Add ntp protocol sniffing

common/sniff/ntp.go

@@ -0,0 +1,58 @@
+package sniff
+
+import (
+	"context"
+	"encoding/binary"
+	"os"
+
+	"github.com/sagernet/sing-box/adapter"
+	C "github.com/sagernet/sing-box/constant"
+)
+
+func NTP(ctx context.Context, metadata *adapter.InboundContext, packet []byte) error {
+	// NTP packets must be at least 48 bytes long (standard NTP header size).
+	pLen := len(packet)
+	if pLen < 48 {
+		return os.ErrInvalid
+	}
+	// Check the LI (Leap Indicator) and Version Number (VN) in the first byte.
+	// We'll primarily focus on ensuring the version is valid for NTP.
+	// Many NTP versions are used, but let's check for generally accepted ones (3 & 4 for IPv4, plus potential extensions/customizations)
+	firstByte := packet[0]
+	li := (firstByte >> 6) & 0x03 // Extract LI
+	vn := (firstByte >> 3) & 0x07 // Extract VN
+	mode := firstByte & 0x07      // Extract Mode
+
+	// Leap Indicator should be a valid value (0-3).
+	if li > 3 {
+		return os.ErrInvalid
+	}
+
+	// Version Check (common NTP versions are 3 and 4)
+	if vn != 3 && vn != 4 {
+		return os.ErrInvalid
+	}
+
+	// Check the Mode field for a client request (Mode 3).  This validates it *is* a request.
+	if mode != 3 {
+		return os.ErrInvalid
+	}
+
+	// Check Root Delay and Root Dispersion. While not strictly *required* for a request,
+	// we can check if they appear to be reasonable values (not excessively large).
+	rootDelay := binary.BigEndian.Uint32(packet[4:8])
+	rootDispersion := binary.BigEndian.Uint32(packet[8:12])
+
+	// Check for unreasonably large root delay and dispersion.  NTP RFC specifies max values of approximately 16 seconds.
+	// Convert to milliseconds for easy comparison.  Each unit is 1/2^16 seconds.
+	if float64(rootDelay)/65536.0 > 16.0 {
+		return os.ErrInvalid
+	}
+	if float64(rootDispersion)/65536.0 > 16.0 {
+		return os.ErrInvalid
+	}
+
+	metadata.Protocol = C.ProtocolNTP
+
+	return nil
+}