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I am new to Ubuntu.

I am getting an error whenever I try to run sudo apt-get update

(407 Proxy authentication required)

Moreover, I am unable to install any software from Ubuntu software center not even using sudo apt-get install. I have tried every answers mentioned in the related questions here but none of them worked for me.

When I try to play any audio file I am getting following error:

Required plugin could not be found Python (v2.7) required to install plugins to play media files of the following type: MPEG-1 Layer 3 (MP3) decoder

For this when I try to run following command, getting error:

sudo apt-get install ubuntu-restricted-extras
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package ubuntu-restricted-extras

I have been trying this since the last 2-3 days with no fruitful result.

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marked as duplicate by mikewhatever, vasa1, qbi, Eric Carvalho, LnxSlck Feb 23 '13 at 13:46

This question was marked as an exact duplicate of an existing question.

Can you add info about your internet connection to the question. Are you behind a proxy or firewall? – mikewhatever Feb 23 '13 at 9:09
i am connecting to the internet via a proxy server that needs authentication – user134816 Feb 23 '13 at 9:18
Looks like your question has been answered a few times: one, two, three. – mikewhatever Feb 23 '13 at 10:43

So, you need to configure this on Internet dialogue.In Gnome 2 is on System > Preferences > Internet

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