What is a rootkit? The following link is a very good read to answer that question.
In Summary, a rootkit is a trojan installed on your Linux server after someone has broken into it. These files are used to cover the hackers tracks, and to give the hacker tools to do more dirty work from your server.
1. su - (change to root user)
2. mkdir /usr/local/chkrootkit
3. wget ftp://ftp.pangeia.com.br/pub/seg/pac/chkrootkit.tar.gz
4. tar -xvzf chkrootkit.tar.gz
5. cd chkrootkit*
6. cp * /usr/local/chkrootkit
7. cd /usr/local/chkrootkit
8. make sense
Now scan your system:
1. cd /usr/local/chkrootkit
chkrootkit may from time to time give false positives. If you ever get a positive or "infected hit" scan a second time. If you do get a positive hit, google the hit to research the issue and steps to correct.
Part 2 - automated chkrootkit, and emailed results.
I'm lazy, and like my server to do the work for me so I have it scan every day, and email me the results.
1. vi /etc/cron.daily/chkrootkit
2. add the following code.
(cd /usr/local/chkrootkit; ./chkrootkit -q 2>&1 | mail -s "Daily chkrootkt scan" email@example.com)
3. chmod 0755 /etc/cron.daily/chkrootkit
This will email firstname.lastname@example.org
every morning with your chkrootkit results. the -q option will only show you exploits.
If you don't like getting the emails or just want to remove this from your server:
1. rm /etc/cron.daily/chkrootkit
2. rm -rf /usr/local/chkrootkit
All files will now be deleted from your server.