software updater failed to download repository information
Another solution is, sometimes it couldn't find the packages from the selected server. You should change it to the best matched server.
- Go to Software & Updates, Ubuntu Software tab,
- Then on Download from click
Other -
Select Best Serverand it will auto update the sources. - Now install it from Software Updater or do
sudo apt-get upgrade
In my experience, there are at least three other causes of the failure: incorrect repository location, repository is down, and incorrect proxy settings.
First, check your software repository location in /etc/apt/sources.list or run software-properties-gtk. Make sure you can reach all of the repository, e.g. by using the web browser to make sure that the site is not currently down.
If it's OK, then check your system-wide proxy settings. Because software update is run by using root privilege, so the proxy settings used would be the root's too. Check this question to set system-wide proxy settings.
Another cause might be firewall, but I'm not sure :).
Hope it helps.
UPDATE
It is not a problem if you get message gpg: /tmp/tmp<somerandom>/trustdb.gpg: trustdb created when running software-properties-gtk from the terminal.
In my case the error messages about not being about to get repository information, checking internet connectivity and not being able to find a "Best Server" were complete red herrings.
The root cause of my problem was that I was trying to upgrade from 13.10 to 14.04 and my update settings were set to only check for LTS updates. Whenever I tried explicitly checking for an update, it said "your system is already up-to-date". I wrongly guessed at first that this was due to a network issue (proxy etc) leading to no updates being found.
When I changed the updater settings to check for ANY new version, I got a message saying 13.10 is no longer supported, with a new button for the upgrade option. Clicking this new button worked fine, with no other network or settings changes.
This looks like a bug to me, since 14.04 is an LTS update, but anyway, it worked!