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Hierarchical systems have been widely used to provide scalable distributed services in the Internet. Unfortunately, such a service hierarchy is vulnerable to DoS attacks. This paper presents HOURS that achieves DoS resilience in an open service hierarchy. HOURS ensures high degree of service accessibility for each surviving node by: 1) augmenting the service hierarchy with hierarchical overlay networks with rich connectivity; 2) making the connectivity of each overlay highly unpredictable; and 3) recovering the overlay when its normal operations are disrupted. We analyze an HOURS-protected open service hierarchy, and demonstrate its high degree of resilience to even large-scale, topology-aware DoS attacks.