Introduction to Intrusion Detection Systems (IDS)

by Tony Bradley
An intrusion detection system (IDS) monitors network traffic and monitors for suspicious activity and alerts the system or network administrator. In some cases the IDS may also respond to anomalous or malicious traffic by taking action such as blocking the user or source IP address from accessing the network.

IDS come in a variety of “flavors” and approach the goal of detecting suspicious traffic in different ways. There are network based (NIDS) and host based (HIDS) intrusion detection systems. There are IDS that detect based on looking for specific signatures of known threats- similar to the way antivirus software typically detects and protects against malware- and there are IDS that detect based on comparing traffic patterns against a baseline and looking for anomalies. There are IDS that simply monitor and alert and there are IDS that perform an action or actions in response to a detected threat. We’ll cover each of these briefly.


Network Intrusion Detection Systems are placed at a strategic point or points within the network to monitor traffic to and from all devices on the network. Ideally you would scan all inbound and outbound traffic, however doing so might create a bottleneck that would impair the overall speed of the network.


Host Intrusion Detection Systems are run on individual hosts or devices on the network. A HIDS monitors the inbound and outbound packets from the device only and will alert the user or administrator of suspicious activity is detected

Signature Based

A signature based IDS will monitor packets on the network and compare them against a database of signatures or attributes from known malicious threats. This is similar to the way most antivirus software detects malware. The issue is that there will be a lag between a new threat being discovered in the wild and the signature for detecting that threat being applied to your IDS. During that lag time your IDS would be unable to detect the new threat.

Anomaly Based

An IDS which is anomaly based will monitor network traffic and compare it against an established baseline. The baseline will identify what is “normal” for that network- what sort of bandwidth is generally used, what protocols are used, what ports and devices generally connect to each other- and alert the administrator or user when traffic is detected which is anomalous, or significantly different, than the baseline.

Passive IDS

A passive IDS simply detects and alerts. When suspicious or malicious traffic is detected an alert is generated and sent to the administrator or user and it is up to them to take action to block the activity or respond in some way.

Reactive IDS

A reactive IDS will not only detect suspicious or malicious traffic and alert the administrator, but will take pre-defined proactive actions to respond to the threat. Typically this means blocking any further network traffic from the source IP address or user.

One of the most well known and widely used intrusion detection systems is the open source, freely available Snort. It is available for a number of platforms and operating systems including both Linux and Windows. Snort has a large and loyal following and there are many resources available on the Internet where you can acquire signatures to implement to detect the latest threats. For other freeware intrusion detection applications you can visit Free Intrusion Detection Software.

There is a fine line between a firewall and an IDS. There is also technology called IPS – Intrusion Prevention System. An IPS is essentially a firewall which combines network-level and application-level filtering with a reactive IDS to proactively protect the network. It seems that as time goes on firewalls, IDS and IPS take on more attributes from each other and blur the line even more.

Essentially, your firewall is your first line of perimeter defense. Best practices recommend that your firewall be explicitly configured to DENY all incoming traffic and then you open up holes where necessary. You may need to open up port 80 to host web sites or port 21 to host an FTP file server. Each of these holes may be necessary from one standpoint, but they also represent possible vectors for malicious traffic to enter your network rather than being blocked by the firewall.

That is where your IDS would come in. Whether you implement a NIDS across the entire network or a HIDS on your specific device, the IDS will monitor the inbound and outbound traffic and identify suspicious or malicious traffic which may have somehow bypassed your firewall or it could possibly be originating from inside your network as well.

An IDS can be a great tool for proactively monitoring and protecting your network from malicious activity, however they are also prone to false alarms. With just about any IDS solution you implement you will need to “tune it” once it is first installed. You need the IDS to be properly configured to recognize what is normal traffic on your network vs. what might be malicious traffic and you, or the administrators responsible for responding to IDS alerts, need to understand what the alerts mean and how to effectively respond.