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    <h1 align="CENTER">Custom error responses</h1>

    <dl>
      <dt>Purpose</dt>

      <dd>
        Additional functionality. Allows webmasters to configure
        the response of Apache to some error or problem. 

        <p>Customizable responses can be defined to be activated in
        the event of a server detected error or problem.</p>

        <p>e.g. if a script crashes and produces a "500 Server
        Error" response, then this response can be replaced with
        either some friendlier text or by a redirection to another
        URL (local or external).</p>
      </dd>

      <dt>Old behavior</dt>

      <dd>NCSA httpd 1.3 would return some boring old error/problem
      message which would often be meaningless to the user, and
      would provide no means of logging the symptoms which caused
      it.<br />
      </dd>

      <dt>New behavior</dt>

      <dd>
        The server can be asked to; 

        <ol>
          <li>Display some other text, instead of the NCSA hard
          coded messages, or</li>

          <li>redirect to a local URL, or</li>

          <li>redirect to an external URL.</li>
        </ol>

        <p>Redirecting to another URL can be useful, but only if
        some information can be passed which can then be used to
        explain and/or log the error/problem more clearly.</p>

        <p>To achieve this, Apache will define new CGI-like
        environment variables, <em>e.g.</em></p>

        <blockquote>
          <code>REDIRECT_HTTP_ACCEPT=*/*, image/gif,
          image/x-xbitmap, image/jpeg<br />
           REDIRECT_HTTP_USER_AGENT=Mozilla/1.1b2 (X11; I; HP-UX
          A.09.05 9000/712)<br />
           REDIRECT_PATH=.:/bin:/usr/local/bin:/etc<br />
           REDIRECT_QUERY_STRING=<br />
           REDIRECT_REMOTE_ADDR=121.345.78.123<br />
           REDIRECT_REMOTE_HOST=ooh.ahhh.com<br />
           REDIRECT_SERVER_NAME=crash.bang.edu<br />
           REDIRECT_SERVER_PORT=80<br />
           REDIRECT_SERVER_SOFTWARE=Apache/0.8.15<br />
           REDIRECT_URL=/cgi-bin/buggy.pl<br />
          </code>
        </blockquote>

        <p>note the <code>REDIRECT_</code> prefix.</p>

        <p>At least <code>REDIRECT_URL</code> and
        <code>REDIRECT_QUERY_STRING</code> will be passed to the
        new URL (assuming it's a cgi-script or a cgi-include). The
        other variables will exist only if they existed prior to
        the error/problem. <strong>None</strong> of these will be
        set if your ErrorDocument is an <em>external</em> redirect
        (<em>i.e.</em>, anything starting with a scheme name like
        <code>http:</code>, even if it refers to the same host as
        the server).</p>
      </dd>

      <dt>Configuration</dt>

      <dd>
        Use of "ErrorDocument" is enabled for .htaccess files when
        the <a href="mod/core.html#allowoverride">"FileInfo"
        override</a> is allowed. 

        <p>Here are some examples...</p>

        <blockquote>
          <code>ErrorDocument 500 /cgi-bin/crash-recover<br />
           ErrorDocument 500 "Sorry, our script crashed. Oh
          dear<br />
           ErrorDocument 500 http://xxx/<br />
           ErrorDocument 404 /Lame_excuses/not_found.html<br />
           ErrorDocument 401
          /Subscription/how_to_subscribe.html</code>
        </blockquote>

        <p>The syntax is,</p>

        <p><code><a
        href="mod/core.html#errordocument">ErrorDocument</a></code>
        &lt;3-digit-code&gt; action</p>

        <p>where the action can be,</p>

        <ol>
          <li>Text to be displayed. Prefix the text with a quote
          ("). Whatever follows the quote is displayed. <em>Note:
          the (") prefix isn't displayed.</em></li>

          <li>An external URL to redirect to.</li>

          <li>A local URL to redirect to.</li>
        </ol>
      </dd>
    </dl>
    <hr />

    <h2>Custom error responses and redirects</h2>

    <dl>
      <dt>Purpose</dt>

      <dd>Apache's behavior to redirected URLs has been modified so
      that additional environment variables are available to a
      script/server-include.</dd>

      <dt>Old behavior</dt>

      <dd>Standard CGI vars were made available to a script which
      has been redirected to. No indication of where the
      redirection came from was provided.</dd>

      <dt>New behavior</dt>

      <dd>A new batch of environment variables will be initialized
      for use by a script which has been redirected to. Each new
      variable will have the prefix <code>REDIRECT_</code>.
      <code>REDIRECT_</code> environment variables are created from
      the CGI environment variables which existed prior to the
      redirect, they are renamed with a <code>REDIRECT_</code>
      prefix, <em>i.e.</em>, <code>HTTP_USER_AGENT</code> becomes
      <code>REDIRECT_HTTP_USER_AGENT</code>. In addition to these
      new variables, Apache will define <code>REDIRECT_URL</code>
      and <code>REDIRECT_STATUS</code> to help the script trace its
      origin. Both the original URL and the URL being redirected to
      can be logged in the access log.</dd>
    </dl>

    <p>If the ErrorDocument specifies a local redirect to a CGI
    script, the script should include a "<samp>Status:</samp>"
    header field in its output in order to ensure the propagation
    all the way back to the client of the error condition that
    caused it to be invoked. For instance, a Perl ErrorDocument
    script might include the following:</p>
<pre>
      :
    print  "Content-type: text/html\n";
    printf "Status: %s Condition Intercepted\n", $ENV{"REDIRECT_STATUS"};
      :
</pre>

    <p>If the script is dedicated to handling a particular error
    condition, such as <samp>404&nbsp;Not&nbsp;Found</samp>, it can
    use the specific code and error text instead.</p>
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