DomainMonitor is a web project that has a RESTful API to get a domain's subdomains and whois data.
You can test the web app using docker:
docker-compose up -d
And go to http://127.0.0.1.
/api/domains/{domain}/whois
- fields: What fields of the whois query the api will return. Should be separated by commas. Example: registrar,creation_date,expiration_date
/api/domains/{domain}/subdomains