NAT Linux

Basic NAT example using iptables

iptables can be configured in two different ways when you boot your machine. The examples will assume you have two Network interface cards. eth0 connected to the internal - private network - and eth1 connected to the external - Internet.

Using a startup script. Example below.
Using iptables save and restore facelity. Example below.

Example network

Using a startup script to configure iptables

When you have a fixed address on the external Interface

When you know your address on the external Interface - not using DHCP - you should use source NAT (SNAT), which is slightly more efficient than using masquerading.
Add the following lines to /etc/rc.local

#  Enable IP Forwading between Interfaces (Routing)
echo "1" > /proc/sys/net/ipv4/ip_forward
# Use iptables Source NAT (SNAT) to translate internal 192.168.1.0/24 IP addresses to the external IP Address 83.90.47.30
# Allow the internal hosts to connect to any IP address on the outside 0.0.0.0/24
iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 0.0.0.0/0 -j SNAT --to 83.90.47.30

When you have a floating address on the external Interface

When you don't know your external IP Address and are fetching it from a DHCP server, you can use masquerading
Add the following lines to /etc/rc.local

#  Enable IP Forwading between Interfaces (Routing)
echo "1" > /proc/sys/net/ipv4/ip_forward
# Use iptables Source NAT (SNAT) to translate internal 192.168.1.0/24 IP addresses to the external IP Address that eth0 has
# Allow the internal hosts to connect to any IP address on the outside 0.0.0.0/24
iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 0.0.0.0/0 -o eth1 -j MASQUERADE

Checking iptables NAT rules and traffic

The iptables list option can be used to see iptables rules and iptables traffic.

use the command iptables -L -t nat to see the rules in the iptables NAT 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
MASQUERADE  all  --  192.168.1.0/24        anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

use the command iptables -L -t nat -v to see the rules in the iptables chains and the traffic for each rule.

[root@bkshost etc]# iptables -L -t nat -v
Chain PREROUTING (policy ACCEPT 275K packets, 19M bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain POSTROUTING (policy ACCEPT 13846 packets, 1355K bytes)
 pkts bytes target     prot opt in     out     source               destination
99447 6447K MASQUERADE  all  --  *      eth1    172.22.0.0/24        0.0.0.0/0

Chain OUTPUT (policy ACCEPT 15307 packets, 1758K bytes)
 pkts bytes target     prot opt in     out     source               destination

WEB server on the inside network

If you have a WEB server on the inside network for example 192.168.1.30, which should be accessible from the outside. (eth1)