Understanding Port States and Network Security Techniques

📋 Course Outline

1. Open|Filtered State
2. SYN Proxy Functionality
3. Firewall and Load Balancer Roles
4. Nmap Half-Open Scan
5. Port Knocking Technique
6. Impact of NAT Devices
7. TLS Mutual Authentication

📖 1. Open|Filtered State

🔑 Key Concepts & Definitions

• Open|Filtered state: When Nmap cannot determine if a port is open or filtered because it does not receive the expected TCP response (SYN/ACK or RST). The port appears as open|filtered, indicating ambiguity (source content).
• Reason for open|filtered classification: Nmap fails to receive TCP responses that distinguish open from filtered ports, often due to network devices like firewalls, load balancers, or routers acting as SYN proxies (TCP intercept), which answer initial SYNs but withhold subsequent responses until the handshake completes (source content).
• Role of missing TCP responses: The absence of SYN/ACK or RST packets during a half-open scan causes Nmap to classify ports as open|filtered because it cannot confirm if the port is open or filtered by a firewall or proxy (source content).
• Effect of SYN proxy: A SYN proxy answers the initial SYN on behalf of the protected host but withholds the internal SYN/ACK until the handshake is fully completed, leading to open|filtered classification in Nmap scans (source content).
• Difference between port states:
  • Open: Port responds with SYN/ACK, indicating readiness to establish a connection.
  • Closed:…