Question

I want to create a Winsock UDP socket that only sends data to a client. I want the kernel to choose an available port for me. On the other hand, I want to indicate which local IP to use, since I’m running a few nics.

I’ve tried combing through the maze of socket options, as well as binding with the port in the socket address set to 0 to no avail.

My code is in Win32 C++.

Answer 1

Please excuse the lack of error checking:

char pkt[...]; size_t pkt_length = ...; sockaddr_in dest; sockaddr_in local; WSAData data; WSAStartup( MAKEWORD( 2, 2 ), &data );  local.sin_family = AF_INET; local.sin_addr.s_addr = inet_addr( <source IP address> ); local.sin_port = 0; // choose any  dest.sin_family = AF_INET; dest.sin_addr.s_addr = inet_addr( <destination IP address> ); dest.sin_port = htons( <destination port number> );  // create the socket SOCKET s = socket( AF_INET, SOCK_DGRAM, IPPROTO_UDP ); // bind to the local address bind( s, (sockaddr *)&local, sizeof(local) ); // send the pkt int ret = sendto( s, pkt, pkt_length, 0, (sockaddr *)&dest, sizeof(dest) );