If you have Joomla installed behind a proxy (like squid, for example) then you will not be able to use the Joomla Update feature - found under Extension Manager / Updates. When you do you'll get errors like:
Update: :Collection: Could not open http://update.joomla.org/core/list.xml
To fix this you will need to specfically enable the http_proxy variable for the apache daemon - it's not enough to have the environment variable set for your own user or cron, for example as the apache user won't pick this up.
The easiest way to fix it to edit the startup script for apache, adding in a line for the proxy e.g.:
Then restart apache.
Add in the user/pass details if these are required. e.g. http_proxy=http://username:password@myproxyserver:3128