[tor-bugs] #15775 [Tor]: Add IPv4 Fallback Directory List to tor, active by default

[Tor Bug Tracker & Wiki]

#15775: Add IPv4 Fallback Directory List to tor, active by default
-------------------------+-------------------------------------------------
     Reporter:  teor     |      Owner:  teor
         Type:           |     Status:  needs_information
  enhancement            |  Milestone:  Tor: 0.2.7.x-final
     Priority:  major    |    Version:  Tor: 0.2.4.7-alpha
    Component:  Tor      |   Keywords:  tor-dist, 027-triaged-1-in,
   Resolution:           |  TorCoreTeam201508
Actual Points:           |  Parent ID:  #15228
       Points:           |
-------------------------+-------------------------------------------------

Comment (by teor):

 ''Directory Failures''

 Directory failures are handled in `directory_get_from_dirserver`:

 When all the dirservers are down, we call(back) directory_all_unreachable
 and wait for an application connection before we try any more dirservers.
 (See #16762 for the most recent change to this code.)

 The some-dirservers-are-down behaviour seems to work just as well - we
 ignore them until we've tried all the other ones.

 I still need to:
 * Add the whitelist/blacklist feature to the fallback directories list
 * Write a wiki page explaining fallback directories
 * Draft an email requesting that relay operators opt-in their ID and IP as
 a fallback directory
 * Convince someone @ torproject.org to send the email to tor-relays
 * someone @ torproject.org collates responses and adds them to the
 whitelist
 * Once we have enough whitelist entries, we merge the whitelisted fallback
 directories into the code

 Open Questions:
 * How many relays should we have in the fallback directory list? 100?
   * Modify the code to report the count at the top of the file
   * Modify the code to `#error` out if we fall below a minimum relay count
 (100)
 * Should we adjust weights so no relay has more than 1% (?) of the weight
 on the list?
   * Modify the code to report the total weight, min, and max proportions
 at the top of the file
   * Modify the code to adjust weights so that no relay gets more than a
 maximum proportion of clients (5%, or 5/(minimum relay count))
 ''Dir Auth vs Fallback Weights''
 * The dir auths don't have set weights, but the fallback dirs do.
 Therefore, the auths get weighted 1.0 (the default weight). The fallback
 weights range from 300K to 300, so the dir auths will hardly ever get
 contacted (about 1 in 2 million clients, or 1 per day). Is this the
 desired behaviour?

 I need guidance on whether the numbers above are sensible.
 But the first step is reporting the actual numbers at the top of the
 output file.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15775#comment:35>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online