Internet Sharing in Linux using IPTABLES

Enable IP forwarding

sysctl -w net.ipv4.ip_forward=1

To enable it in system startup, edit the file /etc/sysctl.conf and set

net.ipv4.ip_forward = 1

iptables

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

service iptables save

Network Configuration

The configuration should be like this

Router:-Connected to the internet provider
IP : 192.168.1.1

Internet Connected System

eth0 (LAN Card 1):-Connected to router

• IP : 192.168.1.10
• Netmask : 255.255.255.0
• Gateway : 192.168.1.1 (IP of the router)

eth1 (LAN Card 2):-Connected to the other system

• IP : 192.168.0.20 (Not the same network as the first card)
• Netmask : 255.255.255.0
• Gateway : 192.168.1.1 (IP of the router)

Second System

LAN Card connected to the first system

• IP : 192.168.0.30
• Netmask : 255.255.255.0
• Gateway : 192.168.0.20 (IP of the second Card in the first system)