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<!--#set var="revision" value="\$Id: checklink.html,v 1.4 2002-10-27 08:26:40 ville Exp $"
--><!--#set var="date" value="\$Date: 2002-10-27 08:26:40 $"
--><!--#set var="title" value="W3C Link Checker documentation"
--><!--#set var="relroot" value="../"
--><!--#include virtual="../header.html" -->
<h1 id="skip">W3C Link Checker documentation</h1>
<ul>
<li><a href="#about">About this service</a></li>
<li><a href="#what">What it does</a></li>
<li><a href="#online">Use it online</a></li>
<li><a href="#install">Install it locally</a></li>
<li><a href="#csb">Comments, suggestions and bugs</a></li>
</ul>
<h2><a name="about" id="about">About this service</a></h2>
<p>
In order to check the validity of the technical reports that W3C
publishes, the Systems Team has developed a link checker.
</p>
<p>
A first version was developed in August 1998 by
<a href="http://www.w3.org/People/Renaud/">Renaud Bruyeron</a>.
Since it was lacking some functionalities,
<a href="http://www.w3.org/People/Hugo/">Hugo Haas</a>
rewrote it more or less from scratch in November 1999.
</p>
<p>
The source code is available publicly under the
<a href="http://www.w3.org/Consortium/Legal/copyright-software">W3C IPR
software notice</a> from
<a href="http://dev.w3.org/cvsweb/validator/httpd/cgi-bin/checklink.pl">CVS</a>.
</p>
<h2><a name="what" id="what">What it does</a></h2>
<p>
The link checker reads an HTML or XHTML document and extracts a list
of anchors and links.
</p>
<p>
It checks that no anchor is defined twice.
</p>
<p>
It then checks that all the links are dereferenceable, including
the fragments. It warns about HTTP redirects, including directory
redirects.
</p>
<p>
It can check recursively a part of a Web site.
</p>
<p>
There is a command-line version and a CGI version. They both
support HTTP basic authentication. This is achieved in the CGI version
by passing through the authorization information from the user browser
to the site tested.
</p>
<p>
The current version has proven to be stable. It could however be
improved:
</p>
<ul id="todo">
<li>
Currently, the URIs are extracted from a defined set of
elements and attributes, whereas the right thing to do would be to
parse the DTD and get the list of elements and attributes to extract
from from the DTD.
</li>
<li>
The program doesn't follow the
<a href="http://www.robotstxt.org/">Robot Exclusion Standard</a>.
</li>
<li>
HTTPS could be supported without much code change.
</li>
<li>
It would be cool to show the source where the error is like the
HTML validator does instead of just giving the line.
</li>
<li>
The link checker should do a GET request when the server replies
501 to a HEAD request.
</li>
<li>
A <code>Referer</code> header should be sent out.
</li>
<li>
Add XML and <a href="http://www.w3.org/XML/Linking">XLink</a> support.
</li>
<li>
Produce a report in <a href="http://www.w3.org/RDF/">RDF</a>.
</li>
<li>
Post annotations to the
<a href="http://www.w3.org/2001/Annotea/">Annotea</a>: when
the document was checked, tag links that are broken, etc.
</li>
<li>
Display an error when both the name and id attributes are used and
their values are different.
</li>
<li>
Probably other things that I haven't thought about.
</li>
</ul>
<p>
If you are interested in making the link checker understand XML,
please <a href="mailto:hugo@w3.org">contact me</a>.
</p>
<h2><a name="online" id="online">Use it online</a></h2>
<p>
There is an
<a href="<!--#echo var="relroot" -->checklink">online version</a>
of the link checker.
</p>
<p>
The number of documents that can be checked recursively is limited
and there is a delay between each document checked to avoid abuses.
</p>
<h2><a name="install" id="install">Install it locally</a></h2>
<p>
The link checker is written in Perl. It is one single file, but it
requires some CPAN modules.
</p>
<p>In order to install it:</p>
<ol>
<li>
Install <a href="http://www.perl.com/">Perl</a>.
</li>
<li>
You will need the following <a href="http://www.cpan.org/">CPAN</a>
distributions, as well as the distributions they possibly depend on.
Depending on your Perl version, you might already have some of
these installed. For an introduction to installing Perl modules,
see <a href="http://www.cpan.org/misc/cpan-faq.html#How_install_Perl_modules">The CPAN FAQ</a>.
<ul>
<li><a href="http://search.cpan.org/dist/libwww-perl/">libwww-perl</a> (version 5.60 or newer if you want HTTP/1.1 with <code>Keep-Alive</code>)</li>
<li><a href="http://search.cpan.org/dist/HTML-Parser/">HTML-Parser</a> (version 3.00 or newer)</li>
<li><a href="http://search.cpan.org/dist/CGI.pm/">CGI.pm</a></li>
<li><a href="http://search.cpan.org/dist/URI/">URI</a></li>
<li><a href="http://search.cpan.org/dist/Time-HiRes/">Time-HiRes</a></li>
<li><a href="http://search.cpan.org/dist/TermReadKey/">TermReadKey</a> (optional but recommended for all platforms; required for password input in command line mode for systems that don't have the <code>stty</code> command, eg. Windows)</li>
</ul>
</li>
<li>
Download the link checker from
<a href="http://dev.w3.org/cvsweb/validator/httpd/cgi-bin/checklink.pl">CVS</a>.
</li>
</ol>
<p>
Calling <code>checklink.pl</code> without any arguments runs the
CGI version, and running <code>checklink.pl --help</code> shows how to
use the command-line version.
</p>
<p>
If you want to enable the authentication capabilities with Apache,
have a look at
<a href="http://lists.w3.org/Archives/Public/www-validator/1999JulSep/0140.html">Steven Drake's hack</a>.
</p>
<h2><a name="csb" id="csb">Comments, suggestions and bugs</a></h2>
<p>
Please send comments, suggestions and bugs about the link checker
to the <a href="mailto:www-validator@w3.org?subject=checklink%3A%20">www-validator mailing list</a>
(<a href="http://lists.w3.org/Archives/Public/www-validator/">archives</a>),
with 'checklink' in the subject.
</p>
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