[pptp-server] WG: forwarding GRE

[Strehle Stefan]

Two things are needed for forwarding of GRE,

a utility called ipfwd and
a kernel patch: 

details on:
http://www.impsec.org/linux/masquerade/VPN-howto/VPN-Masquerade-3.html#ss3.4

The rest of the configuration is straightforward: Allow connection on port
1723 with ipchains, forward traffic coming in on 1723 with ipmasqadm, and
then forward GRE traffic with ipfwd:

ipchains -A input -p tcp -i $extint -s $anywhere -d $vpnserver 1723 -j
ACCEPT
ipmasqadm portfw -a -P tcp -L $extnet 1723 -R $vpnserver 1723
ipfwd --masq $vpnserver 47 &

cheers,

Stefan