For 12.04:
Well, there is already an answer very similar to what I was going to post, but I will post mine too just to make it more clear.
From Ubuntu 11.04 onwards, it's known fact that there have been many changes to Network Settings in Ubuntu. Of course, many people who wanted proxy authentication (like me) had to work a little hard to get it. So, this new short guide is for those who need Proxy settings (with/ without authentication).
Note: In the Screenshots below my Ubuntu is a little bit customized, so the icons may look different in your system.
First of all, either click on the launcher icon (or press super) and it opens Unity panel. Now search for Network, and it will show you some results like this:
Select the first one that says Network.
It will open the Network settings window.
The same can also be opened by searching for System Settings in the Unity and clicking on Network in it like it's shown below:
After the Network Settings window pops up, click on Network Proxy on left like it's shown here:
Now, select Manual on the right and enter the proxy address for each type of server. Let's say that I have a Proxy server at address 172.27.1.1 at port number 5321. The proxy settings would look like this:
Now, if you need authentication to access the proxy, type your username and password as shown below:
Lastly, click on Apply System Wide to apply the settings system wide. This will pop up a one or two authentication windows like shown below.
Enter your password to set the settings and that's it... the System Proxy settings are all set and your Ubuntu is ready to use them!!
i'm in a similar situation here, which not only requires me to turn on and off proxy settings, but sometimes to use a different proxy in another location.
what i do is install squid on my machine (any other proxy capable of using parent proxies can do), and then, point all my applications to this local proxy on my own computer.
i keep a configuration file for squid that uses the internet directly, and a config that lets squid use a parent proxy for each proxy i am required to use.
seen the benefit yet? when i move to another location, i don't have to touch the proxy settings of any of my programs, i just change one file, the one belonging to squid, and reload squid. i name the config files easyily-identifiable names, and the switching goes something like this:
cd /etc/squid
sudo cp squid.conf.work1 squid.conf
sudo service squid reload
the config that lets squid use direct net is "squid.conf.direct" for me.
hope this helps :)
Best Answer
System wide proxy settings are just another consolidated place for proxy settings. It actually modifies the places where different apps' config files. Have you noticed, system wide proxy setting asks for password 2 times? This is mainly because of the modification of two admin files which corresponds to config file.
And yes, from 10.10 and 11.10, some apps have been left out of system wide proxy setting most notable among them is the synaptic.
and moreover, apps can bypass the system wide setting and make use of their own setting again most notable synaptics and Mozilla Firefox(which contains system settings option as well).