[tor-bugs] #28511 [Core Tor/Tor]: Limit the number of open testing circuits, and the total number of testing circuits

[Tor Bug Tracker & Wiki]

#28511: Limit the number of open testing circuits, and the total number of testing
circuits
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     Reporter:  teor     |      Owner:  (none)
         Type:  defect   |     Status:  new
     Priority:  Medium   |  Milestone:  Tor: 0.4.0.x-final
    Component:  Core     |    Version:
  Tor/Tor                |   Keywords:  tor-bwauth, tor-dos, 035-backport,
     Severity:  Normal   |  034-backport-maybe, 033-backport-maybe, 029
                         |  -backport-maybe-not
Actual Points:           |  Parent ID:  #22453
       Points:           |   Reviewer:
      Sponsor:           |
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 Tor relays can open many more testing circuits than they need:

 When Tor is doing its first ORPort reachability test, it initiates one
 testing circuit after the first successful circuit, then one testing
 circuit per second until the ORPort is found reachable. Then it gives up
 after 20 minutes. (1200 circuits is definitely too many.)

 When tor receives any descriptor or consensus, it does another ORPort
 reachability test, and initiates a testing circuit.

 When a testing circuit opens, and there aren't enough testing circuits to
 test bandwidth, then tor initiates another testing circuit.

 When a testing circuit expires, tor doesn't stop opening testing circuits
 to replace it.

 We should place a timeout on bandwidth testing, a limit on the number of
 open testing circuits, and a limit on the total number of testing circuits
 that tor will builds over a certain time. (Maybe an hour?)

 We should be careful to make these limits apply to relays, but not
 authorities. Authorities need to test a large number of relays every hour.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28511>
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