Internet DRAFT - draft-wkumari-dhc-capport
draft-wkumari-dhc-capport
Network Working Group W. Kumari
Internet-Draft Google
Intended status: Standards Track O. Gudmundsson
Expires: March 3, 2016 CloudFlare
P. Ebersman
Comcast
S. Sheng
ICANN
August 31, 2015
Captive-Portal Identification in DHCP / RA
draft-wkumari-dhc-capport-16
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 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.
[ Ed note (remove): This document is being developed in github:
https://github.com/wkumari/draft-wkumari-dhc-capport . ]
Status of This Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on March 3, 2016.
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Copyright Notice
Copyright (c) 2015 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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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 . . . . . . . . . . . . . . . . . . . . 3
2.2. IPv6 DHCP Option . . . . . . . . . . . . . . . . . . . . 4
2.3. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . 4
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
6. Normative References . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 CP 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.
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This document describe 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 byte, and so URIs longer than 255 bytes should not be
used in IPv6 DHCP or IPv6 RA.
In order to avoid having to perform DNS interception, the URI SHOULD
contain an address literal. If the captive portal allows the client
to perform DNS requests to resolve the name, it is then acceptable
for the URI to contain a DNS name. The URI paramter is not null
terminated.
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 (TBA1) (one octet)
o Len: The length, in octets of the URI.
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o URI: The contact URI for the captive portal that the user should
connect to (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 (TBA2) (two octets)
o option-len: The length, in octets of the URI.
o URI: The contact URI for the captive portal that the user should
connect to (encoded following the rules in [RFC3986]).
See [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
Type TBA3
Length 8-bit unsigned integer. The length of the option (including
the Type and Length fields) in units of 8 bytes.
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URI The contact URI for the captive portal that the user should
connect to. For the reasons described above, the implementer
might want to use an IP address literal instead of a DNS name.
This should be padded with NULL (0x0) to make the total option
length (including the Type and Length fields) a multiple of 8
bytes.
3. IANA Considerations
This document defines two DHCP Captive-Portal options, one for IPv4
and one for IPv6. It requires assignment of an option code (TBA1) to
be assigned from "Bootp and DHCP options" registry
(hhttp://www.iana.org/assignments/bootp-dhcp-parameters), as
specified in [RFC2939]. It also requires assignment of an option
code (TBA2) from the "DHCPv6 and DHCPv6 options" registry
(http://www.iana.org/assignments/dhcpv6-parameters).
IANA is also requested to assign an IPv6 RA Option Type code (TBA3)
from the "IPv6 Neighbor Discovery Option Formats" registry. Thanks
IANA!
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
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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
Thanks to Vint Cerf for the initial idea / asking me to write this.
Thanks to Wes George for supplying the IPv6 text. Thanks to Lorenzo
and Erik for the V6 RA kick in the pants.
Thanks to Fred Baker, Paul Hoffman, Barry Leiba, Ted Lemon, Martin
Nilsson, Ole Troan and Asbjorn Tonnesen for detailed review and
comments. Thanks for David Black for review and providing text for
the security considerations. Also great thanks to Joel Jaeggli for
providing feedback and text.
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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[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, <http://www.rfc-editor.org/info/rfc3315>.
[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,
<http://www.rfc-editor.org/info/rfc3986>.
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[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,
<http://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,
<http://www.rfc-editor.org/info/rfc7227>.
Appendix A. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ]
From 15.1 to 16:
Incorporated (missed) comments from David Black's GenART / OpsDir
review.
From 15 to 15.1:
o Incorporated Brian Haberman's IESG review comment: "I think you
need to specify somewhere that the URIs are encoded following the
rules in RFC 3986."
From 14 to 15:
o Incorporated readability comment from Barry Leiba
From 13 to 14:
o Added a bunch of disclaimers explaining that this is not a
complete solution. We expect that the actual interaction bit
should be done in CAPPORT.
From 13.2 to 13(posted):
o Shortened the document by removing most of the [Editors notes],
Section 2, Section 5 and Appendix A. They were mainly background
and have served their purpose. This change suggested by Paul
Hoffman.
From 13.1 to 13.2:
o Moved all of the "what an OS could do with this info" to an
Appendix, to make it even clearer that this is simply an example.
From -12 to -13.1:
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There was a Captive Portal Bar BoF held at the Dallas IETF meeting.
See https://github.com/httpwg/wiki/wiki/Captive-Portals for some
details. This document was discussed, and I got a fair bit of
feedback. Incorporating some of this in -13.
o "In the text discussing why a captive portal notification might be
useful (section 2.2 maybe?), perhaps you should say something
about HSTS and HTTP2.0, since they will further erode the ability
to use common captive portal redirection techniques." - Wes
George.
o Integrated a bunch of useful comments from Martin Nilsson
From -11 to -12:
o Integrated a whole bunch of comments from Ted Lemon, including
missing references, track, missing size of DHCP option,
From 10 to 11:
o Updated Olafur's affiliation.
From 09 to 10:
o Ted Lemon and Joel Jaeggli: there's no benefit to insisting on an
ordering. I think you should just say that the ordering is
indeterminate, and if different mechanisms give non-equivalent
answers, this is likely to cause operational problems in practice.
From 08 to 09:
o Put back the DHCPv6 option, and made the fact that is separate
from the DHCPv4 option clearer (Ted Lemon)
From 07 to 08:
o Incorporated comments from Ted Lemon. Made the document much
shorter.
o Some cleanup.
From 06 to 07:
o Incoroprated a bunch of comments from Asbjorn Tonnesen
o Clarified that this document is only for the DHCP bits, not
everything.
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o CP's *can* do HTTP redirects to DNS names, as long as they allow
access to all needed services.
From 05 to 06:
o Integrated comments from Joel, as below
o Better introduction text, around the "kludgy hacks" section.
o Better "neither condones nor condemns" text
o Fingerprint text.
o Some discussions on the v4 literal stuff.
o More Security Consideration text.
From 04 to 05:
o Integrated comments, primarily from Fred Baker.
From 03 to 04:
o Some text cleanup for readability.
o Some disclaimers about it working better on initial connection
versus CP timeout.
o Some more text explaining that CP interception is
indistinguishable from an attack.
o Connectivity Check test.
o Posting just before the draft cutoff - "I love deadlines. I love
the whooshing noise they make as they go by." -- Douglas Adams,
The Salmon of Doubt
From -02 to 03:
o Removed the DHCPv6 stuff (as suggested / requested by Erik Kline)
o Simplified / cleaned up text (I'm inclined to waffle on, then trim
the fluff)
o This was written on a United flight with in-flight WiFi -
unfortunately I couldn't use it because their CP was borked. :-P
From -01 to 02:
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o Added the IPv6 RA stuff.
From -00 to -01:
o Many nits and editorial changes.
o Whole bunch of extra text and review from Wes George on v6.
From initial to -00.
o Nothing changed in the template!
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
Olafur Gudmundsson
CloudFlare
San Francisco, CA 94107
USA
Email: olafur@cloudflare.com
Paul Ebersman
Comcast
Email: ebersman-ietf@dragon.net
Steve Sheng
Internet Corporation for Assigned Names and Numbers
12025 Waterfront Drive, Suite 300
Los Angeles 90094
United States of America
Phone: +1.310.301.5800
Email: steve.sheng@icann.org
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