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
draft-nottingham-http-portal-02
Abstract
"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
deployments.
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 August 22, 2011.
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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
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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.
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.
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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
entry:
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
Host: www.example.com
User-Agent: ExampleAgent
Upon receiving such a request, the login server would generate a 428
response:
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HTTP/1.1 428 Network Authentication Required
Refresh: 0; url=https://login.example.net/
Content-Type: text/html
<html>
<head>
</head>
<body>
<h1>You are being redirected to log into the network...</h1>
</body>
</html>
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"
<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.
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
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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.
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
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.
Appendix D. Acknowledgements
The author takes all responsibility for errors and omissions.
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Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
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