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.