linkchecker/doc/source/documentation.txt

Documentation
=============

Basic usage
-----------

To check a URL like ``http://www.myhomepage.org/`` it is enough to
execute ``linkchecker http://www.myhomepage.org/``. This will check the
complete domain of www.myhomepage.org recursively. All links pointing
outside of the domain are also checked for validity.

Performed checks
----------------

All URLs have to pass a preliminary syntax test. Minor quoting
mistakes will issue a warning, all other invalid syntax issues
are errors.
After the syntax check passes, the URL is queued for connection
checking. All connection check types are described below.

- HTTP links (``http:``, ``https:``)
  
  After connecting to the given HTTP server the given path
  or query is requested. All redirections are followed, and
  if user/password is given it will be used as authorization
  when necessary.
  Permanently moved pages issue a warning.
  All final HTTP status codes other than 2xx are errors.

- Local files (``file:``)
  
  A regular, readable file that can be opened is valid. A readable
  directory is also valid. All other files, for example device files,
  unreadable or non-existing files are errors.
  
  File contents are checked for recursion.
  
- Mail links (``mailto:``)
  
  A mailto: link eventually resolves to a list of email addresses.
  If one address fails, the whole list will fail.
  For each mail address we check the following things:
  
  1) Check the adress syntax, both of the part before and after
     the @ sign.
  2) Look up the MX DNS records. If we found no MX record,
     print an error.
  3) Check if one of the mail hosts accept an SMTP connection.
     Check hosts with higher priority first.
     If no host accepts SMTP, we print a warning.
  4) Try to verify the address with the VRFY command. If we got
     an answer, print the verified address as an info.

- FTP links (``ftp:``)
  
  For FTP links we do:
  
  1) connect to the specified host
  2) try to login with the given user and password. The default
     user is ``anonymous``, the default password is ``anonymous@``.
  3) try to change to the given directory
  4) list the file with the NLST command

- Telnet links (``telnet:``)
  
  We try to connect and if user/password are given, login to the
  given telnet server.

- NNTP links (``news:``, ``snews:``, ``nntp``)
  
  We try to connect to the given NNTP server. If a news group or
  article is specified, try to request it from the server.

- Ignored links (``javascript:``, etc.)
  
  An ignored link will only print a warning. No further checking
  will be made.
  
  Here is a complete list of recognized, but ignored links. The most
  prominent of them should be JavaScript links.
  
  - ``acap:``      (application configuration access protocol)
  - ``afs:``       (Andrew File System global file names)
  - ``chrome:``    (Mozilla specific)
  - ``cid:``       (content identifier)
  - ``clsid:``     (Microsoft specific)
  - ``data:``      (data)
  - ``dav:``       (dav)
  - ``fax:``       (fax)
  - ``find:``      (Mozilla specific)
  - ``gopher:``    (Gopher)
  - ``imap:``      (internet message access protocol)
  - ``isbn:``      (ISBN (int. book numbers))
  - ``javascript:`` (JavaScript)
  - ``ldap:``      (Lightweight Directory Access Protocol)
  - ``mailserver:`` (Access to data available from mail servers)
  - ``mid:``       (message identifier)
  - ``mms:``       (multimedia stream)
  - ``modem:``     (modem)
  - ``nfs:``       (network file system protocol)
  - ``opaquelocktoken:`` (opaquelocktoken)
  - ``pop:``       (Post Office Protocol v3)
  - ``prospero:``  (Prospero Directory Service)
  - ``rsync:``     (rsync protocol)
  - ``rtsp:``      (real time streaming protocol)
  - ``service:``   (service location)
  - ``shttp:``     (secure HTTP)
  - ``sip:``       (session initiation protocol)
  - ``tel:``       (telephone)
  - ``tip:``       (Transaction Internet Protocol)
  - ``tn3270:``    (Interactive 3270 emulation sessions)
  - ``vemmi:``     (versatile multimedia interface)
  - ``wais:``      (Wide Area Information Servers)
  - ``z39.50r:``   (Z39.50 Retrieval)
  - ``z39.50s:``   (Z39.50 Session)


Recursion
---------

Before descending recursively into a URL, it has to fulfill several
conditions. They are checked in this order:

1. A URL must be valid.

2. A URL must be parseable. This currently includes HTML files,
   Opera bookmarks files, and directories. If a file type cannot
   be determined (for example it does not have a common HTML file
   extension, and the content does not look like HTML), it is assumed
   to be non-parseable.

3. The URL content must be retrievable. This is usually the case
   except for example mailto: or unknown URL types.

4. The maximum recursion level must not be exceeded. It is configured
   with the ``--recursion-level`` option and is unlimited per default.

5. It must not match the ignored URL list. This is controlled with
   the ``--ignore-url`` option.

6. The Robots Exclusion Protocol must allow links in the URL to be
   followed recursively. This is checked by searching for a
   "nofollow" directive in the HTML header data.

Note that the directory recursion reads all files in that
directory, not just a subset like ``index.htm*``.


Frequently asked questions
--------------------------

**Q: LinkChecker produced an error, but my web page is ok with
Mozilla/IE/Opera/...
Is this a bug in LinkChecker?**

A: Please check your web pages first. Are they really ok?
Use the ``--check-html`` option, or check if you are using a proxy
which produces the error.

**Q: I still get an error, but the page is definitely ok.**

A: Some servers deny access of automated tools (also called robots)
like LinkChecker. This is not a bug in LinkChecker but rather a
policy by the webmaster running the website you are checking. Look
the ``/robots.txt`` file which follows the `robots.txt exclusion standard`_.

.. _`robots.txt exclusion standard`:
   http://www.robotstxt.org/wc/norobots-rfc.html

**Q: How can I tell LinkChecker which proxy to use?**

A: LinkChecker works transparently with proxies. In a Unix or Windows
environment, set the http_proxy, https_proxy, ftp_proxy environment
variables to a URL that identifies the proxy server before starting
LinkChecker. For example

::

  $ http_proxy="http://www.someproxy.com:3128"
  $ export http_proxy


**Q: The link "mailto:john@company.com?subject=Hello John" is reported
as an error.**

A: You have to quote special characters (e.g. spaces) in the subject field.
The correct link should be "mailto:...?subject=Hello%20John"
Unfortunately browsers like IE and Netscape do not enforce this.


**Q: Has LinkChecker JavaScript support?**

A: No, it never will. If your page is not working without JS, it is
better checked with a browser testing tool like Selenium_.

.. _Selenium:
   http://seleniumhq.org/


**Q: Is LinkCheckers cookie feature insecure?**

A: Cookies can not store more information as is in the HTTP request itself,
so you are not giving away any more system information.
After storing however, the cookies are sent out to the server on request.
Not to every server, but only to the one who the cookie originated from!
This could be used to "track" subsequent requests to this server,
and this is what some people annoys (including me).
Cookies are only stored in memory. After LinkChecker finishes, they
are lost. So the tracking is restricted to the checking time.
The cookie feature is disabled as default.


**Q: I want to have my own logging class. How can I use it in LinkChecker?**

A: Currently, only a Python API lets you define new logging classes.
Define your own logging class as a subclass of StandardLogger or any other
logging class in the log module.
Then call the addLogger function in Config.Configuration to register
your new Logger.
After this append a new Logging instance to the fileoutput.

::

  import linkcheck, MyLogger
  log_format = 'mylog'
  log_args = {'fileoutput': log_format, 'filename': 'foo.txt'}
  cfg = linkcheck.configuration.Configuration()
  cfg.logger_add(log_format, MyLogger.MyLogger)
  cfg['fileoutput'].append(cfg.logger_new(log_format, log_args)) 


**Q: LinkChecker does not ignore anchor references on caching.**

**Q: Some links with anchors are getting checked twice.**

A: This is not a bug.
It is not necessarily true that if a URL ``ABC#anchor1`` works then
``ABC#anchor2`` works too. That is not specified anywhere and there are
server-side scripts that fail on some anchors and not on others.
This is the reason for always checking URLs with different anchors.
If you really want to disable this, use the ``--no-anchor-caching``
option.


**Q: I see LinkChecker gets a /robots.txt file for every site it
checks. What is that about?**

A: LinkChecker follows the `robots.txt exclusion standard`_. To avoid
misuse of LinkChecker, you cannot turn this feature off.
See the `Web Robot pages`_ and the `Spidering report`_ for more info.

.. _`robots.txt exclusion standard`:
   http://www.robotstxt.org/wc/norobots-rfc.html
.. _`Web Robot pages`:
   http://www.robotstxt.org/wc/robots.html
.. _`Spidering report`:
   http://www.w3.org/Search/9605-Indexing-Workshop/ReportOutcomes/Spidering.txt


**Q: How do I print unreachable/dead documents of my website with
LinkChecker?**

A: No can do. This would require file system access to your web
repository and access to your web server configuration.


**Q: How do I check HTML/XML/CSS syntax with LinkChecker?**

A: Use the ``--check-html`` and ``--check-css`` options.