path: root/modules/pam_cracklib/pam_cracklib.8.xml

<?xml version="1.0" encoding='UTF-8'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
	"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">

<refentry id="pam_cracklib">

  <refmeta>
    <refentrytitle>pam_cracklib</refentrytitle>
    <manvolnum>8</manvolnum>
    <refmiscinfo class="sectdesc">Linux-PAM Manual</refmiscinfo>
  </refmeta>

  <refnamediv id="pam_cracklib-name">
    <refname>pam_cracklib</refname>
    <refpurpose>PAM module to check the password against dictionary words</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis id="pam_cracklib-cmdsynopsis">
      <command>pam_cracklib.so</command>
      <arg choice="opt">
        <replaceable>...</replaceable>
      </arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1 id="pam_cracklib-description">

    <title>DESCRIPTION</title>

    <para>
      This module can be plugged into the <emphasis>password</emphasis> stack of
      a given application to provide some plug-in strength-checking for passwords.
    </para>

    <para>
      The action of this module is to prompt the user for a password and
      check its strength against a system dictionary and a set of rules for
      identifying poor choices.
    </para>

    <para>
      The first action is to prompt for a single password, check its
      strength and then, if it is considered strong, prompt for the password
      a second time (to verify that it was typed correctly on the first
      occasion). All being well, the password is passed on to subsequent
      modules to be installed as the new authentication token.
    </para>

    <para>
      The strength checks works in the following manner: at first the
      <function>Cracklib</function> routine is called to check if the password
      is part of a dictionary; if this is not the case an additional set of
      strength checks is done. These checks are:
    </para>

    <variablelist>
      <varlistentry>
        <term>Palindrome</term>
        <listitem>
          <para>
            Is the new password a palindrome of the old one?
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>Case Change Only</term>
        <listitem>
          <para>
            Is the new password the the old one with  only a change of case?
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>Similar</term>
        <listitem>
          <para>
            Is the new password too much like the old one?
            This is primarily controlled by one argument,
            <option>difok</option> which is a number of characters
            that if different between the old and new are enough to accept
            the new password, this defaults to 10 or 1/2 the size of the
            new password whichever is smaller.
          </para>
          <para>
            To avoid the lockup associated with trying to change a long and
            complicated password, <option>difignore</option> is available.
            This argument can be used to specify the minimum length a new
            password needs to be before the <option>difok</option> value is
            ignored. The default value for <option>difignore</option> is 23.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>Simple</term>
        <listitem>
          <para>
             Is the new password too small?
             This is controlled by 5 arguments <option>minlen</option>,
             <option>dcredit</option>, <option>ucredit</option>,
             <option>lcredit</option>, and <option>ocredit</option>. See the section
             on the arguments for the details of how these work and there defaults.
           </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>Rotated</term>
        <listitem>
          <para>
            Is the new password a rotated version of the old password?
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>Already used</term>
        <listitem>
          <para>
            Was the password used in the past? Previously used passwords
            are to be found in <filename>/etc/security/opasswd</filename>.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>
    <para>
      This module with no arguments will work well for standard unix
      password encryption.  With md5 encryption, passwords can be longer
      than 8 characters and the default settings for this module can make it
      hard for the user to choose a satisfactory new password.  Notably, the
      requirement that the new password contain no more than 1/2 of the
      characters in the old password becomes a non-trivial constraint.  For
      example, an old password of the form "the quick brown fox jumped over
      the lazy dogs" would be difficult to change...  In addition, the
      default action is to allow passwords as small as 5 characters in
      length.  For a md5 systems it can be a good idea to increase the
      required minimum size of a password.  One can then allow more credit
      for different kinds of characters but accept that the new password may
      share most of these characters with the old password.
    </para>

  </refsect1>

  <refsect1 id="pam_cracklib-options">

    <title>OPTIONS</title>
    <para>
      <variablelist>

        <varlistentry>
          <term>
            <option>debug</option>
          </term>
          <listitem>
            <para>
              This option makes the module write information to
              <citerefentry>
                <refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum>
              </citerefentry>
              indicating the behavior of the module (this option does
              not write password information to the log file).
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>type=<replaceable>XXX</replaceable></option>
          </term>
          <listitem>
            <para>
              The default action is for the module to use the
              following prompts when requesting passwords:
              "New UNIX password: " and "Retype UNIX password: ".
              The default word <emphasis>UNIX</emphasis> can
              be replaced with this option.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>retry=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              Prompt user at most <replaceable>N</replaceable> times
              before returning with error. The default is
              <emphasis>1</emphasis>
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>difok=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              This argument will change the default of
              <emphasis>5</emphasis> for the number of characters in
              the new password that must not be present in the old
              password. In addition, if 1/2 of the characters in the
              new password are different then the new password will
              be accepted anyway.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>difignore=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              How many characters should the password have before
              difok will be ignored. The default is
              <emphasis>23</emphasis>.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>minlen=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              The minimum acceptable size for the new password (plus
              one if credits are not disabled which is the default).
              In addition to the number of characters in the new password,
              credit (of +1 in length) is given for each different kind
              of character (<emphasis>other</emphasis>,
              <emphasis>upper</emphasis>, <emphasis>lower</emphasis> and
              <emphasis>digit</emphasis>). The default for this parameter
              is <emphasis>9</emphasis> which is good for a old style UNIX
              password all of the same type of character but may be too low
              to exploit the added security of a md5 system.  Note that
              there is a pair of length limits in
              <emphasis>Cracklib</emphasis> itself, a "way too short" limit
              of 4 which is hard coded in and a defined limit (6) that will
              be checked without reference to <option>minlen</option>.
              If you want to allow passwords as short as 5 characters you
              should not use this module.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>dcredit=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              (N &gt;= 0) This is the maximum credit for having digits in
              the new password. If you have less than or
              <replaceable>N</replaceable>
              digits, each digit will count +1 towards meeting the current
              <option>minlen</option> value. The default for
              <option>dcredit</option> is 1 which is the recommended
              value for <option>minlen</option> less than 10.
            </para>
            <para>
              (N &lt; 0) This is the minimum number of digits that must
              be met for a new password.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>ucredit=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              (N &gt;= 0) This is the maximum credit for having upper
              case letters in the new password.  If you have less than
              or <replaceable>N</replaceable> upper case letters each
              letter will count +1 towards meeting the current
              <option>minlen</option> value. The default for
              <option>ucredit</option> is <emphasis>1</emphasis> which
              is the recommended value for <option>minlen</option> less
              than 10.
            </para>
            <para>
              (N &lt; 0) This is the minimum number of upper
              case letters that must be met for a new password.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>lcredit=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              (N &gt;= 0) This is the maximum credit for having
              lower case letters in the new password. If you have
              less than or <replaceable>N</replaceable> lower case
              letters, each letter will count +1 towards meeting the
              current <option>minlen</option> value. The default for
              <option>lcredit</option> is 1 which is the recommended
              value for <option>minlen</option> less than 10.
            </para>
            <para>
             (N &lt; 0) This is the minimum number of lower
             case letters that must be met for a new password.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>ocredit=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              (N &gt;= 0) This is the maximum credit for having other
              characters in the new password. If you have less than or
              <replaceable>N</replaceable> other characters, each
              character will count +1 towards meeting the current
              <option>minlen</option> value. The default for
              <option>ocredit</option> is 1 which is the recommended
              value for <option>minlen</option> less than 10.
            </para>
            <para>
              (N &lt; 0) This is the minimum number of other
              characters that must be met for a new password.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>minclass=<replaceable>N</replaceable></option>
          </term>
          <listitem>
            <para>
              The minimum number of required classes of characters for
              the new password. The default number is zero. The four
              classes are digits, upper and lower letters and other
              characters.
              The difference to the <option>credit</option> check is
              that a specific class if of characters is not required.
              Instead <replaceable>N</replaceable> out of four of the
              classes are required.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>use_authtok</option>
          </term>
          <listitem>
            <para>
              This argument is used to <emphasis>force</emphasis> the
              module to not prompt the user for a new password but use
              the one provided by the previously stacked
              <emphasis>password</emphasis> module.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>dictpath=<replaceable>/path/to/dict</replaceable></option>
          </term>
          <listitem>
            <para>
              Path to the cracklib dictionaries.
            </para>
          </listitem>
        </varlistentry>

      </variablelist>
    </para>
  </refsect1>

  <refsect1 id="pam_cracklib-services">
    <title>MODULE SERVICES PROVIDED</title>
    <para>
      Only he <option>password</option> service is supported.
    </para>
  </refsect1>

  <refsect1 id='pam_cracklib-return_values'>
    <title>RETURN VALUES</title>
    <para>
      <variablelist>

        <varlistentry>
          <term>PAM_SUCCESS</term>
          <listitem>
            <para>
              The new password passes all checks.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PAM_AUTHTOK_ERR</term>
          <listitem>
            <para>
              No new password was entered,
              the username could not be determined or the new
              password fails the strength checks.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PAM_AUTHTOK_RECOVERY_ERR</term>
          <listitem>
            <para>
              The old password was not supplied by a previous stacked
              module or got not requested from the user.
              The first error can happen if <option>use_authtok</option>
              is specified.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PAM_SERVICE_ERR</term>
          <listitem>
            <para>
              A internal error occured.
            </para>
          </listitem>
        </varlistentry>

      </variablelist>
    </para>
  </refsect1>

  <refsect1 id='pam_cracklib-examples'>
    <title>EXAMPLES</title>
    <para>
      For an example of the use of this module, we show how it may be
      stacked with the password component of
      <citerefentry>
        <refentrytitle>pam_unix</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>
      <programlisting>
#
# These lines stack two password type modules. In this example the
# user is given 3 opportunities to enter a strong password. The
# "use_authtok" argument ensures that the pam_unix module does not
# prompt for a password, but instead uses the one provided by
# pam_cracklib.
#
passwd  password required       pam_cracklib.so retry=3
passwd  password required       pam_unix.so use_authtok
      </programlisting>
    </para>

    <para>
      Another example (in the <filename>/etc/pam.d/passwd</filename> format)
      is for the case that you want to use md5 password encryption:
      <programlisting>
#%PAM-1.0
#
# These lines allow a md5 systems to support passwords of at least 14
# bytes with extra credit of 2 for digits and 2 for others the new
# password must have at least three bytes that are not present in the
# old password
#
password  required pam_cracklib.so \
               difok=3 minlen=15 dcredit= 2 ocredit=2
password  required pam_unix.so use_authtok nullok md5
      </programlisting>
    </para>

    <para>
      And here is another example in case you don't want to use credits:
      <programlisting>
#%PAM-1.0
#
# These lines require the user to select a password with a minimum
# length of 8 and with at least 1 digit number, 1 upper case letter,
# and 1 other character
#
password  required pam_cracklib.so \
               dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 minlen=8
password  required pam_unix.so use_authtok nullok md5
      </programlisting>
    </para>

  </refsect1>

  <refsect1 id='pam_cracklib-see_also'>
    <title>SEE ALSO</title>
    <para>
      <citerefentry>
	<refentrytitle>pam.conf</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>pam.d</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>pam</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>
    </para>
  </refsect1>

  <refsect1 id='pam_cracklib-author'>
    <title>AUTHOR</title>
      <para>
        pam_cracklib was written by Cristian Gafton &lt;gafton@redhat.com&gt;
      </para>
  </refsect1>

</refentry>