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If-None-Match header

files/en-us/web/http/reference/headers/if-none-match/index.md

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The HTTP If-None-Match {{Glossary("request header")}} makes a request conditional. The server returns the requested resource in {{HTTPMethod("GET")}} and {{HTTPMethod("HEAD")}} methods with a {{HTTPStatus("200")}} status, only if it doesn't have an {{HTTPHeader("ETag")}} matching the ones in the If-None-Match header. For other methods, the request will be processed only if the eventually existing resource's {{HTTPHeader("ETag")}} doesn't match any of the values listed.

When the condition fails for {{HTTPMethod("GET")}} and {{HTTPMethod("HEAD")}} methods, the server must return a {{HTTPStatus("304", "304 Not Modified")}} and any of the following header fields that would have been sent in a 200 response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary. For methods that apply server-side changes, the {{HTTPStatus("412", "412 Precondition Failed")}} is used when the condition fails.

The comparison with the stored ETag uses the weak comparison algorithm, meaning two files are considered identical if the content is equivalent — they don't have to be identical byte-by-byte. For example, two pages that differ by their creation date in the footer would still be considered identical.

When used in combination with {{HTTPHeader("If-Modified-Since")}}, If-None-Match has precedence if the server supports it.

There are two common cases for using If-None-Match in requests:

  • For {{HTTPMethod("GET")}} and {{HTTPMethod("HEAD")}} methods, to update a cached entity that has an associated ETag.
  • For other methods, and in particular for {{HTTPMethod("PUT")}}, If-None-Match used with the * value can be used to save a file only if it does not already exist, guaranteeing that the upload won't accidentally overwrite another upload and lose the data of the previous PUT; this problem is a variation of the lost update problem.
<table class="properties"> <tbody> <tr> <th scope="row">Header type</th> <td>{{Glossary("Request header")}}</td> </tr> <tr> <th scope="row">{{Glossary("Forbidden request header")}}</th> <td>No</td> </tr> </tbody> </table>

Syntax

http
If-None-Match: "<etag_value>"
If-None-Match: "<etag_value>", "<etag_value>", …
If-None-Match: *

Directives

  • <etag_value>
    • : Entity tags uniquely representing the requested resources. They are a string of {{Glossary("ASCII")}} characters placed between double quotes (Like "675af34563dc-tr34") and may be prefixed by W/ to indicate that the weak comparison algorithm should be used (this is useless with If-None-Match as it only uses that algorithm).
  • *
    • : The asterisk is a special value representing any resource. They are only useful when uploading a resource, usually with {{HTTPMethod("PUT")}}, to check if another resource with the identity has already been uploaded before.

Examples

http
If-None-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"

If-None-Match: W/"67ab43", "54ed21", "7892dd"

If-None-Match: *

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

  • {{HTTPHeader("ETag")}}
  • {{HTTPHeader("If-Match")}}, {{HTTPHeader("If-Modified-Since")}}, {{HTTPHeader("If-Unmodified-Since")}} conditional request headers
  • {{HTTPStatus("304", "304 Not Modified")}}, {{HTTPStatus("412", "412 Precondition Failed")}} response status codes