sudo apt update always giving Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

Solution 1:

I had the same issue. It turns out that apt-get uses /etc/apt/apt.conf for the proxy settings (my browser settings are already set). Making apt.conf match my browser proxy settings solved the problem.

/etc/apt$ cat apt.conf

Acquire::http::proxy "http://<proxy>";
Acquire::https::proxy "https://<proxy>";
Acquire::ftp::proxy "ftp://<proxy>";
Acquire::socks::proxy "socks:<proxy>";

Solution 2:

Turns out it was a problem in our college's network backend. Issues still exist with it, other WiFi networks work fine.

Solution 3:

I know this is a 3 year old thread but adding my solution so that it could be a help for someone in the future.

I too was facing the same problem and it turned out it was because of exceeding the FUP data cap of my connection. On the browser I was redirected to the ISP's FUP reminder page after which the problem went away.