tcpKeepAlive, err := syscall.GetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_KEEPALIVE)
	if err != nil {
		return
	}
	tcpKeepAliveIdle, err := syscall.GetsockoptInt(fd, syscall.IPPROTO_TCP, syscall_TCP_KEEPIDLE)
	if err != nil {
		return
	}
	tcpKeepAliveInterval, err := syscall.GetsockoptInt(fd, syscall.IPPROTO_TCP, syscall_TCP_KEEPINTVL)
	if err != nil {
		return
	}

		tcpKeepAliveCount        int
	)
	if unix.SupportTCPKeepAliveIdleIntvlCNT() {
		tcpKeepAliveIdle, err = syscall.GetsockoptInt(fd, syscall.IPPROTO_TCP, syscall_TCP_KEEPIDLE)
		if err != nil {
			return
		}
		tcpKeepAliveIdleTime = time.Duration(tcpKeepAliveIdle) * time.Second

		tcpKeepAliveInterval, err = syscall.GetsockoptInt(fd, syscall.IPPROTO_TCP, syscall_TCP_KEEPINTVL)
		if err != nil {
			return
		}

	tcpKeepAlive, err := syscall.GetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_KEEPALIVE)
	if err != nil {
		return
	}
	tcpKeepAliveIdle, err := syscall.GetsockoptInt(fd, syscall.IPPROTO_TCP, syscall_TCP_KEEPIDLE)
	if err != nil {
		return
	}
	tcpKeepAliveInterval, err := syscall.GetsockoptInt(fd, syscall.IPPROTO_TCP, syscall_TCP_KEEPINTVL)
	if err != nil {
		return
	}

	connectFunc       func(int, syscall.Sockaddr) error = syscall.Connect
	listenFunc        func(int, int) error              = syscall.Listen
	getsockoptIntFunc func(int, int, int) (int, error)  = syscall.GetsockoptInt

	sw.stats.getLocked(nso.Cookie).Accepted++
	return ns, sa, nil
}

// GetsockoptInt wraps syscall.GetsockoptInt.
func (sw *Switch) GetsockoptInt(s, level, opt int) (soerr int, err error) {
	so := sw.sockso(s)
	if so == nil {
		return syscall.GetsockoptInt(s, level, opt)
	}
	sw.fmu.RLock()
	f := sw.fltab[FilterGetsockoptInt]
	sw.fmu.RUnlock()

	af, err := f.apply(so)

The [GetsockoptInt] function is now supported on Windows....

	}
	return s, nil
}

func newFileFD(f *os.File) (*netFD, error) {
	s, err := dupSocket(f)
	if err != nil {
		return nil, err
	}
	family := syscall.AF_UNSPEC
	sotype, err := syscall.GetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_TYPE)
	if err != nil {
		poll.CloseFunc(s)
		return nil, os.NewSyscallError("getsockopt", err)
	}
	lsa, _ := syscall.Getsockname(s)
	rsa, _ := syscall.Getpeername(s)

	FilterListen                          // for Listen
	FilterAccept                          // for Accept, Accept4 or AcceptEx
	FilterGetsockoptInt                   // for GetsockoptInt
	FilterClose                           // for Close or Closesocket
)

// A Filter represents a socket system call filter.
//
// It will only be executed before a system call for a socket that has

	msg.Namelen = uint32(SizeofSockaddrAny)
	var iov Iovec
	if len(p) > 0 {
		iov.Base = &p[0]
		iov.SetLen(len(p))
	}
	var dummy byte
	if len(oob) > 0 {
		var sockType int
		sockType, err = GetsockoptInt(fd, SOL_SOCKET, SO_TYPE)
		if err != nil {
			return
		}
		// receive at least one normal byte
		if sockType != SOCK_DGRAM && len(p) == 0 {
			iov.Base = &dummy
			iov.SetLen(1)
		}

	var rsa RawSockaddrAny
	var len _Socklen = SizeofSockaddrAny
	if err = getpeername(fd, &rsa, &len); err != nil {
		return
	}
	return anyToSockaddr(&rsa)
}

func GetsockoptInt(fd, level, opt int) (value int, err error) {
	var n int32
	vallen := _Socklen(4)
	err = getsockopt(fd, level, opt, unsafe.Pointer(&n), &vallen)
	return int(n), err
}