How to Harden SSH on Ubuntu Server with Key Authentication, UFW, and Fail2ban

ByAkensai

Learning how to harden SSH on Ubuntu Server with key authentication, UFW, and Fail2ban is one of the most important steps you can take after setting up a new server. By default, SSH allows password-based logins. This makes your server an easy target for automated brute-force attacks. Attackers scan the internet constantly, looking for open port 22 with weak credentials. In this tutorial, you’ll set up SSH key authentication, disable password logins, configure UFW to restrict access, and install Fail2ban to block repeated failed attempts. By the end, your server will be significantly harder to compromise. This guide targets Ubuntu 20.04 and 22.04, but the steps work on most Debian-based systems.

Prerequisites for How to Harden SSH on Ubuntu Server with Key Authentication, UFW, and Fail2ban

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

– A running Ubuntu 20.04 or 22.04 server
– A non-root user with sudo privileges
– SSH access to the server
– A local machine running Linux, macOS, or Windows with an SSH client
– Basic comfort using the terminal

Estimated time to complete: 30–45 minutes.

You should NOT lock yourself out during this process. Always keep your current SSH session open while testing changes in a second terminal window. This is a critical safety habit. If you close your session before confirming access works, you may lose the ability to log back in.

You can read more about Ubuntu’s official SSH documentation at Ubuntu OpenSSH Server Docs.

Step-by-Step Guide to Harden SSH on Ubuntu Server with Key Authentication, UFW, and Fail2ban

For more strange history, see: How to Harden Ssh on Ubuntu Server with Key-based Authentication and Fail2ban

Step 1: Generate an SSH Key Pair on Your Local Machine

Open a terminal on your local machine. Run the following command to generate a strong ED25519 key pair:

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

When prompted, choose a save location. Press Enter to accept the default. Set a strong passphrase when asked. This adds a second layer of protection if your private key is ever stolen.

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 appends your public key to ~/.ssh/authorized_keys on the server. You’ll be asked for your password one last time. After this step, key-based login will work.

Step 3: Test Key Authentication Before Disabling Passwords

Open a NEW terminal window and test your SSH key login:

ssh -i ~/.ssh/id_ed25519 your_user@your_server_ip

You should log in without entering a password. Do not proceed until this works correctly. Skipping this test is how people get locked out.

Step 4: Disable Password Authentication in SSH Config

On the server, open the SSH daemon configuration file:

sudo nano /etc/ssh/sshd_config

Find and update these lines:

PasswordAuthentication no
PubkeyAuthentication yes
PermitRootLogin no
AuthorizedKeysFile .ssh/authorized_keys

Save and close the file. Then restart the SSH service:

sudo systemctl restart ssh

Test login again in your second terminal window. Confirm it still works before moving on.

Step 5: Configure UFW to Restrict SSH Access

UFW (Uncomplicated Firewall) is built into Ubuntu. Enable it and allow only what you need:

sudo ufw allow OpenSSH
sudo ufw enable
sudo ufw status verbose

If your server runs a web server, also allow HTTP and HTTPS:

sudo ufw allow 80/tcp
sudo ufw allow 443/tcp

If you know your IP address, you can restrict SSH to only your IP:

sudo ufw allow from YOUR_IP to any port 22

This is the most secure option. It blocks SSH from everyone except your specific IP address.

Step 6: Install and Configure Fail2ban

Fail2ban monitors log files and bans IPs that show signs of brute-force activity. Install it with:

sudo apt update
sudo apt install fail2ban -y

Create a local jail configuration file to avoid overwriting defaults during updates:

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

Find the [sshd] section and update it:

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

This configuration bans any IP that fails SSH login 5 times within 10 minutes. The ban lasts one hour. Save the file and restart Fail2ban:

sudo systemctl restart fail2ban
sudo systemctl enable fail2ban

Check that Fail2ban is running correctly:

sudo fail2ban-client status sshd

You’ll see active jails and any currently banned IPs listed in the output.

Step 7: Optional , Change the Default SSH Port

Changing SSH from port 22 to a custom port reduces automated scan traffic. Open /etc/ssh/sshd_config and change:

Port 2222

Update UFW to allow the new port and remove the old rule:

sudo ufw allow 2222/tcp
sudo ufw delete allow OpenSSH
sudo systemctl restart ssh

Test your connection on the new port before closing your session. Use ssh -p 2222 your_user@your_server_ip.

Troubleshooting Common SSH Hardening Issues on Ubuntu

Problem: Locked out after disabling password authentication

If you can’t log in after restarting SSH, you’ll need console access through your hosting provider’s control panel. Most providers offer a web-based console. Use it to re-enable PasswordAuthentication yes temporarily.

Problem: Fail2ban isn’t banning IPs

Check the log path. On Ubuntu 22.04, the log may be at /var/log/auth.log or /var/log/syslog. Confirm with:

sudo tail -f /var/log/auth.log

Also verify the service is active:

sudo systemctl status fail2ban

Problem: UFW blocking legitimate traffic

Run sudo ufw status numbered to see all rules. Delete a specific rule with sudo ufw delete [rule_number]. Be careful not to delete your SSH rule while connected.

Problem: SSH key not accepted

Check permissions on the server. The .ssh directory must be 700 and authorized_keys must be 600:

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

Wrong permissions are the most common reason key authentication fails. You can learn more about Fail2ban configuration at the official Fail2ban documentation.

Conclusion

You’ve now completed the essential steps to harden SSH on Ubuntu Server with key authentication, UFW, and Fail2ban. Your server no longer accepts password logins. It blocks suspicious IPs automatically. And your firewall only allows traffic you’ve explicitly permitted. These three layers work together to dramatically reduce your attack surface. Keeping your system updated is the next important step. Run sudo apt update && sudo apt upgrade regularly. You might also explore setting up two-factor authentication for SSH or configuring automatic security updates with Ubuntu’s unattended-upgrades package. A secure server starts with these fundamentals, and you now have them all in place.

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