Custom Error Responses

Custom Error Responses

Although the Apache HTTP Server provides generic error responses in the event of 4xx or 5xx HTTP status codes, these responses are rather stark, uninformative, and can be intimidating to site users. You may wish to provide custom error responses which are either friendlier, or in some language other than English, or perhaps which are styled more in line with your site layout.

Customized error responses can be defined for any HTTP status code designated as an error condition - that is, any 4xx or 5xx status.

Additionally, a set of values are provided, so that the error document can be customized further based on the values of these variables, using Server Side Includes. Or, you can have error conditions handled by a cgi program, or other dynamic handler (PHP, mod_perl, etc) which makes use of these variables.

Configuration

Custom error documents are configured using the ErrorDocument directive, which may be used in global, virtualhost, or directory context. It may be used in .htaccess files if AllowOverride is set to FileInfo.

ErrorDocument 500 "Sorry, our script crashed. Oh dear"
ErrorDocument 500 /cgi-bin/crash-recover
ErrorDocument 500 http://error.example.com/server_error.html
ErrorDocument 404 /errors/not_found.html
ErrorDocument 401 /subscription/how_to_subscribe.html

The syntax of the ErrorDocument directive is:

ErrorDocument <3-digit-code> <action>

where the action will be treated as:

1. A local URL to redirect to (if the action begins with a "/").
2. An external URL to redirect to (if the action is a valid URL).
3. Text to be displayed (if none of the above). The text must be wrapped in quotes (") if it consists of more than one word.

When redirecting to a local URL, additional environment variables are set so that the response can be further customized. They are not sent to external URLs.

Available Variables

Redirecting to another URL can be useful, but only if some information can be passed which can then be used to explain or log the error condition more clearly.

To achieve this, when the error redirect is sent, additional environment variables will be set, which will be generated from the headers provided to the original request by prepending 'REDIRECT_' onto the original header name. This provides the error document the context of the original request.

For example, you might receive, in addition to more usual environment variables, the following.

REDIRECT_HTTP_ACCEPT=*/*, image/gif, image/jpeg, image/png
REDIRECT_HTTP_USER_AGENT=Mozilla/5.0 Fedora/3.5.8-1.fc12 Firefox/3.5.8
REDIRECT_PATH=.:/bin:/usr/local/bin:/sbin
REDIRECT_QUERY_STRING=
REDIRECT_REMOTE_ADDR=121.345.78.123
REDIRECT_REMOTE_HOST=client.example.com
REDIRECT_SERVER_NAME=www.example.edu
REDIRECT_SERVER_PORT=80
REDIRECT_SERVER_SOFTWARE=Apache/2.2.15
REDIRECT_URL=/cgi-bin/buggy.pl

REDIRECT_ environment variables are created from the 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.

REDIRECT_URL, REDIRECT_STATUS, and REDIRECT_QUERY_STRING are guaranteed to be set, and the other headers will be set only if they existed prior to the error condition.

None of these will be set if the ErrorDocument target is an external redirect (anything starting with a scheme name like http:, even if it refers to the same host as the server).

Customizing Error Responses

If you point your ErrorDocument to some variety of dynamic handler such as a server-side include document, CGI script, or some variety of other handler, you may wish to use the available custom environment variables to customize this response.

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.

Note that if the response contains Location: header (in order to issue a client-side redirect), the script must emit an appropriate Status: header (such as 302 Found). Otherwise the Location: header may have no effect.

Multi Language Custom Error Documents

Provided with your installation of the Apache HTTP Server is a directory of custom error documents translated into 16 different languages. There's also a configuration file in the conf/extra configuration directory that can be included to enable this feature.

In your server configuration file, you'll see a line such as:

# Multi-language error messages
#Include conf/extra/httpd-multilang-errordoc.conf

Uncommenting this Include line will enable this feature, and provide language-negotiated error messages, based on the language preference set in the client browser.

Additionally, these documents contain various of the REDIRECT_ variables, so that additional information can be provided to the end-user about what happened, and what they can do now.

These documents can be customized to whatever degree you wish to provide more useful information to users about your site, and what they can expect to find there.

mod_include and mod_negotiation must be enabled to use this feature.