Introduce DTLS sniffer

common/sniff/dtls.go

@@ -0,0 +1,31 @@
+package sniff
+
+import (
+	"context"
+	"os"
+
+	"github.com/sagernet/sing-box/adapter"
+	C "github.com/sagernet/sing-box/constant"
+)
+
+func DTLSRecord(ctx context.Context, packet []byte) (*adapter.InboundContext, error) {
+	const fixedHeaderSize = 13
+	if len(packet) < fixedHeaderSize {
+		return nil, os.ErrInvalid
+	}
+	contentType := packet[0]
+	switch contentType {
+	case 20, 21, 22, 23, 25:
+	default:
+		return nil, os.ErrInvalid
+	}
+	versionMajor := packet[1]
+	if versionMajor != 0xfe {
+		return nil, os.ErrInvalid
+	}
+	versionMinor := packet[2]
+	if versionMinor != 0xff && versionMinor != 0xfd {
+		return nil, os.ErrInvalid
+	}
+	return &adapter.InboundContext{Protocol: C.ProtocolDTLS}, nil
+}