[tor-bugs] #21394 [Core Tor/Tor]: connection timeouts are affecting Tor Browser usability

Tor Bug Tracker & Wiki blackhole at torproject.org
Wed Nov 1 09:27:02 UTC 2017

#21394: connection timeouts are affecting Tor Browser usability
 Reporter:  arthuredelstein                      |          Owner:  (none)
     Type:  defect                               |         Status:  new
 Priority:  Very High                            |      Milestone:  Tor:
                                                 |  0.3.3.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tbb-performance, tbb-usability,      |  Actual Points:
  performance, tbb-needs                         |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:

Comment (by Sebastian):

 So yes, I think that bugreport is very relevant here. I think this is a
 libevent bug/an unfortunate issue with how libevent and unbound interact.
 By default libevent only allows 64 DNS requests to be inflight at the same
 time. When asking unbound to resolve something and the dns server in
 question nullroutes traffic from unbound, it takes longer than named to
 reply with a SERVFAIL. (named's timeout matches what Tor uses). So if a
 bigger dns server operator decides it has gotten to many queries from a
 relay, reaching that limit of 64 inflight queries happens in absolutely no
 time and all other DNS requests get queued.

 There may be an additional issue where we're marking our local unbound as
 down because it didn't reply quickly enough to a query we made, because
 it's still trying to answer it. Combined this would easily explain why DNS
 gets wedged completely.

 We REALLY need a high-volume exit operator to help us debug these

Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21394#comment:34>
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