I've created a GRE tunnel between LAN A and LAN B. LAN A is behind a router, and LAN B is behind a Linux box and this box has a pppoe interface. When i do tcpdump on Linux box i see that the traffic is routerd to Linux box (tunnel end), so my need is routing the tunnel traffic to internet. I've added these rules to my iptables script, and i stay unable to connect to internet from LAN A Permitting Linux to act as router (necessary to forward packets)

echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -A INPUT -p 47 -j ACCEPT
iptables -A FORWARD -i ppp0 -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -t nat -A POSTROUTING -o gre0 -j MASEQUARADE