How to inspect a host's TLS certificate

1. Enter the hostname

   Type the host (e.g. example.com) into the Hostname field. Up to 253 characters. SNI is set from this name.

2. Pick the port

   Port defaults to 443 (HTTPS). Use 465 for SMTPS, 993 for IMAPS, 995 for POP3S, etc. — anything that speaks TLS at connect time.

3. Press Run

   Our server opens a TLS connection with SNI = Hostname and rejectUnauthorized: false (so expired/self-signed certs still produce a readout), reads the peer certificate, then closes the socket.

4. Read the certificate

   Output covers subject, issuer, validFrom / validTo, SANs (subject alternative names), fingerprint, key algorithm, and serial number. Mismatched SAN or expired dates are common causes of "NET::ERR_CERT…" errors in browsers.