Special Thanks: This article was made possible by support from Linux Professional Institute

Zenmap is the official Nmap Security Scanner GUI. It is a multi-platform, free open source application that provides users with an easy-to-use graphical user interface (GUI) for Nmap. Nmap, or Network Mapper, is the underlying software that is an extremely powerful tool for security auditing, network-attached device discovery, and security reconnaissance. Its primary focus is port scanning, but Nmap is so much more than a port scanner -- it’s a complex security tool that helps you identify operating systems, application/service versions, and vulnerabilities on your network. Nmap's complexity makes the Zenmap program extremely valuable for those who want to get started with security scanning without the long learning curve.

Unlike some GUI tools, Zenmap is not a less flexible tool or less professional tool that is “dumbed down” for those who shun the command line. Sure, it has some “canned” scans that you can select, but you can also enter Nmap commands with options into the Command field for full flexibility and enjoy every aspect of the command-line interface. Zenmap also makes it easy for you to compare scan results, search scan results, and filter hosts.

In this article, I refer to Nmap and Zenmap interchangeably when referring to functionality, because it is really Nmap that is the underlying technology. I refer to Zenmap specifically when addressing GUI features.

One of Zenmap's best features is that it displays the command-line version of the options you select from the menus. This is especially handy if you want to copy/paste those commands into a script for automated scanning or perhaps to learn Nmap’s command-line syntax.

Port Scanning

By default, Nmap’s port scanning features uses the SYN type probe. Because port scanning is a serious security violation, the SYN type might be safe on some networks that aren’t monitored by security information and event management (SIEM) software or other security sentry programs. The SYN scan is a stealthy scan, because it doesn’t make complete TCP connections. In fact, SYN scanning is often referred to as half-open scanning because of this feature.

There are three possible responses to a SYN signal:

• ACK means the port is listening.
• RST means the port is a non-listener.
• No response is marked as filtered.

Operating System Detection

Operating system (OS) detection for exploitative reconnaissance is one reason to perform this type of scan, but for the rest of us, just uncovering what kind of computers are on a network is reason enough to perform it. OS detection has led to some interesting discoveries over the years from finding rogue systems to realizing that an entire subnetwork of unsupported systems existed in a testing center. OS detection isn’t always about finding a vulnerable system to hack.

For IT and security professionals, OS detection can mean ferreting out those systems that are unsupported, unpatched, unauthorized, or those completely unknown to support staff. But even more important perhaps than finding the occasional rogue Windows Vista system on your network is the epiphany through such a scan that some clever person has granted the whole world access to your network by setting up a rogue wireless access point (WAP).

Version Detection

As the Nmap documentation tells you, “While Nmap does many things, its most fundamental feature is port scanning.” This is true, and it’s a basic Nmap usage example. But for hackers and security professionals alike, version detection is the next step above simple port scanning, and it’s much more informative. It’s helpful to know that server01 has ports 22, 25, 80, and 443 open, but which services do those numbers actually represent and which versions of those services are running behind those port numbers.

Yes, ports 22, 25, 80, and 443 are SSH, SMTP, HTTP, and HTTPS, respectively, but what if a naive system administrator decides to run a non-secure service, such as Telnet on port 22? You can’t assume from the port number that the service running will always match. Version detection will identify the service and daemon version for you. The version number tells both the hacker and the security professional if systems are running vulnerable services.