Communication device, system, and method

It is an object of the present invention to achieve improvement of security by a whitelist function and improvement of network reliability by a network redundancy function at the same time. A packet relay device 100 includes packet reception units 200, a packet transfer unit 300, a S/W control unit 400, packet transmission units 500, and an input/output interface 600 and automatically generates a whitelist including an allowed communication rule. It is possible to select whether to perform communication control using the whitelist or to carry out data communication without using the whitelist for each data reception unit 200 that receives data.