c - When binding a client TCP socket to a specific local port with Winsock, SO_REUSEADDR does not have any effect

Question

I'm binding a client TCP socket to a specific local port. To handle the situation where the socket remains in TIME_WAIT state for some time, I use setsockopt() with SO_REUSEADDR on a socket.

It works on Linux, but does not work on Windows, I get WSAEADDRINUSE on connect() call when the previous connection is still in TIME_WAIT.

MSDN is not exactly clear what should happen with client sockets:

[...] For server applications that need to bind multiple sockets to the same port number, consider using setsockopt (SO_REUSEADDR). Client applications usually need not call bind at all—connect chooses an unused port automatically. [...]

How do I avoid this?

Answer 1

When you create a socket with socket(), it has only a type and a protocol family. The ideal is to bind() it to a local address:port too.

The error you mentioned normally happens when the last connection to the same host:port didn't have a graceful shutdown (FIN/ACK FIN/ACK). In these cases, the socket stays in TIME_WAIT state for a certain period of time (OS dependent, but adjustable).

What happens then is when you try to connect() to the same host and same port, it uses the default socket's name/address/port/etc, but this combination is already in use by your zombie socket. To avoid this, you can change the local address:port used to establish the connection by calling bind() after the socket creation, providing the sockaddr struct filled with your local address and a random port.

int main() {
    int ret, fd;
    struct sockaddr_in sa_dst;
    struct sockaddr_in sa_loc;
    char buffer[1024] = "GET / HTTP/1.1
Host: www.google.com

";

    fd = socket(AF_INET, SOCK_STREAM, 0);

    // Local
    memset(&sa_loc, 0, sizeof(struct sockaddr_in));
    sa_loc.sin_family = AF_INET;
    sa_loc.sin_port = htons(LOCAL_RANDOM_PORT);
    sa_loc.sin_addr.s_addr = inet_addr(LOCAL_IP_ADDRESS);

    ret = bind(fd, (struct sockaddr *)&sa_loc, sizeof(struct sockaddr));
    assert(ret != -1);

    // Remote
    memset(&sa_dst, 0, sizeof(struct sockaddr_in));
    sa_dst.sin_family = AF_INET;
    sa_dst.sin_port = htons(80);
    sa_dst.sin_addr.s_addr = inet_addr("64.233.163.104"); // google :)

    ret = connect(fd, (struct sockaddr *)&sa_dst, sizeof(struct sockaddr));
    assert(ret != -1);

    send(fd, buffer, strlen(buffer), 0);
    recv(fd, buffer, sizeof(buffer), 0);
    printf("%s
", buffer);
}

UPDATE: As using a specific local port is a requirement, consider setting SO_LINGER with l_onoff=1 and l_linger=0 so your socket won't block upon close/closesocket, it will just ignore queued data and (hopefully) close the fd. As a last resort you can adjust the TIME_WAIT delay by changing the value of this registry key (highly discouraged!):

HKLMSYSTEMCurrentControlSetServicesTcpipParametersTcpTimedWaitDelay