A flaw in the code for processing byte range headers allows versions 2.2.x of the Apache Web Server to be crippled from a single PC. The vulnerability works by exploiting a feature in web servers that gives you the ability to pause and resume your downloads. There is no patch yet released for this vulnerability on apache, but a few work arounds have been found. These have been posted by The Apache Software Foundation and can be used until a stable fix is released. The Apache software Foundation has Apache Killer” Perl script.
Source: https://thehackernews.com/2011/08/killapache-ddos-tool-half-of-internet.html