Network Working Group W. Kumari
Internet-Draft Google
Intended status: Informational O. Gudmundsson
Expires: July 31, 2015 Shinkuro Inc.
P. Ebersman
Comcast
S. Sheng
ICANN
January 27, 2015
Captive-Portal identification in DHCPv4 / RA
draft-wkumari-dhc-capport-08
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 DHCPv4 option (and an IPv6 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.
Status of This Memo
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This Internet-Draft will expire on July 31, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. DNS Redirection . . . . . . . . . . . . . . . . . . . . . 4
2.2. HTTP Redirection . . . . . . . . . . . . . . . . . . . . 4
2.3. IP Hijacking . . . . . . . . . . . . . . . . . . . . . . 4
3. The Captive-Portal IPv4 DHCP Option . . . . . . . . . . . . . 4
4. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . . . 5
5. Use of the Captive-Portal Option . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
In many environments, users need to connect to a captive portal
device and agree to an acceptable use policy and / or provide billing
information before they can access the Internet.
Many devices perform DNS, HHTP, and / or IP hijacks in order to
present the user with the captive portal web page. These kludgy
workarounds and techniques resemble attacks that DNSSEC and TLS are
intended to protect against. This document describes a DHCPv4 option
(Captive Portal) and an IPv6 Router Advertisement (RA) extension that
informs clients that they are behind a captive portal device and how
to contact it.
This document neither condones nor condemns the use of captive
portals; instead, it recognises that their apparent necessity, and
attempts to improve the user experience.
[ Ed note: This solution complements 802.11U / WiFi Passpoint. It
can be quickly and easily deployed, and works on wired as well ]
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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
Some ISPs implement a captive portal (CP) - a system that intercepts
user requests and redirects them to an interstitial login page - in
order to require the user accept an Acceptable Use Policy (AUP),
provide billing information, or otherwise authenticate a user prior
to allowing them to access the Internet.
Captive portals intercept and redirect 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
until they have satisfied the requirements, captive portals usually
implement IP based filters, or place the user into a restricted VLAN
(or restricted IP range) until after they have been authorized /
satisfied.
These techniques are very similar to attacks that protocols (such as
VPNs, DNSSEC, TLS) are designed to protect against. The interaction
of these protections and the interception leads to poor user
experiences, such as long timeouts, inability to reach the captive
portal web page, etc. The interception may also leak user
information (for example, if the captive portal intercepts and logs
an HTTP Cookie, or URL of the form http://fred:password@example.com).
The user is often unaware of what is causing the issue (their browser
appears to hang, saying something like "Downloading Proxy Script", or
simply "The Internet doesn't work"), and they become frustrated.
This may result in them not purchasing the Internet access provided
by the captive portal. The connectivity attempts may also facilitate
OS fingerprinting even before a client attempts to connect to the
portal itself.
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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, is running their own resolver, is using
a VPN, or already has the DNS information cached.
2.2. HTTP Redirection
In this implementation, the CP acts like a transparent HTTP proxy;
but when it sees an HTTP request from an unauthenticated client using
HTTP/1.0, it intercepts the request and responds with an HTTP status
code 302 to redirect the client to the Captive Portal Login. If the
client is using HTTP/1.1, we respond with a status code 303 See
Other.
This technique has a number of issues, including:
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 issues similar to the HTTP solution, but may also
break other protocols, and may expose more of the user's private
information.
3. The Captive-Portal IPv4 DHCP Option
The Captive Portal DHCP Option (TBA1) informs an IPv4 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; for the foreseeable future (until such time that most
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systems implement this technique) captive portals will still need to
implement the interception techniques to serve legacy clients.
The format of the DHCP 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.
In order to avoid having to perform DNS interception, the URI SHOULD
contain an IPv4 address literal.
For cases requiring SSL/TLS (collection of billing information for
example), the IP literal can redirect to a URI containing a DNS name.
[ED NOTE: Using an address literal is less than ideal, 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 users
would use this to DNS Tunnel out. This would make the CP admin block
external recursives).]
4. 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 TBA2
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Length 8-bit unsigned integer. The length of the option (including
the Type and Length fields) in units of 8 bytes.
URI The URI of the authentication page 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.
5. Use of the Captive-Portal Option
[ED NOTE: This option provides notice to the OS / User applications
that there is a CP. Because of differences in UI design between
Operating Systems, the exact behaviour by OS and Applications is left
to the OS vendor/Application Developer.]
The purpose of the Captive-Portal 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). What is done with this
information is left up to the operating system and application
vendors. This document provides a very high level example of what
could be done with this information.
Many operating systems / applications already include a "connectivity
test" to determine if they are behind a captive portal (for example,
attempting to fetch a specific URL and looking for a specific string
(such as "Success"). These tests sometimes fail or take a long time
to determine when they are behind a CP, but are usually effective for
determining that the captive portal has been satisfied. These tests
will continue to be needed, because there is currently no definitive
signal from the captive portal that it has been satisfied. [ Editor
note: It may be useful to write another document that specifies how a
client can determine that it has passed the CP. This document could
also contain advice to implmentors on only intercepting actually
needed ports, how to advertise that the CP needs to be statisfied
*again*, etc. This should not be done in this document though. ] The
connectivity test may also need to be used if the captive portal
times out the user session and needs the user to re-authenticate.
The operating system may still find the information about the captive
portal URI useful in this case.
When the device is informed that it is behind a captive portal it
should:
1. Not initiate new IP connections until the CP has been satisfied
(other than those to the captive portal browser session and
connectivity checks). Existing connections should be quiesced
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(this will happen more often than some expect -- for example, the
user purchases 1 hour of Internet at a cafe and stays there for 3
hours -- this will "interrupt" the user a few times).
2. Present a dialog box to the user informing them 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, and without a proxy
server. If there is a VPN in place, this connection should be
made outside of the VPN and the user should be informed that
connection is outside the VPN. 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, normal IP connectivity should
resume. The CP success page should contain a string, e.g
"CP_SATISFIED." The OS can then use this string to provide
further information to the user.
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.
6. IANA Considerations
This document defines the DHCP Captive-Portal option and requires
assignment of an option code (TBA1) to be assigned from "Bootp and
DHCP options" registry (http://www.iana.org/assignments/ bootp-dhcp-
parameters/bootp-dhcp-parameters.xml), as specified in [RFC2939].
IANA is also requested to assign an IPv6 RA Option Type code (TBA2)
from the "IPv6 Neighbor Discovery Option Formats" registry. Thanks
IANA!
7. 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. Fake
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DHCP servers / fake RAs are currently a security concern - this
doesn't make them any better or worse.
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.
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. Redirection to a portal where TLS can be used
without hijacking can ameliorate some of the implications of
connecting to a potentially malicious captive portal.
8. 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, Ted Lemon, Ole Troan and Asbjorn Tonnesen for
detailed review and comments. Also great thanks to Joel Jaeggli for
providing feedback and text.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Appendix A. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ]
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
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o Clarified that this document is only for the DHCP bits, not
everything.
o CP's *can* do HTTP redirects to DNS banes, 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 condems" 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)
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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:
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
Shinkuro Inc.
4922 Fairmont Av, Suite 250
Bethesda, MD 20814
USA
Email: ogud@ogud.com
Paul Ebersman
Comcast
Email: ebersman-ietf@dragon.net
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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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