Revision 1117383 of If-None-Match

<div>{{HTTPSidebar}}</div>

<p>The <strong><code>If-None-Match</code></strong> HTTP request header makes the request conditional. For {{HTTPMethod("GET")}} and {{HTTPMethod("HEAD")}} methods, the server will send back the requested resource, with a {{HTTPStatus("200")}} status, only if doesn't have a {{HTTPHeader("ETag")}} matching the given one. 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.</p>

<p>When the condition fails for&nbsp;{{HTTPMethod("GET")}} and {{HTTPMethod("HEAD")}} methods, then the server must return HTTP status code&nbsp;304 (Not Modified). For methods that apply server-side changes, the status code 412 (Precondition Failed) is used. Note that the server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary.</p>

<p>The comparison with the stored {{HTTPHeader("ETag")}} uses the <em>weak comparison algorithm</em>, meaning two files are considered identical not only if they are identical byte to byte, but if the content is equivalent. For example, two pages that would differ only by the date of generation in the footer would be considered as identical.</p>

<p>When used in combination with {{HTTPHeader("If-Modified-Since")}}, it has precedence (if the server supports it).</p>

<p>There are two common use cases:</p>

<ul>
 <li>For {{HTTPMethod("GET")}} and {{HTTPMethod("HEAD")}} methods, to update a cached entity that has an associated {{HTTPHeader("ETag")}}.</li>
 <li>For other methods, and in particular for {{HTTPMethod("PUT")}}, <code>If-None-Match</code> used with the <code>*</code> value can be used to save a file not known to exist, guaranteeing that another upload didn't happen before, losing the data of the previous put; this problems is the variation of the <a href="https://www.w3.org/1999/04/Editing/#3.1">lost update problem</a>.</li>
</ul>

<table class="properties">
 <tbody>
  <tr>
   <th scope="row">Header type</th>
   <td>{{Glossary("Request header")}}</td>
  </tr>
  <tr>
   <th scope="row">{{Glossary("Forbidden header name")}}</th>
   <td>no</td>
  </tr>
 </tbody>
</table>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">
If-None-Match: &lt;etag_value&gt;
If-None-Match: &lt;etag_value&gt;, &lt;etag_value&gt;, …
If-None-Match: *</pre>

<h2 id="Directives">Directives</h2>

<dl>
 <dt>&lt;etag_value&gt;</dt>
 <dd>Entity tags uniquely representing the requested resources. They are a string of ASCII characters placed between double quotes (Like <code>"675af34563dc-tr34"</code>) and may be prefixed by <code>W/</code> to indicate that the weak comparison algorithm should be used (This is useless with <code>If-None-Match</code> as it only uses that algorithm).</dd>
 <dt><code>*</code></dt>
 <dd>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.</dd>
</dl>

<h2 id="Examples">Examples</h2>

<pre>
If-None-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"

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

If-None-Match: *
</pre>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Title</th>
  </tr>
  <tr>
   <td>{{RFC("7232", "If-None-Match", "3.2")}}</td>
   <td>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<p class="hidden">The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out <a href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</p>

<p>{{Compat}}</p>

<h2 id="See_also">See also</h2>

<ul>
 <li>{{HTTPHeader("ETag")}}</li>
 <li>{{HTTPHeader("If-Unmodified-Since")}}</li>
 <li>{{HTTPHeader("If-Modified-Since")}}</li>
 <li>{{HTTPHeader("If-Match")}}</li>
 <li>{{HTTPStatus("304")}}<code> Not Modified</code></li>
 <li>{{HTTPStatus("412")}}<code> Precondition Failed</code></li>
</ul>