1. RFC 6585
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


   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

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

   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

         <title>Too Many Requests</title>
         <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>

   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

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

   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

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

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

   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

      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,

   [Favicon]   Wikipedia, "Favicon", March 2012,

   [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,

   [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/

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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

   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

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

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

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

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  1. RFC 6585