[tor-bugs] #16120 [Tor]: Detect if the network goes down

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#16120: Detect if the network goes down
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     Reporter:  dgoulet      |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  normal       |  Milestone:  Tor: 0.2.7.x-final
    Component:  Tor          |    Version:
   Resolution:               |   Keywords:  TorCore201508
Actual Points:               |  Parent ID:
       Points:               |
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Comment (by teor):

 Replying to [comment:1 arma]:
 > Is this the right layer for solving the problem? How many people
 actually have their network interface go away, vs just go out of range of
 the wireless access point?
 >
 > I've always thought that the right approach here was to distinguish "I
 seem to not be able to reach anything" from "I can reach some things but
 not others", which is tricky to begin with, and extra tricky when you
 consider the time component (if you can't reach anything for a while, but
 then you do reach something, you cannot learn just from that which
 situation you were in).
 >
 > Or to turn it around, if you build the OS-specific code to check if the
 network interface goes away, is it now any less necessary to solve the
 problems I raise above?

 For example, OS X distinguishes two cases:
 * an individual host is "reachable" (there's an entire API for it, see
 https://developer.apple.com/library/mac/documentation/SystemConfiguration/Reference/SCNetworkReachabilityRef/
 )
 * an interface appears or disappears

 If there are no interfaces apart from localhost, we obviously can't get
 out.
 But if there are external interfaces, should we assume that if a certain
 set of well-known servers can't be reached, the entire network is down
 (or, we are under attack)?
 This is a common strategy, but it adds a dependency on external servers
 for proper behaviour.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/16120#comment:4>
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