NMAP and fping deep dive

NMAP and fping are used for scanning and OS footprinting of a network during the information gathering phase of a penetration test.

It is always good to know how to use these tools but also to understand what they’re doing and how they work at a deeper level. So with that in mind, I am going to run these tools and at the same time capture what is going on the wire using Wireshark.

First, let us see how fping works vs the same scan ran using nmap and why the results might be different.

I will run fping on the network to see what hosts are alive. The command I will use is. #>fping -a -g 2>/dev/null

What are the additional parameters of -a and -g doing? The -a parameter is used to only report back hosts that are alive and the -g parameter is telling fping that it should carry out a ping sweep and not just a normal ping against one host. The 2>/dev/null parameter at the end of the command is sending err-out messages to the bit bucket so they’re not displayed while running the command.


After running the command we get the following 7 hosts responding to the ping sweep. Now I will run the same scan but this time using the nmap tool.

The nmap tool is a very powerful tool and it has a lot more capabilities compared to fping. To run the same scan that we did previously but now using nmap we run:

#>nmap -sn

The -sn parameter is requesting nmap to scan the subnet for hosts that are alive.


With the nmap scan we get back 8 hosts that are alive on the network vs the 7 reported by fping. That extra host is but why? Let us take a closer look at what fping is doing vs nmap using the -sn parameter.

fping will first send out arp requests for each host on the subnet it is scanning. If a host replies to the arp request with its MAC address fping will then take that IP address and send an ICMP echo request message to it (ping).


Shown above are the arp request and the arp reply. Next the fping tool sends an ICMP echo request to But if you look at the capture there is no response found! This host has probably been set up to not respond to ICMP messages.


If you compare this to one of the hosts that did reply the output would look like this with an echo request and an echo reply.


So the reason that fping does not show the in its scan results is down to the fact that the host is configured not to respond to the ICMP ping request which is perfectly normal and is a good security practice.

On the other hand, nmap reported it to be alive this is because the nmap -sn scan only sends arp requests out and if a host replies to it with its MAC address nmap marks that host to be alive.

As mentioned earlier nmap is a powerful tool. Let us look at what other scans it can do. Now we know what hosts are alive on the network but that is all we know which lets face it isn’t that much. To see what services (daemons) are running on these hosts we can run another command using nmap.

The command is #>nmap -sS

The -sS parameter is telling nmap to perform a TCP SYN scan which is a stealthier scan because it does not complete a TCP 3-way handshake. When a client wants to communicate with a web server, for example, it first completes a TCP 3-way handshake and then it will start exchanging data. This is usually logged by the web server daemon that a new connection has been made which is bad news for us as it might alert a sysadmin that someone is scanning their network.

A TCP 3-way handshake looks like this.


When running the TCP SYN scan instead of completing the 3-way handshake nmap will send an RST message in reply to the servers SYN/ACK message as shown below. This stops the connection completing and also from the web server daemon logging the connection.


The result of the nmap TCP SYN scan is shown below. It goes through each IP address and sends a SYN message to each well-known port to see if the server will reply with a SYN/ACK  message meaning that the port is open or a RST/ACK message meaning that the port is closed. For the IP of ports 22, 53 and 80 are all open.


That is it for now. I’ll go through more of the capabilities of nmap such as OS fingerprinting in my next post.

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