Captive-Portal Identification in DHCP / RA
draft-ietf-capport-rfc7710bis-06

Abstract

In many environments offering short-term or temporary Internet access (such as coffee shops), it is common to start new connections in a captive portal mode. This highly restricts what the customer can do until the customer has authenticated.

This document describes a DHCPv4 and DHCPv6 option and a Router Advertisement (RA) option to inform clients that they are behind some sort of captive-portal enforcement device, and that they will need to authenticate to get Internet access. It is not a full solution to address all of the issues that clients may have with captive portals; it is designed to be one component of a standardized approach for hosts to interact with such portals. While this document defines how the network operator may convey the captive portal API endpoint to hosts, the specific methods of authenticating to, and interacting with the captive portal are out of scope of this document.

This document replaces RFC 7710. RFC 7710 used DHCP code point 160. Due to a conflict, this document specifies 114.

[ This document is being collaborated on in Github at: https://github.com/capport-wg/7710bis. The most recent version of the document, open issues, etc should all be available here. The authors (gratefully) accept pull requests. Text in square brackets will be removed before publication. ]

Status of This Memo

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

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This Internet-Draft will expire on November 16, 2020.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Requirements Notation
• 2. The Captive-Portal Option
• 2.1. IPv4 DHCP Option
• 2.2. IPv6 DHCP Option
• 2.3. The Captive-Portal IPv6 RA Option
• 3. Precedence of API URIs
• 4. IANA Considerations
• 4.1. Captive Portal Unrestricted Identifier
• 4.2. BOOTP Vendor Extensions and DHCP Options Code Change
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Changes / Author Notes.
• Appendix B. Changes from RFC 7710
• Appendix C. Observations From IETF 106 Network Experiment
• Authors' Addresses

1. Introduction

In many environments, users need to connect to a captive-portal device and agree to an Acceptable Use Policy (AUP) and / or provide billing information before they can access the Internet. Regardless of how that mechanism operates, this document provides functionality to allow the client to know when it is behind a captive portal and how to contact it.

In order to present users with the payment or AUP pages, presently a captive-portal enforcement device has to intercept the user's connections and redirect the user to a captive portal server, using methods that are very similar to man-in-the-middle (MITM) attacks. As increasing focus is placed on security, and end nodes adopt a more secure stance, these interception techniques will become less effective and/or more intrusive.

This document describes a DHCPv4 [RFC2131] and DHCPv6 [RFC8415] option (Captive-Portal) and an IPv6 Router Advertisement (RA) [RFC4861] option that informs clients that they are behind a captive-portal enforcement device and the API endpoint that the host can contact for more information.

This document replaces RFC 7710 [RFC7710].

1.1. Requirements Notation

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.

2. The Captive-Portal Option

The Captive Portal DHCP / RA Option informs the client that it may be behind a captive portal and provides the URI to access an API as defined by [draft-ietf-capport-api]. This is primarily intended to improve the user experience by showing the user the captive portal information faster and more reliably. Note that, for the foreseeable future, captive portals will still need to implement interception techniques to serve legacy clients, and clients will need to perform probing to detect captive portals.

Clients that support the Captive Portal DHCP option SHOULD include the option in the Parameter Request List in DHCPREQUEST messages. DHCP servers MAY send the Captive Portal option without any explicit request.

In order to support multiple "classes" of clients (e.g. IPv4 only, IPv6 only with DHCPv6 ([RFC8415]), and IPv6 only with RA) the captive network can provision the client with the URI via multiple methods (IPv4 DHCP, IPv6 DHCP, and IPv6 RA). The captive portal operator SHOULD ensure that the URIs provisioned by each method are equivalent to reduce the chance of operational problems. The maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, so URIs longer than 255 bytes should not be provisioned via IPv6 DHCP nor IPv6 RA options.

In all variants of this option, the URI MUST be that of the captive portal API endpoint, conforming to the recommendations for such URIs [draft-ietf-capport-api].

A captive portal MAY do content negotiation ([RFC7231] section 3.4) and attempt to redirect clients querying without an explicit indication of support for the captive portal API content type (i.e. without application/capport+json listed explicitly anywhere within an Accept header vis. [RFC7231] section 5.3). In so doing, the captive portal SHOULD redirect the client to the value associated with the "user-portal-url" API key. When performing such content negotiation ([RFC7231] Section 3.4), implementors of captive portals need to keep in mind that such responses might be cached, and therefore SHOULD include an appropriate Vary header field ([RFC7231] Section 7.1.4) or mark them explicitly uncacheable (for example, using Cache-Control: no-store [RFC7234] Section 5.2.2.3).

The URI SHOULD NOT contain an IP address literal. Exceptions to this might include networks with only one operational IP address family where DNS is either not available or not fully functional until the captive portal has been satisfied.

Networks with no captive portals MAY explicitly indicate this condition by using this option with the IANA-assigned URI for this purpose. Clients observing the URI value "urn:ietf:params:capport:unrestricted" MAY forego time-consuming forms of captive portal detection.

2.1. IPv4 DHCP Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Code          | Len           | URI (variable length) ...     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                   ...URI continued...                         .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the IPv4 Captive-Portal DHCP option is shown below.

[RFC2132], Section 2 for more on the format of IPv4 DHCP options.

• Code: The Captive-Portal DHCPv4 Option (114) (one octet)
• Len: The length (one octet), in octets of the URI.
• URI: The URI for the captive portal API endpoint to which the user should connect (encoded following the rules in [RFC3986]).

See

Note that the URI parameter is not null terminated.

2.2. IPv6 DHCP Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          option-code          |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                      URI (variable length)                    .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the IPv6 Captive-Portal DHCP option is shown below.

[RFC7227], Section 5.7 for more examples of DHCP Options with URIs. See [RFC8415], Section 21.1 for more on the format of IPv6 DHCP options.

• option-code: The Captive-Portal DHCPv6Option (103) (two octets)
• option-len: The unsigned 16-bit length, in octets, of the URI.
• URI: The URI for the captive portal API endpoint to which the user should connect (encoded following the rules in [RFC3986]).

See

Note that the URI parameter is not null terminated.

The maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, so URIs longer than 255 bytes should not be provisioned via IPv6 DHCP options.

2.3. The Captive-Portal IPv6 RA Option

This section describes the Captive-Portal Router Advertisement option.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |              URI              .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               .
   .                                                               .
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 2: Captive-Portal RA Option Format

Type

37

Length

8-bit unsigned integer. The length of the option (including the Type and Length fields) in units of 8 bytes.

URI

The URI for the captive portal API endpoint to which the user should connect. This MUST be padded with NULL (0x00) to make the total option length (including the Type and Length fields) a multiple of 8 bytes.

Note that the URI parameter is not guaranteed to be null terminated.

The maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, so URIs longer than 255 bytes should not be provisioned via IPv6 RA options.

3. Precedence of API URIs

A device may learn about Captive Portal API URIs through more than one of (or indeed all of) the above options. Implementations can select their own precedence order (e.g., prefer one of the IPv6 options before the DHCPv4 option, or vice versa, et cetera).

If the URIs learned via more than one option described in Section 2 are not all identical, this condition SHOULD be logged for the device owner or administrator; it is a network configuration error if the learned URIs are not all identical.

4. IANA Considerations

This document requests one new IETF URN protocol parameter ([RFC3553]) entry. This document also requests a reallocation of DHCPv4 option codes (see Appendix C for background).

Thanks IANA!

4.1. Captive Portal Unrestricted Identifier

This document registers a new entry under the IETF URN Sub-namespace for Registered Protocol Parameter Identifiers defined in [RFC3553]:

Registered Parameter Identifier:

capport:unrestricted

Reference:

RFC TBD (this document)

IANA Registry Reference:

https://www.iana.org/assignments/params/params.xml#params-1

Only one value is defined (see URN above). No hierarchy is defined and therefore no sub-namespace registrations are possible.

4.2. BOOTP Vendor Extensions and DHCP Options Code Change

[ RFC Ed: Please remove before publication: RFC7710 uses DHCP Code 160 -- unfortunately, it was discovered that this option code is already widely used by Polycom (see appendix). Option 114 (URL) is currently assigned to Apple (RFC3679, Section 3.2.3 - Contact: Dieter Siegmund, dieter@apple.com - Reason to recover: Never published in an RFC) Tommy Pauly (Apple) and Dieter Siegmund confirm that this codepoint hasn't been used, and Apple is willing to relinquish it for use in CAPPORT. Please see thread: https://mailarchive.ietf.org/arch/msg/captive-portals/TmqQz6Ma_fznD3XbhwkH9m2dB28 for more background. ]

The IANA is requested to update the "BOOTP Vendor Extensions and DHCP Options" registry (https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml) as follows.

   Tag: 114
   Name: DHCP Captive-Portal
   Data Length: N
   Meaning: DHCP Captive-Portal
   Reference: [THIS-RFC]

   Tag: 160
   Name: REMOVED/Unassigned
   Data Length:
   Meaning:
   Reference: [THIS-RFC][RFC7710]

5. Security Considerations

By removing or reducing the need for captive portals to perform MITM hijacking, this mechanism improves security by making the portal and its actions visible, rather than hidden, and reduces the likelihood that users will disable useful security safeguards like DNSSEC validation, VPNs, etc. In addition, because the system knows that it is behind a captive portal, it can know not to send cookies, credentials, etc. By handing out a URI which is protected with TLS, the captive portal operator can attempt to reassure the user that the captive portal is not malicious.

Each of the options described in this document is presented to a node using the same protocols used to provision other information critical to the node's successful configuration on a network. The security considerations applicable to each of these provisioning mechanisms also apply when the node is attempting to learn the information conveyed in these options. In the absence of security measures like RA Guard ([RFC6105], [RFC7113]) or DHCP Shield [RFC7610], an attacker could inject, modify, or block DHCP messages or RAs.

An attacker with the ability to inject DHCP messages or RAs could include an option from this document to force users to contact an address of his choosing. As an attacker with this capability could simply list themselves as the default gateway (and so intercept all the victim's traffic); this does not provide them with significantly more capabilities, but because this document removes the need for interception, the attacker may have an easier time performing the attack.

However, as the operating systems and application that make use of this information know that they are connecting to a captive-portal device (as opposed to intercepted connections) they can render the page in a sandboxed environment and take other precautions, such as clearly labeling the page as untrusted. The means of sandboxing and user interface presenting this information is not covered in this document - by its nature it is implementation specific and best left to the application and user interface designers.

Devices and systems that automatically connect to an open network could potentially be tracked using the techniques described in this document (forcing the user to continually authenticate, or exposing their browser fingerprint). However, similar tracking can already be performed with the presently common captive portal mechanisms, so this technique does not give the attackers more capabilities.

Captive portals are increasingly hijacking TLS connections to force browsers to talk to the portal. Providing the portal's URI via a DHCP or RA option is a cleaner technique, and reduces user expectations of being hijacked - this may improve security by making users more reluctant to accept TLS hijacking, which can be performed from beyond the network associated with the captive portal.

6. Acknowledgements

This document is a -bis of RFC7710. Thanks to all of the original authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve Sheng), and original contributors.

Also thanks to the CAPPORT WG for all of the discussion and improvements including contributions and review from Joe Clarke, Lorenzo Colitti, Dave Dolson, Hans Kuhn, Kyle Larose, Clemens Schimpe, Martin Thomson, Michael Richardson, Remi Nguyen Van, Subash Tirupachur Comerica, Bernie Volz, and Tommy Pauly.

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Changes / Author Notes.

[RFC Editor: Please remove this section before publication ]

From initial to -00.

• Import of RFC7710.

From -00 to -01.

• Remove link-relation text.
• Clarify option should be in DHCPREQUEST parameter list.
• Uppercase some SHOULDs.

Appendix B. Changes from RFC 7710

This document incorporates the following changes from [RFC7710].

1. Clarify that IP string literals are NOT RECOMMENDED.
2. Clarify that the option URI SHOULD be that of the captive portal API endpoint.
3. Clarify that captive portals MAY do content negotiation.
4. Added text about Captive Portal API URI precedence in the event of a network configuration error.
5. Added urn:ietf:params:capport:unrestricted URN.
6. Notes that the DHCP Code changed from 160 to 114.

Appendix C. Observations From IETF 106 Network Experiment

During IETF 106 in Singapore an experiment enabling Captive Portal API compatible clients to discover a venue-info-url (see experiment description for more detail) revealed that some Polycom devices on the same network made use of DHCPv4 option code 160 for other purposes.

The presence of DHCPv4 Option code 160 holding a value indicating the Captive Portal API URL caused these devices to not function as desired. For this reason, this document requests IANA deprecate option code 160 and reallocate different value to be used for the Captive Portal API URL.

Authors' Addresses