How to Harden SSH on a Linux Server: Disable Root Login, Enable Key Authentication, and Configure Fail2ban

ByAkensai

Knowing how to harden SSH on a Linux server: disable root login, enable key authentication, and configure Fail2ban is one of the most important skills for any server administrator. SSH is the primary entry point to your server. If it’s left with default settings, it becomes an easy target for automated brute-force attacks. Every day, thousands of bots scan the internet looking for servers with weak SSH configurations. This tutorial walks you through three critical hardening steps. You’ll disable direct root login, set up SSH key-based authentication, and install Fail2ban to block repeated failed login attempts. By the end, your server will be significantly more resistant to unauthorized access. These steps apply to most Debian-based and Red Hat-based Linux distributions, including Ubuntu, Debian, CentOS, and AlmaLinux.

Prerequisites for How to Harden SSH on a Linux Server: Disable Root Login, Enable Key Authentication, and Configure Fail2ban

Before you start, make sure you have the following in place.

Required access and software:
– A Linux server running Ubuntu 20.04, 22.04, Debian 11/12, or a compatible distribution
– A non-root sudo user already created on the server
– SSH access to the server from your local machine
– A terminal application (Terminal on macOS/Linux, PuTTY or Windows Terminal on Windows)

Assumed knowledge:
– Basic familiarity with the Linux command line
– Ability to open and edit files using nano or vim
– Understanding of what SSH is and how it works

Estimated time: 20–30 minutes

Important warning: Do NOT close your current SSH session until you’ve confirmed a new session works. Locking yourself out of your own server is a real risk if steps are skipped.

You can refer to the official Ubuntu OpenSSH documentation for additional background before proceeding.

Step-by-Step Guide: How to Harden SSH on a Linux Server, Disable Root Login, and Enable Key Authentication

This event shares similarities with: How to Configure Production-ready Ssl/tls Security in Nginx with Tls 1.3 Hardening

Step 1: Generate an SSH key pair on your local machine

Run this command on your local computer, not the server:

ssh-keygen -t ed25519 -C "[email protected]"

Press Enter to accept the default file location. Set a strong passphrase when prompted. This creates two files: a private key and a public key ending in .pub.

Step 2: Copy your public key to the server

Use the ssh-copy-id command to transfer your public key:

ssh-copy-id -i ~/.ssh/id_ed25519.pub your_user@your_server_ip

This adds your public key to ~/.ssh/authorized_keys on the server. Test that key-based login works before moving on. Open a new terminal and connect:

ssh your_user@your_server_ip

You should log in without entering a password (only your passphrase if you set one).

Step 3: Back up the SSH configuration file

Always back up config files before editing them:

sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

Step 4: Disable root login and password authentication

Open the SSH daemon configuration file:

sudo nano /etc/ssh/sshd_config

Find and update these three directives. If they’re commented out with a #, remove the hash and set the values:

PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes

Save the file with Ctrl+O, then exit with Ctrl+X.

Step 5: Optionally change the default SSH port

Changing the port from 22 reduces noise from automated scanners. Find the Port line and change it:

Port 2222

Use any unused port between 1024 and 65535. Remember this port for all future connections.

Step 6: Restart the SSH service

Apply your changes by restarting the SSH daemon:

sudo systemctl restart sshd

Keep your current session open. Open a new terminal window and test the connection:

ssh -p 2222 your_user@your_server_ip

If the connection succeeds, your SSH hardening changes are working correctly.

How to Configure Fail2ban to Complete Your SSH Hardening on a Linux Server

Fail2ban monitors log files and automatically bans IP addresses that show suspicious behavior, like repeated failed login attempts.

Step 7: Install Fail2ban

On Ubuntu or Debian, run:

sudo apt update
sudo apt install fail2ban -y

On CentOS or AlmaLinux, use:

sudo dnf install epel-release -y
sudo dnf install fail2ban -y

Step 8: Create a local Fail2ban configuration

Don’t edit the default config file directly. Create a local override instead:

sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
sudo nano /etc/fail2ban/jail.local

Find the [sshd] section and configure it like this:

[sshd]
enabled = true
port = 2222
filter = sshd
logpath = /var/log/auth.log
maxretry = 5
bantime = 3600
findtime = 600

Here’s what each setting does:
– maxretry = 5 , bans an IP after 5 failed attempts
– bantime = 3600 , keeps the ban active for 1 hour
– findtime = 600 , counts failures within a 10-minute window

Step 9: Enable and start Fail2ban

sudo systemctl enable fail2ban
sudo systemctl start fail2ban

Check that it’s running and the SSH jail is active:

sudo fail2ban-client status sshd

You should see the jail status with current banned IPs and failure counts. You can read more about Fail2ban configuration options in the official Fail2ban documentation on GitHub.

Troubleshooting Common SSH Hardening Issues

Problem: Locked out after restarting SSH

If you can’t connect after restarting, use your hosting provider’s emergency console (like DigitalOcean’s console or AWS EC2 Session Manager). Restore your backup:

sudo cp /etc/ssh/sshd_config.bak /etc/ssh/sshd_config
sudo systemctl restart sshd

Problem: Permission denied (publickey)

This usually means the key wasn’t copied correctly. Check file permissions on the server:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

Problem: Fail2ban not banning IPs

Check the log path matches your system. Ubuntu uses /var/log/auth.log. CentOS uses /var/log/secure. Update logpath in your jail config accordingly.

Problem: Can’t connect on the new port

If you changed the SSH port, your firewall may be blocking it. Allow the new port:

sudo ufw allow 2222/tcp
sudo ufw reload

Tip: Always test new SSH connections in a separate terminal before closing your existing session.

Conclusion

You now know how to harden SSH on a Linux server: disable root login, enable key authentication, and configure Fail2ban to protect against brute-force attacks. These three steps form a solid baseline for SSH security. You’ve blocked direct root access, replaced password logins with cryptographic keys, and set up automatic IP banning for repeated failures. Your server is now much harder to compromise through SSH. From here, consider exploring additional hardening steps like setting up two-factor authentication with Google Authenticator, configuring a firewall with UFW or firewalld, or restricting SSH access to specific IP addresses using AllowUsers in your SSH config. Security is an ongoing process. Revisit your configuration regularly and keep your system packages up to date.

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