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L2TP over IPsec using OpenSwan with Freeradius authentication on Ubuntu 16.04

This article is about the layer 2 tunneling protocol (L2TP) with IPsec to provide end-to-end encryption in the layer 2 VPN. The open source implementations of IPsec are StrongSwan and OpenSwan, both are supported on all Linux distributions. In this tutorial, OpenSwan is used to provide the security channel for L2TP VPN.