detecting-port-scanning-with-fail2ban

name	detecting-port-scanning-with-fail2ban
description	Configures Fail2ban with custom filters and actions to detect port scanning activity, SSH brute force attempts, and network reconnaissance, automatically banning offending IP addresses and alerting security teams to suspicious network probing.
domain	cybersecurity
subdomain	network-security
tags	["network-security","fail2ban","port-scanning","intrusion-prevention","automated-defense"]
version	1.0
author	mahipal
license	Apache-2.0
nist_csf	["PR.IR-01","DE.CM-01","ID.AM-03","PR.DS-02"]
mitre_attack	["T1046","T1040","T1557","T1071","T1595"]
source	https://github.com/mukul975/Anthropic-Cybersecurity-Skills
source_commit	04450304b12645cb2b974ab96d28c0664758a88d
note	Vendored verbatim from an external Apache-2.0 security-skill library, pinned by commit. Exceeds the internal 300-line skill guideline (agent-code-constraints.md) -- kept as-is because this is vendored reference material (forensics/threat-intel procedure), not Yana AI-authored content, and trimming would damage technical accuracy.

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