Why do I get different results with the same Nmap scan?

Question

ubuntu

I'm trying to get information about the firewall using nmap scanning. And I decided to try to do a syn scan with -sS although it is defaulted if you do not specify a scan type. And doing this scan over and over again I get different results.

nmap -sS 192.168.0.199 1 ⚙
Starting Nmap 7.91 ( https://nmap.org ) at 2021-06-16 02:29 EDT
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0
RTTVAR has grown to over 2.3 seconds, decreasing to 2.0

nmap -sS 192.168.0.199 130 ⨯ 1 ⚙
Starting Nmap 7.91 ( https://nmap.org ) at 2021-06-16 02:32 EDT
Nmap scan report for 192.168.0.199
Host is up (0.00062s latency).
All 1000 scanned ports on 192.168.0.199 are filtered

nmap -sS 192.168.0.199 1 ⚙
Starting Nmap 7.91 ( https://nmap.org ) at 2021-06-16 02:32 EDT
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 3.22 seconds

Answer 1

For example, the scanned host banned you with fail2ban (or just rate limit in iptables or something like that).