[tor-bugs] #30472 [Circumvention/Pluggable transport]: Implement a mechanism for PT rechability testing

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#30472: Implement a mechanism for PT rechability testing
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     Reporter:  phw                              |      Owner:  phw
         Type:  project                          |     Status:  assigned
     Priority:  High                             |  Milestone:
    Component:  Circumvention/Pluggable          |    Version:
  transport                                      |   Keywords:
     Severity:  Major                            |  reachability
Actual Points:                                   |  Parent ID:  #30471
       Points:                                   |   Reviewer:
      Sponsor:                                   |
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 Non-vanilla bridges currently have no way to automatically test their
 reachability. Vanilla bridges
 [https://gitweb.torproject.org/torspec.git/tree/path-spec.txt#n193 self-
 test the reachability of their ORPort] by creating a circuit that includes
 themselves, but we cannot do this for, say, obfs4. In practice, this is
 problematic because obfs4 operators won't know if their bridge is
 unreachable; for example due to NAT. In fact, BridgeDB is distributing
 obfs4 bridges that aren't actually reachable.

 We need to build a mechanism that allows non-vanilla bridges to test their
 reachability. Ideally, something would create a circuit over the bridge
 while speaking its respective transport protocol but even a simple TCP or
 UDP-based reachability test would already go a long way.

 Looking at the discussion
 [https://trac.torproject.org/projects/tor/ticket/30331#comment:2 over in
 #30331], tor seems to be the right component to trigger the reachability
 test. In its log files, it can then yell at the operator if the test
 failed. The question is: how should we design the mechanism that
 implements the reachability test?

 One solution would be a simple HTTP API that takes as input an address,
 port, a transport type, and optional parameters, and then tells you if the
 given bridge is reachable, e.g.: the URL https://pt-
 reachable.torproject.org/obfs4/1.2.3.4/9002 may respond with something
 along the lines of `obfs4_reachable: true`. Ideally, if the reachability
 test fails, we should provide details, to help the operator figure out
 what went wrong.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/30472>
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