[tor-bugs] #18580 [Tor]: exit relay fails with 'unbound' DNS resolver when lots of requests time-out

[Tor Bug Tracker & Wiki]

#18580: exit relay fails with 'unbound' DNS resolver when lots of requests time-out
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     Reporter:  Dhalgren  |      Owner:
         Type:  defect    |     Status:  new
     Priority:  Medium    |  Milestone:
    Component:  Tor       |    Version:  Tor: 0.2.7.6
     Severity:  Major     |   Keywords:
Actual Points:            |  Parent ID:
       Points:            |   Reviewer:
      Sponsor:  None      |
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 per

 [tor-relays] What does this message mean in my tor logs?
 https://lists.torproject.org/pipermail/tor-relays/2016-January/008621.html

 [tor-relays] unbound bogs down strangely, degrading exit relay
 https://lists.torproject.org/pipermail/tor-relays/2016-March/008918.html

 Relay daemon ceases to service Tor Browser requests, timing out, when a
 local instance of 'unbound' is the DNS resolver and large numbers of DNS
 requests time-out.

 Works fine when 'named' is swapped in place of 'unbound'.

 GoDaddy DNS stops responding when large numbers of queries are submitted
 and this was observed as the particular trigger.

 To reproduce, configure the SOA+NS records for several thousand dummy
 domains to point to a non-responding IP, then generate large numbers of
 requests against them.

 The commands

 unbound-control dump_requestlist
 unbound-control dump_infra

 are helpful for identifying the state.

 Have debug-level daemon trace taken when relay was in the unresponsive
 condition described.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/18580>
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