I am storing my sitemaps in my web folder. I want web crawlers (Googlebot etc) to be able to access the file, but I don't necessarily want all and sundry to have access to it.

For example, this site (superuser.com), has a site index - as specified by its *robots.txt* file (http://superuser.com/robots.txt).

However, when you type http://superuser.com/sitemap.xml, you are directed to a 404 page.

How can I implement the same thing on my website?

I am running a LAMP website, also I am using a sitemap index file (so I have multiple site maps for the site). I would like to use the same mechanism to make them unavailable via a browser, as described above.