To protect against the scan of ports
PortSentry has the ability to detect portscans(including stealth scans) on the network interfaces of your machine. Upon alarm it can block the attacker via hosts.deny, dropped route or firewall rule. It is part of the Abacus program suite.
Note: If you have no idea what a port/stealth scan is, It's recommended to have a look at http://sf.net/projects/sentrytools/ before installing this package. Otherwise you might easily block hosts you'd better not (e.g. your NFS-server, name-server, etc.).
portsentry added to $PATH
config files within /opt/PortSentry