LINUX As Router

Steps To Make Linux Machine as Router : -

Assumptions: eth0 is internet interface (connected to router for example) and eth1 connected to your internal lan (connected to your HUB/Switch for example).

My Linux   eth0  --> Internet
box        eth1  --> Lan

Step1. Turn on ip forwarding in kernel

open the file  /etc/sysctl.conf  and add/modify following line:

net.ipv4.ip_forward = 1

Step2. Save this configuration using below command

# sysctl -p

Step3. Setup IP forwarding and Masquerading (to act as router), you need to use NAT option of iptables as follows (add following rules to your iptables shell script) :

#  iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE
#  iptables    --append   FORWARD   –in-interface  eth1  -j   ACCEPT

Step4. We  are done with configuration part , Test it with ping or dig:

# ping   ISPdomain.com
# dig    yahoo.com

How To Set the static Route in Linux : -

Scenario :-  We have two lan cards and assigned below mentioned ips

eth0 LAN network 172.0.0.0/8 - gateway IP – 172.8.2.65
eth1 ISP assigned network 202.54.22.128/28 - gateway IP – 202.54.22.129

So we can only ping /access to public server but not the another servers inside LAN , for the solution of this problem we will set the route with route command and static-route file.

Set route using route command

Type the following command:
# route add -net 172.0.0.0 netmask 255.0.0.0 gw 172.8.2.65 eth0
# route -n

The problem with the above 'route' command is that, when linux machine  reboots it will forget static routes. So store them in configuration file: 

# echo '172.0.0.0/8 via 172.8.2.65' >> /etc/sysconfig/network-scripts/route-eth0

Restart the network service

# service network restart

Verify new changes:

# route -n

# ping 172.8.2.65

# ping 172.8.2.10

# ping google.com

# traceroute google.com

# traceroute 172.8.2.10

Understanding Routing Table

What is Routing Table ?

Almost all computers and network devices connected to Internet use routing tables to compute the next hop for a packet. It is electronic table that is stored in a router or a networked computer. The routing table stores the routes (and in some cases, metrics associated with those routes) to particular network destinations. This information contains the topology of the network immediately around it. The construction of routing table is the primary goal of routing protocols and static routes.

Each Linux / UNIX / Windows or any computer that uses TCP/IP need to make routing decision. Routing table is used to control these decisions. To display routing table type the following command at UNIX / Linux shell prompt:

# netstat -r -n
OR
#  route -n

Sample output:
Kernel IP routing table

Destination	Gateway	Genmask	Flags	Metric	Ref	Use	Iface
192.168.2.0	0.0.0.0	255.255.255.0	U	0	0	0	eth0
192.168.1.0	0.0.0.0	255.255.255.0	U	0	0	0	ra0
169.254.0.0	0.0.0.0	255.255.255.0	U	1000	0	0	eth0
0.0.0.0	0.0.0.0	0.0.0.0	UG	100	0	0	ra0

Flag G indicates path to route packets via a gateway. If you would like to visit yahoo.com from your computer you need to go via 192.168.1.1 gateway. This usually means that you have to set up a static route to the gateway beforehand. If you specify the address of one of your local interfaces, it will be used to decide about the interface to which the packets should be routed to. metric field used by routing daemons. This is required on large LAN and wan setups. The output of the kernel routing table is organized in the following columns: