July 4, 2026 · By Rishav Mukherjee
What Is RDAP? How Modern Domain Availability Checking Works
RDAP — Registration Data Access Protocol — is the modern standard for querying domain registration data. It replaced WHOIS, and understanding it explains why domain checkers sometimes return unexpected results.
The Problem with WHOIS
WHOIS dates back to the 1980s. No standard response format — each registry returned text differently. No authentication, no HTTPS, inconsistent field names, easy bulk scraping. It worked, but barely.
What RDAP Fixes
RDAP (defined in RFC 7480–7484) provides: JSON responses with a standardized schema, required HTTPS, authentication support for tiered access, a bootstrap mechanism via IANA for server discovery, and full internationalization support. ICANN mandated RDAP for all registries.
How a Domain Availability Check Works
- Parse the TLD from input (e.g., .io from "example.io")
- Query IANA's bootstrap JSON for that TLD's RDAP server URL
- Send HTTPS GET to that server:
rdap.server/domain/example.io - 200 response = registered. 404 = available. 403/timeout = unknown.
Why Results Are Sometimes "Unknown"
Some RDAP servers rate-limit or block queries from datacenter IP ranges. This prevents bulk scraping. When a server blocks the query, a well-built checker returns "unknown" rather than a false positive or negative. Most common with newer ccTLDs with overprotective registries.
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