I have a mobile version of a site, which is accessed through auto-detection of mobile user-agents.

Since the content is virtually the same as the regular site, I didn’t want it indexed by search engines, both to prevent site visitors from landing on a mobile site, and to avoid SEO penalties for duplicate content.

I used the following RewriteCond/RewriteRule to deliver an alternate robots.txt file for the mobile site.

        RewriteCond %{HTTP_HOST} ^m\..*
        RewriteRule ^robots\.txt$ m.robots.txt [L]

The alternate robots.txt file is:

User-agent: *
Disallow: /

In this case, the site is run by eZ publish, but the same strategy should work with any other mobile site that is run as a subdomain.