Linux Password Policy


Revision as of 22:03, 4 December 2007 by Hutch (Talk | contribs)
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The following examples are on a Red Hat Enterprise Linux AS 4 system.


Password Aging

  • /etc/login.defs for new accounts.
# Password aging controls:
#       PASS_MAX_DAYS   Maximum number of days a password may be used.
#       PASS_MIN_DAYS   Minimum number of days allowed between password changes.
#       PASS_MIN_LEN    Minimum acceptable password length.
#       PASS_WARN_AGE   Number of days warning given before a password expires.

Note that PASS_MIN_LEN in /etc/login.defs has no effect. Minimum password length is controlled by the pam_cracklib module. Note that if minlen= is not specified in pam_cracklib, I believe the default minimum password length is 6 characters.

  • /usr/bin/chage for existing accounts.

Existing account example

User hutchib was already created with essentially no password aging (the default PASS_MAX_DAYS of 99999). To configure the following:

  • A minimum of 7 days between password changes.
  • Password expiration after 90 days.
  • Begin warning about password expiration 14 days in advance.
# /usr/bin/chage -m 7 -M 90 -W 14 hutchib

What happens when your password expires?

  • If the account is inactive (see chage -I and the 7th field in /etc/shadow), you will be unable to login and your password will have to be manually reset by an administrator.

e.g., /var/log/messages

Dec  4 14:33:42 host sshd(pam_unix)[31601]: account hutchib has expired (failed to change password)
  • If the account is expired but not inactive, you are allowed a "grace login" where your old password is accepted and you must immediately change your password. After changing your password, the connection is closed and you must login again.
WARNING: Your password has expired.
You must change your password now and login again!
Changing password for user hutchib.
Changing password for hutchib
(current) UNIX password: 
New UNIX password:
Retype new UNIX password: 
passwd: all authentication tokens updated successfully.
Connection to host closed.

Password Complexity

Password History

Password history--i.e., preventing re-use of old passwords--is maintained using both pam_unix (stores the old password) and pam_cracklib (prevents re-use). By default, password history is disabled.

Example: Prevent re-use of the user's last 24 passwords.

  • Create the password database store.
# touch /etc/security/opasswd
# chown root:root /etc/security/opasswd
# chmod 600 /etc/security/opasswd
  • Configure PAM.

Relevant line in /etc/pam.d/system-auth:

password    sufficient    /lib/security/$ISA/ nullok use_authtok md5 shadow remember=24
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