Nmap Tutorial: An introduction for VARs and security consultants

You may raise a few eyebrows when telling customers that one of the tools you'll be using to check their networks is a free open source tool. Customers may be surprised that you're relying on a free tool with no commercial support and feel that you're putting profits before service. You'll need to explain that you use the best tools available, and many open source tools now compare favorably with commercial alternatives in terms of features, reliability and help forums. One in particular, Nmap, has become the tool of choice for many network and security administrators who want to map networks and test them for vulnerabilities.

Nmap (Network Mapper) Security Scanner, written by Fyodor and now up to version 4, provides a wide range of port-scanning techniques designed to rapidly scan networks, large and small, for network exploration and security auditing. This versatile utility can determine what hosts are available on a network, what services those hosts are offering, and what type of packet filters and firewalls are in use. It also has the ability to remotely fingerprint a machine's operating system. Most Unix and Windows platforms are supported, as is Mac OS X and several handheld devices. It is available in both command line and graphical user interface modes, a blessing for those who are less familiar with working at the command prompt.

If you're tasked with looking after customer networks then you need to run scans to find any open ports and see what information the network is leaking to potential attackers. Nmap is a favorite hacker tool so you'll be able to see exactly what they can find out about your customer's networks. A Windows machine, for example, can use hundreds of ports to communicate with other machines, and each open port is a potential route in to a network for an attacker. Port scanning with Nmap is a fast and effective way to identify which ports are open, what services are running on them and where weak spots exist in the network's defenses. When you have identified which ports are open, you can close any that are not required, thus reducing the number of potentially exploitable services. And when you have mapped the network, you can also see if any unexpected changes have occurred since the last scan. For example, a machine infected by a worm will try to open ports in order to listen for instructions from its controller.

Nmap has won numerous awards, including the Linux Journal's Editor's Choice Award for Best Security Tool. Other accolades include being featured in the film Matrix Reloaded and appearing in a photograph of President Bush visiting the NSA. So if you want to add one of the most versatile network tools to your toolbox and discover what hackers can learn about your customers' networks, you should download your copy at www.insecure.org. In upcoming tips in this series, I explain how to install and configure Nmap on both Linux and Windows machines.