Internet Engineering Task Force K. Larose
Internet-Draft Agilicus
Intended status: Informational D. Dolson
Expires: December 31, 2019 June 29, 2019

CAPPORT Architecture


This document aims to document consensus on the CAPPORT architecture. DHCP or Router Advertisements, an optional signaling protocol, and an HTTP API are used to provide the solution. The role of Provisioning Domains (PvDs) is described.

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Table of Contents

1. Introduction

In this document, "Captive Portal" is used to describe a network to which a device may be voluntarily attached, such that network access is limited until some requirements have been fulfilled. Typically a user is required to use a web browser to fulfill requirements imposed by the network operator, such as reading advertisements, accepting an acceptable-use policy, or providing some form of credentials.

Implementations generally require a web server, some method to allow/block traffic, and some method to alert the user. Common methods of alerting the user involve modifying HTTP or DNS traffic.

Problems with captive portal implementations have been described in [I-D.nottingham-capport-problem]. [If that document cannot be published, consider putting its best parts into an appendix of this document.]

This document standardizes an architecture for implementing captive portals that provides tools for addressing most of those problems. We are guided by these principles:

A side-benefit of the architecture described in this document is that devices without user interfaces are able to identify parameters of captivity. However, this document does not yet describe a mechanism for such devices to escape captivity.

The architecture uses the following mechanisms:

The architecture attempts to provide privacy, authentication, and safety mechanisms to the extent possible.

1.1. Requirements Language

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 RFC 2119.

1.2. Terminology

Captive Network: A network which limits communication of attached devices to restricted hosts until the user has satisfied Captive Portal Conditions, after which access is permitted to a wider set of hosts (typically the internet).

Captive Portal Conditions: site-specific requirements that a user or device must satisfy in order to gain access to the wider network.

Captive Portal Enforcement: The network equipment which enforces the traffic restriction.

Captive Portal User Equipment: Also known as User Equipment. A device which has voluntarily joined a network for purposes of communicating beyond the constraints of the captive network.

Captive Portal Server: The web server providing a user interface for assisting the user in satisfying the conditions to escape captivity.

Captive Portal Signal: A notification from the network used to inform the User Equipment that the state of its captivity could have changed.

Captive Portal Signaling Protocol: Also known as Signaling Protocol. The protocol for communicating Captive Portal Signals.

2. Components

2.1. User Equipment

The User Equipment is the device that a user desires to be attached to a network with full access to all hosts on the network (e.g., to have Internet access). The User Equipment communication is typically restricted by the Captive Portal Enforcement, described in Section 2.4, until site-specific requirements have been met.

At this time we consider only devices with web browsers, with web applications being the means of satisfying Captive Portal Conditions.

None of the above requirements are mandatory because (a) we do not wish to say users or devices must seek access beyond the captive network, (b) the requirements may be fulfilled by manually visiting the captive portal web application, and (c) legacy devices must continue to be supported.

2.2. Provisioning Service

Here we discuss candidate mechanisms for provisioning the User Equipment with the URI of the API to query captive portal state and navigate the portal.

2.2.1. DHCP or Router Advertisements

A standard for providing a portal URI using DHCP or Router Advertisements is described in [RFC7710]. The CAPPORT architecture expects this URI to indicate the API described in Section 2.3.

Although it is not clear from RFC7710 what protocol should be executed at the specified URI, some readers might have assumed it to be an HTML page, and hence there might be User Equipment assuming a browser should open this URI. For backwards compatibility, it is RECOMMENDED that the server check the "Accept" field when serving the URI, and serve HTML pages for "text/html" and serve the API for "application/json". [REVISIT: are these details appropriate?]

2.2.2. Provisioning Domains

Although still a work in progress, [I-D.ietf-intarea-provisioning-domains] proposes a mechanism for User Equipment to be provided with PvD Bootstrap Information containing the URI for a JSON file containing key-value pairs to be downloaded over HTTPS. This JSON file would fill the role of the Captive Portal API described in Section 2.3.

The PvD security model provides secure binding between the information provided by the trusted Router Advertisement and the HTTPS server.

One key-value pair can be used to indicate the network has restricted access, requiring captive portal navigation by a user. E.g., key="captivePortal" and value=<URI of portal>. The key-value pair should provide a different result when access is not restricted. E.g., key="captivePortal" and value="".

This JSON file is extensible, allowing new key-value pairs to indicate such things as network access expiry time, URI for API access by IOT devices, etc.

The PvD server MUST support multiple (repeated) queries from each User Equipment, always returning the current captive portal information. The User Equipment is expected to make this query upon receiving (and validating) a Captive Portal Signal (see Section 2.5).

2.3. Captive Portal API Server

The purpose of a Captive Portal API is to permit a query of Captive Portal state without interrupting the user. This API thereby removes the need for a device to perform clear-text "canary" HTTP queries to check for response tampering.

The URI of this API will have been provisioned to the User Equipment. (Refer to Section 2.2).

This architecture expects the User Equipment to query the API when the User Equipment attaches to the network and multiple times thereafter. Therefore the API MUST support multiple repeated queries from the same User Equipment, returning the current state of captivity for the equipment.

At minimum the API MUST provide: (1) the state of captivity and (2) a URI for a browser to present the portal application to the user. The API SHOULD provide evidence to the caller that it supports the present architecture.

When user equipment receives Captive Portal Signals, the user equipment MAY query the API to check the state. The User Equipment SHOULD rate-limit these API queries in the event of the signal being flooded. (See Section 7.)

The API MUST be extensible to support future use-cases by allowing extensible information elements.

The API MUST use TLS for privacy and server authentication. The implementation of the API MUST ensure both privacy and integrity of any information provided by or required by it.

This document does not specify the details of the API.

2.4. Captive Portal Enforcement

The Captive Portal Enforcement component restricts network access to User Equipment according to site-specific policy. Typically User Equipment is permitted access to a small number of services and is denied general network access until it has performed some action.

The Captive Portal Enforcement component:

2.5. Captive Portal Signal

User Equipment may send traffic outside the captive network prior to the Enforcement device granting it access. The Enforcement Device rightly blocks or resets these requests. However, lacking a signal from the Enforcement Device or interaction with the API server, the User Equipment can only guess at whether it is captive. Consequently, allowing the Enforcement Device to signal to the User Equipment that there is a problem with its connectivity may improve the user's experience.

An Enforcement Device may also want to inform the User Equipment of a pending expiry of its access to the external network, so providing the Enforcement Device the ability to preemptively signal may be desirable.

A specific Captive Portal Signaling Protocol is out of scope for this document. However, in order to ensure that future protocols fit into the architecture, requirements for a Captive Portal Signaling Protocol follow:

  1. The notification SHOULD NOT be easy to spoof. If an attacker can send spoofed notifications to the User Equipment, they can cause the User Equipment to unnecessarily access the API. Rather than relying solely on rate limits to prevent problems, a good protocol will strive to limit the feasibility of such attacks.
  2. It SHOULD be possible to send the notification before the captive portal closes. This will help ensure seamless connectivity for the user, as the User Equipment will not need to wait for a network failure to refresh its login. On receipt of preemptive notification, the User Equipment can prompt the user to refresh.
  3. The signal SHOULD NOT include any information other than an indication that traffic is restricted, which can be used as a prompt to contact the API.

The Captive Portal Signaling Protocol does not provide any means of indicating that the network prevents access to some destinations. The intent is to rely on the Captive Portal API and the web portal to which it points to communicate local network policies.

The Captive Portal Enforcement function MAY send Captive Portal Signals when disallowed User Equipment attempts to send to the network. These signals MUST be rate-limited to a configurable rate.

The signals MUST NOT be sent to the Internet devices. The indications are only sent to the User Equipment.

2.6. Component Diagram

The following diagram shows the communication between each component.

o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o
. CAPTIVE NETWORK                                               .
.                                            +--------------+   .
. +------------+   Provision API URI         | Provisioning |   .
. |            |<---------------------------+|  Service     |   .
. |   User     |                             +--------------+   .
. | Equipment  |   Query CAPPORT status      +-------------+    .
. |            |+--------------------------->| CAPPORT API |    .
. |            |                             |  Server     |    .
. |            |                             +------+------+    .
. |            |                                    | Status    .
. |            |   Portal user interface     +------+------+    .
. |            |+--------------------------->| CAPPORT     |    .
. +------------+                             | web portal  |    .
.     ^   ^ |                                +-------------+    .
.     |   | |   Data                                  |         .
.     |   | +-----------------> +---------------+  Allow/Deny   .
.     |   +--------------------+|               |    Rules      .
.     |                         | Captive Portal|     |         .
.     |   CAPPORT Signal        | Enforcement   | <---+         .
.     +-------------------------+---------------+               .
.                                      ^ |                      .
.                                      | |                      .
.                          Data to/from external network        .
.                                      | |                      .
o . . . . . . . . . . . . . . . . . . .| |. . . . . . . . . . . o
                                       | v
                                  EXTERNAL NETWORK

Figure 1: Captive Portal Architecture Component Diagram

Although the provisioning, API, and web portal functions are shown as discrete blocks, they could of course be combined into a single element.

3. User Equipment Identity

Multiple components in the architecture interact with both the User Equipment and each other. Since the User Equipment is the focus of these interactions, the components must be able to both identify the user equipment from their interactions with it, and be able to agree on the identity of the user equipment when interacting with each other.

The methods by which the components interact restrict the type of information that may be used as an identifying characteristics. This section discusses the identifying characteristics.

3.1. Identifiers

An Identifier is a characteristic of the User Equipment used by the components of a Captive Portal to uniquely determine which specific User Equipment is interacting with them. An Identifier MAY be a field contained in packets sent by the User Equipment to the External Network. Or, an Identifier MAY be an ephemeral property not contained in packets destined for the External Network, but instead correlated with such information through knowledge available to the different components.

3.2. Recommended Properties

The set of possible identifiers is quite large. However, in order to be considered a good identifier, an identifier SHOULD meet the following criteria. Note that the optimal identifier will likely change depending on the position of the components in the network as well as the information available to them. An identifier SHOULD:

An identifier might only apply to the current point of network attachment. If the device moves to a different network location its identity could change.

3.2.1. Uniquely Identify User Equipment

In order to uniquely identify the User Equipment, at most one user equipment interacting with the other components of the Captive Portal MUST have a given value of the identifier.

Over time, the user equipment identified by the value MAY change. Allowing the identified device to change over time ensures that the space of possible identifying values need not be overly large.

Independent Captive Portals MAY use the same identifying value to identify different User Equipment. Allowing independent captive portals to reuse identifying values allows the identifier to be a property of the local network, expanding the space of possible identifiers.

3.2.2. Hard to Spoof

A good identifier does not lend itself to being easily spoofed. At no time should it be simple or straightforward for one User Equipment to pretend to be another User Equipment, regardless of whether both are active at the same time. This property is particularly important when the user equipment is extended externally to devices such as billing systems, or where the identity of the User Equipment could imply liability.

3.2.3. Visible to the API

Since the API will need to perform operations which rely on the identity of the user equipment, such as query whether it is captive, the API needs to be able to relate requests to the User Equipment making the request.

3.2.4. Visible to the Enforcement Device

The Enforcement Device will decide on a per packet basis whether it should be permitted to communicate with the external network. Since this decision depends on which User Equipment sent the packet, the Enforcement Device requires that it be able to map the packet to its concept of the User Equipment.

3.3. Evaluating an Identifier

To evaluate whether an identifier is appropriate, one should consider every recommended property from the perspective of interactions among the components in the architecture. When comparing identifiers, choose the one which best satisfies all of the recommended properties. The architecture does not provide an exact measure of how well an identifier satisfies a given property; care should be taken in performing the evaluation.

3.4. Examples of an Identifier

This section provides some examples of identifiers, along with some evaluation of whether they are good identifiers. The list of identifiers is not exhaustive. Other identifiers may be used. An important point to note is that whether the identifiers are good depends heavily on the capabilities of the components and where in the network the components exist.

3.4.1. Physical Interface

The physical interface by which the User Equipment is attached to the network can be used to identify the User Equipment. This identifier has the property of being extremely difficult to spoof: the User Equipment is unaware of the property; one User Equipment cannot manipulate its interactions to appear as though it is another.

Further, if only a single User Equipment is attached to a given physical interface, then the identifier will be unique. If multiple User Equipment is attached to the network on the same physical interface, then this property is not appropriate.

Another consideration related to uniqueness of the User Equipment is that if the attached User Equipment changes, both the API server and the Enforcement Device must invalidate their state related to the User Equipment.

The Enforcement Device needs to be aware of the physical interface, which constrains the environment: it must either be part of the device providing physical access (e.g., implemented in firmware), or packets traversing the network must be extended to include information about the source physical interface (e.g. a tunnel).

The API server faces a similar problem, implying that it should co-exist with the Enforcement Device, or that the enforcement device should extend requests to it with the identifying information.

3.4.2. IP Address

A natural identifier to consider is the IP address of the User Equipment. At any given time, no device on the network can have the same IP address without causing the network to malfunction, so it is appropriate from the perspective of uniqueness.

However, it may be possible to spoof the IP address, particularly for malicious reasons where proper functioning of the network is not necessary for the malicious actor. Consequently, any solution using the IP address should proactively try to prevent spoofing of the IP address. Similarly, if the mapping of IP address to User Equipment is changed, the components of the architecture must remove or update their mapping to prevent spoofing. Demonstrations of return routeability, such as that required for TCP connection establishment, might be sufficient defense against spoofing, though this might not be sufficient in networks that use broadcast media (such as some wireless networks).

Since the IP address may traverse multiple segments of the network, more flexibility is afforded to the Enforcement Device and the API server: they simply must exist on a segment of the network where the IP address is still unique. However, consider that a NAT may be deployed between the User Equipment and the Enforcement Device. In such cases, it is possible for the components to still uniquely identify the device if they are aware of the port mapping.

In some situations, the User Equipment may have multiple IP addresses, while still satisfying all of the recommended properties. This raises some challenges to the components of the network. For example, if the user equipment tries to access the network with multiple IP addresses, should the enforcement device and API server treat each IP address as a unique User Equipment, or should it tie the multiple addresses together into one view of the subscriber? An implementation MAY do either. Attention should be paid to IPv6 and the fact that it is expected for a device to have multiple IPv6 addresses on a single link. In such cases, identification could be performed by subnet, such as the /64 to which the IP belongs.

4. Solution Workflow

This section aims to improve understanding by describing a possible workflow of solutions adhering to the architecture.

4.1. Initial Connection

This section describes a possible work-flow when User Equipment initially joins a Captive Network.

  1. The User Equipment joins the Captive Network by acquiring a DHCP lease, RA, or similar, acquiring provisioning information.
  2. The User Equipment learns the URI for the Captive Portal API from the provisioning information (e.g., [RFC7710]).
  3. The User Equipment accesses the CAPPORT API to receive parameters of the Captive Network, including web-portal URI. (This step replaces the clear-text query to a canary URL.)
  4. If necessary, the User navigates the web portal to gain access to the external network.
  5. The Captive Portal API server indicates to the Captive Portal Enforcement device that the User Equipment is allowed to access the external network.
  6. The User Equipment attempts a connection outside the captive network
  7. If the requirements have been satisfied, the access is permitted; otherwise the "Expired" behavior occurs.
  8. The User Equipment accesses the network until conditions Expire.

4.2. Conditions About to Expire

This section describes a possible work-flow when access is about to expire.

  1. Precondition: the API server has provided the User Equipment with a duration over which its access is valid
  2. The User Equipment is communicating with the outside network
  3. The User Equipment's UI indicates that the length of time left for its access has fallen below a threshold
  4. The User Equipment visits the API again to validate the expiry time
  5. If expiry is still imminent, the User Equipment prompts the user to access the web-portal URI again
  6. The User extends their access through the web-portal
  7. The User Equipment's access to the outside network continues uninterrupted

5. Acknowledgments

The authors thank Lorenzo Colitti for providing the majority of the content for the Captive Portal Signal requirements.

The authors thank various individuals for their feedback on the mailing list and during the IETF98 hackathon: David Bird, Erik Kline, Alexis La Goulette, Alex Roscoe, Darshak Thakore, and Vincent van Dam.

6. IANA Considerations

This memo includes no request to IANA.

7. Security Considerations

7.1. Trusting the Network

When joining a network, some trust is placed in the network operator. This is usually considered to be a decision by a user on the basis of the reputation of an organization. However, once a user makes such a decision, protocols can support authenticating a network is operated by who claims to be operating it. The Provisioning Domain Architecture [RFC7556] provides some discussion on authenticating an operator.

Given that a user chooses to visit a Captive Portal URI, the URI location SHOULD be securely provided to the user's device. E.g., the DHCPv6 AUTH option can sign this information.

If a user decides to incorrectly trust an attacking network, they might be convinced to visit an attacking web page and unwittingly provide credentials to an attacker. Browsers can authenticate servers but cannot detect cleverly misspelled domains, for example.

7.2. Authenticated APIs

The solution described here assumes that when the User Equipment needs to trust the API server, server authentication will be performed using TLS mechanisms.

7.3. Secure APIs

The solution described here requires that the API be secured using TLS. This is required to allow the user equipment and API server to exchange secrets which can be used to validate future interactions. The API must ensure the integrity of this information, as well as its confidentiality.

7.4. Risk of Nuisance Captive Portal

If a Signaling Protocol is implemented, it may be possible for any user on the Internet to send signals in attempt to cause the receiving equipment to communicate with the Captive Portal API. This has been considered, and implementations may address it in the following ways:

7.5. User Options

The Signal could inform the User Equipment that it is being held captive. There is no requirement that the User Equipment do something about this. Devices MAY permit users to disable automatic reaction to captive-portal indications for privacy reasons. However, there is the trade-off that the user doesn't get notified when network access is restricted. Hence, end-user devices MAY allow users to manually control captive portal interactions, possibly on the granularity of Provisioning Domains.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC7556] Anipko, D., "Multiple Provisioning Domain Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015.
[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.

8.2. Informative References

[I-D.ietf-intarea-provisioning-domains] Pfister, P., Vyncke, E., Pauly, T., Schinazi, D. and W. Shao, "Discovering Provisioning Domain Names and Data", Internet-Draft draft-ietf-intarea-provisioning-domains-05, June 2019.
[I-D.nottingham-capport-problem] Nottingham, M., "Captive Portals Problem Statement", Internet-Draft draft-nottingham-capport-problem-01, April 2016.

Appendix A. Existing captive portal detection implementations

Operating systems and user applications may perform various tests when network connectivity is established to determine if the device is attached to a network with a captive portal present. A common method is to attempt to make a HTTP request to a known, vendor hosted endpoint with a fixed response. Any other response is interpreted as a signal that a captive portal is present. This check is typically not secured with TLS, as a network with a captive portal may intercept the connection, leading to a host name mismatch. Another test that can be performed is a DNS lookup to a known address with an expected answer. Such tests may be less reliable as the captive portal may only be intercepting TCP traffic and deliberately excluding the interception of DNS queries. DNS queries not using UDP may potentially fail this test if operating over TCP or DNS over HTTP. Malicious or misconfigured networks with a captive portal present may not intercept these requests and choose to pass them through or decide to impersonate, leading to the device having a false negative.

Authors' Addresses

Kyle Larose Agilicus EMail:
David Dolson EMail: