In Part I, we examine how to make a Linux system secure. Some of the configurations that are examined include: Someone's single system using a PPP connection A home or shop's small network using a PPP, DSL, or cable modem connection Large networks serving multinational corporations, government, and the military Systems used for simple browsing and e-mail Complex e-commerce sites supporting many systems handling thousands of customer credit cards, offering Web and FTP servers Networks offering secure support for traveling employees
| DSL stands for Digital Subscriber Line, a high-speed digital connection through telephone lines. ADSL, short for Asymmetric DSL, is a popular version of DSL. The Asymmetric means that the download bandwidth is different than the upload bandwidth, typically larger. This is because most people use lots of bandwidth downloading large pages with a browser by issuing short commands of, perhaps, 40 bytes. When e-mail is sent to multiple recipients from Linux, a single copy of the contents is transmitted, along with a recipient list. Thus, if the average e-mail has two recipients, the sender's upload volume will be half of the total download volume. |
These security areas may be addressed individually as time allows. The book is designed so that it may be used as a workbook and as a checklist or to look up individual areas of concern. The book and system security definitely should be re-examined when configurations and circumstances change as well as when problems are suspected. It is extensively indexed and provides useful information in the appendices. Problems in services and circumstances that one does not have may be ignored, though it is recommended that insecure versions of programs that are on the systems but not currently being used be upgraded, disabled (chmod 0), or removed to prevent their accidental use later. 
Problems are rated for danger level with one through five skull-and-crossbones symbols, as shown above. This allows a quick determination of how concerned a system administrator should be about a problem. For solutions (tools and techniques), it suggests how likely they will be valuable for you. The danger level table in Appendix D lists these by danger level, with a cross-reference to the section discussing each danger. The following table explains how to relate the number of skull-and-crossbones symbols to the danger discussed in a particular section. Danger Level Interpretation Danger Level | Interpretation |
|---|
| Minor effect or a risk much less than one percent of systems | 
| DoS potential or minor likelihood of major problem | 
| Has risks; root exploits seen in the past, more not anticipated | 
| Dangerous but acceptable in certain circumstances | 
| Too dangerous to run or a necessity to resolve or a security tool that is a necessity at many sites |
The chapters in this part are: Chapter 2, "Quick Fixes for Common Problems" Chapter 3, "Quick and Easy Hacking and How to Avoid It" Chapter 4, "Common Hacking by Subsystem" Chapter 5, "Common Hacker Attacks" Chapter 6, "Advanced Security Issues" Chapter 7, "Establishing Security Policies" Chapter 8, "Trusting Other Computers" Chapter 9, "Gutsy Break-Ins" Chapter 10, "Case Studies" Chapter 11, "Recent Break-Ins"
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