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

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

<name>mod_brotli</name>
<description>Compress content via Brotli before it is delivered to the
client</description>
<status>Extension</status>
<sourcefile>mod_brotli.c</sourcefile>
<identifier>brotli_module</identifier>
<compatibility>Available in version 2.4.26 and later.</compatibility>
<summary>
    <p>The <module>mod_brotli</module> module provides
    the <code>BROTLI_COMPRESS</code> output filter that allows output from
    your server to be compressed using the brotli compression format before being sent to the client over
    the network. This module uses the Brotli library found at
    <a href="https://github.com/google/brotli">https://github.com/google/brotli</a>.</p>
</summary>
<seealso><a href="../filter.html">Filters</a></seealso>

<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 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 BROTLI_COMPRESS 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 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>BROTLI_COMPRESS</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 BROTLI_COMPRESS
SetEnvIfNoCase Request_URI \.(?:gif|jpe?g|png)$ no-brotli
      </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 BROTLI_COMPRESS text/html
&lt;/Directory&gt;
      </highlight>

      <note><title>Note</title>
        The <code>BROTLI_COMPRESS</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>no-brotli</code>,
        set via <directive module="mod_env">SetEnv</directive>, which
        will disable brotli compression for a particular request, even if
        it is supported by the client.
      </note>

    </section>

</section>

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

    <p>The <module>mod_brotli</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>BROTLI_COMPRESS</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_brotli</module> re-compresses content each
    time a request is made, some performance benefit can be derived by
    pre-compressing the content and telling mod_brotli 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 brotli compressed CSS and JS files if they exist
    # and the client accepts brotli.
    RewriteCond "%{HTTP:Accept-encoding}" "br"
    RewriteCond "%{REQUEST_FILENAME}\.br" "-s"
    RewriteRule "^(.*)\.(js|css)"              "$1\.$2\.br" [QSA]

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


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

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

</section>

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

<usage>
    <p>The <directive>BrotliFilterNote</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">
BrotliFilterNote ratio

LogFormat '"%r" %b (%{ratio}n) "%{User-agent}i"' brotli
CustomLog "logs/brotli_log" brotli
    </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">
BrotliFilterNote Input instream
BrotliFilterNote Output outstream
BrotliFilterNote Ratio ratio

LogFormat '"%r" %{outstream}n/%{instream}n (%{ratio}n%%)' brotli
CustomLog "logs/brotli_log" brotli
    </highlight>
    </example>
</usage>
<seealso><module>mod_log_config</module></seealso>
</directivesynopsis>

<directivesynopsis>
<name>BrotliCompressionQuality</name>
<description>Compression quality</description>
<syntax>BrotliCompressionQuality <var>value</var></syntax>
<default>BrotliCompressionQuality 5</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>BrotliCompressionQuality</directive> directive specifies
    the compression quality (a value between 0 and 11). Higher quality values
    result in better, but also slower compression.
  </p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>BrotliCompressionWindow</name>
<description>Brotli sliding compression window size</description>
<syntax>BrotliCompressionWindow <var>value</var></syntax>
<default>BrotliCompressionWindow 18</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>BrotliCompressionWindow</directive> directive specifies the
    brotli sliding compression window size (a value between 10 and 24). Larger
    window sizes can improve compression quality, but require more memory.</p>
</usage>
</directivesynopsis>

<directivesynopsis>

<name>BrotliCompressionMaxInputBlock</name>
<description>Maximum input block size</description>
<syntax>BrotliCompressionMaxInputBlock <var>value</var></syntax>
<default>(automatic)</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>BrotliCompressionMaxInputBlock</directive> directive specifies
    the maximum input block size between 16 and 24, with the caveat that
    larger block sizes require more memory.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>BrotliAlterETag</name>
<description>How the outgoing ETag header should be modified during compression</description>
<syntax>BrotliAlterETag AddSuffix|NoChange|Remove</syntax>
<default>BrotliAlterETag AddSuffix</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>The <directive>BrotliAlterETag</directive> directive specifies
    how the ETag hader 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.
        In another dynamic compression module, mod_deflate, this has been
        the default since 2.4.0. This setting 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. In another dynamic
        compression module, mod_deflate, this has been the default prior to
        2.4.0. This setting 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>