ubuntu share internet

Before I begin this post, I want to thank Internet Connection Sharing – Ubuntu 10.04 NAT Gateway Setup (Abridged Version) for providing the bulk of the tutorial. I have made some modifications for Ubuntu 12.04.
The setup is simple: a single Ubuntu server will act as a gateway and DHCP server for a local network. All other machines on the local network will receive their IPs from the DHCP server. To make things easier, I’ll call this Ubuntu server “Skyray” for the rest of the post.
Skyray has two network interfaces, eth0 and eth1. eth0 is on the 10.20.30.0/24 subnet and this is the Internet facing interface. eth1 is on the 172.22.22.0/24 subnet, where all other machines are also present. Basically, eth0 will connect to the Internet and eth1 will serve DHCP requests and act as the gateway.

/etc/network/interfaces

First you need to configure eth0 and eth1 for Skyray. Edit the file and make sure it has at least the following settings (or whatever settings are appropriate for your environment).
sudo vim /etc/network/interfaces
(:wq save and quit, i insert mode , esc exit insert mode, :q! exit not save)

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

auto eth1
iface eth1 inet static
    address 192.168.10.1
    netmask 255.255.255.0
    network 192.168.10.0
    broadcast 192.168.10.255

/etc/sysctl.conf

You need to enable IPv4 forwarding. To do so, edit this file.
sudo vim /etc/sysctl.conf
And uncomment the line

# net.ipv4.ip_forward=1

so that it now appears as

net.ipv4.ip_forward=1

Save the file and run the following command to make the change effective without a reboot.
sudo sysctl -w net.ipv4.ip_forward=1

/etc/rc.local

You’ll need to allow iptables rules for NAT to work. Edit the file and save it.
sudo vim /etc/rc.local
Make sure the following two lines appear before the exit 0 line in the file.

/sbin/iptables -P FORWARD ACCEPT
/sbin/iptables --table nat -A POSTROUTING -o eth0 -j MASQUERADE

To make these iptables rules active without rebooting, run the following commands:
sudo iptables -P FORWARD ACCEPT
sudo iptables –-table nat -A POSTROUTING -o eth0 -j MASQUERADE