I have used the function netconn_gethostbyname in my program. I have set the #define LWIP_DNS 1 in lwipopts.h and I also have include the "lwip/api.h" before my main function. When I compile my program, it came cross an error message: undefined reference to `netconn_gethostbyname'. But some other function can be used successfully, such as netconn_connect.