Adding upstream version 3.4.23.upstream/3.4.23upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 243 insertions, 0 deletions

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+PPoossttffiixx BBeeffoorree--QQuueeuuee CCoonntteenntt FFiilltteerr
+
+-------------------------------------------------------------------------------
+
+WWAARRNNIINNGG
+
+The before-queue content filtering feature described in this document limits
+the amount of mail that a site can handle. See the "Pros and Cons" section
+below for details.
+
+TThhee PPoossttffiixx bbeeffoorree--qquueeuuee ccoonntteenntt ffiilltteerr ffeeaattuurree
+
+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. It is roughly equivalent in capabilities to the
+approach described in MILTER_README, except that the latter uses a dedicated
+protocol instead of SMTP.
+
+The before-queue content filter is meant to be used as follows:
+
+                Postfix    BBeeffoorree    Postfix    Postfix    Postfix     smtp
+    Internet ->  SMTP   -> qquueeuuee  ->  SMTP   -> cleanup ->  queue  -<  local
+                server     ffiilltteerr    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
+
+PPrriinncciipplleess ooff ooppeerraattiioonn
+
+As shown in the diagram above, the before-queue filter sits between two Postfix
+SMTP server processes.
+
+  * The before-filter Postfix SMTP server accepts connections from the Internet
+    and does the usual relay access control, SASL authentication, TLS
+    negotiation, 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. Discard or quarantine the mail.
+
+     3. 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.
+
+PPrrooss aanndd ccoonnss ooff bbeeffoorree--qquueeuuee ccoonntteenntt ffiilltteerriinngg
+
+  * 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 limits the amount of mail that a site
+    can handle.
+
+  * Con: Content filtering software can use lots of memory resources. You have
+    to reduce the number of simultaneous content filter processes so that a
+    burst of mail will not drive your system into the ground.
+
+      o With Postfix versions 2.7 and later, SMTP clients will experience an
+        increase in the delay between the time the client sends "end-of-
+        message" and the time the Postfix SMTP server replies (here, the number
+        of before-filter SMTP server processes can be larger than the number of
+        filter processes).
+
+      o With Postfix versions before 2.7, SMTP clients will experience an
+        increase in the delay before they can receive service (here, the number
+        of before-filter SMTP server processes is always equal to the number of
+        filter processes).
+
+CCoonnffiigguurriinngg tthhee PPoossttffiixx SSMMTTPP ppaassss--tthhrroouugghh pprrooxxyy ffeeaattuurree
+
+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-based framework. See: http://
+bent.latency.net/smtpprox/ or https://github.com/jnorell/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
+            # Postfix 2.7 and later performance feature.
+            # -o smtpd_proxy_options=speed_adjust
+        #
+        # After-filter SMTP server. Receive mail from the content filter
+        # on localhost port 10026.
+        #
+        127.0.0.1:10026 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=
+            # Postfix 2.10 and later: specify empty smtpd_relay_restrictions.
+            -o smtpd_relay_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 available in Postfix version 2.2 and later. Earlier
+    Postfix versions will ignore it.
+
+  * 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 TCP port 10025.
+
+  * The "-o smtpd_proxy_options=speed_adjust" tells the before-filter SMTP
+    server that it should receive an entire email message before it connects to
+    a content filter. This reduces the number of simultaneous filter processes.
+
+    NOTE 1: When this option is turned on, a content filter must not
+    selectively reject recipients of a multi-recipient message. Rejecting all
+    recipients is OK, as is accepting all recipients.
+
+    NOTE 2: This feature increases the minimum amount of free queue space by
+    $message_size_limit. The extra space is needed to save the message to a
+    temporary file.
+
+  * Postfix >= 2.3 supports both TCP and UNIX-domain filters. The above filter
+    could be specified as "inet:127.0.0.1:10025". To specify a UNIX-domain
+    filter, specify "unix:pathname". A relative pathname is interpreted
+    relative to the Postfix queue directory.
+
+The after-filter SMTP server is a new master.cf entry:
+
+  * The "127.0.0.1:10026" 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.
+
+CCoonnffiigguurraattiioonn ppaarraammeetteerrss
+
+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 here,
+    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.
+
+HHooww PPoossttffiixx ttaallkkss ttoo tthhee bbeeffoorree--qquueeuuee ccoonntteenntt ffiilltteerr
+
+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. Postfix sends no other SMTP commands.
+
+The content filter should accept the same MAIL FROM and RCPT TO command syntax
+as the before-filter Postfix SMTP server, and should forward the commands
+without modification to the after-filter SMTP server. If the content filter or
+after-filter SMTP server does not support all the ESMTP features that the
+before-filter Postfix SMTP server supports, then the missing features must be
+turned off in the before-filter Postfix SMTP server with the
+smtpd_discard_ehlo_keywords parameter.
+
+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.
+