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Network Working Group                                      M. Nottingham
Internet-Draft                                            August 5, 2010
Intended status: Informational
Expires: February 6, 2011

               Considerations for Captive Portals in HTTP


   "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

   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 February 6, 2011.

Copyright Notice

   Copyright (c) 2010 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
   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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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
   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

   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

   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

   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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