firewall ufw

http://silverlinux.blogspot.hk/2012/05/how-to-pptp-vpn-on-ubuntu-1204-pptpd.html

How To: PPTP VPN on Ubuntu 12.04 (pptpd)
I recently started renting a 128 MB RAM Xen VPS for $15/quarter with a promotional offer for nqhost.com as seen on http://www.lowendbox.com/blog/nqhost-15quarter-128mb-xen-vps-in-dallas/
Since nqhost.com offers unmetered bandwidth through a SoftLayer data center in Texas, I decided to set up a VPN server for my own use.

In this tutorial, I will be showing you how to set up pptpd (poptop) on Ubuntu 12.04 to provide PPTP VPN services.

The following instructions are inspired by http://eran.sandler.co.il/2010/08/30/pptp-vpn-on-ubuntu-10-04-for-your-iphone-ipad/

Install Software
sudo apt-get install pptpd ufw

Allow Ports 22 and 1723 on UFW and Enable UFW
Warning: if you are connected to SSH on a port other than 22, please adjust the first command accordingly so you don't get kicked off.

sudo ufw allow 22
sudo ufw allow 1723
sudo ufw enable

Edit /etc/ppp/pptpd-options
Comment out (by placing a "#" at the beginning of the line) the following lines in "/etc/ppp/pptpd-options":

refuse-pap
refuse-chap
refuse-mschap

If you don't want to require encryption, comment out "require-mppe-128" (might be good to disable it just for testing and re-enable it later)
Add the following:
ms-dns 208.67.222.222
ms-dns 208.67.220.220