Tor design proposals

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Status: Superseded
Superseded-by: 335-middle-only-redux.md

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 and/or
  fingerprints 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

  The MiddleOnly flag would work like most other consensus flags where a
  majority of dirauths have to assign a relay the flag in order for a relay
  to have the MiddleOnly flag.

  Another approach is to make it that only one dirauth is needed to give
  relays this flag, however it would put too much power in the hands of a
  single directory authority servre [3].

5. Acknowledgements

  Thank you so much to nusenu, s7r, David Goulet, 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

  [3] - https://lists.torproject.org/pipermail/tor-dev/2021-September/014630.html