I get unintuitive results from "nmap -A " which I want to clarify.
Setup: sshd (ssh deamon service) is successfully running. postfix is installed and so smpt service is running. However, it only configured to send mail, not to receive. apache not installed, iptables empty, ufw not installed.
From this primary nmap documentation source: "Closed ports have no application listening on them"
Is it possible that some instances of "filtered" results also simply have no application listening on them? Or does "filtered" always mean there is some other reason? (If it's the latter, I would like to find out what that other reason is, which is why I am asking.)
nmap -A xxxxxx.com Starting Nmap 7.01 ( https://nmap.org ) at 2018-01-21 18:13 PST Nmap scan report for xxxxxx.com (45.**.***.***) Host is up (0.058s latency). rDNS record for 45.**.***.***: li****-***.members.linode.com Not shown: 995 closed ports PORT STATE SERVICE VERSION 22/tcp open ssh OpenSSH 7.2p2 Ubuntu 4ubuntu2.2 (Ubuntu Linux; protocol 2.0) | ssh-hostkey: | 2048 **** (RSA) |_ 256 **** (ECDSA) 25/tcp filtered smtp 135/tcp filtered msrpc 139/tcp filtered netbios-ssn 445/tcp filtered microsoft-ds Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel sudo lsof -i -n COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME sshd 3396 root 3u IPv4 15205 0t0 TCP *:ssh (LISTEN) sshd 3396 root 4u IPv6 15214 0t0 TCP *:ssh (LISTEN) master 4988 root 12u IPv4 19670 0t0 TCP 127.0.0.1:smtp (LISTEN) master 4988 root 13u IPv6 19671 0t0 TCP [::1]:smtp (LISTEN) sshd 5582 root 3u IPv4 30352 0t0 TCP **.**.***.***:ssh->**.**.***.***:54224 (ESTABLISHED) sshd 5602 izxzxzn 3u IPv4 30352 0t0 TCP **.**.***.***:ssh->**.**.***.***:54224 (ESTABLISHED) sudo iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination Chain FORWARD (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination