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Securing Linux and UNIX Systems in the Real World

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In the increasingly dangerous world of the Internet, securing your system is of the utmost importance. Bob Toxen provides a checklist every SysAdmin should have covered for real-world Linux security.
This chapter is from the book

Linux is a solid operating system. It is easy to use and install, has very powerful capabilities, runs fast on almost any hardware, and rarely crashes. It has few bugs and its widespread support from a cast of thousands ensures that any remaining bugs get fixed as soon as they are discovered. It is highly versatile and can be made as secure as any UNIX system.

Unfortunately, UNIX and Linux machines are broken into every day, not because they are inherently insecure, but because the steps required to expose a system to the real world safely—the modern Internet—are not always so obvious. The single goal of this book is to teach any Linux or UNIX system administrator how to secure his systems, keep them secure, and feel confident that all necessary steps have been taken.

1.1 Introduction to the Second Edition

Much has happened in the two years since the first edition of Real World Linux Security was published and much that was anticipated has not come to pass. Rather than the anticipated upward-compatible version of IP Chains with additional features and security, we have IP Tables. Transitioning to IP Tables requires a major rewrite of any firewall script, and some features present in IP Chains under the Linux 2.2 kernel are absent from both IP Tables and IP Chains in the 2.4 kernel. IP Tables is addressed in this book in great detail, and I include some fascinating original research and firewall tips and techniques (see ""Firewalls with IP Tables and DMZ" on page 446). You will not find this information elsewhere.

The Internet has become a much more dangerous place. Two years ago an unhardened system stood a reasonable chance of not being compromised for months or even years. Now, an unsecured system probably will be broken into within a week or two, and a complete compromise within one day of being placed on the Web is common. With the popularity of always-on DSL and cable connections, the exposure to possible compromise is increased by a factor of 10, too. It is guaranteed that each system on the Internet gets scanned for various vulnerabilities on a daily basis now.

The cyber warfare of science fiction is now a fact! There is credible evidence that Al Qaeda is preparing to commit damaging attacks against U.S. businesses and government. A number of countries' governments or individuals have staged massive cyber attacks against their enemies. With almost every Internet-connected computer handling credit card data, and crackers (sometimes called hackers) getting more vicious and more interested in financially profiting from their attacks, there is far more at risk.

In 2000 and later, there was much talk about the U.S. Federal Bureau of Investigation (FBI) getting more involved in solving computer crime, and there was much talk worldwide about more laws to reduce computer crime. In 2000, Microsoft promised to make great improvements in Windows security and again promised this in early 2002. I have seen none of this come to pass. While the big Linux vendors have made good on their promises to improve on security, they still have made big goofs in the security of even their latest offerings and even the "rock solid" Apache has not been immune from recent major vulnerabilities.

It is my opinion that even the viruses that have caused billions of dollars in damage due to lost files and wasted time are mild compared to what is possible. Viruses that can attack both Windows and Linux and viruses that can attack Linux and several flavors of UNIX have been demonstrated. As I write this, there are major security vulnerabilities in the current versions of Apache on Linux, UNIX, and Windows (the first in five years), as well as in Internet Explorer and Internet Information Services (IIS) running on most systems. It would not be especially difficult for a hacker to create a virus that attacks all of these platforms, sniffs these systems for credit card, bank account, and investment account numbers, and drains the accounts of money before anyone discovers it.

Real World Linux Security has undergone a major revision in the second edition. Problems that people did not worry about two years ago are now big concerns and have been addressed here. New technologies, such as wireless networks and IP Tables, have been addressed in depth. Hackers now are using more subtle attacks that were rare two years ago. These include attacking at the address resolution protocol (ARP) level. Even the lowly network switch now is being compromised with regularity. All of these situations are addressed.

Better methods for monitoring your network and instantly locking out attackers in this more vicious world are explained in step-by-step detail. Arpwatch, Logcheck, Portsentry, the newest versions of Samba and the GNU Privacy Guard (the GNU answer to Pretty Good Privacy [PGP]), the 2.4 kernel, Red Hat 7.3 and SuSE 8.0, VPNs, and greatly improved Adaptive Firewall techniques all are covered in depth in this book. About 150 pages of new material have been added, while appendices containing listings of programs on the CD-ROM and obsolete material have been removed or revised.

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