WHOIS has been the standard domain-lookup protocol since the 1980s, but it was never formally specified - every registry's output format is slightly different, there's no standard way to ask "is this available" versus "who owns this," and rate limits vary registry to registry with no consistent error format. RDAP (Registration Data Access Protocol, RFC 7482) is IANA's replacement: a real HTTP/JSON API, one predictable response shape, and a published bootstrap registry mapping every TLD to its authoritative RDAP server. This checker queries that server directly - a 404 means no registration record exists (likely available), a 200 means one does (taken), which is exactly what the checker on this site does live, not a simulated result.
"Domain front-running" (sometimes called domain tasting or search-box sniping) is a documented pattern where searching an available name through a registrar's own interface can itself register interest in that name before you complete a purchase - the search becomes a signal the registrar's own systems (or a partner network) can act on faster than you can check out. It's been reported and discussed publicly for years, most notably around Network Solutions in the mid-2000s. The practical defense is simple: check availability against the registry's own authoritative RDAP record instead of a registrar's search interface, and register through a direct handoff once you've confirmed - the same reasoning behind why the checker registers you through Dynadot's own checkout rather than an in-page purchase flow.
RDAP adoption isn't universal. Some ccTLD registries (notably .io and
.me as of this writing) aren't in the IANA bootstrap registry yet, and some
registries that do support RDAP block direct browser queries via CORS. When that happens,
this tool says so plainly rather than falling back to a scraped or estimated result - see
the coverage note on the checker itself for the current supported
list.