Securing Linux and UNIX Systems in the Real World

This chapter is from the book

Real World Linux Security: Intrusion Prevention, Detection and Recovery

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This chapter is from the book

This chapter is from the book 

Real World Linux Security: Intrusion Prevention, Detection and Recovery

Learn More Buy

1.3 How This Book Is Organized

Part I is concerned with increasing the security of your systems. This book is organized with the understanding that some SysAdmins have only a little time right now, but certainly want to fix the most severe holes immediately, before someone breaks into their systems. (The smaller holes also need to be closed, but statistically there is more time to address them before a cracker is likely to try them. Crackers, sometimes incorrectly called hackers, are people who break into computer systems without permission for the fun, challenge, fame, or due to a grudge.) These urgent quick-to-do items are covered in Chapter 2, "Quick Fixes for Common Problems," on page 17. That chapter starts with a discussion of basic security concepts to bring those new to Linux security up to speed and to serve as a "refresher" for veterans. The author estimates that applying just the quick fixes may reduce a system's vulnerability by 70 to 90 percent, based on published reports and incidents discussing probable "points of entry." Many of these solutions are independent from each other so that a SysAdmin may pick the solutions most appropriate to his or her situation and may implement these in almost any order.

The book then progresses into more involved procedures that can be done to increase security, allowing the system administrator to progress to as secure a system as time and desire allows. It even addresses some simple kernel modifications to increase security still further. It can be treated as a workbook, to be worked through a bit at a time, or as a reference book, with relevant areas picked from the Table of Contents or from the extensive Index.

Part II deals with preparing for an intrusion. No computer or network is completely secure and anyone who thinks that his is 100 percent secure is, well, probably due for some "education." Most computer security books deal almost exclusively with securing systems and devote only a few pages to dealing with an intrusion that 10–40 percent of their readers will suffer. This author considers this to be a naive disservice. (All other common platforms are considered even more vulnerable.) In many of the cases that this author has been asked to analyze, the vulnerability that allowed the break-in turned out to be a bug in system software that had not been well known at the time. This proves the point that just securing a system is not sufficient.

This book is called Real World Linux Security: Intrusion Prevention, Detection, and Recovery because in the real world a significant percentage of computers are broken into and the prepared SysAdmin is well prepared for this. Perhaps 5–25 percent of SysAdmins who have secured their Linux boxes still will have to deal with an intrusion. Even the author's own client-side network on broadband suffers hourly intrusion attempts (with no successes so far), but it has been prepared for intrusion attempts and even for fast recovery from a possible successful intrusion.

Switching to another platform will not reduce this risk, in my opinion. I have seen many reports of security bugs in various competing systems. Almost weekly I see a report on a newly discovered severe vulnerability in software long running and widely distributed on these closed-source platforms. Software written by independent vendors also has its share of problems.

Part III deals with detecting intrusions (both attempts and successes) and sophisticated notification and logging in detail. Part IV discusses recovering from intrusions successfully, completely, and quickly! It also covers tracking down the intruder and dealing with law enforcement officers and the courts, and what to expect from them. Outages can cost millions of dollars a day in lost revenue and bad publicity can mean more lost business and worse—the dismissal of the SysAdmins. A quick recovery may get no publicity and might even be blamed on a glitch in the Internet.

This book covers many security problems. These include problems of incorrect configuration, some services whose design prevents them from being made secure, some inherent limitations in the TCP/IP, UDP/IP, ICMP/IP, ARP, and related protocols, bugs in programs that have come with various Linux distributions or which get installed on Linux systems, and even some physical security and human factors (social engineering) matters.

Please do not get the idea that Linux is a hard-to-configure, buggy, half-baked idea not worthy of your attention! Nothing could be further from the truth. Many security experts consider Linux and FreeBSD UNIX to be the most secure general purpose operating systems. This is because the open source allows many more talented white hats² to inspect each line of code for problems and to correct these problems and "fold the fixes back into the master code base" maintained by Linus, the Free Software Foundation, and the creators of the major distributions.

There now is much sharing of code between Linux and the various BSD³ releases of UNIX and even versions of UNIX supported by the various vendors. This is to the advantage of all users of these systems, since there are more developers improving the code. By following the steps in this book, even a major intrusion can be detected and recovered from in a few minutes, rather than the many hours or days that The White House,⁴ Lloyd's of London, eBay.com,⁵ and other major, but apparently unprepared, sites required to recover.

1.3.1 Conventions in This Book

The Table of Contents is designed to allow one to scan it quickly for applicable issues. The Index is extensive and most items are cross-referenced, both by the subsystem or program that is affected and the type of problem, e.g., vulnerability. Some Internet resources (URLs) are listed in whatever sections discuss them; many popular Internet resources are discussed in Appendix A. Many URLs are listed in the Index too. Appendix B discusses non-Internet resources; these include books, CD-ROMs, and videos; some of these are free for the asking. Other appendices contain source code or other data that is too massive to appear in running text. These items also appear on the companion CD-ROM as do a number of open-source tools that are discussed in the text. There is other information on the associated Web site, http://www.realworldlinuxsecurity.com. The Web site also will contain the latest information and errata. There is also appendix of Abbreviations.

New terms being introduced are in italic or "quoted." Examples will appear as follows:

Examples are set with this constant-width font.

Frequently in running text, if something must be named as it might be keyed, such as the more program, it will be set in the constant-width font, as is demonstrated here. Path names and host names in running text are also usually set in the constant-width font.

The three-headed dog on the book's cover is Cerberus, from Greek mythology. He guards the entrance of Hades⁶ to keep the evil demons from escaping into our world and wreaking havoc, chaos, pain, and disaster. He also prevents the living from entering Hades. This is not unlike the security aspects of a system administrator's job and it certainly seems to require three heads to keep ahead of the problems.

Not too many people understand that TCP/IP is the Transmission Control Protocol (TCP) running on top of the Internet Protocol (IP). This means that an incoming TCP/IP packet is first processed by the IP layer of the communications "stack," then by the TCP layer, and then is passed to the program listening on that port. Similarly, UDP/IP is the User Datagram Protocol (UDP) on top of IP and ICMP/IP is the Internet Control Message Protocol on IP. For brevity, these will be referred to as TCP, UDP, and ICMP throughout this book.

1.3.2 Background

You can assume that there are crackers out there with copies of all of the proprietary source code from the UNIX vendors, other operating systems, routers, etc. so the crackers know their vulnerabilities. Unlike Linux, though, there will be far fewer white hats looking over the proprietary code for vulnerabilities and working to get them fixed. While working for free, the Linux volunteers are some of the very best programmers in the world and our goal is the very best code. We will not be limited by time-to-market, development costs, or similar limitations of the commercial world.

While this book is written for the Linux SysAdmin, 95 percent of it is applicable to most UNIX systems as well. The principal difference is that most UNIX SysAdmins do not have access to source code and will need to get most fixes from their vendor. Most vendors release fixes for security holes quite quickly and many of their clients have support contracts to cover this. Some of the security problems to be explored are inherent to the various services and protocols and very similar problems will be found on all platforms, including UNIX, Macs, Windows, VMS, and any other platform supporting the same services.

This book covers types of intruders and their goals, types of security holes and how to plug them, and where to look on the Internet to keep up-to-date on the latest holes and plugs. In many cases, system administration duties are divided between people with different titles, such as Network Administrator, Database Administrator, Webmistress, operator, etc. This book is for these people too. Additionally, it addresses issues of program design that every programmer writing applications, CGIs, shell scripts, etc. must know to avoid creating a security hole.

It is important that the SysAdmin ensure that users have been taught about security too. A user's files or program with improper security can allow intrusion not only into his data but also to the rest of the system and network. This is because some security holes require access to some user's account.

It is important that there be no unauthorized and no unanalyzed bridges between the Internet and internal LANs or WANs, sometimes called Intranets. Producing a written policy to help ensure security, while possibly boring, is an important part of security. If it is on paper, people are less likely to disregard it, particularly if disregarding it could cause a problem that they could be "blamed" for. An entire chapter is devoted to policy.

Intranets are trusted in that confidential unencrypted data flows along them. If the bridging system is not secure, a cracker can come in over the Internet and sniff the Intranet, see the confidential data, and probably break into the important systems.