Communication system, relay device, and diagnosis target device

A relay device comprising a port configured to connect a diagnostic tool, wherein the relay device is configured to

• determine whether specific data whose source appears to be the diagnostic tool is legitimate data from the diagnostic tool when the relay device has received, from a diagnosis target device to be diagnosed by the diagnostic tool, an inquiry signal for inquiring about a monitoring result for the specific data, and
• send the monitoring result including a determination result to outside via a communication bus different from the port.

A diagnosis target device configured to

• connect via a bus to a relay device including a port configured to connect a diagnostic tool in order for the diagnosis target device to be diagnosed by the diagnostic tool, and
• send an inquiry signal for inquiring of the relay device about a monitoring result for specific data whose source appears to be the diagnostic tool when the diagnosis target device has received the specific data.