What is IP Masquerading?

IP Masquerade is a form of Network Address Translation or NAT which NAT allows internally connected computers that do not have one or more registered Internet IP addresses to communicate to the Internet via the Linux server's Internet IP address. The software interface which enables one to configure the kernel for IP masquerading is iptables.

To configure IP Masquerading, you need to have two LAN cards, a PC with CentOS 4 Linux installed.
Install the LAN cards and configure the IP address of your public and private network.

1. Configure the IP address of your public network, it should be similar like this one.

DEVICE=eth0
BOOTPROTO=static
HWADDR=00:50:BA:88:72:D4
IPADDR=xx.xx.xx.xx	#Provided by your ISP
NETMASK=255.255.255.0    # Provided by your ISP
GATEWAY=xx.xx.xx.1    # Provided by your ISP
ONBOOT=yes
TYPE=Ethernet

2. Configure the IP of your private network

DEVICE=eth0
BOOTPROTO=static
HWADDR=00:50:BA:88:72:D4
IPADDR=192.168.0.1
NETMASK=255.255.255.0
ONBOOT=yes
TYPE=Ethernet

3. Configure your /etc/sysconfig/network

NETWORKING=yes
HOSTNAME=localhost.localdomain

4. Check your name server information, if it is configured correctly it should be look like this.

nameserver xx.xx.xx.xx      # Primary DNS Server provided by your ISP
nameserver xx.xx.xx.xx      # Secondary DNS Server provided by your ISP

Reload your network configurations

5. All set, lets configure the iptables to enable IP Masquerading and share your internet connection.

6. Enable packet forwarding

Add this command in /etc/rc.local to automatically start ip forwarding during boot

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

7. Thats it, you can configure your PC in your private network.

IP Address: 192.168.0.2 up to 192.168.0.254
Subnet Mask/Mask/NETMASK: 255.255.255.0
Gateway: 192.168.0.1
DNS: #Your DNS entry should be same in your /etc/resolv.conf
Primary DNS: xx.xx.xx.xx
Secondary DNS: xx.xx.xx.xx

8. Installing DHCP server
To enable DHCP server in your CentOS box, you need to install dhcp package.

Rename your /etc/dhcpd.conf to /etc/dhcpd.conf.old.
Then copy the /usr/share/doc/dhcp-x.x.x/dhcpd.conf.sample to /etc/.

Edit your /etc/dhcpd.conf to similar like this:

ddns-update-style interim;
ignore client-updates;

subnet 192.168.0.0 netmask 255.255.255.0 {
        option routers                  192.168.0.1; #Default Gateway
        option subnet-mask              255.255.255.0;
        option domain-name-servers      xx.xx.xx.xx,xx.xx.xx.xx;       
	range dynamic-bootp 192.168.0.100 192.168.0.200;  #DHCP Range to assign
        default-lease-time 43200;
        max-lease-time 86400;
}

Configure dhcp to run during boot time and start your dhcp server.

9. Disabling and enabling your iptables

To disable your iptables, run this commands.

Type this command if you want to enable your iptables

Have fun...