[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:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
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 Implement a way for tor to detect if the network went down. This has
 multiple use cases (see list of tickets), one for instance is being able
 to differenciate between "We couldn't connect to a Guard because the Guard
 is down" vs "We couldn't connect because the network is down". For a more
 thorough discussion about Guard and network connectivity:
 https://lists.torproject.org/pipermail/tor-dev/2014-August/007346.html

 Furthermore, this also makes a difference when we want to either keep a
 circuit or kill it depending on if the network went down or not.

 We can get this kind of information from the OS, we need an API generic
 enough that allows us to build a compat layer for each OS. Let's also make
 sure it's integrated with the latest control command `GETINFO network-
 liveness` and the event `NETWORK_LIVENESS`.

 Tickets that could benefit from this: #8239, #12595

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