A proxy server in a cloud-based proxy service receives a message that indicates that a domain, whose traffic passes through the proxy server, may be under a denial-of-service (DoS) attack. The proxy server enables 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. In response to receiving a request for a resource of that domain from a visitor, the proxy server presents the set of challenges that, if not passed, are an indication that that the visitor is part of the DoS attack. If the set of challenges are passed, the request may be processed. If the set of challenges are not passed, the request may be dropped.