Network Working Group                                      M. Nottingham
Internet-Draft                                         February 18, 2011
Intended status: Standards Track
Expires: August 22, 2011

          The Network Authentication Required HTTP Status Code


   "Captive portals" are a commonly-deployed means of obtaining access
   credentials and/or payment for a network.  This memo introduces a new
   HTTP status code as a means of addressing issues found in these

   This memo should be discussed on the 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

   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 August 22, 2011.

Copyright Notice

   Copyright (c) 2011 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
   ( 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

Nottingham               Expires August 22, 2011                [Page 1]

Internet-Draft       Network Authentication Required       February 2011

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  428 Network Authentication Required . . . . . . . . . . . . . . 3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   Appendix A.  Using the 428 Status Code  . . . . . . . . . . . . . . 4
   Appendix B.  Issues Raised by Captive Portals . . . . . . . . . . . 5
   Appendix C.  Non-HTTP Applications and Techniques . . . . . . . . . 6
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6

Nottingham               Expires August 22, 2011                [Page 2]

Internet-Draft       Network Authentication Required       February 2011

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.

   However, because port 80 is used for non-browser traffic, a number of
   issues (see Appendix B) have been encountered.

   This memo introduces a new HTTP status code, 428 Network
   Authentication Required, as a solution to some of these issues.
   Appendix A outlines how it might be used in typical deployments.

2.  428 Network Authentication Required

   This status code indicates that the client should authenticate to
   gain network access before resubmitting the request.

   The response body SHOULD indicate how to do this; e.g., with an HTML
   form for submitting credentials.

   Responses with the 428 status code MUST NOT be stored by a cache.

Nottingham               Expires August 22, 2011                [Page 3]

Internet-Draft       Network Authentication Required       February 2011

3.  Security Considerations

   In common use, a response carrying the 428 status code will not come
   from the origin server indicated in the request's URL.  This presents
   many security issues; e.g., an attacking intermediary may be
   inserting cookies into the original domain's name space, may be
   observing cookies or HTTP authentication credentials sent from the
   user agent, and so on.

   However, these risks are not unique to the 428 status code; in other
   words, a captive portal that is not using this status code introduces
   the same issues.

4.  IANA Considerations

   The HTTP Status Codes Registry should be updated with the following

   o  Code: 428
   o  Description: Network Authentication Required
   o  Specification: [ this document ]

Appendix A.  Using the 428 Status Code

   This appendix demonstrates a typical use of the 428 status code; it
   is not normative.

   A network operator wishing to require some authentication, acceptance
   of terms or other user interaction before granting access usually
   does so by identify clients who have not done so ("unknown clients")
   using their MAC addresses.

   Unknown clients then have all traffic blocked, except for that on TCP
   port 80, which is sent to a HTTP server (the "login server")
   dedicated to "logging in" unknown clients, and of course traffic to
   the login server itself.

   For example, a user agent might connect to a network and make the
   following HTTP request on TCP port 80:

   GET /index.htm HTTP/1.1
   User-Agent: ExampleAgent

   Upon receiving such a request, the login server would generate a 428

Nottingham               Expires August 22, 2011                [Page 4]

Internet-Draft       Network Authentication Required       February 2011

   HTTP/1.1 428 Network Authentication Required
   Refresh: 0; url=
   Content-Type: text/html

         <h1>You are being redirected to log into the network...</h1>

   Here, the 428 status code assures that non-browser clients will not
   interpret the response as being from the origin server, and the
   Refresh header redirects the user agent to the login server (an HTML
   META element can be used for this as well).

   Note that the 428 response can itself contain the login interface,
   but it may not be desirable to do so, because browsers would show the
   login interface as being associated with the originally requested
   URL, which may cause confusion.

Appendix B.  Issues Raised by Captive Portals

   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"
   <> 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.

   Another browser-based issue comes about when 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
   <>, CORS
   <> and OAuth
   <> may also be

Nottingham               Expires August 22, 2011                [Page 5]

Internet-Draft       Network Authentication Required       February 2011

   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 <> and CalDAV
   <> 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 <>, 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.

Appendix C.  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
   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.

Appendix D.  Acknowledgements

   The author takes all responsibility for errors and omissions.

Nottingham               Expires August 22, 2011                [Page 6]

Internet-Draft       Network Authentication Required       February 2011

Author's Address

   Mark Nottingham


Nottingham               Expires August 22, 2011                [Page 7]