The host discovery technique that the tester should use is TCP SYN Ping Scan. This technique sends a TCP SYN packet to a specified port on the target host and waits for a response. If the host responds with a TCP SYN/ACK packet, it means the host is alive and the port is open. If the host responds with a TCP RST packet, it means the host is alive but the port is closed. If the host does not respond at all, it means the host is either dead or filtered by a firewall12. TCP SYN Ping Scan can bypass firewall restrictions because it mimics the initial stage of a TCP three-way handshake, which is a common and legitimate network activity. Therefore, most firewalls will allow TCP SYN packets to pass through and reach the target host, unless they are configured to block specific ports or IP addresses3. TCP SYN Ping Scan can also accurately identify live systems because it does not rely on ICMP, which may be blocked or rate-limited by some firewalls or routers.
The other options are not as effective or feasible as TCP SYN Ping Scan for the following reasons:
A. UDP Ping Scan: This technique sends a UDP packet to a specified port on the target host and waits for a response. If the host responds with an ICMP Port Unreachable message, it means the host is alive but the port is closed. If the host does not respond at all, it means the host is either dead, the port is open, or the packet is filtered by a firewall12. UDP Ping Scan may not bypass firewall restrictions because some firewalls may block or drop UDP packets, especially if they are sent to uncommon or reserved ports. UDP Ping Scan may also not accurately identify live systems because it cannot distinguish between open ports and filtered packets, and it may generate false positives or negatives due to packet loss or rate-limiting.
B. ICMP ECHO Ping Scan: This technique sends an ICMP ECHO Request packet to the target host and waits for an ICMP ECHO Reply packet. If the host responds with an ICMP ECHO Reply packet, it means the host is alive. If the host does not respond at all, it means the host is either dead or filtered by a firewall12. ICMP ECHO Ping Scan may not bypass firewall restrictions because some firewalls may block or drop ICMP packets, especially if they are sent to prevent ping sweeps or denial-of-service attacks. ICMP ECHO Ping Scan may also not accurately identify live systems because it may generate false positives or negatives due to packet loss or rate-limiting.
C. ICMP Timestamp Ping Scan: This technique sends an ICMP Timestamp Request packet to the target host and waits for an ICMP Timestamp Reply packet. If the host responds with an ICMP Timestamp Reply packet, it means the host is alive. If the host does not respond at all, it means the host is either dead or filtered by a firewall12. ICMP Timestamp Ping Scan may not bypass firewall restrictions because some firewalls may block or drop ICMP packets, especially if they are sent to prevent ping sweeps or denial-of-service attacks. ICMP Timestamp Ping Scan may also not accurately identify live systems because it may generate false positives or negatives due to packet loss or rate-limiting.
References:
1: Host Discovery in Nmap Network Scanning - GeeksforGeeks
2: nmap Host Discovery Techniques
3: TCP SYN Ping Scan - Nmap
: Ping Sweep - an overview | ScienceDirect Topics
: UDP Ping Scan - Nmap
: UDP Ping Scan - an overview | ScienceDirect Topics
: ICMP Ping Scan - Nmap
: ICMP Ping Scan - an overview | ScienceDirect Topics
