How to Harden SSH on a Linux Server: Disable Root Login, Enforce Key Authentication, and Block Brute-Force Attacks with Fail2ban

ByAkensai

Learning how to harden SSH on a Linux server is one of the most important things you can do to protect your system. Every internet-facing server gets hit with automated bots scanning for weak SSH configurations. These bots try thousands of username and password combinations every hour. If your server uses default SSH settings, it’s only a matter of time before something goes wrong. This tutorial walks you through three essential layers of SSH security: disabling root login, switching to key-based authentication, and installing Fail2ban to block brute-force attacks. By the end, you’ll have a significantly more secure server that’s much harder to compromise. This guide targets Ubuntu and Debian-based systems, but the steps apply to most Linux distributions with minor adjustments.

Prerequisites for Hardening SSH on a Linux Server

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

What you need:
– A Linux server running Ubuntu 20.04, 22.04, or Debian 11/12
– Root or sudo access to the server
– A local machine with a terminal (Linux/macOS) or PuTTY (Windows)
– Basic comfort with the command line

Estimated time: 30–45 minutes

Important warning: Do not close your current SSH session until you’ve tested that key authentication works. Locking yourself out of your own server is a real risk if you skip this step.

You should also make sure your package list is up to date before installing anything. Run this command first:

sudo apt update && sudo apt upgrade -y

If you’re on a cloud provider like DigitalOcean or AWS, keep your provider’s web console open as a backup access method. That way, if something goes wrong, you can still get in.

Step-by-Step Guide to Harden SSH on a Linux Server

This event shares similarities with: How to Set Up Nginx Reverse Proxy with Ssl Termination Using Docker and Let’s Encrypt Certbot

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

You need to create a key pair before you disable password login. Run this on your local machine, 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 (keep this secret) and a public key ending in .pub.

Step 2: Copy Your Public Key to the Server

Now push your public key to the server:

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

If ssh-copy-id isn’t available, copy it manually:

cat ~/.ssh/id_ed25519.pub | ssh your_user@your_server_ip "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"

Step 3: Test Key Authentication Before Changing Anything

Open a new terminal window and test that you can log in with your key:

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

You should get in without entering a password. Don’t proceed until this works.

Step 4: Edit the SSH Configuration File

Now open the SSH daemon configuration file on your server:

sudo nano /etc/ssh/sshd_config

Find and change these settings. If a line starts with #, remove the hash to uncomment it:

PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys
X11Forwarding no
MaxAuthTries 3
LoginGraceTime 20

Setting PermitRootLogin no forces attackers to guess both a username and a key. Setting PasswordAuthentication no eliminates password-based attacks entirely. You can read more about these options in the official sshd_config documentation.

Step 5: Restart the SSH Service

Apply your changes by restarting SSH:

sudo systemctl restart sshd

Keep your current session open. Open another terminal and try logging in again. If it works, your configuration is correct.

Step 6: Install and Configure Fail2ban

Fail2ban monitors log files and automatically bans IP addresses after too many failed login attempts. Install it now:

sudo apt install fail2ban -y

Create a local configuration file. Never edit the default file directly, as package updates can overwrite it:

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

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

[sshd]
enabled = true
port = ssh
filter = sshd
logpath = /var/log/auth.log
maxretry = 3
bantime = 3600
findtime = 600

This configuration bans any IP that fails 3 login attempts within 10 minutes. The ban lasts one hour.

Step 7: Enable and Start Fail2ban

sudo systemctl enable fail2ban
sudo systemctl start fail2ban

Check that it’s running correctly:

sudo fail2ban-client status sshd

You’ll see the number of currently banned IPs and total failed attempts. Learn more about Fail2ban configuration options in the Fail2ban official documentation.

Step 8: Change the Default SSH Port (Optional but Recommended)

Changing SSH from port 22 to a non-standard port reduces noise from automated scanners. In /etc/ssh/sshd_config, change:

Port 2222

If you use UFW, update your firewall rules:

sudo ufw allow 2222/tcp
sudo ufw delete allow 22/tcp
sudo ufw reload

Restart SSH again: sudo systemctl restart sshd

Troubleshooting SSH Hardening Issues on Linux

Problem: Locked out after disabling password authentication

If you can’t get back in, use your cloud provider’s console or recovery mode. Re-enable PasswordAuthentication yes temporarily, restart SSH, then re-check your key setup.

Problem: Permission denied (publickey)

Check permissions on the server. SSH is strict about file permissions:

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

Problem: Fail2ban not banning IPs

Check the log path. On some systems it’s /var/log/secure instead of /var/log/auth.log. Update logpath in your jail configuration accordingly.

Problem: Can’t connect after changing SSH port

Make sure your firewall allows the new port. Also update your Fail2ban port setting to match. If you use a cloud security group, add the new port there too.

Tip: Always keep a second terminal session open while making SSH changes. This gives you a fallback if something breaks mid-configuration.

Conclusion

You’ve now completed the essential steps to harden SSH on a Linux server. Your server no longer accepts root logins or password-based authentication. Fail2ban is actively watching for brute-force attempts and banning offenders automatically. These three changes together make your SSH setup dramatically more secure than a default installation. From here, you might want to look at setting up two-factor authentication for SSH using Google Authenticator, or configuring UFW to whitelist only specific IP ranges. You could also explore Ubuntu’s official firewall documentation for more advanced network-level protection. Small, consistent security improvements add up quickly. Start with SSH, and keep building from there.

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