[tor-bugs] #15642 [- Select a component]: Disable default fallback directories when DirAuthorities, AlternateDirAuthority, or FallbackDir are set

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#15642: Disable default fallback directories when DirAuthorities,
AlternateDirAuthority, or FallbackDir are set
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 Reporter:  teor                  |          Owner:
     Type:  defect                |         Status:  new
 Priority:  minor                 |      Milestone:  Tor: 0.2.7.x-final
Component:  - Select a component  |        Version:  Tor: 0.2.4.7-alpha
 Keywords:  tor-client tor-relay  |  Actual Points:
Parent ID:                        |         Points:
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 Currently, when we set `DirAuthorities` or `AlternateDirAuthority`, we
 don't add the default directory authorities. But, (as long as
 `FallbackDir` isn't set) we do add the default fallback directories.
 However, the list of default fallback directories is currently `NULL`, so
 this issue has no effect.

 But I can't imagine too many scenarios where it would be useful to set an
 `AlternateDirAuthority` or `DirAuthorities`, and still get the default
 `FallbackDir`.

 I can imagine this causing similar issues to #13163, where the default
 authorities were added to a custom set of authorities in some
 circumstances.

 I'll create a patch to fix this, but it won't actually change tor's
 observable behaviour until we add directories to the default fallback
 directory list.

 Bugfix on 90f6071d8dc0 in 0.2.4.7-alpha.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15642>
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