What is an FQDN (Fully Qualified Domain Name) and why does the Captive Portal config require one?

An FQDN is a complete DNS name including all labels up to the root — e.g., captiveportal.mydomain.ch. — fully unambiguous.

FQDN vs hostname:

• captiveportal — just a hostname, ambiguous (which domain?).
• captiveportal.mydomain.ch — FQDN, exactly one host on the entire DNS tree.

Why Captive Portal needs one:

• The TLS certificate's Common Name / SAN must match what the browser typed in the URL bar.
• If the cert is issued for captiveportal.mydomain.ch but the redirect sends the browser to 192.168.10.1, the browser sees a name mismatch and warns.
• The FQDN, the DNS A record, and the cert CN all must agree exactly.

Tip: FQDN ends with an implicit dot (the root). www.example.com. is fully qualified; www alone is not.

Go deeper:

• Fully qualified domain name (Wikipedia) — the complete name from host label to root zone, the unambiguity the cert CN must match.