How to configure iptables to forward a GRE tunnel?

Question

Cisco

Good afternoon, colleagues!
I have never been closely involved in network administration, but there was a need, so I ask for advice.
It is necessary to connect a remote branch through VPN. There is Cisco 1841 on which all settings are made (it was checked before sending, everything works). The branch is tiny and connected via an iRZ RL01w router (TELE2 provider - swears GRE doesn't filter).
Tsiska port 0/0 is connected to Eth1 router, port 0/1 looks inside the branch network. The connection does not rise ...
I realized that I needed to dig in the direction of forwarding GRE traffic through the router by setting up iptables, but I could not find a final solution.
Can you advise what can be done?

Answer 1

-A INPUT -p gre -j ACCEPTor -A FORWARD -p gre -j ACCEPT
- depending on the configuration, but as far as I understood from the question, here is FORWARD. I also have these two
modules to look at: nf_nat_proto_gre nf_conntrack_proto_gre But they were automatically loaded when loading the ip_nat_pptp module (aka nf_nat_pptp ). In short, I have ip_nat_pptp in /etc/modules , and then it goes by itself. PS Well, I don’t have it on OpenWRT, but on an adult Linux (on a PC), you never know what has been cut down on OpenWRT. In short, no guarantee. :)
lsmod|grep proto_gre