Some explanation for the SSH server

Other Network Manager VPN plugins work out of the box, without extra configuration on the remote side. NetworkManager-SSH on the other hand requires certain changes to the SSH server. These three options must be explicitly enabled: ``` AllowTcpForwarding yes PermitTunnel yes GatewayPorts yes ``` I don't know how you can create an ℹ️ button in GTK but it would be nice to have it in the plugin, so that it spewed relevant information.