#6: User Accounts and Strong Password Policy
Use the useradd / usermod commands to create and maintain user accounts. Make sure you have a good and strong password policy. For example, a good password includes at least 8 characters long and mixture of alphabets, number, special character, upper & lower alphabets etc. Most important pick a password you can remember. Use tools such as "John the ripper" to find out weak users passwords on your server. Configure pam_cracklib.so to enforce the password policy.
#6.1: Password Aging
The chage command changes the number of days between password changes and the date of the last password change. This information is used by the system to determine when a user must change his/her password. The /etc/login.defs file defines the site-specific configuration for the shadow password suite including password aging configuration. To disable password aging, enter:
To get password expiration information, enter:
Finally, you can also edit the /etc/shadow file in the following fields:
chage -M 99999 userNameTo get password expiration information, enter:
chage -l userNameFinally, you can also edit the /etc/shadow file in the following fields:
{userName}:{password}:{lastpasswdchanged}:{Minimum_days}:{Maximum_days}:{Warn}:{Inactive}:{Expire}:Where,
- Minimum_days: The minimum number of days required between password changes i.e. the number of days left before the user is allowed to change his/her password.
- Maximum_days: The maximum number of days the password is valid (after that user is forced to change his/her password).
- Warn : The number of days before password is to expire that user is warned that his/her password must be changed.
- Expire : Days since Jan 1, 1970 that account is disabled i.e. an absolute date specifying when the login may no longer be used.
I recommend chage command instead of editing the /etc/shadow by hand:
Recommend readings:
# chage -M 60 -m 7 -W 7 userNameRecommend readings:
- Linux: Force Users To Change Their Passwords Upon First Login
- Linux turn On / Off password expiration / aging
- Lock the user password
- Search for all account without password and lock them
- Use Linux groups to enhance security
#6.2: Restricting Use of Previous Passwords
You can prevent all users from using or reuse same old passwords under Linux. The pam_unix module parameter remember can be used to configure the number of previous passwords that cannot be reused.
#6.3: Locking User Accounts After Login Failures
Under Linux you can use the faillog command to display faillog records or to set login failure limits. faillog formats the contents of the failure log from /var/log/faillog database / log file. It also can be used for maintains failure counters and limits.To see failed login attempts, enter:
To unlock an account after login failures, run:
Note you can use passwd command to lock and unlock accounts:
faillogTo unlock an account after login failures, run:
faillog -r -u userNameNote you can use passwd command to lock and unlock accounts:
# lock account
passwd -l userName
# unlocak account
passwd -u userName#6.4: How Do I Verify No Accounts Have Empty Passwords?
Type the following command
Lock all empty password accounts:
# awk -F: '($2 == "") {print}' /etc/shadowLock all empty password accounts:
# passwd -l accountName#6.5: Make Sure No Non-Root Accounts Have UID Set To 0
Only root account have UID 0 with full permissions to access the system. Type the following command to display all accounts with UID set to 0:
You should only see one line as follows:
# awk -F: '($3 == "0") {print}' /etc/passwdYou should only see one line as follows:
root:x:0:0:root:/root:/bin/bash
If you see other lines, delete them or make sure other accounts are authorized by you to use UID 0.
#7: Disable root Login
Never ever login as root user. You should use sudo to execute root level commands as and when required. sudo does greatly enhances the security of the system without sharing root password with other users and admins. sudo provides simple auditing and tracking features too.
#8: Physical Server Security
You must protect Linux servers physical console access. Configure the BIOS and disable the booting from external devices such as DVDs / CDs / USB pen. Set BIOS and grub boot loader password to protect these settings. All production boxes must be locked in IDCs (Internet Data Center) and all persons must pass some sort of security checks before accessing your server. See also:
#9: Disable Unwanted Services
Disable all unnecessary services and daemons (services that runs in the background). You need to remove all unwanted services from the system start-up. Type the following command to list all services which are started at boot time in run level # 3:
To disable service, enter:
# chkconfig --list | grep '3:on'To disable service, enter:
# service serviceName stop
# chkconfig serviceName off#9.1: Find Listening Network Ports
Use the following command to list all open ports and associated programs:
OR
Use iptables to close open ports or stop all unwanted network services using above service and chkconfig commands.
netstat -tulpnOR
nmap -sT -O localhost
nmap -sT -O server.example.comUse iptables to close open ports or stop all unwanted network services using above service and chkconfig commands.
#9.2: See Also
- update-rc.d like command on Redhat Enterprise / CentOS Linux.
- Ubuntu / Debian Linux: Services Configuration Tool to Start / Stop System Services.
- Get Detailed Information About Particular IP address Connections Using netstat Command.
#10: Delete X Windows
X Windows on server is not required. There is no reason to run X Windows on your dedicated mail and Apache web server. You can disable and remove X Windows to improve server security and performance. Edit /etc/inittab and set run level to 3. Finally, remove X Windows system, enter:
# yum groupremove "X Window System"