Port scanner

All posts tagged Port scanner

Nmap is a popular port scanner. Today we are going to see target specification in nmap. In Nmap target can be specified in six different ways. We are going to look at each one of them. For this, I am using Kali Linux connected internally with IP address

1. nmap -iL <input filename>

This option reads targets from a specific file. Let’s create a file named alpha with a few   newline separated or tab-limited or space separated IP addresses using vi on Desktop.



Then, move to Desktop directory and type the command “nmap -iL alpha” as shown below. It scans the four IP addresses listed in our file.


2. nmap -iR <number of hosts>

This option is used to specify random hosts to scan. Nmap generates its own targets to scan. The <number of hosts> argument tells nmap the number of IP’s to generate. Let’s type command “nmap -iR 5″. We can see that nmap generated five random targets to scan. The scan failed to determine route to the generated IP addresses because my system is not connected to Internet. Why didn’t it generate any internal IP addresses? Because in this scan private, multicast and unallocated address ranges are automatically skipped.



3. nmap <IP address/dir>

This option is used to scan the entire subnet.  Assuming you know CIDR let’s type the command nmap to scan eight IP addresses from to and see the result below.



4. nmap [targets] -exclude[targets]

This option specifies a comma separated list of targets to be excluded from the scan even if they are part of overall network range we specify. For example, in our previous scan we saw that one host which is the system I am working on) is alive. Now I decided to exclude that host and another host from the scan. So I type the command “nmap -exclude,″ and press “Enter”. We can see that nmap has only scanned six IP addresses.



5. nmap [targets] -excludefile[file name]

Now what if the subnet was very big and there are more number of hosts to be excluded from the scan. The above option helps us to achieve this. Remember the file “alpha”( with four IP addresses) we created. Now let’s specify nmap to quit scanning the hosts listed in the file. Type command “nmap -excludefile alpha”. We can see that nmap scanned only four hosts in the subnet.



6. nmap [target1 target2  target3]

This option specfies nmap to scan multiple targets separated by space. Type the command  “nmap″. We can see the result below.



Hope this was helpful.

Scanning plays a very important role in hacking a system. Scanning is a phase in which we  find out the ports which are open and the services listening on those ports. Nmap is the most popular port scanner being used security guys nowadays. However it is very important to understand classification of ports by Nmap while scanning. Nmap classifies ports into six states. They are, open, closed, filtered, unfiltered, open | filtered and closed | filtered. Let us find out when Nmap classifies ports into specific states. For this, I use two virtual machines,

1. Kali Linux as attacker (with IP


2. XP as victim (with IP


On the victim machine, Telnet server is running and an exception is provided for it in windows firewall.


1. Open

Nmap classifies a port as open if an application is actively accepting TCP connections, UDP datagrams or SCTP associations on this port.

When I perform a default Nmap scan from the attacker of port 23 of the victim,

Nmap  –p 23

The result I get is open. This is because the Telnet server is actively accepting connections.


2. Closed

Nmap classifies a port as closed when the port is accessible but there is no application listening on it.  On our victim machine, let’s stop the the telnet service as shown below.


Now when we perform the above scan again, the port is shown as closed because although the port is accessible we don’t have any application listening on it.(i.e telnet is stopped)



3. filtered

Nmap classifies a port as filtered when it can’t determine whether the port is open or closed because packet filtering prevents its probes from reaching the port. On our victim machine, let’s  select ‘Don’t Allow Exceptions’ option in the firewall settings.


When we perform the above scan once again, the port is classified as filtered because firewall filtering blocks the probes of Nmap. When Nmap classifies a port as filtered, it is most likely that a firewall is blocking our probes.


4. Unfiltered.

Nmap classifies a port as unfiltered when a port is accessible but it can’t determine whether it is open or closed. A port is classified as unfiltered only with the ACK scan.

Let’s start the telnet service again on our victim machine and allow an exception for telnet in the firewall.





Then let us perform the ACK scan.

nmap  -sA –p 23


The scan couldn’t determine whether the port is open or closed.

5. open | filtered

A port is classified as open | filtered when Nmap is unable to determine whether a port is open or filtered. This happens for scan types in which open ports give no response. The UDP,IP protocol, FIN, NULL and XMAS scans classify ports this way. Let’s go to our machine and once again block telnet using firewall.


And then perform FIN scan and NULL scan respectively.


The port is classified as open | filtered in both cases because Nmap can’t determine whether the port is open or filtered.

6. closed | filtered

Nmap can’t find out whether a port is closed or filtered. A port is classified this way only for IP IDLE scan. Now what is IDLE scan? Idle scan is a scan in which we use a zombie host to scan the victim. In our example, we use another host with IP as a zombie to perform IDL scan on our victim.


In our victim, firewall is still blocking telnet. Let’s perform a IP IDLE scan.

nmap –sI –p 23


The scan shows result as closed | filtered because it could’nt determine whether a port is closed or filtered.