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>Spam Filtering for Mail Exchangers: </TH
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>Appendix A. Exim Implementation</TD
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><DIV
CLASS="section"
><H1
CLASS="section"
><A
NAME="exim-sa"
></A
>A.10. Adding SpamAssassin</H1
><P
>&#13; Invoking SpamAssassin at SMTP-time is commonly done in either
of two ways in Exim:
</P
><P
></P
><UL
><LI
><P
>&#13; Via the <TT
CLASS="option"
>spam</TT
> condition offered by
<TT
CLASS="option"
>Exiscan-ACL</TT
>. This is the mechanism we
will cover here.
</P
></LI
><LI
><P
>&#13; Via <TT
CLASS="option"
>SA-Exim</TT
>, another utility written by
Marc Merlins (<TT
CLASS="email"
>&#60;<A
HREF="mailto:marc (at) merlins.org"
>marc (at) merlins.org</A
>&#62;</TT
>),
specifically for running SpamAssassin at SMTP time in Exim.
This program operates through Exim's
<TT
CLASS="option"
>local_scan()</TT
> interface, either patched
directly into the Exim source code, or via Marc's own
<TT
CLASS="option"
>dlopen()</TT
> plugin (which, by the way, is
included in Debian's <TT
CLASS="option"
>exim4-daemon-light</TT
>
and <TT
CLASS="option"
>exim4-daemon-heavy</TT
> packages).
</P
><P
>&#13; <TT
CLASS="option"
>SA-Exim</TT
> offers some other features as
well, namely <EM
>greylisting</EM
> and
<EM
>teergrubing</EM
>. However, because the
scan happens after the message data has been received,
neither of these two features may be as useful as they
would be earlier in the SMTP transaction.
</P
><P
>&#13; <TT
CLASS="option"
>SA-Exim</TT
> can be found at:
<A
HREF="http://marc.merlins.org/linux/exim/sa.html"
TARGET="_top"
>http://marc.merlins.org/linux/exim/sa.html</A
>.
</P
></LI
></UL
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="exim-sa-exiscan"
></A
>A.10.1. Invoke SpamAssassin via Exiscan</H2
><P
>&#13; <TT
CLASS="option"
>Exiscan-ACL</TT
>'s
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="option"
>spam</TT
>"</SPAN
> condition passes the
message through either SpamAssassin or Brightmail, and
triggers if these indicate that the message is junk. By
default, it connects to a SpamAssassin daemon
(<TT
CLASS="option"
>spamd</TT
>) running on
<TT
CLASS="option"
>localhost</TT
>. The host address and port can be
changed by adding a <TT
CLASS="option"
>spamd_address</TT
> setting in
the <EM
>main</EM
> section of the Exim
configuration file. For more information, see the
<TT
CLASS="option"
>exiscan-acl-spect.txt</TT
> file included with the
patch.
</P
><P
>&#13; In our implementation, we are going to reject messages
classified as spam. However, we would like to keep a copy of
such messages in a separate mail folder, at least for the time
being. This is so that the user can periodically scan for
<A
HREF="gloss.html#falsepos"
><I
CLASS="glossterm"
>False Positive</I
></A
>s.
</P
><P
>&#13; Exim offers <EM
>controls</EM
> that can be applied
to a message that is accepted, such as
<TT
CLASS="option"
>freeze</TT
>. The Exiscan-ACL patch adds one more
of these controls, namely <TT
CLASS="option"
>fakereject</TT
>.
This causes the following SMTP response:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;550-FAKEREJECT id=<TT
CLASS="parameter"
><I
>message-id</I
></TT
>
550-Your message has been rejected but is being kept for evaluation.
550 If it was a legit message, it may still be delivered to the target recipient(s).
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
>&#13; We can incorporate this feature into our implementation, by
inserting the following snippet in <A
HREF="exim-firstpass.html#acl_data_1"
>acl_data</A
>, prior to the final
<TT
CLASS="option"
>accept</TT
> statement:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13; # Invoke SpamAssassin to obtain $spam_score and $spam_report.
# Depending on the classification, $acl_m9 is set to "ham" or "spam".
#
# If the message is classified as spam, pretend to reject it.
#
warn
set acl_m9 = ham
spam = mail
set acl_m9 = spam
control = fakereject
logwrite = :reject: Rejected spam (score $spam_score): $spam_report
# Add an appropriate X-Spam-Status: header to the message.
#
warn
message = X-Spam-Status: \
${if eq {$acl_m9}{spam}{Yes}{No}} (score $spam_score)\
${if def:spam_report {: $spam_report}}
logwrite = :main: Classified as $acl_m9 (score $spam_score)
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
>&#13; In this example, <TT
CLASS="varname"
>$acl_m9</TT
> is initially set to
<SPAN
CLASS="QUOTE"
>"ham"</SPAN
>. Then SpamAssassin is invoked as the user
<TT
CLASS="option"
>mail</TT
>. If the message is classified as spam,
then <TT
CLASS="varname"
>$acl_m9</TT
> is set to <SPAN
CLASS="QUOTE"
>"spam"</SPAN
>,
and the <TT
CLASS="option"
>FAKEREJECT</TT
> response above is issued.
Finally, an <TT
CLASS="option"
>X-Spam-Status:</TT
> header is added to
the message. The idea is that the <A
HREF="gloss.html#mda"
><I
CLASS="glossterm"
>Mail Delivery Agent</I
></A
> or
the recipient's <A
HREF="gloss.html#mua"
><I
CLASS="glossterm"
>Mail User Agent</I
></A
> can use this header to
filter junk mail into a separate folder.
</P
></DIV
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="exim-sa-config"
></A
>A.10.2. Configure SpamAssassin</H2
><P
>&#13; By default, SpamAssassin presents its report in a verbose,
table-like format, mainly suitable for inclusion in or
attachment to the message body. In our case, we want a terse
report, suitable for the <TT
CLASS="option"
>X-Spam-Status:</TT
>
header in the example above. To do this, we add the following
snippet in its site specific configuration file
(<TT
CLASS="option"
>/etc/spamassassin/local.cf</TT
>,
<TT
CLASS="option"
>/etc/mail/spamassassin/local.cf</TT
>, or similar):
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;### Report template
clear_report_template
report "_TESTSSCORES(, )_"
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
>&#13; Also, a <A
HREF="gloss.html#bayesian"
>Bayesian</A
> scoring
feature is built in, and is turned on by default. We normally
want to turn this off, because it requires training that will
be specific to each user, and thus is not suitable for
system-wide SMTP time filtering:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;### Disable Bayesian scoring
use_bayes 0
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
>&#13; For these changes to take effect, you have to restart the
SpamAssassin daemon (<B
CLASS="command"
>spamd</B
>).
</P
></DIV
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="exim-per-user"
></A
>A.10.3. User Settings and Data</H2
><P
>&#13; Say you have a number of users that want to specify their
individual SpamAssassin preferences, such as the spam
threshold, acceptable languages and character sets,
white/blacklisted senders, and so on. Or perhaps they really
want to be able to make use of SpamAssassin's native Bayesian
scoring (though I don't see why<A
NAME="AEN1828"
HREF="#FTN.AEN1828"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>).
</P
><P
>&#13; As discussed in the <A
HREF="usersettings.html"
>User Settings and Data</A
> section
earlier in the document, there is a way for this to happen.
We need to limit the number of recipients we accept per
incoming mail delivery to one. We accept the first
<B
CLASS="command"
>RCPT TO:</B
> command issued by the caller, then
defer subsequent ones using a <B
CLASS="command"
>451</B
> SMTP
response. As with <A
HREF="exim-greylisting.html"
>greylisting</A
>, if the caller
is a well-behaved MTA it will know how to interpret this
response, and retry later.
</P
><DIV
CLASS="section"
><H3
CLASS="section"
><A
NAME="exim-limit-one-user"
></A
>A.10.3.1. Tell Exim to accept only one recipient per delivery</H3
><P
>&#13; In the <A
HREF="exim-final.html#acl_rcpt_to_final"
>acl_rcpt_to</A
>, we insert the
following statement after validating the recipient address,
but before any <TT
CLASS="option"
>accept</TT
> statements pertaining
to unauthenticated deliveries from remote hosts to local
users (i.e. before any greylist checks, envelope signature
checks, etc):
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13; # Limit the number of recipients in each incoming message to one
# to support per-user settings and data (e.g. for SpamAssassin).
#
# NOTE: Every mail sent to several users at your site will be
# delayed for 30 minutes or more per recipient. This
# significantly slow down the pace of discussion threads
# involving several internal and external parties.
#
defer
message = We only accept one recipient at a time - please try later.
condition = $recipients_count
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
></DIV
><DIV
CLASS="section"
><H3
CLASS="section"
><A
NAME="exim-sa-as-user"
></A
>A.10.3.2. Pass the recipient username to SpamAssassin</H3
><P
>&#13; In <A
HREF="exim-final.html#acl_data_final"
>acl_data</A
>, we modify the
<TT
CLASS="option"
>spam</TT
> condition given in the previous
section, so that it passes on to SpamAssassin the username
specified in the local part of the recipient address.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13; # Invoke SpamAssassin to obtain $spam_score and $spam_report.
# Depending on the classification, $acl_m9 is set to "ham" or "spam".
#
# We pass on the username specified in the recipient address,
# i.e. the portion before any '=' or '@' character, converted
# to lowercase. Multiple recipients should not occur, since
# we previously limited delivery to one recipient at a time.
#
# If the message is classified as spam, pretend to reject it.
#
warn
set acl_m9 = ham
spam = ${lc:${extract{1}{=@}{$recipients}{$value}{mail}}}
set acl_m9 = spam
control = fakereject
logwrite = :reject: Rejected spam (score $spam_score): $spam_report
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
>&#13; Note that instead of using Exim's
<TT
CLASS="option"
>${local_part:...}</TT
> function to get the
username, we manually extracted the portion before any
<SPAN
CLASS="QUOTE"
>"@"</SPAN
> or <SPAN
CLASS="QUOTE"
>"="</SPAN
> character. This is
because we will use the latter character in our <A
HREF="exim-sign.html"
>envelope signature</A
> scheme, to
follow.
</P
></DIV
><DIV
CLASS="section"
><H3
CLASS="section"
><A
NAME="exim-per-user-sa"
></A
>A.10.3.3. Enable per-user settings in SpamAssassin</H3
><P
>&#13; Let us now again look at SpamAssassin. First of all, you
may choose to remove the <TT
CLASS="option"
>use_bayes 0</TT
>
setting that we previously added in its site-wide
configuration file. In any case, each user will now have
the ability to decide whether to override this setting for
themselves.
</P
><P
>&#13; If mailboxes on your system map directly to local UNIX
accounts with home directories, you are done. By default,
the SpamAssassin daemon (<B
CLASS="command"
>spamd</B
>) performs
a <TT
CLASS="option"
>setuid()</TT
> to the username we pass to it,
and stores user data and settings in that user's home
directory.
</P
><P
>&#13; If this is not the case (for instance, if your mail accounts
are managed by Cyrus SASL or by another server), you need to
tell SpamAssassin where to find each user's preferences and
data files. Also, <B
CLASS="command"
>spamd</B
> needs to keep
running as a specific local user instead of attempting to
<TT
CLASS="option"
>setuid()</TT
> to a non-existing user.
</P
><P
>&#13; We do these things by specifying the options passed to
<B
CLASS="command"
>spamd</B
> at startup:
</P
><P
></P
><UL
><LI
><P
>&#13; On a Debian system, edit the <TT
CLASS="option"
>OPTIONS=</TT
>
setting in <TT
CLASS="option"
>/etc/default/spamassassin</TT
>.
</P
></LI
><LI
><P
>&#13; On a RedHat system, edit the
<TT
CLASS="option"
>SPAMDOPTIONS=</TT
> setting in
<TT
CLASS="option"
>/etc/sysconfig/spamassassin</TT
>.
</P
></LI
><LI
><P
>&#13; Others, figure it out.
</P
></LI
></UL
><P
>&#13; The options you need are:
</P
><P
></P
><UL
><LI
><P
>&#13; <TT
CLASS="option"
>-u</TT
> <TT
CLASS="parameter"
><I
>username</I
></TT
> -
specify the user under which <B
CLASS="command"
>spamd</B
>
will run (e.g. <TT
CLASS="option"
>mail</TT
>)
</P
></LI
><LI
><P
>&#13; <TT
CLASS="option"
>-x</TT
> - disable configuration files in
user's home directory.
</P
></LI
><LI
><P
>&#13; <TT
CLASS="option"
>--virtual-config-dir=/var/lib/spamassassin/%u</TT
>
- specify where per-user settings and data are stored.
<SPAN
CLASS="QUOTE"
>"%u"</SPAN
> is replaced with the calling username.
<B
CLASS="command"
>spamd</B
> must be able to create or
modify this directory:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;# mkdir /var/lib/spamassassin
# chown -R mail:mail /var/lib/spamassassin
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
></LI
></UL
><P
>&#13; Needless to say, after making these changes, you need to
restart <B
CLASS="command"
>spamd</B
>.
</P
></DIV
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN1828"
HREF="exim-sa.html#AEN1828"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>&#13; Although it is true that Bayesian training is specific to
each user, it should be noted that SpamAssassin's
Bayesian classifier is, IMHO, not that stellar in any case.
Especially I find this to be the case since spammers have
learned to defeat such systems by seeding random dictionary
words or stories in their mail (e.g. in the metadata of
HTML messages).
</P
></TD
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