我們在使用linux系統設置密碼的時候，經常遇到這樣的問題，系統提示：您的密碼太簡單，或者您的密碼是字典的一部分。那麼系統是如何實現對用戶的密碼的復雜度的檢查的呢？
系統對密碼的控制是有兩部分(我知道的)組成：
1 cracklib
2 login.defs
聲明：login.defs主要是控制密碼的有效期。對密碼進行時間管理。此處不細談
login.defs --shadow password suite configuration
pam_cracklib.so 才是控制密碼復雜度的關鍵文件
redhat公司專門開發了cracklib這個安裝包來判斷密碼的復雜度
可以rpm -ql cracklib查看
密碼的復雜度的判斷是通過pam模塊控制來實現的，具體的模塊是pam_cracklibpam_cracklib 的參數介紹：
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).
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 default word UNIX can be replaced with this option.
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 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.
difignore=N
How many characters should the password have before difok will be ignored. The default is 23.
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 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 Cracklib 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 minlen. If you want to allow passwords as short as 5 characters you should not use this module.
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 recommended 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.
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