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.
