Linux Password Policy

From Brandonhutchinson.com

Current revision

The following examples are on a Red Hat Enterprise Linux AS 4 system.

Password Aging

New Accounts

/etc/login.defs and /etc/default/useradd are the files related to password aging on new accounts.

/etc/login.defs:

# 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.

/etc/default/useradd:

INACTIVE=-1
EXPIRE=

By default, password inactivity (i.e., the number of days after a password is expired in which a user can no longer reset it themselves) and an account expiration date are 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.

Existing Accounts

/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. You may run it with the -d option to modify the last password change field in /etc/shadow.

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 field 7 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 entry:

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, but 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 Length and Complexity

Both pam_cracklib and pam_passwdqc are modules used in enforcing password length and complexity. Although pam_passwordqc is more powerful, I'll be using pam_cracklib as its capabilities meet our site's needs and it is already in the PAM stack.

Example: Require a minimum password length of 9 characters, with at least 1 lowercase character, 1 uppercase character, and 1 digit.

Relevant entry in bold in /etc/pam.d/system-auth:

password    requisite     /lib/security/$ISA/pam_cracklib.so retry=3 minlen=12 lcredit=1 ucredit=1 dcredit=1 ocredit=0

Password History

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    sufficient    /lib/security/$ISA/pam_unix.so nullok use_authtok md5 shadow remember=24

Account Lockout

Account lockout after a number of unsuccessful authentication attempts may be enabled using pam_tally. In this example, accounts are locked out after 5 failed login attempts. Twice an hour, the failed login counter is reset. The failed login counter is also reset with each successful authentication (reset option in PAM configuration).

• Create the pam_tally store for failed login attempts.

# touch /var/log/faillog
# chown root:root /var/log/faillog
# chmod 600 /var/log/faillog

• Configure PAM.

RHEL 3/4 Example

Relevant entries in bold in /etc/pam.d/system-auth:

auth        required      /lib/security/$ISA/pam_env.so
auth        required      /lib/security/$ISA/pam_tally.so onerr=fail no_magic_root
auth        sufficient    /lib/security/$ISA/pam_unix.so likeauth nullok
auth        required      /lib/security/$ISA/pam_deny.so

account     required      /lib/security/$ISA/pam_unix.so
account     sufficient    /lib/security/$ISA/pam_succeed_if.so uid < 100 quiet
account     required      /lib/security/$ISA/pam_permit.so
account     required      /lib/security/$ISA/pam_tally.so deny=5 no_magic_root reset

• pam_tally on RHEL 3/4 does not support the unlock_time parameter. Run the reset_failed_logins script periodically from cron to reset the number of failed logins.

Example root crontab:

# Reset pam_tally counter twice hourly
0,30 * * * * /usr/local/bin/reset_failed_logins

RHEL 5 Example

Modifications needed for pam_tally2 in RHEL 5:

• pam_tally2 on RHEL 5 supports the unlock_time paramter.
• deny is allowed in the auth phase only, not the account phase.
• no_magic_root and reset are not valid options.

Relevant entries in bold in /etc/pam.d/system-auth:

auth        required      /lib/security/$ISA/pam_env.so
auth        required      /lib/security/$ISA/pam_tally.so onerr=fail deny=5 unlock_time=1800
auth        sufficient    /lib/security/$ISA/pam_unix.so likeauth nullok
auth        required      /lib/security/$ISA/pam_deny.so

account     required      /lib/security/$ISA/pam_unix.so
account     sufficient    /lib/security/$ISA/pam_succeed_if.so uid < 100 quiet
account     required      /lib/security/$ISA/pam_permit.so
account     required      /lib/security/$ISA/pam_tally.so

Links

• Enable password aging on Linux systems
• Enforce strong passwords with pam_passwdqc
• How do I lock out a user after a set number of login attempts in Red Hat Enterprise Linux 2.1?
• pam_tally configuration tips
• Strong Passwords with PAM