```
Filename: 277-detect-id-sharing.txt
Title: Detect multiple relay instances running with same ID
Author: Nick Mathewson
Created: 20-Feb-2017
Status: Open
Target: 0.3.??

1. Overview

   This document proposes that we detect multiple relay instances running
   with the same ID, and block them all, or block all but one of each.

2. Motivation

   While analyzing microdescriptor and relay status transitions (see
   proposal XXXX), I found that something like 16/10631 router
   identities from January 2017 were apparently shared by two or
   more relays, based on their excessive number of onion key
   transitions.  This is probably accidental: and if intentional,
   it's probably not achieving whatever the relay operators
   intended.

   Sharing identities causes all the relays in question to "flip" back
   and forth onto the network, depending on which one uploaded its
   descriptor most recently.  One relay's address will be listed; and
   so will that relay's onion key.  Routers connected to one of the
   other relays will believe its identity, but be suspicious of its
   address.  Attempts to extend to the relay will fail because of the
   incorrect onion key.  No more than one of the relays' bandwidths will
   actually get significant use.

   So clearly, it would be best to prevent this.

3. Proposal 1: relay-side detection

   Relays should themselves try to detect whether another relay is using
   its identity.  If a relay, while running, finds that it is listed in
   a fresh consensus using an onion key other than its current or
   previous onion key, it should tell its operator about the problem.

   (This proposal borrows from Mike Perry's ideas related to key theft
   detection.)

4. Proposal 2: offline detection

   Any relay that has a large number of onion-key transitions over time,
   but only a small number of distinct onion keys, is probably two or
   more relays in conflict with one another.

   In this case, the operators can be contacted, or the relay
   blacklisted.

   We could build support for blacklisting all but one of the addresses,
   but it's probably best to treat this as a misconfiguratino serious
   enough that it needs to be resolved.




```