I assume “net.ipv4.ip_forward” is commented in the /etc/sysctl.conf file:
Add or find and comment out the following line
Save, close the file and run the following command to make the changes take effect.
The following iptables firewall rules allow port 1723, GRE and perform NAT
iptables -I INPUT -p tcp --dport 1723 -m state --state NEW -j ACCEPT
iptables -I INPUT -p gre -j ACCEPT
iptables -t nat -I POSTROUTING -o eth0 -j MASQUERADE
In the last rule replace “eth0″ with the interface connecting to the internet on your VPN server. Finally the following rule is required to ensure websites load properly
iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -s 172.20.1.0/24 -j TCPMSS --clamp-mss-to-pmtu
Replace 172.20.1.0/24 with the IP address range used in the “remoteip” option in the /etc/pptpd.conf this firewall rule is used to ensure a proper MTU value is used to prevent fragmentation.