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Linux System Security: The Administrator's Guide to Open Source Security Tools

Linux System Security: The Administrator's Guide to Open Source Security Tools

by Scott Mann, Mitchell Krell, Ellen L. Mitchell

Lock down your Linux system NOW!

  • Up-to-the-minute security techniques for your entire Linux environment!
  • NEW! In-depth coverage of Bastille, the breakthrough Linux lockdown tool!
  • NEW! Intrusion detection with network sniffers and port scanners
  • NEW! Complete coverage of the OpenSSH encryption suite
  • Firewalls, email, Web


Lock down your Linux system NOW!

  • Up-to-the-minute security techniques for your entire Linux environment!
  • NEW! In-depth coverage of Bastille, the breakthrough Linux lockdown tool!
  • NEW! Intrusion detection with network sniffers and port scanners
  • NEW! Complete coverage of the OpenSSH encryption suite
  • Firewalls, email, Web services, filesystems, applications, and more
  • Completely updated for RedHat 7.2

Now there's an up-to-the-minute, hands-on guide to using open source tools to protect any Linux system! Completely updated for the newest tools and distributions, Linux System Security, Second Edition covers virtually every facet of Linux security, from firewalls and intrusion detection to authentication and secure Web services. You'll master over a dozen crucial open source security tools, including sudo, portmap, xinetd, tiger, tripwire, ipchains, pam, crack, and more. Along the way, three long-time Linux sysadmins will show you the "gotchas," rules of thumb, and undocumented tricks it would take you years to learn on your own!

  • Preparing Linux systems for a production environment
  • Using Bastille to lock down Linux systems without unnecessarily compromising their functionality
  • Combatting Trojan horses, backdoors, password cracking, buffer overflows, spoofing, DoS, and more
  • OpenSSH: eliminating eavesdropping, connection hijacking, and other network-level attacks
  • Detecting intrusions with network sniffers and port scanners
  • Firewalls, email, Web services, filesystems, applications, and much more
  • Protecting mixed Linux/UNIX(r) environments
  • Includes a concise introduction to security policies

Want the benefits of Linux without the security risks? Get Linux System Security, Second Edition!

Prentice Hall Series on Computer Networking and Distributed Systems, Radia Perlman, Series Advisor

Editorial Reviews

Mann (SGI specialist in Linux systems) and Mitchell (network analyst at Texas A&M U.) demonstrate exactly how to protect vital resources, using today's most powerful open source security tools. Topics include preparing Linux systems for a production environment; identifying vulnerabilities and planning for security administration; configuring Linux-based firewalls, authentication, and encryption; intrusion detection on Linux systems; securing file systems, e-mail, network services, and other key applications; and protecting Linux/UNIX environments. Annotation c. Book News, Inc., Portland, OR (booknews.com)

Product Details

Pearson Education
Publication date:
Prentice-Hall Series in Computer Networking and Distributed Systems
Edition description:
Product dimensions:
7.32(w) x 9.54(h) x 1.49(d)

Related Subjects

Read an Excerpt

Chapter 1: How did That Happen Vulnerability Survey

Late one night...

"Look! It's world exportable! All I have to do is get a valid user account," aBl-tR3kr (pronounced "able trekker") said aloud.

"What's world exportable?" asked pl3b (pronounced "plebe"), between bites of chocolate donut.

"This home directory on windfall. naive. com."

"Windfall?" asked pl3b.

"Yeah, it'll be our windfall in a moment!"

"What're ya gonna do?" queried pl3b, trying to learn as much as possible.

"Well," smiled aBl-tR3kr, "if they are silly enough to allow NFS over the Internet, they probably aren't using a shadow file either."

"Shadow file?" Now pl3b is really perplexed.

"Yeah. See, all I have to do is create a user account on my Linux box that matches one of theirs ... yep, there's a user called joe." aB1_tR3kr typed rapidly away on the keyboard as pl3b looked over his shoulder. "Now, I'll login here as joe, and whata ya know, I've got Joe's home directory from windfall. naive.com!"

"Wow! That was easy!" pl3b'S thinking that this breaking into UNIX/ Linux systems is simple stuff.

"Now, lemme create the rhos t s file here, and then we'll login remotely." aBl-tR3kr continued with his running commentary as he worked. "OK! We're in as i oe over there. Now, let's check the password file ... whooooya! No shadow file! I'll just e-mail this password file back to myself, like so. Done!"

"So what good is that file? All the passwords are jumbled up," remarked pl3b.

"No problem, my wannabe friend. We'll just use Crack!" aBl-tR3kr smiled confidently. And another system fell into his clutches.

What Happened?

This dialog poses a number of ill-configured services that allowed for unauthorized access to an Internet system. We will see that this is all too often the case as illustrated in Chapter 2.

The first problem with the system windf all. naive. com is the fact that user home directories are exported via the Network File System (NFS) to the world. This could have been prevented by setting access restrictions on the NIPS resources (discussed in Some Major Applications on page 37 in Chapter 3), on the portmap utility (discussed in Chapter 10), through ipchains (discussed in Chapter 16), or preferably through a combination thereof (see Chapter 18).

Our happy cracker, aBl-tR3kr, also took advantage of the trusted host file, rhosts in this case, to gain unauthorized access to the system. The use of the rhosts file in the dialog allowed aBl_tR3kr to log in to windfall. naive. com as joe without a password. We will discuss such files and recommend against their use in Chapter 3. Then we will talk about how to securely replace them in Chapter 11.

The next problem with the system windfall. naive. com is that it doesn't use password shadowing (described in Password Aging and the Shadow File on page 61 in Chapter 4). In this case, the lack of password shadowing means that a world-readable file (/etc/passwd), including each user's hashed (sometimes referred to as "encrypted") password, is available to anyone who can access a valid user account, which aBl tR3kr was able to do. If a shadow file had been in use, he wouldn't have been able to get a list of hashed passwords as easily because the shadow file is readable only by the root user.

While the hashed password cannot be used to log in, the Crack utility (discussed in detail in Chapter 12) may be used to guess the password based on the hashed password. And Crack is pretty good at what it does. You can't prevent the bad-guys from using Crack, but you can make it harder on them by using alternate hashing methods, as we will see in Chapter 5.

Other Cracker Activities

Once an intruder has gained access to an account, there are a variety of things the intruder might do. Among the things that such a malicious user will almost certainly do is create a back door to make returning easier. The intruder will also erase any evidence of his or her activity There are all sorts of freely available utilities that make these tasks simple.

The purpose of this book is to provide ways to make it difficult to break in initially as well as to detect the evidence of the attack quickly. But you must not use this book, or any other books, as your only resource of information for this purpose. New vulnerabilities are identified all the time, and patches and fixes are generated in response. Also, full-disclosure sites (noted in Full-Disclosure Resources on page 9), e-mail lists, and newsgroups will provide additional details regarding the ever changing scene of computer security. Appendix A lists resources that will assist you in staying current.


The purpose of the dialog at the beginning of this chapter is to illustrate the types of activities that occur all too frequently. The intent of this book is to provide you with skills, knowledge, and tools that will allow you to better prepare your systems for use in environments where you do not always know who is accessing what. The techniques and methods discussed are all from the perspective of restricting use to authorized access and making it as difficult as possible for crackers to gain unauthorized access.

This book is how-to oriented. This means that we will discuss the ways in which various utilities may be used to help protect your environment. What you should do in terms of implementing security at your site is largely left to other texts. While we make recommendations throughout this book, nothing can replace a good organizational Security Policy (discussed in Chapter 2). Also, we provide references at the end of each chapter if you wish to further investigate each topic we discuss. You will find that many of these references discuss the particulars of what ought to be done to maintain good security generally.

Before we get into the details of using publicly available tools to help secure your Linux' system(s), we will spend the rest of this chapter discussing...

Meet the Author

Scott Mann currently works for SGI, specializing in Linux systems. Previously, he was an independent consultant providing system, network, and security administration services and education.

Ellen L. Mitchell is a network analyst at Texas A&M University, responsible for campus network security, development, and administration. A consultant specializing in UNIX and network security, she currently maintains the tiger UNIX security package.

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