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434 lines
19 KiB
HTML
434 lines
19 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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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
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#000080"
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ALINK="#FF0000"
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>
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<!--#include virtual="header.html" -->
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<H1 ALIGN="CENTER">Using XSSI and <SAMP>ErrorDocument</SAMP> to configure
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customized international server error responses</H1>
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<P>
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<H2>Index</H2>
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<UL>
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<LI><A HREF="#intro">Introduction</A>
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<LI><A HREF="#createdir">Creating an ErrorDocument directory</A>
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<LI><A HREF="#docnames">Naming the individual error document files</A>
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<LI><A HREF="#headfoot">The common header and footer files</A>
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<LI><A HREF="#createdocs">Creating ErrorDocuments in different languages</A>
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<LI><A HREF="#fallback">The fallback language</A>
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<LI><A HREF="#proxy">Customizing Proxy Error Messages</A>
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<LI><A HREF="#listings">HTML listing of the discussed example</A>
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</UL>
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<HR>
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<H2><A NAME="intro">Introduction</A></H2>
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This document describes an easy way to provide your apache WWW server
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with a set of customized error messages which take advantage of
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<A HREF="../content-negotiation.html">Content Negotiation</A>
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and <A HREF="../mod/mod_include.html">eXtended Server Side Includes (XSSI)</A>
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to return error messages generated by the server in the client's
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native language.
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</P>
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<P>
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By using XSSI, all
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<A HREF="../mod/core.html#errordocument">customized messages</A>
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can share a homogenous and consistent style and layout, and maintenance work
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(changing images, changing links) is kept to a minimum because all layout
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information can be kept in a single file.<BR>
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Error documents can be shared across different servers, or even hosts,
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because all varying information is inserted at the time the error document
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is returned on behalf of a failed request.
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</P>
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<P>
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Content Negotiation then selects the appropriate language version of a
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particular error message text, honoring the language preferences passed
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in the client's request. (Users usually select their favorite languages
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in the preferences options menu of today's browsers). When an error
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document in the client's primary language version is unavailable, the
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secondary languages are tried or a default (fallback) version is used.
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</P>
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<P>
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You have full flexibility in designing your error documents to
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your personal taste (or your company's conventions). For demonstration
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purposes, we present a simple generic error document scheme.
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For this hypothetic server, we assume that all error messages...
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<UL>
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<LI>possibly are served by different virtual hosts (different host name,
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different IP address, or different port) on the server machine,
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<LI>show a predefined company logo in the right top of the message
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(selectable by virtual host),
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<LI>print the error title first, followed by an explanatory text and
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(depending on the error context) help on how to resolve the error,
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<LI>have some kind of standardized background image,
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<LI>display an apache logo and a feedback email address at the bottom
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of the error message.
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</UL>
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</P>
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<P>
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An example of a "document not found" message for a german client might
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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 administrator
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mail address, and even the name and port of the serving virtual host
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are inserted in the error document at "run-time", <EM>i.e.</EM>, when the error
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actually occurs.
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</P>
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<H2><A NAME="createdir">Creating an ErrorDocument directory</A></H2>
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For this concept to work as easily as possible, we must take advantage
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of as much server support as we can get:
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<OL>
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<LI>By defining the <A HREF="../mod/core.html#options">MultiViews option</A>,
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we enable the language selection of the most appropriate language
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alternative (content negotiation).
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<LI>By setting the
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<A HREF="../mod/mod_negotiation.html#languagepriority"
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>LanguagePriority</A>
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directive we define a set of default fallback languages in the situation
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where the client's browser did not express any preference at all.
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<LI>By enabling <A HREF="../mod/mod_include.html">Server Side Includes</A>
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(and disallowing execution of cgi scripts for security reasons),
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we allow the server to include building blocks of the error message,
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and to substitute the value of certain environment variables into the
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generated document (dynamic HTML) or even to conditionally include
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or omit parts of the text.
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<LI>The <A HREF="../mod/mod_mime.html#addhandler">AddHandler</A> and
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<A HREF="../mod/mod_mime.html#addtype">AddType</A> directives are useful
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for automatically XSSI-expanding all files with a <SAMP>.shtml</SAMP>
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suffix to <EM>text/html</EM>.
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<LI>By using the <A HREF="../mod/mod_alias.html#alias">Alias</A> directive,
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we keep the error document directory outside of the document tree
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because it can be regarded more as a server part than part of
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the document tree.
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<LI>The <A HREF="../mod/core.html#directory"><Directory></A>-Block
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restricts these "special" settings to the error document directory
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and avoids an impact on any of the settings for the regular document tree.
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<LI>For each of the error codes to be handled (see RFC2068 for an exact
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description of each error code, or look at
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<CODE>src/main/http_protocol.c</CODE>
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if you wish to see apache's standard messages), an
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<A HREF="../mod/core.html#errordocument">ErrorDocument</A>
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in 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 by a
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custom document will be dealt with by the server in the standard way
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(<EM>i.e.</EM>, a plain error message in english).
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<LI>Finally, the <A HREF="../mod/core.html#allowoverride">AllowOverride</A>
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directive tells apache that it is not necessary to look for
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a .htaccess file in the /errordocs directory: a minor speed
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optimization.
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</OL>
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The resulting <SAMP>httpd.conf</SAMP> configuration would then look
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similar to this: <SMALL>(Note that you can define your own error
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messages using this method for only part of the document tree,
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e.g., a /~user/ subtree. In this case, the configuration could as well
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be put into the .htaccess file at the root of the subtree, and
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the <Directory> and </Directory> directives -but not
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the contained directives- must be 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 with the
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appropriate permissions (readable and executable by the server uid or gid,
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only writable for the administrator).
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<H3><A NAME="docnames">Naming the individual error document files</A></H3>
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By defining the <SAMP>MultiViews</SAMP> option, the server was told to
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automatically scan the directory for matching variants (looking at language
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and content type suffixes) when a requested document was not found.
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In the configuration, we defined the names for the error documents to be
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just their error number (without any suffix).
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<P>
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The names of the individual error documents are now determined like this
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(I'm using 403 as an example, think of it as a placeholder for any of
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the configured error documents):
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<UL>
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<LI>No file errordocs/403 should exist. Otherwise, it would be found and
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served (with the DefaultType, usually text/plain), all negotiation
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would be bypassed.
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<LI>For each language for which we have an internationalized version
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(note that this need not be the same set of languages for each
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error code - you can get by with a single language version until
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you actually <EM>have</EM> translated 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 HREF="#createdocs">see
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below</A>).
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<LI>One fallback document called <SAMP>errordocs/403.shtml</SAMP> is
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created, usually by creating a symlink to the default language
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variant (<A HREF="#fallback">see below</A>).
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</UL>
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<H3><A NAME="headfoot">The common header and footer files</A></H3>
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By putting as much layout information in two special "include files",
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the error documents can be reduced to a bare minimum.
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<P>
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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 the resulting
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error document. These paths are exported as a set of XSSI environment
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variables and are later evaluated by the "footer" special file.
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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 from the main
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error document in a variable called <CODE>title</CODE>.<BR>
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<STRONG>By changing this file, the layout of all generated error
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messages can be changed in a second.</STRONG>
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(By exploiting the features of XSSI, you can easily define different
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layouts based on the current virtual host, or even based on the
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client's domain name).
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<P>
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The second layout file describes the footer to be displayed at the bottom
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of every error message. In this example, it shows an apache logo, the current
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server time, the server version string and adds a mail reference to the
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site's webmaster.
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<P>
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For simplicity, the header file is simply called <CODE>head.shtml</CODE>
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because it contains server-parsed content but no language specific
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information. The footer file exists once for each language translation,
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plus a symlink for the default language.<P>
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<STRONG>Example:</STRONG> for English, French and German versions
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(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 <CODE>foot.shtml.en</CODE><P>
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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 and
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in mod_include.
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<P>
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See <A HREF="#listings">the listings below</A> to see an actual HTML
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implementation of the discussed example.
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<H3><A NAME="createdocs">Creating ErrorDocuments in different languages</A>
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</H3>
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After all this preparation work, little remains to be said about the
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actual documents. They all share a simple common 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 example
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of a [400 Bad Request] error document. Documents as simple as that
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certainly cause no problems to translate or expand.
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<H3><A NAME="fallback">The fallback language</A></H3>
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Do we need a special handling for languages other than those we have
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translations for? We did set the LanguagePriority, didn't we?!
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<P>
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Well, the LanguagePriority directive is for the case where the client does
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not express any language priority at all. But what
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happens in the situation where the client wants one
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of the languages we do not have, and none of those we do have?
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<P>
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Without doing anything, the Apache server will usually return a
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[406 no acceptable variant] error, listing the choices from which the client
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may select. But we're in an error message already, and important error
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information might get lost when the client had to choose a language
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representation first.
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<P>
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So, in this situation it appears to be easier to define a fallback language
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(by copying or linking, <EM>e.g.</EM>, the english version to a language-less version).
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Because the negotiation algorithm prefers "more specialized" variants over
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"more generic" variants, these generic alternatives will only be chosen
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when the normal negotiation did not succeed.
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<P>
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A simple shell script to do it (execute within the errordocs/ dir):
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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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<P>
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</P>
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<H2><A NAME="proxy">Customizing Proxy Error Messages</A></H2>
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<P>
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As of Apache-1.3, it is possible to use the <CODE>ErrorDocument</CODE>
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mechanism for proxy error messages as well (previous versions always
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returned fixed predefined error messages).
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</P>
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<P>
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Most proxy errors return an error code of [500 Internal Server Error].
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To find out whether a particular error document was invoked on behalf
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of a proxy error or because of some other server error, and what the reason
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for the failure was, you can check the contents of the new
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<CODE>ERROR_NOTES</CODE> CGI environment variable:
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if invoked for a proxy error, this variable will contain the actual proxy
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error message text in HTML form.
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</P>
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<P>
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The following excerpt demonstrates how to exploit the <CODE>ERROR_NOTES</CODE>
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variable within an error document:
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</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 NAME="listings">HTML listing of the discussed example</A></H2>
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So, to summarize our example, here's the complete listing of the
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<SAMP>400.shtml.en</SAMP> document. You will notice that it contains
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almost nothing but the error text (with conditional additions).
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Starting with this example, you will find it easy to add more error
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documents, or to translate the error documents to different languages.
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<HR><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="$HTTP_REFERER != ''" -->
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Please inform the owner of
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<A HREF="<!--#echo var="HTTP_REFERER" -->">the referring page</A> about
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the malformed link.
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<!--#else -->
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Please check your request for typing errors and retry.
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<!--#endif -->
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</P>
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<!--#include virtual="foot" -->
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</PRE><HR>
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Here is the complete <SAMP>head.shtml</SAMP> file (the funny line
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breaks avoid empty lines in the document after XSSI processing). Note the
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configuration section at top. That's where you configure the images and logos
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as well as the apache documentation directory. Look how this file displays
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two different logos depending on the content of the virtual host name
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($SERVER_NAME), and that an animated apache logo is shown if the browser
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appears to support it (the latter requires server configuration lines
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of the form <BR><CODE>BrowserMatch "^Mozilla/[2-4]" anigif</CODE><BR>
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for browser types which support animated GIFs).
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<HR><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><HR>
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and this is the <SAMP>foot.shtml.en</SAMP> file:
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<HR><PRE>
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</DIV>
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<HR>
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<DIV ALIGN="right"><SMALL><SUP>Local Server time:
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<!--#echo var="DATE_LOCAL" -->
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</SUP></SMALL></DIV>
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<DIV ALIGN="center">
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<A HREF="<!--#echo var="DOC_Apache" -->">
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<IMG SRC="<!--#echo var="IMG_Apache" -->" BORDER=0 ALIGN="bottom"
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ALT="Powered by <!--#echo var="SERVER_SOFTWARE" -->"></A><BR>
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<SMALL><SUP><!--#set var="var"
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value="Powered by $SERVER_SOFTWARE -- File last modified on $LAST_MODIFIED"
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--><!--#echo var="var" --></SUP></SMALL>
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</DIV>
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<ADDRESS>If the indicated error looks like a misconfiguration, please inform
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<A HREF="mailto:<!--#echo var="SERVER_ADMIN" -->"
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SUBJECT="Feedback about Error message [<!--#echo var="REDIRECT_STATUS"
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-->] <!--#echo var="title" -->, req=<!--#echo var="REQUEST_URI" -->">
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<!--#echo var="SERVER_NAME" -->'s WebMaster</A>.
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</ADDRESS>
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</UL></BODY>
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</HTML>
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</PRE><HR>
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<H3>More welcome!</H3>
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If you have tips to contribute, send mail to <A
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