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Selective denial of service on guard nodes is a well-known attack, in which the malicious guard keeps dropping connections from Tor clients who are not in the set of his targeted victims.

Do Tor authority servers, or probing servers, or those clients who were dropped by the malicious guard have any mechanism to report it to the authority servers so that the malicious guard will be rejected in the next consensus vote?

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Do Tor authority servers, or probing servers, or those clients who were dropped by the malicious guard have any mechanism to report it to the authority servers so that the malicious guard will be rejected in the next consensus vote?

Malicious relays should be reported to bad-relays@lists.torproject.org, along with the malicious activity, the fingerprint of the relay and steps to reproduce it.

However, if the denial of service were to be selective, like you suggest, then it would be difficult to independently verify the malicious activity.

If the relay is found to be acting maliciously then it will be blacklisted and removed from the consensus.

  • Thank you for your answer. I just googled a little bit and found this paper: arxiv.org/abs/1110.5395 . So, do you know if the Detector proposed in this paper is currently implemented in Tor network or not? – 2523fewqf23f Dec 29 '17 at 3:35
  • I don't have any direct knowledge, I used to be part of the bad-relays mailing list (which is invite only) but I know that there are many scanners like exitmap (designed to find bad exits) and other scanners designed to find bad HSDirs. A lot of the cases of targeted denial of service would be part of finding guards for specific users or onion services...but making yourself the guard for that service or user is infeasible, so more commonly such attacks would be temporary and hard to reproduce. – cacahuatl Dec 29 '17 at 5:47
  • I see. Thank you for your quick reply and helpful information. – 2523fewqf23f Dec 29 '17 at 16:07

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