[tor-bugs] #19608 [Core Tor/Tor]: IPv6-only clients can't fetch microdescriptors on 0.2.8.5-rc

[Tor Bug Tracker & Wiki]

#19608: IPv6-only clients can't fetch microdescriptors on 0.2.8.5-rc
------------------------------+--------------------------------
     Reporter:  teor          |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.8.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Major         |   Keywords:  ipv6
Actual Points:                |  Parent ID:
       Points:  0.5           |   Reviewer:
      Sponsor:                |
------------------------------+--------------------------------
 There's a lovely catch-22 when IPv6-only clients try to fetch
 microdescriptors:
 * some fallback directories are on IPv6, and deliver the microdescriptor
 consensus
 * but the microdescriptor consensus doesn't contain any IPv6 addresses
 * but it's a consensus, so IPv6-only clients try to use it to find an IPv6
 relay, and fail

 A solution is to teach IPv6-only clients to get (at least some of) their
 descriptors from the fallback directories, at least until they have some
 IPv6 microdescriptors. Perhaps this should happen automatically when the
 entire consensus fails, but for some reason it doesn't.

 I have no idea how we didn't catch this during testing - perhaps there
 were always cached descriptors? Perhaps we broke falling back to the
 fallbacks for descriptors?

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19608>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online