After Installing Ubuntu 13.10 Saucy, I was not able to update behind the http proxy. Here how i solved this.
root@ubuntu-unixmen:~# apt-get update
Err http://us.archive.ubuntu.com saucy InRelease
Err http://us.archive.ubuntu.com saucy-updates InRelease
Err http://us.archive.ubuntu.com saucy-backports InRelease
Follow the steps bellow to be able to execute apt-get behind a proxy: