IPv6-only stack for guard relay

Support Censorship Circumvention
1

In fact, the design of the Tor network currently does not allow it to run with an IPv6-only stack. Otherwise, if it were possible to connect to the Tor guard relay with an IPv6-only stack, then it would be possible to connect to the Tor network without an obfs bridge.

As for being able to connect to an obfs bridge using IPv6, it is probably because the obfs bridge can currently be connected using an IPv6-only stack, while the Tor guard relay (entry relay) definitely cannot be connected using an IPv6-only stack.

I’m wondering is this true? I’m just scanning all guard relies and it seems only some IPv6 addresses of them are not blocked yet.

2

OK, found it in man page.

       ClientPreferIPv6ORPort 0|1|auto
           If this option is set to 1, Tor prefers an OR port with an IPv6
           address over one with IPv4 if a given entry node has both. (Tor
           also prefers an IPv6 ORPort if IPv4Client is set to 0.) If this
           option is set to auto, Tor bridge clients prefer the configured
           bridge address, and other clients prefer IPv4. Other things may
           influence the choice. This option breaks a tie to the favor of
           IPv6. (Default: auto)

Setting it to 1 currently could circumvent China censorship.