[tor-bugs] #24839 [Core Tor/Tor]: Add a torrc option and descriptor line to opt-in as a FallbackDir

[Tor Bug Tracker & Wiki]

#24839: Add a torrc option and descriptor line to opt-in as a FallbackDir
--------------------------+------------------------------------------------
     Reporter:  teor      |      Owner:  (none)
         Type:  task      |     Status:  new
     Priority:  Medium    |  Milestone:  Tor: 0.3.4.x-final
    Component:  Core      |    Version:
  Tor/Tor                 |
     Severity:  Normal    |   Keywords:  needs-proposal, fallback, tor-spec
Actual Points:            |  Parent ID:  #24786
       Points:  3         |   Reviewer:
      Sponsor:            |
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 This needs:
 * a proposal and a design
 * a patch to dir-spec.txt
 * a patch to the tor man page
 * a tor code patch
 * an updateFallbackDirs.py code patch
 * a wiki update to [[doc/UpdatingFallbackDirectoryMirrors]]

 Here's a quick sketch of a design:

 1. Relay operators set `OfferFallbackDirServer 1` to offer their relay as
 a potential FallbackDir.
 2. Relays with this option set put `offer-fallback-dir-server` in their
 descriptors
 3. updateFallbackDirs.py loads relay fingerprints with `offer-fallback-
 dir-server`, and from the legacy whitelist (#24838 will make this easier)
 4. updateFallbackDirs.py applies the blacklist, does stability checks, and
 generates the fallback list

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24839>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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