Internet Connection Sharing - Simple iptables Example
#!/bin/sh
#
# internet connection sharing wlan0 is the gate way
# eth0 is the lan port this might use a straight ethernet cable to a router wan port or a switch or a single PC
# 192.168.2.2 is the port that is being used by the lan for access I changed it to 192.168.2.254 and set fixed
# addresses for the wan and router
#
# change wlan0 to ppp0 and you can use this for mobile broadband connection sharing
#
ip link set dev eth0 up
ip addr add 192.168.2.1/24 dev eth0
sysctl net.ipv4.ip_forward=1
iptables -t nat -A POSTROUTING -o wlan0 -s 192.168.2.0/24 -j MASQUERADE
iptables -t nat -A PREROUTING -i wlan0 -p tcp --dport 3074 -j DNAT --to-destination 192.168.2.2
iptables -t nat -A PREROUTING -i wlan0 -p udp -m multiport --dports 88,3074 -j DNAT --to-destination 192.168.2.2
iptables -A FORWARD -i wlan0 -d 192.168.2.2 -p tcp --dport 3074 -j ACCEPT
iptables -A FORWARD -i wlan0 -d 192.168.2.2 -p udp -m multiport --dports 88,3074 -j ACCEPT
The article goes on to say...
If things go wrong, the following script should save you when things get badly messed up.
#!/bin/sh
#
# rc.flush-iptables - Resets iptables to default values.
#
# Copyright (C) 2001 Oskar Andreasson
#
# 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; version 2 of the License.
#
# 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 or from the site that you downloaded it
# from; if not, write to the Free Software Foundation, Inc., 59 Temple
# Place, Suite 330, Boston, MA 02111-1307 USA
#
# Configurations
#
IPTABLES="/usr/sbin/iptables"
#
# reset the default policies in the filter table.
#
$IPTABLES -P INPUT ACCEPT
$IPTABLES -P FORWARD ACCEPT
$IPTABLES -P OUTPUT ACCEPT
#
# reset the default policies in the nat table.
#
$IPTABLES -t nat -P PREROUTING ACCEPT
$IPTABLES -t nat -P POSTROUTING ACCEPT
$IPTABLES -t nat -P OUTPUT ACCEPT
#
# reset the default policies in the mangle table.
#
$IPTABLES -t mangle -P PREROUTING ACCEPT
$IPTABLES -t mangle -P POSTROUTING ACCEPT
$IPTABLES -t mangle -P INPUT ACCEPT
$IPTABLES -t mangle -P OUTPUT ACCEPT
$IPTABLES -t mangle -P FORWARD ACCEPT
#
# flush all the rules in the filter and nat tables.
#
$IPTABLES -F
$IPTABLES -t nat -F
$IPTABLES -t mangle -F
#
# erase all chains that's not default in filter and nat table.
#
$IPTABLES -X
$IPTABLES -t nat -X
$IPTABLES -t mangle -X
