http://www.rfc-editor.org/rfc/rfc7710.txt
RFC 7710: Captive-Portal Identification Using DHCP or Router Advertisements (RAs), W. Kumari, O. Gudmundsson, P. Ebersman, S. Sheng2015-12-01T23:00:00-00:00This document describes a DHCP option (and a Router Advertisement (RA) extension) to inform clients that they are behind some sort of captive-portal device and that they will need to authenticate to get Internet access. It is not a full solution to address all of the issues that clients may have with captive portals; it is designed to be used in larger solutions. The method of authenticating to and interacting with the captive portal is out of scope for this document.