Relay device, unauthorized frame detection method, and in-vehicle device

An unauthorized frame detection method for detecting an unauthorized frame by a relay device that, in an in-vehicle network to which a plurality of in-vehicle devices are connected, relays frames among the plurality of in-vehicle devices, the unauthorized frame detection method comprising:

• a step of, in a case where an unauthorized frame is detected by an in-vehicle device among the plurality of in-vehicle devices, receiving a request to start detecting an unauthorized frame from the in-vehicle device;
• and
• a step of, in a case where the request to start is received in a stopped state in which detection processing that detects an unauthorized frame among frames transmitted over the in-vehicle network is stopped, transitioning from the stopped state to an executing state in which the detection processing is executed.

An in-vehicle device connected to an in-vehicle network, the in-vehicle device comprising:

• a detection unit configured to detect an unauthorized frame by determining, in a case where a frame transmitted over the in-vehicle network is received, whether the frame received is an unauthorized frame;
• and
• a transmission unit configured to, in a case where the unauthorized frame is detected by the detection unit, transmits a request to start detecting unauthorized frames to a relay device that relays frames among a plurality of in-vehicle devices in the in-vehicle network.