Detecting Port Forwarding: Can A Know It’s Communicating with C?
No, it’s not possible for machine A to know if machine B is forwarding packets to machine C.
Once the packets reach machine B, machine A loses visibility on them. Machine B will send the packet data to machine C, not the packets themselves. When a packet reaches machine B from machine C, machine B will read the packet, extract its data, and send the data to machine A.