How to force client in UDP to open port when sending with sendto

Question

I have simple server and client in UDP (WinSocks/C++).

I send datagram client -> server via sendto, and reply from server to client using the ip and port obtained from recvfrom function.

I found out that:

Every sendto from client is being sent from different port
When trying to reply from server Windows returns WSAECONNRESET (which mean that port is closed - http://support.microsoft.com/kb/263823)

How can I properly answer client from server (ie force port binding on client when sending using sendto?)

Edit: Adding some source code:

bool InitClient()
{
    internal->sock = socket(PF_INET, SOCK_DGRAM, 0);
    char8 yes = 1;
    setsockopt(internal->sock, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int32));
    return internal->sock != -1;
}
void Send(const IpAddress & target, const uint16 port, const char8 * data, int32 size )
{
    sockaddr_in trgt;
    memset(&trgt, 0, sizeof(trgt));
    trgt.sin_family = AF_INET;
    trgt.sin_port = htons(port);
    trgt.sin_addr.s_addr = target.GetRaw();

    if(sendto(internal->sock, (const char8 *)data, size, 0, (PSOCKADDR)&trgt, sizeof(trgt)) == SOCKET_ERROR)
    {
        LOG("Network sending error: %d", WSAGetLastError());
    }
}

msdn.microsoft.com/en-us/library/windows/desktop/… <- "Note: If a socket is opened, a setsockopt call is made, and then a sendto call is made, Windows Sockets performs an implicit bind function call." And this: cboard.cprogramming.com/networking-device-communication/… — PiotrK
If there had been an implicit bind call made, then I would expect each new use of sendto to re-use the previous port... — sarnold

selbie selbie · Accepted Answer · 2011-12-15T22:39:58

Call the "bind" function to specify a local port to send from. Example of using port 4567 below. Make sure to check the return value from bind.Call this code after you create the socket.

sockaddr_in local = {};
local.family = AF_INET;
local.port = htons(4567);
local.addr = INADDR_ANY;

bind(internal->sock,(sockaddr*)&local, sizeof(local));

If you bind to port zero instead of 4567 then the os will pick a random port for you and use it for all subsequent send and receives. You can call getsockname to discover which port the os picked for you after calling bind.

How to force client in UDP to open port when sending with sendto

1 Answers