Introduce bittorrent related protocol sniffers

* Introduce bittorrent related protocol sniffers

including, sniffers of
1. BitTorrent Protocol (TCP)
2. uTorrent Transport Protocol (UDP)

Signed-off-by: iosmanthus <myosmanthustree@gmail.com>
Co-authored-by: 世界 <i@sekai.icu>

common/sniff/bittorrent.go

@@ -0,0 +1,101 @@
+package sniff
+
+import (
+	"bytes"
+	"context"
+	"encoding/binary"
+	"io"
+	"os"
+
+	"github.com/sagernet/sing-box/adapter"
+	C "github.com/sagernet/sing-box/constant"
+)
+
+const (
+	trackerConnectFlag    = 0
+	trackerProtocolID     = 0x41727101980
+	trackerConnectMinSize = 16
+)
+
+// BitTorrent detects if the stream is a BitTorrent connection.
+// For the BitTorrent protocol specification, see https://www.bittorrent.org/beps/bep_0003.html
+func BitTorrent(_ context.Context, reader io.Reader) (*adapter.InboundContext, error) {
+	var first byte
+	err := binary.Read(reader, binary.BigEndian, &first)
+	if err != nil {
+		return nil, err
+	}
+
+	if first != 19 {
+		return nil, os.ErrInvalid
+	}
+
+	var protocol [19]byte
+	_, err = reader.Read(protocol[:])
+	if err != nil {
+		return nil, err
+	}
+	if string(protocol[:]) != "BitTorrent protocol" {
+		return nil, os.ErrInvalid
+	}
+
+	return &adapter.InboundContext{
+		Protocol: C.ProtocolBitTorrent,
+	}, nil
+}
+
+// UTP detects if the packet is a uTP connection packet.
+// For the uTP protocol specification, see
+//  1. https://www.bittorrent.org/beps/bep_0029.html
+//  2. https://github.com/bittorrent/libutp/blob/2b364cbb0650bdab64a5de2abb4518f9f228ec44/utp_internal.cpp#L112
+func UTP(_ context.Context, packet []byte) (*adapter.InboundContext, error) {
+	// A valid uTP packet must be at least 20 bytes long.
+	if len(packet) < 20 {
+		return nil, os.ErrInvalid
+	}
+
+	version := packet[0] & 0x0F
+	ty := packet[0] >> 4
+	if version != 1 || ty > 4 {
+		return nil, os.ErrInvalid
+	}
+
+	// Validate the extensions
+	extension := packet[1]
+	reader := bytes.NewReader(packet[20:])
+	for extension != 0 {
+		err := binary.Read(reader, binary.BigEndian, &extension)
+		if err != nil {
+			return nil, err
+		}
+
+		var length byte
+		err = binary.Read(reader, binary.BigEndian, &length)
+		if err != nil {
+			return nil, err
+		}
+		_, err = reader.Seek(int64(length), io.SeekCurrent)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return &adapter.InboundContext{
+		Protocol: C.ProtocolBitTorrent,
+	}, nil
+}
+
+// UDPTracker detects if the packet is a UDP Tracker Protocol packet.
+// For the UDP Tracker Protocol specification, see https://www.bittorrent.org/beps/bep_0015.html
+func UDPTracker(_ context.Context, packet []byte) (*adapter.InboundContext, error) {
+	if len(packet) < trackerConnectMinSize {
+		return nil, os.ErrInvalid
+	}
+	if binary.BigEndian.Uint64(packet[:8]) != trackerProtocolID {
+		return nil, os.ErrInvalid
+	}
+	if binary.BigEndian.Uint32(packet[8:12]) != trackerConnectFlag {
+		return nil, os.ErrInvalid
+	}
+	return &adapter.InboundContext{Protocol: C.ProtocolBitTorrent}, nil
+}