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This document describes an easy way to provide your apache WWW server with a set of customized error messages which take advantage of Content Negotiation and eXtended Server Side Includes (XSSI) to return error messages generated by the server in the client's native language.
By using XSSI, all
customized messages
can share a homogenous and consistent style and layout, and maintenance work
(changing images, changing links) is kept to a minimum because all layout
information can be kept in a single file.
Error documents can be shared across different servers, or even hosts,
because all varying information is inserted at the time the error document
is returned on behalf of a failed request.
Content Negotiation then selects the appropriate language version of a particular error message text, honoring the language preferences passed in the client's request. (Users usually select their favorite languages in the preferences options menu of today's browsers). When an error document in the client's primary language version is unavailable, the secondary languages are tried or a default (fallback) version is used.
You have full flexibility in designing your error documents to your personal taste (or your company's conventions). For demonstration purposes, we present a simple generic error document scheme. For this hypothetic server, we assume that all error messages...
An example of a "document not found" message for a german client might
look like this:
All links in the document as well as links to the server's administrator
mail address, and even the name and port of the serving virtual host
are inserted in the error document at "run-time", i.e., when the error
actually occurs.
src/main/http_protocol.c
if you wish to see apache's standard messages), an
ErrorDocument
in the aliased /errordocs directory is defined.
Note that we only define the basename of the document here
because the MultiViews option will select the best candidate
based on the language suffixes and the client's preferences.
Any error situation with an error code not handled by a
custom document will be dealt with by the server in the standard way
(i.e., a plain error message in english).
LanguagePriority en fr de Alias /errordocs /usr/local/apache/errordocs <Directory /usr/local/apache/errordocs> AllowOverride none Options MultiViews IncludesNoExec FollowSymLinks AddType text/html .shtml AddHandler server-parsed .shtml </Directory> # "400 Bad Request", ErrorDocument 400 /errordocs/400 # "401 Authorization Required", ErrorDocument 401 /errordocs/401 # "403 Forbidden", ErrorDocument 403 /errordocs/403 # "404 Not Found", ErrorDocument 404 /errordocs/404 # "500 Internal Server Error", ErrorDocument 500 /errordocs/500The directory for the error messages (here: /usr/local/apache/errordocs/) must then be created with the appropriate permissions (readable and executable by the server uid or gid, only writable for the administrator).
The names of the individual error documents are now determined like this (I'm using 403 as an example, think of it as a placeholder for any of the configured error documents):
One of these layout files defines the HTML document header
and a configurable list of paths to the icons to be shown in the resulting
error document. These paths are exported as a set of XSSI environment
variables and are later evaluated by the "footer" special file.
The title of the current error (which is
put into the TITLE tag and an H1 header) is simply passed in from the main
error document in a variable called title.
By changing this file, the layout of all generated error
messages can be changed in a second.
(By exploiting the features of XSSI, you can easily define different
layouts based on the current virtual host, or even based on the
client's domain name).
The second layout file describes the footer to be displayed at the bottom of every error message. In this example, it shows an apache logo, the current server time, the server version string and adds a mail reference to the site's webmaster.
For simplicity, the header file is simply called head.shtml
because it contains server-parsed content but no language specific
information. The footer file exists once for each language translation,
plus a symlink for the default language.
Example: for English, French and German versions
(default english)
foot.shtml.en,
foot.shtml.fr,
foot.shtml.de,
foot.shtml symlink to foot.shtml.en
Both files are included into the error document by using the
directives <!--#include virtual="head" -->
and <!--#include virtual="foot" -->
respectively: the rest of the magic occurs in mod_negotiation and
in mod_include.
See the listings below to see an actual HTML implementation of the discussed example.
<!--#set var="title" value="error description title" --> <!--#include virtual="head" --> explanatory error text <!--#include virtual="foot" -->In the listings section, you can see an example of a [400 Bad Request] error document. Documents as simple as that certainly cause no problems to translate or expand.
Well, the LanguagePriority directive is for the case where the client does not express any language priority at all. But what happens in the situation where the client wants one of the languages we do not have, and none of those we do have?
Without doing anything, the Apache server will usually return a [406 no acceptable variant] error, listing the choices from which the client may select. But we're in an error message already, and important error information might get lost when the client had to choose a language representation first.
So, in this situation it appears to be easier to define a fallback language (by copying or linking, e.g., the english version to a language-less version). Because the negotiation algorithm prefers "more specialized" variants over "more generic" variants, these generic alternatives will only be chosen when the normal negotiation did not succeed.
A simple shell script to do it (execute within the errordocs/ dir):
for f in *.shtml.en
do
ln -s $f `basename $f .en`
done
As of Apache-1.3, it is possible to use the ErrorDocument
mechanism for proxy error messages as well (previous versions always
returned fixed predefined error messages).
Most proxy errors return an error code of [500 Internal Server Error].
To find out whether a particular error document was invoked on behalf
of a proxy error or because of some other server error, and what the reason
for the failure was, you can check the contents of the new
ERROR_NOTES CGI environment variable:
if invoked for a proxy error, this variable will contain the actual proxy
error message text in HTML form.
The following excerpt demonstrates how to exploit the ERROR_NOTES
variable within an error document:
<!--#if expr="$REDIRECT_ERROR_NOTES = ''" -->
<p>
The server encountered an unexpected condition
which prevented it from fulfilling the request.
</p>
<p>
<A HREF="mailto:<!--#echo var="SERVER_ADMIN" -->"
SUBJECT="Error message [<!--#echo var="REDIRECT_STATUS" -->] <!--#echo var="title" --> for <!--#echo var="REQUEST_URI" -->">
Please forward this error screen to <!--#echo var="SERVER_NAME" -->'s
WebMaster</A>; it includes useful debugging information about
the Request which caused the error.
<pre><!--#printenv --></pre>
</p>
<!--#else -->
<!--#echo var="REDIRECT_ERROR_NOTES" -->
<!--#endif -->
<!--#set var="title" value="Bad Request"
--><!--#include virtual="head" --><P>
Your browser sent a request that this server could not understand:
<BLOCKQUOTE>
<STRONG><!--#echo var="REQUEST_URI" --></STRONG>
</BLOCKQUOTE>
The request could not be understood by the server due to malformed
syntax. The client should not repeat the request without
modifications.
</P>
<P>
<!--#if expr="$HTTP_REFERER != ''" -->
Please inform the owner of
<A HREF="<!--#echo var="HTTP_REFERER" -->">the referring page</A> about
the malformed link.
<!--#else -->
Please check your request for typing errors and retry.
<!--#endif -->
</P>
<!--#include virtual="foot" -->
BrowserMatch "^Mozilla/[2-4]" anigif
<!--#if expr="$SERVER_NAME = /.*\.mycompany\.com/"
--><!--#set var="IMG_CorpLogo"
value="http://$SERVER_NAME:$SERVER_PORT/errordocs/CorpLogo.gif"
--><!--#set var="ALT_CorpLogo" value="Powered by Linux!"
--><!--#else
--><!--#set var="IMG_CorpLogo"
value="http://$SERVER_NAME:$SERVER_PORT/errordocs/PrivLogo.gif"
--><!--#set var="ALT_CorpLogo" value="Powered by Linux!"
--><!--#endif
--><!--#set var="IMG_BgImage" value="http://$SERVER_NAME:$SERVER_PORT/errordocs/BgImage.gif"
--><!--#set var="DOC_Apache" value="http://$SERVER_NAME:$SERVER_PORT/Apache/"
--><!--#if expr="$anigif"
--><!--#set var="IMG_Apache" value="http://$SERVER_NAME:$SERVER_PORT/icons/apache_anim.gif"
--><!--#else
--><!--#set var="IMG_Apache" value="http://$SERVER_NAME:$SERVER_PORT/icons/apache_pb.gif"
--><!--#endif
--><!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<HTML>
<HEAD>
<TITLE>
[<!--#echo var="REDIRECT_STATUS" -->] <!--#echo var="title" -->
</TITLE>
</HEAD>
<BODY BGCOLOR="white" BACKGROUND="<!--#echo var="IMG_BgImage" -->"><UL>
<H1 ALIGN="center">
[<!--#echo var="REDIRECT_STATUS" -->] <!--#echo var="title" -->
<IMG SRC="<!--#echo var="IMG_CorpLogo" -->"
ALT="<!--#echo var="ALT_CorpLogo" -->" ALIGN=right>
</H1>
<HR><!-- ======================================================== -->
<DIV>
</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>
