iptables -t nat -A PREROUTING -j prerouting_rule
[ -z "$WAN" ] || iptables -t nat -A PREROUTING -i "$WAN" -j prerouting_wan
iptables -t nat -A POSTROUTING -j postrouting_rule
iptables -t nat -A PREROUTING -j prerouting_rule
[ -z "$WAN" ] || iptables -t nat -A PREROUTING -i "$WAN" -j prerouting_wan
iptables -t nat -A POSTROUTING -j postrouting_rule