# Using Certificate Transparency Logs to check sub domains

**Date:** 2026-10-01  
**Author:** Kees C. Bakker  
**Original:** https://keestalkstech.com/notes/using-certificate-transparency-logs-to-check-domains/

---

Most domains use TLS certificates for security. Did you know they are published, logged and can be queried? Fun!

You might use it to discover sub domains:

```
curl -sS --retry 5 --retry-all-errors "https://ctlogs.dev/search?q=*.openai.com&output=json" | jq -r '.rows | map(.match) | unique[]'
```