[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:  enhancement                          |         Status:
 Priority:  High                                 |  needs_review
Component:  Tor                                  |      Milestone:  Tor:
 Severity:  Major                                |  0.2.8.x-final
 Keywords:  0.2.8.x-triage, tor-dist,            |        Version:  Tor:
  027-triaged-1-in, DoS, SponsorU, 028-triaged,  |  0.2.4.7-alpha
  TorCoreTeam201512                              |     Resolution:
Parent ID:  #15228                               |  Actual Points:  20
  Sponsor:  SponsorU                             |         Points:
                                                 |  medium/large
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Comment (by teor):

 This task is now focused on making clients fetch from fallback directory
 mirrors. I split off #17709 to make directory downloads happen as follows:
 * authorities generate the consensus
 * FallbackDirs FetchDirInfoExtraEarly
 * Directory Mirrors FetchDirInfoEarly
 * Everything else, including non-directory mirror relays and clients and
 hidden services and bridges, fetches on a client-like schedule
 * Bridge clients then directory_fetches_dir_info_later()

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15775#comment:55>
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