We show that malicious nodes in a peer-to-peer system may impact the external Internet environment, by causing largescale distributed denial of service attacks on nodes not even part of the overlay system. This is in contrast to attacks that disrupt the normal functioning, and performance of the overlay system itself. We formulate several principles critical to the design of membership management protocols robust to such attacks. We show that (i) pull-based mechanisms are preferable to push-based mechanisms; (ii) it is critical to validate membership information received by a node, and even simple probe-based techniques can be quite effective; (iii) validating information by requiring corroboration from multiple sources can provide good security properties with insignificant performance penalties; and (iv) it is important to bound the number of distinct logical identifier (e.g. IDs in a DHT) corresponding to the same physical identifier (e.g., IP address), which a participating node is unable to validate. We demonstrate the importance of these principles in the context of the Kad system for file distribution, and ESM system for video broadcasting. To our knowledge, this is the first systematic study of issues in the design of membership management algorithms in peer-to-peer systems so they may be robust to attacks exploiting them for DDoS attacks on external nodes.
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