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This chapter is from the book

This chapter is from the book

3.4: Implementing and Configuring IDSs

An important part of a security administrator's job is to detect attacks in progress as early as possible so he can minimize the damage and prevent further attacks. An intrusion detection system (IDS) is a software or hardware tool that alerts the administrator to the possibility that an attack is in progress and enables him to take appropriate steps.

Exercise 3.4.1: Understanding IDSs

IDSs can be either network-based or host-based; in other words, they might operate across an entire network or be centered on a specific host. Either type of IDS uses two methods: knowledge-based (also known as signature-based), which identifies known attack signatures similar to the method of virus signatures used by antivirus programs, and behavior-based (also known as anomaly-based), which watches for unusual network behavior that might indicate an attack is in progress. In this exercise, you research these topics and study several commercially available IDSs:

  1. SANS maintains an FAQ site at http://www.sans.org/resources/idfaq/ that answers several hundred questions related to intrusion detection. Peruse the answers to as many questions as you can, but in particular, look for answers to the following:

    • What is intrusion detection, and why is it needed?

    • What is a honeypot?

    • What are host-based and network-based intrusion detection? For more information, access the links provided on these topics.

    • What are some advantages and disadvantages of a knowledge-based IDS? A behavior-based IDS?

    • Why are false positives observed?

    • What are some methods used by attackers to evade IDSs?

    • What are some methods used for intrusion detection on wireless networks? See the paper by David Dobrotka.

    • Note briefly the various types of scans and probes that are discussed.

    • What are some indications of attack on a Unix system?

    • Summarize the types of intrusion-detection products referred to, and note which types of products are most useful under which circumstances.

    • Study and summarize several examples of attacks that they discuss in the last section of their list.

    • You will return to the same site in Exercise 3.4.3 to learn about incident response.


    A false positive refers to an event that is detected as an intrusion when in fact it is a normal occurrence. A false negative refers to a real intrusion that is passed off as a normal occurrence. False negatives can be dangerous because you are not aware that something is going on while the attacker is obtaining data or creating havoc on your systems.


    Be sure that you can compare and contrast the types of IDSs and the circumstances in which each one should be used.

  2. Lawrence Halme and Kenneth Bauer provide a paper on Anti-Intrusion Taxonomy (AINT) at http://www.sans.org/resources/idfaq/aint.php. This paper, which you can reach from the SANS FAQ site, discusses six approaches to IDSs and three means of intrusion prevention. Compare these methods and ask yourself which are the most and least likely means of achieving results.

  3. Another approach to IDSs is a statistical method. Jamil Farshachi presents several types of statistical-based systems at http://www.sans.org/resources/idfaq/statistic_ids.php. Review them and take a look at the Spade anomaly detector. Note in particular how you can set the threshold adjustment. What are the consequences if you set this adjustment too high or too low?

  4. Greg Shipley contrasts network-based and host-based IDSs at http://www.networkcomputing.com/1023/1023f1.html. What are some advantages and disadvantages of each approach, and how does Shipley's solution combine the best of both?

  5. Edward Yakabovicz discusses IDS recommendations at http://searchsecurity.techtarget.com/originalContent/0,289142,sid14_gci779268,00.html. What recommendations for IDS tools and products deployment does he present? In addition, which project management tasks should be undertaken? What are the three tiers that he advocates for deployment of network-based IDSs?

  6. An additional resource for brief Q&A–style information on IDSs and other topics relevant to the Security+ exam appears at http://www.windowsecurity.com/faqs/Intrusion_Detection/. Compare their answers to those obtained from other sources.

  7. Roger Grimes discusses the purpose of honeypots and reviews several commercial honeypots at http://www.winnetmag.com/Articles/Index.cfm (enter 41976 in the InstantDoc ID text box). What are some of the characteristics of attacks that have been uncovered using honeypots? How does a real honeypot differ from a virtual honeypot, and what are some of the advantages and drawbacks of each?

  8. For another viewpoint on real and virtual honeypots, go to Brian Posey's article at http://techrepublic.com.com/5100-6264_11-5195024.html?tag=e064. Summarize his opinions on the advantages and disadvantages of real and virtual honeypots. What conclusion does he draw?


You should be aware that an IDS is unable to detect attacks directed at the network or computer from spoofed email messages.

Although a honeypot is a single computer set up to attract attacks, a honeynet is a network that is set up for the same purpose. You should be able to compare and contrast honeypots with honeynets for the Security+ exam.

Exercise 3.4.2: Configuring an IDS

In this exercise, you download, install, and configure the trial version of a representative host-based IDS, Tripwire for Servers. Perform this exercise on a computer running Windows 2000 Server. The exercise will also work on a computer running Windows 2000 Professional, Windows XP Professional, or Windows Server 2003:

  1. Go to http://www.tripwire.com/products/index.cfm# and read about Tripwire's products.

  2. Select the Find Out More link under Tripwire for Servers.

  3. To obtain information on Tripwire for Servers, select the download of Tripwire for Servers from the right side of the Tripwire for Servers page.

  4. After reading the information presented, click the Go link under Evaluation Downloads.

  5. Click Download Trial Kit Software. You will need to fill out an information form, including a valid email address, to receive a trial license by email and download the product. Follow the instructions provided and click Submit.

  6. In a few minutes, you should receive an email message with instructions and license files. Save these files and download the Windows executable file.

  7. Follow the instructions presented by the installation wizard, and provide the path to the license files when prompted. Click Finish when the installation completes.

  8. Click Start, Programs, Tripwire 4.0 Trial, Trial Guide. Internet Explorer displays the page shown in Figure 3.11. This page contains information on what the trial version does, including a guide that walks you through the performance of several simulated actions.

  9. Figure 3.11Figure 3.11 The demo version of Tripwire Manager contains a Web page from which you can perform a series of simulated actions.

  10. Select the link to run Tripwire Manager.

  11. If you are running Windows XP SP2, you might receive a security alert message. Select Unblock This Program to allow Tripwire Manager to proceed.

  12. Follow instructions on the Web page to perform simulated actions that include adding agent machines to the list, editing the configuration and policy files, creating a baseline, running integrity checks, installing an application, upgrading an existing application, compromising an application, and attacking with a worm.

  13. Note how Tripwire reports on the installation of an unapproved application, as shown in Figure 3.12. This capability enables you to monitor client computers for such activity.

  14. Figure 3.12Figure 3.12 An IDS can alert you to activity such as installation of an unapproved program.

  15. In the last part of the demo exercise, Tripwire reports on a simulated worm attack, as shown in Figure 3.13.

  16. When you are finished, run the last script and observe the conclusions at the bottom of the Web page. Think about how you can use an IDS program like Tripwire to detect changes occurring on a network for which you are responsible.


Many different commercial IDSs are available, and a Web search can provide a lot of information and trial downloads. Downloading and running another IDS or two will prove useful in both advancing your knowledge and preparing you for the Security+ exam.

Figure 3.13Figure 3.13 Tripwire reports on changes made to the Registry by a simulated worm attack.

Exercise 3.4.3: Understanding Incident Response Systems

After you detect an attack in progress, you need to know how to respond. Do you stop the attack immediately, or let it proceed for a period of time to collect additional evidence? What legal rights do you have, and how do you protect them? In this exercise, you study the methods of responding to an attack:

  1. Return to the SANS FAQ site that you accessed in Exercise 3.4.1 at http://www.sans.org/resources/idfaq/. Look at the Incident Handling and Response section and obtain answers to the following:

    • What steps should you take when you suspect that an intrusion is in progress? How should you respond?

    • What legal issues do security administrators need to be aware of?

    • What did we learn from the Melissa virus?

    • How do you preserve the evidence of an attack if you want to pursue legal action? What activities are of questionable legal value?

    • Describe several methods of automated responses. Which are most effective, and what are some of the risks or drawbacks?

    • What are two methods of active response to an attack?

  2. Jim Reavis presents a comprehensive procedure detailing how to respond to an intrusion at http://www.nwfusion.com/newsletters/ sec/0913sec1.html. Which utilities does he recommend for detecting intrusion on Unix systems? What information should you collect when documenting intrusions? What steps does he advocate, and what preparations do you need to make beforehand so that you are ready to deal with an intrusion when it happens?

What Did I Just Learn?

Now that you have looked at intrusion detection systems, let's take a moment to review all the critical items you've experienced in this lab:

  • An IDS can be either network based or host based. A network-based IDS operates across an entire network, whereas a host-based IDS concentrates on a single computer but provides a higher level of intrusion detection.

  • A knowledge-based IDS identifies known attack signatures similar to the method used by antivirus programs.

  • A behavior-based IDS watches for unusual behavior on a network that might indicate an attack is in progress.

  • An active IDS attempts to perform some task that restricts the intruder's access, such as shutting down a server or terminating a connection. A passive IDS merely informs the administrator that an attack is in progress and does not attempt to stop it.

  • A honeypot is a computer that is configured for the purpose of distracting attackers while obtaining information about attacks in progress.

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