I'm trying to set up a GRE tunnel between a linux box and a router (LAN), and I'm having a few problems which seem to depend to my iptables configuration. Watching with tcpdump on linux box, I can see packets coming with flags GREv0, all i need right know is forwarding this data to internet, found here some trace :

iptables -F
iptables -X
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT 

iptables -t nat -F
iptables -t nat -X
iptables -t nat -P PREROUTING ACCEPT
iptables -t nat -P POSTROUTING ACCEPT
iptables -t nat -P OUTPUT ACCEPT 
iptables -t mangle -F
iptables -t mangle -X
iptables -t mangle -P PREROUTING ACCEPT
iptables -t mangle -P OUTPUT ACCEPT 
iptables -A INPUT -p 47 -j ACCEPT 
iptables -A FORWARD -i ppp0 -o cloud -j ACCEPT

iptables -A FORWARD -i cloud -o ppp0 -j ACCEPT

iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT 
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
echo "1" > /proc/sys/net/ipv4/ip_forward
cloud Link encap:UNSPEC HWaddr C4-CE-7A-2E-F2-BF-DD-C0-00-00-00-00-00-00-00-00
inet adr:10.3.3.3 P-t-P:10.3.3.3 Masque:255.255.255.255
UP POINTOPOINT RUNNING NOARP MTU:1476 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:124 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 lg file transmission:0
RX bytes:0 (0.0 B) TX bytes:10416 (10.1 KiB)
Table de routage IP du noyau
Destination Passerelle Genmask Indic MSS Fenêtre irtt Iface
196.206.120.1 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
10.3.3.0 0.0.0.0 255.255.255.0 U 0 0 0 cloud
0.0.0.0 196.206.120.1 0.0.0.0 UG 0 0 0 ppp0
root@aldebaran:~# ip route
196.206.120.1 dev ppp0 proto kernel scope link src 196.206.122.46
192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.18
10.3.3.0/24 dev cloud scope link
default via 196.206.120.1 dev ppp0