Network Working Group M. Nottingham
Internet-Draft August 5, 2010
Intended status: Informational
Expires: February 6, 2011
Considerations for Captive Portals in HTTP
draft-nottingham-http-portal-00
Abstract
"Captive portals" are a commonly-deployed means of obtaining access
credentials and/or payment for a network. This memo discusses issues
of their use for HTTP applications, and proposes one possible
mitigation strategy.
This memo should be discussed on the ietf-http-wg@w3.org mailing
list, although it is not a work item of the HTTPbis WG.
Status of this Memo
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This Internet-Draft will expire on February 6, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. HTTP Issues Encountered . . . . . . . . . . . . . . . . . . . . 3
3. Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. What about Non-HTTP Applications and Techniques? . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
It has become common for networks to require authentication, payment
and/or acceptance of terms of service before granting access.
Typically, this occurs when accessing "public" networks such as those
in hotels, trains, conference centres and similar networks.
While there are several potential means of providing credentials to a
network, these are not yet universally supported, and in some
instances the network administrator requires that information (e.g.,
terms of service, login information) be displayed to end users.
In such cases, it has become widespread practice to use a "captive
portal" that diverts HTTP requests to the administrator's web page.
Once the user has satisfied requirements (e.g., for payment,
acceptance of terms), the diversion is ended and "normal" access to
the network is allowed.
Typically, this diversion is accomplished by one of several possible
techniques;
o IP interception - all requests on port 80 are intercepted and send
to the portal.
o HTTP redirects - all requests on port 80 are intercepted and an
HTTP redirect to the portal's URL is returned.
o DNS interception - all DNS lookups return the portal's IP address.
In each case, the intent is that users connecting to the network will
open a Web browser and see the portal.
This memo examines the HTTP-related issues that these techniques
raise, and proposes a potential mitigation strategy.
2. HTTP Issues Encountered
Since clients cannot differentiate between a portal's response and
that of the HTTP server that they intended to communicate with, a
number of issues arise.
One example is the "favicon.ico"
<http://en.wikipedia.org/wiki/Favicon> commonly used by browsers to
identify the site being accessed. If the favicon for a given site is
fetched from a captive portal instead of the intended site (e.g.,
because the user is unauthenticated), it will often "stick" in the
browser's cache (most implementations cache favicons aggressively)
beyond the portal session, so that it seems as if the portal's
favicon has "taken over" the legitimate site.
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Another browser-based issue comes about when P3P
<http://www.w3.org/TR/P3P/> is supported. Depending on how it is
implemented, it's possible a browser might interpret a portal's
response for the p3p.xml file as the server's, resulting in the
privacy policy (or lack thereof) advertised by the portal being
interpreted as applying to the intended site. Other Web-based
protocols such as WebFinger
<http://code.google.com/p/webfinger/wiki/WebFingerProtocol>, CORS
<http://www.w3.org/TR/cors/> and OAuth
<http://tools.ietf.org/html/draft-ietf-oauth-v2> may also be
vulnerable to such issues.
Although HTTP is most widely used with Web browsers, a growing number
of non-browsing applications use it as a substrate protocol. For
example, WebDAV <http://tools.ietf.org/html/rfc4918> and CalDAV
<http://www.ietf.org/rfc/rfc4791.txt> both use HTTP as the basis (for
network filesystem access and calendaring, respectively). Using
these applications from behind a captive portal can result in
spurious errors being presented to the user, and might result in
content corruption, in extreme cases.
Similarly, other non-browser applications using HTTP can be affected
as well; e.g., widgets <http://www.w3.org/TR/widgets/>, software
updates, and other specialised software such as Twitter clients and
the iTunes Music Store.
It should be noted that it's sometimes believed that using HTTP
redirection to direct traffic to the portal addresses these issues.
However, since many of these uses "follow" redirects, this is not a
good solution.
3. Proposal
The heart of the issues seen is that the client doesn't understand
that a response from the portal does not represent the requested
resource.
As such, the response needs to indicate that it is non-authoritative.
In HTTP, response status codes indicate the type of response, and
therefore defining a new one is the most appropriate way to do this.
Status codes are divided into general classes;
1xx - Informational
2xx - Successful
3xx - Redirection
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4xx - Client errors
5xx - Server errors
Although it's common for captive portals to use redirection status
codes, defining a new 3xx code for them isn't practical; current
implementations won't recognise the new status code, and therefore
won't follow it.
Error status codes, on the other hand, have a nice property in that
browsers will generally display the response content if they don't
understand the status code. The one exception to this is Internet
Explorer, which will display a "friendly" message if the response
body is too small; however, this is easy enough to work around, by
padding the response message as necessary.
HTTP defines 4xx status codes as those where the error lies in the
client; i.e., the client shouldn't retry the same request without
changing something. This is arguably more appropriate than using a
5xx error, where the error is said to lie in the server's area of
responsibility, because clients might automatically retry a request
upon seeing a 5xx error.
In fact, there's already an existing status code with similar (but
not quite suitable) semantics; 407 Proxy Authentication Required.
What's needed is a new status code with the semantics of "Network
Authentication Required."
As such, this memo proposes (but does not yet define) using a new
HTTP response status code in the 4xx range with the semantics
"Network Authentication Required" to mitigate the risks of captive
portals.
Captive portals that deploy this status code will return it for all
requests other than those to the actual portal resources (e.g.,
images). Clients that are unaware of the specific semantics of the
new status code will fall back to treating it as a generic 400 error,
and browsers will display the portal page to users.
Note that this would make the HTTP redirection technique described
above obsolete; the portal page would be served directly with the new
status code.
4. What about Non-HTTP Applications and Techniques?
This memo does not address non-HTTP applications, such as IMAP, POP,
or even TLS-encapsulated HTTP. Since captive portals almost always
target Web browsers (has anyone ever seen one that inserts an e-mail
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into your inbox asking you to authenticate?), this is appropriate.
Instead, it is anticipated that well-behaved portals will block all
non-HTTP ports (especially port 443) until the user has successfully
authenticated.
Overall, there may also be an interesting discussion to be had about
improving network access methods to the point where a user interface
can be presented for the same purposes, without resorting to
intercepting HTTP traffic. However, since such a mechanism would by
necessity require modifying the network stack and operating system of
the client, this memo takes a more modest approach.
5. Security Considerations
This memo does not (yet) define any protocol elements, and therefore
does not (yet) have any security considerations.
6. IANA Considerations
This document has no actions for IANA.
Appendix A. Acknowledgements
The author takes all responsibility for errors and omissions.
Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
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