Instead of having relays maintain a list of directories they are authorized to relay for, directories could opt in via a TXT record or a .well-known/ URI.

This relaxes the anti-DoS semantics of OHTTP (where the concern is a malicious client getting a relay to DoS random servers on the open internet) from per-endpoint opt-in to per-protocol opt-in, which seems appropriate in this context.

Upon receiving an OHTTP request, a payjoin specific OHTTP relay would first check that the host is opting in to receiving OHTTP requests from such relays for the purpose of facilitating payjoin transactions.

Tracking Sequence

• Don't copy URI path & query to gateway request ohttp-relay#46
• RFC 9540 directory and receiver #549
• Opt-in to traffic from arbitrary relays in directory #569
• Implement RFC 9540 ohttp-relay#47
• Gateway opt in mechanism ohttp-relay#58
• Specify OHTTP gateway for opt-in respecting ohttp-relay #570
• specify the allowed_purposes opt in mechanism both
  • in isolation and
  • in BIP 77
• update payjoin-test-services/Cargo.toml after ohttp-relay release
  • Bump 0.0.10 ohttp-relay#64
• Update payjoin-directory
• instruct ohttp relay runners to updates