When a system is used as a server on a public network, it becomes a target for attacks. For this reason, hardening the system and locking down services is of paramount importance for the system administrator.
Before delving into specific issues, review the following general tips for enhancing server security:
Keep all services current, to protect against the latest threats.
Use secure protocols whenever possible.
Serve only one type of network service per machine whenever possible.
Monitor all servers carefully for suspicious activity.