Captive-Portal Identification in DHCP / RA
draft-ekwk-capport-rfc7710bis-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Author | Warren "Ace" Kumari | ||
| Last updated | 2018-07-15 | ||
| Replaced by | draft-ietf-capport-rfc7710bis, RFC 8910 | ||
| RFC stream | (None) | ||
| Formats | |||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ekwk-capport-rfc7710bis-00
Network Working Group W. Kumari
Internet-Draft Google
Updates: 7710 (if approved) E. Kline
Intended status: Standards Track Google Japan
Expires: January 16, 2019 July 15, 2018
Captive-Portal Identification in DHCP / RA
draft-ekwk-capport-rfc7710bis-00
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 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:
https://github.com/wkumari/draft-ekwk-capport-rfc7710bis. 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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 16, 2019.
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Copyright Notice
Copyright (c) 2018 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
(http://trustee.ietf.org/license-info) in effect on the date of
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. The Captive-Portal Option . . . . . . . . . . . . . . . . . . 3
2.1. IPv4 DHCP Option . . . . . . . . . . . . . . . . . . . . 4
2.2. IPv6 DHCP Option . . . . . . . . . . . . . . . . . . . . 4
2.3. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . 5
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
3.1. IETF params Registration . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
6. Normative References . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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
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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 [citation?] 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 Accepts header [citation]). 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.
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Networks with no captive portals MAY explicitly indicate this
condition by using this option with the IANA-assigned URI for this
purpose <iana>. Clients observing the URI value
"urn:ietf:params:capport-unrestricted" MAY forego time-consuming
forms of captive portal detection.
2.1. IPv4 DHCP Option
The format of the IPv4 Captive-Portal DHCP option is shown below.
Code Len Data
+------+------+------+------+------+-- --+-----+
| code | len | URI ... |
+------+------+------+------+------+-- --+-----+
o Code: The Captive-Portal DHCPv4 Option (160) (one octet)
o Len: The length, in octets of the URI.
o URI: The URI for the captive portal API endpoint to which the user
should connect (encoded following the rules in [RFC3986]).
2.2. IPv6 DHCP Option
The format of the IPv6 Captive-Portal DHCP option is shown below.
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) .
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o option-code: The Captive-Portal DHCPv6Option (103) (two octets)
o option-len: The length, in octets of the URI.
o URI: The URI for the captive portal API endpoint to which the user
should connect (encoded following the rules in [RFC3986]).
See [RFC7227], Section 5.7 for more examples of DHCP Options with
URIs.
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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.
3. IANA Considerations
This document requests one new IETF URN protocol parameter
([RFC3553]) entry.
Thanks IANA!
3.1. IETF params Registration
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.
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4. Security Considerations
An attacker with the ability to inject DHCP messages could include
this option 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.
5. 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.
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Also thanks to the CAPPORT WG for all of the discussion and
improvements.
6. 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>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[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, <https://www.rfc-
editor.org/info/rfc2939>.
[RFC3315] Droms, R., Ed., 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, <https://www.rfc-editor.org/info/rfc3315>.
[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, <https://www.rfc-editor.org/info/rfc3553>.
[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>.
[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, <https://www.rfc-
editor.org/info/rfc4861>.
[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,
<https://www.rfc-editor.org/info/rfc7227>.
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Appendix A. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ]
From initial to -00.
o Import of RFC7710.
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
Erik Kline
Google Japan
Roppongi 6-10-1, 44th Floor
Minato, Tokyo 106-6144
Japan
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