23 From query bytes to an authoritative answer

server::respond is the central authoritative pipeline. Its shape is intentionally linear:

1. Reject an unknown opcode from the header without misparsing its body as a standard query.
2. Decode the packet, mapping malformed standard queries to FORMERR.
3. Enforce one question and valid OPT placement.
4. Derive the UDP response limit from EDNS and the transport ceiling.
5. Ask Zone for a typed Lookup.
6. Expand bounded CNAME chains and add relevant target addresses.
7. Normalize RRset TTLs and remove duplicates.
8. Encode or truncate the response.

The code separates mechanism from policy. transport.rs knows UDP datagrams, TCP length prefixes, timeouts, persistent connections, and a fixed worker bound. It knows nothing about zones. The handler knows DNS policy but receives transport limits and client identity as ordinary parameters. That separation lets specialized services reuse the network machinery without pretending to be authoritative zones.

The original djbdns family achieved robustness partly through small processes. rgbdns retains that decomposition while strengthening in-process boundaries. The binaries under src/bin are mostly adapters: environment, configuration, a library call, and the djbdns-compatible fatal exit convention. Small executables remain independently supervisable, but common logic is testable as ordinary Rust functions.