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I have a port forwarded home server running CentOS 7 on my home network. I am trying to fix some issues with it and tried to do an nmap host discovery scan but it doesn't work and says that all hosts are down:

nmap -sL 192.168.1.0/24

I am trying to scan the IP 192.168.1.152 and tried to scan it with nmap:

nmap 192.168.1.152
nmap -Pn 192.168.1.152

It says the host is down. It is not.

I am able to ping 192.168.1.152 succesfully.

I have never had this problem with my server before I started using CentOS so it has to be a CentOS issue, but I have found absolutely no related issues for this online. What's going on?

EDIT: firewall-cmd --list-all output:

public (active)
  target: default
  icmp-block-inversion: no
  interfaces: eth0
  sources: 
  services: ftp http https openvpn ssh
  ports: 1194/tcp 22/tcp
  protocols: 
  masquerade: yes
  forward-ports: 
  source-ports: 
  icmp-blocks: 
  rich rules:
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  • Where are you running the scan from - same subnet? What are the firewall rules on the server? Commented Aug 26, 2020 at 11:56
  • Mostly likely the same issue as in your other question. unix.stackexchange.com/questions/606417/… Commented Aug 26, 2020 at 11:57
  • I will add an edit with the firewall rules. Commented Aug 26, 2020 at 12:01

1 Answer 1

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nmap -sL is a "List scan" that does not send any probes to the targets. It always lists targets as "down" because it has not tried to find out if they are "up." It is intended for expanding network address ranges and performing reverse DNS name resolution, not checking connectivity.

If you can "ping" the target address, meaning you used the ping utility, then it is responsive to ICMP Echo Request messages. Nmap can send these as well, but it requires root/sudo privileges, and for local LAN addresses it sends ARP requests instead. If you do not have sufficient privilege, it will instead try to connect to port 80 and port 443; success or connection reset error is interpreted as the target being "up."

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