Subdomain Finder – Certificate Transparency

Find unique subdomains for one or many websites from public Certificate Transparency records. Enter a domain or URL and receive clean, deduplicated hostnames ready for export or automation in the Apify Dataset.

What this Actor does

The Actor searches public certificate transparency data exposed by crt.name for each submitted website. It normalizes URLs and subdomains to the registrable apex domain, removes duplicate and wildcard entries, filters unrelated hostnames, and returns one structured Dataset item per discovered hostname.

Use cases

• Map a company's public web footprint before security or asset-inventory work
• Find regional, product, documentation, API, staging, and support hostnames
• Enrich domain lists for SEO, competitive research, and lead qualification
• Monitor certificate-derived hostname coverage across multiple domains

How to use

1. Open the Actor in Apify Console.
2. Enter one or more domains or URLs in Domains or URLs.
3. Choose whether to include the apex domain and set an optional result limit.
4. Click Start and open the Dataset when the run finishes.

Input

domains accepts between 1 and 10,000 domains or URLs. Values are converted to lowercase registrable domains and deduplicated, so https://www.similarweb.com/path and similarweb.com produce one search. By default, the apex domain itself is excluded and up to 10,000 hostnames are saved per domain. Request concurrency is managed automatically by the Actor. Large runs can take longer when the public source responds slowly or retries temporary errors.

{
  "domains": [
    "similarweb.com"
  ],
  "includeApex": false,
  "maxResultsPerDomain": 3
}

Output

Results are stored in the default Dataset and can be exported as JSON, CSV, Excel, XML, or RSS. A found item represents one hostname. Domains with no matching hostnames produce one not_found item; failed source requests produce one failed item with a safe error message so batch runs retain their partial results. The default Key-Value Store also contains a SUMMARY record with submitted and normalized domain counts plus aggregate result and billing counts.

This is the complete Dataset item from a real successful run:

{
  "apexDomain": "similarweb.com",
  "subdomain": "academy.similarweb.com",
  "relativeName": "academy",
  "depth": 1,
  "isApex": false,
  "status": "found",
  "totalFoundForDomain": 121,
  "truncated": true,
  "sourceUrl": "https://crt.name/v1/search?apex=similarweb.com",
  "retrievedAt": "2026-08-24T12:51:00.625171Z",
  "error": null
}

Pricing

Each successfully delivered unique subdomain triggers one subdomain-result event. Your Apify subscription tier determines its price:

Apify user tier	Price per found subdomain
Free	$0.0006
Bronze	$0.00054
Silver	$0.00048
Gold	$0.00045
Platinum	$0.00045
Diamond	$0.00045

A charge occurs only for a Dataset item with status: "found". not_found and failed diagnostics, duplicates, retries, and SUMMARY are not charged.

You can set a maximum charge for each run in Apify. If the remaining budget cannot cover every discovered hostname, the Actor saves and charges only the results that fit, marks chargeLimitReached: true in SUMMARY, and stops without delivering unpaid rows.

API and integrations

Run the Actor through the Apify API, schedule recurring searches, trigger runs from webhooks, or connect the Dataset to downstream tools. Dataset exports work with common spreadsheet, database, and automation workflows without additional transformation.

Related Actors

Actor	Best for
Similarweb Website Traffic & SEO Analytics Scraper	Add traffic, ranking, audience, and competitor context to the domains you investigate.
Website Technology Lookup – CMS, Hosting & Tech Stack Data	Identify the published technologies, CMS, and hosting behind discovered web properties.
Bulk Domain Rating Checker	Compare authority metrics for domains after completing asset discovery.
Favicon Extractor	Enrich discovered hostnames with favicon URLs and image metadata.

Limits and data quality

Certificate transparency records show hostnames present in public certificates; they do not prove that a hostname is currently live, reachable, owned by the same organization, or complete. Recently issued certificates may take time to appear, and hostnames that have never appeared in a public certificate cannot be discovered this way. The Actor retries temporary source failures, reports permanent per-domain failures, and marks every returned item with truncated: true when the configured per-domain limit omits additional matches.

Frequently asked questions

Does the Actor check whether each subdomain is online?

No. It discovers certificate-derived hostnames and does not perform DNS resolution, HTTP probing, port scanning, or takeover checks.

Can I submit full URLs or subdomains?

Yes. The Actor extracts the hostname and converts it to the registrable apex domain. Paths, query strings, capitalization, leading www, and duplicate inputs do not create duplicate searches.

Why is a known subdomain missing?

It may not have appeared in a public certificate, the source may not have indexed it yet, or your maxResultsPerDomain value may have truncated the output. Check totalFoundForDomain and truncated in the Dataset item.

Responsible use

Use the Actor only for lawful purposes and handle discovered infrastructure data responsibly. You are responsible for complying with applicable laws, privacy requirements, source terms, and authorization requirements before performing any follow-up testing against discovered hosts.

Support

Open an issue in the Actor's Issues tab and include the run ID, a reproducible input, and the unexpected result. Do not include passwords, API tokens, or other secrets.