My template to configure iptables.

rc.firewall

#!/bin/bash
#rc.firewall
#
#
# Firewall configuration file.
#
#
# AUTHOR: José Lopes Oliveira Jr. <indiecode.com.br>
#
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>. 


##
# GLOBAL VARIABLES
#
IPT="/sbin/iptables"
USAGE="
USAGE: "${0##*/}" [start|stop|restart]
"

# PRIVATE INFO
PRV_IF="eth0"
PRV_IP="10.1.2.1"
PRV_NET="10.1.2.0/24"

# PUBLIC INFO
PUB_IF="eth1"
PUB_IP="10.1.3.1"
PUB_NET="10.1.3.0/24"


##
# FUNCTIONS
#
flush_rules() {
    local table

    for table in filter nat mangle; do
        $IPT -t $table -F
        $IPT -t $table -X
    done

    return $?
}

set_policies() {
# set_policies [ACCEPT|REJECT|DROP]
    $IPT -P INPUT "$1"
    $IPT -P OUTPUT "$1"
    $IPT -P FORWARD "$1"

    return $?
}

packet_forwarding() {
# Enable/Disable packet forwarding.
    if [ "$1" = "ON" ]; then
        echo "1" > /proc/sys/net/ipv4/ip_forward
    else
        echo "0" > /proc/sys/net/ipv4/ip_forward
    fi

    return $?
}

load_modules() {
# load_modules foo bar barfoo foobar
    local module

    for module in $@; do
        echo modprobe $module
    done

    return $?
}


##
# ARGUMENTS PROCESSING
# But first figure out if is root.
if [ "$(id -u)" != "0" ]; then
    echo "ERROR: This script must be run as root."
    exit 1
fi

if [ "$#" != "1" ]; then
    echo "$USAGE"
  exit 1

else
    case "$1" in
        "stop")
            echo "stopping firewall"
            packet_forwarding OFF
            flush_rules
            set_policies ACCEPT
            exit 0
            ;;

        "start" | "restart")
            echo "starting firewall"
            packet_forwarding ON
            flush_rules
            set_policies DROP
            load_modules ip_tables iptable_filter ip_nat_ftp iptable_mangle iptable_nat ip_conntrack_ftp ipt_LOG ipt_MASQUERADE
            ;;

        *)
            echo "$USAGE"
            exit 1
    esac
fi


##
# FIREWALL RULES
# In Firewall we trust.
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT

# PRIVATE NETWORK TRUST
# We trust the private network and will forward all traffic from it.
#$IPT -A FORWARD -i $PRV_IF -o $PUB_IF -j ACCEPT
#$IPT -A FORWARD -i $PUB_IF -o $PRV_IF -m state --state ESTABLISHED,RELATED -j ACCEPT

# NETWORK TRUST
# We assume that anyone on the private network can be trusted...
#$IPT -A INPUT -i $PRV_IF -s $PRV_NET -j ACCEPT
#$IPT -A OUTPUT -o $PRV_IF -d $PRV_NET -j ACCEPT
#... and no-one on the public network can be trusted.
#$IPT -A OUTPUT -o $PUB_IF -j ACCEPT
#$IPT -A INPUT -i $PUB_IF -m state --state ESTABLISHED,RELATED -j ACCEPT

# TRICKS
# Some security tricks.
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route  # Disable source routing
echo 1 > /proc/sys/net/ipv4/tcp_syncookies  # Enable SYN Cookies --to avoid SYN Flood attacks
echo 1 > /proc/sys/net/ipv4/conf/default/rp_filter  # Firewall will answer in the original interface

# ICMP
echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects  # Disable ICMP redirects
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts  # Disable ping broadcasts
$IPT -A INPUT -p icmp --icmp-type 0 -j ACCEPT  # Echo response
$IPT -A INPUT -p icmp --icmp-type 3 -j ACCEPT  # Destination unreachable
$IPT -A INPUT -p icmp --icmp-type 11 -j ACCEPT  # Time exceeded
$IPT -A INPUT -p icmp --icmp-type 8 -m limit --limit 1/second -j ACCEPT  # Prevent Ping of Death

# BCP 38 COMPLIANCE
# tools.ietf.org/html/bcp38
#$IPT -A FORWARD -i $PRV_IF -s ! $PRV_NET -j DROP  # From LAN
#$IPT -A FORWARD -i $PUB_IF -s $PRV_NET -j DROP  # To LAN

# BLOCKINGS
# Specific hosts
#$IPT -A INPUT -i $PUB_IF -s 10.220.231.236 -j REJECT --reject-with icmp-host-prohibited

# Network
#$IPT -A INPUT -i $PUB_IF -s 10.67.232.0/24 -j REJECT --reject-with icmp-net-prohibited

# PUBLISHINGS WITH PORT FORWARDING
# A webmail in the private network.
#$IPT -t nat -A PREROUTING -i $PUB_IF -d $PUB_IP -p tcp --dport smtp -j DNAT --to 192.168.1.254
#$IPT -A FORWARD -i $PUB_IF -p tcp --dport smtp -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

# A webserver in the private network.
#$IPT -t nat -A PREROUTING -i $PUB_IF -d PUB_IP -p tcp --dport http -j DNAT --to 192.168.1.253
#$IPT -A FORWARD -i $PUB_IF -p tcp --dport http -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

##
# PUT THE RULES HERE
#

##
# END OF RULES
#

##
# LOGGING
# If the packet gets here, it will be dropped.  But logged first.
$IPT -A INPUT -i $PUB_IF -j LOG --log-prefix="INPUT "
$IPT -A OUTPUT -o $PUB_IF -j LOG --log-prefix="OUTPUT "
$IPT -A FORWARD -j $LOG --log-prefix="FORWARD "

exit $?