The Basics of Rootkits: Leave No Trace
Subtle and insubstantial, the expert leaves no trace; divinely mysterious, he is inaudible. Thus he is the master of his enemy's fate.
Many books discuss how to penetrate computer systems and software. Many authors have already covered how to run hacker scripts, write buffer-overflow exploits, and craft shellcode. Notable examples include the texts Exploiting Software,  The Shellcoder's Handbook,  and Hacking Exposed. 
This book is different. Instead of covering the attacks, this book will teach you how attackers stay in after the break-in. With the exception of computer forensics books, few discuss what to do after a successful penetration. In the case of forensics, the discussion is a defensive one—how to detect the attacker and how to reverse-engineer malicious code. In this book we take an offensive approach. This book is about penetrating a computer system without being detected. After all, for a penetration to be successful over time, it cannot be detected.
In this chapter we will introduce you to rootkit technology and the general principals of how it works. Rootkits are only part of the computer-security spectrum, but they are critical for many attacks to be successful.
Rootkits are not, in and of themselves, malicious. However, rootkits can be used by malicious programs. Understanding rootkit technology is critical if you are to defend against modern attacks.
Understanding Attackers' Motives
A back door in a computer is a secret way to get access. Back doors have been popularized in many Hollywood movies as a secret password or method for getting access to a highly secure computer system. But back doors are not just for the silver screen—they are very real, and can be used for stealing data, monitoring users, and launching attacks deep into computer networks.
An attacker might leave a back door on a computer for many reasons. Breaking into a computer system is hard work, so once an attacker succeeds, she will want to keep the ground she has gained. She may also want to use the compromised computer to launch additional attacks deeper into the network.
A major reason attackers penetrate computers is to gather intelligence. To gather intelligence, the attacker will want to monitor keystrokes, observe behavior over time, sniff packets from the network, and exfiltrate  data from the target. All of this requires establishing a back door of some kind. The attacker will want to leave software running on the target system that can perform intelligence gathering.
Attackers also penetrate computers to destroy them, in which case the attacker might leave a logic bomb on the computer, which she has set to destroy the computer at a specific time. While the bomb waits, it needs to stay undetected. Even if the attacker does not require subsequent back-door access to the system, this is a case where software is left behind and it must remain undetected.
The Role of Stealth
To remain undetected, a back-door program must use stealth. Unfortunately, most publicly available "hacker" back-door programs aren't terribly stealthy. Many things can go wrong. This is mostly because the developers want to build everything including the proverbial kitchen sink into a back-door program. For example, take a look at the Back Orifice or NetBus programs. These back-door programs sport impressive lists of features, some as foolish as ejecting your CD-ROM tray. This is fun for office humor, but not a function that would be used in a professional attack operation.  If the attacker is not careful, she may reveal her presence on the network, and the whole operation may sour. Because of this, professional attack operations usually require specific and automated back-door programs—programs that do only one thing and nothing else. This provides assurance of consistent results.
If computer operators suspect that their computer or network has been penetrated, they may perform forensic discovery, looking for unusual activity or back-door programs.  The best way to counter forensics is with stealth: If no attack is suspected, then no forensics are likely to be applied to the system. Attackers may use stealth in different ways. Some may simply try to step lightly by keeping network traffic to a minimum and avoiding storing files on the hard drive. Others may store files but employ obfuscation techniques that make forensics more difficult. If stealth is used properly, forensics will never be applied to a compromised system, because the intrusion will not have been detected. Even if an attack is suspected and forensics end up being used a good stealth attack will store data in obfuscated ways to escape detection.
When Stealth Doesn't Matter
Sometimes an attacker doesn't need to be stealthy. For instance, if the attacker wants to penetrate a computer only long enough to steal something, such as an e-mail spool, perhaps she doesn't care if the attack is eventually detected.
Another time when stealth is not required is when the attacker simply wants to crash the target computer. For example, perhaps the target computer is controlling an anti-aircraft system. In this case, stealth is not a concern—just crashing the system is enough to achieve the objective. In most cases, a computer crash will be obvious (and disturbing) to the victim. If this is the kind of attack you want to learn more about, this book will not help you.
Now that you have a basic understanding of attackers' motives, we'll spend the rest of this chapter discussing rootkits in general, including some background on the subject as well as how rootkits work.