Ubuntu 12.04 WARNING: The following packages cannot be authenticated!

We were getting the below message today in one of our servers:

WARNING: The following packages cannot be authenticated!

Sometimes just running a couple of commands does the trick:

sudo apt-get update
sudo apt-get upgrade

Sometimes you need to delete the repositories list, reimport the keys, clearn, purge, update, upgrade ets. Just Google about this error to convince yourself about how many things could be originating this issue (or similar).

Perhaps one of the first places to look for problems is running command 'sudo apt-get update', a WARNING might reveal the problem like:

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EE5793634EF4A35

You know you need to authorize the above key to resolve it:

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 16126D3A3E5C1192

After that you should be able to install packages that were failing before with errors like:

E: There are problems and -y was used without --force-yes

Some other times there is security imposed in your network by devices and/or appliances which will interfere with the upgrade. Especially firewalls use either Proxying and Packet Filtering and those might be the culprits. If you face this error I would recommend installing the same version of the OS in a same-subnetwork-machine and compare the results with an installation in a machine in a different network. You might have to run strace, tcpdump and God knows what else before you convince the Firewall admin that there is something funky going on with the Network.