Comment 8 for bug 446552

@Michael: Well, it is just simple authentication. Here's what i did:

$ sudo env | grep http_proxy; sudo /usr/bin/do-release-upgrade
[sudo] password for user:
http_proxy=http://user:<email address hidden>:3128/
Checking for a new ubuntu release
No new release found

If i modifiy the mentioned python script to print out the error code it says 407.

A "wget http://changelogs.ubuntu.com/meta-release" works without problems.