Adding upstream version 3.4.23.upstream/3.4.23upstream

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

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+PPoossttffiixx BBaacckkssccaatttteerr HHoowwttoo
+
+-------------------------------------------------------------------------------
+
+OOvveerrvviieeww
+
+This document describes features that require Postfix version 2.0 or later.
+
+Topics covered in this document:
+
+  * What is backscatter mail?
+  * How do I block backscatter mail to random recipient addresses?
+  * How do I block backscatter mail to real recipient addresses?
+
+      o Blocking backscatter mail with forged mail server information
+      o Blocking backscatter mail with forged sender information
+      o Blocking backscatter mail with other forged information
+      o Blocking backscatter mail from virus scanners
+
+The examples use Perl Compatible Regular Expressions (Postfix pcre: tables),
+but also provide a translation to POSIX regular expressions (Postfix regexp:
+tables). PCRE is preferred primarily because the implementation is often
+faster.
+
+WWhhaatt iiss bbaacckkssccaatttteerr mmaaiill??
+
+When a spammer or worm sends mail with forged sender addresses, innocent sites
+are flooded with undeliverable mail notifications. This is called backscatter
+mail. With Postfix, you know that you're a backscatter victim when your logfile
+goes on and on like this:
+
+    Dec  4 04:30:09 hostname postfix/smtpd[58549]: NOQUEUE: reject:
+    RCPT from xxxxxxx[x.x.x.x]: 550 5.1.1 <yyyyyy@your.domain.here>:
+    Recipient address rejected: User unknown; from=<>
+    to=<yyyyyy@your.domain.here> proto=ESMTP helo=<zzzzzz>
+
+What you see are lots of "user unknown" errors with "from=<>". These are error
+reports from MAILER-DAEMONs elsewhere on the Internet, about email that was
+sent with a false sender address in your domain.
+
+HHooww ddoo II bblloocckk bbaacckkssccaatttteerr mmaaiill ttoo rraannddoomm rreecciippiieenntt aaddddrreesssseess??
+
+If your machine receives backscatter mail to random addresses, configure
+Postfix to reject all mail for non-existent recipients as described in the
+LOCAL_RECIPIENT_README and STANDARD_CONFIGURATION_README documentation.
+
+If your machine runs Postfix 2.0 and earlier, disable the "pause before reject"
+feature in the SMTP server. If your system is under stress then it should not
+waste time.
+
+    /etc/postfix/main.cf:
+        # Not needed with Postfix 2.1 and later.
+        smtpd_error_sleep_time = 0
+
+        # Not needed with Postfix 2.4 and later.
+        unknown_local_recipient_reject_code = 550
+
+HHooww ddoo II bblloocckk bbaacckkssccaatttteerr mmaaiill ttoo rreeaall rreecciippiieenntt aaddddrreesssseess??
+
+When backscatter mail passes the "unknown recipient" barrier, there still is no
+need to despair. Many mail systems are kind enough to attach the message
+headers of the undeliverable mail in the non-delivery notification. These
+message headers contain information that you can use to recognize and block
+forged mail.
+
+BBlloocckkiinngg bbaacckkssccaatttteerr mmaaiill wwiitthh ffoorrggeedd mmaaiill sseerrvveerr iinnffoorrmmaattiioonn
+
+Although my email address is "wietse@porcupine.org", all my mail systems
+announce themselves with the SMTP HELO command as "hostname.porcupine.org".
+Thus, if returned mail has a Received: message header like this:
+
+    Received: from porcupine.org ...
+
+Then I know that this is almost certainly forged mail (almost; see next section
+for the fly in the ointment). Mail that is really sent by my systems looks like
+this:
+
+    Received: from hostname.porcupine.org ...
+
+For the same reason the following message headers are very likely to be the
+result of forgery:
+
+    Received: from host.example.com ([1.2.3.4] helo=porcupine.org) ...
+    Received: from [1.2.3.4] (port=12345 helo=porcupine.org) ...
+    Received: from host.example.com (HELO porcupine.org) ...
+    Received: from host.example.com (EHLO porcupine.org) ...
+
+Some forgeries show up in the way that a mail server reports itself in
+Received: message headers. Keeping in mind that all my systems have a mail
+server name of hostname.porcupine.org, the following is definitely a forgery:
+
+    Received: by porcupine.org ...
+    Received: from host.example.com ( ... ) by porcupine.org ...
+
+Another frequent sign of forgery is the Message-ID: header. My systems produce
+a Message-ID: of <stuff@hostname.porcupine.org>. The following are forgeries,
+especially the first one:
+
+    Message-ID: <1cb479435d8eb9.2beb1.qmail@porcupine.org>
+    Message-ID: <yulszqocfzsficvzzju@porcupine.org>
+
+To block such backscatter I use header_checks and body_checks patterns like
+this:
+
+    /etc/postfix/main.cf:
+        header_checks = pcre:/etc/postfix/header_checks
+        body_checks = pcre:/etc/postfix/body_checks
+
+    /etc/postfix/header_checks:
+        # Do not indent the patterns between "if" and "endif".
+        if /^Received:/
+        /^Received: +from +(porcupine\.org) +/
+            reject forged client name in Received: header: $1
+        /^Received: +from +[^ ]+ +\(([^ ]+ +[he]+lo=|[he]+lo +)
+    (porcupine\.org)\)/
+            reject forged client name in Received: header: $2
+        /^Received:.* +by +(porcupine\.org)\b/
+            reject forged mail server name in Received: header: $1
+        endif
+        /^Message-ID:.* <!&!/ DUNNO
+        /^Message-ID:.*@(porcupine\.org)/
+            reject forged domain name in Message-ID: header: $1
+
+    /etc/postfix/body_checks:
+        # Do not indent the patterns between "if" and "endif".
+        if /^[> ]*Received:/
+        /^[> ]*Received: +from +(porcupine\.org) /
+            reject forged client name in Received: header: $1
+        /^[> ]*Received: +from +[^ ]+ +\(([^ ]+ +[he]+lo=|[he]+lo +)
+    (porcupine\.org)\)/
+            reject forged client name in Received: header: $2
+        /^[> ]*Received:.* +by +(porcupine\.org)\b/
+            reject forged mail server name in Received: header: $1
+        endif
+        /^[> ]*Message-ID:.* <!&!/ DUNNO
+        /^[> ]*Message-ID:.*@(porcupine\.org)/
+            reject forged domain name in Message-ID: header: $1
+
+Notes:
+
+  * The example uses pcre: tables mainly for speed; with minor modifications,
+    you can use regexp: tables as explained below.
+
+  * The example is simplified for educational purposes. In reality my patterns
+    list multiple domain names, as "(domain|domain|...)".
+
+  * The "\." matches "." literally. Without the "\", the "." would match any
+    character.
+
+  * The "\(" and "\)" match "(" and ")" literally. Without the "\", the "(" and
+    ")" would be grouping operators.
+
+  * The "\b" is used here to match the end of a word. If you use regexp:
+    tables, specify "[[:>:]]" (on some systems you should specify "\>" instead;
+    for details see your system documentation).
+
+  * The "if /pattern/" and "endif" eliminate unnecessary matching attempts. DO
+    NOT indent lines starting with /pattern/ between the "if" and "endif"!
+
+  * The two "Message-ID:.* <!&!" rules are workarounds for some versions of
+    Outlook express, as described in the caveats section below.
+
+CCaavveeaattss
+
+  * Netscape Messenger (and reportedly, Mozilla) sends a HELO name that is
+    identical to the sender address domain part. If you have such clients then
+    the above patterns would block legitimate email.
+
+    My network has only one such machine, and to prevent its mail from being
+    blocked I have configured it to send mail as user@hostname.porcupine.org.
+    On the Postfix server, a canonical mapping translates this temporary
+    address into user@porcupine.org.
+
+        /etc/postfix/main.cf:
+            canonical_maps = hash:/etc/postfix/canonical
+
+        /etc/postfix/canonical:
+            @hostname.porcupine.org @porcupine.org
+
+    This is of course practical only when you have very few systems that send
+    HELO commands like this, and when you never have to send mail to a user on
+    such a host.
+
+    An alternative would be to remove the hostname from
+    "hostname.porcupine.org" with address masquerading, as described in the
+    ADDRESS_REWRITING_README document.
+
+  * Reportedly, Outlook 2003 (perhaps Outlook Express, and other versions as
+    well) present substantially different Message-ID headers depending upon
+    whether or not a DSN is requested (via Options "Request a delivery receipt
+    for this message").
+
+    When a DSN is requested, Outlook 2003 uses a Message-ID string that ends in
+    the sender's domain name:
+
+        Message-ID: <!&! ...very long string... ==@example.com>
+
+    where example.com is the domain name part of the email address specified in
+    Outlook's account settings for the user. Since many users configure their
+    email addresses as username@example.com, messages with DSN turned on will
+    trigger the REJECT action in the previous section.
+
+    If you have such clients then you can to exclude their Message-ID strings
+    with the two "Message-ID:.* <!&!" patterns that are shown in the previous
+    section. Otherwise you will not be able to use the two backscatter rules to
+    stop forged Message ID strings. Of course this workaround may break the
+    next time Outlook is changed.
+
+BBlloocckkiinngg bbaacckkssccaatttteerr mmaaiill wwiitthh ffoorrggeedd sseennddeerr iinnffoorrmmaattiioonn
+
+Like many people I still have a few email addresses in domains that I used in
+the past. Mail for those addresses is forwarded to my current address. Most of
+the backscatter mail that I get claims to be sent from these addresses. Such
+mail is obviously forged and is very easy to stop.
+
+    /etc/postfix/main.cf:
+        header_checks = pcre:/etc/postfix/header_checks
+        body_checks = pcre:/etc/postfix/body_checks
+
+    /etc/postfix/header_checks:
+        /^(From|Return-Path):.*\b(user@domain\.tld)\b/
+            reject forged sender address in $1: header: $2
+
+    /etc/postfix/body_checks:
+        /^[> ]*(From|Return-Path):.*\b(user@domain\.tld)\b/
+            reject forged sender address in $1: header: $2
+
+Notes:
+
+  * The example uses pcre: tables mainly for speed; with minor modifications,
+    you can use regexp: tables as explained below.
+
+  * The example is simplified for educational purposes. In reality, my patterns
+    list multiple email addresses as "(user1@domain1\.tld|user2@domain2\.tld)".
+
+  * The two "\b" as used in "\b(user@domain\.tld)\b" match the beginning and
+    end of a word, respectively. If you use regexp: tables, specify "[[:<:]]
+    and [[:>:]]" (on some systems you should specify "\< and \>" instead; for
+    details see your system documentation).
+
+  * The "\." matches "." literally. Without the "\", the "." would match any
+    character.
+
+BBlloocckkiinngg bbaacckkssccaatttteerr mmaaiill wwiitthh ootthheerr ffoorrggeedd iinnffoorrmmaattiioonn
+
+Another sign of forgery can be found in the IP address that is recorded in
+Received: headers next to your HELO host or domain name. This information must
+be used with care, though. Some mail servers are behind a network address
+translator and never see the true client IP address.
+
+BBlloocckkiinngg bbaacckkssccaatttteerr mmaaiill ffrroomm vviirruuss ssccaannnneerrss
+
+With all the easily recognizable forgeries eliminated, there is one category of
+backscatter mail that remains, and that is notifications from virus scanner
+software. Unfortunately, some virus scanning software doesn't know that viruses
+forge sender addresses. To make matters worse, the software also doesn't know
+how to report a mail delivery problem, so that we cannot use the above
+techniques to recognize forgeries.
+
+Recognizing virus scanner mail is an error prone process, because there is a
+lot of variation in report formats. The following is only a small example of
+message header patterns. For a large collection of header and body patterns
+that recognize virus notification email, see http://www.dkuug.dk/keld/virus/ or
+http://www.t29.dk/antiantivirus.txt.
+
+    /etc/postfix/header_checks:
+        /^Subject: *Your email contains VIRUSES/ DISCARD virus notification
+        /^Content-Disposition:.*VIRUS1_DETECTED_AND_REMOVED/
+            DISCARD virus notification
+        /^Content-Disposition:.*VirusWarning.txt/ DISCARD virus notification
+
+Note: these documents haven't been updated since 2004, so they are useful only
+as a starting point.
+
+A plea to virus or spam scanner operators: please do not make the problem worse
+by sending return mail to forged sender addresses. You're only harassing
+innocent people. If you must return mail to the purported sender, please return
+the full message headers, so that the sender can filter out the obvious
+forgeries.
+