Linux VPN pptp dev / ip forward

# iptables -A FORWARD -i tun0 -o tun0 -j ACCEPT # vpn to vpn
# iptables -A FORWARD -i tun0 -o eth0 -j ACCEPT # vpn to ethernet
# iptables -A FORWARD -i eth0 -o tun0 -j ACCEPT # ethernet to vpn

Reference :
iptables - Linux IP Forwarding for OpenVPN - correct firewall setup? - Server Fault