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ClamAV Virus Scanning

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:

yum install clamav clamav-db clamd
/etc/init.d/clamd start

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:

rm -rf /usr/share/doc/clamav-0.95.3/test/

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:

emacs /etc/cron.daily/clamscan_daily

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#!/bin/bash
 
# email subject
SUBJECT="VIRUS DETECTED ON `hostname`!!!"
# Email To ?
EMAIL="me@domain.com"
# 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: alert@domain.com" >>  ${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
chmod +x /etc/cron.daily/clamscan_daily

The Hourly Scan:

emacs /etc/cron.hourly/clamscan_hourly

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#!/bin/bash
 
# email subject
SUBJECT="VIRUS DETECTED ON `hostname`!!!"
# Email To ?
EMAIL="me@domain.com"
# 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: alert@domain.com" >>  ${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
chmod +x /etc/cron.hourly/clamscan_hourly

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.

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