Maybe this is old topic…
but this is quite important for people who are using ubuntu behind firewalll
and have difficulties on updating and synchronizing repository sources …

This is the simplest solution :
1. create (if doesn’t exist) or update file /etc/apt/apt.conf
sudo gedit /etc/apt/apt.conf
2. add the following line
Acquire::proxy:http://user@password:proxyaddress:portaddress/
3. then invoke the following command to test the configuration
sudo apt-get update

Voila …
now your ubuntu box has an access to ubuntu repository defined on your sources.list