[tor-bugs] #17372 [Tor]: Use Guard as Fallback Directory

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#17372: Use Guard as Fallback Directory
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     Reporter:  teor         |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  High         |  Milestone:  Tor: 0.2.8.x-final
    Component:  Tor          |    Version:
     Severity:  Normal       |   Keywords:
Actual Points:               |  Parent ID:
       Points:  medium       |    Sponsor:
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 When a client hasn't used the Tor network for a few days, its consensus
 expires. If the authorities are blocked while it's offline, it can't
 bootstrap.

 However, it has (almost) enough information to bootstrap from a semi-
 trusted relay - its guard.

 This change requires:
 * a proposal
 * an additional line `GuardFallbackDir` in the state file, formatted like
 a FallbackDir line, if the guard is also a directory mirror

 When #12538 is implemented, every relay (and therefore every Guard) will
 be a directory cache by default.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17372>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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