# Documentation

## Basic usage

To check a URL like ``http://www.example.org/`` it is enough to
type ``linkchecker www.example.org`` on the command line or
type ``www.example.org`` in the GUI application. This will check the
complete domain of ``http://www.example.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.
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 (status code 301) issue a warning.
  All final HTTP status codes other than 2xx are errors.
  
  For HTTPS links, the SSL certificate is checked against the
  given hostname. If it does not match, a warnings is printed.

- Local files (``file:``)
  
  A regular, readable file that can be opened is valid. A readable
  directory is also valid. All other files, for example unreadable,
  non-existing or device files are errors.
  
  File contents are checked for recursion. If they are parseable
  files (for example HTML files), all links in that file will be
  checked.
  
- Mail links (``mailto:``)
  
  A mailto: link resolves to a list of email addresses.
  If one email address fails the whole list will fail.
  For each email address the following things are checked:
  
  1. Check the address syntax, both the part before and after
     the @ sign.
  2. Look up the MX DNS records. If no MX record is found,
     print an error.
  3. Check if one of the MX mail hosts accept an SMTP connection.
     Check hosts with higher priority first.
     If none of the hosts accept SMTP, a warning is printed.
  4. Try to verify the address with the VRFY command. If there is
     an answer, the verified address is printed as an info.

- FTP links (``ftp:``)
  
  For FTP links the following is checked:
  
  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:``)
  
  A connect and if user/password are given, login to the
  given telnet server is tried.

- NNTP links (``news:``, ``snews:``, ``nntp``)
  
  A connect is tried to connect to the given NNTP server. If a news group or
  article is specified, it will be requested from the server.

- Ignored links (``javascript:``, etc.)
  
  An ignored link will print a warning, but no error. No further checking
  will be made.
  
  Here is the complete list of recognized, but ignored links. The most
  prominent of them are 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)
  - ``irc:``       (internet relay chat)
  - ``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)
  - ``skype:``     (Skype)
  - ``steam:``     (Steam browser 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. The conditions are checked in this order:

1. The URL must be valid.
2. The URL must be parseable. This currently includes HTML files,
   Bookmarks files (Opera, Chrome or Safari), directories and on
   Windows systems MS Word files if Word and the Pywin32 module
   is installed on your system.
   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`` command line option, the recursion
   level GUI option, or through the configuration file.
   The recursion level is unlimited by default.
5. It must not match the ignored URL list. This is controlled with
   the ``--ignore-url`` command line option or through the
   configuration file.
6. The Robots Exclusion Protocol must allow links in the URL to be
   followed recursively. This is checked by evaluating the servers
   robots.txt file and searching for a "nofollow" directive in the
   HTML header data.

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


## Configuration file

Each user can edit a configuration with advanced options for
checking or filtering.

On Unix or OS X systems the user configuration file is at

- ``~/.linkchecker/linkcheckerrc``

On Windows systems the user configuration file is at

- ``%HOMEPATH%\.linkchecker\linkcheckerrc``