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

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#15775: Add IPv4 Fallback Directory List to tor, active by default
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     Reporter:  teor         |      Owner:  teor
         Type:  enhancement  |     Status:  needs_information
     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):

 We want to start with opt-in via email to tor-relays (with proposed list)
 and operator contact info. This could be drafted by anyone, but needs to
 be sent from a torproject.org email address.

 Operators who opt-in would have their relay fingerprints and IPs added to
 the list, and be included on the list if both the fingerprint and IP
 match.

 Other considerations, from IRC discussions:

 There is a tradeoff between a small list, where a single operator learns
 about more clients, and a large list, which may become unmanageable. (And
 it might slow down bootstrap if too many unreliable relays are included.)

 Every release containing a list of fallback directories will forever
 contact those directories on initial bootstrap. We'll need to ensure that
 the list is large enough that:
 * no one relay / IP gets overwhelming traffic
 * there is always an unblocked relay which can provide an updated
 consensus (in most cases, the authorities will do this, as long as they
 are unblocked)

 This situation will improve once #15942 (exponential backoff) and #15935
 (advisory-only old client disabling) are implemented.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15775#comment:31>
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