An intrusion detection system (IDS) works like a smoke detector. It detects a risk, issues a warning, but does not take any further steps to prevent the attack. A full-blown intrusion prevention system (IPS), on the other hand, can take additional steps to stop the attack. However, an IPS is much more elaborate (and often expensive), and it is often overkill for a small home network. For many users on small networks, the notification is the most important part, and an IDS can deliver that notification for far less trouble and a smaller learning curve.

An IDS takes a deep look at the IP packets passing through. If the analyzed content appears suspicious, the system alerts the user. The IDS references a database populated with known attack patterns to decide whether or not a packet contains a possible attack. This makes it similar to a virus scanner, which examines data and compares the data against known patterns.

An IDS is part of the standard equipment at any data center today. At a modern data center, a fast appliance fields data streams from all servers via a multi-gigabit network interface card and reports its discoveries to the network operations team. This might sound like an expensive solution, but the technology can just as easily be applied to home networks. This article describes how to set up a simple Raspberry Pi or comparable single-board system to act as an IDS. The downsized IDS receives a copy of the incoming packets from the router. You'll need a router that is capable of capturing and forwarding incoming packets to the Rasp Pi for monitoring purposes. Some routers might include this feature directly. In other cases, you can set up forwarding on the router if it comes with Bash scripting support. The examples in this article are based on a Fritz!Box 7583 router.

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