Setting Up NAT Server in Linux

Here I have mentioned How to setup a NAT server in Linux in just few servers with the help  of iptable.

In my scenario I have tested with Red Hat enterprise Linux 6(Kernal 3.5) but I think I should work with most of linux kernels .

In my Environment there are two lan card on my linux system where I want to setup NAT server.

I am assuming a well configured linux system where iptable  and network connectivity is working with two NIC card and  IP address  configured are :

For eth0

IP=110.234.6.180/24 and gateway =110.234.6.170

(In most of practical scenario eth0 is given as public ip provided by your ISP)

For eth0

IP=192.168.0.251/24

Now login to terminal with superuser (root)

First you need to tell your kernel that you want to allow IP forwarding.

#echo 1 > /proc/sys/net/ipv4/ip_forward

Now  configure iptables to forward the packets from your internal network(192.168.0.0), on /dev/eth1, to your external network(110.234.6.180 i.e provided by your ISP) on /dev/eth0. You do this will the following commands:

# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

#  iptables -A FORWARD -i eth0 -o eth1 -m state   --state RELATED,ESTABLISHED -j ACCEPT

# iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

Now now open /etc/sysctl.conf in your editor (in my case I am using vim)

#vim . /etc/sysctl.conf 

Change the line that says net.ipv4.ip_forward = 0to net.ipv4.ip_forward = 1

Now now edit  /etc/sysconfig/iptables-config in your editor (in my case I am using vim) make sure that:

IPTABLES_MODULES_UNLOAD, IPTABLES_SAVE_ON_STOP, IPTABLES_SAVE_ON_RESTART are all set to 'yes'.

Reboot machine and you are done.