PPTP in Linux -cannot port forward, certain website not working

Having problem with VPN in Linux? Ubuntu, Debian, etc?

These two should help with your problem!

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
or
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE && iptables-sav

add NAT rule to your iptables. sweet.

and...

Ooops some website can't work on your vpn? Easy. Just add this:
iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu




Source:
http://www.tldp.org/HOWTO/IP-Masquerade-HOWTO/mtu-issues.html