public static String Login(String usr, char[] pwd) throws Exception {
    InetAddress ip = InetAddress.getLocalHost();
    NetworkInterface network = NetworkInterface.getByInetAddress(ip);
    byte[] mac = network.getHardwareAddress();
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < mac.length; i++) {
        sb.append(String.format("%02x%s", mac[i], (i < mac.length - 1) ? ":" : ""));
    }
    mac_address = sb.toString();
    String urlencode_pwd = URLEncoder.encode(pwdEncode(String.valueOf(pwd)), "utf-8");
    String data =
            "action=login&username="
                    + usrEncode(usr)
                    + "&password="
                    + urlencode_pwd
                    + "&drop=0&pop=1&type=2&n=117&mbytes=0&minutes=0&ac_id=1&mac="
                    + mac_address;
    String response = HttpPost("http://172.16.154.130:69/cgi-bin/srun_portal", data);
    if (response.contains("login_ok")) {
        isLinked = true;
        return "success";
    } else {
        isLinked = false;
        return response;
    }
}