The nc -vz -u command is used to test if a server is listening on a specific port on a remote host using UDP protocol. The -v option enables verbose mode, which will print information about the progress of the connection attempt. The -z option enables "scanning" mode, which tells nc to not send any data and instead just test if the port is open or closed. The -u option specifies that UDP protocol should be used.
Here is an example of how you might use this command:
nc -vz -u example.com 80
This would try to connect to port 80 on the host example.com using UDP protocol. If the port is open and the server is listening, you should see output similar to this:
Connection to example.com 80 port [udp/http] succeeded!
If the port is closed or the host is unreachable, you should see an error message indicating that the connection could not be established.