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<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.2</p>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.2</a></div><div id="page-content"><div id="preamble"><h1>Environment Variables in Apache</h1>
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<p><span>Available Languages: </span><a href="./en/env.html" title="English"> en </a> |
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<a href="./ja/env.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
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<a href="./ko/env.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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<p>The Apache HTTP Server provides a mechanism for storing
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information in named variables that are called <em>environment
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variables</em>. This information can be used to control various
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operations such as logging or access control. The variables are
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also used as a mechanism to communicate with external programs
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such as CGI scripts. This document discusses different ways to
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manipulate and use these variables.</p>
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<p>Although these variables are referred to as <em>environment
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variables</em>, they are not the same as the environment
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variables controlled by the underlying operating system.
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Instead, these variables are stored and manipulated in an
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internal Apache structure. They only become actual operating
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system environment variables when they are provided to CGI
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scripts and Server Side Include scripts. If you wish to
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manipulate the operating system environment under which the
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server itself runs, you must use the standard environment
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manipulation mechanisms provided by your operating system
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<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#setting">Setting Environment Variables</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#using">Using Environment Variables</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#special">Special Purpose Environment Variables</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#examples">Examples</a></li>
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<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<h2><a name="setting" id="setting">Setting Environment Variables</a></h2>
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<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_env.html">mod_env</a></code></li><li><code class="module"><a href="./mod/mod_rewrite.html">mod_rewrite</a></code></li><li><code class="module"><a href="./mod/mod_setenvif.html">mod_setenvif</a></code></li><li><code class="module"><a href="./mod/mod_unique_id.html">mod_unique_id</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_setenvif.html#browsermatch">BrowserMatch</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#browsermatchnocase">BrowserMatchNoCase</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#passenv">PassEnv</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#setenv">SetEnv</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#setenvifnocase">SetEnvIfNoCase</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#unsetenv">UnsetEnv</a></code></li></ul></td></tr></table>
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<h3><a name="basic-manipulation" id="basic-manipulation">Basic Environment Manipulation</a></h3>
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<p>The most basic way to set an environment variable in Apache
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is using the unconditional <code class="directive"><a href="./mod/mod_env.html#setenv">SetEnv</a></code> directive. Variables may also be passed from
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the environment of the shell which started the server using the
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<code class="directive"><a href="./mod/mod_env.html#passenv">PassEnv</a></code> directive.</p>
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<h3><a name="conditional" id="conditional">Conditional Per-Request Settings</a></h3>
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<p>For additional flexibility, the directives provided by
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mod_setenvif allow environment variables to be set on a
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per-request basis, conditional on characteristics of particular
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requests. For example, a variable could be set only when a
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specific browser (User-Agent) is making a request, or only when
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a specific Referer [sic] header is found. Even more flexibility
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is available through the mod_rewrite's <code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> which uses the
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<code>[E=...]</code> option to set environment variables.</p>
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<h3><a name="unique-identifiers" id="unique-identifiers">Unique Identifiers</a></h3>
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<p>Finally, mod_unique_id sets the environment variable
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<code>UNIQUE_ID</code> for each request to a value which is
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guaranteed to be unique across "all" requests under very
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specific conditions.</p>
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<h3><a name="standard-cgi" id="standard-cgi">Standard CGI Variables</a></h3>
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<p>In addition to all environment variables set within the
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Apache configuration and passed from the shell, CGI scripts and
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SSI pages are provided with a set of environment variables
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containing meta-information about the request as required by
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the <a href="http://cgi-spec.golux.com/">CGI
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specification</a>.</p>
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<h3><a name="caveats" id="caveats">Some Caveats</a></h3>
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<li>It is not possible to override or change the standard CGI
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variables using the environment manipulation directives.</li>
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<li>When <code class="program"><a href="./programs/suexec.html">suexec</a></code> is used to launch
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CGI scripts, the environment will be cleaned down to a set of
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<em>safe</em> variables before CGI scripts are launched. The
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list of <em>safe</em> variables is defined at compile-time in
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<code>suexec.c</code>.</li>
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<li>For portability reasons, the names of environment
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variables may contain only letters, numbers, and the
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underscore character. In addition, the first character may
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not be a number. Characters which do not match this
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restriction will be replaced by an underscore when passed to
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CGI scripts and SSI pages.</li>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="using" id="using">Using Environment Variables</a></h2>
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<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_authz_host.html">mod_authz_host</a></code></li><li><code class="module"><a href="./mod/mod_cgi.html">mod_cgi</a></code></li><li><code class="module"><a href="./mod/mod_ext_filter.html">mod_ext_filter</a></code></li><li><code class="module"><a href="./mod/mod_headers.html">mod_headers</a></code></li><li><code class="module"><a href="./mod/mod_include.html">mod_include</a></code></li><li><code class="module"><a href="./mod/mod_log_config.html">mod_log_config</a></code></li><li><code class="module"><a href="./mod/mod_rewrite.html">mod_rewrite</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_authz_host.html#allow">Allow</a></code></li><li><code class="directive"><a href="./mod/mod_log_config.html#customlog">CustomLog</a></code></li><li><code class="directive"><a href="./mod/mod_authz_host.html#deny">Deny</a></code></li><li><code class="directive"><a href="./mod/mod_ext_filter.html#extfilterdefine">ExtFilterDefine</a></code></li><li><code class="directive"><a href="./mod/mod_headers.html#header">Header</a></code></li><li><code class="directive"><a href="./mod/mod_log_config.html#logformat">LogFormat</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewritecond">RewriteCond</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code></li></ul></td></tr></table>
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<h3><a name="cgi-scripts" id="cgi-scripts">CGI Scripts</a></h3>
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<p>One of the primary uses of environment variables is to
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communicate information to CGI scripts. As discussed above, the
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environment passed to CGI scripts includes standard
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meta-information about the request in addition to any variables
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set within the Apache configuration. For more details, see the
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<a href="howto/cgi.html">CGI tutorial</a>.</p>
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<h3><a name="ssi-pages" id="ssi-pages">SSI Pages</a></h3>
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<p>Server-parsed (SSI) documents processed by mod_include's
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<code>INCLUDES</code> filter can print environment variables
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using the <code>echo</code> element, and can use environment
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variables in flow control elements to makes parts of a page
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conditional on characteristics of a request. Apache also
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provides SSI pages with the standard CGI environment variables
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as discussed above. For more details, see the <a href="howto/ssi.html">SSI tutorial</a>.</p>
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<h3><a name="access-control" id="access-control">Access Control</a></h3>
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<p>Access to the server can be controlled based on the value of
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environment variables using the <code>allow from env=</code>
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and <code>deny from env=</code> directives. In combination with
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<code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code>, this
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allows for flexible control of access to the server based on
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characteristics of the client. For example, you can use these
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directives to deny access to a particular browser (User-Agent).
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<h3><a name="logging" id="logging">Conditional Logging</a></h3>
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<p>Environment variables can be logged in the access log using
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the <code class="directive"><a href="./mod/mod_log_config.html#logformat">LogFormat</a></code>
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option <code>%e</code>. In addition, the decision on whether
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or not to log requests can be made based on the status of
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environment variables using the conditional form of the
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<code class="directive"><a href="./mod/mod_log_config.html#customlog">CustomLog</a></code>
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directive. In combination with <code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code> this allows for flexible control of which
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requests are logged. For example, you can choose not to log
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requests for filenames ending in <code>gif</code>, or you can
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choose to only log requests from clients which are outside your
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<h3><a name="response-headers" id="response-headers">Conditional Response Headers</a></h3>
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<p>The <code class="directive"><a href="./mod/mod_headers.html#header">Header</a></code>
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directive can use the presence or
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absence of an environment variable to determine whether or not
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a certain HTTP header will be placed in the response to the
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client. This allows, for example, a certain response header to
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be sent only if a corresponding header is received in the
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request from the client.</p>
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<h3><a name="external-filter" id="external-filter">External Filter Activation</a></h3>
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<p>External filters configured by <code class="module"><a href="./mod/mod_ext_filter.html">mod_ext_filter</a></code>
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using the <code class="directive"><a href="./mod/mod_ext_filter.html#extfilterdefine">ExtFilterDefine</a></code> directive can
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by activated conditional on an environment variable using the
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<code>disableenv=</code> and <code>enableenv=</code> options.</p>
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<h3><a name="url-rewriting" id="url-rewriting">URL Rewriting</a></h3>
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<p>The <code>%{ENV:...}</code> form of <em>TestString</em> in
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the <code class="directive"><a href="./mod/mod_rewrite.html#rewritecond">RewriteCond</a></code>
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allows mod_rewrite's rewrite
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engine to make decisions conditional on environment variables.
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Note that the variables accessible in mod_rewrite without the
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<code>ENV:</code> prefix are not actually environment
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variables. Rather, they are variables special to mod_rewrite
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which cannot be accessed from other modules.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="special" id="special">Special Purpose Environment Variables</a></h2>
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<p>Interoperability problems have led to the introduction of
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mechanisms to modify the way Apache behaves when talking to
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particular clients. To make these mechanisms as flexible as
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possible, they are invoked by defining environment variables,
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typically with <code class="directive"><a href="./mod/mod_setenvif.html#browsermatch">BrowserMatch</a></code>, though <code class="directive"><a href="./mod/mod_env.html#setenv">SetEnv</a></code> and <code class="directive"><a href="./mod/mod_env.html#passenv">PassEnv</a></code> could also be used, for example.</p>
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<h3><a name="downgrade" id="downgrade">downgrade-1.0</a></h3>
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<p>This forces the request to be treated as a HTTP/1.0 request
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even if it was in a later dialect.</p>
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<h3><a name="force-gzip" id="force-gzip">force-gzip</a></h3>
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<p>If you have the <code>DEFLATE</code> filter activated, this
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environment variable will ignore the accept-encoding setting of
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your browser and will send compressed output unconditionally.</p>
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<h3><a name="force-no-vary" id="force-no-vary">force-no-vary</a></h3>
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<p>This causes any <code>Vary</code> fields to be removed from
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the response header before it is sent back to the client. Some
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clients don't interpret this field correctly; setting this
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variable can work around this problem. Setting this variable
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also implies <strong>force-response-1.0</strong>.</p>
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<h3><a name="force-response" id="force-response">force-response-1.0</a></h3>
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<p>This forces an HTTP/1.0 response to clients making an HTTP/1.0
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request. It was originally
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implemented as a result of a problem with AOL's proxies. Some
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HTTP/1.0 clients may not behave correctly when given an HTTP/1.1
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response, and this can be used to interoperate with them.</p>
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<h3><a name="gzip-only-text-html" id="gzip-only-text-html">gzip-only-text/html</a></h3>
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<p>When set to a value of "1", this variable disables the DEFLATE
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output filter provided by <code class="module"><a href="./mod/mod_deflate.html">mod_deflate</a></code> for
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content-types other than <code>text/html</code>. If you'd rather
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use statically compressed files; <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code>
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evaluates the variable as well (not only for gzip, but for all
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encodings that differ from "identity").</p>
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<h3><a name="no-gzip" id="no-gzip">no-gzip</a></h3>
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<p>When set, the <code>DEFLATE</code> filter of
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<code class="module"><a href="./mod/mod_deflate.html">mod_deflate</a></code> will be turned off and
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<code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> will refuse to deliver encoded
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<h3><a name="nokeepalive" id="nokeepalive">nokeepalive</a></h3>
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<p>This disables <code class="directive"><a href="./mod/core.html#keepalive">KeepAlive</a></code>
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<h3><a name="prefer-language" id="prefer-language">prefer-language</a></h3>
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<p>This influences <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code>'s behaviour. If
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it contains a language tag (such as <code>en</code>, <code>ja</code>
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or <code>x-klingon</code>), <code class="module"><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> tries
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to deliver a variant with that language. If there's no such variant,
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the normal <a href="content-negotiation.html">negotiation</a> process
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<h3><a name="redirect-carefully" id="redirect-carefully">redirect-carefully</a></h3>
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<p>This forces the server to be more careful when sending a redirect
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to the client. This is typically used when a client has a known
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problem handling redirects. This was originally implemented as a
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result of a problem with Microsoft's WebFolders software which has
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a problem handling redirects on directory resources via DAV
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<h3><a name="suppress-error-charset" id="suppress-error-charset">suppress-error-charset</a></h3>
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<p><em>Available in versions after 2.0.54</em></p>
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<p>When Apache issues a redirect in response to a client request,
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the response includes some actual text to be displayed in case
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the client can't (or doesn't) automatically follow the redirection.
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Apache ordinarily labels this text according to the character set
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which it uses, which is ISO-8859-1.</p>
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<p> However, if the redirection is to a page that uses a different
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character set, some broken browser versions will try to use the
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character set from the redirection text rather than the actual page.
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This can result in Greek, for instance, being incorrectly rendered.</p>
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<p>Setting this environment variable causes Apache to omit the character
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set for the redirection text, and these broken browsers will then correctly
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use that of the destination page.</p>
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<h3><a name="proxy" id="proxy">force-proxy-request-1.0, proxy-nokeepalive, proxy-sendchunked, proxy-sendcl</a></h3>
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<p>These directives alter the protocol behavior of
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<code class="module"><a href="./mod/mod_proxy.html">mod_proxy</a></code>. See the <code class="module"><a href="./mod/mod_proxy.html">mod_proxy</a></code>
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documentation for more details.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="examples" id="examples">Examples</a></h2>
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<h3><a name="misbehaving" id="misbehaving">Changing protocol behavior with misbehaving clients</a></h3>
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<p>Earlier versions recommended that the following lines be included in
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httpd.conf to deal with known client problems. Since the affected clients
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are no longer seen in the wild, this configuration is likely no-longer
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<div class="example"><pre>
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# The following directives modify normal HTTP response behavior.
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# The first directive disables keepalive for Netscape 2.x and browsers that
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# spoof it. There are known problems with these browser implementations.
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# The second directive is for Microsoft Internet Explorer 4.0b2
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# which has a broken HTTP/1.1 implementation and does not properly
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# support keepalive when it is used on 301 or 302 (redirect) responses.
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BrowserMatch "Mozilla/2" nokeepalive
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BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0
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# The following directive disables HTTP/1.1 responses to browsers which
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# are in violation of the HTTP/1.0 spec by not being able to grok a
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# basic 1.1 response.
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BrowserMatch "RealPlayer 4\.0" force-response-1.0
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BrowserMatch "Java/1\.0" force-response-1.0
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BrowserMatch "JDK/1\.0" force-response-1.0</pre></div>
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<h3><a name="no-img-log" id="no-img-log">Do not log requests for images in the access log</a></h3>
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<p>This example keeps requests for images from appearing in the
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access log. It can be easily modified to prevent logging of
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particular directories, or to prevent logging of requests
376
coming from particular hosts.</p>
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<div class="example"><pre>
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SetEnvIf Request_URI \.gif image-request
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SetEnvIf Request_URI \.jpg image-request
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SetEnvIf Request_URI \.png image-request
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CustomLog logs/access_log common env=!image-request</pre></div>
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<h3><a name="image-theft" id="image-theft">Prevent "Image Theft"</a></h3>
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<p>This example shows how to keep people not on your server
388
from using images on your server as inline-images on their
389
pages. This is not a recommended configuration, but it can work
390
in limited circumstances. We assume that all your images are in
391
a directory called /web/images.</p>
392
<div class="example"><pre>
393
SetEnvIf Referer "^http://www.example.com/" local_referal
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# Allow browsers that do not send Referer info
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SetEnvIf Referer "^$" local_referal
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<Directory /web/images>
399
Allow from env=local_referal
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</Directory></pre></div>
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<p>For more information about this technique, see the
403
ApacheToday tutorial " <a href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">
404
Keeping Your Images from Adorning Other Sites</a>".</p>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="./en/env.html" title="English"> en </a> |
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<a href="./ja/env.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
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<a href="./ko/env.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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</div><div id="footer">
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<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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