| Internet-Draft | Captive Portal API | February 2020 |
| Pauly & Thakore | Expires 7 August 2020 | [Page] |
This document describes an HTTP API that allows clients to interact with a Captive Portal system.¶
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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:¶
This document leverages the terminology and components described in [I-D.ietf-capport-architecture] and additionally uses the following association:¶
The Captive Portal Architecture defines several categories of interaction between clients and Captive Portal systems:¶
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.¶
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 example.org, the URI's of the API could be:¶
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 "example.com"). 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.¶
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:¶
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.¶
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 Host: example.org 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": "https://example.org/portal.html",
"venue-info-url": "https://flight.example.com/entertainment",
"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.¶
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.¶
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.¶
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).¶
This document registers the media type for Captive Portal API JSON text, "application/captive+json".¶
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:¶
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.¶
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.¶