Flexible network security is achieved while permitting a trusted process. Port monitoring is used for extracting information about a server port being used through a network communication program. Internal permitted program storage is used for extracting information about a program for which communication is permitted by the firewall. The system determines whether a destination port of a packet of inbound traffic has been registered in the internal permitted port storage, and if the destination port has not been registered, transmitting the corresponding packet to the firewall, and if the destination port has been registered, allowing the corresponding packet to as inbound traffic as a transmission to a permitted port as a hooked original function of a permitted trusted process.