lwip does not accept a server request

I am trying to write a modbus slave based on the enet_lwip example and echo server example from lwip website using the Raw API. When a modbus master was connected, connection was not established. When debugged, the modus_init() method was successfully completed and tcp pcb bound. The problem seems to be that the tcp acept method is not called. The lwip docs says that ethernetif() method must be called from the main program, but the enet_lwip does not call such a method. How  do I get around with this problem?