Network Working Group W. Kumari
Internet-Draft Google
Intended status: Informational O. Gudmundsson
Expires: January 5, 2015 Shinkuro Inc.
P. Ebersman
Comcast
S. Sheng
ICANN
July 4, 2014
Captive-Portal identification in DHCP / RA
draft-wkumari-dhc-capport-04
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 and / or
authenticated.
This document describes a DHCP option (and an 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 January 5, 2015.
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Copyright Notice
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . 5
3. The Captive-Portal DHCP Option . . . . . . . . . . . . . . . 5
4. The Captive-Portal RA Option . . . . . . . . . . . . . . . . 6
5. Use of the Captive-Portal Option . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 9
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.
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, which makes the user experience
sub-optimal.
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This document describes a DHCP 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 captive portals; instead,
it recognises that they are here to stay, and attempts to improve the
user experience.
The technique described in this document mainly improve the user
experience when first connecting to a network behind a captive
portal. It may also help if the captive portal access times out
after connecting, but this is less reliable.
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 the user to accept an Acceptable Use Policy (AUP) and
/ or provides billing information (such as their last name and room
number in a hotel, credit card information, etc.) through a web
interface before the user can access the Internet.
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
until they have satisfied the requirements, captive portals usually
implement IP based filters, or place the user into a restricted VLAN
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(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 the 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 often results in them not purchasing the Internet access
provided by the captive portal.
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, it
intercepts the request and responds with an HTTP status code 302 to
redirect the client to the Captive Portal Login.
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.
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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 user's private
information.
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. This is primarily intended to improve the user
experience; for the foreseeable future 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 for DHCPv4, TBA2 for
DHCPv6)
o Len: The length, in octets of the URI.
o URI: The URI of the authentication page that the user should
connect to.
The URI MUST be a URL with an IP-literal for the host portion (to
remove the need to allow DNS from unauthenticated clients). The
DHCPv4 URI MUST contain an IPv4 address.
[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).]
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4. The Captive-Portal RA Option
[Ed: I'm far from an RA expert. I think there are only 8 bits for
Type, is it worth burning an option code on this? I have also
specified that the option length should padded to multiples of 8 byte
to better align with the examples I've seen. Is this required /
preferred, or is smaller RAs better? ]
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.
URI The URI of the authentication page that the user should connect
to. 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 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 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.
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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. 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 /
pay again. 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 page and connectivity
checks). Existing connections should be quiesced (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. 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. This document does not define how to know that the user
has authenticated [Ed: Should it? And option would be for the
"Thank you for paying" page to contain a unique string (e.g:
"CP_SATISFIED"]. Operating system vendors may wish to provide a
public service that their devices can use as a connectivity
check.
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
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continue until the network changes, or a new DHCP message without
the CP is received.
6. IANA Considerations
This document defines DHCPv4 Captive-Portal option which requires
assignment of DHCPv4 option code TBA1 assigned from "Bootp and DHCP
options" registry (http://www.iana.org/assignments/ bootp-dhcp-
parameters/bootp-dhcp-parameters.xml), as specified in [RFC2939].
The IANA is also requested at assign an IPv6 RA Type code (TBA3) from
the [TODO] 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
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.
8. 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.
Thanks to Wes George for supplying the IPv6 text. Thanks to Lorenzo
and Erik for the V6 RA kick in the pants.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. 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 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:
o Added the IPv6 RA stuff.
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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
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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