A penetration tester is assessing a network protected by a signature-based Intrusion Detection System (IDS). Initial scans using default Nmap settings are being blocked. The tester decides to use scripting and Nmap's advanced options to customize the scan and evade detection. Which of the following approaches would be most effective at modifying the scan's traffic pattern to bypass the IDS?
Using the -sS (SYN scan) and -A (Aggressive scan) options together to quickly identify all open ports and services.
Writing a Bash script to execute nmap -sT (TCP Connect scan) on all 65,535 ports for each host sequentially.
Implementing a script that randomizes target hosts, fragments packets, and introduces significant, variable time delays between probes.
Utilizing the --system-dns option to resolve hostnames for all IPs in the target subnet before scanning.
To evade a signature-based IDS, a penetration tester should alter the scan to avoid patterns the IDS is configured to detect. Default scans and aggressive options create highly recognizable traffic. The most effective evasion strategy involves combining techniques that mimic benign or unusual traffic. Randomizing targets breaks up sequential scanning patterns, fragmenting packets (-f) can bypass simple signature matching, and introducing variable time delays (--scan-delay with randomization) helps to avoid tripping rate-based detection rules. A full TCP Connect scan (-sT) is easily logged and detected, and simply changing DNS resolution methods does not hide the scan itself.
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