[tor-bugs] #17834 [Tor]: Add ClientPreferIPv6DirPort

[Tor Bug Tracker & Wiki]

#17834: Add ClientPreferIPv6DirPort
-----------------------------+--------------------------------
     Reporter:  teor         |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  Medium       |  Milestone:  Tor: 0.2.8.x-final
    Component:  Tor          |    Version:
     Severity:  Normal       |   Keywords:  ipv6
Actual Points:               |  Parent ID:  #17811
       Points:               |    Sponsor:
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 Add ClientPreferIPv6DirPort to express a preference for IPv6 directory
 connections over IPv4 directory connections.

 Tor already has ClientPreferIPv6ORPort, which expresses a preference for
 IPv6 OR connections over IPv4 OR connections.

 (ClientUseIPv6 allows the use of IPv6 connections,
 ClientPreferIPv6ORPort/ClientPreferIPv6DirPort are the tie-breakers if
 IPv4 and IPv6 are both equally likely.)

 While we're doing this, let's warn on nonsensical situations like
 ClientPreferIPv6* 1 ClientUseIPv6 0. (Although ClientPreferIPv6* 0
 ClientUseIPv6 1 should be an info or notice, as ClientPreferIPv6* 0 is the
 default.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17834>
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