IETF RFC 7710 PDF

In many environments offering short-term or temporary Internet access (such as coffee shops), it is common to start new connections in a captive-portal mode. This highly restricts what the customer can do until the customer has authenticated.

This 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.