Why nmap -PU can't find any host available,even the target machine return an ICMP port unreachable p

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本文探讨了使用Nmap进行主机发现时遇到的问题，特别是当目标机器返回ICMP端口不可达包时，Nmap未能正确识别主机为在线状态的情况。文中详细介绍了Nmap的-PU选项的工作原理，并通过Wireshark捕获的数据包来辅助分析问题所在。

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Why nmap -PU can't find any host available,even the target machine return an ICMP port unreachable packet.

I run [nmap -sn -n -PU xxx.xxx.xxx.xxx/24]for host discovery，The description of -PU in official Documents:

[Upon hitting a closed port on the target machine, the UDP probe should elicit an ICMP port unreachable packet in return. This signifies to Nmap that the machine is up and available.]

Meaning that when the target host returns ICMP unreachable [Type3 Code3].Nmap will report this host is available.
I use wireshark to capture.And wireshark displays many hosts replied ICMP port unreachable,but the final result is shown by nmap:"Nmap done: 256 IP addresses (0 hosts up)"，Meaning is no host is available. Selinux and iptables are disable.

My computer:
Centos 6.5

Nmap 6.40

下面是在windows XP下运行的截图，在centos下运行一样，因为截图不方便所以在window下上图。