How to share Internet Connection in Ubuntu
This answer by Owais Lone should help you from ubuntu at the bottom of this page
Right click the network manager and click edit connections. The under Wired tab, Add a new connection. Under the IPV4 Setting tab, select "Shared to others" for Method.
Now other machines should connect to LAN and get Internet access automatically.
Also, look at https://help.ubuntu.com/community/Internet/ConnectionSharing