Postfix Before-Queue Content Filter

-------------------------------------------------------------------------------

WARNING WARNING WARNING

The before-queue content filtering feature described in this document is
suitable only for low-traffic sites. See the "Pros and Cons" section below for
details.

The Postfix before-queue content filter feature

As of version 2.1, the Postfix SMTP server can forward all incoming mail to a
content filtering proxy server that inspects all mail BEFORE it is stored in
the Postfix mail queue.

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

                Postfix    Before    Postfix    Postfix    Postfix     smtp
    Internet ->  SMTP   -> queue  ->  SMTP   -> cleanup ->  queue  -<  local
                server     filter    server     server                virtual

The before-queue content filter is not to be confused with the approach
described in the FILTER_README document, where mail is filtered AFTER it is
stored in the Postfix mail queue.

This document describes the following topics:

  * Principles of operation
  * Pros and cons of before-queue content filtering
  * Configuring the Postfix SMTP pass-through proxy feature
  * Configuration parameters
  * How Postfix talks to the before-queue content filter
  * Transparency

Principles of operation

The before-filter Postfix SMTP server receives mail from the Internet and does
the usual relay access control, SASL authentication, RBL lookups, rejecting
non-existent sender or recipient addresses, etc. The before-queue filter
receives unfiltered mail content from Postfix and does one of the following:

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

 2. Reject the mail by sending a suitable SMTP status code back to Postfix.
    Postfix passes the status back to the remote SMTP client. This way, Postfix
    does not have to send a bounce message.

The after-filter Postfix SMTP server receives mail from the content filter.
From then on Postfix processes the mail as usual.

The before-queue content filter described here works just like the after-queue
content filter described in the FILTER_README document. In many cases you can
use the same software, within the limitations as discussed in the "Pros and
Cons" section below.

Pros and cons of before-queue content filtering

  * Pro: Postfix can reject mail before the incoming SMTP mail transfer
    completes, so that Postfix does not have to send rejected mail back to the
    sender (which is usually forged anyway). Mail that is not accepted remains
    the responsibility of the remote SMTP client.

  * Con: The remote SMTP client expects an SMTP reply within a deadline. As the
    system load increases, fewer and fewer CPU cycles remain available to
    answer within the deadline, and eventually you either have to stop
    accepting mail or you have to stop filtering mail. It is for this reason
    that the before-queue content filter can be used only on low-traffic sites.

  * Con: Content filtering software can use lots of memory resources. In order
    to not run out of memory you have to reduce the number of before-filter
    SMTP server processes so that a burst of mail will not drive your system
    into the ground with too many content filter processes. This, in turn,
    means that SMTP clients have to wait for a long time before they receive
    service.

Configuring the Postfix SMTP pass-through proxy feature

In the following example, the before-filter Postfix SMTP server gives mail to a
content filter that listens on localhost port 10025. The after-filter Postfix
SMTP server receives mail from the content filter via localhost port 10026.
From then on mail is processed as usual.

The content filter itself is not described here. You can use any filter that is
SMTP enabled. 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/.

                                               Postfix
                  Postfix      filter on     SMTP server    Postfix    Postfix
    Internet -> SMTP server -> localhost  ->     on      -> cleanup -> incoming
                on port 25     port 10025     localhost     server      queue
                                             port 10026

This is configured by editing the master.cf file:

    /etc/postfix/master.cf:
        # =============================================================
        # service type  private unpriv  chroot  wakeup  maxproc command
        #               (yes)   (yes)   (yes)   (never) (100)
        # =============================================================
        #
        # Before-filter SMTP server. Receive mail from the network and
        # pass it to the content filter on localhost port 10025.
        #
        smtp      inet  n       -       n       -       20      smtpd
            -o smtpd_proxy_filter=127.0.0.1:10025
            -o smtpd_client_connection_count_limit=10
        #
        # After-filter SMTP server. Receive mail from the content filter
        # on localhost port 10026.
        #
        :10026    inet  n       -       n       -        -      smtpd
            -o smtpd_authorized_xforward_hosts=127.0.0.0/8
            -o smtpd_client_restrictions=
            -o smtpd_helo_restrictions=
            -o smtpd_sender_restrictions=
            -o smtpd_recipient_restrictions=permit_mynetworks,reject
            -o smtpd_data_restrictions=
            -o mynetworks=127.0.0.0/8
            -o receive_override_options=no_unknown_recipient_checks

Note: do not specify spaces around the "=" or "," characters.

The before-filter SMTP server entry is a modified version of the default
Postfix SMTP server entry that is normally configured at the top of the
master.cf file:

  * The number of SMTP sessions is reduced from the default 100 to only 20.
    This prevents a burst of mail from running your system into the ground with
    too many content filter processes.

  * The "-o smtpd_client_connection_count_limit=10" prevents one SMTP client
    from using up all 20 SMTP server processes. This limit is not necessary if
    you receive all mail from a trusted relay host.

    Note: this setting is ignored by the stable Postfix 2.1 release. The
    feature will be available only in the experimental release until Postfix
    2.2.

  * The "-o smtpd_proxy_filter=127.0.0.1:10025" tells the before filter SMTP
    server that it should give incoming mail to the content filter that listens
    on localhost port 10025.

The after-filter SMTP server is a new master.cf entry:

  * The ":10026" makes the after-filter SMTP server listen on the localhost
    address only, without exposing it to the network. NEVER expose the after-
    filter SMTP server to the Internet :-)

  * The "-o smtpd_authorized_xforward_hosts=127.0.0.0/8" allows the after-
    filter SMTP server to receive remote SMTP client information from the
    before filter SMTP server, so that the after-filter Postfix daemons log the
    remote SMTP client information instead of logging localhost[127.0.0.1].

  * The other after-filter SMTP server settings avoid duplication of work that
    is already done in the "before filter" SMTP server.

By default, the filter has 100 seconds to do its work. If it takes longer then
Postfix gives up and reports an error to the remote SMTP client. You can
increase this time limit (see configuration parameter section below) but doing
so is pointless because you can't control when the remote SMTP client times
out.

Configuration parameters

Parameters that control proxying:

  * smtpd_proxy_filter (syntax: host:port): The host and TCP port of the
    before-queue content filter. When no host or host: is specified, localhost
    is assumed.

  * smtpd_proxy_timeout (default: 100s): Timeout for connecting to the before-
    queue content filter and for sending and receiving commands and data. All
    proxy errors are logged to the maillog file. For privacy reasons, all the
    remote SMTP client sees is "451 Error: queue file write error". It would
    not be right to disclose internal details to strangers.

  * smtpd_proxy_ehlo (default: $myhostname): The hostname to use when sending
    an EHLO command to the before-queue content filter.

How Postfix talks to the before-queue content filter

The before-filter Postfix SMTP server connects to the content filter, delivers
one message, and disconnects. While sending mail into the content filter,
Postfix speaks ESMTP but uses no command pipelining. Postfix generates its own
EHLO, XFORWARD (for logging the remote client IP address instead of localhost
[127.0.0.1]), DATA and QUIT commands, and forwards unmodified copies of all the
MAIL FROM and RCPT TO commands that the before-filter Postfix SMTP server
didn't reject itself. The SMTP proxy server should accept the same MAIL FROM
and RCPT TO command syntax as the Postfix SMTP server. Postfix sends no other
SMTP commands.

The content filter is expected to pass on unmodified SMTP commands from a
before-filter Postfix SMTP server to an after-filter Postfix SMTP server that
usually listens on a non-standard port. When the filter rejects content, it
should send a negative SMTP response back to the before-filter Postfix SMTP
server, and it should abort the connection with the after-filter Postfix SMTP
server without completing the SMTP conversation with the after-filter Postfix
SMTP server.

More detail on the postfix-to-proxy interaction is in the section titled
"Transparency".

Transparency

The before-filter Postfix SMTP server forwards the MAIL FROM, RCPT TO and DATA
commands that it has approved, but it does not forward other commands such as
TLS or SASL commands. It can therefore not be transparent.

The real-time content filter, on the other hand, has to be transparent. In
order to support non-transparent real-time content filters, Postfix would have
to reconcile the before-filter Postfix ESMTP feature set with the feature set
that Postfix receives from the real-time content filter.

  * When a future Postfix version supports DSN, but the content filter does not
    announce DSN support in the EHLO reply, then the before-filter SMTP server
    would have to either 1) suppress the DSN feature in its EHLO announcement,
    or 2) duplicate all the work that needs to be done when delivering DSN-
    aware mail to a non-DSN destination.

  * When the content filter does not announce 8BITMIME support in the EHLO
    reply, then the before-filter SMTP server would have to either 1) suppress
    the 8BITMIME feature in its EHLO announcement, or 2) convert the content to
    quoted-printable before giving it to the content filter.

  * Performance: when Postfix has to suppress elements from the before-filter
    EHLO reply because they are incompatible with the real-time content filter,
    then Postfix has to connect to the content filter as soon as the client
    sends a valid EHLO command. This wastes a lot of resources when all the
    MAIL FROM or RCPT TO commands are rejected.

Therefore, the Postfix SMTP server cannot be transparent with respect to the
before-queue content filter.

