SSH, The Secure Shell: The Definitive Guide, 2nd Edition

Chapter 5. Serverwide Configuration

After installing an SSH server (sshd),^[42] it’s time to make informed decisions about your server’s operation. Which authentication techniques should be permitted? How many bits should the server key contain? Should idle connections be dropped after a time limit or left connected indefinitely? These and other questions must be considered carefully. sshd has reasonable defaults, but don’t accept them blindly. Your server should conform to a carefully planned security policy. Fortunately, sshd is highly configurable, so you can make it do all kinds of interesting tricks.

This chapter covers serverwide configuration, in which a system administrator controls the global runtime behavior of the SSH server. This includes a large, rich set of features, such as TCP/IP settings, encryption, authentication, access control, and error logging. Some features are controlled by modifying a serverwide configuration file, and others by command-line options passed to the server at invocation.

Serverwide configuration is just one of three levels for controlling the behavior of SSH servers. The other two levels are compile-time configuration (Chapter 4), in which the server is compiled with or without certain functionality; and per-account configuration (Chapter 8), in which the server’s behavior is modified by end users for their accounts only. We’ll discuss the distinction between the three levels in more detail later. [5.2]

This chapter covers only the OpenSSH and ...