Relay join request forwarding rules support #6729

Summary

We should support the Join Request filtering feature of the Relay specification.

Current Situation

All join requests are forwarded by the relays.

Why do we need this? Who uses it, and when?

In order to limit which (JoinEUI, DevEUI) prefixes are forwarded by the relay.

Proposed Implementation

Similar to the current uplink_forwarding_rules, but with a default rule.

Contributing

• I can help by doing more research.
• I can help by implementing the feature after the proposal above is approved.
• I can help by testing the feature before it's released.

Code of Conduct

• I agree to follow TTN's Community Code of Conduct.