Discovery

Subdomain Finder

Discover subdomains via certificate transparency and DNS bruteforce. Highlights mail subdomains and ESP CNAMEs.

About this check

Discover subdomains and identify mail-sending services.

Frequently asked questions

What does the Subdomain Finder actually check?: Discover subdomains and identify mail-sending services.
Is this really free?: Yes. Every on.bo tool is free, no signup required, no rate-limit tricks.
Do you store the data I check?: We log minimal metadata to keep the service healthy. Lookups themselves are not retained beyond short-term caching.