Multi Domain Robots.txt
If you are under apache, you can use mod_rewrite to deliver others robots.txt :

  RewriteCond %{HTTP_HOST} ^domain1.com$
  RewriteRule ^robots.txt$ robots.domain1.com.txt [L]
  RewriteCond %{HTTP_HOST} ^domain2.com$
  RewriteRule ^robots.txt$ robots.domain2.com.txt [L]


Note you may also generate your robots.txt in some controller for each site depends on config and fallback to default if not defined.