This is an integrated & simplified intro about installing pptp/l2tp server on ubuntu 12.04.
pptpd, xl2tpd, openswan are needed.
ppp should be installed by default, you can
apt-get install ppp just for sure.
dnsmasq is needed to simplify dns issue.
So, just type commands:
sudo apt-get install pptpd openswan l2tpd dnsmasq
Server ip is assigned as: 192.168.210.1
Client ip are in : 192.168.210.100~192.168.210.200