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      <h3>Apache HTTP Server Version 2.0</h3>
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    <h1 align="center">Environment Variables in Apache</h1>

    <p>The Apache HTTP Server provides a mechanism for storing
    information in named variables that are called <em>environment
    variables</em>. This information can be used to control various
    operations such as logging or access control. The variables are
    also used as a mechanism to communicate with external programs
    such as CGI scripts. This document discusses different ways to
    manipulate and use these variables.</p>

    <p>Although these variables are referred to as <em>environment
    variables</em>, they are not the same as the environment
    variables controlled by the underlying operating system.
    Instead, these variables are stored and manipulated in an
    internal Apache structure. They only become actual operating
    system environment variables when they are provided to CGI
    scripts and Server Side Include scripts. If you wish to
    manipulate the operating system environment under which the
    server itself runs, you must use the standard environment
    manipulation mechanisms provided by your operating system
    shell.</p>

    <ul>
      <li><a href="#setting">Setting Environment Variables</a></li>

      <li><a href="#using">Using Environment Variables</a></li>

      <li><a href="#special">Special Purpose Environment
      Variables</a></li>

      <li><a href="#examples">Examples</a></li>
    </ul>
    <hr />

    <h2><a id="setting" name="setting">Setting Environment
    Variables</a></h2>

    <table border="1">
      <tr>
        <td valign="top"><strong>Related Modules</strong><br />
         <br />
         <a href="mod/mod_env.html">mod_env</a><br />
         <a href="mod/mod_rewrite.html">mod_rewrite</a><br />
         <a href="mod/mod_setenvif.html">mod_setenvif</a><br />
         <a href="mod/mod_unique_id.html">mod_unique_id</a><br />
         </td>

        <td valign="top"><strong>Related Directives</strong><br />
         <br />
         <a
        href="mod/mod_setenvif.html#BrowserMatch">BrowserMatch</a><br />
         <a
        href="mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</a><br />
         <a href="mod/mod_env.html#passenv">PassEnv</a><br />
         <a
        href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
         <a href="mod/mod_env.html#setenv">SetEnv</a><br />
         <a
        href="mod/mod_setenvif.html#SetEnvIf">SetEnvIf</a><br />
         <a
        href="mod/mod_setenvif.html#SetEnvIfNoCase">SetEnvIfNoCase</a><br />
         <a href="mod/mod_env.html#unsetenv">UnsetEnv</a><br />
         </td>
      </tr>
    </table>

    <h3>Basic Environment Manipulation</h3>

    <p>The most basic way to set an environment variable in Apache
    is using the unconditional <code>SetEnv</code> directive.
    Variables may also be passed from the environment of the shell
    which started the server using the <code>PassEnv</code>
    directive.</p>

    <h3>Conditional Per-Request Settings</h3>

    <p>For additional flexibility, the directives provided by
    mod_setenvif allow environment variables to be set on a
    per-request basis, conditional on characteristics of particular
    requests. For example, a variable could be set only when a
    specific browser (User-Agent) is making a request, or only when
    a specific Referer [sic] header is found. Even more flexibility
    is available through the mod_rewrite's <code>RewriteRule</code>
    which uses the <code>[E=...]</code> option to set environment
    variables.</p>

    <h3>Unique Identifiers</h3>

    <p>Finally, mod_unique_id sets the environment variable
    <code>UNIQUE_ID</code> for each request to a value which is
    guaranteed to be unique across "all" requests under very
    specific conditions.</p>

    <h3>Standard CGI Variables</h3>

    <p>In addition to all environment variables set within the
    Apache configuration and passed from the shell, CGI scripts and
    SSI pages are provided with a set of environment variables
    containing meta-information about the request as required by
    the <a href="http://cgi-spec.golux.com/">CGI
    specification</a>.</p>

    <h3>Some Caveats</h3>

    <ul>
      <li>It is not possible to override or change the standard CGI
      variables using the environment manipulation directives.</li>

      <li>When <a href="suexec.html">suexec</a> is used to launch
      CGI scripts, the environment will be cleaned down to a set of
      <em>safe</em> variables before CGI scripts are launched. The
      list of <em>safe</em> variables is defined at compile-time in
      <code>suexec.c</code>.</li>

      <li>For portability reasons, the names of environment
      variables may contain only letters, numbers, and the
      underscore character. In addition, the first character may
      not be a number. Characters which do not match this
      restriction will be replaced by an underscore when passed to
      CGI scripts and SSI pages.</li>
    </ul>
    <hr />

    <h2><a id="using" name="using">Using Environment
    Variables</a></h2>

    <table border="1">
      <tr>
        <td valign="top"><strong>Related Modules</strong><br />
         <br />
         <a href="mod/mod_access.html">mod_access</a><br />
         <a href="mod/mod_cgi.html">mod_cgi</a><br />
         <a href="mod/mod_headers.html">mod_headers</a><br />
         <a href="mod/mod_include.html">mod_include</a><br />
         <a href="mod/mod_log_config.html">mod_log_config</a><br />
         <a href="mod/mod_rewrite.html">mod_rewrite</a><br />
         </td>

        <td valign="top"><strong>Related Directives</strong><br />
         <br />
         <a href="mod/mod_access.html#allow">Allow</a><br />
         <a
        href="mod/mod_log_config.html#customlog">CustomLog</a><br />
         <a href="mod/mod_access.html#deny">Deny</a><br />
         <a href="mod/mod_headers.html#Header">Header</a><br />
         <a
        href="mod/mod_log_config.html#logformat">LogFormat</a><br />
         <a
        href="mod/mod_rewrite.html#RewriteCond">RewriteCond</a><br />
         <a
        href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
         </td>
      </tr>
    </table>

    <h3>CGI Scripts</h3>

    <p>One of the primary uses of environment variables is to
    communicate information to CGI scripts. As discussed above, the
    environment passed to CGI scripts includes standard
    meta-information about the request in addition to any variables
    set within the Apache configuration. For more details, see the
    <a href="howto/cgi.html">CGI tutorial</a>.</p>

    <h3>SSI Pages</h3>

    <p>Server-parsed (SSI) documents processed by mod_include's
    <code>INCLUDES</code> filter can print environment variables
    using the <code>echo</code> element, and can use environment
    variables in flow control elements to makes parts of a page
    conditional on characteristics of a request. Apache also
    provides SSI pages with the standard CGI environment variables
    as discussed above. For more details, see the <a
    href="howto/ssi.html">SSI tutorial</a>.</p>

    <h3>Access Control</h3>

    <p>Access to the server can be controlled based on the value of
    environment variables using the <code>allow from env=</code>
    and <code>deny from env=</code> directives. In combination with
    <code>SetEnvIf</code>, this allows for flexible control of
    access to the server based on characteristics of the client.
    For example, you can use these directives to deny access to a
    particular browser (User-Agent).</p>

    <h3>Conditional Logging</h3>

    <p>Environment variables can be logged in the access log using
    the <code>LogFormat</code> option <code>%e</code>. In addition,
    the decision on whether or not to log requests can be made
    based on the status of environment variables using the
    conditional form of the <code>CustomLog</code> directive. In
    combination with <code>SetEnvIf</code> this allows for flexible
    control of which requests are logged. For example, you can
    choose not to log requests for filenames ending in
    <code>gif</code>, or you can choose to only log requests from
    clients which are outside your subnet.</p>

    <h3>Conditional Response Headers</h3>

    <p>The <code>Header</code> directive can use the presence or
    absence of an environment variable to determine whether or not
    a certain HTTP header will be placed in the response to the
    client. This allows, for example, a certain response header to
    be sent only if a corresponding header is received in the
    request from the client.</p>

    <h3>URL Rewriting</h3>

    <p>The <code>%{ENV:...}</code> form of <em>TestString</em> in
    the <code>RewriteCond</code> allows mod_rewrite's rewrite
    engine to make decisions conditional on environment variables.
    Note that the variables accessible in mod_rewrite without the
    <code>ENV:</code> prefix are not actually environment
    variables. Rather, they are variables special to mod_rewrite
    which cannot be accessed from other modules.</p>
    <hr />

    <h2><a id="special" name="special">Special Purpose Environment
    Variables</a></h2>

    <p>Interoperability problems have led to the introduction of
    mechanisms to modify the way Apache behaves when talking to
    particular clients. To make these mechanisms as flexible as
    possible, they are invoked by defining environment variables,
    typically with <a
    href="mod/mod_setenvif.html#browsermatch">BrowserMatch</a>,
    though <a href="mod/mod_env.html#setenv">SetEnv</a> and <a
    href="mod/mod_env.html#passenv">PassEnv</a> could also be used,
    for example.</p>

    <h2>downgrade-1.0</h2>

    <p>This forces the request to be treated as a HTTP/1.0 request
    even if it was in a later dialect.</p>

    <h2>force-no-vary</h2>

    <p>This causes any <code>Vary</code> fields to be removed from
    the response header before it is sent back to the client. Some
    clients don't interpret this field correctly (see the <a
    href="misc/known_client_problems.html">known client
    problems</a> page); setting this variable can work around this
    problem. Setting this variable also implies
    <strong>force-response-1.0</strong>.</p>

    <h2>force-response-1.0</h2>

    <p>This forces an HTTP/1.0 response when set. It was originally
    implemented as a result of a problem with AOL's proxies. Some
    clients may not behave correctly when given an HTTP/1.1
    response, and this can be used to interoperate with them.</p>

    <h2>nokeepalive</h2>

    <p>This disables <a
    href="mod/core.html#keepalive">KeepAlive</a> when set.</p>

    <h2>redirect-carefully</h2>

    <p>This forces the server to be more careful when sending a redirect
    to the client.  This is typically used when a client has a known
    problem handling redirects.  This was originally implemented as a
    result of a problem with Microsoft's WebFolders software which has
    a problem handling redirects on directory resources via DAV 
    methods.</p>

    <hr />
    <h2><a id="examples" name="examples">Examples</a></h2>

    <h3>Changing protocol behavior with misbehaving clients</h3>

    <p>We recommend that the following lines be included in
    httpd.conf to deal with known client problems.</p>
<pre>
#
# The following directives modify normal HTTP response behavior.
# The first directive disables keepalive for Netscape 2.x and browsers that
# spoof it. There are known problems with these browser implementations.
# The second directive is for Microsoft Internet Explorer 4.0b2
# which has a broken HTTP/1.1 implementation and does not properly
# support keepalive when it is used on 301 or 302 (redirect) responses.
#
BrowserMatch "Mozilla/2" nokeepalive
BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0

#
# The following directive disables HTTP/1.1 responses to browsers which
# are in violation of the HTTP/1.0 spec by not being able to grok a
# basic 1.1 response.
#
BrowserMatch "RealPlayer 4\.0" force-response-1.0
BrowserMatch "Java/1\.0" force-response-1.0
BrowserMatch "JDK/1\.0" force-response-1.0
</pre>

    <h3>Do not log requests for images in the access log</h3>

    <p>This example keeps requests for images from appearing in the
    access log. It can be easily modified to prevent logging of
    particular directories, or to prevent logging of requests
    coming from particular hosts.</p>
<pre>
    SetEnvIf Request_URI \.gif image-request
    SetEnvIf Request_URI \.jpg image-request
    SetEnvIf Request_URI \.png image-request
    CustomLog logs/access_log env=!image-request
</pre>

    <h3>Prevent "Image Theft"</h3>

    <p>This example shows how to keep people not on your server
    from using images on your server as inline-images on their
    pages. This is not a recommended configuration, but it can work
    in limited circumstances. We assume that all your images are in
    a directory called /web/images.</p>
<pre>
    SetEnvIf Referer "^http://www.example.com/" local_referal
    # Allow browsers that do not send Referer info
    SetEnvIf Referer "^$" local_referal
    &lt;Directory /web/images&gt;
       Order Deny,Allow
       Deny from all
       Allow from env=local_referal
    &lt;/Directory&gt;
</pre>

    <p>For more information about this technique, see the
    ApacheToday tutorial " <a
    href="http://apachetoday.com/news_story.php3?ltsn=2000-06-14-002-01-PS">
    Keeping Your Images from Adorning Other Sites</a>".</p>
        <hr />

    <h3 align="CENTER">Apache HTTP Server Version 2.0</h3>
    <a href="./"><img src="images/index.gif" alt="Index" /></a>



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