How Domain Availability Checking Works
When you search for a domain on a registrar's website, they're doing two things behind the scenes:
- DNS query: If the domain returns NXDOMAIN (non-existent domain), it's likely available
- RDAP/WHOIS lookup: Confirms registration status — a domain can exist in DNS but be in a pending-delete state
The Gotchas
DNS Alone Isn't Enough
Some registered domains have no DNS records configured. They'll return NXDOMAIN but are actually taken. That's why you need to check RDAP/WHOIS too.
Pending Delete Domains
When a domain expires, it goes through a lifecycle: expired → redemption period → pending delete → available. Domains in pending-delete will show as registered in WHOIS but are about to become available.
Premium Domains
Some TLDs have premium pricing for short or dictionary-word domains. Availability doesn't guarantee standard pricing.
Checking Availability via API
Single domain check:
curl "https://api.dnstoolkit.io/dns/available?domain=my-cool-startup.com"Bulk check up to 50 domains at once:
curl -X POST "https://api.dnstoolkit.io/dns/available/bulk" \
-H "Content-Type: application/json" \
-d '{"domains": ["startup.io", "startup.dev", "startup.app"]}'TLD Search
Want to find which TLDs are available for your keyword? The TLD search endpoint checks across 200+ extensions:
curl "https://api.dnstoolkit.io/dns/tld-search?keyword=startup"This returns availability status for .com, .io, .dev, .app, .ai, and hundreds more — perfect for building a domain search tool.