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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>