This is a set of related programs, usually located at a network gateway server, that protects the resources of a private network from other networks.

A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It acts as a barrier between a trusted internal network and untrusted external networks like the Internet. A sandbox isolates programs, a rootkit hides malicious access, and password crackers are attack tools - none fit the definition.