@aha_1980
ok, thanks help again.
regards.

@bublee_jessie And what is the problem? On the server side check the client IP which is trying to connect and only accept connection if correct IP and not connected yet.

@aha_1980
ok, i understood what you said.

As I said earlier, very difficult to get the status ethernet using UDP. UDP is fire & forget protocol. You need to build your own mechanism to monitor the status. It could be simple TCP connection to see if the connection alive with remote server. If not you need to get the interfaces status periodically through QNetworkInterface & check if the interface is up. You can use QNetworkInterface::flags() & QNetworkInterface::IsUp to check the status.