Hi all
I wonder if it might be worth having a discussion on how to detect malicious and/or suspicious relays. To my knowledge, the project currently only scans for MITM and tries to detect larger Sybil attacks (but doesn't always act when detected).
We have a lot of knowledge now about types of attacks and the unusual behaviours that might present if they are being deployed. For example:
1) Relays changing their ident every 24 hours are likely trying to be in a position to be the directory node for a HS (there are several of these relays active today).
2) Many relays being launched on sequential IP addresses, and/or with two nodes per IP - again - likely intercepting DHT publications or Sybil.
3) I have spotted several times a large number of relays in the same subnet, or adjacent /24 subnets. You could extend this to ASes.
4) Honeypot relays that try to spot unusual cells or traffic patterns traversing the tor network. For example, this could have detected the RELAY_EARLY attack if it was based on a different code base. One could define very robust and tight rules for what is permitted - flagging nodes sending unusual traffic. Additionally, PADDING cells are not currently used by the official client but are used very widely in traffic confirmation attacks - whilst intermediate relays wont be able to detect this, clients can and could flag it with an authority (via a 3-hop circuit).
5) One could scan for unusual descriptors being returned too - e.g., the descriptor is currently with in tight size bounds - but one could pad with bytes to support traffic confirmation if PADDING cells are put on the red flag list.
6) We also have the wider question of traffic tampering by exits, like the recent binary patching exit which I believe was not detected by the project.
7) And finally, Exits that only exit ports which permit tampering - e.g. the exits that only exit Bitcoin traffic for example.
The question of course is where is the threshold and what does one do in the event of one of these.. Personally I am of the view that suspicious relays are not worth keeping in favour of diversity - but that view does contradict the project's I think.