Linux Password Policy
The following examples are on a Red Hat Enterprise Linux AS 4 system.
/etc/login.defs and /etc/default/useradd are the files related to password aging on 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.
By default, password inactivity and expire date is disabled.
Note that PASS_MIN_LEN in /etc/login.defs has no effect. Minimum password length is controlled by the pam_cracklib module. If minlen= is not specified in pam_cracklib, the default minimum password length is 6 characters.
/usr/bin/chage is used to modify password aging on existing accounts. chage does not update the last password change field (field 3) in /etc/shadow, so passwords could expire immediately after running it.
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 field 7 in /etc/shadow), you will be unable to login and your password will have to be manually reset by an administrator.
Dec 4 14:33:42 host sshd(pam_unix): 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 history--i.e., preventing re-use of old passwords--may be enabled 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 each 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 entry in bold in /etc/pam.d/system-auth:
password required /lib/security/$ISA/pam_cracklib.so retry=3 type= password sufficient /lib/security/$ISA/pam_unix.so nullok use_authtok md5 shadow remember=24