I have configured a linux NAT box which has two network interfaces one is wired and the other is wireless.With this setup and few firewall rules I am able to share internet connection with other wireless clients.


Internet&SIPXPBX======NAT Box======== Wireless Client
Proxy ============(Masq-Iptables)===========(SIP SJPHONE)
eth0=============eth0+++++++ath0=============ath0
10.1.20.1==========IP-1=======IP-2=========192.168.20.45

IP-1 : 10.1.20.12
IP-2 : 192.168.20.30


Apart from internet sharing I am also trying to configure my SIP client(SJ- Soft Phone) to get registered with the SIP server but I am unable to do that the below given rules I have already applied and to support VoIP I have added some more rules.My SIP client is in different subnet and the Proxy is in different subnet.The registration request raised by this softphone will be for the IP 10.1.20.1 and the dest port 5060.This I have confirmed through packet sniffer(ethereal).Though I am able to telnet this destination port from the wireless client (and I am able to establish telnet connection too) the SIP user agent is not getting registered with the SIP proxy server.The account information is also created at the proxy end. It will be really appreciable if somebody can help me out in this


#!/bin/sh
INTIF="ath0"
EXTIF="eth0"
EXTIP=10.1.20.1
/sbin/depmod -a
/sbin/modprobe ip_tables
/sbin/modprobe ip_conntrack
/sbin/modprobe ip_conntrack_ftp
/sbin/modprobe ip_conntrack_irc
/sbin/modprobe iptable_nat
/sbin/modprobe ip_nat_ftp
echo "1" > /proc/sys/net/ipv4/ip_forward
echo "1" > /proc/sys/net/ipv4/ip_dynaddr
iptables -P INPUT ACCEPT
iptables -F INPUT
iptables -P OUTPUT ACCEPT
iptables -F OUTPUT
iptables -P FORWARD DROP
iptables -F FORWARD
iptables -t nat -F
iptables -A FORWARD -i $EXTIF -o $INTIF -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT
iptables -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE

As I am a beginner with iptables I have given the steps that I have followed. very sorry for this lengthy post.