For quick connection to VPN server from Ubuntu
Step by step manual for Ubuntu 18.04:
git clone https://github.com/oleksiikoshlatyi/l2tp-client.git
cd l2tp-client
sudo ./connect_vpn.sh 'your_vpn_server_ip_or_domain_name' 'your_ipsec_pre_shared_key' 'your_vpn_username' 'your_vpn_password'