[tor-bugs] #33375 [Core Tor/Tor]: Stop advertising an IPv6 exit policy when DNS is broken for IPv6

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#33375: Stop advertising an IPv6 exit policy when DNS is broken for IPv6
-------------------------+-------------------------------------------------
     Reporter:  teor     |      Owner:  (none)
         Type:  defect   |     Status:  new
     Priority:  Medium   |  Milestone:  Tor: unspecified
    Component:  Core     |    Version:  Tor: 0.2.9.14
  Tor/Tor                |
     Severity:  Normal   |   Keywords:  ipv6, tor-client, tor-exit, tor-dns
Actual Points:           |  Parent ID:  #24833
       Points:           |   Reviewer:
      Sponsor:           |
-------------------------+-------------------------------------------------
 When `dns_seems_to_be_broken_for_ipv6()`, exits should stop advertising an
 IPv6 exit policy.

 Here's a rough design:
 * when `dns_seems_to_be_broken_for_ipv6()` is first set to 1, mark the
 relay descriptor dirty
 * when rebuilding the descriptor, check
 `dns_seems_to_be_broken_for_ipv6()` before including an IPv6 exit policy
 * reset `dns_seems_to_be_broken_for_ipv6()` periodically, maybe every 1-3
 days?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33375>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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