Often, one needs to install Ubuntu from behind a corporate firewall. While this seems like it should just work, the installer will get stuck while trying to access the package repositories and check for security updates. At this point in the process, the installer is attempting to contact an http address, which will be inaccessible unless a proxy server has been set up. This tip describes how to teach the Ubuntu installer about any proxies that may be protecting your network environment.
It turns out that this is really easy to do from the command line. Simply open up a terminal ( Applications -> Accessories -> Terminal ) and type the following command:
% http_proxy=http://<your proxy url>:<port> ubiquity gtk_ui
This is essentially the same command that is invoked when you double click on the Install icon on the desktop, but adds information about your proxy server to the environment before starting. Follow the remaining install steps as you would normally.
After the install is complete, you will need to again configure your system to use your proxy for network accesses. The special command used to launch the installer will not cause your system to be pre-configured to use your proxy.