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  <title>Custom Error Responses</title>

  <summary>
    <p>Additional functionality allows webmasters to configure the response 
    of Apache to some error or problem.</p>

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

    <p>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>
  </summary>

  <section id="behavior">
    <title>Behavior</title>

    <section>
      <title>Old Behavior</title>

      <p>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.</p>
    </section>

    <section>
      <title>New Behavior</title>

      <p>The server can be asked to:</p>

      <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:</p>

      <example>
        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
      </example>

      <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 <directive module="core">ErrorDocument</directive> is an 
      <em>external</em> redirect (anything starting with a 
      scheme name like <code>http:</code>, even if it refers to the same host 
      as the server).</p>
    </section>
  </section>

  <section id="configuration">
    <title>Configuration</title>

    <p>Use of <directive module="core">ErrorDocument</directive> is enabled 
    for .htaccess files when the 
    <directive module="core">AllowOverride</directive> is set accordingly.</p>

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

    <example>
      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
    </example>

    <p>The syntax is,</p>

    <example>
      ErrorDocument &lt;3-digit-code&gt; &lt;action&gt;
    </example>

    <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>
  </section>

  <section id="custom">
    <title>Custom Error Responses and Redirects</title>

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

    <section>
      <title>Old behavior</title>

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

    <section>
      <title>New behavior</title>

      <p>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.</p>

      <p>If the ErrorDocument specifies a local redirect to a CGI
      script, the script should include a "<code>Status:</code>"
      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>

      <example>
        ... <br />
        print  "Content-type: text/html\n"; <br />
        printf "Status: %s Condition Intercepted\n", $ENV{"REDIRECT_STATUS"}; <br />
        ...
      </example>

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

      <p>Note that the script <em>must</em> emit an appropriate
      <code>Status:</code> header (such as <code>302&nbsp;Found</code>), if the
      response contains a <code>Location:</code> header (in order to issue a
      client side redirect). Otherwise the <code>Location:</code> header may
      have no effect.</p>
    </section>
  </section>
</manualpage>
