[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 dgoulet):

 Replying to [comment:8 teor]:
 > Authorities do not use guards for anything.

 Well they do for couple reason I can find with the logs. First, self
 reachability testing, it goes through a 3 path length circuit and thus
 requiring a Guard (`consider_testing_reachability()`)

 Second, client hidden service preemptive circuit
 (`needs_hs_client_circuits()`). This one happens quite a bit when CBT is
 learning (#24228).

 So what I mean here is that we should definitely investigate why
 authorities (so far I can only see them hitting this issue) don't consider
 any nodes a Guard and makes them fallback to the entire routerset.
 Actually, it is a bit worst then that because of:

 {{{
     flags &= ~ (CRN_NEED_UPTIME|CRN_NEED_CAPACITY|CRN_NEED_GUARD|
                 CRN_PREF_ADDR);
     choice = router_choose_random_node(
                      excludedsmartlist, excludedset, flags);
 }}}

 BUT, as it turns out, this is definitely not the problem of this
 ticket....

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21534#comment:9>
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