Detecting Network Scanning with IDS Signatures

Overview

Network scanning is typically the first phase of an attack, where adversaries enumerate live hosts, open ports, running services, and OS versions using tools like Nmap, Masscan, ZMap, and custom scanners. Detecting this reconnaissance activity provides early warning of potential attacks. IDS/IPS systems like Suricata and Snort can identify scanning through signature-based detection (matching known scanner packet patterns), threshold-based detection (counting connection attempts over time), and anomaly detection (identifying unusual traffic patterns). This skill covers writing and deploying IDS signatures for scan detection, configuring threshold-based alerting, and correlating scan activity with downstream attack indicators.

Prerequisites

Suricata 7.0+ or Snort 3.0+ deployed in IDS/IPS mode
Network TAP or SPAN port for traffic visibility
Emerging Threats ruleset enabled
Logging infrastructure for alert analysis (ELK Stack, Splunk)
Baseline understanding of normal network traffic patterns

Core Concepts

Scanning Techniques and Detection Indicators

Scan Type	Nmap Flag	Packet Characteristics	Detection Method
TCP SYN	`-sS`	SYN flag only, no completion	SYN without SYN/ACK response pattern
TCP Connect	`-sT`	Full 3-way handshake	Multiple connections from single source
TCP FIN	`-sF`	FIN flag only	FIN to closed port (RST response)
TCP Xmas	`-sX`	FIN+PSH+URG flags	Unusual flag combination
TCP NULL	`-sN`	No flags set	Zero-flag TCP packet
UDP Scan	`-sU`	UDP to many ports	ICMP port unreachable responses
ACK Scan	`-sA`	ACK flag only (firewall probing)	Unsolicited ACK packets
SYN/ACK Scan	Custom	SYN+ACK without prior SYN	State violation
OS Fingerprint	`-O`	Unusual TCP options/window sizes	Specific option combinations
Version Detect	`-sV`	Service probe strings	Known probe payloads

Nmap Timing Templates

Template	Nmap Flag	Speed	Detection Difficulty
Paranoid	`-T0`	1 probe/5 min	Very difficult
Sneaky	`-T1`	1 probe/15 sec	Difficult
Polite	`-T2`	1 probe/0.4 sec	Moderate
Normal	`-T3`	Default parallelism	Easy
Aggressive	`-T4`	Parallel, 1.25s timeout	Very easy
Insane	`-T5`	Maximum parallelism	Trivial

Implementation Steps

Step 1: Deploy Suricata Scan Detection Rules

Create /var/lib/suricata/rules/scan-detection.rules:

# === TCP Scan Detection ===

# Detect TCP SYN scan (high volume SYN without completion)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN TCP SYN Scan Detected"; flags:S,12; threshold:type both,track by_src,count 30,seconds 10; classtype:attempted-recon; sid:5000001; rev:2;)

# Detect TCP FIN scan
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN TCP FIN Scan"; flags:F,12; threshold:type both,track by_src,count 20,seconds 60; classtype:attempted-recon; sid:5000002; rev:1;)

# Detect TCP Xmas scan (FIN+PSH+URG)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN TCP Xmas Tree Scan"; flags:FPU,12; classtype:attempted-recon; sid:5000003; rev:1;)

# Detect TCP NULL scan (no flags)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN TCP NULL Scan"; flags:0,12; classtype:attempted-recon; sid:5000004; rev:1;)

# Detect ACK scan (firewall probing)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN TCP ACK Scan"; flags:A,12; flow:stateless; threshold:type both,track by_src,count 50,seconds 30; classtype:attempted-recon; sid:5000005; rev:1;)

# Detect SYN+ACK scan (unusual stateless probe)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN TCP SYN-ACK Scan"; flags:SA,12; flow:stateless; threshold:type both,track by_src,count 30,seconds 30; classtype:attempted-recon; sid:5000006; rev:1;)

# === UDP Scan Detection ===

# Detect UDP port scan
alert udp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN UDP Port Scan"; threshold:type both,track by_src,count 30,seconds 10; classtype:attempted-recon; sid:5000010; rev:1;)

# === Nmap Specific Detection ===

# Detect Nmap OS fingerprinting (T1 probe - ECN SYN)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN Nmap OS Fingerprint Probe"; flags:SEC,12; window:1; classtype:attempted-recon; sid:5000020; rev:1;)

# Detect Nmap window scan (specific window size patterns)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN Nmap Window Size Probe"; flags:A,12; flow:stateless; window:1024; threshold:type both,track by_src,count 10,seconds 30; classtype:attempted-recon; sid:5000021; rev:1;)

# Detect Nmap version detection probes
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN Nmap Service Version Probe"; flow:established; content:"HELP"; depth:4; threshold:type both,track by_src,count 5,seconds 60; classtype:attempted-recon; sid:5000022; rev:1;)

# Detect Nmap scripting engine (NSE)
alert http $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN Nmap NSE HTTP Script"; http.user_agent; content:"Nmap Scripting Engine"; classtype:attempted-recon; sid:5000023; rev:1;)

# === Masscan Detection ===

# Detect Masscan SYN scan (specific window size)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN Masscan SYN Scan Detected"; flags:S,12; window:1024; threshold:type both,track by_src,count 100,seconds 10; classtype:attempted-recon; sid:5000030; rev:1;)

# === Internal Scan Detection ===

# Detect internal host scanning (lateral movement recon)
alert tcp $HOME_NET any -> $HOME_NET any (msg:"SCAN Internal Network Scan Detected"; flags:S,12; threshold:type both,track by_src,count 50,seconds 30; classtype:attempted-recon; sid:5000040; rev:1;)

# Detect internal ICMP sweep
alert icmp $HOME_NET any -> $HOME_NET any (msg:"SCAN Internal ICMP Sweep"; itype:8; threshold:type both,track by_src,count 30,seconds 10; classtype:attempted-recon; sid:5000041; rev:1;)

Step 2: Configure Threshold-Based Detection

Edit /etc/suricata/threshold.config:

# Suppress scan alerts from authorized vulnerability scanners
suppress gen_id 1, sig_id 5000001, track by_src, ip 10.0.5.100
suppress gen_id 1, sig_id 5000001, track by_src, ip 10.0.5.101

# Rate-limit scan alerts to prevent log flooding
rate_filter gen_id 1, sig_id 5000001, track by_src, count 5, seconds 300, new_action alert, timeout 600
rate_filter gen_id 1, sig_id 5000040, track by_src, count 3, seconds 300, new_action alert, timeout 600

# Event filter for critical internal scans
event_filter gen_id 1, sig_id 5000040, type both, track by_src, count 1, seconds 60

Step 3: Scan Detection Analysis Script

#!/usr/bin/env python3
"""Analyze IDS alerts for network scanning activity and generate reports."""

import json
import sys
from collections import defaultdict
from datetime import datetime


class ScanDetector:
    """Correlate IDS alerts to identify scanning campaigns."""

    def __init__(self):
        self.scan_events = defaultdict(lambda: {
            'source_ip': '',
            'target_ips': set(),
            'target_ports': set(),
            'scan_types': set(),
            'alert_count': 0,
            'first_seen': None,
            'last_seen': None,
            'signatures': defaultdict(int),
        })

    def process_eve_json(self, filepath: str):
        """Process Suricata EVE JSON alert log."""
        with open(filepath, 'r') as f:
            for line in f:
                try:
                    event = json.loads(line)
                    if event.get('event_type') != 'alert':
                        continue

                    alert = event.get('alert', {})
                    sig = alert.get('signature', '')

                    if 'SCAN' not in sig:
                        continue

                    src_ip = event.get('src_ip', '')
                    dst_ip = event.get('dest_ip', '')
                    dst_port = event.get('dest_port', 0)
                    ts = datetime.fromisoformat(
                        event['timestamp'].replace('Z', '+00:00')
                    )

                    scanner = self.scan_events[src_ip]
                    scanner['source_ip'] = src_ip
                    scanner['target_ips'].add(dst_ip)
                    scanner['target_ports'].add(dst_port)
                    scanner['alert_count'] += 1
                    scanner['signatures'][sig] += 1

                    if 'SYN' in sig:
                        scanner['scan_types'].add('SYN Scan')
                    elif 'FIN' in sig:
                        scanner['scan_types'].add('FIN Scan')
                    elif 'Xmas' in sig:
                        scanner['scan_types'].add('Xmas Scan')
                    elif 'NULL' in sig:
                        scanner['scan_types'].add('NULL Scan')
                    elif 'UDP' in sig:
                        scanner['scan_types'].add('UDP Scan')
                    elif 'Nmap' in sig:
                        scanner['scan_types'].add('Nmap Detected')
                    elif 'Masscan' in sig:
                        scanner['scan_types'].add('Masscan Detected')
                    elif 'Internal' in sig:
                        scanner['scan_types'].add('Internal Scan')

                    if scanner['first_seen'] is None or ts < scanner['first_seen']:
                        scanner['first_seen'] = ts
                    if scanner['last_seen'] is None or ts > scanner['last_seen']:
                        scanner['last_seen'] = ts

                except (json.JSONDecodeError, KeyError, ValueError):
                    continue

    def generate_report(self):
        """Generate scan detection report."""
        scanners = sorted(
            self.scan_events.values(),
            key=lambda x: x['alert_count'],
            reverse=True
        )

        print(f"\n{'='*70}")
        print("NETWORK SCAN DETECTION REPORT")
        print(f"{'='*70}")
        print(f"Unique Scanning Sources: {len(scanners)}\n")

        for scanner in scanners:
            targets = len(scanner['target_ips'])
            ports = len(scanner['target_ports'])
            duration = (scanner['last_seen'] - scanner['first_seen']).total_seconds() \
                if scanner['first_seen'] and scanner['last_seen'] else 0

            is_internal = scanner['source_ip'].startswith(('10.', '172.', '192.168.'))
            severity = "CRITICAL" if is_internal else \
                       "HIGH" if targets > 50 or ports > 100 else "MEDIUM"

            print(f"[{severity}] Scanner: {scanner['source_ip']}")
            print(f"  Type: {'INTERNAL' if is_internal else 'EXTERNAL'}")
            print(f"  Scan Types: {', '.join(scanner['scan_types'])}")
            print(f"  Target Hosts: {targets}, Target Ports: {ports}")
            print(f"  Total Alerts: {scanner['alert_count']}")
            print(f"  Duration: {duration:.0f}s")
            print(f"  First Seen: {scanner['first_seen']}")
            print(f"  Top Signatures:")
            for sig, count in sorted(
                scanner['signatures'].items(), key=lambda x: x[1], reverse=True
            )[:5]:
                print(f"    - {sig}: {count}")
            print()


if __name__ == '__main__':
    detector = ScanDetector()
    log_file = sys.argv[1] if len(sys.argv) > 1 else '/var/log/suricata/eve.json'
    detector.process_eve_json(log_file)
    detector.generate_report()

Response Playbook

Triage - Determine if scan is from authorized scanner or unknown source
Enrich - Look up source IP in threat intelligence feeds
Assess Scope - Count unique targets and ports to gauge scan breadth
Block - Add aggressive external scanners to firewall block list
Investigate Internal - Internal scans may indicate compromised host; isolate and investigate
Correlate - Check if scan was followed by exploitation attempts

Best Practices

Whitelist Authorized Scanners - Suppress alerts from known vulnerability scanner IPs
Focus on Internal Scans - Internal scanning is higher severity than external (indicates compromise)
Threshold Tuning - Adjust thresholds based on environment; a /16 network sees more scan noise
Correlate with Other Alerts - Combine scan detection with exploitation alerts for kill chain visibility
Time-Based Analysis - Scans at unusual hours (3 AM) warrant higher priority
Rate Limit Alerts - Prevent scan floods from overwhelming the SIEM with noise

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