I Present you Robots.php
So you have like 2-3 or 4 domains pakerd in the same place :
cars.com , cars.co.uk, cars.fr, cars.ru
Same content but made for different visitors, perhaps different languages. The problems come when google visits your site and indexes all sites creating duplicate content (same content on different websites in this case)
This could be a serios problem for you , but what can you do?
You could make a 301 redirect for the domains you dont want to be indexed by Google but in this case if you would redirect cars.ru to cars.com the russian user would get redirected to cars.com to, and i suspect sometimes you just dont wanna do that
Well i suggest the following aproch :
Allowing google just on one of your sites, lets say cars.com (because is international and all) without redirecting the users to the cars.com using robots.txt and rewrite urls
You have attached the robots.php and the rewrite rule is :
RewriteRule ^robots\.txt$ robots.php