Detecting Port Scanning with Fail2ban

When to Use

• Automatically blocking IP addresses that perform port scans against internet-facing servers
• Defending SSH, HTTP, FTP, and other services against brute force attacks with automated IP banning
• Creating custom detection filters for organization-specific attack patterns in log files
• Reducing noise from automated scanning bots before traffic reaches IDS/IPS for deeper analysis
• Implementing defense-in-depth by adding host-based automated response to network monitoring

Do not use as the sole network security control, for protecting against distributed attacks from many source IPs, or as a replacement for proper firewall rules and network segmentation.

Prerequisites

• Fail2ban 0.11+ installed (fail2ban-client --version)
• Root/sudo access for iptables/nftables manipulation
• Services logging connection attempts to parseable log files (syslog, auth.log, access.log)
• iptables or nftables installed and operational as the host firewall
• Optional: SMTP server for email notifications on ban events

Workflow

Step 1: Install and Configure Fail2ban

# Install Fail2ban
sudo apt install -y fail2ban

# Create local configuration (never edit jail.conf directly)
sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local

# Configure global defaults
sudo tee /etc/fail2ban/jail.local << 'EOF'
[DEFAULT]
# Ban duration (1 hour default, escalates for repeat offenders)
bantime = 3600
# Detection window
findtime = 600
# Max failures before ban
maxretry = 5
# Ban action using iptables
banaction = iptables-multiport
banaction_allports = iptables-allports
# Email notifications
destemail = security@example.com
sender = fail2ban@example.com
mta = sendmail
action = %(action_mwl)s

# Ignore internal networks
ignoreip = 127.0.0.1/8 ::1 10.10.0.0/16

# Use systemd journal backend where available
backend = systemd

[sshd]
enabled = true
port = ssh
filter = sshd
logpath = /var/log/auth.log
maxretry = 3
bantime = 7200
findtime = 300

[sshd-ddos]
enabled = true
port = ssh
filter = sshd-ddos
logpath = /var/log/auth.log
maxretry = 6
bantime = 3600
EOF

Step 2: Create Custom Port Scan Detection Filter

# Create iptables logging rule for dropped connections
sudo iptables -N PORTSCAN
sudo iptables -A PORTSCAN -j LOG --log-prefix "PORTSCAN_DETECTED: " --log-level 4
sudo iptables -A PORTSCAN -j DROP

# Log SYN packets to closed ports (indicates scanning)
sudo iptables -A INPUT -p tcp --tcp-flags SYN,ACK,FIN,RST SYN -m state --state NEW \
  -m recent --name portscan --set
sudo iptables -A INPUT -p tcp --tcp-flags SYN,ACK,FIN,RST SYN -m state --state NEW \
  -m recent --name portscan --rcheck --seconds 10 --hitcount 20 -j PORTSCAN

# Create Fail2ban filter for port scanning
sudo tee /etc/fail2ban/filter.d/portscan.conf << 'EOF'
[Definition]
# Match iptables port scan log entries
failregex = PORTSCAN_DETECTED: .* SRC=<HOST> DST=\S+ .* DPT=\d+
ignoreregex =
datepattern = {^LN-BEG}
EOF

# Create Fail2ban filter for Nmap detection via kernel logs
sudo tee /etc/fail2ban/filter.d/nmap-scan.conf << 'EOF'
[Definition]
# Detect rapid connection attempts to multiple ports from same source
failregex = kernel: \[.*\] PORTSCAN_DETECTED: .* SRC=<HOST>
            iptables: .* PORTSCAN .* SRC=<HOST>
ignoreregex =
datepattern = {^LN-BEG}
EOF

# Create filter for HTTP scanning/probing
sudo tee /etc/fail2ban/filter.d/http-scan.conf << 'EOF'
[Definition]
# Detect scanners probing for common vulnerabilities
failregex = ^<HOST> .* "(GET|POST|HEAD) /(wp-login|wp-admin|phpmyadmin|admin|.env|xmlrpc|wp-content/uploads).*" (403|404|444)
            ^<HOST> .* "(GET|POST) /.*\.(php|asp|aspx|jsp|cgi)\?.*" (403|404)
            ^<HOST> .* "() .*" 400
            ^<HOST> .* "(GET|POST) /.*" 400
ignoreregex =
datepattern = {^LN-BEG}
EOF

Step 3: Configure Jail for Port Scanning

# Add port scan jails to jail.local
sudo tee -a /etc/fail2ban/jail.local << 'EOF'

[portscan]
enabled = true
filter = portscan
logpath = /var/log/kern.log
maxretry = 10
findtime = 60
bantime = 86400
banaction = iptables-allports
action = %(action_mwl)s

[nmap-scan]
enabled = true
filter = nmap-scan
logpath = /var/log/kern.log
maxretry = 5
findtime = 30
bantime = 86400
banaction = iptables-allports
action = %(action_mwl)s

[http-scan]
enabled = true
filter = http-scan
logpath = /var/log/nginx/access.log
maxretry = 10
findtime = 300
bantime = 3600
banaction = iptables-multiport
port = http,https


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