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334-middle-only-flag.txt
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334-middle-only-flag.txt
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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Status: Superseded
Superseded-by: 335-middle-only-redux.md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