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.
When to Use
- When investigating security incidents that require detecting network scanning with ids signatures
- When building detection rules or threat hunting queries for this domain
- When SOC analysts need structured procedures for this analysis type
- When validating security monitoring coverage for related attack techniques
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 |
Workflow
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
"""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':
alert = event.get(, {})
sig = alert.get(, )
sig:
src_ip = event.get(, )
dst_ip = event.get(, )
dst_port = event.get(, )
ts = datetime.fromisoformat(
event[].replace(, )
)
scanner = .scan_events[src_ip]
scanner[] = src_ip
scanner[].add(dst_ip)
scanner[].add(dst_port)
scanner[] +=
scanner[][sig] +=
sig:
scanner[].add()
sig:
scanner[].add()
sig:
scanner[].add()
sig:
scanner[].add()
sig:
scanner[].add()
sig:
scanner[].add()
sig:
scanner[].add()
sig:
scanner[].add()
scanner[] ts < scanner[]:
scanner[] = ts
scanner[] ts > scanner[]:
scanner[] = ts
(json.JSONDecodeError, KeyError, ValueError):
():
scanners = (
.scan_events.values(),
key= x: x[],
reverse=
)
()
()
()
()
scanner scanners:
targets = (scanner[])
ports = (scanner[])
duration = (scanner[] - scanner[]).total_seconds() \
scanner[] scanner[]
is_internal = scanner[].startswith((, , ))
severity = is_internal \
targets > ports >
()
()
()
()
()
()
()
()
sig, count (
scanner[].items(), key= x: x[], reverse=
)[:]:
()
()
__name__ == :
detector = ScanDetector()
log_file = sys.argv[] (sys.argv) >
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
References
