nmap: Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn

Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn means Nmap's host discovery phase got no answer, so it skipped the port scan. Host discovery sends ICMP echo and a few TCP probes; many hosts and firewalls drop all of them. If you know the host is up, run the scan with -Pn, which treats the host as online and goes straight to port scanning: nmap -Pn -p 22,80,443 HOST. Nmap still exits with status 0, as the run itself succeeded.

What it looks like

Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn
Nmap done: 1 IP address (0 hosts up) scanned in N seconds

Fixes

nmap -Pn HOST                        # skip discovery, scan the top 1000 ports
nmap -Pn -p 22,443 HOST              # only the ports you care about (much faster with -Pn)
sudo nmap HOST                       # root: ICMP, timestamp, SYN to 443 and ACK to 80 as discovery
sudo nmap -PS22,443 -PA80 HOST       # custom discovery probes the firewall allows
nmap -Pn --reason -p 22 HOST         # show why each port got its state

With -Pn and many targets, scans take longer: every address is scanned even if nothing lives there. On a local network, sudo nmap -sn 192.168.1.0/24 uses ARP, which firewalls on the hosts cannot block.

Which discovery probes are used