|Captive Portal API
|Pauly & Thakore
This document describes an HTTP API that allows clients to interact with a Captive Portal system. With this API, clients can discover how to get out of captivity and fetch state about their Captive Portal sessions.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
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This document describes a HyperText Transfer Protocol (HTTP) Application Programming 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 [CAPPORT-ARCH]. Specifically, the API provides:¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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. A set of mechanisms for discovering the API server endpoint is defined in [RFC8910].¶
The API server endpoint MUST be accessed over HTTP using an https URI [RFC2818] and SHOULD use the default https port. For example, if the Captive Portal API server is hosted at "example.org", the URI of the API could be "https://example.org/captive-portal/api".¶
The client SHOULD NOT assume that the URI of the API server for a given network will stay the same and SHOULD rely on the discovery or provisioning process each time it joins the network.¶
As described in Section 3 of [CAPPORT-ARCH], the identity of the client needs to be visible to the Captive Portal API server in order for the server to correctly reply with the client's portal state. If the identifier used by the Captive Portal system is the client's set of IP addresses, the system needs to ensure that the same IP addresses are visible to both the API server and the enforcement device.¶
If the API server needs information about the client identity that is not otherwise visible to it, the URI provided to the client during provisioning SHOULD be distinct per client. Thus, depending on how the Captive Portal system is configured, the URI will be unique for each client host and between sessions for the same client host.¶
For example, a Captive Portal system that uses per-client session URIs could use "https://example.org/captive-portal/api/X54PD39JV" as its API URI.¶
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; second, it provides the client of the API an opportunity to authenticate the server that is hosting the API. This authentication allows the client to ensure that the entity providing the Captive Portal API has a valid certificate for the hostname provisioned by the network using the mechanisms defined in [RFC8910], by validating that a DNS-ID [RFC6125] on the certificate is equal to the provisioned hostname.¶
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, especially for clients that do not support OCSP stapling, a captive network SHOULD permit connections to OCSP responders or Certificate Revocation Lists (CRLs) that are referenced by certificates provided by the API server. For more discussion on certificate revocation checks, see Section 6.5 of BCP 195 [RFC7525]. 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 mechanisms 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 MUST 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 if the network redirects a cleartext webpage to a web portal.¶
The following key MUST be included in the top level of the JSON structure returned by the API server:¶
|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 will still be allowed enough access for it to perform server authentication (Section 4.1).
The following keys can be optionally included in the top level of the JSON structure returned by the API server:¶
|Provides the URL of a web portal that MUST be accessed over TLS with which a user can interact.
|Provides the URL of a webpage or site that SHOULD be accessed over TLS 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).
|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.
|An integer that 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 the client session is time-limited and SHOULD omit this value for captive clients (i.e., captive=true) or when the session is not time-limited.
|An integer that indicates the number of bytes remaining, after which the client will be placed into a captive state. The byte count represents the sum of the total number of IP packet (layer 3) bytes sent and received by the client, including IP headers. Captive Portal systems might not count traffic to whitelisted servers, such as the API server, but clients cannot rely on such behavior. The API server SHOULD include this value if the client is not captive (i.e., captive=false) and the client session is byte-limited and SHOULD omit this value for captive clients (i.e., captive=true) or when the session is not byte-limited.
The valid JSON keys can be extended by adding entries to the Captive Portal API Keys Registry (Section 8.2). 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.¶
Captive Portal JSON content can contain per-client data that is not appropriate to store in an intermediary cache. Captive Portal API servers SHOULD set the Cache-Control header field in any responses to "private" or a more restrictive value, such as "no-store" [RFC7234].¶
Client behavior for issuing requests for updated JSON content is implementation specific and can be based on user interaction or the indications of seconds and bytes remaining in a given session. If at any point the client does not receive valid JSON content from the API server, either due to an error or due to receiving no response, the client SHOULD continue to apply the most recent valid content it had received or, if no content had been received previously, proceed to interact with the captive network as if the API capabilities were not present.¶
Upon discovering the URI of the API server, a client connected to a captive network will query the API server to retrieve information about its captive state and conditions to escape captivity. In this example, the client discovered the URI "https://example.org/captive-portal/api/X54PD39JV" using one of the mechanisms defined in [RFC8910].¶
To request the Captive Portal JSON content, a client sends an HTTP GET request:¶
The server then responds with the JSON content for that client:¶
Upon receiving this information, the client will use it to direct the user to 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 external network access, the client SHOULD query the API server again to verify that it is no longer captive.¶
When the client requests the Captive Portal JSON content after gaining external network access, the server responds with updated JSON content:¶
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. 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 the hostname that was provisioned by the network using [RFC8910]. Note that this validation only confirms that the API server matches what the network's provisioning mechanism (such as DHCP or IPv6 Router Advertisements) provided; it is not validating the security of those provisioning mechanisms or the user's trust relationship to the network.¶
Information passed between a client and the user-facing web portal may include a user's personal information, such as a full name and credit card details. Therefore, it is important that both the user-facing web portal and the API server that points a client to the web portal are only accessed over encrypted connections.¶
It is important to note that although communication to the user-facing web portal requires use of TLS, the authentication only validates that the web portal server matches the name in the URI provided by the API server. Since this is not a name that a user typed in, the hostname of the website that would be presented to the user may include "confusable characters", which can mislead the user. See Section 12.5 of [RFC8264] for a discussion of confusable characters.¶
This document registers the media type for Captive Portal API JSON text, "application/captive+json".¶
IANA has created a new registry called "Captive Portal API Keys", which reserves JSON keys for use in Captive Portal API data structures. The initial contents of this registry are provided in Section 5.¶
Each entry in the registry contains the following fields:¶
New assignments for the "Captive Portal API Keys" registry will be administered by IANA using the Specification Required policy [RFC8126]. The designated expert is expected to validate the existence of documentation describing new keys in a permanent, publicly available specification, such as an Internet-Draft or RFC. 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 nongeneric use cases, particularly ones that are not specified as part of an IETF document, are encouraged to use a domain-specific prefix.¶