NAT Linux

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

• Ubunto
• Redhat
• Centos

Basic NAT example using IP Tables

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 IP Tables 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)

Example network

iptables -A PREROUTING -t nat -i eth1 -p tcp --dport 80 -j DNAT --to 192.168.1.30:80

OLD

Add the following lines to /etc/rc.local to configure Routing at boot.

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.