How I Built a Domain Availability Checker with PHP and RDAP

Domain availability checking looks simple: type a domain, get a "yes" or "no." But when you need to check hundreds of domains across dozens of TLDs — reliably, quickly, and without paying for a commercial API — the problem gets interesting.

I recently built Site Tools Box, a free online toolbox for webmasters that includes a bulk domain availability checker. Here's how the backend works, and why I chose RDAP over raw WHOIS.

Why Raw WHOIS Doesn't Scale

Traditional WHOIS is a plain-text protocol over port 43. It works, but it has real problems for a modern tool:

• No standard format. Every registry returns text differently. Parsing "No match for" vs "Not found" vs "Status: available" becomes a fragile regex nightmare.
• Rate limiting. Some WHOIS servers simply stop responding if you query too fast. There's no standard Retry-After header.
• No concurrency support. You open a raw TCP socket for each query. That doesn't scale.

For a bulk checker that handles up to 200 domains per request, this approach falls apart quickly.

The Solution: RDAP with WHOIS Fallback

RDAP (Registration Data Access Protocol) is the modern replacement for WHOIS, mandated by ICANN. It's JSON-based, supports HTTP status codes, and most major registries now support it.

The idea is simple: use RDAP first, fall back to WHOIS only when RDAP isn't available.

Step 1: Map TLDs to RDAP Endpoints

Each registry has its own RDAP endpoint. I built a simple lookup table:

function domainRdapEndpoint($tld) {
    $map = [
        'com' => 'https://rdap.verisign.com/com/v1/domain/',
        'io'  => 'https://rdap.identitydigital.services/rdap/domain/',
        'ai'  => 'https://rdap.identitydigital.services/rdap/domain/',
        'xyz' => 'https://rdap.centralnic.com/xyz/domain/',
        // ... and more
    ];
    return $map[$tld] ?? null;
}

If the TLD isn't in the map, the tool falls back to a WHOIS socket connection for that domain.

Step 2: Parse the RDAP Response

RDAP returns JSON. A 200 status means the domain is registered, and a 404 means it's available. The JSON body also contains registrar info, registration date, and expiration date:

function parseRdapBody($domain, $body, $source = 'RDAP') {
    $d = json_decode($body, true);
    if (!$d) return null;

    $registrar = '';
    if (!empty($d['entities'])) {
        foreach ($d['entities'] as $entity) {
            if (!empty($entity['roles']) && in_array('registrar', $entity['roles'])) {
                foreach ($entity['vcardArray'][1] as $v) {
                    if ($v[0] === 'fn') { $registrar = $v[3]; break; }
                }
            }
        }
    }

    return [
        'domain' => $domain,
        'registered' => true,
        'registrar' => $registrar ?: '-',
    ];
}

Step 3: Handle Bulk Checks with curl_multi

For bulk requests, I use curl_multi to fire up to 30 concurrent RDAP queries. This is significantly faster than a sequential loop, and it respects the rate limits of most registries:

function domainCheckBatch($domains, $concurrency = 30) {
    $rdapDomains = [];
    foreach ($domains as $d) {
        $tld = substr(strrchr($d, '.'), 1);
        if (domainRdapEndpoint($tld)) $rdapDomains[] = $d;
    }

    for ($i = 0; $i < count($rdapDomains); $i += $concurrency) {
        $chunk = array_slice($rdapDomains, $i, $concurrency);
        $mh = curl_multi_init();
        // Add handles, execute, collect results
    }
}

Domains without an RDAP endpoint (like .cn or .uk) go through the WHOIS fallback, which uses a raw socket connection on port 43.

Three Things I Learned

1. RDAP is far more reliable than WHOIS. The JSON structure means no more parsing fragile text. HTTP status codes are unambiguous.

2. curl_multi is the right tool for bulk checks. Sequential queries make the tool painfully slow. A concurrency of 30 is the sweet spot before registries start throttling.

3. Always have a fallback. Not every TLD supports RDAP. The WHOIS fallback handles the rest, even if it's slower and messier.

Bonus: Scoring Available Domains

Since I was already building the checker, I added a small scoring algorithm for available domains. It looks at five dimensions:

• Extension (.com scores highest)
• Length (shorter is better)
• Meaning (real English words or pinyin)
• Pronounceability (vowel-consonant alternation)
• Character quality (pure letters, no numbers)

It's not a professional valuation — just a rough starting point. But it helps users quickly filter hundreds of available domains.

Try It Out

You can try the domain availability checker here: Site Tools Box

It supports single and bulk checks, and gives available domains a rough value score.

If you're building something similar and have questions about the RDAP implementation, drop a comment below. I'm happy to share more of the code.

Comments

[Furqan Ashraf]

Since you asked what's missing: a "last checked" timestamp and the source (RDAP or WHOIS) next to each result would help a lot. A 404 from RDAP is a strong hint, but reserved and premium names can still show as available when they aren't. Maybe flag those as "likely available" rather than a hard yes. Also curious how the hard-coded domainRdapEndpoint() map holds up over time. Pulling from IANA's dns.json bootstrap file and caching it might save you updating the map by hand.

[laing lii]

Thanks for the great feedback, Furqan! These are incredibly actionable.

You are 100% right about the 404 issue—premium/reserved domains can be tricky, so labeling them as "likely available" is a much better approach.

I also love the idea of using IANA's bootstrap file to replace my hard-coded endpoint map. That will definitely save me a lot of manual work going forward.

I'll be adding the "last checked" timestamp and source next. Really appreciate you pointing these out!

[laing lii]

Hi HN, I'm the author of Site Tools Box.

I built this because I kept running into the same problem: needing a quick domain check, DNS lookup, or HTTP status test, and every existing site was either stuffed with ads, required a sign-up, or sent my data to a server to do something simple.

Site Tools Box is a collection of 20+ free tools for webmasters and developers: domain availability, WHOIS, DNS, HTTP status, dead link scanner, JSON formatter, and more. No sign-ups, no ads.

The domain availability checker and appraisal tool are probably the most useful ones. The appraisal tool gives a rough value range based on length, extension, and pronounceability — not a professional valuation, just a starting point.

Built with vanilla PHP and JS. No frameworks, no build step.

sitetoolsbox.com

Feedback welcome — especially on which tools are useful and what's missing.