Installation guide

Chapter 9. SSH Protocol 133
• ssh_host_key.pub — The RSA public key used by the sshd daemon for version 1 of the SSH
protocol.
• ssh_host_rsa_key — The RSA private key used by the sshd daemon for version 2 of the SSH
protocol.
• ssh_host_rsa_key.pub — The RSA public key used by the sshd for version 2 of the SSH
protocol.
User-specific SSH configuration information is stored in the user’s home directory within the
~/.ssh/ directory:
• authorized_keys — This file holds a list of "authorized" public keys for servers. When the
client connects to a server, the server authenticates the client by checking its signed public key
stored within this file. This is an optional authentication method.
• id_dsa — Contains the DSA authentication identity of the user.
• id_dsa.pub — The DSA public key of the user.
• id_rsa — The RSA public key used by sshd for version 2 of the SSH protocol.
• identity — The RSA private key used by sshd for version 1 of the SSH protocol.
• known_hosts — This file contains DSA host keys of SSH servers accessed by the user. This file is
very important for ensuring that the SSH client is connecting the correct SSH server. If a host’s key
has changed, and you are not absolutely certain why, you should contact the system administrator
of the SSH server to make sure that the server has not been compromised. If a server’s host keys
are legitimately altered by a re-installation of Red Hat Linux the next time you log into that server
you will be notified that the host key stored in the known_hosts file does not match. To connect
to the server, the user must open the known_hosts file in a text editor and delete the key for that
host. This allows the SSH client to create a new host key.
See the man pages for ssh and sshd for information concerning the various directives available in
the SSH configuration files.
9.5. More Than a Secure Shell
A secure command line interface is just the beginning of the many ways SSH can be used. Given the
proper amount of bandwidth, X11 sessions can be directed over an SSH channel. Or, by using TCP/IP
forwarding, previously insecure port connections between systems can be mapped to specific SSH
channels.
9.5.1. X11 Forwarding
Opening an X11 session over an established SSH connection is as easy as running an X program on
the local machine. When an X program is run from the secure shell prompt, the SSH client and server
create a new secure channel, and the X program data is sent over that channel to your client machine
transparently.
X11 forwarding can be very useful. For example, you can use X11 forwarding to create a secure,
interactive session with up2date on the server to update packages. To do this, connect to the server
using ssh and type:
up2date &
You will be asked to supply the root password for the server. Then, Red Hat Update Agent will
appear and you can update your packages on the server as though you were sitting in front of the
machine.