How to show closed ports in NMap

filtered

Nmap cannot determine whether the port is open because packet filtering prevents its probes from reaching the port. The filtering could be from a dedicated firewall device, router rules, or host-based firewall software. These ports frustrate attackers because they provide so little information. Sometimes they respond with ICMP error messages such as type 3 code 13 (destination unreachable: communication administratively prohibited), but filters that simply drop probes without responding are far more common. This forces Nmap to retry several times just in case the probe was dropped due to network congestion rather than filtering. This slows down the scan dramatically.

You could ask nmap to try a TCP Connect ...

nmap -PN -sT -p 33491 example.com

What's the difference between -sS and -sT on nmap?

SCAN TYPES

-sS TCP SYN scan:

This technique is often referred to as "half-open" scanning, because you don’t open a full TCP connection. You send a SYN packet, as if you are going to open a real connection and you wait for a response. A SYN|ACK indicates the port is listen- ing. A RST is indicative of a non-listener. If a SYN|ACK is received, a RST is immediately sent to tear down the connection (actually our OS kernel does this for us). The primary advantage to this scanning technique is that fewer sites will log it. Unfortunately you need root privileges to build these custom SYN packets. This is the default scan type for privileged users.

-sT TCP connect() scan:

This is the most basic form of TCP scanning. The connect() system call provided by your operating system is used to open a connection to every interesting port on the machine. If the port is listening, connect() will succeed, oth erwise the port isn’t reachable. One strong advantage to this technique is that you don’t need any special privileges. Any user on most UNIX boxes is free to use this call.

This sort of scan is easily detectable as target host logs will show a bunch of connection and error messages for the services which accept() the connection just to have it immediately shut- down. This is the default scan type for unprivileged users.

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