func (this *SocksServer) handleUDP(reader io.Reader, writer *v2io.BufferedWriter) error {
	response := protocol.NewSocks5Response()
	response.Error = protocol.ErrorSuccess

	udpAddr := this.udpAddress

	response.Port = udpAddr.Port()
	switch {
	case udpAddr.Address().IsIPv4():
		response.SetIPv4(udpAddr.Address().IP())
	case udpAddr.Address().IsIPv6():
		response.SetIPv6(udpAddr.Address().IP())
	case udpAddr.Address().IsDomain():
		response.SetDomain(udpAddr.Address().Domain())
	}

	response.Write(writer)
	err := writer.Flush()

	if err != nil {
		log.Error("Socks: failed to write response: ", err)
		return err
	}

	// The TCP connection closes after this method returns. We need to wait until
	// the client closes it.
	// TODO: get notified from UDP part
	<-time.After(5 * time.Minute)

	return nil
}

func (this *SocksServer) handleSocks4(reader *v2io.BufferedReader, writer *v2io.BufferedWriter, auth protocol.Socks4AuthenticationRequest) error {
	result := protocol.Socks4RequestGranted
	if auth.Command == protocol.CmdBind {
		result = protocol.Socks4RequestRejected
	}
	socks4Response := protocol.NewSocks4AuthenticationResponse(result, auth.Port, auth.IP[:])

	socks4Response.Write(writer)

	if result == protocol.Socks4RequestRejected {
		log.Warning("Socks: Unsupported socks 4 command ", auth.Command)
		return ErrorUnsupportedSocksCommand
	}

	reader.SetCached(false)
	writer.SetCached(false)

	dest := v2net.TCPDestination(v2net.IPAddress(auth.IP[:]), auth.Port)
	packet := v2net.NewPacket(dest, nil, true)
	this.transport(reader, writer, packet)
	return nil
}

func (this *SocksServer) handleSocks5(reader *v2io.BufferedReader, writer *v2io.BufferedWriter, auth protocol.Socks5AuthenticationRequest) error {
	expectedAuthMethod := protocol.AuthNotRequired
	if this.config.AuthType == AuthTypePassword {
		expectedAuthMethod = protocol.AuthUserPass
	}

	if !auth.HasAuthMethod(expectedAuthMethod) {
		authResponse := protocol.NewAuthenticationResponse(protocol.AuthNoMatchingMethod)
		err := protocol.WriteAuthentication(writer, authResponse)
		writer.Flush()
		if err != nil {
			log.Error("Socks: failed to write authentication: ", err)
			return err
		}
		log.Warning("Socks: client doesn't support any allowed auth methods.")
		return ErrorUnsupportedAuthMethod
	}

	authResponse := protocol.NewAuthenticationResponse(expectedAuthMethod)
	err := protocol.WriteAuthentication(writer, authResponse)
	writer.Flush()
	if err != nil {
		log.Error("Socks: failed to write authentication: ", err)
		return err
	}
	if this.config.AuthType == AuthTypePassword {
		upRequest, err := protocol.ReadUserPassRequest(reader)
		if err != nil {
			log.Error("Socks: failed to read username and password: ", err)
			return err
		}
		status := byte(0)
		if !this.config.HasAccount(upRequest.Username(), upRequest.Password()) {
			status = byte(0xFF)
		}
		upResponse := protocol.NewSocks5UserPassResponse(status)
		err = protocol.WriteUserPassResponse(writer, upResponse)
		writer.Flush()
		if err != nil {
			log.Error("Socks: failed to write user pass response: ", err)
			return err
		}
		if status != byte(0) {
			log.Warning("Socks: Invalid user account: ", upRequest.AuthDetail())
			return proxy.ErrorInvalidAuthentication
		}
	}

	request, err := protocol.ReadRequest(reader)
	if err != nil {
		log.Error("Socks: failed to read request: ", err)
		return err
	}

	if request.Command == protocol.CmdUdpAssociate && this.config.UDPEnabled {
		return this.handleUDP(reader, writer)
	}

	if request.Command == protocol.CmdBind || request.Command == protocol.CmdUdpAssociate {
		response := protocol.NewSocks5Response()
		response.Error = protocol.ErrorCommandNotSupported
		response.Port = v2net.Port(0)
		response.SetIPv4([]byte{0, 0, 0, 0})

		response.Write(writer)
		writer.Flush()
		if err != nil {
			log.Error("Socks: failed to write response: ", err)
			return err
		}
		log.Warning("Socks: Unsupported socks command ", request.Command)
		return ErrorUnsupportedSocksCommand
	}

	response := protocol.NewSocks5Response()
	response.Error = protocol.ErrorSuccess

	// Some SOCKS software requires a value other than dest. Let's fake one:
	response.Port = v2net.Port(1717)
	response.SetIPv4([]byte{0, 0, 0, 0})

	response.Write(writer)
	if err != nil {
		log.Error("Socks: failed to write response: ", err)
		return err
	}

	reader.SetCached(false)
	writer.SetCached(false)

	dest := request.Destination()
	log.Info("Socks: TCP Connect request to ", dest)

	packet := v2net.NewPacket(dest, nil, true)
	this.transport(reader, writer, packet)
	return nil
}