[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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Comment (by teor):

 No, just a small amount of file loading code, which will work much like
 `torrc.defaults` or the `geoip` files. So I can use them as my example.
 Then I can access the configured list in
 `add_default_fallback_dir_servers`.

 I think that the priority should be: (lowest to highest)
 * `torrc.defaults` `FallbackDir` lines
 * default `FallbackDirFile`
 * `torrc` (including any custom `FallbackDirFile`)
 * command-line arguments

 Generally, I'd expect a union of all those lists, except when it comes to
 plusses and minuses and the weird option overriding interactions I don't
 quite understand. I'll trust the config code to do the right thing here.

 In a test network, we'll also need to disable the default
 `FallbackDirFile`, otherwise test networks could fallback to the public
 tor network. Which would be weird and unhelpful.

 Most test networks are configured to just override the authorities, and I
 wouldn't want to break that with this change.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15775#comment:11>
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