Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top
telnet 33491
Connected to
Escape character is '^]'
telnet> close
Connection closed.

As you can see, the above telnet command shows port 33491 is open.

Running NMAP with -PN on that same IP and port shows it as closed:

root@Ubuntu:~# nmap -PN -p33491
Starting Nmap 5.00 ( ) at 2011-10-18 18:18 EDT
Interesting ports on (
33491/tcp filtered unknown

Nmap done: 1 IP address (1 host up) scanned in 2.07 seconds

In the above example, telnet shows the port open, but NMAP shows the port as 'filtered' aka closed.

How can I get NMAP to show open ports for remote hosts (that disable ping)?

share|improve this question
Could it be UDP? nmap -sU -PN -p33491 – Alex Forbes Oct 19 '11 at 16:28
@al4: telnet 33491 uses TCP and succeeds. – RedGrittyBrick Oct 19 '11 at 16:36
up vote 4 down vote accepted


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
share|improve this answer

Nmap responding that a port is filtered does not mean that it is closed. It means that NMap is not able to make that determination - it could be either open or closed.

Per the manual Chapter 15

Filtered means that a firewall, filter, or other network obstacle is blocking the port so that Nmap cannot tell whether it is open or closed.

share|improve this answer

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .