Detecting Network Scanning With Ids Signatures

使用 IDS 签名检测网络扫描

概述

网络扫描（network scanning）通常是攻击的第一阶段，攻击者使用 Nmap、Masscan、ZMap 和自定义扫描工具枚举活跃主机、开放端口、运行中的服务和操作系统版本。检测这种侦察活动可以为潜在攻击提供早期预警。Suricata 和 Snort 等 IDS/IPS 系统可通过三种方式识别扫描：基于签名的检测（匹配已知扫描工具的数据包模式）、基于阈值的检测（统计一段时间内的连接尝试次数）和异常检测（识别异常流量模式）。本技能涵盖编写和部署用于扫描检测的 IDS 签名、配置基于阈值的告警，以及将扫描活动与下游攻击指标进行关联。

前置条件

• 以 IDS/IPS 模式部署 Suricata 7.0+ 或 Snort 3.0+
• 用于流量可见性的网络 TAP 或 SPAN 端口
• 启用 Emerging Threats 规则集
• 用于告警分析的日志基础设施（ELK Stack、Splunk）
• 了解正常网络流量模式的基线知识

核心概念

扫描技术与检测指标

Nmap 时序模板

实施步骤

步骤 1：部署 Suricata 扫描检测规则

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

# === TCP 扫描检测 ===

# 检测 TCP SYN 扫描（大量 SYN 但未完成）
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;)

# 检测 TCP FIN 扫描
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;)

# 检测 TCP Xmas 扫描（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;)

# 检测 TCP NULL 扫描（无标志）
alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SCAN TCP NULL Scan"; flags:0,12; classtype:attempted-recon; sid:5000004; rev:1;)

# 检测 ACK 扫描（探测防火墙）
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;)

# 检测 SYN+ACK 扫描（异常无状态探测）
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 扫描检测 ===

# 检测 UDP 端口扫描
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 专项检测 ===

# 检测 Nmap 操作系统指纹识别（T1 探测——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;)

# 检测 Nmap 窗口扫描（特定窗口大小模式）
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;)

# 检测 Nmap 版本检测探测
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;)

# 检测 Nmap 脚本引擎（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 检测 ===

# 检测 Masscan SYN 扫描（特定窗口大小）
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;)

# === 内部扫描检测 ===

# 检测内部主机扫描（横向移动侦察）
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;)

# 检测内部 ICMP 扫描
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;)

步骤 2：配置基于阈值的检测

编辑 /etc/suricata/threshold.config：

# 对已授权的漏洞扫描器抑制扫描告警
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

# 限制扫描告警速率，防止日志洪泛
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

# 关键内部扫描的事件过滤
event_filter gen_id 1, sig_id 5000040, type both, track by_src, count 1, seconds 60

步骤 3：扫描检测分析脚本

#!/usr/bin/env python3
"""分析 IDS 告警中的网络扫描活动并生成报告。"""

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


class ScanDetector:
    """关联 IDS 告警以识别扫描活动。"""

    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):
        """处理 Suricata EVE JSON 告警日志。"""
        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 扫描')
                    elif 'FIN' in sig:
                        scanner['scan_types'].add('FIN 扫描')
                    elif 'Xmas' in sig:
                        scanner['scan_types'].add('Xmas 扫描')
                    elif 'NULL' in sig:
                        scanner['scan_types'].add('NULL 扫描')
                    elif 'UDP' in sig:
                        scanner['scan_types'].add('UDP 扫描')
                    elif 'Nmap' in sig:
                        scanner['scan_types'].add('检测到 Nmap')
                    elif 'Masscan' in sig:
                        scanner['scan_types'].add('检测到 Masscan')
                    elif 'Internal' in sig:
                        scanner['scan_types'].add('内部扫描')

                    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):
        """生成扫描检测报告。"""
        scanners = sorted(
            self.scan_events.values(),
            key=lambda x: x['alert_count'],
            reverse=True
        )

        print(f"\n{'='*70}")
        print("网络扫描检测报告")
        print(f"{'='*70}")
        print(f"唯一扫描来源：{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 = "严重" if is_internal else \
                       "高" if targets > 50 or ports > 100 else "中"

            print(f"[{severity}] 扫描源：{scanner['source_ip']}")
            print(f"  类型：{'内部' if is_internal else '外部'}")
            print(f"  扫描类型：{', '.join(scanner['scan_types'])}")
            print(f"  目标主机数：{targets}，目标端口数：{ports}")
            print(f"  告警总数：{scanner['alert_count']}")
            print(f"  持续时间：{duration:.0f} 秒")
            print(f"  首次发现：{scanner['first_seen']}")
            print(f"  主要签名：")
            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()

响应剧本

1. 分类（Triage）——确定扫描是否来自已授权的扫描器或未知来源
2. 信息丰富（Enrich）——在威胁情报 feeds 中查询源 IP
3. 评估范围——统计唯一目标和端口数量，评估扫描广度
4. 封锁——将激进的外部扫描器添加到防火墙封锁列表
5. 调查内部——内部扫描可能表明主机被入侵；隔离并调查
6. 关联——检查扫描后是否发生了利用尝试

最佳实践

• 将授权扫描器加白名单——对已知漏洞扫描器 IP 抑制告警
• 重点关注内部扫描——内部扫描比外部扫描严重性更高（表明已被入侵）
• 阈值调整——根据环境调整阈值；/16 网络的扫描噪声更多
• 与其他告警关联——将扫描检测与利用告警结合，获得杀伤链可见性
• 基于时间的分析——非常规时间（凌晨 3 点）的扫描需优先处理
• 限制告警速率——防止扫描洪泛使 SIEM 充斥噪声

参考资料

• Suricata 规则文档
• Nmap IDS 规避技术
• OPNsense Suricata Nmap 检测规则
• Emerging Threats 规则集