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Tanvir Rahman
Tanvir Rahman

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Secure Your Linux Server with Fail2Ban (Step-by-Step Guide)

#linux #devops #fail2ban #security

Brute force attacks on SSH, mail and web services are a constant threat to any internet-facing server. Fail2Ban is a powerful, lightweight tool that helps mitigate these attacks by automatically banning IPs that exhibit malicious behavior.

In this guide, we’ll walk through the complete installation and configuration of Fail2Ban on a Linux server to secure SSH and Postfix (mail server). We’ll also set up email alerts and configure persistent bans.

🧰 Prerequisites

  • A Linux server (Debian/Ubuntu or CentOS/RHEL)
  • Root or sudo access
  • SSH access to the server
  • (Optional) A mail server (Postfix) if you want to monitor email service

🛠️ 1. Update System Packages

Before installing anything, make sure your package manager is up-to-date.

sudo apt update && sudo apt upgrade -y  # For Debian/Ubuntu
# or
sudo yum update -y                      # For CentOS/RHEL
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📦 2. Install Fail2Ban and Dependencies

Install Fail2Ban using your system’s package manager:

sudo apt install -y fail2ban            # Debian/Ubuntu
# or
sudo yum install -y fail2ban            # CentOS/RHEL
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📁 3. Create Custom Jail Configuration

Fail2Ban uses jails to define which services to monitor. Let’s back up the default config and create a custom one.

Backup the Default Config 

sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
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Modify Basic Settings

Set a 24-hour ban time, 10-minute find time, and allow a maximum of 5 retries:

sudo sed -i 's/bantime  = 10m/bantime  = 24h/;s/findtime  = 10m/findtime  = 600/;s/maxretry = 5/maxretry = 5/' /etc/fail2ban/jail.local
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📄 4. Configure Jails for SSH and Postfix

Now, let’s create a separate jail configuration file:

sudo tee /etc/fail2ban/jail.d/custom.conf << 'EOF'
[sshd]
enabled = true
port = 22
filter = sshd
logpath = /var/log/auth.log
maxretry = 5

[postfix]
enabled = true
port = 25
filter = postfix
logpath = /var/log/mail.log
maxretry = 5
EOF
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📧 5. Configure Email Notifications

Ensure mailutils is installed and the mail command is available:

which mail
# Should return: /usr/bin/mail
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Then, add your email settings in jail.local:

sudo tee -a /etc/fail2ban/jail.local << 'EOF'

[DEFAULT]
banaction = iptables-multiport
banaction_allports = iptables-allports
loglevel = DEBUG
ignoreself = false
action = %(action_mwl)s
sender = fail2ban@yourdomain.com
destemail = admin@yourdomain.com
mta = sendmail

EOF
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Replace yourdomain.com and the email addresses with your actual values.

📊 6. Enable and Start Fail2Ban

Enable the service to start on boot and start it now:

sudo systemctl enable fail2ban
sudo systemctl start fail2ban
sudo systemctl status fail2ban
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Restart Fail2Ban to Apply All Changes 

sudo systemctl restart fail2ban
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🧪 7. Testing & Monitoring

Simulate a Ban

Try multiple failed SSH login attempts from another IP.

Check Logs 

sudo tail -f /var/log/fail2ban.log
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View Real-Time Jail Status 

sudo fail2ban-client status sshd
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Unban an IP 

sudo fail2ban-client set sshd unbanip <IP_ADDRESS>
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🔧 Optional: Adjust UFW if Installed

If you use ufw, remove conflicting rules and allow SSH manually:

sudo ufw delete limit 22/tcp
sudo ufw delete limit 22/tcp "v6"
sudo ufw allow 22/tcp
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✅ Summary

Here’s what we’ve accomplished:

  • ✅ Updated system packages
  • ✅ Installed and configured Fail2Ban
  • ✅ Created custom jails for SSH and Postfix
  • ✅ Set ban time (24h), find time (10m), max retry (5)
  • ✅ Configured email alerts
  • ✅ Enabled persistent bans and logging
  • ✅ Tested the setup

Your server is now better protected against brute force attacks. 🎉

📌 Next Steps

  • Add additional jails (e.g., Nginx, Apache) if you run web servers
  • Whitelist internal or VPN IPs (/etc/fail2ban/jail.d/whitelist.conf)
  • Monitor logs regularly for suspicious activity
  • Fine-tune thresholds if you experience false positives

Stay safe and secure! 🔐

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