This paper considers a type of application-level DDoS attacks aiming at search engines. The attackers issue search requests randomly drawn from a predefined word pool. Without any detection mechanism, a search engine can become quite unusable given enough attackers. We design a scheme to counter these attacks. Then, we evaluate the performance of the scheme. We also study the false positive and false negative ratios yielded by the scheme. Keywords-DDoS, application level security, request queueI.