Something

for T in filter nat; do
  iptables -t $T -F
  iptables -t $T -X
done

iptables -N input_rule
iptables -N output_rule
iptables -N forwarding_rule
iptables -t nat -N prerouting_rule
iptables -t nat -N postrouting_rule

iptables -P INPUT DROP
iptables -A INPUT -m state --state INVALID -j DROP
iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A INPUT -p tcp --tcp-flags SYN SYN --tcp-option \! 2 -j  DROP

iptables -A INPUT -j input_rule

iptables -A INPUT ${eth1:+\! -i eth1} -j ACCEPT
iptables -A INPUT -p icmp     -j ACCEPT 
iptables -A INPUT -p gre      -j ACCEPT

iptables -A INPUT -p tcp -j REJECT --reject-with tcp-reset
iptables -A INPUT -j REJECT --reject-with icmp-port-unreachable

iptables -P OUTPUT DROP         
iptables -A OUTPUT -m state --state INVALID -j DROP
iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

iptables -A OUTPUT -j output_rule

iptables -A OUTPUT -j ACCEPT 

iptables -A OUTPUT -p tcp -j REJECT --reject-with tcp-reset
iptables -A OUTPUT -j REJECT --reject-with icmp-port-unreachable

iptables -P FORWARD DROP
iptables -A FORWARD -m state --state INVALID -j DROP
iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT

iptables -A FORWARD -j forwarding_rule

iptables -A FORWARD -i br0 -o br0 -j ACCEPT
iptables -A FORWARD -i br0 -o eth1 -j ACCEPT

iptables -t nat -A prerouting_rule -i eth1 -p tcp --dport 22 -j ACCEPT
iptables        -A input_rule      -i $WAN -p tcp --dport 22 -j ACCEPT

iptables -t nat -A PREROUTING -j prerouting_rule
iptables -t nat -A POSTROUTING -j postrouting_rule
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE

iptables -F input_rule
iptables -F output_rule
iptables -F forwarding_rule
iptables -t nat -F prerouting_rule
iptables -t nat -F postrouting_rule