506 Variant Also Negotiates

The HTTP 506 Variant Also Negotiates server error response status code is returned during Transparent Content Negotiation (TCN) when there is recursive loop in the process of selecting a resource.

Transparent 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.

Transparent Content Negotiation is not supported in most modern browsers due to complexity in implementations, lack of standardization of how clients automatically choose from responses, and the additional round-trips that slow down client-server interaction. 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 (Accept-Language, Accept, etc.).

Status

http
506 Variant Also Negotiates

Specifications

Specification
RFC 2295
# section-8.1

Examples

Resource with variants

In the following example, a client requests a page in the fr locale using the 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>

See also