1) Create separate robots.txt file (name it robots_testing.txt, for example).
2) Add this rule into your .htaccess in website root folder:
RewriteCond %{HTTP_HOST} =testing.example.com
RewriteRule ^robots\.txt$ /robots_testing.txt [L]
It will rewrite (internal redirect) any request for robots.txt
to robots_testing.txt
IF domain name = testing.example.com
.
Alternatively, do opposite -- rewrite all requests for robots.txt
to robots_disabled.txt
for all domains except example.com
:
RewriteCond %{HTTP_HOST} !=example.com
RewriteRule ^robots\.txt$ /robots_disabled.txt [L]