I have tried put in apt config Acquire::http::proxy, and in Network>Network proxy with and without user:password@server:port and using environment variables http_proxy and https_proxy. Nothing seems to work, not even a single proxy authentication popup window.
The only proxy configuration working is within Firefox Edit> Preferences> Advanced> Network> Connection> Settings> Manual Proxy Configuration.
- Using gconf to manually set the proxy doesn't work
Using Synaptic in previous Ubuntu versions was too simple.