Network Scanning
Core Technology
Network scanning may carry a negative connotation, but it doesn't mean you shouldn't look for weak spots in your network.
Imagine you are administering a small office or home network. Perhaps you want to know what hosts in this subnet are currently online, or which service that Internet of Things (IoT) device keeps open to the world. Network scanners are tools built to do just that.
Even if a host is properly secured and has unused ports closed, a network scanner may tell quite a lot about it. There are slight discrepancies in how popular operating systems (OSs) implement network protocols such as TCP. A tool that knows these nuances can make an educated guess about which OS the host runs. This is known as OS fingerprinting, and many network scanners implement it as well. Sometimes, it can even give you an uptime estimate!
As you guessed already, this Core Tech is about network scanning. Before we dive in, a usual word of warning: As with many technologies, network scanning can be used for good and for evil. Many network attacks begin with it, so it is deemed illegal in some provider and corporate networks. Never scan a network you don't really own unless you have permission to do so. When in doubt, a purpose-built scan target, scanme.nmap.org, is a good choice.
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