlined in the "advanced" or "simple" content filtering examples above, except that you must not specify "content_filter" or "receive_override_options" in the main.cf file.

Set up MX records in the DNS that route each domain to the proper SMTP server instance.

FILTER actions in access or header/body tables

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.

To turn on content filtering with an access(5) table rule: 

/etc/postfix/access:
    whatever       FILTER foo:bar

To turn on content filtering with a header_checks(5) or body_checks(5) table pattern: 

/etc/postfix/header_checks:
    /whatever/     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: