[tor-bugs] #17835 [Tor]: Make ClientPreferIPv6ORPort/DirPort smarter

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#17835: Make ClientPreferIPv6ORPort/DirPort smarter
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     Reporter:  teor    |      Owner:
         Type:  defect  |     Status:  new
     Priority:  Medium  |  Milestone:  Tor: 0.2.9.x-final
    Component:  Tor     |    Version:
     Severity:  Normal  |   Keywords:
Actual Points:          |  Parent ID:  #17811
       Points:          |    Sponsor:
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 It's hard for users on dual-stack machines to know whether their IPv4 or
 IPv6 connections provides better connectivity or censorship-resistance.

 Tor could help with this by:
 * Making ClientPreferIPv6ORPort/DirPort into an autobool, and setting the
 default to auto (infer), rather than 0 (IPv4)
 * Infer initial IPv4/IPv6 preference by looking at the machine's network
 interfaces, and:
   * Ignoring localhost (tor_addr_is_localhost) and link-local addresses (a
 new tor_addr_is_link_local)
   * Checking for (other) private addresses (tor currently considers link-
 local addresses private, this shouldn't change in the rest of the
 codebase, but it is important to distinguish here, as IPv4/IPv6 hosts with
 no IPv6 connectivity still auto-configure a link-local IPv6 address)
   * Checking for publicly routable addresses
   * Preferring IP versions that have a public address, over those that
 have a private address
   * (I'd also say to avoid IP versions that only have localhost or link-
 local addresses, and don't use IP versions that have no addresses, but
 that might cause issues with proxies, or with machines that prohibit
 enumeration of local addresses.)
   * Repeat these checks every N minutes in case the network interfaces
 have changed (relays do this every 20 minutes at the moment)
 * Infer a preference for IPv4 or IPv6 based on the number of failed
 connections
   * Globally track counts of attempted IPv4 and IPv6 connections
   * Globally track failures of attempted IPv4 and IPv6 connections
   * If there have been at least M attempts (2? 3?) on (one? each?) IP
 version, then:
     * If an IP version has 100% failure, avoid it
     * IP an IP version has a significantly (2x?) larger failure rate,
 avoid it

 I'm deferring this to 0.2.9 because:
   * IPv4/IPv6 Tor clients might work fine without any of these changes.
   * It's complex code, and it's not clear that it's the best way to
 determine preferred IP versions. (Let's get some experience with dual-
 stack clients first.)
   * We only have 4/9 authorities and ~25% of fallback directory mirrors on
 IPv6, so let's prefer IPv4 by default at the moment for load-balancing
 reasons. (For similar reasons, ClientUseIPv6 could default to 1 in future,
 but now might not be a good time to do that. However, Tor Browser sets it
 to 1 at the moment.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17835>
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