Linux as Internet Gateway

Step 1. Add 2 Network cards to the Linux box

Step 2. Verify the Network cards, check if they installed properly or not

Step 3. Configure eth0 for Internet with a Public (External network or Internet)
# cat 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 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=192.168.1.1 # 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 208.67.222.222 # Primary DNS Server provided by the ISP
nameserver 208.67.220.220 # Secondary DNS Server provided by the ISP
Step 8. NAT configuration with IP Tables
First of all you have to flush and delete existing Firewall rules. So flush rules by typing in terminal:
iptables -F
iptables -t nat -F
iptables -t mangle -F
Now delete these chains:
iptables -X
iptables -t nat -X
iptables -t mangle -X
# Set up IP FORWARDing and Masquerading
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth1 -j ACCEPT
# Enables packet forwarding by kernel (save this setting in /etc/sysctl.conf file)
>echo 1 > /proc/sys/net/ipv4/ip_forward
#Apply the configuration
service iptables save
service iptables restart
# Check if iptables is set to start during boot up
chkconfig –list iptables
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 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:
“Gateway”: Use the internal network IP address of the Linux box. (192.168.1.1)
“DNS Configuration”: Use the IP addresses of the ISP Domain Name Servers.
“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