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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>6.2. pam_cracklib - checks the password against dictionary words</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot"><link rel="home" href="Linux-PAM_SAG.html" title="The Linux-PAM System Administrators' Guide"><link rel="up" href="sag-module-reference.html" title="Chapter 6. A reference guide for available modules"><link rel="prev" href="sag-pam_access.html" title="6.1. pam_access - logdaemon style login access control"><link rel="next" href="sag-pam_debug.html" title="6.3. pam_debug - debug the PAM stack"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">6.2. pam_cracklib - checks the password against dictionary words</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="sag-pam_access.html">Prev</a> </td><th width="60%" align="center">Chapter 6. A reference guide for available modules</th><td width="20%" align="right"> <a accesskey="n" href="sag-pam_debug.html">Next</a></td></tr></table><hr></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="sag-pam_cracklib"></a>6.2. pam_cracklib - checks the password against dictionary words</h2></div></div></div><div class="cmdsynopsis"><p><code class="command">pam_cracklib.so</code> [ <em class="replaceable"><code>...</code></em> ]</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="sag-pam_cracklib-description"></a>6.2.1. DESCRIPTION</h3></div></div></div><p> This module can be plugged into the <span class="emphasis"><em>password</em></span> stack of a given application to provide some plug-in strength-checking for passwords. </p><p> 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. </p><p> 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. </p><p> The strength checks works in the following manner: at first the <code class="function">Cracklib</code> 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: </p><div class="variablelist"><dl class="variablelist"><dt><span class="term">Palindrome</span></dt><dd><p> Is the new password a palindrome? </p></dd><dt><span class="term">Case Change Only</span></dt><dd><p> Is the new password the the old one with only a change of case? </p></dd><dt><span class="term">Similar</span></dt><dd><p> Is the new password too much like the old one? This is primarily controlled by one argument, <code class="option">difok</code> which is a number of character changes (inserts, removals, or replacements) between the old and new password that are enough to accept the new password. This defaults to 5 changes. </p></dd><dt><span class="term">Simple</span></dt><dd><p> Is the new password too small? This is controlled by 6 arguments <code class="option">minlen</code>, <code class="option">maxclassrepeat</code>, <code class="option">dcredit</code>, <code class="option">ucredit</code>, <code class="option">lcredit</code>, and <code class="option">ocredit</code>. See the section on the arguments for the details of how these work and there defaults. </p></dd><dt><span class="term">Rotated</span></dt><dd><p> Is the new password a rotated version of the old password? </p></dd><dt><span class="term">Same consecutive characters</span></dt><dd><p> Optional check for same consecutive characters. </p></dd><dt><span class="term">Too long monotonic character sequence</span></dt><dd><p> Optional check for too long monotonic character sequence. </p></dd><dt><span class="term">Contains user name</span></dt><dd><p> Optional check whether the password contains the user's name in some form. </p></dd></dl></div><p> 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. </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="sag-pam_cracklib-options"></a>6.2.2. OPTIONS</h3></div></div></div><p> </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"> <code class="option">debug</code> </span></dt><dd><p> This option makes the module write information to <span class="citerefentry"><span class="refentrytitle">syslog</span>(3)</span> indicating the behavior of the module (this option does not write password information to the log file). </p></dd><dt><span class="term"> <code class="option">authtok_type=<em class="replaceable"><code>XXX</code></em></code> </span></dt><dd><p> The default action is for the module to use the following prompts when requesting passwords: "New UNIX password: " and "Retype UNIX password: ". The example word <span class="emphasis"><em>UNIX</em></span> can be replaced with this option, by default it is empty. </p></dd><dt><span class="term"> <code class="option">retry=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> Prompt user at most <em class="replaceable"><code>N</code></em> times before returning with error. The default is <span class="emphasis"><em>1</em></span>. </p></dd><dt><span class="term"> <code class="option">difok=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> This argument will change the default of <span class="emphasis"><em>5</em></span> for the number of character changes in the new password that differentiate it from the old password. </p></dd><dt><span class="term"> <code class="option">minlen=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> 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 (<span class="emphasis"><em>other</em></span>, <span class="emphasis"><em>upper</em></span>, <span class="emphasis"><em>lower</em></span> and <span class="emphasis"><em>digit</em></span>). The default for this parameter is <span class="emphasis"><em>9</em></span> 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 <span class="emphasis"><em>Cracklib</em></span> 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 <code class="option">minlen</code>. If you want to allow passwords as short as 5 characters you should not use this module. </p></dd><dt><span class="term"> <code class="option">dcredit=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> (N >= 0) This is the maximum credit for having digits in the new password. If you have less than or <em class="replaceable"><code>N</code></em> digits, each digit will count +1 towards meeting the current <code class="option">minlen</code> value. The default for <code class="option">dcredit</code> is 1 which is the recommended value for <code class="option">minlen</code> less than 10. </p><p> (N < 0) This is the minimum number of digits that must be met for a new password. </p></dd><dt><span class="term"> <code class="option">ucredit=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> (N >= 0) This is the maximum credit for having upper case letters in the new password. If you have less than or <em class="replaceable"><code>N</code></em> upper case letters each letter will count +1 towards meeting the current <code class="option">minlen</code> value. The default for <code class="option">ucredit</code> is <span class="emphasis"><em>1</em></span> which is the recommended value for <code class="option">minlen</code> less than 10. </p><p> (N < 0) This is the minimum number of upper case letters that must be met for a new password. </p></dd><dt><span class="term"> <code class="option">lcredit=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> (N >= 0) This is the maximum credit for having lower case letters in the new password. If you have less than or <em class="replaceable"><code>N</code></em> lower case letters, each letter will count +1 towards meeting the current <code class="option">minlen</code> value. The default for <code class="option">lcredit</code> is 1 which is the recommended value for <code class="option">minlen</code> less than 10. </p><p> (N < 0) This is the minimum number of lower case letters that must be met for a new password. </p></dd><dt><span class="term"> <code class="option">ocredit=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> (N >= 0) This is the maximum credit for having other characters in the new password. If you have less than or <em class="replaceable"><code>N</code></em> other characters, each character will count +1 towards meeting the current <code class="option">minlen</code> value. The default for <code class="option">ocredit</code> is 1 which is the recommended value for <code class="option">minlen</code> less than 10. </p><p> (N < 0) This is the minimum number of other characters that must be met for a new password. </p></dd><dt><span class="term"> <code class="option">minclass=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> 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 <code class="option">credit</code> check is that a specific class if of characters is not required. Instead <em class="replaceable"><code>N</code></em> out of four of the classes are required. </p></dd><dt><span class="term"> <code class="option">maxrepeat=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> Reject passwords which contain more than N same consecutive characters. The default is 0 which means that this check is disabled. </p></dd><dt><span class="term"> <code class="option">maxsequence=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> Reject passwords which contain monotonic character sequences longer than N. The default is 0 which means that this check is disabled. Examples of such sequence are '12345' or 'fedcb'. Note that most such passwords will not pass the simplicity check unless the sequence is only a minor part of the password. </p></dd><dt><span class="term"> <code class="option">maxclassrepeat=<em class="replaceable"><code>N</code></em></code> </span></dt><dd><p> Reject passwords which contain more than N consecutive characters of the same class. The default is 0 which means that this check is disabled. </p></dd><dt><span class="term"> <code class="option">reject_username</code> </span></dt><dd><p> Check whether the name of the user in straight or reversed form is contained in the new password. If it is found the new password is rejected. </p></dd><dt><span class="term"> <code class="option">gecoscheck</code> </span></dt><dd><p> Check whether the words from the GECOS field (usualy full name of the user) longer than 3 characters in straight or reversed form are contained in the new password. If any such word is found the new password is rejected. </p></dd><dt><span class="term"> <code class="option">enforce_for_root</code> </span></dt><dd><p> The module will return error on failed check also if the user changing the password is root. This option is off by default which means that just the message about the failed check is printed but root can change the password anyway. Note that root is not asked for an old password so the checks that compare the old and new password are not performed. </p></dd><dt><span class="term"> <code class="option">use_authtok</code> </span></dt><dd><p> This argument is used to <span class="emphasis"><em>force</em></span> the module to not prompt the user for a new password but use the one provided by the previously stacked <span class="emphasis"><em>password</em></span> module. </p></dd><dt><span class="term"> <code class="option">dictpath=<em class="replaceable"><code>/path/to/dict</code></em></code> </span></dt><dd><p> Path to the cracklib dictionaries. </p></dd></dl></div><p> </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="sag-pam_cracklib-types"></a>6.2.3. MODULE TYPES PROVIDED</h3></div></div></div><p> Only the <code class="option">password</code> module type is provided. </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="sag-pam_cracklib-return_values"></a>6.2.4. RETURN VALUES</h3></div></div></div><p> </p><div class="variablelist"><dl class="variablelist"><dt><span class="term">PAM_SUCCESS</span></dt><dd><p> The new password passes all checks. </p></dd><dt><span class="term">PAM_AUTHTOK_ERR</span></dt><dd><p> No new password was entered, the username could not be determined or the new password fails the strength checks. </p></dd><dt><span class="term">PAM_AUTHTOK_RECOVERY_ERR</span></dt><dd><p> The old password was not supplied by a previous stacked module or got not requested from the user. The first error can happen if <code class="option">use_authtok</code> is specified. </p></dd><dt><span class="term">PAM_SERVICE_ERR</span></dt><dd><p> A internal error occurred. </p></dd></dl></div><p> </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="sag-pam_cracklib-examples"></a>6.2.5. EXAMPLES</h3></div></div></div><p> For an example of the use of this module, we show how it may be stacked with the password component of <span class="citerefentry"><span class="refentrytitle">pam_unix</span>(8)</span> </p><pre class="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 </pre><p> </p><p> Another example (in the <code class="filename">/etc/pam.d/passwd</code> format) is for the case that you want to use md5 password encryption: </p><pre class="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 </pre><p> </p><p> And here is another example in case you don't want to use credits: </p><pre class="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 </pre><p> </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="sag-pam_cracklib-author"></a>6.2.6. AUTHOR</h3></div></div></div><p> pam_cracklib was written by Cristian Gafton <gafton@redhat.com> </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sag-pam_access.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sag-module-reference.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sag-pam_debug.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">6.1. pam_access - logdaemon style login access control </td><td width="20%" align="center"><a accesskey="h" href="Linux-PAM_SAG.html">Home</a></td><td width="40%" align="right" valign="top"> 6.3. pam_debug - debug the PAM stack</td></tr></table></div></body></html>