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351 lines
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351 lines
14 KiB
Plaintext
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Environment Variables in Apache</title>
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</head>
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
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<!--#include virtual="header.html" -->
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<h1 align="center">Environment Variables in Apache</h1>
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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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shell.</p>
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<ul>
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<li><a href="#setting">Setting Environment Variables</a></li>
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<li><a href="#using">Using Environment Variables</a></li>
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<li><a href="#special">Special Purpose Environment
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Variables</a></li>
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<li><a href="#examples">Examples</a></li>
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</ul>
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<hr />
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<h2><a id="setting" name="setting">Setting Environment
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Variables</a></h2>
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<table border="1">
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<tr>
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<td valign="top"><strong>Related Modules</strong><br />
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<br />
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<a href="mod/mod_env.html">mod_env</a><br />
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<a href="mod/mod_rewrite.html">mod_rewrite</a><br />
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<a href="mod/mod_setenvif.html">mod_setenvif</a><br />
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<a href="mod/mod_unique_id.html">mod_unique_id</a><br />
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</td>
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<td valign="top"><strong>Related Directives</strong><br />
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<br />
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<a
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href="mod/mod_setenvif.html#BrowserMatch">BrowserMatch</a><br />
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<a
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href="mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</a><br />
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<a href="mod/mod_env.html#passenv">PassEnv</a><br />
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<a
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href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
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<a href="mod/mod_env.html#setenv">SetEnv</a><br />
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<a
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href="mod/mod_setenvif.html#SetEnvIf">SetEnvIf</a><br />
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<a
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href="mod/mod_setenvif.html#SetEnvIfNoCase">SetEnvIfNoCase</a><br />
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<a href="mod/mod_env.html#unsetenv">UnsetEnv</a><br />
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</td>
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</tr>
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</table>
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<h3>Basic Environment Manipulation</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>SetEnv</code> directive.
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Variables may also be passed from the environment of the shell
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which started the server using the <code>PassEnv</code>
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directive.</p>
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<h3>Conditional Per-Request Settings</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>RewriteRule</code>
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which uses the <code>[E=...]</code> option to set environment
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variables.</p>
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<h3>Unique Identifiers</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>Standard CGI Variables</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>Some Caveats</h3>
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<ul>
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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 <a href="suexec.html">suexec</a> 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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</ul>
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<hr />
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<h2><a id="using" name="using">Using Environment
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Variables</a></h2>
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<table border="1">
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<tr>
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<td valign="top"><strong>Related Modules</strong><br />
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<br />
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<a href="mod/mod_access.html">mod_access</a><br />
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<a href="mod/mod_cgi.html">mod_cgi</a><br />
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<a href="mod/mod_headers.html">mod_headers</a><br />
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<a href="mod/mod_include.html">mod_include</a><br />
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<a href="mod/mod_log_config.html">mod_log_config</a><br />
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<a href="mod/mod_rewrite.html">mod_rewrite</a><br />
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</td>
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<td valign="top"><strong>Related Directives</strong><br />
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<br />
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<a href="mod/mod_access.html#allow">Allow</a><br />
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<a
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href="mod/mod_log_config.html#customlog">CustomLog</a><br />
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<a href="mod/mod_access.html#deny">Deny</a><br />
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<a href="mod/mod_headers.html#Header">Header</a><br />
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<a
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href="mod/mod_log_config.html#logformat">LogFormat</a><br />
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<a
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href="mod/mod_rewrite.html#RewriteCond">RewriteCond</a><br />
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<a
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href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
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</td>
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</tr>
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</table>
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<h3>CGI Scripts</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>SSI Pages</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
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href="howto/ssi.html">SSI tutorial</a>.</p>
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<h3>Access Control</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>SetEnvIf</code>, this allows for flexible control of
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access to the server based on characteristics of the client.
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For example, you can use these directives to deny access to a
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particular browser (User-Agent).</p>
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<h3>Conditional Logging</h3>
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<p>Environment variables can be logged in the access log using
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the <code>LogFormat</code> option <code>%e</code>. In addition,
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the decision on whether or not to log requests can be made
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based on the status of environment variables using the
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conditional form of the <code>CustomLog</code> directive. In
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combination with <code>SetEnvIf</code> this allows for flexible
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control of which requests are logged. For example, you can
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choose not to log requests for filenames ending in
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<code>gif</code>, or you can choose to only log requests from
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clients which are outside your subnet.</p>
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<h3>Conditional Response Headers</h3>
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<p>The <code>Header</code> 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>URL Rewriting</h3>
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<p>The <code>%{ENV:...}</code> form of <em>TestString</em> in
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the <code>RewriteCond</code> 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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<hr />
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<h2><a id="special" name="special">Special Purpose Environment
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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 <a
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href="mod/mod_setenvif.html#browsermatch">BrowserMatch</a>,
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though <a href="mod/mod_env.html#setenv">SetEnv</a> and <a
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href="mod/mod_env.html#passenv">PassEnv</a> could also be used,
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for example.</p>
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<h2>downgrade-1.0</h2>
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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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<h2>force-no-vary</h2>
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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 (see the <a
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href="misc/known_client_problems.html">known client
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problems</a> page); setting this variable can work around this
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problem. Setting this variable also implies
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<strong>force-response-1.0</strong>.</p>
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<h2>force-response-1.0</h2>
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<p>This forces an HTTP/1.0 response when set. It was originally
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implemented as a result of a problem with AOL's proxies. Some
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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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<h2>nokeepalive</h2>
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<p>This disables <a
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href="mod/core.html#keepalive">KeepAlive</a> when set.</p>
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<h2>redirect-carefully</h2>
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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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methods.</p>
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<hr />
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<h2><a id="examples" name="examples">Examples</a></h2>
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<h3>Changing protocol behavior with misbehaving clients</h3>
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<p>We recommend that the following lines be included in
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httpd.conf to deal with known client problems.</p>
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<pre>
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#
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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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#
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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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#
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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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#
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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
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</pre>
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<h3>Do not log requests for images in the access log</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
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coming from particular hosts.</p>
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<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 env=!image-request
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</pre>
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<h3>Prevent "Image Theft"</h3>
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<p>This example shows how to keep people not on your server
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from using images on your server as inline-images on their
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pages. This is not a recommended configuration, but it can work
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in limited circumstances. We assume that all your images are in
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a directory called /web/images.</p>
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<pre>
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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>
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Order Deny,Allow
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Deny from all
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Allow from env=local_referal
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</Directory>
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</pre>
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<p>For more information about this technique, see the
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ApacheToday tutorial " <a
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href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">
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Keeping Your Images from Adorning Other Sites</a>".</p>
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<!--#include virtual="footer.html" -->
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</body>
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</html>
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