Can't answer in detail, but if there's no major differences between 3.7 and 4.2, the place to start looking is MACNET_process_rx_bds() in macnet_receive.c. Code in that function examines ethernet packets, determines if RTCS wants the packet [ ENET_find_receiver() ] and passes them into the stack if so. I'd guess you could graft your code somewhere in there.