Captive Portal WG M. Donnelly
Internet-Draft M. Cullen
Intended status: Informational Painless Security
Expires: January 4, 2018 July 3, 2017

Captive Portal (CAPPORT) API


This document describes an HTTP API that allows User Equipment to detect the existence of a Captive Portal on the local network and determine the properties of the Captive Portal.

Status of This Memo

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This Internet-Draft will expire on January 4, 2018.

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Table of Contents

1. Introduction

This document describes a HyperText Transfer Protocol (HTTP) Application Program Interface (API) that allows User Equipment to detect the existence of a Captive Portal (CAPPORT) on the local network and determine the properties of the Captive Portal. The API defined in this document has been designed to meet the requirements of the CAPPORT API, as discussed in the CAPPORT Architecture [I-D.larose-capport-architecture].

2. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in[RFC2119].

3. Workflow

The CAPPORT protocol consists of two phases. In the first phase User Equipment acquires an IP address and determines the URL of the local CAPPORT API Server, if any. The second phase consists of the User Equipment querying the CAPPORT API Server for the requirements for accessing its protected networks.

During the first phase, User Equipment uses the Dynamic Host Configuration Protocol (DHCP) or IPv6 Router Advertisements (RAs) to acquire an IP address and to determine the URL for the local CAPPORT API Server. This details for the first phase are described in RFC 7710[RFC7710], and the rest of this document assumes that the User Equipments already has a URL to reach the CAPPORT API Server.

The second phase begins with the User Equipment accessing the URL provided in the first phase with JSON content type. The CAPPORT API Server responds with the current status of the User Equipment's access to the protected networks.

The User Equipment SHOULD request the same URL with an HTML content type to start the process to gain access to the Captive Portal.

4. Use of the DHCP Captive-Portal Option

As decribed above, to use the CAPPORT API, User Equipment needs a URL that can be used to reach the CAPPORT API Server. DHCP Servers and IPv6 Routers SHOULD provide, and User Equipment SHOULD obtain, the required URL using the DCHP Captive-Portal Option or the IPv6 RA Captive-Portal Option, as described in [RFC7710].

To provide backwards compatibility with the original use of the DHCP and RA options described in RFC7710, the CAPPORT API defined in this document is exclusively accessed using HTTP with an Accept header value of "application/json". Captive Portals that implement the CAPPORT API SHOULD respond to an HTTP GET that has an Accept header of "text/html" with HTML content that, when displayed in a web browser, will allow the user to interactively meet the Captive Portal requirements for network access.


This section defines the CAPPORT API.

5.1. URLs and HTTP Methods

This section describes the URLs that can be used to access the CAPPORT API.

5.1.1. Associating User Equipment with its URL

The CAPPORT API Server SHOULD associate an incoming request with a particular User Equipment consistently. [TODO: specify how this would happen.]

5.1.2. Interactive URL

The CAPPORT API Server SHOULD respond to HTTP GET requests to the provided URL that specify an Accept header value of "text/html" with HTML content instead of this protocol. When the User Equipment wants to satisfy the conditions for network access, it SHOULD display this interactive URL in a web browser to allow the user to complete the network access outside of this protocol.


The CAPPORT API Server SHOULD respond to HTTP POST requests to the provided URL that specify an Accept header value of "application/json" with the CAPPORT API protocol.

5.2. JSON Data Structures

The CAPPORT API data structures are specified in JavaScript Object Notation (JSON)[RFC7159]. This document specifies the structure of the JSON structures and message using the JSON Content Rules (JCR) defined in draft-newton-json-content-rules [I-D.newton-json-content-rules].

5.2.1. CAPPORT Common Elements

This section describes structures that are shared between requests and responses. Toplevel Object

The CAPPORT API will contain JSON-formatted data. The toplevel object contains a networks object whose value is an array of zero or more network objects.

    $toplevel = {
      $networks ,
      $session_token ?

The toplevel object MUST contain a networks object.

The CAPPORT API Server responses MUST contain a session_token object. The session-token object contains a session token which will be used in ICMP requests as discussed in RFC 7710.

QUESTION: Should the session token just be provided by the server, or should it be negotiated between the client and server using something like a DH exchange? Networks Object

The networks object represents the list of networks being acted on in this CAPPORT session.

    $networks = {
      ( "DEFAULT" || // ) = $network +

The networks object is a JSON object whose keys are network names and whose values are network objects. Thus a single response could be used in gaining access to multiple protected networks at once. The first request to the CAPPORT API Server will contain no networks, and acts as a discovery request.

The CAPPORT API Server SHOULD use the special name DEFAULT for one network that provides access to the greater Internet. Network Object

The network object represents a network protected by the Captive Portal.

    $network = {
      "conditions" : [ $condition + ] ,
      "state" : $network_state ? ,
      "details" : $network_details ?

The network object MUST contain a 'conditions' key whose value is an array of one or more $condition objects, which represent the unmet conditions for gaining access to this network. The conditions object SHOULD NOT contain conditions that have already been met.

CAPPORT API Server responses MUST contain the 'state' key, whose value is the $network_state object, which represents the state of access that the User Equipment has to the network.

CAPPORT API Server responses SHOULD contain the 'details' key, whose value is the $network_details object, which provides relevant information about the network.

5.2.2. User Equipment Request

For the initial CAPPORT request from the User Equipment, the JSON object will consist of the toplevel object with its required networks objects. The networks object will contain no networks, and the session_token object will be empty. This acts as a discovery request.

                "networks" : {}
                "session-token" : ""

Figure 1 CAPPORT API Server Response Network State Object

The network_state object details the current state of the User Equipment access to the protected network.

                    $network_state = {
                    "permitted" : boolean ,
                    "expires" : datetime ? ,
                    "bytes_remaining" : integer ?

The network_state object MUST contain the "permitted" key, whose boolean value indicates whether the User Equipment is permitted to access the protected network.

The network_state object SHOULD contain the "expires" key if the access to the protected network will expire at a known time in the future. The value is a datetime object of the time the access will expire. If there is not a known expiration time, the key SHOULD be omitted.

The network_state object SHOULD contain the "bytes_remaining" key if the access to the protected network will expire after the User Equipment transfers a known number of bytes. The value is an integer of the number of bytes remaining. If there is not a known limit for this User Equipment, the key MAY be omitted or its value MAY be -1.

6. IANA Considerations

This document does not require any IANA allocations. Please remove this section before RFC publication.

7. Security Considerations

The CAPPORT API described in this document is intended to automate a process that is currently accomplished by a user filling out a HTML form in a Web Browser. Therefore, this mechanism should meet the requirement of being no less secure than presenting the user with a HTML form for completion in a Web Browser, and submitting that form to a Captive Portal.

TBD: Provide complete security requirements and analysis.

7.1. Privacy Considerations

Information passed in this protocol may include a user's personal information, such as a full name and credit card details. Therefore, it is important that CAPPORT API Servers do not allow access to the CAPPORT API over unecrypted sessions.

8. Acknowledgements

This document was written using xml2rfc, as described in [RFC7749]

9. References

9.1. Normative References

[I-D.larose-capport-architecture] Larose, K. and D. Dolson, "CAPPORT Architecture", Internet-Draft draft-larose-capport-architecture-01, June 2017.
[I-D.newton-json-content-rules] Newton, A. and P. Cordell, "A Language for Rules Describing JSON Content", Internet-Draft draft-newton-json-content-rules-08, March 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014.
[RFC7710] Kumari, W., Gudmundsson, O., Ebersman, P. and S. Sheng, "Captive-Portal Identification Using DHCP or Router Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710, December 2015.

9.2. Informative References

[RFC7749] Reschke, J., "The "xml2rfc" Version 2 Vocabulary", RFC 7749, DOI 10.17487/RFC7749, February 2016.

Authors' Addresses

Mark Donnelly Painless Security 14 Summer Street, Suite 202 Malden, MA 02148 USA EMail: URI:
Margaret Cullen Painless Security 14 Summer Street, Suite 202 Malden, MA 02148 USA Phone: +1 781 405-7464 EMail: URI: