inp2p/pkg/nat/detect.go

// Package nat provides NAT type detection via UDP and TCP STUN.
package nat

import (
	"encoding/json"
	"fmt"
	"net"
	"time"

	"github.com/openp2p-cn/inp2p/pkg/protocol"
)

const (
	detectTimeout = 5 * time.Second
)

// DetectResult holds the NAT detection outcome.
type DetectResult struct {
	Type     protocol.NATType
	PublicIP string
	Port1    int // external port seen on STUN server port 1
	Port2    int // external port seen on STUN server port 2
}

// stunReq is sent to the STUN endpoint.
type stunReq struct {
	ID int `json:"id"`
}

// stunRsp is received from the STUN endpoint.
type stunRsp struct {
	IP   string `json:"ip"`
	Port int    `json:"port"`
	ID   int    `json:"id"`
}

// DetectUDP sends probes from the same local port to two different server
// UDP ports. If both return the same external port → Cone; different → Symmetric.
func DetectUDP(serverIP string, port1, port2 int) DetectResult {
	result := DetectResult{Type: protocol.NATUnknown}

	// Bind a single local UDP port
	conn, err := net.ListenPacket("udp", ":0")
	if err != nil {
		return result
	}
	defer conn.Close()

	r1, err1 := probeUDP(conn, serverIP, port1, 1)
	r2, err2 := probeUDP(conn, serverIP, port2, 2)

	if err1 != nil || err2 != nil {
		return result // timeout → NATUnknown
	}

	result.PublicIP = r1.IP
	result.Port1 = r1.Port
	result.Port2 = r2.Port

	if r1.Port == r2.Port {
		result.Type = protocol.NATCone
	} else {
		result.Type = protocol.NATSymmetric
	}

	// Check if public IP equals local IP → no NAT
	localIP := conn.LocalAddr().(*net.UDPAddr).IP.String()
	if localIP == r1.IP || r1.IP == "" {
		// might be public
	}

	return result
}

func probeUDP(conn net.PacketConn, serverIP string, port, id int) (stunRsp, error) {
	addr, err := net.ResolveUDPAddr("udp", fmt.Sprintf("%s:%d", serverIP, port))
	if err != nil {
		return stunRsp{}, err
	}

	frame, _ := protocol.Encode(protocol.MsgNAT, protocol.SubNATDetectReq, stunReq{ID: id})

	conn.SetWriteDeadline(time.Now().Add(detectTimeout))
	if _, err := conn.WriteTo(frame, addr); err != nil {
		return stunRsp{}, err
	}

	buf := make([]byte, 1024)
	conn.SetReadDeadline(time.Now().Add(detectTimeout))
	n, _, err := conn.ReadFrom(buf)
	if err != nil {
		return stunRsp{}, err
	}

	var rsp stunRsp
	if n > protocol.HeaderSize {
		json.Unmarshal(buf[protocol.HeaderSize:n], &rsp)
	}
	return rsp, nil
}

// DetectTCP connects to two different TCP ports on the server and compares
// the observed external port. This is the fallback when UDP is blocked.
func DetectTCP(serverIP string, port1, port2 int) DetectResult {
	result := DetectResult{Type: protocol.NATUnknown}

	r1, err1 := probeTCP(serverIP, port1, 1)
	r2, err2 := probeTCP(serverIP, port2, 2)

	if err1 != nil || err2 != nil {
		return result
	}

	result.PublicIP = r1.IP
	result.Port1 = r1.Port
	result.Port2 = r2.Port

	if r1.Port == r2.Port {
		result.Type = protocol.NATCone
	} else {
		result.Type = protocol.NATSymmetric
	}
	return result
}

func probeTCP(serverIP string, port, id int) (stunRsp, error) {
	conn, err := net.DialTimeout("tcp", fmt.Sprintf("%s:%d", serverIP, port), detectTimeout)
	if err != nil {
		return stunRsp{}, err
	}
	defer conn.Close()

	frame, _ := protocol.Encode(protocol.MsgNAT, protocol.SubNATDetectReq, stunReq{ID: id})
	conn.SetWriteDeadline(time.Now().Add(detectTimeout))
	if _, err := conn.Write(frame); err != nil {
		return stunRsp{}, err
	}

	buf := make([]byte, 1024)
	conn.SetReadDeadline(time.Now().Add(detectTimeout))
	n, err := conn.Read(buf)
	if err != nil {
		return stunRsp{}, err
	}

	var rsp stunRsp
	if n > protocol.HeaderSize {
		json.Unmarshal(buf[protocol.HeaderSize:n], &rsp)
	}
	return rsp, nil
}

// Detect runs UDP detection first, falls back to TCP if UDP is blocked.
func Detect(serverIP string, udpPort1, udpPort2, tcpPort1, tcpPort2 int) DetectResult {
	result := DetectUDP(serverIP, udpPort1, udpPort2)
	if result.Type != protocol.NATUnknown {
		return result
	}
	// UDP blocked, fallback to TCP
	return DetectTCP(serverIP, tcpPort1, tcpPort2)
}

// ─── Server-side STUN handler ───

// ServeUDPSTUN listens on a UDP port and echoes back the sender's observed IP:port.
func ServeUDPSTUN(port int, quit <-chan struct{}) error {
	addr, err := net.ResolveUDPAddr("udp", fmt.Sprintf(":%d", port))
	if err != nil {
		return err
	}
	conn, err := net.ListenUDP("udp", addr)
	if err != nil {
		return err
	}
	defer conn.Close()

	go func() {
		<-quit
		conn.Close()
	}()

	buf := make([]byte, 1024)
	for {
		n, remoteAddr, err := conn.ReadFromUDP(buf)
		if err != nil {
			select {
			case <-quit:
				return nil
			default:
				continue
			}
		}

		// Parse request
		var req stunReq
		if n > protocol.HeaderSize {
			json.Unmarshal(buf[protocol.HeaderSize:n], &req)
		}

		// Reply with observed address
		rsp := stunRsp{
			IP:   remoteAddr.IP.String(),
			Port: remoteAddr.Port,
			ID:   req.ID,
		}
		frame, _ := protocol.Encode(protocol.MsgNAT, protocol.SubNATDetectRsp, rsp)
		conn.WriteToUDP(frame, remoteAddr)
	}
}

// ServeTCPSTUN listens on a TCP port. Each connection: read one req, write one rsp with observed addr.
func ServeTCPSTUN(port int, quit <-chan struct{}) error {
	ln, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
	if err != nil {
		return err
	}
	defer ln.Close()

	go func() {
		<-quit
		ln.Close()
	}()

	for {
		conn, err := ln.Accept()
		if err != nil {
			select {
			case <-quit:
				return nil
			default:
				continue
			}
		}
		go func(c net.Conn) {
			defer c.Close()
			remoteAddr := c.RemoteAddr().(*net.TCPAddr)

			buf := make([]byte, 1024)
			c.SetReadDeadline(time.Now().Add(10 * time.Second))
			n, err := c.Read(buf)
			if err != nil {
				return
			}

			var req stunReq
			if n > protocol.HeaderSize {
				json.Unmarshal(buf[protocol.HeaderSize:n], &req)
			}

			rsp := stunRsp{
				IP:   remoteAddr.IP.String(),
				Port: remoteAddr.Port,
				ID:   req.ID,
			}
			frame, _ := protocol.Encode(protocol.MsgNAT, protocol.SubNATDetectRsp, rsp)
			c.SetWriteDeadline(time.Now().Add(5 * time.Second))
			c.Write(frame)
		}(conn)
	}
}