Linux Server Hardening Security Tips
Linux Server Hardening Security Tips
Linux Server Hardening Checklist and Tips
The following instructions assume that you are using CentOS/RHEL or Ubuntu/Debian based Linux distribution.
Minimize Software to Minimize Vulnerability
Do you really need all sort of web services installed? Avoid installing unnecessary software to avoid vulnerabilities in software. Use the RPM package manager such as yum or apt-get and/or dpkg to review all installed set of software packages on a system. Delete all unwanted packages.
# yum list installed
# yum list packageName
# yum remove packageName
OR
# dpkg --list
# dpkg --info packageName
# apt-get remove packageName
Keep Linux Kernel and Software Up to Date
Applying security patches is an important part of maintaining Linux server. Linux provides all necessary tools to keep your system updated, and also allows for easy upgrades between versions. All security update should be reviewed and applied as soon as possible. Again, use the RPM package manager such as yum and/or apt-get and/or dpkg to apply all security updates.
# yum update
OR
# apt-get update && apt-get upgrade
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.
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:
chage -M 99999 userName
To get password expiration information, enter:
chage -l userName
Finally, you can also edit the /etc/shadow file in the following fields:
{userName}:{password}:{lastpasswdchanged}:{Minimum_days}:{Maximum_days}:{Warn}:{Inactive}:{Expire}:
- 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.
# chage -M 60 -m 7 -W 7 userName
Recommend 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
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.
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:
faillog
To unlock an account after login failures, run:
faillog -r -u userName
Note you can use passwd command to lock and unlock accounts:
# lock account
passwd -l userName
# unlocak account
passwd -u userName
How Do I Verify No Accounts Have Empty Passwords?
Type the following command
# awk -F: '($2 == "") {print}' /etc/shadow
Lock all empty password accounts:
# passwd -l accountName
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:
# awk -F: '($3 == "0") {print}' /etc/passwd
You 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.
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.
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:
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:
# chkconfig --list | grep '3:on'
To disable service, enter:
# service serviceName stop
# chkconfig serviceName off
Find Listening Network Ports
Use the following command to list all open ports and associated programs:
netstat -tulpn
OR
nmap -sT -O localhost
nmap -sT -O server.example.com
Use iptables to close open ports or stop all unwanted network services using above service and chkconfig commands.
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"
Linux Kernel /etc/sysctl.conf Hardening
/etc/sysctl.conf file is used to configure kernel parameters at runtime. Linux reads and applies settings from /etc/sysctl.conf at boot time. Sample /etc/sysctl.conf:
# Turn on execshield
kernel.exec-shield=1
kernel.randomize_va_space=1
# Enable IP spoofing protection
net.ipv4.conf.all.rp_filter=1
# Disable IP source routing
net.ipv4.conf.all.accept_source_route=0
# Ignoring broadcasts request
net.ipv4.icmp_echo_ignore_broadcasts=1
net.ipv4.icmp_ignore_bogus_error_messages=1
# Make sure spoofed packets get logged
net.ipv4.conf.all.log_martians = 1
Separate Disk Partitions
Separation of the operating system files from user files may result into a better and secure system. Make sure the following filesystems are mounted on separate partitions:
- /usr
- /home
- /var and /var/tmp
- /tmp
Create separate partitions for Apache and FTP server roots. Edit /etc/fstab file and make sure you add the following configuration options:
- noexec – Do not set execution of any binaries on this partition (prevents execution of binaries but allows scripts).
- nodev – Do not allow character or special devices on this partition (prevents use of device files such as zero, sda etc).
- nosuid – Do not set SUID/SGID access on this partition (prevent the setuid bit).
Sample /etc/fstab entry to to limit user access on /dev/sda5 (ftp server root directory):
/dev/sda5 /ftpdata ext3 defaults,nosuid,nodev,noexec 1 2
Disable Unwanted SUID and SGID Binaries
All SUID/SGID bits enabled file can be misused when the SUID/SGID executable has a security problem or bug. All local or remote user can use such file. It is a good idea to find all such files. Use the find command as follows:
#See all set user id files:
find / -perm +4000
# See all group id files
find / -perm +2000
# Or combine both in a single command
find / ( -perm -4000 -o -perm -2000 ) -print
find / -path -prune -o -type f -perm +6000 -ls
You need to investigate each reported file. See reported file man page for further details.
Secure OpenSSH Server
The SSH protocol is recommended for remote login and remote file transfer. However, ssh is open to many attacks. See how to secure OpenSSH server:
Other Recommendation:
- Backups – It cannot be stressed enough how important it is to make a backup of your Linux system. A proper offsite backup allows you to recover from cracked server i.e. an intrusion. The traditional UNIX backup programs are dump and restore are also recommended.
- How to: Looking for Rootkits on Linux based server.
- Howto: Enable ExecShield Buffer Overflows Protection on Linux based server.
- Subscribe to Redhat or Debian Linux security mailing list or RSS feed.
Recommend readings:
- Red Hat Enterprise Linux – Security Guide.
- Linux security cookbook- A good collections of security recipes for new Linux admin.
- Snort 2.1 Intrusion Detection, Second Edition – Good introduction to Snort and Intrusion detection under Linux.
- Hardening Linux – Hardening Linux identifies many of the risks of running Linux hosts and applications and provides practical examples and methods to minimize those risks.
- Linux Security HOWTO.
In the next part of this series I will discuss how to secure specific applications (such as Proxy, Mail, LAMP, Database) and a few other security tools. Did I miss something? Please add your favorite system security tool or tip in the comments.
