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this is true, RTB may not work and im not sure if badspot keeps logs of IPs on his forum account outside of the last logged in IP.
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also, im not sure if getting his IP will help. finding it wont draw a line of evidence, because i dont think OP has access to a log of IPs that connect to his server, AKA the packets used in the attack. (unless he uses RTB, perhaps. i think that keeps a log of that kind of thing.)