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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 16:18:56 +0000
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+<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN"
+ "http://www.w3.org/TR/html4/loose.dtd">
+
+<html>
+
+<head>
+
+<title>Postfix Backscatter Howto</title>
+
+<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+
+</head>
+
+<body>
+
+<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix
+Backscatter Howto</h1>
+
+<hr>
+
+<h2>Overview </h2>
+
+<p> This document describes features that require Postfix version
+2.0 or later. </p>
+
+<p> Topics covered in this document: </p>
+
+<ul>
+
+<li><a href="#wtf">What is backscatter mail?</a>
+
+<li><a href="#random">How do I block backscatter mail to random
+recipient addresses?</a>
+
+<li><a href="#real">How do I block backscatter mail to real
+recipient addresses?</a>
+
+<ul>
+
+<li><a href="#forged_helo">Blocking backscatter mail with forged
+mail server information</a>
+
+<li><a href="#forged_sender">Blocking backscatter mail with forged
+sender information</a>
+
+<li><a href="#forged_other">Blocking backscatter mail with other
+forged information</a>
+
+<li><a href="#scanner">Blocking backscatter mail from virus
+scanners</a>
+
+</ul>
+
+</ul>
+
+<p> 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.</p>
+
+<h2><a name="wtf">What is backscatter mail?</a></h2>
+
+<p> 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:
+</p>
+
+<blockquote>
+<pre>
+Dec 4 04:30:09 hostname postfix/smtpd[58549]: NOQUEUE: reject:
+RCPT from xxxxxxx[x.x.x.x]: 550 5.1.1 &lt;yyyyyy@your.domain.here&gt;:
+Recipient address rejected: User unknown; from=&lt;&gt;
+to=&lt;yyyyyy@your.domain.here&gt; proto=ESMTP helo=&lt;zzzzzz&gt;
+</pre>
+</blockquote>
+
+<p> What you see are lots of "user unknown" errors with "from=&lt;&gt;".
+These are error reports from MAILER-DAEMONs elsewhere on the Internet,
+about email that was sent with a false sender address in your domain.
+</p>
+
+<h2><a name="random">How do I block backscatter mail to random
+recipient addresses?</a></h2>
+
+<p> 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. </p>
+
+<p> 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. </p>
+
+<blockquote>
+<pre>
+/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
+</pre>
+</blockquote>
+
+<h2><a name="real">How do I block backscatter mail to real
+recipient addresses?</a></h2>
+
+<p> 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.
+</p>
+
+<h3><a name="forged_helo">Blocking backscatter mail with forged
+mail server information</a></h3>
+
+<p> 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: </p>
+
+<blockquote>
+<pre>
+Received: from porcupine.org ...
+</pre>
+</blockquote>
+
+<p> Then I know that this is almost certainly forged mail (almost;
+see <a href="#caveats">next section</a> for the fly in the ointment).
+Mail that is really
+sent by my systems looks like this: </p>
+
+<blockquote>
+<pre>
+Received: from hostname.porcupine.org ...
+</pre>
+</blockquote>
+
+<p> For the same reason the following message headers are very likely
+to be the result of forgery:</p>
+
+<blockquote>
+<pre>
+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) ...
+</pre>
+</blockquote>
+
+<p> 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 <i>hostname</i>.porcupine.org,
+the following is definitely a forgery:</p>
+
+<blockquote>
+<pre>
+Received: by porcupine.org ...
+Received: from host.example.com ( ... ) by porcupine.org ...
+</pre>
+</blockquote>
+
+<p> Another frequent sign of forgery is the Message-ID: header. My
+systems produce a Message-ID: of
+&lt;<i>stuff</i>@<i>hostname</i>.porcupine.org&gt;. The following
+are forgeries, especially the first one:
+
+<blockquote>
+<pre>
+Message-ID: &lt;1cb479435d8eb9.2beb1.qmail@porcupine.org&gt;
+Message-ID: &lt;yulszqocfzsficvzzju@porcupine.org&gt;
+</pre>
+</blockquote>
+
+<p> To block such backscatter I use header_checks and body_checks
+patterns like this: </p>
+
+<blockquote>
+<pre>
+/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:.* &lt;!&amp;!/ 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 /^[&gt; ]*Received:/
+ /^[&gt; ]*Received: +from +(porcupine\.org) /
+ reject forged client name in Received: header: $1
+ /^[&gt; ]*Received: +from +[^ ]+ +\(([^ ]+ +[he]+lo=|[he]+lo +)(porcupine\.org)\)/
+ reject forged client name in Received: header: $2
+ /^[&gt; ]*Received:.* +by +(porcupine\.org)\b/
+ reject forged mail server name in Received: header: $1
+ endif
+ /^[&gt; ]*Message-ID:.* &lt;!&amp;!/ DUNNO
+ /^[&gt; ]*Message-ID:.*@(porcupine\.org)/
+ reject forged domain name in Message-ID: header: $1
+</pre>
+</blockquote>
+
+<p> Notes: </p>
+
+<ul>
+
+<li> <p> The example uses pcre: tables mainly for speed; with minor
+modifications, you can use regexp: tables as explained below. </p>
+
+<li> <p> The example is simplified for educational purposes. In
+reality my patterns list multiple domain names, as
+"<tt>(domain|domain|...)</tt>". </p>
+
+<li> <p> The "<tt>\.</tt>" matches "<tt>.</tt>" literally. Without
+the "<tt>\</tt>", the "<tt>.</tt>" would match any character. </p>
+
+<li> <p> The "<tt>\(</tt>" and "<tt>\)</tt>" match "<tt>(</tt>"
+and "<tt>)</tt>" literally. Without the "<tt>\</tt>", the "<tt>(</tt>"
+and "<tt>)</tt>" would be grouping operators. </p>
+
+<li> <p> The "<tt>\b</tt>" is used here to match the end of a word.
+If you use regexp: tables, specify "<tt>[[:&gt;:]]</tt>" (on some
+systems you should specify "<tt>\&gt;</tt>" instead; for details
+see your system documentation).
+
+<li> <p> The "if /pattern/" and "endif" eliminate unnecessary
+matching attempts. DO NOT indent lines starting with /pattern/
+between the "if" and "endif"! </p>
+
+<li> <p> The two "<tt>Message-ID:.* &lt;!&amp;!</tt>" rules are
+workarounds for some versions of Outlook express, as described in
+the <a href="#caveats"> caveats </a> section below.
+
+</ul>
+
+<p><a name="caveats"><strong>Caveats</strong></a></p>
+
+<ul>
+
+<li>
+
+<p> 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.
+</p>
+
+<p> 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.
+</p>
+
+<blockquote>
+<pre>
+/etc/postfix/main.cf:
+ canonical_maps = hash:/etc/postfix/canonical
+
+/etc/postfix/canonical:
+ @hostname.porcupine.org @porcupine.org
+</pre>
+</blockquote>
+
+<p> 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. </p>
+
+<p> An alternative would be to remove the hostname from
+"hostname.porcupine.org" with address
+masquerading, as described in the ADDRESS_REWRITING_README document.
+</p>
+
+<li> <p> 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"). </p>
+
+<p> When a DSN is requested, Outlook 2003 uses a Message-ID string
+that ends in the sender's domain name: </p>
+
+<blockquote>
+<pre>
+Message-ID: &lt;!&amp;! ...very long string... ==@example.com&gt;
+</pre>
+</blockquote>
+
+<p> where <i>example.com</i> 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 <i>username@example.com</i>,
+messages with DSN turned on will trigger the REJECT action in the
+previous section. </p>
+
+<p> If you have such clients then you can exclude their Message-ID
+strings with the two "<tt>Message-ID:.* &lt;!&amp;!</tt>" 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. </p>
+
+</ul>
+
+<h3><a name="forged_sender">Blocking backscatter mail with forged
+sender information</a></h3>
+
+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.
+
+<blockquote>
+<pre>
+/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:
+ /^[&gt; ]*(From|Return-Path):.*\b(user@domain\.tld)\b/
+ reject forged sender address in $1: header: $2
+</pre>
+</blockquote>
+
+<p> Notes: </p>
+
+<ul>
+
+<li> <p> The example uses pcre: tables mainly for speed; with minor
+modifications, you can use regexp: tables as explained below. </p>
+
+<li> <p> The example is simplified for educational purposes. In
+reality, my patterns list multiple email addresses as
+"<tt>(user1@domain1\.tld|user2@domain2\.tld)</tt>". </p>
+
+<li> <p> The two "<tt>\b</tt>" as used in "<tt>\b(user@domain\.tld)\b</tt>"
+match the beginning and end of a word, respectively. If you use
+regexp: tables, specify "<tt>[[:&lt;:]]</tt> and <tt>[[:&gt;:]]</tt>"
+(on some systems you should specify "<tt>\&lt;</tt> and <tt>\&gt;</tt>"
+instead; for details see your system documentation). </p>
+
+<li> <p> The "<tt>\.</tt>" matches "<tt>.</tt>" literally. Without
+the "<tt>\</tt>", the "<tt>.</tt>" would match any character. </p>
+
+</ul>
+
+<h3><a name="forged_other">Blocking backscatter mail with other
+forged information</a></h3>
+
+<p> 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. </p>
+
+<h3><a name="scanner">Blocking backscatter mail from virus
+scanners</a></h3>
+
+<p> 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. </p>
+
+<p> 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
+https://web.archive.org/web/20100317123907/http://std.dkuug.dk/keld/virus/
+or http://www.t29.dk/antiantivirus.txt. </p>
+
+<blockquote>
+<pre>
+/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
+</pre>
+</blockquote>
+
+<p> Note: these documents haven't been updated since 2004, so they
+are useful only as a starting point. </p>
+
+<p> 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. </p>
+
+</body>
+
+</html>