NAT Linux

Linux IP Tables

IP Tables is used for packet filtering and NAT/PAT translation on several Linux distributions including

• Ubunto
• Redhat
• Centos

Enable IP Forwarding (Routing) between interfaces

Before you can route packets from the Inside network Interface to the outside network Interface, you need to enable IP forwarding.

Enabling forwarding until next boot

To enable IP forwarding you need to toggle a switch in the kernel, you can do that with the following command. But remember, next time you boot the machine, it will be switched of again.

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

Enable IP forwarding permanently

To enable IP forwarding permanently, you either need to issue the command below in a boot-script. /etc/rc.local or similar
or
Change the file /etc/sysctl.conf to include the following line. Perhaps you only need to uncomment a line. (Remove the # from beginning of the line)

net.ipv4.ip_forward=1

Note that altering /etc/sysctl.conf will first be effective after reboot.

Basic NAT example using IP Tables

In the example below the internal network 192.168.1.0/24 is Source Natted (SNAT) to the external IP Address 83.90.47.30. Source nat also makes port translations. So the example uses NAT/PAT, and would be sufficient as a NAT/PAT solution for a private network.

iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 0.0.0.0/0 -j SNAT --to 83.90.47.30

use the command iptables -L -t nat to see the rule in the IPTABLES chains.

[root@bkshost etc]# iptables -L -t nat
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
SNAT       all  --  192.168.1.0/24        anywhere           to:83.90.47.30

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Webserver located on internal network

To redirect WEB traffic originating from the outside to a WEB-server on the inside you would use a rule as showed below.