A hands-on guide to protecting Linux data from security risks. Introduces readers to the seven deadly sins of Linux security, showing how to set up firewalls, break in case studies, block spam, develop internal security policies, and recover from an intrusion quickly. Softcover. DLC: Linux.
"synopsis" may belong to another edition of this title.
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
Who Should Read This Book?
This book will aid Linux and UNIX System Administrators (SysAdmins) in making their systems and networks as secure as possible from intruders and improper action of the users. It covers both quick and simple solutions, and some more involved solutions to eliminate every possible vulnerability.
It is organized to allow the busy SysAdmin to increase the security of the systems one piece at a time. It is recognized that one cannot take a system down for a week and work exclusively on its security for that week. In the real world, a SysAdmin's time is divided up by many tasks that cannot wait and systems are too critical to stay down for long.
In the real world, some systems will be broken into despite the best efforts of talented SysAdmins. This book devotes over 60,000 words to dealing with a possible break-in. It deals with how to prepare for it, how to detect it, and how to recover from it quickly and completely with minimal loss of confidential data and money, with minimal inconvenience to one's customers and employees, and with minimal publicity. This is considered one of the unique features of this book. On March 30, 2000, 350 "hackers" from around the world gathered in Israel for a conference. Organizers there said that they were able to break into 28 percent of Israeli computers that they tried and that this percentage was typical worldwide. This was with the permission of the computers' owners, who were convinced that their computers were invulnerable. The quoted statistics were not broken down by operating system type. Both John Draper ("Captain Crunch") and Kevin Mitnick were there.
The book is designed to be used by both the veteran of many years of Linux and UNIX experience, as well as the new SysAdmin. It does assume that the reader is somewhat knowledgeable in system administration; Prentice Hall has other fine books to help people hone their SysAdmin skills. There are many useful details here, both for the person with a single Linux box at home and for those supporting multinational corporations and large government agencies with very large networks comprised of multiple types of operating systems.1.2
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 15. 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 theirs 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.Innovative solutions are presented to even the most daunting problems, such as keeping customers' credit card numbers secure even if the Web server and the entire internal network are completely compromised! This solves a major widespread problem with e-commerce companies.
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 quiet site on a Dynamic IP over PPP suffers weekly 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 hats 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 BSD 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, Lloyd's of London, eBay, and other major, but apparently unprepared, sites required to recover.1.2.1 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. These are mirrored on the associated Web site, realworldlinuxsecurity. The Web site also will contain the latest information and errata. There is also a Glossary of Acronyms.
The three-headed dog on the book's cover is Cerberus from Greek mythology. He guards the entrance of Hades to keep the evil demons from escaping into our world and wreak 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.1.2.2 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.1.3
What Are You Protecting?
There are essentially four things that you need to protect against.1.
An intruder reading your confidential data
An intruder could see your product designs, competitive plans for the future, names and addresses of customers, customers' credit card and bank account information, your bank account numbers and contents, sensitive system data including modem phone numbers, passwords, etc.
Frequently the greater harm will happen if the intruder makes the data available to others. While a cracker herself knowing about your product design may not be a severe problem, publishing it on the Internet where your competitors can get it is a severe problem. If your customers' credit card numbers are revealed and it becomes publicly known (as has happened to America Online) people will be afraid to do business with you.2.
An intruder changing your data
This is perhaps the most scary and damaging intrusion. An intruder can alter designs and data without your people discovering it. This could cause loss of life and very severe liability. What if the formulation of a pharmaceutical company's medicine is changed, the design of a automobile or airplane is changed, or the program operating a factory or patient X-ray or Gamma ray device is changed. Patients' medical records could be altered. Any of these situations could result in death. They also could result in large lawsuits.
An intruder may not even realize the harm that his actions could do. In a case in Berkeley, California, crackers were in a system that controlled a cyclotron that sometimes was used for cancer treatments. Intruders have caused banks' ATMs to spit out money to no one in particular and made embarrassing changes to agencies' Web pages, including the Central Intelligence Agency's. 3.
An intruder removing your data
The harm here is self-evident and a good backup program limits the damage that can be done if it is detected.4.
Denial of Service
This is when an intruder causes a computer or network to be "less available" or "not available." Less available includes the system slowing down substantially because of intruder-induced loads or rescheduling, less modems or ports being available to legitimate users, due to intrFrom the Back Cover:
"You have in your hands a book I've been waiting to read for years-a practical, hands-on guide to hardening your Linux system."
—From the foreword by Eric S. Raymond
"A comprehensive guide to system security-covers everything from hardening a system to system-recovery after an attack. "
—Steve Bourne, Creator of the Bourne Shell
Your enemy is coming—are you ready?
It's not a question of "if" but "when." Will you be ready to protect your system when a cracker comes to call? Real World Linux Security goes beyond the books that merely detail system vulnerabilities; it offers system administrators practical solutions for safeguarding Linux systems and actively responding to break-in attempts. Veteran Bob Toxen shows you how to know your enemies and stop them at the front gate, before they can damage your system.
The hands-on guide to protecting your Linux data—and yourself
About the CD-ROM
The accompanying CD contains original software that locks out crackers and alerts system administrators. In addition, it includes programs that monitor system health and report suspicious activities, detect network sniffers, and speed backup and recovery.
About the Author
Bob Toxen has 26 years of UNIX/Linux experience, and is one of the 168 recognized developers of Berkeley UNIX. He learned about security as a student at UC Berkeley, when he played for "the other team," successfully cracking several of the original UNIX systems there. He is president of Fly-By-Day Consulting, specializing in Linux security, client/server creation, system administration, porting, and C programming.
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