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506 Variant Also Negotiates

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The HTTP 506 Variant Also Negotiates server error response status code is returned during content negotiation when there is recursive loop in the process of selecting a resource.

Agent-driven content negotiation enables a client and server to collaboratively decide the best variant of a given resource when the server has multiple variants. A server sends a 506 status code due to server misconfiguration that results in circular references when creating responses.

Lack of standardization of how clients automatically choose from responses, and the additional round-trips that slow down client-server interaction mean this mechanism is rarely used. Server-driven content negotiation is far more common, where a server directly chooses the most appropriate resource for the client based on the request headers ({{HTTPHeader("Accept-Language")}}, {{HTTPHeader("Accept")}}, etc.).

Status

http
506 Variant Also Negotiates

Examples

Resource with variants

In the following example, a client requests a page in the fr locale using the {{HTTPHeader("Accept-Language")}} header. This can be performed using curl:

bash
curl  -H "Negotiate: trans" -H "Accept-Language: fr;" http://example.com/index

This produces the following request:

http
GET /index HTTP/1.1
Host: example.com
User-Agent: curl/8.7.1
Accept: */*
Negotiate: trans
Accept-Language: fr

Due to server misconfiguration, the variant response for fr points to a type map which itself causes transparent negotiation to be performed. The server may detect this condition by the presence of a TCN header in a choice response before it is sent:

http
HTTP/1.1 506 Variant Also Negotiates
Date: Mon, 22 Jul 2024 10:00:00 GMT
Server: Apache/2.4.41 (Unix)
Content-Type: text/html; charset=UTF-8
Content-Length: 233
TCN: list
Vary: negotiate,accept-language
Alternates: {"index.html.en" 1 {type text/html} {language en} {length 48}}, {"another-map.html.fr.map" 1 {type text/html} {language fr} {length 45}}}}

<html>
<head>
  <title>506 Variant Also Negotiates</title>
</head>
<body>
  <h1>Variant Also Negotiates</h1>
  <p>A variant for the requested resource is itself a negotiable resource. This indicates a configuration error.</p>
</body>
</html>

Specifications

{{Specifications}}

See also