If you encounter the error WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! while connecting to your remote machine through SSH, it means the identification key on the server is not the same with last connection. To fix this, simply remove the host's key from ~/.ssh/known_hosts.
Default file permissions on Linux is 777, which means everyone can read, write and execute the file. It's fine for most of the scenario, but it's another story when comes to SSH private key. If you try to establish a SSH connection with a too open private key, WARNING: UNPROTECTED PRIVATE KEY FILE! will be shown.
To turn off automatic system upgrade and package lists update on your Debian/Ubuntu based Linux distros is quite easy. And we'll be covering editing apt configuration through both CLI (Command-Line Interface) and GUI (Graphical User Interface) so it can be done by ssh or desktop.