I have a machine that is connected to another host over a permanent GRE tunnel. I have put the machine behind a Linux firewall (Smoothwall), and NAT-ed all GRE packets to the machine, using the rules:
/sbin/iptables -t nat -A PREROUTING -p 47 --src $tunnel_server_ip -j DNAT --to-destination $false_ip /sbin/iptables -t nat -A POSTROUTING -p 47 --src $false_ip -j SNAT --to-source $real_ip /sbin/iptables -t nat -A INPUT -p 47 -j ACCEPT
It all works fine, but after a period of inactivity on the tunnel, of approximately 15 minutes, I cannot connect from the internet to the machine over the tunnel. After I send any packet from the machine to the internet, ping, open a web page in a browser, the tunnel gets activated again. As a workaround I now use a cron script that starts curl to a page in the internet.
Please help me understand what causes the tunnel to stop functioning, and tell me what can I do to replace the curl hack.