This is for Ubuntu Hardy 8.04 earlier and later versions may differ.
The proxy is an important part of the connectivity quotient, since most users do not have 100% direct Internet access and will have to use a proxy server at some point in their lives to supplement bandwidth. Normally one would either set the proxy in the relevant browser-application such as Firefox or Filezilla or set this information for the overall system using System > Preferences > Network Proxy.
In the twisted genius that is Linux and which effect the Ubuntu Community distribution, the Synaptic Packet manager appears to be classified as a separate application and the usual laws of physics do not apply. If you’re having trouble downloading applications, then try setting the proxy in Synaptic. You will find the proxy settings in Synapic by opening it and choosing Settings > Preferences > Network
You can also check your proxy environment by using the following command:
typing "printenv" in your terminal.
If you need to update or correct the proxy information use the following
export http_proxy=www.your.proxy:port (IP:Port is also possible) or export http_proxy=http://username:firstname.lastname@example.org:port/
Update Manager is another part of the system which appears to exist as a thing unto itself. To set the proxy you have to use the following workaround, as their is no graphical facility to do this currently.
Should pop up a box where you can input the appropriate information. This issue is tagged as Bug 21536