IP source address spoofing has plagued the Internet for many years. Attackers spoof source addresses to mount attacks and redirect blame. Researchers have proposed many mechanisms to defend against spoofing, with varying levels of success. With the defense mechanisms available today, where do we stand? How do the various defense mechanisms compare? This article first looks into the current state of IP spoofing, then thoroughly surveys the current state of IP spoofing defense. It evaluates data from the Spoofer Project, and describes and analyzes host-based defense methods, router-based defense methods, and their combinations. It further analyzes what obstacles stand in the way of deploying those modern solutions and what areas require further research.