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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.)