Manual WHOIS checks work for one-off lookups. For competitor monitoring, lead verification, or brand protection you need automation through a WHOIS API.

Popular WHOIS API Services

Service	Free tier	Price	Features
WhoisXML API	500 requests/mo	from $19/mo	Normalized JSON, history
Whoxy.com	100/mo	from $10/mo	Reverse WHOIS, bulk
ip-api.com	45/min (IP)	Free	IP geolocation only
domain-coupler.io	10/mo	from $5/mo	Simple, fast

PHP Example

// WHOIS lookup via the WhoisXML API
$apiKey = 'YOUR_API_KEY';
$domain = 'example.com';

$url = "https://www.whoisxmlapi.com/whoisserver/WhoisService"
     . "?apiKey={$apiKey}&domainName={$domain}&outputFormat=JSON";

$response = file_get_contents($url);
$data = json_decode($response, true);

echo "Registrar: " . $data['WhoisRecord']['registrarName'] . "
";
echo "Expires: " . $data['WhoisRecord']['expiresDate'] . "
";
echo "NS: " . implode(', ', $data['WhoisRecord']['nameServers']['hostNames']) . "
";

Python Example

import requests

API_KEY = 'YOUR_API_KEY'
domains = ['example.com', 'competitor.com', 'anotherdomain.net']

for domain in domains:
    url = f"https://www.whoisxmlapi.com/whoisserver/WhoisService"
    params = {
        'apiKey': API_KEY,
        'domainName': domain,
        'outputFormat': 'JSON'
    }
    resp = requests.get(url, params=params).json()
    record = resp.get('WhoisRecord', {})
    print(f"{domain}: expires {record.get('expiresDate', 'N/A')}")

Using the whois Python Library

pip install python-whois

import whois

w = whois.whois('example.com')
print(w.expiration_date)  # Expiration date
print(w.registrar)        # Registrar
print(w.name_servers)     # NS servers

💡 Competitor monitoring: Run a WHOIS check on competitor domains every week. If a domain has moved to a new owner or its NS servers changed, that signals a hosting move or a business sale.

⚠️ Rate limiting: Public WHOIS servers throttle requests (usually 1–5/sec). Run bulk checks only through paid APIs with official rate limits.

Common Errors When Working with a WHOIS API

Error	Cause
HTTP 401 Unauthorized	An invalid or expired API key, or the key was passed in the wrong parameter — different services expect apiKey, api_key, or a dedicated header
HTTP 429 Too Many Requests	The plan's request limit was exceeded: checking a list of domains in bulk with no pause between requests quickly hits the rate limit
WhoisRecord.registrarName: null in the response	The domain uses "thin" WHOIS (common for some ccTLDs) — the registrar does not publish this data in the shared protocol, only through the zone registry's RDAP
SSL certificate problem: unable to get local issuer certificate	An outdated CA bundle on the server, or the openssl extension is not enabled in PHP — curl cannot verify the API's certificate chain
WhoisCommandFailed: No match for domain	The domain is available, or the registry's whois server is temporarily down; python-whois does not retry automatically
Quota exceeded / HTTP 402 Payment Required	The free tier's monthly quota ran out before the renewal date — the service's counter resets on the first of the month, not 30 days after signup

When a WHOIS API Is Not the Right Tool

You need a real-time check right inside a site's signup form — public WHOIS servers are too slow and too rate-limited for a synchronous request on every keystroke; RDAP with your own cache fits better
There is no budget for a paid tier, but the volume of checks is large — free limits run out quickly, and bulk requests to public WHOIS servers end in an IP ban
You need historical WHOIS data from past years — a regular lookup only returns the current state; history requires a separate paid service such as WhoisXML Historical WHOIS
The domain is in a zone with WHOIS closed by default (most .eu, many .de) — the owner's personal data will not come back through any API, since the restriction sits at the registry level, not the service

Verifying the Result

Before wiring the API into a production script, confirm the key actually works and the limit is not already exhausted — it saves time debugging the whole pipeline later. The basics of WHOIS and manual lookups are covered in the articles on what WHOIS is and on checking a domain online.

# Confirm the key works with a single curl request
curl -s "https://www.whoisxmlapi.com/whoisserver/WhoisService?apiKey=YOUR_KEY&domainName=example.com&outputFormat=JSON" | head -c 200

# Count how many domains from the list actually got a response
grep -c '"expiresDate"' results.json