Learn how to secure your Linux server's SSH access with seven essential security measures, including key-based authentication, two-factor authentication, firewall configuration, and more.
Publish date: 11/9/2024
Secure Shell (SSH) is a fundamental tool for remotely managing Linux servers, providing encrypted communication channels for secure remote access and file transfers.
While SSH is inherently secure due to its encryption protocols, its widespread use across millions of servers makes it a prime target for cybercriminals and automated attacks. Every day, servers face countless unauthorized access attempts through SSH, ranging from simple brute-force attacks to sophisticated exploitation techniques.
Default SSH configurations, while functional, often prioritize convenience over security. This can leave your server vulnerable to various attack vectors, potentially compromising your entire system if breached.
Basically, as a system administrator or server owner, implementing proper SSH security measures isn't just best practice – it's essential for protecting your infrastructure, data, and services from unauthorized access. Otherwise, it's only a matter of time.
That said, in this article, we'll be sharing seven quick and easy methods to improve SSH security on your Linux server. These techniques are proven, and practical, and can be implemented within minutes by nearly anyone. Let's dive in.
Allowing the root user to log in directly via SSH poses a significant security risk.
If an attacker gains access to the root account, they have unrestricted control over your server.
Open the SSH configuration file:
sudo nano /etc/ssh/sshd_config
Note: While the underlying information is very similar, some steps throughout may slightly differ depending on your Linux distribution. For example, your default editor may differ from Nano, such as Vim. Also, if you're using a RHEL-based distribution (Red Hat Enterprise Linux, CentOS Stream, Fedora, AlmaLinux, Rocky Linux, etc.), you'll need to use dnf instead of apt.
Find the line that says PermitRootLogin and change it to:
PermitRootLogin no
Save the file and restart SSH:
sudo systemctl restart sshd
By disabling root login, users must log in with a non-privileged account and escalate privileges using sudo, adding an extra layer of security.
Password-based authentication is susceptible to brute-force attacks.
SSH key-based authentication uses cryptographic keys, which are significantly more secure.
Generate SSH keys on your local machine:
ssh-keygen -t ed25519 -C "[email protected]"
Copy the public key to the server:
ssh-copy-id username@server_ip
Edit the SSH configuration file:
sudo nano /etc/ssh/sshd_config
Set:
PasswordAuthentication no
Restart SSH:
sudo systemctl restart sshd
Changing the default SSH port (22) can reduce the number of automated attacks and bots targeting your server.
Open the SSH configuration file:
sudo nano /etc/ssh/sshd_config
Find the commented-out line Port 22 and uncomment it while changing it to a non-standard port, such as:
Port 2222
Then update your firewall to allow the new port (the following is using UFW, which is standard on Ubuntu and can easily be installed on Debian):
sudo ufw allow 2222/tcp
For FirewallD (RHEL/AlmaLinux/Rocky Linux/Fedora systems):
sudo firewall-cmd --permanent --add-port=2222/tcp
sudo firewall-cmd --reload
Restart SSH:
sudo systemctl restart sshd
Note: Ensure that the chosen port is not used by other services and is allowed through any external firewalls.
Allow SSH on the new port:
sudo ufw allow 2222/tcp
Enable the firewall:
sudo ufw enable
Verify the status:
sudo ufw status
You can also restrict SSH access to specific IP addresses or ranges:
sudo ufw allow from your_ip_address to any port 2222
Allow SSH on the new port:
sudo firewall-cmd --permanent --add-port=2222/tcp
Reload FirewallD to apply changes:
sudo firewall-cmd --reload
Verify the status:
sudo firewall-cmd --list-all
Allow SSH access from specific IP address:
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="your_ip_address" port port="2222" protocol="tcp" accept'
Allow SSH access from specific subnet:
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="192.168.1.0/24" port port="2222" protocol="tcp" accept'
Remove default SSH port if you've changed it:
sudo firewall-cmd --permanent --remove-service=ssh
Check rich rules:
sudo firewall-cmd --list-rich-rules
Note: As mentioned earlier, FirewallD is the default firewall management tool on RHEL-based systems, while UFW is more common on Debian-based systems (Ubuntu, Debian). Choose the appropriate firewall tool based on your distribution.
Install Fail2Ban:
sudo apt-get install fail2ban
Note: Remember, if you're using a RHEL-based distribution, you'll need to use dnf.
Create a local Fail2Ban configuration file:
sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
sudo nano /etc/fail2ban/jail.local
Restart Fail2Ban:
sudo systemctl restart fail2ban
Install Google Authenticator:
sudo apt-get install libpam-google-authenticator
Configure Google Authenticator for each user:
google-authenticator
Modify the SSH configuration:
sudo nano /etc/ssh/sshd_config
Add:
ChallengeResponseAuthentication yes
Edit the PAM SSH configuration:
sudo nano /etc/pam.d/sshd
Add the following line at the end:
auth required pam_google_authenticator.so
Restart SSH:
sudo systemctl restart sshd
Update package lists:
sudo apt-get update
Upgrade installed packages:
sudo apt-get upgrade
Upgrade distribution (if applicable):
sudo apt-get dist-upgrade
Set up automatic security updates:
sudo apt-get install unattended-upgrades
sudo dpkg-reconfigure unattended-upgrades
Update package lists:
sudo dnf check-update
Upgrade installed packages:
sudo dnf upgrade
For major system upgrades:
sudo dnf system-upgrade download --releasever=<version_number>
sudo dnf system-upgrade reboot
Set up automatic security updates:
# Install dnf-automatic
sudo dnf install dnf-automatic
# Enable and start the service
sudo systemctl enable --now dnf-automatic.timer
# Configure automatic updates
sudo nano /etc/dnf/automatic.conf
For either system, reboot if necessary (some updates require a system reboot):
sudo reboot
By implementing these seven methods, you can significantly improve your basic SSH security. And it takes very little time.
Why not give at least a few things on this list a go?
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