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Internet Engineering Task Force (IETF)                     M. NottinghamRequest for Comments: 6585                                     RackspaceUpdates:2616                                                R. FieldingCategory: Standards Track                                          AdobeISSN: 2070-1721                                               April 2012Additional HTTP Status CodesAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Nottingham & Fielding        Standards Track                    [Page 1]

RFC 6585              Additional HTTP Status Codes            April 2012Table of Contents1. Introduction ....................................................22. Requirements ....................................................23. 428 Precondition Required .......................................24. 429 Too Many Requests ...........................................35. 431 Request Header Fields Too Large .............................46. 511 Network Authentication Required .............................47. Security Considerations .........................................68. IANA Considerations .............................................79. References ......................................................7Appendix A. Acknowledgements .......................................9Appendix B. Issues Raised by Captive Portals .......................91.  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/htmlNottingham & Fielding        Standards Track                    [Page 2]

RFC 6585              Additional HTTP Status Codes            April 2012   <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.Nottingham & Fielding        Standards Track                    [Page 3]

RFC 6585              Additional HTTP Status Codes            April 20125.  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.Nottingham & Fielding        Standards Track                    [Page 4]

RFC 6585              Additional HTTP Status Codes            April 20126.1.  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.Nottingham & Fielding        Standards Track                    [Page 5]

RFC 6585              Additional HTTP Status Codes            April 20127.  Security Considerations7.1.  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.Nottingham & Fielding        Standards Track                    [Page 6]

RFC 6585              Additional HTTP Status Codes            April 20128.  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.  References9.1.  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.Nottingham & Fielding        Standards Track                    [Page 7]

RFC 6585              Additional HTTP Status Codes            April 2012   [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>.Nottingham & Fielding        Standards Track                    [Page 8]

RFC 6585              Additional HTTP Status Codes            April 2012Appendix 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.Nottingham & Fielding        Standards Track                    [Page 9]

RFC 6585              Additional HTTP Status Codes            April 2012Authors' 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/Nottingham & Fielding        Standards Track                   [Page 10]