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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).

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).