I have a few questions about using a robots.txt file in DNN.

1. If you put a robots.txt file containing "Disallow: /" into the root directory of your DNN installation (the highest-level portal), does that prevent spiders from accessing all child portals also?

2. How would you create/place a robots.txt file to operate only in the top-level portal (not in child portals)?

3. How would you create/place a robots.txt file to operate only in a particular child portal?

4. I notice this statement in all DNN pages: meta name="ROBOTS" content="INDEX, FOLLOW"
    Would a robots.txt file override that meta tag?

5. Anything else I should know about this subject?

Considering that DNN is desgned to promote your site with SEO by default; and and the fact that any content can be hidden or made available to any user role, nearly any configuration is possible from site to page to module with standard DNN security.

For your private portal, you have to have at least one page open to the public to allow users to login. If you hide your home page using security, DNN automaticaly takes to you a default login page.  To hide portal menu and content you will need to setup at least one page for public access. Add a new page with name Public Login, uncheck include in menu and check all users to view page. Then add an account login module. Assuming all your modules have inherit view permissions from page, set permissions for every page (except Public Login) by checking registered users to view page and copy to children (descendants). Now use site settings, and change the login page to Public Login.

That's it.

To clean up left over artifacts, a blank page title displays the url so change it to   (note explorer titlebar), then add a space to the page description and keywords. The best way to change the footer is to use a copy of the portal skin, rename it and remove the appropriate tokens. (Warning: you can break your site by playing with a skin so make sure you understand what you are doing.) Use site settings to set the portal skin to the limited edition.

This is the best technique to hide a single portal from search engines yet allow SEO for other portals on the same DNN installation.