How to Check Domain Availability for Free with RDAP

The Registration Data Access Protocol (RDAP) is the standard way to look up who has registered a domain name. It’s the official successor to WHOIS, and ICANN requires accredited registries and registrars for generic top-level domains (.com, .net, .org and the newer gTLDs) to support it. Many country-code registries do too.

For our purposes, an RDAP lookup answers one question very cheaply: is this domain registered?

rdap.org is a free redirector. It looks up which registry runs a TLD and forwards your request there, so you don’t need to know the right server:

# 404 = no registration found (likely available)
# 200 = registered (taken)
curl -s -o /dev/null -w "%{http_code}\n" -L https://rdap.org/domain/smithplumbing.com

Drop the -o /dev/null -w … part to see the full JSON for a registered domain: registrar, creation and expiry dates, nameservers and status codes such as client transfer prohibited.

Because RDAP is just HTTPS and JSON, you don’t need a paid domain availability API for basic checks:

// Node 18+ / any modern runtime: no key, no SDK
async function isAvailable(domain) {
  const res = await fetch(`https://rdap.org/domain/${domain}`, { redirect: 'follow' })
  if (res.status === 404) return true    // not registered
  if (res.ok) return false               // registered
  return null                            // rate-limited, unsupported TLD, or error: unknown
}

The three-way result is deliberate. A failed check isn’t the same as “taken”, and treating errors as “unavailable” hides good domains from you. PresenceZero stores the same three states: available, taken, and “check failed”.

• Unsupported TLDs. Some country-code registries don’t offer RDAP yet. Check the IANA RDAP bootstrap file: if a TLD isn’t listed, an error says nothing about availability.
• Reserved and premium names. A registry can withhold a name, or price it far above normal, without it being “registered”. RDAP may report not-found while your registrar shows a premium price.
• Recently dropped domains. Domains in redemption or pending-delete aren’t available to register yet, even though they no longer work.
• Race conditions. Availability can change at any moment. The registrar’s checkout is the final word.

RDAP servers are public infrastructure, and many rate-limit aggressively. If you check domains in bulk:

• keep to roughly one request per second overall;
• back off exponentially when you get 429 Too Many Requests;
• cache results: “taken” rarely changes, so cache it for weeks; “available” can change, so cache it for about a day.

That’s exactly how PresenceZero’s RDAP checker behaves: a global one-per-second throttle, exponential backoff on 429s, and a shared cache (24 hours for available, 30 days for taken), so the same domain is never looked up twice across scans.

When you’re pitching a website, the domain you check should be the one the owner would pick themselves. A simple recipe:

1. Lowercase the name and strip accents and punctuation (“Café Élise” becomes cafeelise).
2. Turn & into “and”.
3. Drop trailing legal suffixes such as Inc, LLC, Ltd, Corp or GmbH.
4. Try both the full form and a compact one without stop words (“the”, “and”, “of”).
5. Check .com first, then the business’s country domain (.ca, .co.uk…).

An available exact-match .com is one of the strongest openers you can bring to a pitch, which is why it carries its own weight in PresenceZero’s lead score.