Scanning Beyond IDS and Firewall

Comprehensive Guide to Scanning Beyond IDS and Firewall

Introduction

Understanding how to scan networks through intrusion detection systems (IDS) and firewalls is essential for cybersecurity professionals, particularly for those preparing for the Certified Ethical Hacker (CEH) exam. This guide covers the importance, mechanisms, and exam preparation tips for this critical topic.

Why Scanning Beyond IDS and Firewall is Important

Security professionals need to understand how attackers bypass defensive measures for several reasons:
- To properly assess network vulnerabilities
- To implement effective countermeasures
- To ensure comprehensive security testing
- To protect against sophisticated attacks that evade standard security controls
- To meet compliance requirements for thorough security testing

What is Scanning Beyond IDS and Firewall?

This concept refers to techniques used to conduct network reconnaissance and scanning while avoiding detection by security measures like firewalls and intrusion detection systems. These techniques allow attackers (or ethical hackers during security assessments) to gather information about target systems even when protective measures are in place.

How Scanning Beyond IDS and Firewall Works

1. IDS Evasion Techniques:
- Fragmentation: Splitting packets into smaller fragments to avoid signature-based detection
- Timing techniques: Slow scanning that stays below threshold alerts
- Source port manipulation: Using trusted ports like 53, 80, or 443
- Decoys: Generating noise from multiple IP addresses to obscure the real scan source
- Encryption: Using encrypted channels to mask scanning activity

2. Firewall Bypass Methods:
- IP address spoofing: Disguising source addresses
- Using proxy servers or TOR: Hiding original scan source
- Source routing: Specifying the route packets take through networks
- ACK scanning: Using only ACK packets which may pass through simple firewalls
- FTP bounce attacks: Using FTP servers as intermediaries
- ICMP tunneling: Hiding data in ICMP packets that may be permitted through firewalls

3. Tools for Advanced Scanning:
- Nmap with specific options (e.g., -f for fragmentation, -D for decoy scanning)
- Hping3 for crafting custom packets
- Metasploit scanning modules
- Nessus with IDS evasion plugins
- Proxychains for routing scans through proxies

Exam Tips: Answering Questions on Scanning Beyond IDS and Firewall

1. Key Areas to Focus On:
- Understand the differences between stateful and stateless firewalls
- Know common port numbers that firewalls typically allow
- Memorize Nmap commands and switches for evasion techniques
- Learn protocol behaviors that can be exploited for evasion
- Know the limitations of signature-based versus anomaly-based detection

2. Common Question Types:
- Scenario-based questions asking for the best evasion technique in a given situation
- Tool-specific questions about which Nmap flags provide certain evasion capabilities
- Questions comparing different scanning techniques' effectiveness against security controls
- Identification of signatures or patterns that might trigger an IDS alert

3. Strategy for Answering:
- Read carefully for clues about the security infrastructure described in the question
- Consider which evasion technique best matches the scenario constraints
- Remember that slower, more stealthy techniques are often preferred when evading IDS
- For firewall evasion, consider which protocols are most likely to be permitted outbound
- Look for the most efficient technique that achieves the objective with minimal detection risk

4. Practical Application Knowledge:
- Questions may require you to select the correct command syntax
- Understand the expected results of different scan types
- Know how to interpret scan results to identify successfully bypassed security controls
- Be familiar with logs and how evasion techniques might appear in them

Conclusion

Scanning beyond IDS and firewalls represents an advanced skill set in the ethical hacker's toolkit. Mastery of these techniques demonstrates a deep understanding of network security fundamentals, protocol behaviors, and the limitations of security controls. For the CEH exam, focus on recognizing the appropriate techniques for specific scenarios and understanding the technical details of how each evasion method works.