.htaccess Rewrite with authentication

I've been looking for a htaccess rewrite and authentication solution but I can't understand the rewrite at all :(

What I need is when a user goes to http://sub.do开发者_如何学运维main.com/folder or http://sub.domain.com/folder/ it will be asked for authentication through htaccess but if they go directly to that folder through a different domain they won't be asked for password.

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Do you want just to disable directory listing? Put this into .htaccess file

Options -Indexes