source: freewrt/package/iptables/files/firewall.init@ 141418f

#!/bin/sh

. /etc/rc.conf

if [ X"${firewall}" == X"NO" ]; then
        exit 0
fi

. /etc/functions.sh

WAN=$(nvram get wan_ifname)
LAN=$(nvram get lan_ifname)

case $1 in
        start)
                iptables -N input_rule
                iptables -N output_rule
                iptables -N forwarding_rule

                iptables -t nat -N prerouting_rule
                iptables -t nat -N postrouting_rule

                ### 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
                iptables -A INPUT -p tcp --tcp-flags SYN SYN --tcp-option \! 2 -j  DROP

                #
                # insert accept rule or to jump to new accept-check table here
                #
                iptables -A INPUT -j input_rule

                # allow
                iptables -A INPUT ${WAN:+\! -i $WAN} -j ACCEPT  # allow from all interfaces except for wan
                iptables -A INPUT -p icmp       -j ACCEPT       # allow ICMP
                iptables -A INPUT -p gre        -j ACCEPT       # allow GRE
                # allow ssh from remote
                iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 22 -j ACCEPT 
                iptables        -A input_rule      -i $WAN -p tcp --dport 22 -j ACCEPT

                # reject (what to do with anything not allowed earlier)
                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

                #
                # insert accept rule or to jump to new accept-check table here
                #
                iptables -A OUTPUT -j output_rule

                # allow
                iptables -A OUTPUT -j ACCEPT            #allow everything out

                # 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

                ### FORWARDING
                ### (connections routed through the router)

                # base case
                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

                #
                # insert accept rule or to jump to new accept-check table here
                #
                iptables -A FORWARD -j forwarding_rule

                # allow
                iptables -A FORWARD -i br0 -o br0 -j ACCEPT
                [ -z "$WAN" ] || iptables -A FORWARD -i $LAN -o $WAN -j ACCEPT

                # reject (what to do with anything not allowed earlier)
                # uses the default -P DROP

                ### MASQ
                iptables -t nat -A PREROUTING -j prerouting_rule
                iptables -t nat -A POSTROUTING -j postrouting_rule
                [ -z "$WAN" ] || iptables -t nat -A POSTROUTING -o $WAN -j MASQUERADE
                ;;
        stop)
                ## CLEAR TABLES
                for T in filter nat; do
                        iptables -t $T -F
                        iptables -t $T -X
                done
                ;;
        restart)
                $0 stop
                $0 start
                ;;
        *)
                echo "Usage: $0 {start | stop | restart}"
                ;;
esac
exit 0