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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<HTML>
<HEAD>
<TITLE>Custom error responses</TITLE>
</HEAD>
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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>
<3-digit-code> action
<P>where the action can be,
<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>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 Not Found</SAMP>, it can use the specific code and
error text instead.
</P>
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