Building a cheap, powerful intrusion-detection system

If your overall security system doesn't include network-based intrusion detection, now is an excellent time to consider implementing an IDS package

Administering the IDS

After a successful installation, pointing a Web browser to the IDS will produce a summary alert window.

From here, intrusion-detection data may be analyzed efficiently.

Each alert can be analyzed individually or as a group. In the above example, the majority of the alerts generated constituted false positives because the alerts were on regular traffic that may have had abnormal but perfectly harmless characteristics. For example, one such alert was generated when a valid remote desktop session ended abruptly, possibly by a user not closing the remote desktop application correctly.

I previously noted that the IDS sensor should always be placed between the firewall and the LAN. Suppose the alert was indicative of a valid attack, the firewall could then be configured to deny all traffic from that source address. No new alerts should be logged after the firewall configuration, thereby effectively eliminating the threat.

Going forward

Building a functional IDS sensor is only the first step. Once installed, the IDS administrator should spend a significant amount of time exploring the alerts and capabilities of the system. One doesn't begin a major building project after setting up and operating a table saw for the first time, and such is the case with Snort/BASE.

As threats emerge, rules must be added to the system to match the signatures of those threats. Snort offers a subscription service for access to emerging rules for a minimal fee or free access to the same rules to registered users for 30 days after they are released to the subscription service. Oinkmaster is an excellent tool for updating rules regularly.

In addition, signatures may be created manually, or pass options may be added to signatures that are determined to produce an abundance of false positives. Determining if alerts are in fact normal network traffic or an actual threat is obviously necessary, as it would be foolish to disable a signature simply because it's producing many alerts. Other open-source tools such as MRTG, ntop and tcpdump, in conjunction with server and network equipment log analysis, can provide the data needed to streamline the IDS configuration

Snort can be deployed in a centrally managed distributed environment in which multiple sensors report back to a single database server. In large enterprise networks, this can be useful in correlating events as well as simply parsing information from multiple points on the network. It isn't uncommon to deploy Snort sensors at borders between security zones in a LAN, such as between administrative servers and local users.

A signature-based network IDS is simply a tool to enforce your company's security policy. Expecting that installing an IDS (or any single security solution, for that matter) will eliminate all threats is flirting with a false sense of security. However, delving into the world of open-source IDS is a path that can produce immediate and significant returns.

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Greg Schaffer

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