A rootkit is a set of binaries, that overwrite the server binaries and are made to hide the presence and actions of the hacker.

If you notice any suspicious or out of the ordinary activity you will want to run a check for rootkits, and also check for infected binaries.

A great tool for this is RootKit Hunter:
http://rootkit.nl/projects/rootkit_hunter.html

This will check for over 50 rootkits.

You can install that as follows:

Code:
wget http://internap.dl.sourceforge.net/sourceforge/rkhunter/rkhunter-1.3.2.tar.gz
tar -xzvf rkhunter-1.3.2.tar.gz
cd rkhunter-1.3.2
./installer.sh --layout /usr/local --install

Now to run it type:

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