apache-http-server/docs/manual/mod/mod_deflate.xml

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<modulesynopsis metafile="mod_deflate.xml.meta">

<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">Filters</a></seealso>

<section id="supportedencodings"><title>Supported Encodings</title>
  <p>The <code>gzip</code> encoding is the only one supported to ensure complete compatibility
  with old browser implementations. The <code>deflate</code> encoding is not supported, 
  please check the <a href="https://zlib.net/zlib_faq.html#faq39">zlib's documentation</a>
  for a complete explanation.
  </p>
</section>

<section id="recommended"><title>Sample Configurations</title>
    <note type="warning"><title>Compression and TLS</title>
        <p>Some web applications are vulnerable to an information disclosure
        attack when a TLS connection carries deflate compressed data. For more
        information, review the details of the "BREACH" family of attacks.</p>
    </note>
    <p>This is a simple configuration that compresses common text-based content types.</p>

    <example><title>Compress only a few types</title>
    <highlight language="config">
AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript application/javascript
    </highlight>
    </example>

</section>

<section id="enable"><title>Enabling Compression</title>
    <note type="warning"><title>Compression and TLS</title>
        <p>Some web applications are vulnerable to an information disclosure
        attack when a TLS connection carries deflate compressed data. For more
        information, review the details of the "BREACH" family of attacks.</p>
    </note>

    <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>

      <highlight language="config">
SetOutputFilter DEFLATE
SetEnvIfNoCase Request_URI "\.(?:gif|jpe?g|png)$" no-gzip
      </highlight>

      <p>If you want to restrict the compression to particular MIME types
      in general, you may use the <directive module="mod_filter"
      >AddOutputFilterByType</directive> directive. Here is an example of
      enabling compression only for the html files of the Apache
      documentation:</p>

      <highlight language="config">
&lt;Directory "/your-server-root/manual"&gt;
    AddOutputFilterByType DEFLATE text/html
&lt;/Directory&gt;
      </highlight>

      <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>
      <note><title>Note</title>
        There is an environment variable <code>force-gzip</code>,
        set via <directive module="mod_env">SetEnv</directive>, which
        will ignore the accept-encoding setting of your browser and will
        send compressed output.
      </note>

    </section>
    <section id="inflate"><title>Output Decompression</title>
      <p>The <module>mod_deflate</module> module also provides a filter for
      inflating/uncompressing a gzip compressed response body. In order to activate
      this feature you have to insert the <code>INFLATE</code> filter into
      the output filter chain using <directive module="core"
      >SetOutputFilter</directive> or <directive module="mod_mime"
      >AddOutputFilter</directive>, for example:</p>

      <highlight language="config">
&lt;Location "/dav-area"&gt;
    ProxyPass "http://example.com/"
    SetOutputFilter INFLATE
&lt;/Location&gt;
      </highlight>

      <p>This Example will uncompress gzip'ed output from example.com, so other
      filters can do further processing with it.
      </p>

    </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>

      <highlight language="config">
&lt;Location "/dav-area"&gt;
    SetInputFilter DEFLATE
&lt;/Location&gt;
      </highlight>

      <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>

    <highlight language="config">
Header append Vary User-Agent
    </highlight>

    <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>
    <highlight language="config">
Header set Vary *
    </highlight>
    </example>
</section>

<section id="precompressed"><title>Serving pre-compressed
content</title>

    <p>Since <module>mod_deflate</module> re-compresses content each
    time a request is made, some performance benefit can be derived by
    pre-compressing the content and telling <module>mod_deflate</module> to serve them
    without re-compressing them. This may be accomplished using a
    configuration like the following:</p>

    <highlight language="config">
&lt;IfModule mod_headers.c&gt;
    # Serve gzip compressed CSS and JS files if they exist
    # and the client accepts gzip.
    RewriteCond "%{HTTP:Accept-encoding}" "gzip"
    RewriteCond "%{REQUEST_FILENAME}\.gz" -s
    RewriteRule "^(.*)\.(css|js)"         "$1\.$2\.gz" [QSA]

    # Serve correct content types, and prevent mod_deflate double gzip.
    RewriteRule "\.css\.gz$" "-" [T=text/css,E=no-gzip:1]
    RewriteRule "\.js\.gz$"  "-" [T=text/javascript,E=no-gzip:1]


    &lt;FilesMatch "(\.js\.gz|\.css\.gz)$"&gt;
      # Serve correct encoding type.
      Header append Content-Encoding gzip

      # Force proxies to cache gzipped &amp;
      # non-gzipped css/js files separately.
      Header append Vary Accept-Encoding
    &lt;/FilesMatch&gt;
&lt;/IfModule&gt;
    </highlight>

</section>

<directivesynopsis>
<name>DeflateFilterNote</name>
<description>Places the compression ratio in a note for logging</description>
<syntax>DeflateFilterNote [<var>type</var>] <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>
    <highlight language="config">
DeflateFilterNote ratio

LogFormat '"%r" %b (%{ratio}n) "%{User-agent}i"' deflate
CustomLog "logs/deflate_log" deflate
    </highlight>
    </example>

    <p>If you want to extract more accurate values from your logs, you
    can use the <var>type</var> argument to specify the type of data
    left as a note for logging. <var>type</var> can be one of:</p>

    <dl>
      <dt><code>Input</code></dt>
      <dd>Store the byte count of the filter's input stream in the note.</dd>

      <dt><code>Output</code></dt>
      <dd>Store the byte count of the filter's output stream in the note.</dd>

      <dt><code>Ratio</code></dt>
      <dd>Store the compression ratio (<code>output/input * 100</code>)
      in the note. This is the default, if the <var>type</var> argument
      is omitted.</dd>
    </dl>

    <p>Thus you may log it this way:</p>

    <example><title>Accurate Logging</title>
    <highlight language="config">
DeflateFilterNote Input instream
DeflateFilterNote Output outstream
DeflateFilterNote Ratio ratio

LogFormat '"%r" %{outstream}n/%{instream}n (%{ratio}n%%)' deflate
CustomLog "logs/deflate_log" deflate
</highlight>
    </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. If the compressed response size is bigger than the one specified
    by this directive then httpd will switch to chunked encoding 
    (HTTP header <code>Transfer-Encoding</code> set to <code>Chunked</code>), with the
    side effect of not setting any <code>Content-Length</code> HTTP header. This is particularly 
    important when httpd works behind reverse caching proxies or when httpd is configured with 
    <module>mod_cache</module> and <module>mod_cache_disk</module> because
    HTTP responses without any <code>Content-Length</code> header might not be cached.
  </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>

<directivesynopsis>
<name>DeflateCompressionLevel</name>
<description>How much compression do we apply to the output</description>
<syntax>DeflateCompressionLevel <var>value</var></syntax>
<default>Zlib's default</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>DeflateCompressionLevel</directive> directive specifies
        what level of compression should be used, the higher the value,
        the better the compression, but the more CPU time is required to
        achieve this.</p>
    <p>The value must between 1 (less compression) and 9 (more compression).</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>DeflateInflateLimitRequestBody</name>
<description>Maximum size of inflated request bodies</description>
<syntax>DeflateInflateLimitRequestBody <var>value</var></syntax>
<default>None, but LimitRequestBody applies after deflation</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context><context>.htaccess</context></contextlist>
<override>All</override>
<compatibility>2.4.10 and later</compatibility>

<usage>
    <p>The <directive>DeflateInflateLimitRequestBody</directive> directive
        specifies the maximum size of an inflated request body. If it is unset,
        <directive module="core">LimitRequestBody</directive> is applied to the
        inflated body.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>DeflateInflateRatioLimit</name>
<description>Maximum inflation ratio for request bodies</description>
<syntax>DeflateInflateRatioLimit <var>value</var></syntax>
<default>DeflateInflateRatioLimit 200</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context><context>.htaccess</context></contextlist>
<override>All</override>
<compatibility>2.4.10 and later</compatibility>

<usage>
    <p>The <directive>DeflateInflateRatioLimit</directive> directive
        specifies the maximum ratio of deflated to inflated size of an
        inflated request body. This ratio is checked as the body is
        streamed in, and if crossed more than
        <directive module="mod_deflate">DeflateInflateRatioBurst</directive>
        times, the request will be terminated.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>DeflateInflateRatioBurst</name>
<description>Maximum number of times the inflation ratio for request bodies
             can be crossed</description>
<syntax>DeflateInflateRatioBurst <var>value</var></syntax>
<default>DeflateInflateRatioBurst 3</default>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context><context>.htaccess</context></contextlist>
<override>All</override>
<compatibility>2.4.10 and later</compatibility>

<usage>
    <p>The <directive>DeflateInflateRatioBurst</directive> directive
       specifies the maximum number of times the
       <directive module="mod_deflate">DeflateInflateRatioLimit</directive> can
       be crossed before terminating the request.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>DeflateAlterETag</name>
<description>How the outgoing ETag header should be modified during compression</description>
<syntax>DeflateAlterETag AddSuffix|NoChange|Remove</syntax>
<default>DeflateAlterETag AddSuffix</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<compatibility>Available in Apache 2.4.58 and later</compatibility>

<usage>
    <p>The <directive>DeflateAlterETag</directive> directive specifies
    how the ETag header should be altered when a response is compressed.</p>
    <dl>
    <dt>AddSuffix</dt>
    <dd><p>Append the compression method onto the end of the ETag, causing
        compressed and uncompressed representations to have unique ETags.
        This has been the default since 2.4.0, but prevents serving
        "HTTP Not Modified" (304) responses to conditional requests for
        compressed content.</p></dd>
    <dt>NoChange</dt>
    <dd><p>Don't change the ETag on a compressed response. This was the default
        prior to 2.4.0, but does not satisfy the HTTP/1.1 property that all
        representations of the same resource have unique ETags.</p></dd>
    <dt>Remove</dt>
    <dd><p>Remove the ETag header from compressed responses. This prevents
        some conditional requests from being possible, but avoids the
        shortcomings of the preceding options.</p></dd>
    </dl>
</usage>
</directivesynopsis>

</modulesynopsis>