In the event of a problem or error, Apache httpd can be configured to do one of four things,

1. output a simple hardcoded error message
2. output a customized message
3. internally redirect to a local URL-path to handle the problem/error
4. redirect to an external URL to handle the problem/error

The first option is the default, while options 2-4 are configured using the ErrorDocument directive, which is followed by the HTTP response code and a URL or a message. Apache httpd will sometimes offer additional information regarding the problem/error.

From 2.4.13, expression syntax can be used inside the directive to produce dynamic strings and URLs.

URLs can begin with a slash (/) for local web-paths (relative to the DocumentRoot ), or be a full URL which the client can resolve. Alternatively, a message can be provided to be displayed by the browser. Note that deciding whether the parameter is an URL, a path or a message is performed before any expression is parsed. Examples:

ErrorDocument 500 http://example.com/cgi-bin/server-error.cgi
ErrorDocument 404 /errors/bad_urls.php
ErrorDocument 401 /subscription_info.html
ErrorDocument 403 "Sorry, can't allow you access today"
ErrorDocument 403 Forbidden!
ErrorDocument 403 /errors/forbidden.py?referrer=%{escape:%{HTTP_REFERER}}

Additionally, the special value default can be used to specify Apache httpd's simple hardcoded message. While not required under normal circumstances, default will restore Apache httpd's simple hardcoded message for configurations that would otherwise inherit an existing ErrorDocument .

ErrorDocument 404 /cgi-bin/bad_urls.pl
<Directory "/web/docs">
 ErrorDocument 404 default
</Directory>

Note that when you specify an ErrorDocument that points to a remote URL (ie. anything with a method such as http in front of it), Apache HTTP Server will send a redirect to the client to tell it where to find the document, even if the document ends up being on the same server. This has several implications, the most important being that the client will not receive the original error status code, but instead will receive a redirect status code. This in turn can confuse web robots and other clients which try to determine if a URL is valid using the status code. In addition, if you use a remote URL in an ErrorDocument 401 , the client will not know to prompt the user for a password since it will not receive the 401 status code. Therefore, if you use an ErrorDocument 401 directive, then it must refer to a local document.

Microsoft Internet Explorer (MSIE) will by default ignore server-generated error messages when they are "too small" and substitute its own "friendly" error messages. The size threshold varies depending on the type of error, but in general, if you make your error document greater than 512 bytes, then MSIE will show the server-generated error rather than masking it. More information is available in Microsoft Knowledge Base article Q294807.

Although most error messages can be overridden, there are certain circumstances where the internal messages are used regardless of the setting of ErrorDocument . In particular, if a malformed request is detected, normal request processing will be immediately halted and the internal error message returned. This is necessary to guard against security problems caused by bad requests.

If you are using mod_proxy, you may wish to enable ProxyErrorOverride so that you can provide custom error messages on behalf of your Origin servers. If you don't enable ProxyErrorOverride, Apache httpd will not generate custom error documents for proxied content.

See also

• documentation of customizable responses