openvpn is the client to connect to a openvpn server on debian. To use it, you first need an openvpn client configuration file, say config.ovpn, which is obtained from the server.

Then, install openvpn client with the commands below.

> sudo apt install openvpn network-manager-openvpn-gnome

Next, click settings > network > vpn > import from file, import the file config.ovpn, and enjoy the magic!

A failed way I experienced is to use the command line

> sudo openvpn --config config.ovpn

don't know why, but find the ip a result lacks a noprefixrout word.