Well, it’s been a long time since I posted the first article on this. My time or lack thereof got the best of me. To counter this attack is actually very very easy. The first thing you do is you find out who is the referrer. This is simply done by tailing the logs. If you have a single domain, this can be fairly easy. Otherwise my preferred method involves using “watch ls -l” and seeing which log grows the fastest. This tends to be the one getting hit, or a likely suspect. I will probably write a perl script later to check this and tell me which log grows the most in say 10 seconds eventually. After this, you can use tail in the manner of:
tail -f /etc/httpd/domlogs/domain.log
When you do this, you will see what IPs are querying the page and the source they are being referred from. Look for any thing that doesn’t look like a search engine. To actually block them after they are identified what you do is you block the attack based on a referrer in the .htaccess. See the convenient rewrite code I jacked off another web site (about the same I did when I really saw the attack.)
RewriteEngine on
# Options +FollowSymlinks
RewriteCond %{HTTP_REFERER} attacker\.com [NC]
RewriteRule .* – [F]
So, why does this work you may ask? In the case of the scenario I saw the person was attacking a “high value” target. This means a page that hits the database and has dynamically generated content with no caching. Server side configuration CAN make these sort of attacks a lot harder to perpetrate as well. Anything that you can do to increase the robustness of a server will help with a DoS. When you add a rule like this where it denies access to the referrer basically what happens is you pull up static content instead. Static content uses virtually no resources compared to something PHP based and backed by a databse. It’s a good idea to know about this sort of attack, as I could see it being bigger in the future. Black hat SEO is very common these days, and if you have the SEO part down the resources to do the rest of this attack are virtually nothing compared to what it does. You can click here to go on Freshlinks and learn more about SEO and outreach strategies. It could also be plausible we will see this attack combined with “conventional, network level” type DoSing to increase its effectiveness.