iptables -A forwarding_rule -i tun+ -j ACCEPT
iptables -A forwarding_rule -o tun+ -j ACCEPT

Chain forwarding_rule (1 references)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  tun+   any     anywhere             anywhere
    7   564 ACCEPT     all  --  any    tun+    anywhere             anywhere