Table of Contents for
Practical UNIX and Internet Security, 3rd Edition

Version ebook / Retour

Cover image for bash Cookbook, 2nd Edition Practical UNIX and Internet Security, 3rd Edition by Alan Schwartz Published by O'Reilly Media, Inc., 2003
  1. Cover
  2. Practical Unix & Internet Security, 3rd Edition
  3. A Note Regarding Supplemental Files
  4. Preface
  5. Unix “Security”?
  6. Scope of This Book
  7. Which Unix System?
  8. Conventions Used in This Book
  9. Comments and Questions
  10. Acknowledgments
  11. A Note to Would-Be Attackers
  12. I. Computer Security Basics
  13. 1. Introduction: Some Fundamental Questions
  14. What Is Computer Security?
  15. What Is an Operating System?
  16. What Is a Deployment Environment?
  17. Summary
  18. 2. Unix History and Lineage
  19. History of Unix
  20. Security and Unix
  21. Role of This Book
  22. Summary
  23. 3. Policies and Guidelines
  24. Planning Your Security Needs
  25. Risk Assessment
  26. Cost-Benefit Analysis and Best Practices
  27. Policy
  28. Compliance Audits
  29. Outsourcing Options
  30. The Problem with Security Through Obscurity
  31. Summary
  32. II. Security Building Blocks
  33. 4. Users, Passwords, and Authentication
  34. Logging in with Usernames and Passwords
  35. The Care and Feeding of Passwords
  36. How Unix Implements Passwords
  37. Network Account and Authorization Systems
  38. Pluggable Authentication Modules (PAM)
  39. Summary
  40. 5. Users, Groups, and the Superuser
  41. Users and Groups
  42. The Superuser (root)
  43. The su Command: Changing Who You Claim to Be
  44. Restrictions on the Superuser
  45. Summary
  46. 6. Filesystems and Security
  47. Understanding Filesystems
  48. File Attributes and Permissions
  49. chmod: Changing a File’s Permissions
  50. The umask
  51. SUID and SGID
  52. Device Files
  53. Changing a File’s Owner or Group
  54. Summary
  55. 7. Cryptography Basics
  56. Understanding Cryptography
  57. Symmetric Key Algorithms
  58. Public Key Algorithms
  59. Message Digest Functions
  60. Summary
  61. 8. Physical Security for Servers
  62. Planning for the Forgotten Threats
  63. Protecting Computer Hardware
  64. Preventing Theft
  65. Protecting Your Data
  66. Story: A Failed Site Inspection
  67. Summary
  68. 9. Personnel Security
  69. Background Checks
  70. On the Job
  71. Departure
  72. Other People
  73. Summary
  74. III. Network and Internet Security
  75. 10. Modems and Dialup Security
  76. Modems: Theory of Operation
  77. Modems and Security
  78. Modems and Unix
  79. Additional Security for Modems
  80. Summary
  81. 11. TCP/IP Networks
  82. Networking
  83. IP: The Internet Protocol
  84. IP Security
  85. Summary
  86. 12. Securing TCP and UDP Services
  87. Understanding Unix Internet Servers and Services
  88. Controlling Access to Servers
  89. Primary Unix Network Services
  90. Managing Services Securely
  91. Putting It All Together: An Example
  92. Summary
  93. 13. Sun RPC
  94. Remote Procedure Call (RPC)
  95. Secure RPC (AUTH_DES)
  96. Summary
  97. 14. Network-Based Authentication Systems
  98. Sun’s Network Information Service (NIS)
  99. Sun’s NIS+
  100. Kerberos
  101. LDAP
  102. Other Network Authentication Systems
  103. Summary
  104. 15. Network Filesystems
  105. Understanding NFS
  106. Server-Side NFS Security
  107. Client-Side NFS Security
  108. Improving NFS Security
  109. Some Last Comments on NFS
  110. Understanding SMB
  111. Summary
  112. 16. Secure Programming Techniques
  113. One Bug Can Ruin Your Whole Day . . .
  114. Tips on Avoiding Security-Related Bugs
  115. Tips on Writing Network Programs
  116. Tips on Writing SUID/SGID Programs
  117. Using chroot( )
  118. Tips on Using Passwords
  119. Tips on Generating Random Numbers
  120. Summary
  121. IV. Secure Operations
  122. 17. Keeping Up to Date
  123. Software Management Systems
  124. Updating System Software
  125. Summary
  126. 18. Backups
  127. Why Make Backups?
  128. Backing Up System Files
  129. Software for Backups
  130. Summary
  131. 19. Defending Accounts
  132. Dangerous Accounts
  133. Monitoring File Format
  134. Restricting Logins
  135. Managing Dormant Accounts
  136. Protecting the root Account
  137. One-Time Passwords
  138. Administrative Techniques for Conventional Passwords
  139. Intrusion Detection Systems
  140. Summary
  141. 20. Integrity Management
  142. The Need for Integrity
  143. Protecting Integrity
  144. Detecting Changes After the Fact
  145. Integrity-Checking Tools
  146. Summary
  147. 21. Auditing, Logging, and Forensics
  148. Unix Log File Utilities
  149. Process Accounting: The acct/pacct File
  150. Program-Specific Log Files
  151. Designing a Site-Wide Log Policy
  152. Handwritten Logs
  153. Managing Log Files
  154. Unix Forensics
  155. Summary
  156. V. Handling Security Incidents
  157. 22. Discovering a Break-in
  158. Prelude
  159. Discovering an Intruder
  160. Cleaning Up After the Intruder
  161. Case Studies
  162. Summary
  163. 23. Protecting Against Programmed Threats
  164. Programmed Threats: Definitions
  165. Damage
  166. Authors
  167. Entry
  168. Protecting Yourself
  169. Preventing Attacks
  170. Summary
  171. 24. Denial of Service Attacks and Solutions
  172. Types of Attacks
  173. Destructive Attacks
  174. Overload Attacks
  175. Network Denial of Service Attacks
  176. Summary
  177. 25. Computer Crime
  178. Your Legal Options After a Break-in
  179. Criminal Hazards
  180. Criminal Subject Matter
  181. Summary
  182. 26. Who Do You Trust?
  183. Can You Trust Your Computer?
  184. Can You Trust Your Suppliers?
  185. Can You Trust People?
  186. Summary
  187. VI. Appendixes
  188. A. Unix Security Checklist
  189. Preface
  190. Chapter 1: Introduction: Some Fundamental Questions
  191. Chapter 2: Unix History and Lineage
  192. Chapter 3: Policies and Guidelines
  193. Chapter 4: Users, Passwords, and Authentication
  194. Chapter 5: Users, Groups, and the Superuser
  195. Chapter 6: Filesystems and Security
  196. Chapter 7: Cryptography Basics
  197. Chapter 8: Physical Security for Servers
  198. Chapter 9: Personnel Security
  199. Chapter 10: Modems and Dialup Security
  200. Chapter 11: TCP/IP Networks
  201. Chapter 12: Securing TCP and UDP Services
  202. Chapter 13: Sun RPC
  203. Chapter 14: Network-Based Authentication Systems
  204. Chapter 15: Network Filesystems
  205. Chapter 16: Secure Programming Techniques
  206. Chapter 17: Keeping Up to Date
  207. Chapter 18: Backups
  208. Chapter 19: Defending Accounts
  209. Chapter 20: Integrity Management
  210. Chapter 21: Auditing, Logging, and Forensics
  211. Chapter 22: Discovering a Break-In
  212. Chapter 23: Protecting Against Programmed Threats
  213. Chapter 24: Denial of Service Attacks and Solutions
  214. Chapter 25: Computer Crime
  215. Chapter 26: Who Do You Trust?
  216. Appendix A: Unix Security Checklist
  217. Appendix B: Unix Processes
  218. Appendixes C, D, and E: Paper Sources, Electronic Sources, and Organizations
  219. B. Unix Processes
  220. About Processes
  221. Signals
  222. Controlling and Examining Processes
  223. Starting Up Unix and Logging In
  224. C. Paper Sources
  225. Unix Security References
  226. Other Computer References
  227. D. Electronic Resources
  228. Mailing Lists
  229. Web Sites
  230. Usenet Groups
  231. Software Resources
  232. E. Organizations
  233. Professional Organizations
  234. U.S. Government Organizations
  235. Emergency Response Organizations
  236. Index
  237. Index
  238. Index
  239. Index
  240. Index
  241. Index
  242. Index
  243. Index
  244. Index
  245. Index
  246. Index
  247. Index
  248. Index
  249. Index
  250. Index
  251. Index
  252. Index
  253. Index
  254. Index
  255. Index
  256. Index
  257. Index
  258. Index
  259. Index
  260. Index
  261. Index
  262. Index
  263. About the Authors
  264. Colophon
  265. Copyright

Some Last Comments on NFS

Here are a few final pieces of advice about making NFS as secure as possible.

Well-Known Bugs

NFS depends on NIS or NIS+ on many machines. Both NFS and NIS implementations have had some well-known implementation flaws and bugs in recent years. Not only are these flaws well-known, but there are also a number of hacker toolboxes available that include programs to take advantage of these flaws. Therefore, if you are running NFS, you should be certain that you are up to date on vendor patches and bug fixes. In particular:

  • Make sure that your version of the RPC portmapper does not allow proxy requests and that your own system is not in the export list for a partition. Otherwise, a faked packet sent to your RPC system can be made to fool your NFS system into acting as if the packet was valid and came from your own machine.

  • Make sure that your NFS uses either Secure RPC or examines the full 32 bits of the UIDs that are passed in. Some early versions of NFS examined only the least significant 16 bits of the passed-in UID for some tests, so accesses could be crafted that would function as root accesses instead of being mapped to nobody.

  • Make sure that your version of NFS does not allow remote users to issue mknod commands on partitions they import from your servers. A user creating a new /dev/kmem file on your partition has made a big first step towards a complete compromise of your system.

  • Make sure that your NFS does the correct thing when someone does a cd . in the top level of an directory imported from your server. Some older versions of NFS would return a file handle to the server’s real parent directory instead of the parent to the client’s mount point. Because NFS doesn’t know how you get file handles, and it applies permissions on whole partitions rather than mount points, this process could lead to your server’s security being compromised.

    In particular, when a server would export a subdirectory as the root partition for a diskless workstation, a user on the workstation could do cd /; cd .., and instead of getting the root directory again, he would have access to the parent directory on the server! Further compounding this scenario, the export of the partition needed to be done with root= access. As a result, clients would have unrestricted access to the server’s disks!

    Admittedly, this was fixed a long time ago, in Version 2 of NFS. However, we have repeatedly seen mistakes reappear in reimplementations and ports to new platforms. By documenting this problem, perhaps we can help keep it from appearing again.

  • Make sure that your server parses the export option list correctly. Some past NFS implementations did not implement access control correctly. In particular, in these implementations, if you specify access= with either the rw= or root= option on the same line, the system sometimes forgot the access= specification and exported the partition without host restriction.

For Real Security, Don’t Use NFS

NFS and other distributed filesystems provide some wonderful functions. They are also a source of continuing headaches. You should consider whether you really need all the flexibility and power of NFS and distributed systems. By reexamining your fundamental assumptions, you may find that you can reconfigure your systems to avoid NFS problems completely—by eliminating NFS.

Let’s look at the reasons that organizations typically feel that they need NFS:

Software synchronization

One reason that is often given for having NFS is to easily keep software in sync on many machines at once. However, that argument was more valid before the days of high-speed local networks and cheap disks. You might be better served by equipping each workstation in your enterprise with a 20 GB or 40 GB disk, with a complete copy of all of your applications residing on each machine. You can use a facility such as rdist or rsync over SSH to make necessary updates. Not only will this configuration give you better security, but it will also provide better fault tolerance: if the server or network goes down, each system has everything necessary to continue operation. This configuration also facilitates system customization.

Home account access

A second argument for network filesystems is that they allow users to access their home accounts with greater ease, no matter which machine they use. But while this may make sense in a university’s student lab, most employees almost always use the same machine, so there is no reason to access multiple machines as if they were equivalent.

Network filesystems are sometimes used to share large databases from multiple points. But network filesystems are a poor choice for this application because locking the database and synchronizing updates is usually more difficult than sharing a single machine using remote logins. In fact, with the X Window System, opening a window on a central database machine is convenient and often as fast as (or faster than) accessing the data via a network filesystem. Alternatively, you can use a database server with client programs that are run locally.

Cost

The argument is also made that sharing filesystems over the network results in lower cost. In point of fact, such a configuration may be more expensive than the alternatives. For instance, putting high-resolution color X display terminals on each desktop and connecting them with a 100 MB switched Ethernet to a multiprocessor server equipped with RAID disk may be more cost-effective, provide better security, give better performance, and use less electricity. The result may be a system that is cheaper to buy, operate, and maintain. The only loss is the cachet of equipping each user with a top-of-the-line workstation on their desktops when all they really need is access to a keyboard, mouse, and fast display.

Security

Ironically, the only argument for network filesystems may be security—provided that you manage your system carefully. Today, most X terminals have no support for encryption.[225] On client-/server-based systems that use Kerberos or DCE, you can avoid sending unencrypted passwords and user data over the network. But be careful: you will only get the data confidentiality aspects of this approach if your remote filesystem encrypts all user data—most don’t.

Questioning your basic assumptions may simultaneously save you time and money, and also improve your security.



[225] When we published the second edition of this book, we expected this situation to change in the near future. It still hasn’t. Dedicated X terminal hardware still does not encrypt data. On the other hand, it has become increasingly popular to use low-cost PCs as software-based X terminals, and many X Window System products for PCs do include the ability to make connections through SSH.