How do I call TCPConn.SetLinger()?

I'm using Go 1.2 and trying to call TCPConn.SetLinger(0) on a socket that my server is closing due to fatal errors in the input. However, I can't figure out how I would do this. Both net.Listener.Accept() and TCPListener.Accept() return net.Conn, not net.TCPConn. net.Conn does not implement SetLinger. I tried doing a type switch, but then the compiler complains of an "impossible type switch case: conn (type net.Conn) cannot have dynamic type net.TCPConn (missing Close method)". How can net.TCPConn be missing a Close method?? I haven't checked the source, but the documentation lists a close method (and what kind of connection can you not close, anyway?).

It seems like you cannot cast net.Conn to net.TCPConn (because of no Close method), and you can't get a TCPConn on a server, because all the Accept()s return a net.Conn. So how do I actually call SetLinger? (Or for that matter, SetKeepAlive, SetNoDelay?)

After looking more closely, it looks like there is a net.TCPListener.AcceptTCP() which returns a TCPListener, is that really what I have to do? Why can't I get a TCPConn from Accept()? I started it off with net.Listen("tcp", ":4321"), so I know it has to be a TCPConn underneath...

You could use TCPListener.AcceptTCP(), Accept just calls AcceptTCP and returns *TCPConn as an interface.

From http://golang.org/src/pkg/net/tcpsock_posix.go?s=7727:7771#L233

func (l *TCPListener) Accept() (Conn, error) {
	c, err := l.AcceptTCP() //notice this part
	if err != nil {
		return nil, err
	}
	return c, nil
}

Or like James mentioned in the comments, you could simply cast it to tcpcon := conn.(*net.TCPConn).

Per How to Set Go net/http Socket Options - setsockopt() example

We could set SO_LINGER through Control of net.Dialer

	dialer := &net.Dialer{
		Control: func(network, address string, conn syscall.RawConn) error {
			var operr error
			if err := conn.Control(func(fd uintptr) {
				operr = syscall.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_LINGER, 1)
			}); err != nil {
				return err
			}
			return operr
		},
	}

	client := http.Client{Transport: &http.Transport{DialContext: dialer.DialContext}}

Another solution could be done through DialContext of http.Transport

type connDialer struct {
	c net.Conn
}

func (cd connDialer) DialContext(ctx context.Context, network, addr string) (net.Conn, error) {
	tcpConn, ok := cd.c.(*net.TCPConn)
	if ok {
		tcpConn.SetLinger(0)
		return tcpConn, nil
	}
	return cd.c, nil
}

dialer := net.Dialer{}
conn, _ := dialer.DialContext(ctx, network, addr)

client := http.Client{Transport: &http.Transport{DialContext: connDialer{c: conn}.DialContext}}