Comment 4 for bug 1509176

If you want to run "do-release-upgrade" with apt-cacher then make sure that the apt-cacher option "ubuntu_release_names" contains the new release code name you want to upgrade to.

I wanted to update Ubuntu via console from 15.04 "vivid" to 15.10 "wily" and got the following errors on console:

Checking for a new Ubuntu release
Err Upgrade tool signature
  403 Sorry, not allowed to fetch that type of file: wily.tar.gz.gpg
Err Upgrade tool
  403 Sorry, not allowed to fetch that type of file: wily.tar.gz
Fetched 0 B in 0s (0 B/s)
WARNING:root:file 'wily.tar.gz.gpg' missing
Failed to fetch
Fetching the upgrade failed. There may be a network problem.

To fix this problem I created a new config file (/etc/apt-cacher/conf.d/99ubuntu) from the default configuration (/etc/apt-cacher/apt-cacher.conf), edited it to uncomment the line and added all missing code names from http://releases.ubuntu.com/

grep '^#*ubuntu_release_names' /etc/apt-cacher/apt-cacher.conf >/etc/apt-cacher/conf.d/99ubuntu
nano /etc/apt-cacher/conf.d/99ubuntu

Then "do-release-upgrade" worked with apt-cacher.

Kind regards
Maddes