Automating Linux Anti-Virus Using ClamAV and Cron

Thankfully Linux isn’t a platform which has a significant problem with Viruses, however it is always better to be safe than sorry. Luckily ClamAV is an excellent free anti-virus solution for Linux servers. However, at least on RedHat Enterprise 5 (RHEL5) the default install doesn’t offer any automated scanning and alerting. So here is what I’ve done:

The following steps assume you are using RHEL5, but should apply to other Linux distributions as well.

First, you’ll want to install ClamAV:

[bash light=”true”] yum install clamav clamav-db clamd
/etc/init.d/clamd start[/bash]

On RHEL5 at least this automatically sets up a daily cron job that uses freshclam to update the virus definitions, so that’s good.

Next I recommend removing the test virus files, although you can save this until after you test the rest of the setup:

[bash light=”true”] rm -rf /usr/share/doc/clamav-0.95.3/test/[/bash]

Now we want to setup our automation. I have a daily cron job that scans the entire server which can take several minutes, and then an hourly cron job that only scans files which were created or modified within the last hour. This should provide rapid notification of any infection without bogging your server down for 5 minutes every hour. The hourly scans run in a couple of seconds.

Each scanning script then checks the scan logs to see if there were any infected files found, and if so immediately sends you a notification e-mail (you could set this address to your mobile phone’s SMS account if you wanted).

The Daily Scan:

[bash light=”true”] emacs /etc/cron.daily/clamscan_daily[/bash]

Paste in:

[bash] #!/bin/bash

# email subject
SUBJECT="VIRUS DETECTED ON `hostname`!!!"
# Email To ?
EMAIL="[email protected]"
# Log location
LOG=/var/log/clamav/scan.log

check_scan () {

# Check the last set of results. If there are any "Infected" counts that aren’t zero, we have a problem.
if [ `tail -n 12 ${LOG} | grep Infected | grep -v 0 | wc -l` != 0 ] then
EMAILMESSAGE=`mktemp /tmp/virus-alert.XXXXX`
echo "To: ${EMAIL}" >> ${EMAILMESSAGE}
echo "From: [email protected]" >> ${EMAILMESSAGE}
echo "Subject: ${SUBJECT}" >> ${EMAILMESSAGE}
echo "Importance: High" >> ${EMAILMESSAGE}
echo "X-Priority: 1" >> ${EMAILMESSAGE}
echo "`tail -n 50 ${LOG}`" >> ${EMAILMESSAGE}
sendmail -t < ${EMAILMESSAGE}
fi

}

clamscan -r / –exclude-dir=/sys/ –quiet –infected –log=${LOG}

check_scan[/bash]

[bash light=”true”] chmod +x /etc/cron.daily/clamscan_daily[/bash]

The Hourly Scan:

[bash light=”true”] emacs /etc/cron.hourly/clamscan_hourly[/bash]

Paste in:

[bash] #!/bin/bash

# email subject
SUBJECT="VIRUS DETECTED ON `hostname`!!!"
# Email To ?
EMAIL="[email protected]"
# Log location
LOG=/var/log/clamav/scan.log

check_scan () {

# Check the last set of results. If there are any "Infected" counts that aren’t zero, we have a problem.
if [ `tail -n 12 ${LOG} | grep Infected | grep -v 0 | wc -l` != 0 ] then
EMAILMESSAGE=`mktemp /tmp/virus-alert.XXXXX`
echo "To: ${EMAIL}" >> ${EMAILMESSAGE}
echo "From: [email protected]" >> ${EMAILMESSAGE}
echo "Subject: ${SUBJECT}" >> ${EMAILMESSAGE}
echo "Importance: High" >> ${EMAILMESSAGE}
echo "X-Priority: 1" >> ${EMAILMESSAGE}
echo "`tail -n 50 ${LOG}`" >> ${EMAILMESSAGE}
sendmail -t < ${EMAILMESSAGE}
fi

}

find / -not -wholename ‘/sys/*’ -and -not -wholename ‘/proc/*’ -mmin -61 -type f -print0 | xargs -0 -r clamscan –exclude-dir=/proc/ –exclude-dir=/sys/ –quiet –infected –log=${LOG}
check_scan

find / -not -wholename ‘/sys/*’ -and -not -wholename ‘/proc/*’ -cmin -61 -type f -print0 | xargs -0 -r clamscan –exclude-dir=/proc/ –exclude-dir=/sys/ –quiet –infected –log=${LOG}
check_scan

[/bash]
[bash light=”true”] chmod +x /etc/cron.hourly/clamscan_hourly[/bash]

Protected System

You should now have a well protected system with low impact to system performance and rapid alerting. Anti-Virus is only one piece of protecting a server, but hopefully this makes it easy to implement for everyone.