Gre Tunnel Cisco Linux traffic forwarding

I setup a gre tunnel a cisco router and a Linux machine, the tunnel interface in the Linux box named pic.
Well i have to forward traffic coming from cisco through the Linux box.
the rules i’ve set in the Linux box is described as follow:


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 -i pic  -o ppp0 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables  -A FORWARD -i ppp0 -o pic -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables  -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE


I see the traffic coming from tunnel and forwarded to internet but no reply from sent packet.

May i miss something like a routing rule.

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