From
http://pptpclient.sourceforge.net/routing.phtml
iptables -A INPUT -s 192.168.10.0/24 -d 0.0.0.0/0.0.0.0 -j ACCEPT -i (internal interface)
iptables -A FORWARD s 0.0.0.0/0.0.0.0 -d192.168.10.0/24
-j ACCEPT -o (external interface)
iptables -A FORWARD -s 192.168.10.0/24 -d 0.0.0.0/0.0.0.0
-j ACCEPT
iptables --table nat --append POSTROUTING \
--out-interface ${IFNAME} --jump MASQUERADE