At an SDN controller executing using a processor and a memory, a tracing packet is configured with a default value in a tag field. The tracing packet is inserted from the SDN controller into the SDN at a switch in the SDN. A returned packet and a port identifier is received at the controller, from the switch, the returned packet including a modified content in a location of the tracing packet that is different from the tag field. The port identifier corresponds to a port of the switch on which the switch received the returned packet from an middlebox. A function of the middlebox is identified by analyzing a modification applied to the modified content by the middlebox. The function of the middlebox and a location of the middlebox in the SDN are saved. The location includes the port identifier and an identifier of the switch.