Last week was a very frustrating time for me. For whatever reason, an unusually number of botnets decided to zero in on my Drupal site and created what I call an unintentional Denial of Service attack (DOS). The attack was actually from spambots looking looking for script vulnerabilities found mainly in older versions of e107 and WordPress. Since the target of these spambots were non-Drupal pages, my Drupal site responded by delivering an unusually large number of "page not found" and "access denied" error pages. Eventually, these requests from a multitude of IPs were too many for my server to handle and for all intents and purposes the botnet attack caused a distributed denial of service that prevented me and my users from accessing the site.
These type of attacks on Drupal sites are nothing new and have been observed and discussed at great length at Drupal.org. However, my search at Drupal.org as well as Google didn't really find a solution that completely addressed my problem. Trying to prevent a DDoS attack isn't easy to begin with and at first the answers alluded me.
I originally looked at Drupal for the solution to my problems. While I've used Mollom for months, Mollom is designed to fight off comment spam while the bots attacking my sight were looking for script vulnerabilities that didn't exist. So with Mollom being the wrong tool to fight off this kind of attack, I decided to take a look at the Drupal contributed model Bad Behavior. Bad Behavior is a set of PHP scripts which prevents spambots from accessing your site by analyzing their actual HTTP requests and comparing them to profiles from known spambots then blocks such access and logs their attempts. I actually installed an "unofficial" version of the Bad Behavior module which packages the Bad Behavior 2.1 scripts and utilizes services from Project Honey Pot.
As I had already suspected, looking for Drupal to solve this botnet attack wasn't the answer. Pretty much all Bad Behavior did for me was to take the time Drupal was spending delivering "page not found" error pages and use it to deliver "access denied" error pages. My Drupal site is likely safer with the Bad Behavior module installed, but it was the wrong tool to help me reduce the botnets from overtaxing Drupal running on my server. Ideally, you would like to prevent the attacks ever reaching your server by taking a look at such things as the firewall, router, and switches. However, since I didn't have access to the hardware, I decided it was time to look at my Apache configuration.
