Most of what arrives at my mail servers is unwanted: viruses, spam, and executable garbage. Even if you're running something other than Windows on the desktop, the sudden appearance of a new virus can overwhelm your inbox. If you're an administrator, your users likely aren't as reliable about not clicking on attachments as we'd all like. Combined with the flood of spam and random garbage, putting a mail server on the Internet without filtering is like covering yourself with barbecue sauce and breaking into the Charity Home for Badgers with Rabies. Decent spam and virus protection measures can save you a lot of time and effort.
While the systems I use for this article are FreeBSD, the tools and techniques will work on any UNIX-like operating system running modern Sendmail.
Many commercial vendors provide garbage mail protection for Sendmail, but if I go around buying software then I'm never going to be able to afford that 17" laptop. Instead, I use MIMEDefang for generic content handling, SpamAssassin to identify unwanted bulk mail, and ClamAV to reject viruses. This combination eliminates almost all unwanted mail, while letting the good stuff through.
Sendmail(8) provides a
milter (or mail filter) API for
third-party programs. This means that it's fairly straightforward to add
functionality to Sendmail with very little overhead. We can attach MIMEDefang
to Sendmail via
milter. MIMEDefang can call both SpamAssassin and ClamAV. To
make this work, however, you must install and configure the various programs in
the correct order.
Install ClamAV from /usr/ports/security/clamav by running
make install. Do not run
make clean yet! The
work subdirectory contains some sample viruses that we'll use to confirm that the program works correctly. The port will install the main virus scanner,
clamd(8), a command-line console
preliminary virus signature database, and an assortment of documentation and
ancillary programs. Under normal operation, the
scanner should be running at all times. Other programs can send files to
clamd(8) to learn if they are infected.
clamd from the command line. Though it requires
configuration, we'll be able to test it before doing our custom setup. While
you're still in the port directory, use the console command to scan the
subdirectory for viruses.
# clamscan -r -l testoutput.txt
-r recursively scans the current directory and the
clamscan to log the output to the text file
testoutput.txt. When you run this command,
will check every file under the current directory for viruses and print out its
status. A condensed version of the results, containing only the path to the
infected files and a list of statistics, will appear in the logfile. For
ClamAV 0.65, this test should discover five infected files.
Now, let's configure ClamAV to cooperate with MIMEDefang. The main
configuration file is /usr/local/etc/clamav.conf. Many of the
settings are tweakable as you desire. See the
page for all of the options. The most important change is that you should set
the user to mailnull, the same user that Sendmail and MIMEDefang
#User clamav User mailnull
By changing this setting, you also need to change the permissions on the various directories to which ClamAV writes.
# chown -R mailnull:mailnull /var/run/clamav/ # chown -R mailnull:mailnull /var/log/clamav/
Now that you have a basic virus scanner, you can update your virus
definitions. Virus definitions are maintained by volunteers from around the
world. When a major new virus hits, you can expect to see a definition
available within hours. As I write this, the ClamAV database has signatures
for 10,131 popular viruses. First, run
root to confirm that your software can successfully contact one of
the virus signature mirrors and download the latest definitions. You should
see the program check the freshness of main.cvd and
daily.cvd before returning to the command line.
Once you know the update process works, enable the
daemon to check for updates. There is one minor complication, however;
freshclam runs as clamav and
as mailnull, so by default you won't be able to write
freshclam reports in the
clamav log directory. While
you can muck around with file and directory ownership or change the user that
freshclam runs as, the simplest thing to do is put your
freshclam log elsewhere. When you're done, your
/etc/rc.conf should have the following new lines:
clamav_clamd_enable="YES" clamav_freshclam_enable="YES" clamav_freshclam_flags="--checks=1 --datadir=/usr/local/share/clamav \ --daemon-notify=/usr/local/etc/clamav.conf --log=/var/log/freshclam.log"
The next time you reboot, the system will start the ClamAV virus scanner and
updater. You might need to
touch /var/log/freshclam.log and
chown clamav:clamav /var/log/freshclam.log if
freshclam(8) has trouble starting.
By default, ClamAV puts its UNIX socket in
/var/run/clamav/clamd. MIMEDefang expects to find it in
/var/spool/MIMEDefang/clamd.sock. One or the other must change.
If you do not change the socket, you will see errors in the mail log where
MIMEDefang cannot communicate with the virus scanner. I consistently change
ClamAV to use the MIMEDefang location, simply because it doesn't make any
difference to the programs, but it's easier for me to remember. Change this in
the clamav.conf file under the
Once you change this in the configuration, however, ClamAV will not run until you've installed MIMEDefang. Despite advances in computing technology, placing sockets in nonexistent directories still presents difficulties.
SpamAssassin is perhaps the most celebrated piece of anti-spam software. It's a Perl package that uses pattern-matching to assign each piece of email a score. Key phrases, such as "Make money fast" and "Work from home," will increase the mail's score, as will bogus headers and an origination IP of a known spam source. You can set your mail client to delete or filter mail that has a score above your preferred limit.
Install SpamAssassin from /usr/ports/mail/p5-Mail-SpamAssassin.
As SpamAssassin is managed as part of MIMEDefang, we don't need to configure SpamAssassin itself; its configuration has been assimilated into MIMEDefang. Proceed directly to installing MIMEDefang.
MIMEDefang can add the common legalese found at the bottom of some email messages, provide very fine-grained access control, and alter emails in almost any other way desired. Our main interest is in using MIMEDefang to strip out unwanted attachment types and coordinate ClamAV and SpamAssassin activities. MIMEDefang's port lives under /usr/ports/mail/mimedefang. Like SpamAssassin, it's written in Perl. Like ClamAV, it runs in daemon mode, so that the system can avoid the massive overhead of starting up a Perl program for every email that passes through the system. When you install MIMEDefang, the configure script will automatically detect that you have both ClamAV and SpamAssassin and will build itself appropriately.
MIMEDefang's configuration file, /usr/local/etc/mimedefang/mimedefang-filter, contains fragments of Perl code that integrate with the main MIMEDefang daemon. If you are not comfortable with Perl, don't worry; just be certain that you implement new functions exactly as they are shown and you won't have any trouble. (I also highly recommend that you spend a few hours with an introductory Perl text; even though I'm a systems administrator and not a programmer, Perl is far too useful in my day-to-day work.)
You should set a few variables in
mimedefang, such as
$AdminName. If you have a separate
MIMEDefang administrator, you can enter a specific name and email address, but
otherwise, just use your network's generic mail management information. (For
those of you unfamiliar with Perl, the single and double quotation marks are
very important and must be as shown.)
$AdminAddress = 'firstname.lastname@example.org'; $AdminName = "Mail Administrator";
MIMEDefang will occasionally send an email about an action it has taken.
$DaemonAddress variable to the email address you want it
$DaemonAddress = 'email@example.com';
Everything else in this file is strictly optional. Take a look at some of the settings, however, as you might find them useful in your network. There are examples of blocking emails with too many MIME parts, as well as configuring where MIMEDefang will place its alerts. The default includes antispam and antivirus functions, as well as mail quarantine.
Personally, I find that MIMEDefang's mail quarantine functions are reliable enough that I'm comfortable simply rejecting viral emails. There's a comment in the mimedefang-filter file much like this:
# But quarantine the part for examination later. Comment # the next line out if you don't want to bother. action_quarantine($entity, "A known virus was discovered and deleted. \ Virus-scanner messages follow:\n$VirusScannerMessages\n\n");
By commenting out the line beginning with
MIMEDefang will make Sendmail reject incoming viruses as soon as possible.
Finally, the variable
$bad_exts contains a list of extensions
that MIMEDefang will block. Extend this list as needed. For example, if
some evil mastermind were to write a high-powered virus that propagated via
files with the .doc extension, you could block them by adding .doc to this
list and restarting MIMEDefang.
To make MIMEDefang start at boot time, go into /usr/local/etc/rc.d and copy mimedefang.sh-dist to mimedefang.sh. By default, this program does not start when the
system boots. You must have
clamd(8) and MIMEDefang running;
SpamAssassin is included as part of MIMEDefang, so you don't need
spamd(8) as you would for a standalone SpamAssassin
MIMEDefang includes a configuration file for its SpamAssassin calls: /usr/local/etc/mimedefang/spamassassin/sa-mimedefang.cf. If you want to make any changes to the global SpamAssassin settings, change this file. Editing other SpamAssassin configuration files will have no effect whatsoever on SpamAssassin. You can configure SpamAssassin in almost endless ways; we'll cover only the bare bones.
Adjust the sensitivity of the filter with the
variable. The default is 5. When a piece of email scores more than 5 points
on the spam-o-meter, SpamAssassin flags it as a piece of spam. Users can
choose to filter as they see fit.
Another important feature is the whitelist, which allows you to list email
addresses that should never be considered spam sources. For example, my sister
sends HTML-laden, image-heavy email from a known spam sewer. By listing her
email address on a
whitelist_from line, SpamAssassin will let it
pass, even if she forwards me a piece of pornographic spam and asks me where it
came from. Similarly,
blacklist_from variables allow me never to
see email from chosen addresses.
One surprise for users familiar with SpamAssassin is that MIMEDefang does not allow SpamAssassin to alter the email in any way. Instead, SpamAssassin reports a score back to MIMEDefang and lets MIMEDefang change the message. The MIMEDefang FAQ includes several suggestions for editing the filter configuration so as to display SpamAssassin information as desired.
By default, MIMEDefang displays a single header that contains the SpamAssassin score and a series of asterisks, much as the example below shows.
X-Spam-Score: 21.207 (*********************)
Many email clients have difficulty filtering on this header; their filtering rules will not let them compare the numerical score. By making one minor change in how MIMEDefang marks spam email, you can make life much easier for these users.
In /usr/local/etc/mimedefang/mimedefang-filter, you'll see a line like this:
action_change_header("X-Spam-Score", "$hits ($score) $names");
If you exchange the
$score and the
as shown below, your users will be able to filter on the number of asterisks
action_change_header("X-Spam-Score", "$score ($hits) $names");
Your email header will then look something like this:
X-Spam-Score: ************* (13.002)
You can write a rule that searches the X-Spam-Score header for a certain number of asterisks and filters those messages away. With this header setup, users can easily adjust their own spam tolerances; you can be harsh on spam detection on the server, knowing that all you're really doing is adding a header. If a user decides to set his sensitivity to two asterisks and loses some vital email, that's not your fault.
Now that you have a working MIMEDefang/SpamAssassin/ClamAV installation, how
do you tie all this into Sendmail's
milter interface? A modern Sendmail system
Makefiles that simplify creating a configuration file.
You need a custom Sendmail .mc file, so go into
/etc/mail and copy freebsd.mc to a file named after
the host. For example, on bewilderbeast.blackhelicopters.org, I
use a .mc file called bewilderbeast.mc. Next, enter
this filename in /etc/make.conf as the
Whenever you rebuild your system, this tells
make(1) to use
your custom .mc file instead of the default. This retains your
customizations, instead of making your spam protection fail after every
make installworld. (After an upgrade, be certain you compare the
newly updated freebsd.mc with your custom file, just in case
something important changes.) Now add the lines below to the end of your custom
MAIL_FILTER(`mimedefang', `S=local:/var/spool/MIMEDefang/mimedefang.sock, \ F=T, T=C:15m;S:4m;R:4m;E:10m')dnl define(`confINPUT_MAIL_FILTERS', `mimedefang')dnl
Stay in the /etc/mail directory and run
make all install
restart. This will rebuild your sendmail.cf file from your
customized configuration file and restart Sendmail with it.
tail -f /var/log/maillog, you will be able to see log
entries from MIMEDefang as it processes every message. Viruses will bounce at
the border, while spam will be conveniently flagged for your users. Even a
small mail server such as mine is hit with hundreds of viruses and thousands of
pieces of spam a day; now, it's all flagged or rejected. Just as much junk will travel
across the Internet, but you won't be so aware of it. Today, that's as good as
you're going to get.
Michael W. Lucas
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