$ sysctl -d net.inet6.ip6.v6only net.inet6.ip6.v6only: This toggle sets whether or not wildcard bindings for ipv6 (::) are v6only or also bind to a v4 socket (0.0.0.0)
Looks like there are many undocumented sysctls in this MIB namespace. I'll take a sweep at them. :)
Added review D5336 addressing this PR.
Fixed in base r312307. Please mark as resolved.
Closing per comment 3.