There is! RFC 7710[1] specifies a DHCP option/RA extension that encodes the URL of the captive portal, s.t. when, e.g. DHCP completes, the connecting machine im

There is! RFC 7710[1] specifies a DHCP option/RA extension that encodes the URL of the captive portal, s.t. when, e.g. DHCP completes, the connecting machine immediately knows what the captive portal URL is, and doesn't have to get MitM'd to know it. [1]: https://tools.ietf.org/html/rfc7710 https://tools.ietf.org/html/rfc7710