How We Check Domain Availability
Domain Search King asks the registry behind each extension whether it holds a record for the name, using RDAP. A 404 means no record and a 200 means registered. Where a 404 can hide reserved or premium names, we also read WHOIS. Some answers are cached, and no check can promise a price.
Published by Domain Search King · Updated
What is RDAP and how does a lookup work?
RDAP, the Registration Data Access Protocol, is the HTTP service registries use to publish registration records as JSON. RFC 9082 sets the query format: the server's base URL, then domain/, then the name. RFC 9083 sets the shape of the JSON that comes back. RFC 7480, section 5.3 covers the negative case, where a server with no matching data returns 404.
This is a real exchange with the .com registry, trimmed to the fields we read. The lookups came back like this on November 1, 2026:
GET https://rdap.verisign.com/com/v1/domain/example.com
HTTP 200
"ldhName": "EXAMPLE.COM"
"status": ["client delete prohibited", "client transfer prohibited", "client update prohibited"]
"events": registration 1995-08-14, expiration 2027-08-13
GET https://rdap.verisign.com/com/v1/domain/qzvplorbinx.com
HTTP 404The second name is a made-up string. Our code turns the status code into one of three answers:
| Registry reply | What we report | What happens next |
|---|---|---|
| HTTP 404 | Available, subject to the second check below | On .com and .net we stop here |
| HTTP 200 | Registered | We can also read status and dates from the body |
| HTTP 429 | Nothing yet | Wait about 600 ms and try once more |
| Timeout or network error | Nothing yet | Wait about 250 ms and try once more |
| Two failed tries | Registry WHOIS decides | If WHOIS is silent too, the answer is Unknown |
| Any other status | Unknown | Unknown never counts as available |
Which registries do we query?
Eight extensions are pinned in our code to the RDAP servers below. Everything else is looked up in IANA's RDAP bootstrap file, which we keep for up to 12 hours per server instance. An extension with no RDAP server at all goes to its registry's WHOIS service on port 43.
| Extension | RDAP server |
|---|---|
| .com | rdap.verisign.com/com/v1/ |
| .net | rdap.verisign.com/net/v1/ |
| .org | rdap.publicinterestregistry.org/rdap/ |
| .io, .ai, .info | rdap.identitydigital.services/rdap/ |
| .co | rdap.registry.co/co/ |
| .xyz | rdap.centralnic.com/xyz/ |
Most checks go first through our own caching proxy, which sits between our site and the registries so that they see fewer requests. If the proxy is down, or returns no answer for a name, our server asks the registry directly.
What gets checked depends on the page. A homepage search checks each name on .com, .ai and .io. Switching off ".com only" adds .co, .net and .org. The domain verify page takes a single name on any extension, as long as it is a registrable name. It refuses subdomains, because a registry has no record for those and would answer 404.
When is a 404 not enough?
A 404 proves the registry has no registration on file. On many extensions it says nothing about names the registry holds back or sells at a higher price, because those can return 404 as well. So the rule depends on the extension:
| Extensions | What we need before saying available |
|---|---|
| .com, .net | RDAP 404. No WHOIS step. |
| .io, .ai, .xyz, .tv, .cc, .me, .ly, .sh, .gg, .vc | RDAP 404 and a WHOIS answer saying the name is free. A silent WHOIS gives Unknown. |
| Other generic extensions (.org, .app, .shop and so on), plus .co | RDAP 404, then WHOIS. A reserved or premium answer turns the result into Registered. If the registry has retired WHOIS, we report the name as not registered, with the premium price unverified. A WHOIS server that exists but gives no usable answer leaves the result as Unknown. |
| Other two-letter country codes | RDAP or WHOIS not found is taken as free. |
These three names all returned 404 from RDAP on November 1, 2026. Asked directly, the WHOIS servers said something else, and our route returned false for each:
| Name | RDAP | Registry WHOIS said | Our answer |
|---|---|---|---|
| remember.ai | 404 | This name is reserved by the Registry. | available: false, registry-reserved |
| postimg.org | 404 | This name is reserved by the Registry in accordance with ICANN Policy. | available: false, registry-reserved |
| yacht.xyz | 404 | This domain has been reserved by the registry, and is not available for registration. | available: false, registry-reserved |
In batch searches the WHOIS step runs against a deadline. Names it does not reach in time stay Unknown and are left out of the available list.
What is cached, and for how long?
Several layers can answer before a registry is asked again. The ages below come from our code and are defaults unless stated.
| Layer | Applies to | Age of the answer |
|---|---|---|
| RDAP proxy | Homepage batches, naming, MCP tools | Available answers up to about 10 minutes, taken answers up to about 24 hours |
| Keyword cache | First paint for popular keywords on the homepage | Up to about 30 hours by default. The page then rechecks those names in the background. |
| Generator landing pages | The name grid in the page HTML | Most pages carry a stored list, checked when it was built, with that time shown beside the grid. The CDN keeps those pages for a day and can serve an older copy for up to a week while it refreshes. A page with no stored list is kept for one hour. |
| Niche pages under /ideas | The name lists on those pages | The same pattern: stored lists for a day at the CDN, generated lists for one hour, and an older copy is possible while a page refreshes. |
| /verify and /api/rdap | One name you typed | Asks the proxy for a fresh answer. Our response is marked no-store. |
The Register button in homepage search rechecks a name when clicked. Register links on the cached landing-page grids and on /ideas go straight to the registrar with no recheck, so an older answer can reach you there.
Where can a result still be wrong?
Premium names
A registry can charge far more than the normal fee for a name nobody has registered. WHOIS sometimes says so, and we match that wording. When a registry publishes no WHOIS, we say the price could not be verified. On .com and .net we run no WHOIS step at all, and we never show a registry's premium price from RDAP, because the 404 does not carry one.
Reserved names
The WHOIS step catches the wording we know, as in the table above. A registry that words its reservation differently could slip past on an extension without strict confirmation, and the registrar would then refuse the name.
Pending delete
An expired name stays on file through its deletion, so RDAP keeps returning 200 and we report it as registered. That is accurate, though it can surprise someone looking at a dead website. kombuchahaus.com expired on September 4, 2026, and on November 1 RDAP still showed it with the status "redemption period" while DNS had no record of it. ICANN's guide to domain status codes describes the sequence. After the purge the registry answers 404, and our 24-hour cache of taken answers can keep showing the name as registered for up to a day. A backorder service may also get it first.
Timing
Someone can register a name a minute after any check, ours included. A cached "available" answer widens that gap by the ages listed above.
The guide to names you can't register covers what to do in each case, and the guide to a registered domain with no website covers the dead-website case.
How can you confirm a result yourself?
- Paste the name into the verify page for a single check.
- Open the domain check report to see the registrar, dates and status codes on a registered name.
- Query the registry yourself. Paste an RDAP URL from the table above into a browser, with
domain/and the name on the end, or use ICANN Lookup. - For names RDAP calls not found on .ai, .io or .xyz, query the registry's WHOIS server, such as whois.nic.ai.
- Put the name in a registrar's cart. Only the registrar can show the final price for your account and confirm the sale.
Check a Name Against the Registry
Paste one domain and see what the registry holds for it right now.
Open the verify pageFAQ
Does checking a name reserve it?
No. A check reserves nothing and sends nothing to a registrar. The outside query goes to the registry's RDAP or WHOIS service. We do record the name you checked, or the keyword you searched, in our own event log, which is how we count usage.
Is RDAP the same as WHOIS?
They serve the same purpose. RDAP runs over HTTP and returns structured JSON, while WHOIS is an older text service on port 43. We read RDAP first and use WHOIS as a fallback and as a second check for reserved and premium names.
Does Domain Search King show the price of a domain?
Search results can show Spaceship's price for a name when we have it from Spaceship's API. Other registrars' prices are not shown, and RDAP does not report prices. The registrar's cart has the final figure.
What does Unknown mean?
The registry did not answer, gave a reply we could not read, or a required WHOIS confirmation failed. Wait a minute and retry on the verify page, or ask a registrar directly.