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 (resetoption 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
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