Network Working Group W. Kumari
Internet-Draft Google
Updates: 7710 (if approved) E. Kline
Intended status: Standards Track Loon
Expires: September 12, 2019 March 11, 2019

Captive-Portal Identification in DHCP / RA


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 DHCP option (and a Router Advertisement (RA) extension) to inform clients that they are behind some sort of captive-portal 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 used in larger solutions. The method of authenticating to, and interacting with the captive portal is out of scope of this document.

[ This document is being collaborated on in Github at: 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. ]

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This Internet-Draft will expire on September 12, 2019.

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Table of Contents

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. It is anticipated that the IETF will work on a more fully featured protocol at some point, to ease interaction with Captive Portals. Regardless of how that protocol operates, it is expected that this document will provide needed functionality because the client will need 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, the captive-portal device has to intercept the user's connections and redirect the user to the captive portal, 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 DHCP ([RFC2131]) option (Captive-Portal) and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that informs clients that they are behind a captive-portal device and how to contact it.

1.1. Requirements Notation

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 [RFC2119].

2. The Captive-Portal Option

The Captive Portal DHCP / RA Option informs the client that it is behind a captive portal and provides the URI to access an authentication page. This is primarily intended to improve the user experience by getting them to the captive portal faster; for the foreseeable future, captive portals will still need to implement the interception techniques to serve legacy clients, and clients will need to perform probing to detect captive portals.

In order to support multiple "classes" of clients (e.g. IPv4 only, IPv6 only with DHCPv6 ([RFC3315]), IPv6 only with RA) the captive portal can provide the URI via multiple methods (IPv4 DHCP, IPv6 DHCP, IPv6 RA). The captive portal operator should ensure that the URIs handed out 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 used in IPv6 DHCP or IPv6 RA.

In all variants of this option, the URI SHOULD be that of the captive portal API endpoint, conforming to the recommendations for such URIs [cite:API] (i.e. the URI SHOULD contain a DNS name and SHOULD reference a secure transport, e.g. https). 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.

The URI SHOULD NOT contain an IP address literal.

The URI parameter is not null terminated.

Networks with no captive portals MAY explicitly indicate this condition by using this option with the IANA-assigned URI for this purpose (see Section 5.1.1). Clients observing the URI value "urn:ietf:params:capport-unrestricted" MAY forego time-consuming forms of captive portal detection.

2.1. IPv4 DHCP Option

    Code   Len           Data
   +------+------+------+------+------+--   --+-----+
   | code | len  |  URI                  ...        |
   +------+------+------+------+------+--   --+-----+

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

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.


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

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

3. The Captive-Portal Link Relation Type

Some captive portal network deployments may be unable to change, or unwilling to risk changing, the network infrastructure necessary to use any of the above options. In such deployments, when clear text HTTP intercept and redirection are used, a Link relation header ([RFC8288], Section 3.3) MAY be inserted to convey to a HTTP client (user agent) the associated Captive Portal API URI.

HTTP user agents MUST ignore this link relation in any context other than when explicitly probing to detect the presence of a captive portal. Failure to do so could allow an attacker to inject a Captive Portal API URI other than the correct URI for a given network or for networks where there is no captive portal present at all.

4. 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. It is a network configuration error if the learned URIs are not all identical.

However, if the URIs learned are not in fact all identical the captive device MUST prioritize URIs learned from network provisioning or configuration mechanisms before all other URIs. Specifically, URIs learned via any of the options in Section 2 should take precedence over any URI learned via a mechanism like the one described in Section 3.

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. URI precedence in this situation is not specified by this document.

5. IANA Considerations

This document requests two new IETF URN protocol parameter ([RFC3553]) entries.

Thanks IANA!

5.1. IETF params Registration

5.1.1. Registry name: Captive Portal Unrestricted Identifier

Registry name: Captive Portal Unrestricted Identifier

URN: urn:ietf:params:capport-unrestricted

Specification: RFC TBD (this document)

Repository: RFC TBD (this document)

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

5.1.2. Registry name: Captive Portal API Link Relation Type

Registry name: Captive Portal API Link Relation Type

URN: urn:ietf:params:capport-api

Specification: RFC TBD (this document)

Repository: RFC TBD (this document)

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

6. Security Considerations

An attacker with the ability to inject DHCP messages, RAs, or HTTP headers into cleartext HTTP communications could include an option or link relation from this document and so force users to contact an address of his choosing. As an attacker with this capability could simply list himself 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. 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 standard 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.

By simplifying the interaction with the captive portal systems, and doing away with the need for interception, we think that users will be less likely to 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 using which is protected with TLS, the captive portal operator can attempt to reassure the user that the captive portal is not malicious.

7. 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.

8. 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.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997.
[RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition of New DHCP Options and Message Types", BCP 43, RFC 2939, DOI 10.17487/RFC2939, September 2000.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003.
[RFC3553] Mealling, M., Masinter, L., Hardie, T. and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2003.
[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.
[RFC4861] Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007.
[RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S. and S. Krishnan, "Guidelines for Creating New DHCPv6 Options", BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014.
[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.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017.

Appendix A. Changes / Author Notes.

[RFC Editor: Please remove this section before publication ]

From initial to -00.

Appendix B. Differences from RFC 7710

This document incorporates the following differences from [RFC7710].

Authors' Addresses

Warren Kumari Google 1600 Amphitheatre Parkway Mountain View, CA, 94043 US EMail:
Erik Kline Loon 1600 Amphitheatre Parkway Mountain View, CA, 94043 US EMail: