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Does the Windows TCP connector support SO_KEEPALIVE by default?

I encountered a strange error with TCP sockets. It seems to be SO_KEEPALIVEenabled by default on all sockets.

I wrote a short test case for creating a socket and connecting to a server. Immediately after connecting, I check SO_KEEPALIVEfor getsockopt. A value other than zero, which, according to MSDN, means that support is supported. Perhaps I do not understand this.

I recently had a strange error when the server shutdown twice in a row. Some clients were in a state where they were sending login information and waiting for a response. Although a “t23” was imposed on the socket connected to the server, a termination was not sent to notify the client of the server failure, so I assume that the socket was not completely closed.

After about 2 hours (actually about 1 hour, 59 minutes and 19 seconds), a completion packet was sent to read, notifying the client that the connection was no longer open. Here I began to suspect SO_KEEPALIVE.

I am trying to understand why this happened. This caused some problems, because clients that lost connection for any reason should automatically connect to the server; in this case, since the disconnect notification was not notified, the client did not connect until 2 hours later.

An explicit fix is ​​to set a timeout, but I would like to know how this situation could happen.

SO_KEEPALIVE not installed on the socket by my application server or client.

// Error checking is removed for this snippet, but all winsock calls succeed.
int main() {
    WORD wVersionRequested;
    WSADATA wsaData;
    int err;

    wVersionRequested = MAKEWORD(2, 2);
    err = WSAStartup(wVersionRequested, &wsaData);

    SOCKET foo = WSASocket(AF_INET, SOCK_STREAM, IPPROTO_TCP, 0, 0, 0);

    DWORD optval;
    int optlen = sizeof(optval);
    int test = 0;
    test = getsockopt(foo, SOL_SOCKET, SO_KEEPALIVE, (char*)&optval, &optlen);
    std::cout << "Returned " << optval << std::endl;

    sockaddr_in clientService; 
    clientService.sin_family = AF_INET;
    clientService.sin_addr.s_addr = inet_addr("127.0.0.1");
    clientService.sin_port = htons(446);

    connect(foo, (SOCKADDR*) &clientService, sizeof(clientService));

    test = getsockopt(foo, SOL_SOCKET, SO_KEEPALIVE, (char*)&optval, &optlen);
    std::cout << "Returned " << optval << std::endl;

    std::cin.get();
    return 0;
}

// Example output:
// Returned 2883584
// Returned 2883584
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1 answer

First run your test on a clean installation of the operating system in a virtual machine. I suspect that something else that you installed has been shaken by the keep alive setting.

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