Routing (NAT)

                                                  Step By Step Configuration of NAT

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.

WAN = eth0 with public IP (120.50.4.162)
LAN = eth1 with Private IP (192.168.1.100)

ISP Line:

IP Address: 120.50.4.160/30
Usable IP (ISP): 120.50.4.161
Usable IP (OFFICE): 120.50.4.162
IP Address: 120.50.4.162
Subnet mask: 255.255.255.252
Gateway: 120.50.4.161
Primary DNS: 203.112.194.243
Secondary DNS: 203.112.194.15

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

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

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

Ethernet Card Configuration: (For Public IP address)
[root@gateway ~]# vim /etc/sysconfig/network‐scripts/ifcfg‐eth0
DEVICE=eth0
BOOTPROTO=none
BROADCAST=120.50.4.163                         # Optional Entry
HWADDR=00:21:27:c7:c0:0f
IPADDR=120.50.4.162
NETMASK=255.255.255.252                      # Provided by the ISP
NETWORK=120.50.4.160                           # Optional
ONBOOT=yes
TYPE=Ethernet
GATEWAY=120.50.4.161                            # Provided by the ISP

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

Ethernet Card Configuration: (For Private IP address)

[root@gateway ~]# vim /etc/sysconfig/network‐scripts/ifcfg‐eth1
DEVICE=eth1
BOOTPROTO=none
BROADCAST=192.168.1.255
HWADDR=00:24:21:9a:6a:9a
IPADDR=192.168.1.100                                 # Gateway of the LAN
NETMASK=255.255.255.0                            # Specify based on your requirement
NETWORK=192.168.1.0
ONBOOT=yes
TYPE=Ethernet

Step #5. Host Configuration (Optional)

[root@gateway ~]# cat /etc/hosts


127.0.0.1           gateway              localhost.localdomain localhost

Step #6. Gateway Configuration

[root@gateway ~]# vim /etc/sysconfig/network
NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=gateway
GATEWAY=120.50.4.161            # Internet Gateway, provided by the ISP

Step #7. DNS Configuration

[root@gateway ~]# vim /etc/resolv.conf
nameserver 203.112.194.243                  # Primary DNS Server provided by the ISP
nameserver 203.112.194.15                    # 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.

[root@gateway ~]# cat /etc/sysconfig/iptables
[root@gateway ~]# iptables –L
[root@gateway ~]# service iptables stop
[root@gateway ~]# iptables ‐F
[root@gateway ~]# service iptables save
[root@gateway ~]# iptables ‐t nat ‐A POSTROUTING ‐s 192.168.1.0/24 ‐j SNAT ‐‐tosource
120.50.4.162
[root@gateway ~]# service iptables save
[root@gateway ~]# service iptables status
[root@gateway ~]# service iptables restart
[root@gateway ~]# cat /etc/sysconfig/iptables
num        target    pro     opt        source            destination
1             SNAT   all      ‐‐    192.168.1.0/24     0.0.0.0/0     to:120.50.4.162

Step #8. Ip Forwarding Enable

[root@gateway ~]# vim /etc/sysctl.conf
net.ipv4.ip_forward = 1      [ Change “0” to “1” for ip forwarding enable]
Step #9. Testing
[root@gateway ~]# ping www.google.com
[root@gateway ~]# ping 192.168.2.1
[root@gateway ~]# route ‐n

Destination            Gateway           Genmask         Flags Metric Ref Use           Iface
120.50.4.160        0.0.0.0        255.255.255.252    U        0      0    0               eth0
192.168.1.0          0.0.0.0        255.255.255.0        U        0      0    0               eth1
169.254.0.0           0.0.0.0       255.255.0.0           U         0      0   0                eth1
0.0.0.0                120.50.4.161   0.0.0.0                UG      0      0    0               eth0

Client Configuration in Windows XP

THE END