Table of Contents for
SSH, The Secure Shell: The Definitive Guide, 2nd Edition

Version ebook / Retour

Cover image for bash Cookbook, 2nd Edition SSH, The Secure Shell: The Definitive Guide, 2nd Edition by Robert G. Byrnes Published by O'Reilly Media, Inc., 2005
  1. Cover
  2. SSH, the Secure Shell, 2nd Edition
  3. Preface
  4. Protect Your Network with SSH
  5. Intended Audience
  6. Reading This Book
  7. Our Approach
  8. Which Chapters Are for You?
  9. Supported Platforms
  10. Disclaimers
  11. Conventions Used in This Book
  12. Comments and Questions
  13. Safari Enabled
  14. Acknowledgments
  15. 1. Introduction to SSH
  16. What Is SSH?
  17. What SSH Is Not
  18. The SSH Protocol
  19. Overview of SSH Features
  20. History of SSH
  21. Related Technologies
  22. Summary
  23. 2. Basic Client Use
  24. A Running Example
  25. Remote Terminal Sessions with ssh
  26. Adding Complexity to the Example
  27. Authentication by Cryptographic Key
  28. The SSH Agent
  29. Connecting Without a Password or Passphrase
  30. Miscellaneous Clients
  31. Summary
  32. 3. Inside SSH
  33. Overview of Features
  34. A Cryptography Primer
  35. The Architecture of an SSH System
  36. Inside SSH-2
  37. Inside SSH-1
  38. Implementation Issues
  39. SSH and File Transfers (scp and sftp)
  40. Algorithms Used by SSH
  41. Threats SSH Can Counter
  42. Threats SSH Doesn’t Prevent
  43. Threats Caused by SSH
  44. Summary
  45. 4. Installation and Compile-Time Configuration
  46. Overview
  47. Installing OpenSSH
  48. Installing Tectia
  49. Software Inventory
  50. Replacing r-Commands with SSH
  51. Summary
  52. 5. Serverwide Configuration
  53. Running the Server
  54. Server Configuration: An Overview
  55. Getting Ready: Initial Setup
  56. Authentication: Verifying Identities
  57. Access Control: Letting People In
  58. User Logins and Accounts
  59. Forwarding
  60. Subsystems
  61. Logging and Debugging
  62. Compatibility Between SSH-1 and SSH-2 Servers
  63. Summary
  64. 6. Key Management and Agents
  65. What Is an Identity?
  66. Creating an Identity
  67. SSH Agents
  68. Multiple Identities
  69. PGP Authentication in Tectia
  70. Tectia External Keys
  71. Summary
  72. 7. Advanced Client Use
  73. How to Configure Clients
  74. Precedence
  75. Introduction to Verbose Mode
  76. Client Configuration in Depth
  77. Secure Copy with scp
  78. Secure, Interactive Copy with sftp
  79. Summary
  80. 8. Per-Account Server Configuration
  81. Limits of This Technique
  82. Public-Key-Based Configuration
  83. Hostbased Access Control
  84. The User rc File
  85. Summary
  86. 9. Port Forwarding and X Forwarding
  87. What Is Forwarding?
  88. Port Forwarding
  89. Dynamic Port Forwarding
  90. X Forwarding
  91. Forwarding Security: TCP-Wrappers and libwrap
  92. Summary
  93. 10. A Recommended Setup
  94. The Basics
  95. Compile-Time Configuration
  96. Serverwide Configuration
  97. Per-Account Configuration
  98. Key Management
  99. Client Configuration
  100. Remote Home Directories (NFS, AFS)
  101. Summary
  102. 11. Case Studies
  103. Unattended SSH: Batch or cron Jobs
  104. FTP and SSH
  105. Pine, IMAP, and SSH
  106. Connecting Through a Gateway Host
  107. Scalable Authentication for SSH
  108. Tectia Extensions to Server Configuration Files
  109. Tectia Plugins
  110. 12. Troubleshooting and FAQ
  111. Debug Messages: Your First Line of Defense
  112. Problems and Solutions
  113. Other SSH Resources
  114. 13. Overview of Other Implementations
  115. Common Features
  116. Covered Products
  117. Other SSH Products
  118. 14. OpenSSH for Windows
  119. Installation
  120. Using the SSH Clients
  121. Setting Up the SSH Server
  122. Public-Key Authentication
  123. Troubleshooting
  124. Summary
  125. 15. OpenSSH for Macintosh
  126. Using the SSH Clients
  127. Using the OpenSSH Server
  128. 16. Tectia for Windows
  129. Obtaining and Installing
  130. Basic Client Use
  131. Key Management
  132. Accession Lite
  133. Advanced Client Use
  134. Port Forwarding
  135. Connector
  136. File Transfers
  137. Command-Line Programs
  138. Troubleshooting
  139. Server
  140. 17. SecureCRT and SecureFX for Windows
  141. Obtaining and Installing
  142. Basic Client Use
  143. Key Management
  144. Advanced Client Use
  145. Forwarding
  146. Command-Line Client Programs
  147. File Transfer
  148. Troubleshooting
  149. VShell
  150. Summary
  151. 18. PuTTY for Windows
  152. Obtaining and Installing
  153. Basic Client Use
  154. File Transfer
  155. Key Management
  156. Advanced Client Use
  157. Forwarding
  158. Summary
  159. A. OpenSSH 4.0 New Features
  160. Server Features: sshd
  161. Client Features: ssh, scp, and sftp
  162. ssh-keygen
  163. B. Tectia Manpage for sshregex
  164. Regex Syntax: Egrep Patterns
  165. Regex Syntax: ZSH_FILEGLOB (or Traditional) Patterns
  166. Character Sets for Egrep and ZSH_FILEGLOB
  167. Regex Syntax: SSH Patterns
  168. Authors
  169. See Also
  170. C. Tectia Module Names for Debugging
  172. D. SSH-1 Features of OpenSSH and Tectia
  173. OpenSSH Features
  174. Tectia Features
  175. E. SSH Quick Reference
  176. Legend
  177. sshd Options
  178. sshd Keywords
  179. ssh Options
  180. scp Options
  181. ssh and scp Keywords
  182. ssh-keygen Options
  183. ssh-agent Options
  184. ssh-add Options
  185. Identity and Authorization Files, OpenSSH
  186. Identity and Authorization Files, Tectia
  187. Environment Variables
  188. Index
  189. Index
  190. Index
  191. Index
  192. Index
  193. Index
  194. Index
  195. Index
  196. Index
  197. Index
  198. Index
  199. Index
  200. Index
  201. Index
  202. Index
  203. Index
  204. Index
  205. Index
  206. Index
  207. Index
  208. Index
  209. Index
  210. Index
  211. Index
  212. Index
  213. Index
  214. About the Authors
  215. Colophon
  216. Copyright

The Architecture of an SSH System

SSH has about a dozen distinct, interacting components that produce the features we’ve covered. [3.1] Figure 3-1 illustrates the major components and their relationships to one another.

SSH architecture

Figure 3-1. SSH architecture

By “component” we don’t necessarily mean “program”: SSH also has keys, sessions, and other fun things. In this section we provide a brief overview of all the components, so you can begin to get the big picture of SSH:

Server

A program that allows incoming SSH connections to a machine, handling authentication, authorization, and so forth. In most Unix SSH implementations, the server is sshd.

Client

A program that connects to SSH servers and makes requests, such as “log me in” or “copy this file.” In OpenSSH and Tectia, the major clients are ssh, scp, and sftp.

Session

An ongoing connection between a client and a server. It begins after the client successfully authenticates to a server and ends when the connection terminates. Sessions may be interactive or batch.

Key

A relatively small amount of data, generally from tens of to 1,000 or 2,000 bits, used as a parameter to cryptographic algorithms such as encryption or message authentication. The key binds the algorithm operation in some way to the key holder: in encryption, it ensures that only someone else holding that key (or a related one) can decrypt the message; in authentication, it allows you to verify later that the key holder actually signed the message. There are two kinds of keys : symmetric or secret key, and asymmetric or public key. [3.2.2] An asymmetric key has two parts: the public and private components. SSH has several types of keys, as summarized in Table 3-1.

Table 3-1. Keys, keys, keys

Name

Lifetime

Created by

Type

Purpose

User key

Persistent

User

Public

Identify a user to the server

Host key

Persistent

Administrator

Public

Identify a server/machine

Session key

One session

Client (and server)

Secret

Protect communications

User key

A persistent, asymmetric key used by clients as proof of a user’s identity. (A single user may have many keys/identities.)

Host key

A persistent, asymmetric key used by a server as proof of its identity, as well as by a client when proving its host’s identity as part of hostbased authentication. [3.4.3.6] If a machine runs a single SSH server, the host key also uniquely identifies the machine. (If a machine is running multiple SSH servers, each may have a different host key, or they may share.)

Session key

A randomly generated, symmetric key for encrypting the communication between an SSH client and server. It is shared by the two parties in a secure manner during the SSH connection setup so that an eavesdropper can’t discover it. Both sides then have the session key, which they use to encrypt their communications. When the SSH session ends, the key is destroyed.

Tip

An SSH connection has several session keys: each direction (server to client, and client to server) has keys for encryption and others for integrity checking. In our discussions we treat all the session keys as a unit and speak of “the session key” for convenience; they are all derived from a single master secret, anyway. If the context requires it, we identify the individual key we mean.

Key generator

A program that creates persistent keys (user keys and host keys) for SSH. OpenSSH and Tectia have the program ssh-keygen.

Known-hosts database

A collection of host keys. Clients and servers refer to this database to authenticate one another.

Agent

A program that caches user keys in memory, so users needn’t keep retyping their passphrases. The agent responds to requests for key-related operations, such as signing an authenticator, but it doesn’t disclose the keys themselves. It is a convenience feature. OpenSSH and Tectia have the agent ssh-agent, and the program ssh-add loads and unloads the key cache.

Signer

A program that signs hostbased authentication packets. We explain this in our discussion of hostbased authentication. [3.4.3.6]

Random seed

A pool of random data used by SSH components to initialize software pseudo-random number generators.

Configuration file

A collection of settings to tailor the behavior of an SSH client or server.

Not all these components are required in an implementation of SSH. Certainly servers, clients, and keys are mandatory, but many implementations don’t have an agent, and some don’t even include a key generator.