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490 lines
20 KiB
HTML
490 lines
20 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>International Customized Server Error Messages</title>
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</head>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
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vlink="#000080" alink="#FF0000">
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<!--#include virtual="header.html" -->
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<h1 align="center">Using XSSI and <samp>ErrorDocument</samp> to
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configure customized international server error responses</h1>
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<h2>Index</h2>
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<ul>
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<li><a href="#intro">Introduction</a></li>
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<li><a href="#createdir">Creating an ErrorDocument
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directory</a></li>
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<li><a href="#docnames">Naming the individual error document
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files</a></li>
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<li><a href="#headfoot">The common header and footer
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files</a></li>
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<li><a href="#createdocs">Creating ErrorDocuments in
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different languages</a></li>
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<li><a href="#fallback">The fallback language</a></li>
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<li><a href="#proxy">Customizing Proxy Error
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Messages</a></li>
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<li><a href="#listings">HTML listing of the discussed
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example</a></li>
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</ul>
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<hr />
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<h2><a id="intro" name="intro">Introduction</a></h2>
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This document describes an easy way to provide your apache WWW
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server with a set of customized error messages which take
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advantage of <a href="../content-negotiation.html">Content
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Negotiation</a> and <a href="../mod/mod_include.html">eXtended
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Server Side Includes (XSSI)</a> to return error messages
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generated by the server in the client's native language. <br />
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<br />
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<p>By using XSSI, all <a
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href="../mod/core.html#errordocument">customized messages</a>
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can share a homogenous and consistent style and layout, and
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maintenance work (changing images, changing links) is kept to a
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minimum because all layout information can be kept in a single
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file.<br />
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Error documents can be shared across different servers, or
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even hosts, because all varying information is inserted at the
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time the error document is returned on behalf of a failed
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request.</p>
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<p>Content Negotiation then selects the appropriate language
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version of a particular error message text, honoring the
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language preferences passed in the client's request. (Users
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usually select their favorite languages in the preferences
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options menu of today's browsers). When an error document in
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the client's primary language version is unavailable, the
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secondary languages are tried or a default (fallback) version
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is used.</p>
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<p>You have full flexibility in designing your error documents
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to your personal taste (or your company's conventions). For
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demonstration purposes, we present a simple generic error
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document scheme. For this hypothetic server, we assume that all
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error messages...</p>
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<ul>
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<li>possibly are served by different virtual hosts (different
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host name, different IP address, or different port) on the
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server machine,</li>
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<li>show a predefined company logo in the right top of the
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message (selectable by virtual host),</li>
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<li>print the error title first, followed by an explanatory
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text and (depending on the error context) help on how to
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resolve the error,</li>
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<li>have some kind of standardized background image,</li>
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<li>display an apache logo and a feedback email address at
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the bottom of the error message.</li>
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</ul>
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<br />
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<br />
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<p>An example of a "document not found" message for a german
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client might look like this:<br />
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<img src="../images/custom_errordocs.gif"
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alt="[Needs graphics capability to display]" /><br />
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All links in the document as well as links to the server's
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administrator mail address, and even the name and port of the
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serving virtual host are inserted in the error document at
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"run-time", <em>i.e.</em>, when the error actually occurs.</p>
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<h2><a id="createdir" name="createdir">Creating an
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ErrorDocument directory</a></h2>
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For this concept to work as easily as possible, we must take
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advantage of as much server support as we can get:
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<ol>
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<li>By defining the <a
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href="../mod/core.html#options">MultiViews option</a>, we
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enable the language selection of the most appropriate
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language alternative (content negotiation).</li>
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<li>By setting the <a
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href="../mod/mod_negotiation.html#languagepriority">LanguagePriority</a>
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directive we define a set of default fallback languages in
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the situation where the client's browser did not express any
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preference at all.</li>
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<li>By enabling <a href="../mod/mod_include.html">Server Side
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Includes</a> (and disallowing execution of cgi scripts for
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security reasons), we allow the server to include building
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blocks of the error message, and to substitute the value of
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certain environment variables into the generated document
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(dynamic HTML) or even to conditionally include or omit parts
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of the text.</li>
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<li>The <a
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href="../mod/mod_mime.html#addhandler">AddHandler</a> and <a
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href="../mod/mod_mime.html#addtype">AddType</a> directives
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are useful for automatically XSSI-expanding all files with a
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<samp>.shtml</samp> suffix to <em>text/html</em>.</li>
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<li>By using the <a
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href="../mod/mod_alias.html#alias">Alias</a> directive, we
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keep the error document directory outside of the document
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tree because it can be regarded more as a server part than
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part of the document tree.</li>
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<li>The <a
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href="../mod/core.html#directory"><Directory></a>-Block
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restricts these "special" settings to the error document
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directory and avoids an impact on any of the settings for the
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regular document tree.</li>
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<li>For each of the error codes to be handled (see RFC2068
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for an exact description of each error code, or look at
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<code>src/main/http_protocol.c</code> if you wish to see
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apache's standard messages), an <a
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href="../mod/core.html#errordocument">ErrorDocument</a> in
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the aliased <samp>/errordocs</samp> directory is defined.
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Note that we only define the basename of the document here
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because the MultiViews option will select the best candidate
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based on the language suffixes and the client's preferences.
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Any error situation with an error code <em>not</em> handled
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by a custom document will be dealt with by the server in the
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standard way (<em>i.e.</em>, a plain error message in
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english).</li>
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<li>Finally, the <a
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href="../mod/core.html#allowoverride">AllowOverride</a>
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directive tells apache that it is not necessary to look for a
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.htaccess file in the /errordocs directory: a minor speed
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optimization.</li>
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</ol>
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The resulting <samp>httpd.conf</samp> configuration would then
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look similar to this: <small>(Note that you can define your own
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error messages using this method for only part of the document
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tree, e.g., a /~user/ subtree. In this case, the configuration
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could as well be put into the .htaccess file at the root of the
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subtree, and the <Directory> and </Directory>
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directives -but not the contained directives- must be
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omitted.)</small>
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<pre>
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LanguagePriority en fr de
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Alias /errordocs /usr/local/apache/errordocs
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<Directory /usr/local/apache/errordocs>
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AllowOverride none
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Options MultiViews IncludesNoExec FollowSymLinks
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AddType text/html .shtml
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<FilesMatch "\.shtml[.$]">
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SetOutputFilter INCLUDES
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</FilesMatch>
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</Directory>
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# "400 Bad Request",
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ErrorDocument 400 /errordocs/400
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# "401 Authorization Required",
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ErrorDocument 401 /errordocs/401
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# "403 Forbidden",
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ErrorDocument 403 /errordocs/403
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# "404 Not Found",
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ErrorDocument 404 /errordocs/404
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# "500 Internal Server Error",
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ErrorDocument 500 /errordocs/500
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</pre>
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The directory for the error messages (here:
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<samp>/usr/local/apache/errordocs/</samp>) must then be created
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with the appropriate permissions (readable and executable by
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the server uid or gid, only writable for the administrator).
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<h3><a id="docnames" name="docnames">Naming the individual
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error document files</a></h3>
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By defining the <samp>MultiViews</samp> option, the server was
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told to automatically scan the directory for matching variants
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(looking at language and content type suffixes) when a
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requested document was not found. In the configuration, we
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defined the names for the error documents to be just their
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error number (without any suffix).
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<p>The names of the individual error documents are now
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determined like this (I'm using 403 as an example, think of it
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as a placeholder for any of the configured error
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documents):</p>
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<ul>
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<li>No file errordocs/403 should exist. Otherwise, it would
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be found and served (with the DefaultType, usually
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text/plain), all negotiation would be bypassed.</li>
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<li>For each language for which we have an internationalized
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version (note that this need not be the same set of languages
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for each error code - you can get by with a single language
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version until you actually <em>have</em> translated
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versions), a document
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<samp>errordocs/403.shtml.<em>lang</em></samp> is created and
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filled with the error text in that language (<a
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href="#createdocs">see below</a>).</li>
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<li>One fallback document called
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<samp>errordocs/403.shtml</samp> is created, usually by
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creating a symlink to the default language variant (<a
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href="#fallback">see below</a>).</li>
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</ul>
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<h3><a id="headfoot" name="headfoot">The common header and
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footer files</a></h3>
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By putting as much layout information in two special "include
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files", the error documents can be reduced to a bare minimum.
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<p>One of these layout files defines the HTML document header
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and a configurable list of paths to the icons to be shown in
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the resulting error document. These paths are exported as a set
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of XSSI environment variables and are later evaluated by the
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"footer" special file. The title of the current error (which is
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put into the TITLE tag and an H1 header) is simply passed in
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from the main error document in a variable called
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<code>title</code>.<br />
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<strong>By changing this file, the layout of all generated
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error messages can be changed in a second.</strong> (By
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exploiting the features of XSSI, you can easily define
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different layouts based on the current virtual host, or even
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based on the client's domain name).</p>
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<p>The second layout file describes the footer to be displayed
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at the bottom of every error message. In this example, it shows
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an apache logo, the current server time, the server version
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string and adds a mail reference to the site's webmaster.</p>
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<p>For simplicity, the header file is simply called
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<code>head.shtml</code> because it contains server-parsed
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content but no language specific information. The footer file
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exists once for each language translation, plus a symlink for
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the default language.</p>
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<p><strong>Example:</strong> for English, French and German
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versions (default english)<br />
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<code>foot.shtml.en</code>,<br />
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<code>foot.shtml.fr</code>,<br />
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<code>foot.shtml.de</code>,<br />
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<code>foot.shtml</code> symlink to
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<code>foot.shtml.en</code></p>
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<p>Both files are included into the error document by using the
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directives <code><!--#include virtual="head" --></code>
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and <code><!--#include virtual="foot" --></code>
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respectively: the rest of the magic occurs in mod_negotiation
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and in mod_include.</p>
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<p>See <a href="#listings">the listings below</a> to see an
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actual HTML implementation of the discussed example.</p>
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<h3><a id="createdocs" name="createdocs">Creating
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ErrorDocuments in different languages</a></h3>
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After all this preparation work, little remains to be said
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about the actual documents. They all share a simple common
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structure:
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<pre>
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<!--#set var="title" value="<em>error description title</em>" -->
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<!--#include virtual="head" -->
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<em>explanatory error text</em>
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<!--#include virtual="foot" -->
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</pre>
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In the <a href="#listings">listings section</a>, you can see an
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example of a [400 Bad Request] error document. Documents as
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simple as that certainly cause no problems to translate or
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expand.
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<h3><a id="fallback" name="fallback">The fallback
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language</a></h3>
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Do we need a special handling for languages other than those we
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have translations for? We did set the LanguagePriority, didn't
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we?!
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<p>Well, the LanguagePriority directive is for the case where
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the client does not express any language priority at all. But
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what happens in the situation where the client wants one of the
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languages we do not have, and none of those we do have?</p>
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<p>Without doing anything, the Apache server will usually
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return a [406 no acceptable variant] error, listing the choices
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from which the client may select. But we're in an error message
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already, and important error information might get lost when
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the client had to choose a language representation first.</p>
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<p>So, in this situation it appears to be easier to define a
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fallback language (by copying or linking, <em>e.g.</em>, the
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english version to a language-less version). Because the
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negotiation algorithm prefers "more specialized" variants over
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"more generic" variants, these generic alternatives will only
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be chosen when the normal negotiation did not succeed.</p>
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<p>A simple shell script to do it (execute within the
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errordocs/ dir):</p>
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<pre>
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for f in *.shtml.en
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do
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ln -s $f `basename $f .en`
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done
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</pre>
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<h2><a id="proxy" name="proxy">Customizing Proxy Error
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Messages</a></h2>
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<p>As of Apache-1.3, it is possible to use the
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<code>ErrorDocument</code> mechanism for proxy error messages
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as well (previous versions always returned fixed predefined
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error messages).</p>
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<p>Most proxy errors return an error code of [500 Internal
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Server Error]. To find out whether a particular error document
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was invoked on behalf of a proxy error or because of some other
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server error, and what the reason for the failure was, you can
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check the contents of the new <code>ERROR_NOTES</code> CGI
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environment variable: if invoked for a proxy error, this
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variable will contain the actual proxy error message text in
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HTML form.</p>
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<p>The following excerpt demonstrates how to exploit the
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<code>ERROR_NOTES</code> variable within an error document:</p>
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<pre>
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<!--#if expr="$REDIRECT_ERROR_NOTES = ''" -->
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<p>
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The server encountered an unexpected condition
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which prevented it from fulfilling the request.
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</p>
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<p>
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<A HREF="mailto:<!--#echo var="SERVER_ADMIN" -->"
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SUBJECT="Error message [<!--#echo var="REDIRECT_STATUS" -->] <!--#echo var="title" --> for <!--#echo var="REQUEST_URI" -->">
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Please forward this error screen to <!--#echo var="SERVER_NAME" -->'s
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WebMaster</A>; it includes useful debugging information about
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the Request which caused the error.
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<pre><!--#printenv --></pre>
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</p>
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<!--#else -->
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<!--#echo var="REDIRECT_ERROR_NOTES" -->
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<!--#endif -->
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</pre>
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<h2><a id="listings" name="listings">HTML listing of the
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discussed example</a></h2>
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So, to summarize our example, here's the complete listing of
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the <samp>400.shtml.en</samp> document. You will notice that it
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contains almost nothing but the error text (with conditional
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additions). Starting with this example, you will find it easy
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to add more error documents, or to translate the error
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documents to different languages.
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<hr />
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<pre>
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<!--#set var="title" value="Bad Request"
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--><!--#include virtual="head" --><P>
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Your browser sent a request that this server could not understand:
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<BLOCKQUOTE>
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<STRONG><!--#echo var="REQUEST_URI" --></STRONG>
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</BLOCKQUOTE>
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The request could not be understood by the server due to malformed
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syntax. The client should not repeat the request without
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modifications.
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</P>
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<P>
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<!--#if expr="$HTTP_REFERER != ''" -->
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Please inform the owner of
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<A HREF="<!--#echo var="HTTP_REFERER" -->">the referring page</A> about
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the malformed link.
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<!--#else -->
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Please check your request for typing errors and retry.
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<!--#endif -->
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</P>
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<!--#include virtual="foot" -->
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</pre>
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<hr />
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Here is the complete <samp>head.shtml</samp> file (the funny
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line breaks avoid empty lines in the document after XSSI
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processing). Note the configuration section at top. That's
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where you configure the images and logos as well as the apache
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documentation directory. Look how this file displays two
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different logos depending on the content of the virtual host
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name ($SERVER_NAME), and that an animated apache logo is shown
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if the browser appears to support it (the latter requires
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server configuration lines of the form <br />
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<code>BrowserMatch "^Mozilla/[2-4]" anigif</code><br />
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for browser types which support animated GIFs).
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<hr />
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<pre>
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<!--#if expr="$SERVER_NAME = /.*\.mycompany\.com/"
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--><!--#set var="IMG_CorpLogo"
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value="http://$SERVER_NAME:$SERVER_PORT/errordocs/CorpLogo.gif"
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--><!--#set var="ALT_CorpLogo" value="Powered by Linux!"
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--><!--#else
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--><!--#set var="IMG_CorpLogo"
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value="http://$SERVER_NAME:$SERVER_PORT/errordocs/PrivLogo.gif"
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--><!--#set var="ALT_CorpLogo" value="Powered by Linux!"
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--><!--#endif
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--><!--#set var="IMG_BgImage" value="http://$SERVER_NAME:$SERVER_PORT/errordocs/BgImage.gif"
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--><!--#set var="DOC_Apache" value="http://$SERVER_NAME:$SERVER_PORT/Apache/"
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--><!--#if expr="$anigif"
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--><!--#set var="IMG_Apache" value="http://$SERVER_NAME:$SERVER_PORT/icons/apache_anim.gif"
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--><!--#else
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--><!--#set var="IMG_Apache" value="http://$SERVER_NAME:$SERVER_PORT/icons/apache_pb.gif"
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--><!--#endif
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--><!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<HTML>
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<HEAD>
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<TITLE>
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[<!--#echo var="REDIRECT_STATUS" -->] <!--#echo var="title" -->
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</TITLE>
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</HEAD>
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<BODY BGCOLOR="white" BACKGROUND="<!--#echo var="IMG_BgImage" -->"><UL>
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<H1 ALIGN="center">
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[<!--#echo var="REDIRECT_STATUS" -->] <!--#echo var="title" -->
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<IMG SRC="<!--#echo var="IMG_CorpLogo" -->"
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ALT="<!--#echo var="ALT_CorpLogo" -->" ALIGN=right>
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</H1>
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<HR><!-- ======================================================== -->
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<DIV>
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</pre>
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<hr />
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and this is the <samp>foot.shtml.en</samp> file:
|
|
<hr />
|
|
<pre>
|
|
</DIV>
|
|
<HR>
|
|
<DIV ALIGN="right"><SMALL><SUP>Local Server time:
|
|
<!--#echo var="DATE_LOCAL" -->
|
|
</SUP></SMALL></DIV>
|
|
<DIV ALIGN="center">
|
|
<A HREF="<!--#echo var="DOC_Apache" -->">
|
|
<IMG SRC="<!--#echo var="IMG_Apache" -->" BORDER=0 ALIGN="bottom"
|
|
ALT="Powered by <!--#echo var="SERVER_SOFTWARE" -->"></A><BR>
|
|
<SMALL><SUP><!--#set var="var"
|
|
value="Powered by $SERVER_SOFTWARE -- File last modified on $LAST_MODIFIED"
|
|
--><!--#echo var="var" --></SUP></SMALL>
|
|
</DIV>
|
|
<ADDRESS>If the indicated error looks like a misconfiguration, please inform
|
|
<A HREF="mailto:<!--#echo var="SERVER_ADMIN" -->"
|
|
SUBJECT="Feedback about Error message [<!--#echo var="REDIRECT_STATUS"
|
|
-->] <!--#echo var="title" -->, req=<!--#echo var="REQUEST_URI" -->">
|
|
<!--#echo var="SERVER_NAME" -->'s WebMaster</A>.
|
|
</ADDRESS>
|
|
</UL></BODY>
|
|
</HTML>
|
|
</pre>
|
|
<hr />
|
|
|
|
<h3>More welcome!</h3>
|
|
If you have tips to contribute, send mail to <a
|
|
href="mailto:martin@apache.org">martin@apache.org</a>
|
|
<!--#include virtual="footer.html" -->
|
|
</body>
|
|
</html>
|
|
|