Status of This Memo

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.¶

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/rfc8910.¶

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 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

Table of Contents

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 [RFC8908]. 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; nonetheless, the mechanism provided by this document provides a more reliable and performant way to do so, and is therefore the preferred mechanism for captive portal detection.¶

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 identical to reduce the chance of operational problems. As the maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, URIs longer than this SHOULD NOT be provisioned by any of the IPv6 options described in this document. In IPv6-only environments, this restriction can be relaxed.¶

In all variants of this option, the URI MUST be that of the captive portal API endpoint ([RFC8908]).¶

A captive portal MAY do content negotiation (Section 3.4 of [RFC7231]) 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 field as described in Section 5.3 of [RFC7231]). 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 (Section 3.4 of [RFC7231]), implementors of captive portals need to keep in mind that such responses might be cached, and therefore SHOULD include an appropriate Vary header field (Section 7.1.4 of [RFC7231]) or set the Cache-Control header field in any responses to "private" or a more restrictive value such as "no-store" (Section 5.2.2.3 of [RFC7234]).¶

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. Use of IP Address certificates ([RFC3779]) adds considerations that are out of scope for this document.¶

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.¶

    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...                         .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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)                    .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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              .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               .
   .                                                               .
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

IANA has registered a new IETF URN protocol parameter ([RFC3553]). IANA has also reallocated two DHCPv4 option codes (see Appendix B for background) and updated the references for previously registered DHCPv6 and IPv6 ND options.¶

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 order to interact with the captive portal. 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 that is protected with TLS, the captive portal operator can attempt to reassure the user that the captive portal is not malicious.¶

Clients processing these options SHOULD validate that the option's contents conform to the validation requirements for URIs, including those described in [RFC3986].¶

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 DHCPv6-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 the attacker's choosing. 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(s) that make use of this information know that they are connecting to a captive portal device (as opposed to intercepted connections where the OS/application may not know that they are connecting to a captive portal or hostile device), 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 a 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 resatisfy the Captive Portal conditions 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.¶

Appendix A. Changes from RFC 7710

This document incorporates the following changes from [RFC7710].¶

1. Clarified that IP string literals are NOT RECOMMENDED.¶
2. Clarified that the option URI MUST be that of the captive portal API endpoint.¶
3. Clarified 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. Noted that the DHCPv4 Option Code changed from 160 to 114.¶

Appendix B. Observations from IETF 106 Network Experiment

During IETF 106 in Singapore, an experiment enabling clients compatible with the Captive Portal API 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, IANA has deprecated option code 160 and allocated a different value to be used for the Captive Portal API URL.¶

Authors' Addresses