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Introduction

Our goal in writing Network Intrusion Detection, Third Edition has been to empower you as an analyst. We believe that if you read this book cover to cover, and put the material into practice as you go, you will be ready to enter the world of intrusion analysis. Many people have read our books, or attended our live class offered by SANS, and the lights have gone on; then, they are off to the races. We will cover the technical material, the workings of TCP/IP, and also make every effort to help you understand how an analyst thinks through dozens of examples.

Network Intrusion Detection, Third Edition is offered in five parts. "TCP/IP," begins with Chapter 1, ranging from an introduction to the fundamental concepts of the Internet protocol to a discussion of Remote Procedure Calls (RPCs). We realize that it has become stylish to begin a book saying a few words about TCP/IP, but the system Judy and I have developed has not only taught more people IP but a lot more about IP as well—more than any other system ever developed. We call it "real TCP" because the material is based on how packets actually perform on the network, not theory. Even if you are familiar with IP, give the first part of the book a look. We are confident you will be pleasantly surprised. Perhaps the most important chapter in Part I is Chapter 5, "Stimulus and Response." Whenever you look at a network trace, the first thing you need to determine is if it is a stimulus or a response. This helps you to properly analyze the traffic. Please take the time to make sure you master this material; it will prevent analysis errors as you move forward.

Tip

Whenever you look at a network trace, the first thing you need to determine is if it is a stimulus or a response.

The book continues in Part II, "Traffic Analysis" with a discussion of traffic analysis. By this, we mean analyzing the network traffic by consideration of the header fields of the IP and higher protocol fields. Although ASCII and hex signatures are a critical part of intrusion detection, they are only tools in the analyst's tool belt. Also in Part II, we begin to show you the importance of each field, how they are rich treasures to understanding. Every field has meaning, and fields provide information both about the sender of the packet and its intended purpose. As this part of the book comes to a close, we tell you stories from the perspective of an analyst

seeing network patterns for the first time. The goal is to help you prepare for the day when you will face an unknown pattern.

Although there are times a network pattern is so obvious it almost screams its message, more often you have to search for events of interest. Sometimes, you can do this with a well-known signature, but equally often, you must search for it. Whenever attackers write software for denial of service, or exploits, the software tends to leave a signature that is the result of crafting the packet. This is similar to the way that a bullet bears the marks of the barrel of the gun that fired it, and experts can positively identify the gun by the bullet. In of the book, "Filters/Rules for Network Monitoring" we build the skills to examine any field in the packet and the knowledge to determine what is normal and what is anomalous. In this section, we practice these skills both with TCPdump and also Snort. In Part IV, we consider the larger framework of intrusion detection. We discuss where you should place sensors, what a console needs to support for data analysis, and automated and manual response issues to intrusion detection. In addition, this section helps arm the analyst with information about how the intrusion detection capability fits in with the business model of the organization.

Finally, this book provides three appendixes that reference common signatures of well-known reconnaissance, denial of service, and exploit scans. We believe you will find this to be no fluff, packed with data from the first to the last page.

Network Intrusion Detection, Third Edition has not been developed by professional technical writers. Judy and I have been working as analysts since 1996 and have faced a number of new patterns. We are thankful for this opportunity to share our experiences and insights with you and hope this book will be of service to you in your journey as an intrusion analyst.