Internet-Draft Captive Portal API February 2020
Pauly & Thakore Expires 7 August 2020 [Page]
Captive Portal Interaction
Intended Status:
Standards Track
T. Pauly, Ed.
Apple Inc.
D. Thakore, Ed.

Captive Portal API


This document describes an HTTP API that allows clients to interact with a Captive Portal system.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 7 August 2020.

Table of Contents

1. Introduction

This document describes a HyperText Transfer Protocol (HTTP) Application Program Interface (API) that allows clients to interact with a Captive Portal system. The API defined in this document has been designed to meet the requirements in the Captive Portal Architecture [I-D.ietf-capport-architecture]. Specifically, the API provides:

2. Terminology

This document leverages the terminology and components described in [I-D.ietf-capport-architecture] and additionally uses the following association:

3. Workflow

The Captive Portal Architecture defines several categories of interaction between clients and Captive Portal systems:

  1. Provisioning, in which a client discovers that a network has a captive portal, and learns the URI of the API server.
  2. API Server interaction, in which a client queries the state of the captive portal and retrieves the necessary information to get out of captivity.
  3. Enforcement, in which the enforcement device in the network blocks disallowed traffic.

This document defines the mechanisms used in the second category. It is assumed that the location of the Captive Portal API server has been discovered by the client as part of Provisioning. The mechanism for discovering the API Server endpoint is not covered by this document.

4. API Details

4.1. URI of Captive Portal API endpoint

The URI of the API endpoint MUST be accessed using HTTP over TLS (HTTPS) and SHOULD be served on port 443 [RFC2818]. The client SHOULD NOT assume that the URI for a given network attachment will stay the same, and SHOULD rely on the discovery or provisioning process each time it joins the network. Depending on how the Captive Portal system is configured, the URI might be unique for each client host and between sessions for the same client host.

For example, if the Captive Portal API server is hosted at, the URI's of the API could be:

  • ""
  • ""

4.1.1. Server Authentication

The purpose of accessing the Captive Portal API over an HTTPS connection is twofold: first, the encrypted connection protects the integrity and confidentiality of the API exchange from other parties on the local network; and second, it provides the client of the API an opportunity to authenticate the server that is hosting the API. This authentication is aimed at allowing a user to be reasonably confident that the entity providing the Captive Portal API has a valid certificate for the hostname in the URI (such as ""). The hostname of the API SHOULD be displayed to the user in order to indicate the entity which is providing the API service.

Clients performing revocation checking will need some means of accessing revocation information for certificates presented by the API server. Online Certificate Status Protocol [RFC6960] (OCSP) stapling, using the TLS Certificate Status Request extension [RFC6066] SHOULD be used. OCSP stapling allows a client to perform revocation checks without initiating new connections. To allow for other forms of revocation checking, a captive network could permit connections to OCSP responders or Certificate Revocation Lists (CRLs) that are referenced by certificates provided by the API server. In addition to connections to OCSP responders and CRLs, a captive network SHOULD also permit connections to Network Time Protocol (NTP) [RFC5905] servers or other time-sync mechnisms to allow clients to accurately validate certificates.

Certificates with missing intermediate certificates that rely on clients validating the certificate chain using the URI specified in the Authority Information Access (AIA) extension [RFC5280] SHOULD NOT be used by the Captive Portal API server. If the certificates do require the use of AIA, the captive network will need to allow client access to the host specified in the URI.

If the client is unable to validate the certificate presented by the API server, it MUST NOT proceed with any of the behavior for API interaction described in this document. The client will proceed to interact with the captive network as if the API capabilities were not present. It may still be possible for the user to access the network by being redirected to a web portal.

4.2. JSON Keys

The Captive Portal API data structures are specified in JavaScript Object Notation (JSON) [RFC8259]. Requests and responses for the Captive Portal API use the "application/captive+json" media type. Clients SHOULD include this media type as an Accept header in their GET requests, and servers MUST mark this media type as their Content-Type header in responses.

The following keys are defined at the top-level of the JSON structure returned by the API server:

  • "captive" (required, boolean): indicates whether the client is in a state of captivity, i.e it has not satisfied the conditions to access the external network. If the client is captive (i.e. captive=true), it can still be allowed enough access for it to perform server authentication Section 4.1.1.
  • "user-portal-url" (optional, string): provides the URL of a web portal with which a user can interact.
  • "venue-info-url" (optional, string): provides the URL of a webpage or site on which the operator of the network has information that it wishes to share with the user (e.g., store info, maps, flight status, or entertainment).
  • "can-extend-session" (optional, boolean): indicates that the URL specified as "user-portal-url" allows the user to extend a session once the client is no longer in a state of captivity. This provides a hint that a client system can suggest accessing the portal URL to the user when the session is near its limit in terms of time or bytes.
  • "seconds-remaining" (optional, integer): indicates the number of seconds remaining, after which the client will be placed into a captive state. The API server SHOULD include this value if the client is not captive (i.e. captive=false) and SHOULD omit this value for captive clients.
  • "bytes-remaining" (optional, integer): indicates the number of bytes remaining, after which the client will be in placed into a captive state. The byte count represents the total number of IP packet (layer 3) bytes sent and received by the client. Captive portal systems might not count traffic to whitelisted servers, such as the API server, but clients cannot rely on such behavior.

The valid JSON keys can be extended by adding entries to the Captive Portal API Keys Registry Section 6. If a client receives a key that it does not recognize, it MUST ignore the key and any associated values. All keys other than the ones defined in this document as "required" will be considered optional.

4.3. Example Interaction

A client connected to a captive network upon discovering the URI of the API server will query the API server to retrieve information about its captive state and conditions to escape captivity. To request the Captive Portal JSON content, a client sends an HTTP GET request:

GET /captive-portal/api/X54PD
Accept: application/captive+json

The server then responds with the JSON content for that client:

HTTP/1.1 200 OK
Cache-Control: private
Date: Mon, 02 Mar 2020 05:07:35 GMT
Content-Type: application/captive+json

   "captive": true,
   "user-portal-url": "",
   "venue-info-url": "",
   "seconds-remaining": 326,
   "can-extend-session": true

Upon receiving this information the client will provide this information to the user so that they may navigate the web portal (as specified by the user-portal-url value) to enable access to the external network. Once the user satisfies the requirements for extenal network access, the client SHOULD query the API server again to verify that it is no longer captive.

5. Security Considerations

One of the goals of this protocol is to improve the security of the communication between client hosts and Captive Portal systems. Client traffic is protected from passive listeners on the local network by requiring TLS-encrypted connections between the client and the Captive Portal API server, as described in Section 4. All communication between the clients and the API server MUST be encrypted.

In addition to encrypting communications between clients and Captive Portal systems, this protocol requires a basic level of authentication from the API server, as described in Section 4.1.1. Specifically, the API server MUST present a valid certificate on which the client can perform revocation checks. This allows the client to ensure that the API server has authority for a hostname that can be presented to a user.

It is important to note that while the server authentication checks can validate a specific hostname, it is certainly possible for the API server to present a valid certificate for a hostname that uses non-standard characters or is otherwise designed to trick the user into believing that its hostname is some other, more trustworthy, name. This is a danger of any scenario in which a hostname is not typed in by a user.

5.1. 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 Captive Portal API Servers do not allow access to the Captive Portal API over unencrypted sessions.

6. IANA Considerations

IANA is requested to create a registration for an "application/captive+json" media type (Section 6.1) and a registry for fields in that format (Section 6.2).

6.1. Captive Portal API JSON Media Type Registration

This document registers the media type for Captive Portal API JSON text, "application/captive+json".

Type name:
Subtype name:
Required parameters:
Optional parameters:
Encoding considerations:
Encoding considerations are identical to those specified for the "application/json" media type.
Security considerations:
See Section 5
Interoperability considerations:
This document specifies format of conforming messages and the interpretation thereof.
Published specification:
This document
Applications that use this media type:
This media type is intended to be used by servers presenting the Captive Portal API, and clients connecting to such captive networks.
Additional information:
Person & email address to contact for further information:
See Authors' Addresses section.
Intended usage:
Restrictions on usage:
Change controller:

6.2. Captive Portal API Keys Registry

IANA is asked to create and maintain a new registry called "Captive Portal API Keys", which will reserve JSON keys for use in Captive Portal API data structures. The initial contents of this registry are provided in Section 4.2.

Each entry in the registry contains the following fields:

The JSON key being registered, in string format.
The type of the JSON value to be stored, as one of the value types defined in [RFC8259].
A brief description explaining the meaning of the value, how it might be used, and/or how it should be interpreted by clients.

New assignments for Captive Portal API Keys Registry will be administered by IANA through Expert Review [RFC8126]. The Designated Expert is expected to validate the existence of documentation describing new keys in a permanent publicly available specification. The expert is expected to validate that new keys have a clear meaning and do not create unnecessary confusion or overlap with existing keys. Keys that are specific to non-generic use cases, particularly ones that are not specified as part of an IETF document, are encouraged to use a domain-specific prefix.

7. Acknowledgments

This work in this document was started by Mark Donnelly and Margaret Cullen. Thanks to everyone in the CAPPORT Working Group who has given input.

8. References

8.1. Normative References

Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, , <>.
Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, , <>.
Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, DOI 10.17487/RFC5905, , <>.
Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, , <>.
Santesson, S., Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 6960, DOI 10.17487/RFC6960, , <>.
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <>.
Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, , <>.

8.2. Informative References

Larose, K. and D. Dolson, "CAPPORT Architecture", Work in Progress, Internet-Draft, draft-ietf-capport-architecture-05, , <>.

Authors' Addresses

Tommy Pauly (editor)
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
Darshak Thakore (editor)
858 Coal Creek Circle
Louisville, CO 80027,
United States of America