SSH, The Secure Shell: The Definitive Guide

SSH, The Secure Shell: The Definitive Guide

by Daniel J. Barrett, Richard E. Silverman

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This guide covers the Secure Shell in detail for both system administrators and end users. It demystifies the SSH man pages and includes thorough coverage of basic to advanced use of SSH client tools for UNIX, Windows and Macintosh.  See more details below


This guide covers the Secure Shell in detail for both system administrators and end users. It demystifies the SSH man pages and includes thorough coverage of basic to advanced use of SSH client tools for UNIX, Windows and Macintosh.

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Chapter 8: Per-Account Server Configuration

In this chapter:
Limits of This Technique
Public Key-Based Configuration
Trusted-Host Access Control
The User rc File

We've seen two techniques for controlling the SSH server's behavior globally: compile-time configuration (Chapter 4) and serverwide configuration (Chapter 5). These techniques affect all incoming SSH connections to a given server machine. Now it's time to introduce a third, finer-grained method of server control: per-account configuration.

As the name implies, per-account configuration controls the SSH server differently for each user account on the server machine. For example, a user account sandy can accept incoming SSH connections from any machine on the Internet, while rick permits connections only from the domain, and fraidycat refuses key-based connections. Each user configures his or her own account, using the facilities highlighted in Figure 8-1, without needing special privileges or assistance from the system administrator.

We have already seen a simple type of per-account configuration. A user may place a public key into her authorization file, instructing the SSH server to permit logins to her account by public-key authentication. But per-account configuration can go further, becoming a powerful tool for access control and playing some fun tricks with your account. Accepting or rejecting connections by particular keys or hosts is just the beginning. For instance, you can make an incoming SSH connection run a program of your choice, instead of the client's choice. This is called a forced command, and we'll cover quite a few interesting applications.

Per-account configuration may control only incoming SSH connections to your account. If you're interested in configuring outgoing SSH connections by running SSH clients, refer to Chapter 7.

Limits of This Technique

Per-account configuration can do many interesting things, but it has some restrictions that we will discuss:

  • It can't defeat security measures put in place by compile-time or serverwide configuration. (Thank goodness.)

  • It is most flexible and secure if you use public-key authentication. Trusted-host and password authentication provide a much narrower range of options.

Overriding Serverwide Settings

SSH settings in a user's account may only restrict the authentication of incoming connections. They can't enable any SSH features that have been turned off more globally, and they can't permit a forbidden user or host to authenticate. For example, if your SSH server rejects all connections from the domain, you can't override this restriction within your account by per-account configuration.[1]

This limitation makes sense. No end-user tool should be able to violate a server security policy. However, end users should be (and are) allowed to restrict incoming connections to their accounts.

A few features of the server may be overridden by per-account configuration. The most notable one is the server's idle timeout, which may be extended beyond the serverwide setting. But such features can't coerce the server to accept a connection it has been globally configured to reject.

If you are an end user, and per-account configuration doesn't provide enough flexibility, you can run your own instance of the SSH server, which you may configure to your heart's content. Be cautious, though, since this is seldom the right thing to do. The restrictions you're trying to circumvent are part of the security policy defined for the machine by its administrators, and you shouldn't run a program that flouts this policy just because you can. If the machine in question is under your administrative control, simply configure the main SSH server as you wish. If not, then installing and running your own sshd might violate your usage agreement and/or certainly annoy your sysadmin. And that's never a wise thing to do.

Authentication Issues

To make the best use of per-account configuration, use public-key authentication. Password authentication is too limited, since the only way to control access is with the password itself. Trusted-host authentication permits a small amount of flexibility, but not nearly as much as public-key authentication.

If you're still stuck in the password-authentication dark ages, let this be another reason to switch to public keys. Even though passwords and public-key passphrases might seem similar (you type a secret word, and voilà, you're logged in), public keys are far more flexible for permitting or denying access to your account. Read on and learn how.

Public Key-Based Configuration

To set up public-key authentication in your account on an SSH server machine, you create an authorization file, typically called authorized_keys (SSH1, OpenSSH/1), authorized_keys2 (OpenSSH/2), or authorization (SSH2), and list the keys that provide access to your account. Well, we've been keeping a secret. Your authorization file can contain not only keys but also other keywords or options to control the SSH server in powerful ways. We will discuss:

  • The full format of an authorization file

  • Forced commands for limiting the set of programs that the client may invoke on the server

  • Restricting incoming connections from particular hosts

  • Setting environment variables for remote programs

  • Setting an idle timeout so clients will be forcibly disconnected if they aren't sending data

  • Disabling certain features of the incoming SSH connection, such as port forwarding and tty allocation

As we demonstrate how to modify your authorization file, remember that the file is consulted by the SSH server only at authentication time. Therefore, if you change your authorization file, only new connections will use the new information. Any existing connections are already authenticated and won't be affected by the change.

Also remember that an incoming connection request won't reach your authorization file if the SSH server rejects it for other reasons, namely, failing to satisfy the serverwide configuration. If a change to your authorization file doesn't seem to be having an effect, make sure it doesn't conflict with a (more powerful) serverwide configuration setting.

SSH1 Authorization Files

Your SSH1 authorized_keys file, generally found in ~/.ssh/authorized_keys, is a secure doorway into your account via the SSH-1 protocol. Each line of the file contains a public key and means the following: "I give permission for SSH-1 clients to access my account, in a particular way, using this key as authentication." Notice the words "in a particular way." Until now, public keys have provided unlimited access to an account. Now we'll see the rest of the story.

Each line of authorized_keys contains up to three items in order, some optional and some required:

  • A set of options (optional, surprise, surprise).

  • The public key (required). This appears in three parts:

  • The number of bits in the key, typically a small integer such as 1024

  • The exponent of the key: an integer

  • The modulus of the key: a very large integer, typically several hundred digits long

  • A descriptive comment (optional). This can be any text, such as "Bob's public key" or "My home PC using SecureCRT 3.1."

Public keys and comments are generated by ssh-keygen in .pub files, you may recall, and you typically insert them into authorized_keys by copying. Options, however, are usually typed into authorized_keys with a text editor.[2]

An option may take two forms. It may be a keyword, such as:

# SSH1, OpenSSH: Turn off port forwarding

or it may be a keyword followed by an equals sign and a value, such as:

# SSH1, OpenSSH: Set idle timeout to five minutes

Multiple options may be given together, separated by commas, with no whitespace between the options:

# SSH1, OpenSSH

If you mistakenly include whitespace:

# THIS IS ILLEGAL: whitespace between the options
no-port-forwarding, idle-timeout=5m 

your connection by this key won't work properly. If you connect with debugging turned on (ssh1 -v), you will see a "syntax error" message from the SSH server.

Many SSH users aren't aware of options or neglect to use them. This is a pity because options provide extra security and convenience. The more you know about the clients that access your account, the more options you can use to control that access.

SSH2 Authorization Files

An SSH2 authorization file, typically found in ~/.ssh2/authorization,[3] has a different format from its SSH1 ancestor. Instead of public keys, it contains keywords and values, much like other SSH configuration files we've seen. Each line of the file contains one keyword followed by its value. The most commonly used keywords are Key and Command.

Public keys are indicated using the Key keyword. Key is followed by whitespace, and then the name of a file containing a public key. Relative filenames refer to files in ~/.ssh2. For example:

# SSH2 only

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Meet the Author

Daniel J. Barrett, Ph.D., has been immersed in Internet technology since 1985. Currently a software engineer and vice president at a well-known financial services company, Dan has also been a heavy metal singer, Unix system administrator, university lecturer, web designer, and humorist.

Dan has written several other O'Reilly books, including NetResearch: Finding Information Online and Bandits on the Information Superhighway, as well as monthly columns for Compute! and Keyboard Magazine. He and his family reside in Boston.

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