[tor-bugs] #7691 [Tor]: Path bias code should probe unusable circuits

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#7691: Path bias code should probe unusable circuits
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 Reporter:  mikeperry        |          Owner:  mikeperry         
     Type:  enhancement      |         Status:  new               
 Priority:  major            |      Milestone:  Tor: 0.2.4.x-final
Component:  Tor              |        Version:                    
 Keywords:  MikePerry201212  |         Parent:                    
   Points:                   |   Actualpoints:                    
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 There are a couple of cases where the path bias "use" accounting from
 #7440 can run into issues. In particular, circuits used in attempts to
 connect to unresponsive external hosts are indistinguishable from
 malicious failure. Also, cannibalized circuits have a similar problem, in
 that they are technically immediately "dirty" but they are actually
 unused.

 So the plan is to issue a probe RELAY_BEGIN cell upon circuit close to an
 internal address such as 0.a.b.c:25. This will cause well-behaved exit
 nodes to kick us an EXITPOLICY RELAY_END cell back, which we can then use
 to declare the circuit as functional, avoiding the path bias false
 positive.

 For some additional best-practice checks, we should perhaps locally track
 the a.b.c tuple for each probe to ensure it is the same in the response
 (yes, the IP is echoed, but not the port), and we should ensure no other
 unexpected/corrupted RELAY cells arrive on that same circuit, otherwise we
 should close it and mark it failed. Hopefully this latter property is
 already always enforced. If not, we should probably enforce it while we're
 at it.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/7691>
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