http://blog.z-proj.com/enabling-l2tp-over-ipsec-on-ubuntu-16-04/
https://raymii.org/s/tutorials/IPSEC_L2TP_vpn_with_Ubuntu_14.04.html
http://askubuntu.com/questions/569763/l2tp-ipsec-vpn-client-on-ubuntu-14-10/578881
https://raymii.org/s/tutorials/IPSEC_L2TP_vpn_with_Ubuntu_14.04.html
http://askubuntu.com/questions/569763/l2tp-ipsec-vpn-client-on-ubuntu-14-10/578881
http://askubuntu.com/questions/617785/how-to-connect-to-l2tp-over-ipsec-vpn
http://blog.csdn.net/hhbgk/article/details/52549816