After migrating a VM to OpenVZ I noticed that the VPN server inside of the container no longer worked. I was not able to setup OpenVZ correct to get the VPN working again. So I used a Xen based VM with Ubuntu 12.04 LTS to setup the VPN again. I found a nice tutorial for that: IPSEC/L2TP VPN on Ubuntu 12.04 with OpenSwan, xl2tpd and ppp. The site has also tutorials for Raspberry Pi with Arch Linux ARM, CentOS 7, CentOS 6, Ubuntu 14.04 LTS and other (older) Ubuntu versions.