Message(s) are received from each one of multiple proxy servers, which are anycasted to the same IP address, that indicate source IP addresses of packets that are received that are directed to that same IP address. These proxy servers receive the packets as result of domain(s) resolving to that same IP address, and a particular one of the proxy servers receives the packets as a result of an anycast protocol implementation selecting that proxy server. Based on these message(s) from each of the proxy servers, a determination of the likelihood of a packet having a particular source IP address being legitimately received at each of the proxy servers is determined. A message is transmitted to each of the proxy servers that indicates which source IP addresses of packets are not likely to be legitimately received at that proxy server.