Step-By-Step Configuration of NAT with iptables

Submitted by ganesh35 (Contact Author) (Forums) on Wed, 2006-11-08 19:53. :: Kernel | Linux

Step-By-Step Configuration of NAT with iptables

This tutorial shows how to set up network-address-translation (NAT) on a Linux system with iptables rules so that the system can act as a gateway and provide internet access to multiple hosts on a local network using a single public IP address. This is achieved by rewriting the source and/or destination addresses of IP packets as they pass through the NAT system. 

Requirements:

CPU - PII or more
OS - Any Linux distribution
Software - Iptables
Network Interface Cards: 2

Here is my considerations:

Replace xx.xx.xx.xx with your WAN IP

Replace yy.yy.yy.yy with your LAN IP

(i.e. 192.168.0.0/16, 172.16.0.0/12,  10.0.0.0/8 as suggested by Mr. tzs)

WAN = eth0 with public IP xx.xx.xx.xx
LAN = eth1 with private IP yy.yy.yy.yy/ 255.255.0.0

 

Step by Step Procedure

Step #1. Add 2 Network cards to the Linux box

Step #2. Verify the Network cards, Wether they installed properly or not

ls /etc/sysconfig/network-scripts/ifcfg-eth* | wc -l

    ( The output should be "2")

Step #3. Configure eth0 for Internet with a Public ( IP External network or Internet)

cat /etc/sysconfig/network-scripts/ifcfg-eth0

DEVICE=eth0
BOOTPROTO=none
BROADCAST=xx.xx.xx.255    # Optional Entry
HWADDR=00:50:BA:88:72:D4    # Optional Entry
IPADDR=xx.xx.xx.xx
NETMASK=255.255.255.0    # Provided by the ISP
NETWORK=xx.xx.xx.0       # Optional
ONBOOT=yes
TYPE=Ethernet
USERCTL=no
IPV6INIT=no
PEERDNS=yes
GATEWAY=xx.xx.xx.1    # Provided by the ISP

Step #4. Configure eth1 for LAN with a Private IP (Internal private network)

cat /etc/sysconfig/network-scripts/ifcfg-eth1

BOOTPROTO=none
PEERDNS=yes
HWADDR=00:50:8B:CF:9C:05    # Optional
TYPE=Ethernet
IPV6INIT=no
DEVICE=eth1
NETMASK=255.255.0.0        # Specify based on your requirement
BROADCAST=""
IPADDR=192.168.2.1        # Gateway of the LAN
NETWORK=192.168.0.0        # Optional
USERCTL=no
ONBOOT=yes

Step #5. Host Configuration    (Optional)

cat /etc/hosts

    127.0.0.1       nat localhost.localdomain   localhost

Step #6. Gateway Configuration

cat /etc/sysconfig/network

    NETWORKING=yes
    HOSTNAME=nat
    GATEWAY=xx.xx.xx.1    # Internet Gateway, provided by the ISP

Step #7. DNS Configuration

cat /etc/resolv.conf

    nameserver 203.145.184.13      # Primary DNS Server provided by the ISP
    nameserver 202.56.250.5        # Secondary DNS Server provided by the ISP

Step #8. NAT configuration with IP Tables

    # Delete and flush. Default table is "filter". Others like "nat" must be explicitly stated.

iptables --flush            # Flush all the rules in filter and nat tables

iptables --table nat --flush

iptables --delete-chain

# Delete all chains that are not in default filter and nat table

iptables --table nat --delete-chain

# Set up IP FORWARDing and Masquerading

iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE

iptables --append FORWARD --in-interface eth1 -j ACCEPT

# Enables packet forwarding by kernel 

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

 #Apply the configuration

service iptables restart

Step #9. Testing

 # Ping the Gateway of the network from client system

ping 192.168.2.1

Try it on your client systems

ping google.com

 Configuring PCs on the network (Clients)

•    All PC's on the private office network should set their "gateway" to be the local private network IP address of the Linux gateway computer.
•    The DNS should be set to that of the ISP on the internet.
Windows '95, 2000, XP,  Configuration:

•    Select "Start" + Settings" + "Control Panel"
•    Select the "Network" icon
•    Select the tab "Configuration" and double click the component "TCP/IP" for the ethernet card. (NOT the TCP/IP -> Dial-Up Adapter)
•    Select the tabs:
o    "Gateway": Use the internal network IP address of the Linux box. (192.168.2.1)
o    "DNS Configuration": Use the IP addresses of the ISP Domain Name Servers. (Actual internet IP address)
o    "IP Address": The IP address (192.168.XXX.XXX - static) and netmask (typically 255.255.0.0 for a small local office network) of the PC can also be set here.


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Submitted by mynamewastaken (registered user) on Sun, 2007-02-04 03:52.

(i.e. 192.168.0.0/16, 172.16.0.0/12,  10.0.0.0/8 as suggested by Mr. tzs)

 Unless you are doing some subnetting here, I would suggest keeping those internal addresses at their defaults which are:

192.168.1.0/24, ie mask 255.255.255.0 for a class c address.

172.16.0.0/16 mask 255.255.0.0 for class b

10.0.0.0/8 255.0.0.0 for class a

Submitted by tzs (registered user) on Fri, 2006-11-10 03:04.

1. The example is using 190.1.0.0/16 for private IP addresses. This is bad. That block is real live addresses, allocated to ISPs in Latin America and the Carribean. Private IP addresses should be choosen from one of the following blocks:

192.168.0.0/16

172.16.0.0/12

 10.0.0.0/8

 2. The example uses 190.1.7.1 as the address of the gateway on the LAN in step #4, but pings it at 190.1.6.1 in step #9.