[tor-bugs] #15775 [Tor]: Add IPv4 Fallback Directory List to add_default_fallback_dir_servers()

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#15775: Add IPv4 Fallback Directory List to add_default_fallback_dir_servers()
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     Reporter:  teor         |      Owner:  teor
         Type:  enhancement  |     Status:  accepted
     Priority:  major        |  Milestone:  Tor: 0.2.7.x-final
    Component:  Tor          |    Version:  Tor: 0.2.4.7-alpha
   Resolution:               |   Keywords:  tor-dist, 027-triaged-1-in
Actual Points:               |  Parent ID:  #15228
       Points:               |
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Changes (by teor):

 * status:  needs_information => accepted


Comment:

 nickm and I spoke on IRC, and he said that we might actually want to test
 the fallback directories feature in a test network sometime. That would be
 prudent.

 So, I suggest that we:
 * Stick the default fallback directories in `torrc.defaults`
   * The final set of FallbackDirs will then be the union of
 `torrc.defaults`, `torrc`, and command-line
 * Create a new `FallbackDirsMirrorDefaultDirAuths` option set to true by
 default
 * Only use FallbackDirs if the current authories satisfy
 `FallbackDirsMirrorDefaultDirAuths`, that is:
   * If `FallbackDirsMirrorDefaultDirAuths` is true, and the default
 directory authorities are being used, use the configured FallbackDirs
   * If `FallbackDirsMirrorDefaultDirAuths` is false, and custom directory
 authorities are being used, use the configured FallbackDirs
   * Otherwise, the FallbackDirs don't mirror the authorities, so don't use
 any FallbackDirs.

 Then, anyone wanting to test FallbackDirs in a test network will need to:
 * Set `FallbackDirsMirrorDefaultDirAuths` to false
 * Disable the default fallback directories in `torrc.defaults`, or, more
 likely, disable the use of `torrc.defaults` entirely on the command-line

 I think it's the simplest way to avoid a regression to #15642. Otherwise
 test networks could fallback to the public tor network.

 This avoids an additional `FallbackDirFile`, and also avoids weird
 special-casing in the code. It puts an additional configuration burden on
 test network operators who want to use FallbackDirs (a tiny group), and
 gets it right by default for everyone else.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15775#comment:13>
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