Link1
https://expats-in-china.com/t/setup-a-vpn-ipsec-on-centos-7-using-strongswan/27
Link2
https://www.vultr.com/docs/using-strongswan-for-ipsec-vpn-on-centos-7
Link3
http://blog.leanote.com/post/shazi/Untitled-552ba6ca38f4117de8000169-5
New links
https://raymii.org/s/tutorials/IPSEC_vpn_with_CentOS_7.html