想用kotlin写一个自定义dns工具，用到了VpnService，能正常连接vpn，但是连上就没网了，有没有android大佬指点一下 
class MyVpnService : VpnService() {
    private var vpnTunnel: ParcelFileDescriptor? = null
    private var vpnJob: Job? = null

    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
        StrictMode.setThreadPolicy(ThreadPolicy.Builder().permitAll().build())
        if (vpnTunnel == null) {
       ...