Fallback directory mirrors are hard-coded into Tor. You need to have a fast, stable relay with the same IP (v4 and v6) for at least 1-2 years to be considered. More information about fallback directories here: https://trac.torproject.org/projects/tor/wiki/doc/FallbackDirectoryMirrors
Once or twice a year, a script chooses 150-200 relays from the potential list to be added to the official list. (source)
If your relay qualifies, send a message to the tor-relays mailing list and ask to have your relay added: https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-relays
You need to set
To get an IPv6 ORPort, your relay needs to have an IPv6 connection and a configured IPv6 ORPort. You can find your IPv6 address using
ifconfig or similar commands.
Once you have your IPv6 address, specify an IPv6 ORPort in your
Obviously, use your own IPv6 address and your preferred port. More information about IPv6 relays can be found here: https://trac.torproject.org/projects/tor/wiki/doc/IPv6RelayHowto