Hi Roger, On 2021-09-12 20:48, Roger Dingledine wrote:
On Sun, Sep 12, 2021 at 12:17:37PM -0700, Neel Chauhan wrote:If a relay has the MiddleOnly flag, we do not allow it to be used for thefollowing purposes: * Entry GuardWhile we're trying to be exhaustive here, "Directory Guard" might be agood addition to this list. (But trying to be exhaustive is risky because Tor's design will change over time and we'll forget to update this list.)On an onion service host, when a INTRODUCE2 cell is received, if therendevous point has a MiddleOnly flag, the onion service host should closethe circuit and therefore not proceed with the protocol.Two thoughts on this part: (A) If we're teaching Tors to actively avoid touching these MiddleOnly relays even when other people specify them, the rendezvous point isn't the only one to look for. The next one that comes to mind is the introduction point, i.e. if a client gets an onion descriptor that lists an introduction point that has the flag, they would want to avoidit. And now that we've got two examples, I bet there's a third, and evenif there isn't a third now, it's the sort of thing where future design changes will forget to consider this part. (B) There's a bigger problem here, stemming from desynchronized networkknowledge. For example, if my Tor doesn't think a relay has the MiddleOnly flag, but your Tor thinks it does (e.g. because I have the consensus fromthis hour and you have the one from last hour), then you'll refuse to interact with me. First, this situation can leak to me which consensus you're using,which could build into other attacks. See this classic paper on this risk:https://www.freehaven.net/anonbib/#danezis-pet2008 And second, this situation introduces hard-to-debug robustness issues, which wouldn't be just a theoretical concern, since they would happen each time the flag transitions on a given relay. My suggestion would be to drop this idea of having Tors refuse to use MiddleOnly relays in risky roles when other people specify them. We already make sure to build our own path using relays we wanted to use, before reaching those risky roles. Let's trust the other side to do it too and not worry about it if it doesn't. In the case of the two examples we've identified so far, theattacker could use any relay they like in the next hop after that relay,and we wouldn't know whether they're doing it. And for the rendezvous point case in particular, it doesn't even need to be a relay that's in the consensus right now (in part because we didn't want to get into theinformation desync situation there too), so putting only this constrainton what is an acceptable rendezvous point would be weird.That is, I think these extra restrictions (avoiding the relays) would be a slight improvement to security in theory, but I see that as outweighedby the loss of robustness and by the other security angle (avoiding letting people probe our internal network knowledge). --Roger
Roger and George, thank you so much for your feedback.I was worried restricting MiddleOnly relays too far would become too ambitious and hard to implement a la Windows "Longhorn"/Vista (disclaimer: I work at Microsoft but not on Windows). I guess it's true.
I have an updated Prop334 attached. -Neel
Filename: 334-middle-only-flag.txt Title: A Directory Authority flag to mark Relays as Middle-only Author: Neel Chauhan Created: 2021-09-07 Status: Open 1. Introduction The Health Team often deals with a large number of relays with an incorrect configuration (e.g. not all relays in MyFamily), or needs validation that requires contacting the relay operator. It is desirable to put the said relays in a less powerful position, such as a middle only flag that prevents a relay from being used in more powerful positions like an entry guard or an exit relay. [1] 1.1. Motivation The proposed middle-only flag is needed by the Health Team to prevent misconfigured relays from being used in positions capable of deanonymizing users while the team evaluates the relay's risk to the network. An example of this scenario is when a guard and exit relay run by the same operator has an incomplete MyFamily, and the same operator's guard and exit are used in a circuit. The reason why we won't play with the Guard and Exit flags or weights to achieve the same goal is because even if we were to reduce the guard and exit weights of a misconfigured relay, it could keep some users at risk of deanonymization. Even a small fraction of users at risk of deanonymization isn't something we should aim for. One case we could look out for is if all relays are exit relays (unlikely), or if walking onions are working on the current Tor network. This proposal should not affect those scenarios, but we should watch out for these cases. 2. The MiddleOnly Flag We propose a consensus flag MiddleOnly. As mentioned earlier, relays will be assigned this flag from the directory authorities. What this flag does is that a relay must not be used as an entry guard or exit relay. This is to prevent issues with a misconfigured relay as described in Section 1 (Introduction) while the Health Team assesses the risk with the relay. 3. Implementation details The MiddleOnly flag can be assigned to relays whose IP addresses are configured at the directory authority level, similar to how the BadExit flag currently works. In short, if a relay's IP is designated as middle-only, it must assign the MiddleOnly flag, otherwise we must not assign it. Relays which haven't gotten the Guard or Exit flags yet but have IP addresses that aren't designated as middle-only in the dirauths must not get the MiddleOnly flag. This is to allow new entry guards and exit relays to enter the Tor network, while giving relay administrators flexibility to increase and reduce bandwidth, or change their exit policy. 3.1. Client Implementation Clients should interpret the MiddleOnly flag while parsing relay descriptors to determine whether a relay is to be avoided for non-middle purposes. If a client parses the MiddleOnly flag, it must not use MiddleOnly-designated relays as entry guards or exit relays. 3.2. MiddleOnly Relay Purposes If a relay has the MiddleOnly flag, we do not allow it to be used for the following purposes: * Entry Guard * Directory Guard * Exit Relay The reason for this is to prevent a misconfigured relay from being used in places where they may know about clients or destination traffic. This is in case certain misconfigured relays are used to deanonymize clients. We could also bar a MiddleOnly relay from other purposes such as rendezvous and fallback directory purposes. However, while more secure in theory, this adds unnecessary complexity to the Tor design and has the possibility of breaking clients that aren't MiddleOnly-aware [2]. 4. Consensus Considerations 4.1. Consensus Methods We propose a new consensus method 32, which is to only use this flag if and when all authorities understand the flag and agree on it. This is because the MiddleOnly flag impacts path selection for clients. 4.2. Consensus Requirements On the directory authorities, similar to the BadExit flag, if one dirauth gives a relay the MiddleOnly flag, we should mark the MiddleOnly flag for the relay even if other dirauths didn't add the flag. This is to help prevent a malicious relay from harming the network while the majority of dirauths' administrators wait to give the said relay a MiddleOnly flag. 5. Acknowledgements Thank you so much to nusenu, s7r, and Roger Dingledine for your suggestions to Prop334. My proposal wouldn't be what it is without you. 6. Citations [1] - https://gitlab.torproject.org/tpo/core/tor/-/issues/40448 [2] - https://lists.torproject.org/pipermail/tor-dev/2021-September/014627.html
_______________________________________________ tor-dev mailing list tor-dev@xxxxxxxxxxxxxxxxxxxx https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev