I do get the following error message(s) when running
Err http://nl.archive.ubuntu.com precise-backports/multiverse i386 Packages
503 Service Unavailable
The server is not behing a proxy, but there is a firewall, I suspect that the firewall is causing me troubles (transparant proxy?).
What kind of checks can I perform to be sure it is the firewall and not something else? Which specific ports should I check in which way to get conclusive evidence?