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<title>W3C Link Checker Documentation</title>
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<div id="banner"><h1 id="title"><a href="http://www.w3.org/" title="W3C"><img alt="W3C" id="logo" src="../images/no_w3c.png" width="110" height="61" /></a> <a href="../checklink"><span>Link Checker</span></a></h1>
<p id="tagline">Check links and anchors in Web pages or full Web sites</p></div>
<div id="main">
<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="#bot">Robots exclusion</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
Since it was lacking some functionalities,
rewrote it more or less from scratch in November 1999.
It has been improved by Ville Skyttä and many other volunteers since.
</p>
<p>
The source code is available publicly under the
software notice</a> from
title="Comprehensive Perl Archive Network">CPAN</abbr></a> (released
versions) and a
(development and archived release versions).
</p>
<h2><a name="what" id="what">What it does</a></h2>
<p>
The link checker reads an HTML or XHTML document or a CSS style sheet
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
<abbr title="Common Gateway Interface">CGI</abbr> version. They both
authentication</a>. This is achieved in the CGI version
by passing through the authorization information from the user browser
to the site tested.
</p>
<h2><a name="online" id="online">Use it online</a></h2>
<p>
There is an
of the link checker.
</p>
<p>
In the online version (and in general, when run as a CGI script),
the number of documents that can be checked recursively is limited.
</p>
<p id="wait">
Both the command line version and the online one sleep at least one
second between requests to each server to avoid abuses and target
server congestion.
</p>
<h3>Access keys</h3>
<p>
The following access keys are implemented throughout the
site in an attempt to help users using screen readers.
</p>
<ol>
<li>Home: access key "1" leads back to the service's home page.</li>
<li>Downloads: access key "2" leads to downloads.</li>
<li>Documentation: access key "3" leads to the documentation index for
the service.</li>
<li>Feedback: access key "4" leads to the feedback instructions.</li>
</ol>
<h2><a name="install" id="install">Install it locally</a></h2>
<p>
The link checker is written in Perl. It is packaged as a standard
a few other modules which are also available from CPAN.
</p>
<h3 id="install-CPAN">Install with the CPAN utility</h3>
<p>If you system has a working installation of Perl, you should be able to install the link checker and its dependencies with a single line from the commandline shell:</p>
<p><kbd>sudo perl -MCPAN -e 'install W3C::LinkChecker'</kbd> (use without the <kbd>sudo</kbd> command if installing from an administrator account).</p>
<p>If this is the first time you use the CPAN utility, you may have to answer a few setup questions before the tool downloads, builds and installs the link checker.</p>
<h3 id="install-manual">Install by hand</h3>
<p>If for any reason the technique described above is not working or if you prefer installing each package by hand, follow the instructions below:</p>
<ol>
<li>
or newer.
</li>
<li>
distributions, as well as the distributions they possibly depend on.
Depending on your Perl version, you might already have some of
these installed. Also, the latest versions of these may require a
recent version of Perl. As long as the minimum version requirement(s)
below are satisfied, everything should be fine. The latest version
should not be needed, just get an older version that works with your
Perl. For an introduction to installing Perl modules,
<ul>
</ul>
</li>
<li>
Optionally install the link checker configuration file,
<code>etc/checklink.conf</code> contained in the link checker
distribution package into <code>/etc/w3c/checklink.conf</code>
or set the <code>W3C_CHECKLINK_CFG</code> environment variable to the
location where you installed it.
</li>
<li>
Optionally, install the <code>checklink</code> script into a location
in your web server which allows execution of CGI scripts (typically a
directory named <code>cgi-bin</code> somewhere below your web server's
root directory).
</li>
<li>
See also the <code>README</code> and <code>INSTALL</code> file(s)
included in the above distributions.
</li>
</ol>
<p>
Running <kbd>checklink --help</kbd> shows how to
use the command line version. The distribution package also includes
more extensive <abbr title="Plain Old Documentation">POD</abbr>
documentation, use
to view it.
</p>
<p>
<abbr title="Secure Sockets Layer">SSL</abbr>/<abbr title="Transport Layer Security">TLS</abbr>v1
support for <code>https</code> in the link checker needs support for
it in libwww-perl; see
in the libwww-perl distribution for more information.
</p>
<p>
In online mode, link checker's output should not be buffered to avoid
browser timeouts. The link checker itself does not buffer its output,
but in some cases output buffering needs to be explicitly disabled for
it in the web server running it. One such case is Apache's mod_deflate
compression module which as a side effect results in output buffering;
one way to disable it for the link checker (while leaving it enabled for
other resources if configured so elsewhere) is to add the following
section to an appropriate place in the Apache configuration (assuming the
link checker script's filename is <code>checklink</code>):
</p>
<blockquote><pre>
<Files checklink>
SetEnv no-gzip
</Files>
</pre></blockquote>
<p>
If you want to enable the authentication capabilities with Apache,
have a look at
</p>
<p>
The link checker honors proxy settings from the
<code><em>scheme</em>_proxy</code> environment variables. See
<code>env_proxy</code></a> method for more information.
</p>
<p>
Some environment variables affect the way how the link checker uses
In particular, passive mode is the default. See
for more information.
</p>
<p>
There are multiple alternatives for configuring the default
server for use with <code>news:</code> URIs without explicit hostnames,
see
for more information.
</p>
<h2><a name="bot" id="bot">Robots exclusion</a></h2>
<p>
The link checker honors
rules</a>. To place rules specific to the W3C Link Checker in
<code>/robots.txt</code> files, sites can use the
<code>W3C-checklink</code> user agent string. For example, to allow
the link checker to access all documents on a server and to disallow
all other robots, one could use the following:
</p>
<pre>
User-Agent: *
Disallow: /
User-Agent: W3C-checklink
Disallow:
</pre>
<p>
Robots exlusion support in the link checker is based on the
Perl module. It currently supports the
of the standard. The robots META tag, ie.
<code><meta name="robots" content="..."></code>, is not supported.
Other than that, the link checker's implementation goes all the way
in trying to honor robots exclusion rules; if a
<code>/robots.txt</code> disallows it, not even the first document
submitted as the root for a link checker run is fetched.
</p>
<p>
Note that <code>/robots.txt</code> rules affect only user agents
that honor it; it is not a generic method for access control.
</p>
<h2><a name="csb" id="csb">Comments, suggestions and bugs</a></h2>
<p>
The current version has proven to be stable. It could however be
</p>
<p>
Please send comments, suggestions and bug reports about the link checker
to the <a href="mailto:www-validator@w3.org?subject=checklink%3A%20">www-validator mailing list</a>
with 'checklink' in the subject. See examples below
</p>
<dl>
<dt>Good</dt>
<dd>Subject: online checklink times out when accessed with Iceweasel 2.1.12</dd>
<dt>Bad</dt>
<dd>Subject: checklink</dd>
<dt>Bad</dt>
<dd>Subject: checklink does not work</dd>
</dl>
<h3><a name="issues" id="issues">Known issues</a></h3>
<p>
If a link checker run in "summary only" mode takes a long time, some
user agents may stop loading the results page due to a timeout. We
have placed workarounds hoping to avoid this in the code, but have not
yet found one that would work reliably for all browsers. If you
experience these timeouts, try avoiding "summary only" mode, or try
using the link checker with another browser.
</p>
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