Among others, especially if the machine is online on an external network, it is better to be interested in :

 -> Iptables : the firewall

 -> denyhosts : which blocks unwanted IP addresses

 -> portsentry : which protects from forcing to connection

 -> fai2ban : like portsentry

 -> chrootkit : Against malware that wants to take the place of the administrator

 -> clamav : the antivirus

 -> kali : which makes it possible to test the resistance to attacks of a machine,

and surely other weapons of defense...