Acquire::http::Proxy "http://xxx.xxx.xxx.xxx:3128"; Acquire::https::Proxy "http://xxx.xxx.xxx.xxx:3128"; Acquire::ftp::Proxy "http://xxx.xxx.xxx.xxx:3128";
Try this:
In etc/apt/apt.conf:
Acquire::http::Proxy "http://xxx.xxx.xxx.xxx:3128";
Acquire::https::Proxy "http://xxx.xxx.xxx.xxx:3128";
Acquire::ftp::Proxy "http://xxx.xxx.xxx.xxx:3128";