docs/manual/mod/mod_deflate.xml

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<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<modulesynopsis>

<name>mod_deflate</name>
<description>Compress content before it is delivered to the
client</description>
<status>Extension</status>
<sourcefile>mod_deflate.c</sourcefile>
<identifier>deflate_module</identifier>

<summary>
    <p>The <module>mod_deflate</module> module provides
    the <code>DEFLATE</code> output filter that allows output from
    your server to be compressed before being sent to the client over
    the network.</p>
</summary>
<seealso><a href="../filter.html">The filter documentation</a></seealso>

<section id="recommended"><title>Recommended Configuration</title>
    <p>This is a sample configuration for the impatient. But please take
    the time and read the sections below for a detailed description!</p>

    <example><title>Compress only a few types</title>
      AddOutputFilterByType DEFLATE text/html text/plain text/xml
    </example>

    <example><title>Compress everything except images</title>
      &lt;Location /&gt;<br />
      <indent>
        # Insert filter<br />
        SetOutputFilter DEFLATE<br />
        <br />
        # Netscape 4.x has some problems...<br />
        BrowserMatch ^Mozilla/4         gzip-only-text/html<br />
        <br />
        # Netscape 4.06-4.08 have some more problems<br />
        BrowserMatch ^Mozilla/4\.0[678] no-gzip<br />
        <br />
        # MSIE masquerades as Netscape, but it is fine<br />
        BrowserMatch \bMSIE             !no-gzip !gzip-only-text/html<br />
        <br />
        # Don't compress images<br />
        SetEnvIfNoCase Request_URI \<br />
        <indent>
          \.(?:gif|jpe?g|png)$ no-gzip dont-vary<br />
        </indent>
        <br />
        # Make sure proxies don't deliver the wrong content<br />
        Header append Vary User-Agent env=!dont-vary<br />
      </indent>
      &lt;/Location&gt;
    </example>

</section>

<section id="enable"><title>Enabling Compression</title>

    <section id="output"><title>Output Compression</title>
      <p>Compression is implemented by the <code>DEFLATE</code>
      <a href="../filter.html">filter</a>. The following directive
      will enable compression for documents in the container where it
      is placed:</p>

      <example>
        SetOutputFilter DEFLATE
      </example>

      <p>Some popular browsers cannot handle compression of all content
      so you may want to set the <code>gzip-only-text/html</code> note to
      <code>1</code> to only allow html files to be compressed (see
      below). If you set this to <em>anything but <code>1</code></em> it
      will be ignored.</p>
      
      <p>If you want to restrict the compression to particular MIME types
      in general, you may use the <directive module="core"
      >AddOutputFilterByType</directive> directive. Here is an example of
      enabling compression only for the html files of the Apache
      documentation:</p>

      <example>
        &lt;Directory "/your-server-root/manual"&gt;<br />
        <indent>
          AddOutputFilterByType DEFLATE text/html<br />
        </indent>
        &lt;/Directory&gt;
      </example>

      <p>For browsers that have problems even with compression of all file
      types, use the <directive module="mod_setenvif"
      >BrowserMatch</directive> directive to set the <code>no-gzip</code>
      note for that particular browser so that no compression will be
      performed. You may combine <code>no-gzip</code> with <code
      >gzip-only-text/html</code> to get the best results. In that case
      the former overrides the latter. Take a look at the following
      excerpt from the <a href="#recommended">configuration example</a>
      defined in the section above:</p>

      <example>
        BrowserMatch ^Mozilla/4         gzip-only-text/html<br />
        BrowserMatch ^Mozilla/4\.0[678] no-gzip<br />
        BrowserMatch \bMSIE             !no-gzip !gzip-only-text/html
      </example>

      <p>At first we probe for a <code>User-Agent</code> string that
      indicates a Netscape Navigator version of 4.x. These versions
      cannot handle compression of types other than
      <code>text/html</code>. The versions 4.06, 4.07 and 4.08 also
      have problems with decompressing html files. Thus, we completely
      turn off the deflate filter for them.</p>

      <p>The third <directive module="mod_setenvif">BrowserMatch</directive>
      directive fixes the guessed identity of the user agent, because
      the Microsoft Internet Explorer identifies itself also as "Mozilla/4"
      but is actually able to handle requested compression. Therefore we
      match against the additional string "MSIE" (<code>\b</code> means
      "word boundary") in the <code>User-Agent</code> Header and turn off
      the restrictions defined before.</p>

      <note><title>Note</title>
        The <code>DEFLATE</code> filter is always inserted after RESOURCE
        filters like PHP or SSI. It never touches internal subrequests.
      </note>
    </section>

    <section id="input"><title>Input Decompression</title>
      <p>The <module>mod_deflate</module> module also provides a filter for
      decompressing a gzip compressed request body . In order to activate
      this feature you have to insert the <code>DEFLATE</code> filter into
      the input filter chain using <directive module="core"
      >SetInputFilter</directive> or <directive module="mod_mime"
      >AddInputFilter</directive>, for example:</p>

      <example>
        &lt;Location /dav-area&gt;<br />
        <indent>
          SetInputFilter DEFLATE<br />
        </indent>
        &lt;/Location&gt;
      </example>
      
      <p>Now if a request contains a <code>Content-Encoding:
      gzip</code> header, the body will be automatically decompressed.
      Few browsers have the ability to gzip request bodies. However,
      some special applications actually do support request
      compression, for instance some <a
      href="http://www.webdav.org">WebDAV</a> clients.</p>

      <note type="warning"><title>Note on Content-Length</title>
        <p>If you evaluate the request body yourself, <em>don't trust
        the <code>Content-Length</code> header!</em>
        The Content-Length header reflects the length of the
        incoming data from the client and <em>not</em> the byte count of
        the decompressed data stream.</p>
      </note>
    </section>
</section>

<section id="proxies"><title>Dealing with proxy servers</title>

    <p>The <module>mod_deflate</module> module sends a <code>Vary:
    Accept-Encoding</code> HTTP response header to alert proxies that
    a cached response should be sent only to clients that send the
    appropriate <code>Accept-Encoding</code> request header.  This
    prevents compressed content from being sent to a client that will
    not understand it.</p>

    <p>If you use some special exclusions dependent
    on, for example, the <code>User-Agent</code> header, you must 
    manually configure an addition to the <code>Vary</code> header
    to alert proxies of the additional restrictions.  For example,
    in a typical configuration where the addition of the <code>DEFLATE</code>
    filter depends on the <code>User-Agent</code>, you should add:</p>

    <example>
      Header append Vary User-Agent
    </example>
    
    <p>If your decision about compression depends on other information
    than request headers (<em>e.g.</em> HTTP version), you have to set the
    <code>Vary</code> header to the value <code>*</code>. This prevents
    compliant proxies from caching entirely.</p>

    <example><title>Example</title>
      Header set Vary *
    </example>
</section>

<directivesynopsis>
<name>DeflateFilterNote</name>
<description>Places the compression ratio in a note for logging</description>
<syntax>DeflateFilterNote <var>notename</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>DeflateFilterNote</directive> directive
    specifies that a note about compression ratios should be attached
    to the request. The name of the note is the value specified for
    the directive. You can use that note for statistical purposes by
    adding the value to your <a href="../logs.html#accesslog"
    >access log</a>.</p>

    <example><title>Example</title>
      DeflateFilterNote ratio<br />
      <br />
      LogFormat '"%r" %b (%{ratio}n) "%{User-agent}i"' deflate<br />
      CustomLog logs/deflate_log deflate
    </example>
</usage>
<seealso><module>mod_log_config</module></seealso>
</directivesynopsis>

<directivesynopsis>
<name>DeflateBufferSize</name>
<description>Fragment size to be compressed at one time by zlib</description>
<syntax>DeflateBufferSize <var>value</var></syntax>
<default>DeflateBufferSize 8096</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>DeflateBufferSize</directive> directive specifies
    the size in bytes of the fragments that zlib should compress at one
    time.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>DeflateWindowSize</name>
<description>Zlib compression window size</description>
<syntax>DeflateWindowSize <var>value</var></syntax>
<default>DeflateWindowSize 15</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>DeflateWindowSize</directive> directive specifies the
    zlib compression window size (a value between 1 and 15). Generally, the
    higher the window size, the higher can the compression ratio be expected.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>DeflateMemLevel</name>
<description>How much memory should be used by zlib for compression</description>
<syntax>DeflateMemLevel <var>value</var></syntax>
<default>DeflateMemLevel 9</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>DeflateMemLevel</directive> directive specifies
    how much memory should be used by zlib for compression
    (a value between 1 and 9).</p>
</usage>
</directivesynopsis>

</modulesynopsis>