Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) on VirtualBox [duplicate]
Replace <proxy> with your proxy address.
For example, if your proxy server is 1.2.3.4 and the port is 1234 then the first line should be
Acquire::http::proxy "http://1.2.3.4:123
Use this for all other lines.