SET UP INTERNET CONNECTION SHARING ON UBUNTU: If you want to share your internet connection on Ubuntu, just follow these steps. But first, some prerequisites:
-2 network cards on the computer the internet is coming from
-1 network cable (not crossover) to connect the two computers
-1 network card on the second computer
For the computer the internet connection is coming from or we shall call it computer 1, follow the steps for setting it up:
Step-1. Open a terminal and use this command:
ifconfig ethX ip
*Where 'ethX' is the network card that is going to be connected to the second computer and 'ip' is your desired server IP address (Usually 192.168.0.1 is used)
Step-2. Configure NAT by:
iptables -t nat -A POSTROUTING -o ethX -j MASQUERADE
*Where 'ethX' is the network card that the Internet connection is coming from. Then type:
echo 1 > /proc/sys/net/ipv4/ip_forward
Step-3. Now, we will have to install the packages dnsmasq and ipmasq:
sudo apt-get install dnsmasq ipmasq
Step-4. Restart dnsmasq using this command:
Step-5. You should reconfigure ipmasq to run after networking has been started:
Step-6. Repeat steps 1 and 2.
Step-7. Add the line "net.ipv4.ip_forward = 1" to /etc/sysctl.conf by using a text editor:
Configuring the second computer (computer 2):
Set the network card ip to: 192.168.0.2
Netmask: (like the first computer)
You can also enter the internet DNS's from the computer connected to the internet.
Note: The DNS's can be found in the following file: /etc/resolv.conf and have this syntax: nameserver ip
PS: If using Ubuntu Jaunty and later, you must add a line to /etc/rc.local so that this is persistent after restarting your computer.
gksu gedit /etc/rc.local
Paste this line:
sudo iptables -A POSTROUTING -t nat -j MASQUERADE
That's about. Now enjoy sharing your internet connection in Ubuntu.