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<H1 ALIGN="CENTER">Custom error responses</H1>

<DL>

<DT>Purpose

  <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>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>

<DT>Old behavior

  <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>

      <P>

<DT>New behavior

  <DD>The server can be asked to;
  <OL>
    <LI>Display some other text, instead of the NCSA hard coded messages, or
    <LI>redirect to a local URL, or
    <LI>redirect to an external URL.
  </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>To achieve this, Apache will define new CGI-like environment
     variables, <EM>e.g.</EM>

  <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>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>

<DT>Configuration

  <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...

  <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><CODE><A HREF="mod/core.html#errordocument">ErrorDocument</A></CODE>
&lt;3-digit-code&gt; action

  <P>where the action can be,

  <OL>
    <LI>Text to be displayed.  Prefix the text with a quote (&quot;). Whatever
        follows the quote is displayed. <EM>Note: the (&quot;) prefix isn't
        displayed.</EM>

    <LI>An external URL to redirect to.

    <LI>A local URL to redirect to.

  </OL>
</DL>

<P><HR><P>

<H2>Custom error responses and redirects</H2>

<DL>

<DT>Purpose

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

<DT>Old behavior

  <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.

  <P>

<DT>New behavior
  <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.

</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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