Scanning tools are useful both to hackers and system administrators.  At work we’ll be using a tool called Discovery and Dependency Mapping which basically scans the entire network and gathers information on all the devices on the network and their dependency on eachother.  This is probably a little more advanced than Nmap, but a real-life example of sys. admins wanting to know what’s on the network.

Nmap is a very useful, free scanning utility.  The first objective is to find out what hosts are out on a network.  Issuing the command “nmap -sP 192.168.100.*” would bring back all the hosts on the 192.168.100.0 network.

example of using nmap -sP

The next step is to stack fingerprint the network using the command “nmap -sT 192.168.100.102″:

nmap -sT

To see what Nmap is doing, you can use Wireshark to capture the packets that go to and from the target computer.  Here’s an example of nmap hitting port 80 on the target computer:

This command (nmap -sT) shows you the ports open on the target system, as well as the services running on those ports.  This can help you identify what operating system the computer is running which allow you to probe deeper and find out, for example, the type of server and version.  If you’re worried about your scan showing up on the target computer’s logs (as evident in the three-way handshake completed when identifying port 80 as open in the above image), you can do a stealth mode scan with “nmap -sS x.x.x.x”.

Running the command “nmap -O x.x.x.x” will have Nmap guess the operating system on the machine.  I didn’t have much success with that command (from the command line), but using the GUI and command “nmap -T4 –version-light -sV -F -O 192.168.100.102″, it guessed the OS as Windows along with a list of possible versions.

Nmap GUI

Network traffic generated from using Nmap, shown by its signature, can be very detectable.  Nmap can be configured to mask its signature from being easily detected.

The three-way handshake is the process by which two computers create a reliable connection to eachother using TCP (Transmission Control Protocol).  The computer requesting the connection sends out a synchronize packet (SYN), when the second computer receives this packet it responds by sending a synchronize packet and an acknowledgement packet (SYN/ACK).  When the initiating computer receives the acknowledgement from the requested computer, it then sends an ACK packet as well, completing the three-way handshake.  There now exists an open-communication channel between the two computers until one issues a “FIN” or “RST” packet or the connection times out.

This is a very important concept in IT security, and it is also very exploitable.  When you think about it, the requesting computer is making sure it is connecting to the right computer before completing the connection.  What happens if someone intercepts the SYN packet (disguised as the designated computer) and sends a spoofed SYN/ACK and the requestor then completes the connection thinking it’s connected to the desired computer?  Or someone can listen in on the connection you’ve made (if it isn’t secure) and disguise information/malware to look like it came from the computer you’re talking to.

Another vulnerability occurs when someone maliciously sends out a flood of SYN packets from a spoofed IP address to a server, causing the server to consume large amounts of resources trying to keep up with these malicious packets.  This is a form of a denial-of-service attack (DoS).  This vulnerability is less of a worry with modern networks.

Sources: wikipedia, Computer Security Lab Manual