UnixPower on Networking
  http://www.unix-power.net/

 
 Top - Linux - CentOS7 パスワードポリシーの設定




デフォルトのCentOS7では簡単なパスワードを設定しようとすると警告的なメッセージは出るものの、無視して設定することは可能となっています。例えば test / test といういかにもありがちなユーザアカウントとパスワードを設定しようとしたとき。

# useradd test
# passwd test
ユーザー test のパスワードを変更。
新しいパスワード:
よくないパスワード: このパスワードは 8 未満の文字列です。
新しいパスワードを再入力してください:
passwd: すべての認証トークンが正しく更新できました。

設定できちゃいました。これをIPアドレス等のアクセス制限なく外部インターネットに公開したらあっという間にアカウントを取られてしまいそうです。

こういった安易なパスワードを設定させないために、パスワードを設定する際には英字でそれぞれ大文字、小文字をそれぞれ1文字以上、数字も1文字以上、全体で何文字以上といったポリシーを設定しておき、それにあったものでないと設定できないよう強制することが可能となっております。

設定ファイルは/etc/pam.d/system-authとなります。このファイルの15行目あたりを編集します。

# vi /etc/pam.d/system-auth

<省略>
password    requisite     pam_pwquality.so try_first_pass local_users_only retry=3 authtok_type= \
                          minlen=8 dcredit=-1 ucredit=-1 lcredit=-1 ocredit=0 enforce_for_root
<省略>

上のそれぞれの意味は下記の通りです。

項目 説明 
minlen  パスワード全体でN文字以上でなければならない
dcredit パスワードに数字が最低含まれていなければならない数を指定。minlenと併用の場合は-をつける
ucredit パスワードに大文字の英字が最低含まれていなければならない数を指定。minlenと併用の場合は-をつける
lcredit パスワードに小文字の英字が最低含まれていなければならない数を指定。minlenと併用の場合は-をつける
ocredit パスワードに記号が最低含まれていなければならない数を指定。minlenと併用の場合は-をつける
enforce_for_root rootでパスワード変更する場合においても上で設定した条件を満たさないと変更できないようにする

上記の設定例では全体で8文字以上でなおかつ英数字 ( 大文字/小文字をそれぞれ含む ) のパスワードでなければ設定できないということになります。一般ユーザでパスワード変更するときはもちろん、rootで変更する場合も条件を満たさないと変更できないようになっています。但し、enforce_for_rootオプションを指定しなければrootであれば他のオプションを指定していても任意のパスワードに変更できるということになります。

その他にも指定できるオプションは上記以外にもありますが、ここでは割愛しています。man pam_pwqualityで参照可能となっております。

# man pam_pwquality
PAM_PWQUALITY(8)                System Manager's Manual                PAM_PWQUALITY(8)

NAME
       pam_pwquality - PAM module to perform password quality checking

SYNOPSIS
       pam_pwquality.so [...]

DESCRIPTION
       This module can be plugged into the password stack of a given service to provide
       some plug-in strength-checking for passwords. The code was originally  based  on
       pam_cracklib module and the module is backwards compatible with its options.

       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.

       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 ver‐
       ify  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 authenti‐
       cation token.

       The strength checks works in the following manner: at first the Cracklib 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:

       Palindrome
           Is the new password a palindrome?

       Case Change Only
           Is the new password the the old one with only a change of case?

       Similar
           Is  the new password too much like the old one? This is primarily controlled
           by one argument, difok 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.

       Simple
           Is the new password too small? This is controlled  by  6  arguments  minlen,
           maxclassrepeat,  dcredit,  ucredit, lcredit, and ocredit. See the section on
           the arguments for the details of how these work and there defaults.

       Rotated
           Is the new password a rotated version of the old password?

       Same consecutive characters
           Optional check for same consecutive characters.

       Too long monotonic character sequence
           Optional check for too long monotonic character sequence.

       Contains user name
           Optional check whether the password contains the user's name in some form.

       These checks are configurable either by use of the module arguments or by  modi‐
       fying the /etc/security/pwquality.conf configuration file.

OPTIONS
       debug
           This  option  makes the module write information to syslog(3) indicating the
           behavior of the module (this option does not write password  information  to
           the log file).

       authtok_type=XXX
           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  UNIX  can be replaced with this option, by default it is
           empty.

       retry=N
           Prompt user at most N times before returning with error. The default is 1.

       difok=N
           This argument will change the default of 5 for the number of changes in  the
           new password from the old password.

       minlen=N
           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 (other, upper, lower and  digit).  The  default  for  this
           parameter  is  9 . Note that there is a pair of length limits also in Crack‐
           lib, which is used for dictionary checking, a "way too  short"  limit  of  4
           which  is  hard  coded  in  and  a build time defined limit (6) that will be
           checked without reference to minlen.

       dcredit=N
           (N >= 0) This is the maximum credit for having digits in the  new  password.
           If  you have less than or N digits, each digit will count +1 towards meeting
           the current minlen value. The default for dcredit is 1 which is  the  recom‐
           mended value for minlen less than 10.

           (N  <  0)  This  is  the minimum number of digits that must be met for a new
           password.

       ucredit=N
           (N >= 0) This is the maximum credit for having upper case letters in the new
           password.  If  you  have  less than or N upper case letters each letter will
           count +1 towards meeting the current minlen value. The default  for  ucredit
           is 1 which is the recommended value for minlen less than 10.

           (N  <  0)  This is the minimum number of upper case letters that must be met
           for a new password.

       lcredit=N
           (N >= 0) This is the maximum credit for having lower case letters in the new
           password.  If  you  have less than or N lower case letters, each letter will
           count +1 towards meeting the current minlen value. The default  for  lcredit
           is 1 which is the recommended value for minlen less than 10.

           (N  <  0)  This is the minimum number of lower case letters that must be met
           for a new password.

       ocredit=N
           (N >= 0) This is the maximum credit for having other characters in  the  new
           password.  If  you have less than or N other characters, each character will
           count +1 towards meeting the current minlen value. The default  for  ocredit
           is 1 which is the recommended value for minlen less than 10.

           (N  < 0) This is the minimum number of other characters that must be met for
           a new password.

       minclass=N
           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 credit check is  that  a
           specific  class  if  of characters is not required. Instead N out of four of
           the classes are required.

       maxrepeat=N
           Reject passwords which contain more than N same consecutive characters.  The
           default is 0 which means that this check is disabled.

       maxsequence=N
           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  pass‐
           word.

       maxclassrepeat=N
           Reject  passwords  which  contain  more than N consecutive characters of the
           same class.  The default is 0 which means that this check is disabled.

       gecoscheck=N
           If nonzero, check whether the individual words longer than 3 characters from
           the  passwd  GECOS field of the user are contained in the new password.  The
           default is 0 which means that this check is disabled.

       badwords=<list of words>
           The words more than 3 characters long from this  space  separated  list  are
           individually searched for and forbidden in the new password.  By default the
           list is empty which means that this check is disabled.

       enforce_for_root
           The module will return error on failed check even 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.

       local_users_only
           The module will not test the password quality for users that are not present
           in  the /etc/passwd file. The module still asks for the password so the fol‐
           lowing modules in the stack can use the use_authtok option.  This option  is
           off by default.

       use_authtok
           This  argument  is used to force the module to not prompt the user for a new
           password but use the one provided by the previously stacked password module.

       dictpath=/path/to/dict
           Path to the cracklib dictionaries.

MODULE TYPES PROVIDED
       Only the password module type is provided.

RETURN VALUES
       PAM_SUCCESS
           The new password passes all checks.

       PAM_AUTHTOK_ERR
           No new password was entered, the username could not be determined or the new
           password fails the strength checks.

       PAM_AUTHTOK_RECOVERY_ERR
           The  old  password  was not supplied by a previous stacked module or got not
           requested from the user. The first error can happen if use_authtok is speci‐
           fied.

       PAM_SERVICE_ERR
           A internal error occurred.

EXAMPLES
       For an example of the use of this module, we show how it may be stacked with the
       password component of pam_unix(8)

           #
           # 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_pwquality.
           #
           passwd  password required       pam_pwquality.so retry=3
           passwd  password required       pam_unix.so use_authtok

       Another example (in the /etc/pam.d/passwd format) is for the case that you  want
       to use md5 password encryption:

           #%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_pwquality.so \
                          difok=3 minlen=15 dcredit= 2 ocredit=2
           password  required pam_unix.so use_authtok nullok md5

       And here is another example in case you don´t want to use credits:

           #%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_pwquality.so \
                          dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 minlen=8
           password  required pam_unix.so use_authtok nullok md5

SEE ALSO
       pwscore(1), pwquality.conf(5), pam_pwquality(8), pam.conf(5), PAM(8)

AUTHORS
       Tomas Mraz <tmraz@redhat.com>
       Original author of pam_cracklib module Cristian Gafton <gafton@redhat.com>

Red Hat, Inc.                         10 Nov 2011                      PAM_PWQUALITY(8)



 Copyright(C) 2003-2015 UnixPower on Networking All rights reserved.