gutenberg.org blocks access to all hosting services as per http://www.gutenberg.org/error403.php
How to achieve this? Is there a list of hosting ips? Also, would these be added to CSF?
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It's achieved with a blacklist. They have a list of IP address blocks allocated to big hosting services, and redirest traffic coming from those IPs to the 403 page you included. So that's how you'd do it, yes. Of course, as with any blacklist, it's not comprehensive and the smaller or more obscure hosts tend to slip under the radar.