Linux as a Router

Here is good news for you, if you have an old PC hardware. Dont throw it away. It can eliminate the

need of purchasing a router for your small network.

A Linux system could work as a powerful router in small network. You only have to install a Linux Distro

on your PC/Laptop and install an extra NIC if you only have one. It only takes two minutes to configure a

Linux machine as a router. This blog show you how to configure Linux as a router in few simple steps.

Prerequisites

1. A Linux system with two or more interfaces.

2. Internet Connection.

3. IP tables basic knowledge. (need not to be a geek)

4. Another system on LAN for testing.

5. Dnsmasq. (if a DHCP is not working in your LAN environment)

Topology

Figure 1 Topology

Linux as a Router

Steps

1. Configuring ip address

edit network connection setting from the setting menu

2. Enable ip forwarding

Run this command in terminal

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

This will give output as

1 /proc/sys/net/ipv4/ip_forward

To make the change permanent so after next boot you don't have to enable ip forwarding again. Edit

/etc/systcl.conf

and uncomment the line

net.ipv4.ip_forward=1

Figure 2 Config IP address Figure 3 Config IP address

Linux as a Router

3. For enabling NATing issue the command:

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

4. Configuring firewall to allow inbound and outbound transit traffic by the follwoing commands:

sudo iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

sudo iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT

5. Configuring dnsmasq

Dnsmasq is a lightweight dns cum dhcp/tftp server and it is easily configurable. Just uncomment the line

#dhcp-range=192.168.0.50,192.168.0.150,12h

Here u can change the range with your choice of subnet, no need to change other things dnsmasq will

automatically provide default router and DNS server ip address as the ip of machine it is running on.

Figure 4

Linux as a Router

(NOTE: if you have a DHCP serer already in your environment you need not to follow this step just

configure your DHCP to provide gateway ip address as the ip address of LAN facing interface of Linux

machine or set "option 3 " as the ip address of Linux machine.)

6. Test the connectivity to internet on test pc

(NOTE: The interface used here i.e. eth0 & eth1 could be different on your machine i.e. the machine can

be connected to internet via a PPP (point to point) or wireless connection, so make change in

configuration according to your need.)

Figure 5