PPTP VPN Installation, configuration and setup. CentOS 5 and 6 for 32 and 64bit and OpenVZ / XEN / KVM

1. Install ppp via yum:
$ yum install ppp -y

2. Download and install pptpd (the daemon for point-to-point tunneling). You can find the correct package at this website http://poptop.sourceforge.net/yum/stable/packages/ :
$ cd /usr/local/src $ wget http://poptop.sourceforge.net/yum/stable/packages/pptpd-1.3.4-2.el6.x86_64.rpm $ rpm -Uhv pptpd-1.3.4-2.el6.x86_64.rpm

3. Once installed, open /etc/pptpd.conf using text editor and add following line:
localip 209.85.227.26 remoteip 209.85.227.27-30

4. Open /etc/ppp/options.pptpd and add authenticate method, encryption and DNS resolver value:
require-mschap-v2 require-mppe-128 ms-dns 8.8.8.8

5. Lets create user to access the VPN server. Open /etc/ppp/chap-secrets and add the user as below:
vpnuser pptpd myVPN$99 * The format is: [username] [space] [server] [space] [password] [space][IP addresses]

6. We need to allow IP packet forwarding for this server. Open /etc/sysctl.conf via text editor and change line below:
net.ipv4.ip_forward = 1

7. Run following command to take effect on the changes:
$ sysctl -p

8. Allow IP masquerading in IPtables by executing following line:
$ iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE $ service iptables save $ service iptables restart

Next, we need to allow TCP port 1723 and the GRE protocol through iptables.
iptables -A INPUT -i eth0 -p tcp --dport 1723 -j ACCEPT iptables -A INPUT -i eth0 -p gre -j ACCEPT iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

The following iptables rules are necessary if you want to be able to route all your internet traffic through the VPN server.
iptables -A FORWARD -i ppp+ -o eth0 -j ACCEPT iptables -A FORWARD -i eth0 -o ppp+ -j ACCEPT

Update: Once you have done with step 8, check the rules at /etc/sysconfig/iptables. Make sure that the POSTROUTING rules is above any REJECT rules.

9. Turn on the pptpd service at startup and reboot the server:
$ chkconfig pptpd on $ init 6

Once the server is online after reboot, you should now able to access the PPTP server from the VPN client. You can monitor /var/log/messages for ppp and pptpd related log. Cheers!