DCV Inspector helps you inspect the DNS, HTTP, and SMTP requests made by a certificate authority during domain validation. You can use it to detect violations of domain validation rules, such as the use of Delegated Third Parties.
DCV Inspector creates a unique subdomain for each test. All DNS queries, HTTP requests, and emails to this subdomain (including descendants of the subdomain) are recorded and presented to you for inspection.
Tip: stop your test immediately after getting a certificate, so the results don't get polluted by bots which crawl hostnames found in Certificate Transparency logs.