root@OpenWrt:~# cat /etc/firewall.user 
#!/bin/sh
. /etc/functions.sh

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

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

### BIG FAT DISCLAIMER
## The "-i $WAN" is used to match packets that come in via the $WAN interface.
## it WILL NOT MATCH packets sent from the $WAN ip address -- you won't be able
## to see the effects from within the LAN.

### Open port to WAN
## -- This allows port 22 to be answered by (dropbear on) the router
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

### Port forwarding
## -- This forwards port 8080 on the WAN to port 80 on 192.168.1.2
# iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 8080 -j DNAT --to 192.168.1.2:80
# iptables        -A forwarding_rule -i $WAN -p tcp --dport 80 -d 192.168.1.2 -j ACCEPT

iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 993 -j DNAT --to-destination 192.168.0.2
iptables        -A forwarding_rule -i $WAN -p tcp --dport 993 -d 192.168.0.2 -j ACCEPT

iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 60000:60021 -j DNAT --to-destination 192.168.0.10
iptables        -A forwarding_rule -i $WAN -p tcp --dport 60000:60021 -d 192.168.0.10 -j ACCEPT
iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 60022 -j DNAT --to 192.168.0.10:22
iptables        -A forwarding_rule -i $WAN -p tcp --dport 22 -d 192.168.0.10 -j ACCEPT
iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 60023:60999 -j DNAT --to-destination 192.168.0.10
iptables        -A forwarding_rule -i $WAN -p tcp --dport 60023:60999 -d 192.168.0.10 -j ACCEPT
iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 5900 -j DNAT --to-destination 192.168.0.10
iptables        -A forwarding_rule -i $WAN -p tcp --dport 5900 -d 192.168.0.10 -j ACCEPT

### DMZ
## -- Connections to ports not handled above will be forwarded to 192.168.1.2
# iptables -t nat -A prerouting_rule -i $WAN -j DNAT --to 192.168.1.2
# iptables        -A forwarding_rule -i $WAN -d 192.168.1.2 -j ACCEPT

root@OpenWrt:~# cat /etc/init.d/S35firewall 
#!/bin/sh

## Please make changes in /etc/firewall.user

. /etc/functions.sh
WAN="$(nvram get wan_ifname)"
WANDEV="$(nvram get wan_device)"
LAN="$(nvram get lan_ifname)"
WDS="$(nvram get wds0_ifname)"

## CLEAR TABLES
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 -N LAN_ACCEPT
[ -z "$WAN" ] || iptables -A LAN_ACCEPT -i "$WAN" -j RETURN
[ -z "$WANDEV" -o "$WANDEV" = "$WAN" ] || iptables -A LAN_ACCEPT -i "$WANDEV" -j RETURN
iptables -A LAN_ACCEPT -j ACCEPT

### INPUT
###  (connections with the router as destination)

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

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

  # allow
  iptables -A INPUT -j LAN_ACCEPT       # allow from lan/wifi interfaces 
  iptables -A INPUT -p icmp     -j ACCEPT       # allow ICMP
  iptables -A INPUT -p gre      -j ACCEPT       # allow GRE

  # 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 REJECT
  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 REJECT
  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 $LAN -o $LAN -j ACCEPT
  iptables -A FORWARD -i $WDS -o $LAN -j ACCEPT
  iptables -A FORWARD -i $LAN -o $WDS -j ACCEPT
  iptables -A FORWARD -i $LAN -o $WAN -j ACCEPT

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

### MASQ
  iptables -t nat -A PREROUTING -j prerouting_rule
  iptables -t nat -A POSTROUTING -j postrouting_rule
  iptables -t nat -A POSTROUTING -o $WAN -j MASQUERADE

## USER RULES
[ -f /etc/firewall.user ] && . /etc/firewall.user
[ -e /etc/config/firewall ] && {
        awk -f /usr/lib/common.awk -f /usr/lib/firewall.awk /etc/config/firewall | ash
}