[tor-bugs] #15775 [Tor]: Add IPv4 Fallback Directory List to tor, active by default

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#15775: Add IPv4 Fallback Directory List to tor, active by default
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     Reporter:  teor     |      Owner:  teor
         Type:           |     Status:  needs_review
  enhancement            |  Milestone:  Tor: 0.2.8.x-final
     Priority:  major    |    Version:  Tor: 0.2.4.7-alpha
    Component:  Tor      |   Keywords:  0.2.8.x-triage, tor-dist,
   Resolution:           |  027-triaged-1-in, DoS, SponsorU,
Actual Points:  20       |  TorCoreTeam201510, 028-triaged
       Points:           |  Parent ID:  #15228
  medium/large           |    Sponsor:  SponsorU
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Comment (by teor):

 Added feature from prop 220 to script:
 * Target fallback count is 20% of guard count

 See `feature15775-fallback-v7` on https://github.com/teor2345/tor.git

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15775#comment:49>
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