save and exit

5. Now, you can add user account and password

vi /etc/ppp/chap-secrets  

# Secrets for authentication using CHAP
# client server secret IP addresses
username pptpd password *

save and exit

6. Setup Forwarding (Enable network forwarding)

nano /etc/sysctl.conf

change ‘net.ipv4.ip_forward’ from 0 to 1

7. Configure iptables to do NAT
If XEN/Dedicate:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

and if OpenVZ:

iptables -t nat -A POSTROUTING -o venet0 -j SNAT –to-source x.x.x.x

change x.x.x.x with your IP Server

8. change  NAT rule untuk iptables to configure firewall

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
iptables -A FORWARD -i ppp+ -o eth0 -j ACCEPT
iptables -A FORWARD -i eth0 -o ppp+ -j ACCEPT
iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
service iptables save
service iptables restart

9. Start PPTP VPN server

service pptpd start

For log pptp server  /var/log/messages


NOTE:
You must add this command in your terminal

iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

if you feel there are website not loading anyway (ex you load speedtest.net, kompas.com, twitter images, or anything website) see there 

Source: http://jadoel.info/2013/08/cara-install-pptp-vpn-di-centos-5-vps/