apache/docs/manual/custom-error.xml

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<manualpage metafile="custom-error.xml.meta">

  <title>Custom Error Responses</title>

  <summary>

    <p>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.</p>

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

    <p>Additionally, a set of values are provided, so
    that the error document can be customized further based on the
    values of these variables, using <a href="howto/ssi.html">Server
    Side Includes</a>. 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.</p>

  </summary>

  <section id="configuration"><title>Configuration</title>

    <p>Custom error documents are configured using the <directive
    module="core">ErrorDocument</directive> directive,
    which may be used in global,
    virtualhost, or directory context. It may be used in .htaccess files
    if <directive module="core">AllowOverride</directive> is set to
    FileInfo.</p>

    <highlight language="config">
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
    </highlight>

    <p>The syntax of the <code>ErrorDocument</code> directive is:</p>

    <highlight language="config">
      ErrorDocument &lt;3-digit-code&gt; &lt;action&gt;
    </highlight>

    <p>where the action will be treated as:</p>

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

    <p>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.</p>

  </section>

  <section id="variables"><title>Available Variables</title>

      <p>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.</p>

      <p>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.</p>

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

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

      <p><code>REDIRECT_</code> environment variables are created from
      the 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>.</p>

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

      <p><strong>None</strong> of these will be
      set if the <directive module="core">ErrorDocument</directive> target 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 id="custom"><title>Customizing Error Responses</title>

      <p>If you point your <code>ErrorDocument</code> 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.</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>

       <highlight language="perl">
...
print  "Content-type: text/html\n";
printf "Status: %s Condition Intercepted\n", $ENV{"REDIRECT_STATUS"};
...
      </highlight>

      <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 if the response contains <code>Location:</code>
      header (in order to issue a client-side redirect), the script
      <em>must</em> emit an appropriate <code>Status:</code> header
      (such as <code>302&nbsp;Found</code>). Otherwise the
      <code>Location:</code> header may have no effect.</p>

  </section>

  <section id="multi-lang"><title>Multi Language Custom Error Documents</title>

    <p>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
    <code>conf/extra</code> configuration directory that can be included
    to enable this feature.</p>

    <p>In your server configuration file, you'll see a line such as:</p>

    <highlight language="config">
# Multi-language error messages
#Include conf/extra/httpd-multilang-errordoc.conf
    </highlight>

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

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

    <p>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.</p>

    <p><module>mod_include</module> and <module>mod_negotiation</module>
    must be enabled to use this feature.</p>

 </section>

</manualpage>