RDAP vs. WHOIS: A Technical Comparison for Developers

If you are building anything that queries domain registration data — availability checkers, portfolio tools, monitoring systems, compliance software — WHOIS and RDAP are the two protocols you will encounter. WHOIS is the legacy system. RDAP is the replacement. The differences matter more than most developers expect.

WHOIS: How It Works

WHOIS is a simple query-response protocol defined in RFC 3912. You open a TCP connection to port 43 on a WHOIS server, send a domain name, and receive plain text in response.

whois example.com

The response is unstructured text. The format varies by registry — Verisign's .com WHOIS returns fields in a different order and with different field names than Nominet's .co.uk WHOIS. Parsing WHOIS requires registry-specific parsers that break with any registry format change.

WHOIS also has a discovery problem. There is no standard way to find which WHOIS server handles a given TLD. You need a lookup table, which goes out of date.

RDAP: How It Works

RDAP is defined in RFCs 7480-7484. The protocol uses HTTPS (port 443), with standard HTTP methods (GET), and returns JSON-LD structured data.

curl https://rdap.verisign.com/com/v1/domain/example.com

The JSON response has a defined structure. Fields have consistent names across registries. You parse it once.

Discovery uses the IANA bootstrap file. The bootstrap file at https://data.iana.org/rdap/dns.json maps TLDs to their RDAP base URLs. Cache this file (it updates infrequently) and your client can find the correct RDAP server for any TLD automatically.

Response Structure Comparison

A WHOIS response looks like this:

Domain Name: EXAMPLE.COM
Registry Domain ID: 2336799_DOMAIN_COM-VRSN
Registrar WHOIS Server: whois.iana.org
Updated Date: 2023-08-14T07:01:38Z
Creation Date: 1995-08-14T04:00:00Z
Registrar: RESERVED-INTERNET ASSIGNED NUMBERS AUTHORITY

The same data in RDAP:

{
  "objectClassName": "domain",
  "handle": "2336799_DOMAIN_COM-VRSN",
  "ldhName": "example.com",
  "status": ["client delete prohibited", "client transfer prohibited"],
  "events": [
    {"eventAction": "last changed", "eventDate": "2023-08-14T07:01:38Z"},
    {"eventAction": "registration", "eventDate": "1995-08-14T04:00:00Z"}
  ],
  "entities": [
    {
      "objectClassName": "entity",
      "roles": ["registrar"],
      "vcardArray": [["version", {}, "text", "4.0"], ["fn", {}, "text", "IANA"]]
    }
  ]
}

The RDAP structure is consistent, machine-readable without regex parsing, and unambiguous about what each field means.

Key Differences for Developers

Availability checking. In WHOIS, detecting availability requires parsing the text response for patterns like "No match for domain" (Verisign) or "NOT FOUND" (other registries) — different strings per registry. In RDAP, a 404 HTTP response definitively means the domain is not registered. A 200 means it is. One status code, no string parsing required.

Rate limiting. WHOIS servers impose rate limits that are enforced inconsistently and often undocumented. RDAP servers use standard HTTP rate limiting with proper 429 status codes, Retry-After headers, and documented limits.

Authentication. RDAP supports tiered access: unauthenticated requests return public data; authenticated requests can return additional contact data (for accredited users). WHOIS has no authentication model.

DNSSEC data. RDAP returns DNSSEC information (DS records, key data) in structured format. WHOIS returns it as unparseable text in a free-form field.

IPv6 and HTTPS. RDAP is served over HTTPS. WHOIS runs over plain TCP. For security-conscious tooling, RDAP is the correct choice.

WHOIS Deprecation Timeline

ICANN's 2023 temporary specification and subsequent policy decisions moved registries toward RDAP as the primary access protocol. Several large registries have deprecated their WHOIS servers or redirected them to RDAP endpoints.

For new tooling, build on RDAP. For existing tools, plan migration. WHOIS servers are still running as of 2027, but the protocol is on a deprecation path with no fixed end date but clear directional intent.

Implementation Note

BatchDomain uses RDAP exclusively for availability checks. The IANA bootstrap resolves the correct endpoint per TLD, and the 404/200 response pattern means accurate results without per-registry parsing logic. This is the architecture to follow for any new availability checking infrastructure.