Typo-squatting refers to the practice of registering domain names that are typo variations of popular websites. We propose a new approach, called Strider Typo-Patrol, to discover large-scale, systematic typosquatters. We show that a large number of typosquatting domains are active and a large percentage of them are parked with a handful of major domain parking services, which serve syndicated advertisements on these domains. We also describe the Strider URL Tracer, a tool that we have released to allow website owners to systematically monitor typo-squatting domains of their sites. 1