Nmap (“Network Mapper”) is a free and open source utility for
network exploration and security auditing. Many systems and network
administrators also find it useful for tasks such as network
inventory, managing service upgrade schedules, and monitoring host or
service uptime. Nmap uses raw IP packets in novel ways to determine
what hosts are available on the network, what services (application
name and version) those hosts are offering, what operating systems
(and OS versions) they are running, what type of packet
filters/firewalls are in use, and dozens of other characteristics. It
was designed to rapidly scan large networks, but works fine against
single hosts. Nmap runs on all major computer operating systems, and
both console and graphical versions are available.
This chapter uses fictional stories to provide a broad overview
of Nmap and how it is typically used. An important legal section
helps users avoid (or at least be aware of) controversial usage that
could lead to ISP account cancellation or even civil and criminal
charges. It also discusses the risks of crashing remote machines as
well as miscellaneous issues such as the Nmap license (GNU GPL),
copyright, and export control restrictions.