NAT Configuration in Linux

NAT ( Network Address Translation) Configuration:

What is NAT ?
Network Address Translation is the process where a network device, usually a firewall, assigns a public address to a computer (or group of computers) inside a private network. The main use of NAT is to limit the number of public IP addresses an organization or company must use, for both economy and security purposes.

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 NAT Configure:
You apply system that solve you nat problems----------

Configure Step by Step

Step #1. Add Two 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 "Two")

Step #3. Configure eth0 for Internet with a Public ( IP Address 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  

cat /etc/hosts

   127.0.0.1       nat localhost.localdomain   localhost

Step #6. Gateway Configuration  (really set Getway) 

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 102.108.184.25      # Primary DNS Server provided by the ISP
    nameserver 102.56.250.5        # 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.

Common Problems in Linux:

The most common problem or question I receive about this is related to DNS. The instructions above setup the network to route the packets for you, but if your DNS isn't setup correctly (or at all) you won't be able to reference sites by their domain or hostnames. If at first you think this isn't working for you, plese try to ping an external IP address. If you can do this, then your problem is with DNS and not iptables.

Related Books And Tips :

• Linux iptables Pocket Reference
• Linux Networking Cookbook
• Linux Network Administrator's Guide
• What is a router?
• What is a private IP address?
• What is firewall?
• What is DHCP?
• www.newstips24.blogspot.com