    Internet Engineering Task Force (IETF)                     M. Nottingham
    Request for Comments: 6585                                     Rackspace
    Updates: 2616                                                R. Fielding
    Category: Standards Track                                          Adobe
    ISSN: 2070-1721                                               April 2012

                          Additional HTTP Status Codes

    Abstract

       This document specifies additional HyperText Transfer Protocol (HTTP)
       status codes for a variety of common situations.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6585.

    Copyright Notice

       Copyright (c) 2012 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
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

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    Table of Contents

       1. Introduction ....................................................2
       2. Requirements ....................................................2
       3. 428 Precondition Required .......................................2
       4. 429 Too Many Requests ...........................................3
       5. 431 Request Header Fields Too Large .............................4
       6. 511 Network Authentication Required .............................4
       7. Security Considerations .........................................6
       8. IANA Considerations .............................................7
       9. References ......................................................7
       Appendix A. Acknowledgements .......................................9
       Appendix B. Issues Raised by Captive Portals .......................9

    1.  Introduction

       This document specifies additional HTTP [RFC2616] status codes for a
       variety of common situations, to improve interoperability and avoid
       confusion when other, less precise status codes are used.

       Note that these status codes are optional; servers cannot be required
       to support them.  However, because clients will treat unknown status
       codes as a generic error of the same class (e.g., 499 is treated as
       400 if it is not recognized), they can be safely deployed by existing
       servers (see [RFC2616] Section 6.1.1 for more information).

    2.  Requirements

       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].

    3.  428 Precondition Required

       The 428 status code indicates that the origin server requires the
       request to be conditional.

       Its typical use is to avoid the "lost update" problem, where a client
       GETs a resource's state, modifies it, and PUTs it back to the server,
       when meanwhile a third party has modified the state on the server,
       leading to a conflict.  By requiring requests to be conditional, the
       server can assure that clients are working with the correct copies.

       Responses using this status code SHOULD explain how to resubmit the
       request successfully.  For example:

       HTTP/1.1 428 Precondition Required
       Content-Type: text/html

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       <html>
          <head>
             <title>Precondition Required</title>
          </head>
          <body>
             <h1>Precondition Required</h1>
             <p>This request is required to be conditional;
             try using "If-Match".</p>
          </body>
       </html>

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

    4.  429 Too Many Requests

       The 429 status code indicates that the user has sent too many
       requests in a given amount of time ("rate limiting").

       The response representations SHOULD include details explaining the
       condition, and MAY include a Retry-After header indicating how long
       to wait before making a new request.

       For example:

       HTTP/1.1 429 Too Many Requests
       Content-Type: text/html
       Retry-After: 3600

       <html>
          <head>
             <title>Too Many Requests</title>
          </head>
          <body>
             <h1>Too Many Requests</h1>
             <p>I only allow 50 requests per hour to this Web site per
                logged in user.  Try again soon.</p>
          </body>
       </html>

       Note that this specification does not define how the origin server
       identifies the user, nor how it counts requests.  For example, an
       origin server that is limiting request rates can do so based upon
       counts of requests on a per-resource basis, across the entire server,
       or even among a set of servers.  Likewise, it might identify the user
       by its authentication credentials, or a stateful cookie.

       Responses with the 429 status code MUST NOT be stored by a cache.

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    5.  431 Request Header Fields Too Large

       The 431 status code indicates that the server is unwilling to process
       the request because its header fields are too large.  The request MAY
       be resubmitted after reducing the size of the request header fields.

       It can be used both when the set of request header fields in total is
       too large, and when a single header field is at fault.  In the latter
       case, the response representation SHOULD specify which header field
       was too large.

       For example:

       HTTP/1.1 431 Request Header Fields Too Large
       Content-Type: text/html

       <html>
          <head>
             <title>Request Header Fields Too Large</title>
          </head>
          <body>
             <h1>Request Header Fields Too Large</h1>
             <p>The "Example" header was too large.</p>
          </body>
       </html>

       Responses with the 431 status code MUST NOT be stored by a cache.

    6.  511 Network Authentication Required

       The 511 status code indicates that the client needs to authenticate
       to gain network access.

       The response representation SHOULD contain a link to a resource that
       allows the user to submit credentials (e.g., with an HTML form).

       Note that the 511 response SHOULD NOT contain a challenge or the
       login interface itself, because browsers would show the login
       interface as being associated with the originally requested URL,
       which may cause confusion.

       The 511 status SHOULD NOT be generated by origin servers; it is
       intended for use by intercepting proxies that are interposed as a
       means of controlling access to the network.

       Responses with the 511 status code MUST NOT be stored by a cache.

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    6.1.  The 511 Status Code and Captive Portals

       The 511 status code is designed to mitigate problems caused by
       "captive portals" to software (especially non-browser agents) that is
       expecting a response from the server that a request was made to, not
       the intervening network infrastructure.  It is not intended to
       encourage deployment of captive portals -- only to limit the damage
       caused by them.

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

       Unknown clients then have all traffic blocked, except for that on TCP
       port 80, which is sent to an 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

       Upon receiving such a request, the login server would generate a 511
       response:

       HTTP/1.1 511 Network Authentication Required
       Content-Type: text/html

       <html>
          <head>
             <title>Network Authentication Required</title>
             <meta http-equiv="refresh"
                   content="0; url=https://login.example.net/">
          </head>
          <body>
             <p>You need to <a href="https://login.example.net/">
             authenticate with the local network</a> in order to gain
             access.</p>
          </body>
       </html>

       Here, the 511 status code assures that non-browser clients will not
       interpret the response as being from the origin server, and the META
       HTML element redirects the user agent to the login server.

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    7.  Security Considerations

    7.1.  428 Precondition Required

       The 428 status code is optional; clients cannot rely upon its use to
       prevent "lost update" conflicts.

    7.2.  429 Too Many Requests

       When a server is under attack or just receiving a very large number
       of requests from a single party, responding to each with a 429 status
       code will consume resources.

       Therefore, servers are not required to use the 429 status code; when
       limiting resource usage, it may be more appropriate to just drop
       connections, or take other steps.

    7.3.  431 Request Header Fields Too Large

       Servers are not required to use the 431 status code; when under
       attack, it may be more appropriate to just drop connections, or take
       other steps.

    7.4.  511 Network Authentication Required

       In common use, a response carrying the 511 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 511 status code; in other
       words, a captive portal that is not using this status code introduces
       the same issues.

       Also, note that captive portals using this status code on a Secure
       Socket Layer (SSL) or Transport Layer Security (TLS) connection
       (commonly, port 443) will generate a certificate error on the client.

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    8.  IANA Considerations

       The HTTP Status Codes registry has been updated with the following
       entries:

          Value: 428
          Description: Precondition Required
          Reference: [RFC6585]

          Value: 429
          Description: Too Many Requests
          Reference: [RFC6585]

          Value: 431
          Description: Request Header Fields Too Large
          Reference: [RFC6585]

          Value: 511
          Description: Network Authentication Required
          Reference: [RFC6585]

    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.

       [RFC2616]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                   Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                   Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

    9.2.  Informative References

       [CORS]      van Kesteren, A., Ed., "Cross-Origin Resource Sharing",
                   W3C Working Draft WD-cors-20100727, July 2010,
                   <http://www.w3.org/TR/cors/>.

       [Favicon]   Wikipedia, "Favicon", March 2012,
                   <http://en.wikipedia.org/w/
                   index.php?title=Favicon&oldid=484627550>.

       [OAuth2.0]  Hammer-Lahav, E., Ed., Recordon, D., and D. Hardt, "The
                   OAuth 2.0 Authorization Protocol", Work in Progress,
                   March 2012.

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       [P3P]       Marchiori, M., Ed., "The Platform for Privacy Preferences
                   1.0 (P3P1.0) Specification", W3C Recommendation
                   REC-P3P-20020416, April 2002,
                   <http://www.w3.org/TR/P3P/>.

       [RFC4791]   Daboo, C., Desruisseaux, B., and L. Dusseault,
                   "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
                   March 2007.

       [RFC4918]   Dusseault, L., Ed., "HTTP Extensions for Web Distributed
                   Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

       [WIDGETS]   Caceres, M., Ed., "Widget Packaging and XML
                   Configuration", W3C Recommendation REC-widgets-20110927,
                   September 2011, <http://www.w3.org/TR/widgets/>.

       [WebFinger] WebFinger Project, "WebFingerProtocol (Draft)",
                   January 2010, <http://code.google.com/p/webfinger/wiki/
                   WebFingerProtocol>.

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    Appendix A.  Acknowledgements

       Thanks to Jan Algermissen and Julian Reschke for their suggestions
       and feedback.

    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.  The 511 status code is intended to help
       mitigate some of them.

       One example is the "favicon.ico" [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 the Platform for Privacy
       Preferences [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
       [WebFinger], Cross-Origin Resource Sharing [CORS], and Open
       Authorization [OAuth2.0] 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, Web Distributed Authoring and Versioning (WebDAV) [RFC4918]
       and Calendaring Extensions to WebDAV (CalDAV) [RFC4791] both use HTTP
       as the basis (for remote authoring 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 [WIDGETS], software updates, and other
       specialized 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.

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    Authors' Addresses

       Mark Nottingham
       Rackspace

       EMail: mnot@mnot.net
       URI:   http://www.mnot.net/

       Roy T. Fielding
       Adobe Systems Incorporated
       345 Park Ave.
       San Jose, CA  95110
       USA

       EMail: fielding@gbiv.com
       URI:   http://roy.gbiv.com/

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