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I am able to ping a server on an vpn. However, when I use a normal nmap scan it does not pickup on the host. Doing a scan with host discovery turned off (nmap -Pn host scan) does show the host.

If I am not mistaken, should my ping not fail if the regular nmap scan (nmap host) fails?

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2 Answers 2

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From nmap.org:

If no host discovery options are given, Nmap sends an ICMP echo request, a TCP SYN packet to port 443, a TCP ACK packet to port 80, and an ICMP timestamp request. ... For unprivileged Unix shell users, the default probes are a SYN packet to ports 80 and 443 using the connect system call.

I am guessing you ran nmap as an unprivileged user, so ICMP was not used for host discovery. Using -Pn "works" because it skips host discovery and treats the host as up no matter what.

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The ping protocol is a layer 3 protocol, so, it doesn't made any TCP/UDP connection and it doesn't use any port, maybe the server that you ran nmap against it is simply block TCP/UDP connection or Block them just if they come from unknown IP.

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    Either TCP/UDP and ICMP (the protocol used by ping) is layer 3. This answer is incorrect.
    – slayer
    May 21, 2021 at 3:01
  • @slayer ICMP is often presented as a layer 3 protocol (which I do disagree with)
    – multithr3at3d
    May 22, 2021 at 3:55
  • @slayer, TCP/UDP are transport layer protocols but the ICMP is a network layer protocol.
    – Mohamed Marrouchi
    May 24, 2021 at 15:54
  • @MohamedMarrouchi The TCP/UDP on transport (4) layer doesn’t provide a guarantee of packet delivery. It is the charge of the network (3) layer to assurance the connection-oriented and connectionless service, AFAIK. The ICMP is a connectionless protocol.
    – slayer
    May 24, 2021 at 16:13
  • @MohamedMarrouchi Ah! I noticed a mess here. Now I understood what you mean. When you affirmed, ICMP protocol is the network layer to clarify that it is a connectionless protocol, I had confused because TCP/UDP is also layer 3. So, I had understood that you were dealing with TCP/UDP working only at the transport layer. Your answer was confusing to me, but now that I understand, I agree. lol
    – slayer
    May 24, 2021 at 17:01

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