[tor-bugs] #17576 [Tor]: Add torrc option to disable default fallback directories

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#17576: Add torrc option to disable default fallback directories
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 Reporter:  teor               |          Owner:
     Type:  enhancement        |         Status:  new
 Priority:  Medium             |      Milestone:  Tor: 0.2.8.x-final
Component:  Tor                |        Version:
 Severity:  Normal             |     Resolution:
 Keywords:  TorCoreTeam201511  |  Actual Points:
Parent ID:  #15775             |         Points:
  Sponsor:                     |
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Comment (by teor):

 What we want to be able to do:
 * Use hard-coded fallbacks (if any): FallbackDirDisable 0 (default)
 * Replace hard-coded fallbacks (if any) with one or more custom fallbacks:
 FallbackDir ... (regardless of whether FallbackDirDisable is 0 or 1)
 * Disable hard-coded fallbacks and just use the authorities:
 FallbackDirDisable 1

 What I don't think we really need (and it might just be confusing):
 * Augment (add to) hard-coded fallbacks with custom fallbacks
   * we can always supply a list of the hard-coded fallbacks, so users can
 augment it themselves

 How to change torrc.defaults fallback options on the command-line:
 * augment FallbackDir ... with +FallbackDir ...
 * replace FallbackDir ... with FallbackDir ...
 * delete FallbackDir ... with /FallbackDir
 * replace FallbackDirDisable 0|1 with FallbackDirDisable 1|0

 I think this covers it all.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17576#comment:2>
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