Internet Engineering Task Force (IETF) K. Larose

Internet Engineering Task Force (IETF) K. Larose
Request for Comments: 8952 Agilicus
Category: Informational D. Dolson
ISSN: 2070-1721
H. Liu
Google
November 2020

Captive Portal Architecture

Abstract

This document describes a captive portal architecture. Network
provisioning protocols such as DHCP or Router Advertisements (RAs),
an optional signaling protocol, and an HTTP API are used to provide
the solution.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

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). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8952.

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
1.1. Requirements Language
1.2. Terminology
2. Components
2.1. User Equipment
2.2. Provisioning Service
2.2.1. DHCP or Router Advertisements
2.2.2. Provisioning Domains
2.3. Captive Portal API Server
2.4. Captive Portal Enforcement Device
2.5. Captive Portal Signal
2.6. Component Diagram
3. User Equipment Identity
3.1. Identifiers
3.2. Recommended Properties
3.2.1. Uniquely Identify User Equipment
3.2.2. Hard to Spoof
3.2.3. Visible to the API Server
3.2.4. Visible to the Enforcement Device
3.3. Evaluating Types of Identifiers
3.4. Example Identifier Types
3.4.1. Physical Interface
3.4.2. IP Address
3.4.3. Media Access Control (MAC) Address
3.5. Context-Free URI
4. Solution Workflow
4.1. Initial Connection
4.2. Conditions about to Expire
4.3. Handling of Changes in Portal URI
5. IANA Considerations
6. Security Considerations
6.1. Trusting the Network
6.2. Authenticated APIs
6.3. Secure APIs
6.4. Risks Associated with the Signaling Protocol
6.5. User Options
6.6. Privacy
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Existing Captive Portal Detection Implementations
Acknowledgments
Authors' Addresses

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 of captive portals generally require a web server,
some method to allow/block traffic, and some method to alert the
user. Common methods of alerting the user in implementations prior
to this work involve modifying HTTP or DNS traffic.

This document describes an architecture for implementing captive
portals while addressing most of the problems arising for current
captive portal mechanisms. The architecture is guided by these
requirements:

* Current captive portal solutions typically implement some
variations of forging DNS or HTTP responses. Some attempt man-in-
the-middle (MITM) proxy of HTTPS in order to forge responses.
Captive portal solutions should not have to break any protocols or
otherwise act in the manner of an attacker. Therefore, solutions
MUST NOT require the forging of responses from DNS or HTTP servers
or from any other protocol.

* Solutions MUST permit clients to perform DNSSEC validation, which
rules out solutions that forge DNS responses. Solutions SHOULD
permit clients to detect and avoid TLS man-in-the-middle attacks
without requiring a human to perform any kind of "exception"
processing.

* To maximize universality and adoption, solutions MUST operate at
the layer of Internet Protocol (IP) or above, not being specific
to any particular access technology such as cable, Wi-Fi, or
mobile telecom.

* Solutions SHOULD allow a device to query the network to determine
whether the device is captive, without the solution being coupled
to forging intercepted protocols or requiring the device to make
sacrificial queries to "canary" URIs to check for response
tampering (see Appendix A). Current captive portal solutions that
work by affecting DNS or HTTP generally only function as intended
with browsers, breaking other applications using those protocols;
applications using other protocols are not alerted that the
network is a captive portal.

* The state of captivity SHOULD be explicitly available to devices
via a standard protocol, rather than having to infer the state
indirectly.

* The architecture MUST provide a path of incremental migration,
acknowledging the existence of a huge variety of pre-existing
portals and end-user device implementations and software versions.
This requirement is not to recommend or standardize existing
approaches, but rather to provide device and portal implementors a
path to a new standard.

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 describe a mechanism for
such devices to negotiate for unrestricted network access. A future
document could provide a solution to devices without user interfaces.
This document focuses on devices with user interfaces.

The architecture uses the following mechanisms:

* Network provisioning protocols provide end-user devices with a
Uniform Resource Identifier (URI) [RFC3986] for the API that end-
user devices query for information about what is required to
escape captivity. DHCP, DHCPv6, and Router Advertisement options
for this purpose are available in [RFC8910]. Other protocols
(such as RADIUS), Provisioning Domains [CAPPORT-PVD], or static
configuration may also be used to convey this Captive Portal API
URI. A device MAY query this API at any time to determine whether
the network is holding the device in a captive state.

* A Captive Portal can signal User Equipment in response to
transmissions by the User Equipment. This signal works in
response to any Internet protocol and is not done by modifying
protocols in band. This signal does not carry the Captive Portal
API URI; rather, it provides a signal to the User Equipment that
it is in a captive state.

* Receipt of a Captive Portal Signal provides a hint that User
Equipment could be captive. In response, the device MAY query the
provisioned API to obtain information about the network state.
The device can take immediate action to satisfy the portal
(according to its configuration/policy).

The architecture attempts to provide confidentiality, 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", "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.

1.2. Terminology

Captive Portal
A network that limits the 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 Device
The network equipment that enforces the traffic restriction. Also
known as "Enforcement Device".

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

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

Captive Portal API
An HTTP API allowing User Equipment to query information about its
state of captivity within the Captive Portal. This information
might include how to obtain full network access (e.g., by visiting
a URI). Also known as "API".

Captive Portal API Server
A server hosting the Captive Portal API. Also known as "API
Server".

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

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

Captive Portal Session
Also referred to simply as the "Session", a Captive Portal Session
is the association for a particular User Equipment instance that
starts when it interacts with the Captive Portal and gains open
access to the network and ends when the User Equipment moves back
into the original captive state. The Captive Network maintains
the state of each active Session and can limit Sessions based on a
length of time or a number of bytes used. The Session is
associated with a particular User Equipment instance using the
User Equipment's identifier (see Section 3).

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 Enforcement Device, described in Section 2.4, until
site-specific requirements have been met.

This document only considers devices with web browsers, with web
applications being the means of satisfying Captive Portal Conditions.
An example of such User Equipment is a smart phone.

The User Equipment:

* SHOULD support provisioning of the URI for the Captive Portal API
(e.g., by DHCP).

* SHOULD distinguish Captive Portal API access per network
interface, in the manner of Provisioning Domain Architecture
[RFC7556].

* SHOULD have a non-spoofable mechanism for notifying the user of
the Captive Portal.

* SHOULD have a web browser so that the user may navigate to the
User Portal.

* SHOULD support updates to the Captive Portal API URI from the
Provisioning Service.

* MAY prevent applications from using networks that do not grant
full network access. For example, a device connected to a mobile
network may be connecting to a captive Wi-Fi network; the
operating system could avoid updating the default route to a
device on the captive Wi-Fi network until network access
restrictions have been lifted (excepting access to the User
Portal) in the new network. This has been termed "make before
break".

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

If User Equipment supports the Captive Portal API, it MUST validate
the API Server's TLS certificate (see [RFC2818]) according to the
procedures in [RFC6125]. The API Server's URI is obtained via a
network provisioning protocol, which will typically provide a
hostname to be used in TLS server certificate validation, against a
DNS-ID in the server certificate. If the API Server is identified by
IP address, the iPAddress subjectAltName is used to validate the
server certificate. An Enforcement Device SHOULD allow access to any
services that User Equipment could need to contact to perform
certificate validation, such as Online Certificate Status Protocol
(OCSP) responders, Certificate Revocation Lists (CRLs), and NTP
servers; see Section 4.1 of [RFC8908] for more information. If
certificate validation fails, User Equipment MUST NOT make any calls
to the API Server.

The User Equipment can store the last response it received from the
Captive Portal API as a cached view of its state within the Captive
Portal. This state can be used to determine whether its Captive
Portal Session is near expiry. For example, the User Equipment might
compare a timestamp indicating when the Session expires to the
current time. Storing state in this way can reduce the need for
communication with the Captive Portal API. However, it could lead to
the state becoming stale if the User Equipment's view of the relevant
conditions (byte quota, for example) is not consistent with the
Captive Portal API's.

2.2. Provisioning Service

The Provisioning Service is primarily responsible for providing a
Captive Portal API URI to the User Equipment when it connects to the
network, and later if the URI changes. The Provisioning Service
could also be the same service that is responsible for provisioning
the User Equipment for access to the Captive Portal (e.g., by
providing it with an IP address). This section discusses two
mechanisms that may be used to provide the Captive Portal API URI to
the User Equipment.

2.2.1. DHCP or Router Advertisements

A standard for providing a Captive Portal API URI using DHCP or
Router Advertisements is described in [RFC8910]. The captive portal
architecture expects this URI to indicate the API described in
Section 2.3.

2.2.2. Provisioning Domains

[CAPPORT-PVD] proposes a mechanism for User Equipment to be provided
with Provisioning Domain (PvD) Bootstrap Information containing the
URI for the API described in Section 2.3.

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 User Equipment to perform clear-text "canary" (see
Appendix A) 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 and return the state of
captivity for the equipment.

At minimum, the API MUST provide the state of captivity. Further,
the API MUST be able to provide a URI for the User Portal. The
scheme for the URI MUST be "https" so that the User Equipment
communicates with the User Portal over TLS.

If the API receives a request for state that does not correspond to
the requesting User Equipment, the API SHOULD deny access. Given
that the API might use the User Equipment's identifier for
authentication, this requirement motivates Section 3.2.2.

A caller to the API needs to be presented with evidence that the
content it is receiving is for a version of the API that it supports.
For an HTTP-based interaction, such as in [RFC8908], this might be
achieved by using a content type that is unique to the protocol.

When User Equipment receives Captive Portal Signals, the User
Equipment MAY query the API to check its state of captivity. The
User Equipment SHOULD rate-limit these API queries in the event of
the signal being flooded. (See Section 6.)

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

The API MUST use TLS to ensure server authentication. The
implementation of the API MUST ensure both confidentiality 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 Device

The Enforcement Device component restricts the network access of User
Equipment according to the site-specific policy. Typically, User
Equipment is permitted access to a small number of services
(according to the policies of the network provider) and is denied
general network access until it satisfies the Captive Portal
Conditions.

The Enforcement Device component:

* Allows traffic to pass for User Equipment that is permitted to use
the network and has satisfied the Captive Portal Conditions.

* Blocks (discards) traffic according to the site-specific policy
for User Equipment that has not yet satisfied the Captive Portal
Conditions.

* Optionally signals User Equipment using the Captive Portal
Signaling Protocol if certain traffic is blocked.

* Permits User Equipment that has not satisfied the Captive Portal
Conditions to access necessary APIs and web pages to fulfill
requirements for escaping captivity.

* Updates allow/block rules per User Equipment in response to
operations from the User Portal.

2.5. Captive Portal Signal

When User Equipment first connects to a network, or when there are
changes in status, the Enforcement Device could generate a signal
toward the User Equipment. This signal indicates that the User
Equipment might need to contact the API Server to receive updated
information. For instance, this signal might be generated when the
end of a Session is imminent or when network access was denied. For
simplicity, and to reduce the attack surface, all signals SHOULD be
considered equivalent by the User Equipment as a hint to contact the
API. If future solutions have multiple signal types, each type
SHOULD be rate-limited independently.

An Enforcement Device MUST rate-limit any signal generated in
response to these conditions. See Section 6.4 for a discussion of
risks related to a Captive Portal Signal.

2.6. Component Diagram

The following diagram shows the communication between each component
in the case where the Captive Portal has a User Portal and the User
Equipment chooses to visit the User Portal in response to discovering
and interacting with the API Server.

o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o
. CAPTIVE PORTAL .
. +------------+ Join Network +--------------+ .
. | |+--------------------------->| Provisioning | .
. | | Provision API URI | Service | .
. | |<---------------------------+| | .
. | User | +--------------+ .
. | Equipment | Query captivity status +-------------+ .
. | |+--------------------------->| API | .
. | | Captivity status response | Server | .
. | |<---------------------------+| | .
. | | +------+------+ .
. | | | Status .
. | | Portal UI page requests +------+------+ .
. | |+--------------------------->| | .
. | | Portal UI pages | User Portal | .
. | |<---------------------------+| | .
. +------------+ | | .
. ^ ^ | +-------------+ .
. | | | Data to/from ext. network | .
. | | +-----------------> +---------------+ Allow/Deny .
. | +--------------------+| | Rules .
. | | Enforcement | | .
. | Captive Portal Signal | Device |<----+ .
. +-------------------------+---------------+ .
. ^ | .
. | | .
. Data to/from external network .
. | | .
o . . . . . . . . . . . . . . . . . . .| |. . . . . . . . . . . o
| v
EXTERNAL NETWORK

Figure 1: Captive Portal Architecture Component Diagram

In the diagram:

* During provisioning (e.g., DHCP), and possibly later, the User
Equipment acquires the Captive Portal API URI.

* The User Equipment queries the API to learn of its state of
captivity. If captive, the User Equipment presents the portal
user interface from the User Portal to the user.

* Based on user interaction, the User Portal directs the Enforcement
Device to either allow or deny external network access for the
User Equipment.

* The User Equipment attempts to communicate to the external network
through the Enforcement Device.

* The Enforcement Device either allows the User Equipment's packets
to the external network or blocks the packets. If blocking
traffic and a signal has been implemented, it may respond with a
Captive Portal Signal.

The Provisioning Service, API Server, and User Portal are described
as discrete functions. An implementation might provide the multiple
functions within a single entity. Furthermore, these functions,
combined or not, as well as the Enforcement Device, could be
replicated for redundancy or scale.

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 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 characteristic. 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 instance is interacting with them. An identifier can
be a field contained in packets sent by the User Equipment to the
external network. Or, an identifier can 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:

* uniquely identify the User Equipment

* be hard to spoof

* be visible to the API Server

* be visible to the Enforcement Device

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

The Captive Portal MUST associate the User Equipment with an
identifier that is unique among all of the User Equipment interacting
with the Captive Portal at that time.

Over time, the User Equipment assigned to an identifier 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 instances. 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
instance to pretend to be another User Equipment instance, regardless
of whether both are active at the same time. This property is
particularly important when the User Equipment identifier is
referenced externally by devices such as billing systems or when the
identity of the User Equipment could imply liability.

3.2.3. Visible to the API Server

Since the API Server will need to perform operations that rely on the
identity of the User Equipment, such as answering a query about
whether the User Equipment is captive, the API Server needs to be
able to relate a request 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 the
packet should be forwarded to the external network. Since this
decision depends on which User Equipment instance 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 Types of Identifiers

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

3.4. Example Identifier Types

This section provides some example identifier types, along with some
evaluation of whether they are suitable types. The list of
identifier types is not exhaustive; other types may be used. An
important point to note is that whether a given identifier type is
suitable 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
type has the property of being extremely difficult to spoof: the User
Equipment is unaware of the property; one User Equipment instance
cannot manipulate its interactions to appear as though it is another.

Further, if only a single User Equipment instance is attached to a
given physical interface, then the identifier will be unique. If
multiple User Equipment instances are attached to the network on the
same physical interface, then this type 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 type 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
routability, 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
(either IPv4, IPv6, or a dual-stack [RFC4213] combination) 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 instance, 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.

3.4.3. Media Access Control (MAC) Address

The MAC address of a device is often used as an identifier in
existing implementations. This document does not discuss the use of
MAC addresses within a captive portal system, but they can be used as
an identifier type, subject to the criteria in Section 3.2.

3.5. Context-Free URI

A Captive Portal API needs to present information to clients that is
unique to that client. To do this, some systems use information from
the context of a request, such as the source address, to identify the
User Equipment.

Using information from context rather than information from the URI
allows the same URI to be used for different clients. However, it
also means that the resource is unable to provide relevant
information if the User Equipment makes a request using a different
network path. This might happen when User Equipment has multiple
network interfaces. It might also happen if the address of the API
provided by DNS depends on where the query originates (as in split
DNS [RFC8499]).

Accessing the API MAY depend on contextual information. However, the
URIs provided in the API SHOULD be unique to the User Equipment and
not dependent on contextual information to function correctly.

Though a URI might still correctly resolve when the User Equipment
makes the request from a different network, it is possible that some
functions could be limited to when the User Equipment makes requests
using the Captive Portal. For example, payment options could be
absent or a warning could be displayed to indicate the payment is not
for the current connection.

URIs could include some means of identifying the User Equipment in
the URIs. However, including unauthenticated User Equipment
identifiers in the URI may expose the service to spoofing or replay
attacks.

4. Solution Workflow

This section aims to improve understanding by describing a possible
workflow of solutions adhering to the architecture. Note that the
section is not normative; it describes only a subset of possible
implementations.

4.1. Initial Connection

This section describes a possible workflow when User Equipment
initially joins a Captive Portal.

1. The User Equipment joins the Captive Portal 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., [RFC8910]).

3. The User Equipment accesses the Captive Portal API to receive
parameters of the Captive Portal, including the User Portal URI.
(This step replaces the clear-text query to a canary URI.)

4. If necessary, the user navigates to the User Portal to gain
access to the external network.

5. If the user interacted with the User Portal to gain access to the
external network in the previous step, the User Portal indicates
to the Enforcement Device that the User Equipment is allowed to
access the external network.

6. The User Equipment attempts a connection outside the Captive
Portal.

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 workflow when access is about to
expire.

1. Precondition: the API 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 detects that the length of time left for its
access has fallen below a threshold by comparing its stored
expiry time with the current time.

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 User Portal URI again.

6. The user accepts the prompt displayed by the User Equipment.

7. The user extends their access through the User Portal via the
User Equipment's user interface.

8. The User Equipment's access to the outside network continues
uninterrupted.

4.3. Handling of Changes in Portal URI

A different Captive Portal API URI could be returned in the following
cases:

* If DHCP is used, a lease renewal/rebind may return a different
Captive Portal API URI.

* If RA is used, a new Captive Portal API URI may be specified in a
new RA message received by end User Equipment.

When the Provisioning Service updates the Captive Portal API URI, the
User Equipment can retrieve updated state from the URI immediately,
or it can wait as it normally would until the expiry conditions it
retrieved from the old URI are about to expire.

5. IANA Considerations

This document has no IANA actions.

6. Security Considerations

6.1. 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 that a network is
operated by who claims to be operating it. The Provisioning Domain
Architecture [RFC7556] provides some discussion on authenticating an
operator.

The user makes an informed choice to visit and trust the Captive
Portal URI. Since the network provides the Captive Portal URI to the
User Equipment, the network SHOULD do so securely so that the user's
trust in the network can extend to their trust of the Captive Portal
URI. For example, 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.

Further, the possibility of an on-path attacker in an attacking
network introduces some risks. The attacker could redirect traffic
to arbitrary destinations. The attacker could analyze the user's
traffic leading to loss of confidentiality, or the attacker could
modify the traffic inline.

6.2. Authenticated APIs

The solution described here requires that when the User Equipment
needs to access the API Server, the User Equipment authenticates the
server; see Section 2.1.

The Captive Portal API URI might change during the Captive Portal
Session. The User Equipment can apply the same trust mechanisms to
the new URI as it did to the URI it received initially from the
Provisioning Service.

6.3. 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 that can be used to validate future interactions.
The API MUST ensure the integrity of this information, as well as its
confidentiality.

An attacker with access to this information might be able to
masquerade as a specific User Equipment instance when interacting
with the API, which could then allow them to masquerade as that User
Equipment instance when interacting with the User Portal. This could
give them the ability to determine whether the User Equipment has
accessed the portal, deny the User Equipment service by ending their
Session using mechanisms provided by the User Portal, or consume that
User Equipment's quota. An attacker with the ability to modify the
information could deny service to the User Equipment or cause them to
appear as different User Equipment instances.

6.4. Risks Associated with the Signaling Protocol

If a Signaling Protocol is implemented, it may be possible for any
user on the Internet to send signals in an 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:

* The signal only signals to the User Equipment to query the API.
It does not carry any information that may mislead or misdirect
the User Equipment.

* Even when responding to the signal, the User Equipment securely
authenticates with API Servers.

* The User Equipment limits the rate at which it accesses the API,
reducing the impact of an attack attempting to generate excessive
load on either the User Equipment or API. Note that because there
is only one type of signal and one type of API request in response
to the signal, this rate-limiting will not cause loss of signaling
information.

6.5. User Options

The Captive Portal Signal could signal to 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 Signal indications for
privacy reasons. However, there would be 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.

6.6. Privacy

Section 3 describes a mechanism by which all components within the
Captive Portal are designed to use the same identifier to uniquely
identify the User Equipment. This identifier could be abused to
track the user. Implementers and designers of Captive Portals should
take care to ensure that identifiers, if stored, are stored securely.
Likewise, if any component communicates the identifier over the
network, it should ensure the confidentiality of the identifier on
the wire by using encryption such as TLS.

There are benefits to choosing mutable anonymous identifiers. For
example, User Equipment could cycle through multiple identifiers to
help prevent long-term tracking. However, if the components of the
network use an internal mapping to map the identity to a stable,
long-term value in order to deal with changing identifiers, they need
to treat that value as sensitive information; an attacker could use
it to tie traffic back to the originating User Equipment, despite the
User Equipment having changed identifiers.

7. References

7.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.

[RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<https://www.rfc-editor.org/info/rfc7556>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8910] Kumari, W. and E. Kline, "Captive-Portal Identification in
DHCP and Router Advertisements (RAs)", RFC 8910,
DOI 10.17487/RFC8910, September 2020,
<https://www.rfc-editor.org/info/rfc8910>.

7.2. Informative References

[CAPPORT-PVD]
Pfister, P. and T. Pauly, "Using Provisioning Domains for
Captive Portal Discovery", Work in Progress, Internet-
Draft, draft-pfister-capport-pvd-00, 30 June 2018,
<https://tools.ietf.org/html/draft-pfister-capport-pvd-
00>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.

[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213,
DOI 10.17487/RFC4213, October 2005,
<https://www.rfc-editor.org/info/rfc4213>.

[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.

[RFC8908] Pauly, T., Ed. and D. Thakore, Ed., "Captive Portal API",
RFC 8908, DOI 10.17487/RFC8908, September 2020,
<https://www.rfc-editor.org/info/rfc8908>.

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 an 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. This
has been referred to as a "canary" request because, like the canary
in the coal mine, it can be the first sign that something is wrong.

Another test that can be performed is a DNS lookup to a known address
with an expected answer. If the answer differs from the expected
answer, the equipment detects that a captive portal is present. DNS
queries over TCP or HTTPS are less likely to be modified than DNS
queries over UDP due to the complexity of implementation.

The different tests may produce different conclusions, varying by
whether or not the implementation treats both TCP and UDP traffic and
by which types of DNS are intercepted.

Malicious or misconfigured networks with a captive portal present may
not intercept these canary requests and choose to pass them through
or decide to impersonate, leading to the device having a false
negative.

Acknowledgments

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

The authors thank Benjamin Kaduk for providing the content related to
TLS certificate validation of the API Server.

The authors thank Michael Richardson for providing wording requiring
DNSSEC and TLS to operate without the user adding exceptions.

The authors thank various individuals for their feedback on the
mailing list and during the IETF 98 hackathon: David Bird, Erik
Kline, Alexis La Goulette, Alex Roscoe, Darshak Thakore, and Vincent
van Dam.

Authors' Addresses

Kyle Larose
Agilicus

Email: [email protected]

David Dolson

Email: [email protected]

Heng Liu
Google

Email: [email protected]