Anthropic-Cybersecurity-Skills/skills/detecting-port-scanning-with-fail2ban/SKILL.md

---
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
---
# 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

```bash
# 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

```bash
# 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

```bash
# 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

[recidive]
enabled = true
filter = recidive
logpath = /var/log/fail2ban.log
bantime = 604800
findtime = 86400
maxretry = 3
banaction = iptables-allports
action = %(action_mwl)s
EOF
```

### Step 4: Configure Advanced Ban Actions

```bash
# Create custom action that blocks and sends webhook notification
sudo tee /etc/fail2ban/action.d/iptables-webhook.conf << 'EOF'
[Definition]
actionstart = <iptables> -N f2b-<name>
              <iptables> -A f2b-<name> -j RETURN
              <iptables> -I <chain> -p <protocol> -j f2b-<name>

actionstop = <iptables> -D <chain> -p <protocol> -j f2b-<name>
             <iptables> -F f2b-<name>
             <iptables> -X f2b-<name>

actioncheck = <iptables> -n -L <chain> | grep -q 'f2b-<name>[ \t]'

actionban = <iptables> -I f2b-<name> 1 -s <ip> -j <blocktype>
            curl -s -X POST "<webhook_url>" \
              -H "Content-Type: application/json" \
              -d '{"text":"[Fail2ban] Banned <ip> from <name> jail (failures: <failures>)"}'

actionunban = <iptables> -D f2b-<name> -s <ip> -j <blocktype>

[Init]
chain = INPUT
blocktype = DROP
webhook_url = https://hooks.slack.com/services/XXXX/YYYY/ZZZZ
EOF

# Create escalating ban action for repeat offenders
sudo tee /etc/fail2ban/action.d/escalating-ban.conf << 'EOF'
[Definition]
actionban = <iptables> -I f2b-<name> 1 -s <ip> -j DROP
            echo "$(date) BAN <ip> jail=<name> failures=<failures> bantime=<bantime>" >> /var/log/fail2ban-bans.log

actionunban = <iptables> -D f2b-<name> -s <ip> -j DROP
              echo "$(date) UNBAN <ip> jail=<name>" >> /var/log/fail2ban-bans.log
EOF
```

### Step 5: Test and Validate Detection

```bash
# Restart Fail2ban
sudo systemctl restart fail2ban

# Verify jails are active
sudo fail2ban-client status
sudo fail2ban-client status sshd
sudo fail2ban-client status portscan

# Test the port scan filter with a regex check
sudo fail2ban-regex /var/log/kern.log /etc/fail2ban/filter.d/portscan.conf

# Test the HTTP scan filter
sudo fail2ban-regex /var/log/nginx/access.log /etc/fail2ban/filter.d/http-scan.conf

# Simulate a port scan from a test machine (authorized)
# From the test machine:
nmap -sS -p 1-1000 <target_ip>

# Verify the scanner gets banned
sudo fail2ban-client status portscan
# Should show the test IP in the banned list

# Check iptables for the ban rule
sudo iptables -L f2b-portscan -n

# Unban the test IP
sudo fail2ban-client set portscan unbanip <test_ip>
```

### Step 6: Monitor and Maintain

```bash
# View real-time ban activity
sudo tail -f /var/log/fail2ban.log | grep -E "Ban|Unban"

# Generate daily summary report
sudo tee /usr/local/bin/fail2ban-report.sh << 'SCRIPT'
#!/bin/bash
echo "=== Fail2ban Daily Report $(date) ==="
echo ""
echo "Active Jails:"
sudo fail2ban-client status | grep "Jail list"
echo ""
echo "Currently Banned IPs:"
for jail in $(sudo fail2ban-client status | grep "Jail list" | sed 's/.*://;s/,//g'); do
    count=$(sudo fail2ban-client status "$jail" | grep "Currently banned" | awk '{print $NF}')
    if [ "$count" -gt 0 ]; then
        echo "  $jail: $count banned"
        sudo fail2ban-client status "$jail" | grep "Banned IP"
    fi
done
echo ""
echo "Last 24 hours - Ban count by jail:"
grep "Ban " /var/log/fail2ban.log | grep "$(date +%Y-%m-%d)" | awk '{print $NF}' | sort | uniq -c | sort -rn
SCRIPT
chmod +x /usr/local/bin/fail2ban-report.sh

# Schedule daily report
echo "0 8 * * * root /usr/local/bin/fail2ban-report.sh | mail -s 'Fail2ban Report' security@example.com" | sudo tee /etc/cron.d/fail2ban-report

# Persist iptables rules across reboots
sudo apt install iptables-persistent
sudo netfilter-persistent save
```

## Key Concepts

| Term | Definition |
|------|------------|
| **Jail** | Fail2ban configuration unit that combines a filter (what to detect), an action (what to do), and parameters (thresholds, timing) for a specific service |
| **Filter** | Regular expression patterns that Fail2ban applies to log files to identify failed authentication attempts, scanning, or other malicious activity |
| **Recidive Jail** | Meta-jail that monitors Fail2ban's own log for repeat offenders, applying escalating ban durations to IPs banned multiple times |
| **Find Time** | Time window in seconds during which Fail2ban counts matching log entries; maxretry failures within findtime triggers a ban |
| **Ban Action** | Command or script executed when an IP is banned, typically adding firewall rules but extensible to webhooks, SIEM alerts, or blocklist updates |
| **Ignore IP** | Whitelist of IP addresses or CIDR ranges that are never banned, preventing lockout of trusted networks and monitoring systems |

## Tools & Systems

- **Fail2ban 0.11+**: Log-parsing intrusion prevention framework that bans IP addresses based on pattern matching across any log file
- **iptables/nftables**: Linux kernel firewall used by Fail2ban ban actions to block offending IP addresses at the network layer
- **fail2ban-regex**: Testing utility for validating filter regular expressions against actual log files before deploying to production
- **fail2ban-client**: Command-line management tool for querying jail status, manually banning/unbanning IPs, and reloading configuration
- **rsyslog/syslog-ng**: System logging daemons that generate the log files Fail2ban monitors for attack detection

## Common Scenarios

### Scenario: Defending a Public-Facing Web Server Against Automated Scanning

**Context**: A company runs a public web server that receives thousands of automated scan attempts daily from bots probing for vulnerable paths (/wp-admin, /phpmyadmin, /.env). The security team wants to automatically block scanners while allowing legitimate traffic. The server runs Nginx on Ubuntu 22.04.

**Approach**:
1. Install Fail2ban and configure it to monitor Nginx access logs for scanning patterns (404/403 responses to known vulnerability paths)
2. Create a custom `http-scan` filter matching common scanner signatures and vulnerability probing URIs
3. Set maxretry to 10 within a 5-minute findtime, with a 1-hour bantime for first offense
4. Enable the recidive jail to escalate ban duration to 7 days for repeat offenders
5. Configure webhook notifications to Slack for real-time visibility of banning activity
6. Add iptables logging rules for SYN packets to closed ports to detect port scanning
7. Create a daily report script showing banned IPs, attack patterns, and geographic distribution

**Pitfalls**:
- Setting maxretry too low (e.g., 1-2), causing legitimate users who mistype URLs to get banned
- Not whitelisting monitoring systems (Nagios, UptimeRobot) that may trigger filters with their health checks
- Forgetting to persist iptables rules, losing all bans after a reboot
- Not testing filters with fail2ban-regex before deploying, resulting in no matches or excessive false positives

## Output Format

```
## Fail2ban Port Scan Defense Report

**Server**: web-prod-01 (203.0.113.50)
**Reporting Period**: 2024-03-15 00:00 to 2024-03-16 00:00 UTC

### Active Jails

| Jail | Filter | Max Retry | Ban Time | Currently Banned |
|------|--------|-----------|----------|------------------|
| sshd | sshd | 3 | 2 hours | 12 IPs |
| portscan | portscan | 10 | 24 hours | 47 IPs |
| http-scan | http-scan | 10 | 1 hour | 89 IPs |
| recidive | recidive | 3 | 7 days | 8 IPs |

### 24-Hour Summary
- Total ban events: 347
- Unique IPs banned: 156
- Top attacking country: CN (67 IPs), RU (34 IPs), US (21 IPs)
- Most targeted service: HTTP scanning (214 bans)
- Recidive escalations: 8 IPs banned for 7 days

### Top 5 Banned IPs
| IP Address | Jail | Ban Count | First Seen | Last Seen |
|------------|------|-----------|------------|-----------|
| 45.33.32.156 | portscan | 12 | 00:15 | 23:47 |
| 198.51.100.23 | http-scan | 8 | 02:30 | 18:22 |
| 203.0.113.100 | sshd | 6 | 05:12 | 21:33 |
```