Question

I am learning WinSock UDP. Most tutorials I’ve seen ask me to:

On server side:

SOCKET socked = socket(PF_INET, SOCK_DGRAM, 0)
bind(...);
for(;;) { recvfrom(..); }

On client side:

SOCKET socked = socket(PF_INET, SOCK_DGRAM, 0);
sendto(...);

But I wonder how can I do other way, send data from server to client. I don’t know client’s port (I figured out it vary every sendto call), and I want to be able to connect client behihd NAT to server and still recive data from server.

How it is done in modern games?

Answer 1

There are two questions in there.

Finding the address

The function recvfrom is declared:

int recvfrom(
  __in         SOCKET s,
  __out        char *buf,
  __in         int len,
  __in         int flags,
  __out        struct sockaddr *from, /* <--- */
  __inout_opt  int *fromlen           /* <--- */
);

So you can use from and fromlen in sendto.

sendto(..., (SOCKADDR *)from, *fromlen);

NAT

The NAT-box should automatically allow the server to “answer” to the datagram sent by the client. So, the mere fact that the client sent a datagram causes the NAT-box to ephemerally (a few minutes?) open a port to allow traffic return.