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.
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Table of Contents
1. 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).
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
<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.
HTTP/1.1 429 Too Many Requests
<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.
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.
HTTP/1.1 431 Request Header Fields Too Large
<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.
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
Upon receiving such a request, the login server would generate a 511
HTTP/1.1 511 Network Authentication Required
<title>Network Authentication Required</title>
<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.
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
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.
Appendix A. Acknowledgements
Thanks to Jan Algermissen and Julian Reschke for their suggestions
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
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