-# initial rules
-$IPT -t filter -A INPUT -m state --state INVALID -j DROP
-$IPT -t filter -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-$IPT -t filter -A FORWARD -m state --state INVALID -j DROP
-$IPT -t filter -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
+iptables -N input_rule
+iptables -N output_rule
+iptables -N forwarding_rule
+
+iptables -t nat -N prerouting_rule
+iptables -t nat -N postrouting_rule
+
+### Port forwarding
+# iptables -t nat -A prerouting_rule -p tcp --dport 22 -j DNAT --to 192.168.1.2
+# iptables -A forwarding_rule -p tcp --dport 22 -d 192.168.1.2 -j ACCEPT
+
+### INPUT
+### (connections with the router as destination)
+
+ # base case
+ iptables -P INPUT DROP
+ iptables -A INPUT -m state --state INVALID -j DROP
+ iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
+
+ # allow
+ iptables -A INPUT -i \! $WAN -j ACCEPT # allow from lan/wifi interfaces
+ iptables -A INPUT -p icmp -j ACCEPT # allow ICMP
+ iptables -A INPUT -p 47 -j ACCEPT # allow GRE
+ #
+ # insert accept rule or to jump to new accept-check table here
+ #
+ iptables -A INPUT -j input_rule
+
+ # reject (what to do with anything not allowed earlier)
+ iptables -A INPUT -p tcp --syn --tcp-option \! 2 -j DROP
+ iptables -A INPUT -p tcp -j REJECT --reject-with tcp-reset
+ iptables -A INPUT -j REJECT --reject-with icmp-port-unreachable
+
+### OUTPUT
+### (connections with the router as source)
+
+ # base case
+ iptables -P OUTPUT DROP
+ iptables -A OUTPUT -m state --state INVALID -j DROP
+ iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
+
+ # allow
+ iptables -A OUTPUT -j ACCEPT #allow everything out
+ #
+ # insert accept rule or to jump to new accept-check table here
+ #
+ iptables -A OUTPUT -j output_rule
+
+ # reject (what to do with anything not allowed earlier)
+ iptables -A OUTPUT -p tcp -j REJECT --reject-with tcp-reset
+ iptables -A OUTPUT -j REJECT --reject-with icmp-port-unreachable