Linux - Turn Computer Into Router for Internal Network

Setup

For this, I am using a Ubuntu computer with two ethernet cards.

• eth0 connects to the internet on a DHCP connection
• eth1 connects to a switch which all the other computers on the local network are connected to.
• eth0 has ip 192.168.1.x where x can change
• eth1 set to static ip 192.168.2.1 as it will be the gateway.
• All other computers on the network have ips set manually to 192.168.2.x with gateway set to 192.168.2.1
• In network manager, eth1 connection set to 192.168.1.2 with gateway 192.168.1.x which matches the ip of eth0

Steps

Enter the following commands each time computer starts:

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

You will need to allow packet forwarding.
To do this in ubuntu, edit the /etc/sysctl.conf file and uncomment the line shown here (remove the # at the front)

#net.ipv4.ip_forward=1

Run the following command:

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

Source

linux-as-router