type ProtocolVersion struct {
  VER uint8
  NMETHODS uint8
  METHODS []uint8
}


func (s *ProtocolVersion) handshake(conn net.Conn) error {
  b := make([]byte, 255)
  n, err := conn.Read(b)
  if err != nil {
    log.Println(err)
    return err
  }
  s.VER = b[0] //ReadByte reads and returns a single byte，第一個參數為socks的版本號
  s.NMETHODS = b[1] //nmethods是記錄methods的長度的。nmethods的長度是1個字節
  if n != int(2+s.NMETHODS) {
    return errors.New("協議錯誤, sNMETHODS不對")
  }
  s.METHODS = b[2:2+s.NMETHODS] //讀取指定長度信息，讀取正好len(buf)長度的字節。如果字節數不是指定長度，則返回錯誤信息和正確的字節數

  if s.VER != 5 {
    return errors.New("該協議不是socks5協議")
  }

  //服務器回應客戶端消息:
  //第一個參數表示版本號為5，即socks5協議，
  // 第二個參數表示服務端選中的認證方法，0即無需密碼訪問, 2表示需要用戶名和密碼進行驗證。 
  resp :=[]byte{5, 0} 
  conn.Write(resp)
  return nil
} 

type Socks5Auth struct {
  VER uint8
  ULEN uint8
  UNAME string
  PLEN uint8
  PASSWD string
}

func (s *Socks5Auth) authenticate(conn net.Conn) error {
  b := make([]byte, 128)
  n, err := conn.Read(b)
  if err != nil{
    log.Println(err)
    return err
  }

  s.VER = b[0]
  if s.VER != 5 {
    return errors.New("該協議不是socks5協議")
  }

  s.ULEN = b[1]
  s.UNAME = string(b[2:2+s.ULEN])
  s.PLEN = b[2+s.ULEN+1]
  s.PASSWD = string(b[n-int(s.PLEN):n])
  log.Println(s.UNAME, s.PASSWD)
  if username != s.UNAME || passwd != s.PASSWD {
    return errors.New("賬號密碼錯誤")
  }

  /**
  回應客戶端,響應客戶端連接成功
  The server verifies the supplied UNAME and PASSWD, and sends the
  following response:
              +----+--------+
              |VER | STATUS |
              +----+--------+
              | 1 |  1  |
              +----+--------+
  A STATUS field of X'00' indicates success. If the server returns a
  `failure' (STATUS value other than X'00') status, it MUST close the
  connection.
  */
 resp := []byte{0x05, 0x00}
  conn.Write(resp) 

  return nil
}

type Socks5Resolution struct {
  VER uint8
  CMD uint8
  RSV uint8
  ATYP uint8
  DSTADDR []byte
  DSTPORT uint16
  DSTDOMAIN string
  RAWADDR *net.TCPAddr
}

func (s *Socks5Resolution) lstRequest(conn net.Conn) error {
  b := make([]byte, 128)
  n, err := conn.Read(b)
  if err != nil || n  7 {
    log.Println(err)
    return errors.New("請求協議錯誤")
  }
  s.VER = b[0]
  if s.VER != 5 {
    return errors.New("該協議不是socks5協議")
  }

  s.CMD = b[1]
  if s.CMD != 1 { 
    return errors.New("客戶端請求類型不為代理連接, 其他功能暫時不支持.")
  }
  s.RSV = b[2] //RSV保留字端，值長度為1個字節

  s.ATYP = b[3]

  switch s.ATYP {
  case 1:
    // IP V4 address: X'01'
    s.DSTADDR = b[4 : 4+net.IPv4len]
  case 3:
    // DOMAINNAME: X'03'
    s.DSTDOMAIN = string(b[5:n-2])
    ipAddr, err := net.ResolveIPAddr("ip", s.DSTDOMAIN)
 if err != nil {
  return err
 }
    s.DSTADDR = ipAddr.IP
  case 4:
    // IP V6 address: X'04'
    s.DSTADDR = b[4 : 4+net.IPv6len]
 default:
 return errors.New("IP地址錯誤")
  }

  s.DSTPORT = binary.BigEndian.Uint16(b[n-2:n])
  // DSTADDR全部換成IP地址，可以防止DNS污染和封殺
  s.RAWADDR = net.TCPAddr{
 IP:  s.DSTADDR,
 Port: int(s.DSTPORT),
  }
  
  /**
  回應客戶端,響應客戶端連接成功
    +----+-----+-------+------+----------+----------+
    |VER | REP | RSV | ATYP | BND.ADDR | BND.PORT |
    +----+-----+-------+------+----------+----------+
    | 1 | 1 | X'00' | 1  | Variable |  2   |
    +----+-----+-------+------+----------+----------+
  */
 resp := []byte{0x05, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
  conn.Write(resp) 

  return nil
}

  wg := new(sync.WaitGroup)
  wg.Add(2)

  go func() {
 defer wg.Done()
 defer dstServer.Close()
    io.Copy(dstServer, client)
  }()

  go func() {
 defer wg.Done()
    defer client.Close()
    io.Copy(client, dstServer)
  }()

  wg.Wait()