How to make GRE tunnel iptables port redirect on Linux

Load the ip_nat_pptp and ip_conntrack_pptp kernel module
modprobe ip_nat_pptp
modprobe ip_conntrack_pptp

vi /etc/sysconfig/iptables-config
IPTABLES_MODULES=”ip_nat_pptp”

Ref. http://www.pc-freak.net/blog/how-to-make-gre-tunnel-iptables-port-redirect-on-linux/
http://download.parallels.com/doc/pcs/html/Parallels_Cloud_Server_Users_Guide/30254.htm