Consider the following example: you have a router between two networks, and you want to cut off the
router from the outside world using some iptables rules. However, all traffic that is forwarded by the router between the two networks basically is to be ignored by iptables (i.e., the router does not play firewall for any of the two networks).
Currently, if conntrack is loaded on the router, then conntrack -L on the router lists all the connections, not only those to and from the router, but also all connections between the two. Certainly, it takes some CPU cycles for the router to keep track of all the connections. Also, the number of connections that conntrack can take of is limited.
So is there a way to let Linux "bypass" conntrack and maybe other netfilter stuff when it comes to forwarded packets?