DHC W. Kumari
Internet-Draft Google
Intended status: Informational O. Gudmundsson
Expires: July 17, 2014 Shinkuro Inc.
P. Ebersman
Infoblox
S. Sheng
ICANN
January 13, 2014
Captive-Portal identification in DHCP
draft-wkumari-dhc-capport-00
Abstract
In many environments (such as hotels, coffee shops and other
establishments that offer Internet service to customers) it is common
to start new connections in a captive portal mode, i.e. highly
restrict what the customer can do until the customer has accepted
terms of service, provided payment information or authenticated.
This document describes a DHCP option to inform clients that they are
behind some sort of captive portal device, and that they will need to
authenticate to get Internet Access.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 17, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
Kumari, et al. Expires July 17, 2014 [Page 1]
Internet-Draft DHCP Captive-Portal January 2014
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
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
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. DNS Redirection . . . . . . . . . . . . . . . . . . . . . 3
2.2. HTTP Redirection . . . . . . . . . . . . . . . . . . . . 4
2.3. IP Hijacking . . . . . . . . . . . . . . . . . . . . . . 4
3. The Captive-Portal DHCP Option . . . . . . . . . . . . . . . 4
4. Use of the Captive-Portal Option . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
In many environments (coffee shops and hotels) users need to connect
to a captive portal device and agree to an acceptable use policy or
provide billing information before they can access the Internet.
In order to present the user with the captive portal web page, many
devices perform DNS and / or HTTP and / or IP hijacks. As well as
being kludgy hacks, these techniques looks very similar to attacks
that DNSSEC and TLS protect against.
This document describes a DHCP option (Captive-Portal) that informs
DHCP clients that they are behind a captive portal device, and how to
contact it.
This document neither condones nor condemns captive portals; instead
it recognises that they are here to stay, and attempts to improve the
user's experience.
Kumari, et al. Expires July 17, 2014 [Page 2]
Internet-Draft DHCP Captive-Portal January 2014
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. Background
Many Internet Service Providers (ISPs) that offer public Internet
access require theuser to first accept an Acceptable Use Policy (AUP)
and / or provides billing information (such as their last name and /
or room number in a hotel, credit card information, etc) through a
web interface.
In order to meet this requirement, some ISPs implement a captive
portal (CP) - a system that intercepts user requests and redirects
them to an interstitial login page.
Captive portals intercept and redirects user requests in a number of
ways, including:
o DNS Redirection
o IP Redirection
o HTTP Redirection
o Restricted scope addresses
o Traffic blocking (until the user is authenticated)
In order to ensure that the user is unable to access the Internet,
captive portals usually implement IP based filters, or place the user
in to a restricted VLAN or restricted IP range until after they have
been authorized.
2.1. DNS Redirection
The CP either intercepts all DNS traffic or advertises itself (for
example using DHCP) as the recursive server for the network. Until
the user has authenticated to the captive portal, the CP responds to
all DNS requests listing the address of its web portal. Once the
user has authenticated the CP returns the "correct" addresses.
This technique has many shortcomings. It fails if the client is
performing DNSSEC validation, or if the client already has the DNS
information cached.
Kumari, et al. Expires July 17, 2014 [Page 3]
Internet-Draft DHCP Captive-Portal January 2014
2.2. HTTP Redirection
In this implmentation, the CP acts like a transparent HTTP proxy, but
when it sees an HTTP request from an unauthenticated client it
intercepts the request and responds with an HTTP status code 302 to
redirect the client to the Captive Portal Login.
The issues with this technique include:
o It fails if the user is only using HTTPS
o It exposes various private user information, such as HTTP Cookies,
etc.
o It doesn't work if the client has a VPN and / or proxies their web
traffic to an external web proxy.
2.3. IP Hijacking
In this scenario, the captive portal intercepts connections to any IP
address. It spoofs the destination IP address and "pretends" to be
whatever the user tried to access.
This technique has similar issues as the HTTP solution, but may also
break other protocols, and may expose more of the users private
information, etc.
3. The Captive-Portal DHCP Option
The Captive Portal DHCP Option (TBA1) informs the DHCP client that it
is behind a captive portal and provides the URI to access the
authentication page.
The format of the DHCPv4 Captive-Portal DHCP option is shown below.
Code Len Data
+------+------+------+------+------+-- --+-----+
| code | len | URI ... |
+------+------+------+------+------+-- --+-----+
o Code: The Captive-Portal DHCP Option (TBA1)
o Len: The length, in octets of the URI.
o URI: The URI of the authentication page that the user should
connect to.
Kumari, et al. Expires July 17, 2014 [Page 4]
Internet-Draft DHCP Captive-Portal January 2014
The URI SHOULD be a URL with an IP-literal for the host portion (to
remove the need to allow DNS from unauthenticated clients).
[ED NOTE: Using an address literal is less than awesome, but better
than the alternatives. Recommending a DNS name means that the CP
would need to allow DNS from unauthenticated clients (as we don't
want to force users to use the CP's provided DNS) and some folk would
use this to DNS Tunnel out. This would make the CP admin block
external recursives).]
4. Use of the Captive-Portal Option
[ED NOTE: This section is, and probably will remain, fairly hand
wavy. This option provides notice to the OS / User applications that
there is a CP, but I think that the UI / etc is best designed /
handled by the Operating System vendors / Application developers. ]
The purpose of the Captive-Portal DHCP Option is to inform the
operating system and applications that they are behind a captive
portal type device, and will need to authenticate before getting
network access (and how to reach the authentication page).
The exact method that the interaction with the user occurs is device
/ operating system / application dependent, the below is simply one
option.
When the device receives a DHCP response with the Captive-Portal
Option it SHOULD:
1. Not initiate new IP connections until the CP has been satisfied.
[TODO(Someone): Existing connections should be placed on hold
(need better text). This will happen more often than some expect
-- you buy 1h of Internet at a cafe and stay there for 3h -- this
will "interrupt" you a few times).]
2. Present a dialog box to the user informing that they are behind a
captive portal and ask if they wish to proceed.
3. If the user elects to proceed, the device should initiate a
connection to the captive portal login page using a web browser
configured with a separate cookie store. Some captive portals
send the user a cookie when they authenticate (so that the user
can re-authenticate more easily in the future - the browser
should keep these CP cookies separate from other cookies.
4. Once the user has authenticated (how does it know? HTTP
response?! Probe (ugh?)) normal IP connectivity should resume.
Kumari, et al. Expires July 17, 2014 [Page 5]
Internet-Draft DHCP Captive-Portal January 2014
5. The device should (using an OS dependent method) expose to the
user / user applications that they have connected though a
captive portal (for example by creating a file in /proc/net/
containing the interface and captive portal URI). This should
continue until the network changes, or a new DHCP message without
the CP is received.
5. IANA Considerations
[ This section stolen from draft-ietf-dhc-access-network-identifier.
:-) ]
This document defines DHCPv4 Captive-Portal option which requires
assignment of DHCPv4 option code TBD1 assigned from "Bootp and DHCP
options" registry (http://www.iana.org/assignments/ bootp-dhcp-
parameters/bootp-dhcp-parameters.xml), as specified in [RFC2939].
6. Security Considerations
An attacker with the ability to inject DHCP messages could include
this option and so force users to contact him. As an attacker with
this capability could simply list himself as the default gateway (and
so see all the victims traffic) we do not think this gives them
significantly more capabilities. Fake DHCP servers are currently a
security concern - this doesn't make them any better or worse.
Devices and systems that automatically connect to open network could
potentially be tracked using the techniques described in this
document (forcing the user to continually authenticate, or exposing
their browser fingerprint), but similar tracking could already be
performed with the standard captive portal mechanisms, so this
doesn't seem to give the attackers more capabilities.
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.
7. Acknowledgements
The primary author has discussed this idea with a number of folk, and
asked them to assist by becoming co-authors. Unfortunately he has
forgotten who many of them were; if you were one of them, I
apologize.
Thanks to Vint Cerf for the initial idea / asking me to write this.
Kumari, et al. Expires July 17, 2014 [Page 6]
Internet-Draft DHCP Captive-Portal January 2014
8. References
8.1. Normative References
[IANA.AS_Numbers]
IANA, "Autonomous System (AS) Numbers",
<http://www.iana.org/assignments/as-numbers>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[I-D.ietf-sidr-iana-objects]
Manderson, T., Vegoda, L., and S. Kent, "RPKI Objects
issued by IANA", draft-ietf-sidr-iana-objects-03 (work in
progress), May 2011.
Appendix A. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ]
From -00 to -01.
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
Shinkuro Inc.
4922 Fairmont Av, Suite 250
Bethesda, MD 20814
USA
Email: ogud@ogud.com
Kumari, et al. Expires July 17, 2014 [Page 7]
Internet-Draft DHCP Captive-Portal January 2014
Paul Ebersman
Infoblox
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
Kumari, et al. Expires July 17, 2014 [Page 8]