Prop 312: 3.2.5. Use Directory Header IPv6 Addresses
This is a parent ticket.
If relays are unable to discover their IPv6 address in any other way, they should get their IPv6 address from the X-Your-Address-Is HTTP header in tor directory documents. To support this change, we propose that relays start fetching directory documents over IPv4 and IPv6.
We propose that bridges continue to only fetch directory documents over IPv4, because they try to imitate clients. Therefore, they can't use X-Your-Address-Is HTTP headers to find their IPv6 addresses.
See proposal 312, section 3.2.5: https://gitweb.torproject.org/torspec.git/tree/proposals/312-relay-auto-ipv6-addr.txt#n429