RootPrompt.org: Intrusion Detection: Knowing when someone is knocking on your doorJun 26, 2000, 18:59 (5 Talkback[s])
(Other stories by Lance E. Spitzner)
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"Your network is being scanned for vulnerabilities. This may happen only once a month or twice a day, regardless, there are people out there probing your network and systems for weaknesses. I can say this with confidence because I have yet to work on a network that has not been probed. My personal network of six systems at home is on a dedicated ISDN line. This network has no valuable data, nor represents any organization, yet I get probed two to four times a week. If you have a system or network connected to the Internet, you become a target. This article will discuss how you can protect yourself by detecting these intrusion attempts. I will then cover what you can do when you discover these attempts."
"There are a variety of different probes hackers will attempt. The first type we will prepare for is one of the most common, port scans. Port scans are where an inidvidual attempts to connect to a variety of different ports. The scans can be used on a specific target, or used to scan entire IP ranges, often chosen at random This is one of the most popular information gathering methods used by hackers today as it identifies what ports and services are open."
"To detect these scans, we will build a system that emails us alerts whenever someone connects to a predetermined port. First, we identify three to five of the most commonly scanned ports. Then we select two to three systems to listen on these ports. When an intruder scans our network, he will most likely hit our systems listening on these ports. When these ports are scanned, the systems log the attempt, execute various predetermined actions, then email an alert to a point of contact."
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