Custom error responses

### Apache HTTP Server Version 1.3

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# Custom error responses

Purpose
  Additional functionality. Allows webmasters to configure the response of Apache to some error or problem.

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

  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).

Old behavior
  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.
New behavior
  The server can be asked to;
  1. Display some other text, instead of the NCSA hard coded messages, or
  2. redirect to a local URL, or
  3. redirect to an external URL.

  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.

  To achieve this, Apache will define new CGI-like environment variables, e.g.

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

  note the `REDIRECT_` prefix.

  At least `REDIRECT_URL` and `REDIRECT_QUERY_STRING` 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. None of these will be set if your ErrorDocument is an external redirect (i.e., anything starting with a scheme name like `http:`, even if it refers to the same host as the server).

Configuration
  Use of "ErrorDocument" is enabled for .htaccess files when the "FileInfo" override is allowed.

  Here are some examples...

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

  The syntax is,

  `ErrorDocument` <3-digit-code> action

  where the action can be,

  1. Text to be displayed. Prefix the text with a quote ("). Whatever follows the quote is displayed. Note: the (") prefix isn't displayed.
  2. An external URL to redirect to.
  3. A local URL to redirect to.

## Custom error responses and redirects

Purpose
  Apache's behavior to redirected URLs has been modified so that additional environment variables are available to a script/server-include.
Old behavior
  Standard CGI vars were made available to a script which has been redirected to. No indication of where the redirection came from was provided.
New behavior
  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 `REDIRECT_`. `REDIRECT_` environment variables are created from the CGI environment variables which existed prior to the redirect, they are renamed with a `REDIRECT_` prefix, i.e., `HTTP_USER_AGENT` becomes `REDIRECT_HTTP_USER_AGENT`. In addition to these new variables, Apache will define `REDIRECT_URL` and `REDIRECT_STATUS` to help the script trace its origin. Both the original URL and the URL being redirected to can be logged in the access log.

If the ErrorDocument specifies a local redirect to a CGI script, the script should include a "`Status:`" 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:

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

If the script is dedicated to handling a particular error condition, such as `404 Not Found`, it can use the specific code and error text instead.

### Apache HTTP Server