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You want to see if the machine at IP address 192.168.1.150 is running SMTP. You want to do this in as quiet a manner as possible. Which of the following nmap commands should you use to accomplish this?
nmap -sO 192.168.1.150
nmap -A -T4 192.168.1.150
nmap -sS -p 25 192.168.1.150
nmap -sS 192.168.1.150/25
Of the options provided, the best choice is nmap -sS -p 25 192.168.1.150. This command will cause nmap to initiate a SYN (or "half-open") scan against the target 192.168.1.150 and will test only TCP port 25, the default port for SMTP.
nmap -sS 192.168.1.150/25 would cause nmap to initiate a SYN scan against all hosts in the network 192.168.1.128/25, scanning all hosts in that subnet for any open ports in the well-known port range. This would likely reveal if the target in question is listening on TCP 25, but is a rather noisy approach in this case since we are interested in a specific target.
nmap -A -T4 192.168.1.150 will scan only the intended target, and is likely to reveal if it is listening on TCP port 25. The -A option will enable OS detection, version detection, script scanning and traceroute. This is not, however, a particularly stealthy option. This syntax will cause nmap to scan all ports in the well-known port range. The -T4 option will enable aggressive timing. This is a noisy scan and much more likely to trigger an IDS than the syntax nmap -sS -p 25 192.168.1.150.
nmap -sO 192.168.1.150 will initiate an IP protocol scan against the target.
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