Network Working Group W. Kumari
Internet-Draft Google
Obsoletes: 7710 (if approved) E. Kline
Intended status: Standards Track Loon
Expires: October 1, 2020 March 30, 2020
Captive-Portal Identification in DHCP / RA
draft-ietf-capport-rfc7710bis-03
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 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 used in larger solutions. The method of
authenticating to, and interacting with the captive portal is out of
scope of this document.
RFC7710 used DHCP code point 160. Due to a conflict, this document
specifies TBD.
[ 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. ]
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This Internet-Draft will expire on October 1, 2020.
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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 . . . . . . . . . . . . . . . . . . . . 5
2.3. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . 5
3. Precedence of API URIs . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
4.1. IETF params Registration . . . . . . . . . . . . . . . . 6
4.1.1. Registry name: Captive Portal Unrestricted Identifier 6
4.2. BOOTP Vendor Extensions and DHCP Options Code Change . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 10
Appendix B. Changes from RFC 7710 . . . . . . . . . . . . . . . 10
Appendix C. Observations From IETF 106 Network Experiment . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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
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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 DHCP ([RFC2131]) option (Captive-Portal)
and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that
informs clients that they are behind a captive-portal enforcement
device and how to contact an API for more information.
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 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 the 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 ([RFC3315]), 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 or IPv6 RA
either.
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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 server MAY redirect requests that do not have an
Accept header field ([RFC7231] Section 5.3) containing a field item
whose content-type is "application/capport+json" to the URL conveyed
in the "user-portal-url" API key. When performing such content
negotiation ([RFC7231] Section 3.4), captive portals implementors
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).
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.
Networks with no captive portals MAY explicitly indicate this
condition by using this option with the IANA-assigned URI for this
purpose (see Section 4.1.1). Clients observing the URI value
"urn:ietf:params:capport-unrestricted" MAY forego time-consuming
forms of captive portal detection.
2.1. IPv4 DHCP Option
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 (TBD) (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]).
Note that the URI parameter is not null terminated.
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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.
Note that the URI parameter is not null terminated.
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
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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.
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. 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 some other mechanism, such as a
redirect.
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. Implementations can select their own
precedence order.
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. IETF params Registration
4.1.1. 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.
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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: [RFC7710][Deprecated]
5. Security Considerations
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 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.
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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.
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 which is protected with TLS,
the captive portal operator can attempt to reassure the user that the
captive portal is not malicious.
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 Thompson, Michael Richardson, Remi Nguyen Van, Bernie
Volz, and Tommy Pauly.
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>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
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[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>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[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, <https://www.rfc-editor.org/info/rfc7710>.
7.2. URIs
[1] https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments
[2] https://tickets.meeting.ietf.org/wiki/CAPPORT
[3] https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP-
Standardization-160-vs-66/td-p/72577
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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.
From -00 to -01.
o Remove link-relation text.
o Clarify option should be in DHCPREQUEST parameter list.
o 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 [1] enabling Captive
Portal API compatible clients to discover a venue-info-url (see
experiment description [2] for more detail) revealed that some
Polycom devices on the same network made use of DHCPv4 option code
160 for other purposes [3].
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.
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Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
Erik Kline
Loon
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: ek@loon.com
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