Fix utp sniffer

common/protocol/bittorrent/bittorrent.go

@@ -3,11 +3,8 @@ package bittorrent
 import (
 	"encoding/binary"
 	"errors"
-	"math"
-	"time"
 
 	"github.com/xtls/xray-core/common"
-	"github.com/xtls/xray-core/common/buf"
 )
 
 type SniffHeader struct{}
@@ -39,50 +36,44 @@ func SniffUTP(b []byte) (*SniffHeader, error) {
 		return nil, common.ErrNoClue
 	}
 
-	buffer := buf.FromBytes(b)
-
-	var typeAndVersion uint8
-
-	if binary.Read(buffer, binary.BigEndian, &typeAndVersion) != nil {
-		return nil, common.ErrNoClue
-	} else if b[0]>>4&0xF > 4 || b[0]&0xF != 1 {
+	// type 4 (ST_SYN), version 1
+	if b[0] != 0x41 {
 		return nil, errNotBittorrent
 	}
 
-	var extension uint8
-
-	if binary.Read(buffer, binary.BigEndian, &extension) != nil {
-		return nil, common.ErrNoClue
-	} else if extension != 0 && extension != 1 {
+	// timestamp_difference is always 0 in new connections
+	if binary.BigEndian.Uint32(b[8:12]) != 0 {
 		return nil, errNotBittorrent
 	}
 
+	// Walk the extension chain. Selective ack (1) and extension bits (2)
+	extension, offset := b[1], 20
 	for extension != 0 {
-		if extension != 1 {
+		if len(b) < offset+2 {
 			return nil, errNotBittorrent
 		}
-		if binary.Read(buffer, binary.BigEndian, &extension) != nil {
-			return nil, common.ErrNoClue
-		}
-
-		var length uint8
-		if err := binary.Read(buffer, binary.BigEndian, &length); err != nil {
-			return nil, common.ErrNoClue
+		length := int(b[offset+1])
+		switch extension {
+		case 1: // selective ack
+			if length < 4 || length%4 != 0 {
+				return nil, errNotBittorrent
+			}
+		case 2: // extension bits: fixed 8 bytes, sent in ST_SYN by µTorrent
+			if length != 8 {
+				return nil, errNotBittorrent
+			}
+		default:
+			return nil, errNotBittorrent
 		}
-		if common.Error2(buffer.ReadBytes(int32(length))) != nil {
-			return nil, common.ErrNoClue
+		if len(b) < offset+2+length {
+			return nil, errNotBittorrent
 		}
+		extension = b[offset]
+		offset += 2 + length
 	}
 
-	if common.Error2(buffer.ReadBytes(2)) != nil {
-		return nil, common.ErrNoClue
-	}
-
-	var timestamp uint32
-	if err := binary.Read(buffer, binary.BigEndian, &timestamp); err != nil {
-		return nil, common.ErrNoClue
-	}
-	if math.Abs(float64(time.Now().UnixMicro()-int64(timestamp))) > float64(24*time.Hour) {
+	// extensions should consume all ST_SYN payload
+	if len(b) != offset {
 		return nil, errNotBittorrent
 	}
 

common/protocol/bittorrent/bittorrent_test.go

@@ -0,0 +1,67 @@
+package bittorrent
+
+import (
+	"encoding/binary"
+	"testing"
+
+	"github.com/xtls/xray-core/common"
+)
+
+// utpPacket builds the fixed 20-byte header defined by BEP 29.
+func utpPacket(packetType, extension byte, tsDiff uint32, payload ...byte) []byte {
+	b := make([]byte, 20)
+	b[0] = packetType<<4 | 1
+	b[1] = extension
+	binary.BigEndian.PutUint16(b[2:4], 0x4a3f)     // connection_id, random
+	binary.BigEndian.PutUint32(b[4:8], 0x8c3a91d2) // timestamp_microseconds, sender's clock
+	binary.BigEndian.PutUint32(b[8:12], tsDiff)
+	binary.BigEndian.PutUint32(b[12:16], 0x00100000) // wnd_size
+	binary.BigEndian.PutUint16(b[16:18], 0x71ee)     // seq_nr, random in libutp/libtorrent
+	binary.BigEndian.PutUint16(b[18:20], 0x0000)     // ack_nr
+	return append(b, payload...)
+}
+
+func TestSniffUTP(t *testing.T) {
+	selectiveAck := []byte{0, 4, 0xff, 0x00, 0xff, 0x00}
+	wrongVersion := utpPacket(4, 0, 0)
+	wrongVersion[0] = 4<<4 | 2
+
+	cases := []struct {
+		name    string
+		payload []byte
+		err     error
+	}{
+		{"syn", utpPacket(4, 0, 0), nil},
+		{"syn with selective ack", append(utpPacket(4, 1, 0), selectiveAck...), nil},
+		{"syn with extension bits", append(utpPacket(4, 2, 0), 0, 8, 1, 2, 3, 4, 5, 6, 7, 8), nil},
+		{"extension bits with wrong length", append(utpPacket(4, 2, 0), 0, 4, 1, 2, 3, 4), errNotBittorrent},
+		{"syn with nonzero timestamp_difference", utpPacket(4, 0, 0x1234), errNotBittorrent},
+		{"syn with trailing payload", utpPacket(4, 0, 0, 'x'), errNotBittorrent},
+		// txid 0x4100, no EDNS0: the worst case colliding with the uTP header
+		{"dns query", []byte{
+			0x41, 0x00, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x01, 'a', 0x02, 'c', 'o', 0x00, 0x00, 0x01, 0x00, 0x01,
+		}, errNotBittorrent},
+		{"established connection packets", utpPacket(0, 0, 0x5678, 'x', 'y', 'z'), errNotBittorrent},
+		{"state", utpPacket(2, 0, 0x5678), errNotBittorrent},
+		{"fin", utpPacket(1, 0, 0x5678), errNotBittorrent},
+		{"wrong version", wrongVersion, errNotBittorrent},
+		{"unknown packet type", utpPacket(5, 0, 0), errNotBittorrent},
+		{"unknown extension", utpPacket(4, 2, 0), errNotBittorrent},
+		{"extension chain past the datagram", utpPacket(4, 1, 0, 0, 8, 0xff), errNotBittorrent},
+		{"selective ack not in multiples of 4", append(utpPacket(4, 1, 0), 0, 3, 0xff, 0x00, 0xff), errNotBittorrent},
+		{"shorter than the header", utpPacket(4, 0, 0)[:19], common.ErrNoClue},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			h, err := SniffUTP(c.payload)
+			if err != c.err {
+				t.Fatalf("expected error %v, got %v", c.err, err)
+			}
+			if err == nil && h == nil {
+				t.Fatal("expected a sniff header, got nil")
+			}
+		})
+	}
+}