Anti-Knock

Functionality
This module monitors HTTP 404 Not Found errors from requests that reach Drupal and blocks IP addresses that exceed a configured threshold.
This module only blocks the IP addresses of visitors who are not logged in. Logged-in users requests are not evaluated.

About
Anti-Knock automatically blocks continuous requests intent on Directory Busting / Denial Of Service attacks using the functionality of the Drupal Ban module.

Drupal's .htaccess file provides an excellent layer of protection against 404 requests containing malicious patterns, but this does not usually stop more sophisticated probing.
In general, an attacker may probe a site using this method looking for paths that might exist to help them profile an application.
Besides being a nuisance and filling up the logs, getting accurate information about an applications structure can assist attackers in later stage DOS attacks.
Poorly constructed or malicious bots or web spiders can also cause this problem.
Requests to non-existent or expired paths to content can trigger this module to block an IP, so we recommend redirecting such paths appropriately.