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.
Although my email address is "[email protected]", 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) ...
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: <[email protected]>
Message-ID: <[email protected]>
To block such backscatter I use
header_checks and
body_checks
patterns like this:
/etc/postfix/
main.cf:
header_checks =
regexp:/etc/postfix/header_checks
body_checks =
regexp:/etc/postfix/body_checks
/etc/postfix/header_checks:
/^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
/^Message-ID:.*@(porcupine\.org)/
reject forged domain name in Message-ID: header: $1
/etc/postfix/body_checks:
/^[> ]*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
/^[> ]*Message-ID:.*@(porcupine\.org)/
reject forged domain name in Message-ID: header: $1
Notes:
-
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.
Caveats
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
[email protected]. On the Postfix server, a canonical
mapping translates this temporary address into [email protected].
/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.
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 =
regexp:/etc/postfix/header_checks
body_checks =
regexp:/etc/postfix/body_checks
/etc/postfix/header_checks:
/^(From|Return-Path):.*[[:<:]](user@domain\.tld)[[:>:]]/
reject forged sender address in $1: header: $2
/etc/postfix/body_checks:
/^[> ]*(From|Return-Path):.*[[:<:]](user@domain\.tld)[[:>:]]/
reject forged sender address in $1: header: $2
Notes:
-
The example is simplified for educational purposes. In
reality, my patterns list multiple email addresses as
"(user1@domain1\.tld|user2@domain2\.tld)".
-
The "[[:<:]]" and "[[:>:]]" match
the beginning and end of a word, respectively. On some systems you
should specify "\<" and "\>" instead. For
details see your system documentation.
-
The "\." matches "." literally. Without
the "\", the "." would match any character.
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.
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
https://www.dkuug.dk/keld/virus/
or
https://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.
