Additional HTTP Status Codes

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

This document specifies additional HTTP status codes for a variety of common situations, to improve interoperability and avoid confusion when other, less precise status codes are used. Feedback should occur on the ietf-http-wg@w3.org mailing list, although this draft is NOT a work item of the IETF HTTPbis Working Group.

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 .

This status code indicates that the client needs to authenticate to gain network access. The response representation SHOULD indicate how to do this; e.g., with an HTML form for submitting credentials. The 428 status SHOULD NOT be generated by origin servers; it is intended for use by intermediaries. Responses with the 428 status code MUST NOT be stored by a cache.

This section demonstrates a typical use of the 428 status code; it is not normative. See for an explanation of the issues that motivate this status code. 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:

Upon receiving such a request, the login server would generate a 428 response:

Network Authentication Required

You are being redirected to log into the network...

]]> 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.

This status code indicates that the client has exceeded some usage limit on the origin server, such as sending too many requests in a given amount of time ("rate limiting"). The response representations SHOULD include details explaining the limit, and MAY include a Retry-After header indicating how long to wait before making a new request. For example:

Limit Exceeded

I only allow 50 requests per hour to this Web site. Try again soon.

]]> Note that this specification does not define specific limits, nor their scope. 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. Responses with the 429 status code MUST NOT be stored by a cache.

This 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 are 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:

Request Header Fields Too Large

The "Example" header was too large.

]]> Responses with the 430 status code MUST NOT be stored by a cache.

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.

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.

Servers are not required to use the 430 status code; when under attack, it may be more appropriate to just drop connections, or take other steps.

The HTTP Status Codes Registry should be updated with the following entries: Code: 428 Description: Network Authentication Required Specification: [ this document ] Code: 429 Description: Limit Exceeded Specification: [ this document ] Code: 430 Description: Request Header Fields Too Large Specification: [ this document ]

&rfc2119; &rfc2616; &rfc4918; &rfc4791;

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" 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 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 , CORS and OAuth 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, WebDAV and CalDAV 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 , 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.