NAT configuration in cent OS

Step-By-Step Configuration of NAT with iptables

 

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

Source : http://www.howtoforge.com/nat_iptables

Also execute the below commands if it still does not work:

  iptables -A FORWARD -i eth1 -j ACCEPT  iptables -A FORWARD -o eth1 -j ACCEPT    iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE