[tor-bugs] #21534 [Core Tor/Tor]: "Client asked me to extend back to the previous hop" in small networks

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#21534: "Client asked me to extend back to the previous hop" in small networks
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 Reporter:  teor                                 |          Owner:  (none)
     Type:  defect                               |         Status:  new
 Priority:  Very High                            |      Milestone:  Tor:
                                                 |  0.3.2.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  regression?, guard-selection,        |  Actual Points:
  dirauth                                        |
Parent ID:  #21573                               |         Points:  1
 Reviewer:                                       |        Sponsor:
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Comment (by teor):

 Authorities do not use guards for anything.

 And chutney uses TestingTorNetwork, which turns off the IPv4 /16
 restriction, which normally stops us choosing a guard and middle that are
 the same relay. When it is turned off, we still need to avoid choosing
 exactly the same relay for two hops in the same circuit.

 I'm not sure why this is happening on the live network and the test
 network. Because the /16 restriction should prevent any node from choosing
 the same relay twice in a path.

 Does the /16 restriction work when we're using an IPv6 address to extend
 to the guard?

 I think we should add an unconditional same-id restriction, and bug-log
 (and log the ip addresses and fingerprints in the path) when it's
 triggered when the /16 restriction is on. And we should check if IPv6
 guards trigger it. And if anything else triggers it.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21534#comment:8>
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