Patent ID: 8856924

Claim:
A method in a proxy server in a cloud-based proxy service, wherein the proxy server is situated between client computing devices that request network resources and origin servers that serve network resources, the method comprising: receiving a first message that indicates that a domain, whose traffic passes through the proxy server, is suspected to be under a denial-of-service (DoS) attack; in response to receiving the first message, enabling a rule for the domain that specifies that future requests for resources at that domain are subject to at least initially passing a set of one or more challenges; while the rule is enabled and responsive to receiving a first request for a resource of that domain from a first visitor, automatically presenting the set of challenges based on the enabled rule that if not passed are an indication that the first visitor is part of the DoS attack, wherein automatically presenting the set of challenges includes automatically embedding a client-side script into a page and transmitting the page to the first visitor, wherein the page is not the requested resource, and wherein the client-side script, when executed by a client network application that supports client-side script execution, solves a math or other computationally expensive problem and transmits a second message to the proxy server with the solution to the math or other computationally expensive problem to allow the proxy server to determine a likelihood of whether the first request originated from a web browser.