It could be appear the typical post for filling content of a blog... But currently I am working in a project with VPNs and I would like to have a diary about how a VPN IPSec works and how to configure it in Linux (at least one configuration). It is not the first time I am working with a VPN IPSec, and now that I am re-reading all of the RFCs I am getting a better vision about that. TL;DR …