Chapter 9.
SSH Protocol
SSH  allows users to log into host systems remotely. Unlike
rlogin
or
telnet
SSH encrypts the
login session, making it impossible for intruders to collect clear text passwords.
SSH is designed to replace older, less secure terminal applications used to log into remote systems,
such as telnet or rsh. A related program called
scp
replaces older programs designed to copy files
between hosts, such as ftp or rcp. Because these older applications do not encrypt passwords between
the client and the server, you should avoid them whenever possible. Using secure methods to remotely
log into other systems will decrease the security risks for both your system and the system you are
logging into.
9.1. Features of SSH
SSH (or Secure SHell) is a protocol for creating a secure connection between two systems using a
client server architecture. Under the SSH protocol, the client machine initiates all connections to a
server.
The SSH protocol provides the following safeguards:
After an initial connection, the client verifies it is connecting to the same server during subsequent
sessions.
The client transmits its authentication information to the server, such as a username and password,
in an encrypted format.
All data sent and received during the connection is transferred using strong, 128 bit encryption,
making intercepted transmissions extremely difficult to decrypt and read.
The client has the ability to use X11
1
applications launched from the shell prompt. This technique,
called X11 forwarding, provides a secure means to use graphical applications over a network.
Because the SSH protocol encrypts everything it sends and receives, it can be used to secure otherwise
insecure protocols. Using a technique called port forwarding, an SSH server can become a conduit to
secure insecure protocols, like POP, increasing overall system and data security.
Red Hat Linux 8.0 includes the general OpenSSH package (
openssh
), the OpenSSH server
(
openssh server
) and client (
openssh clients
) packages. Please see the chapter titled
OpenSSH in the Official Red Hat Linux Customization Guide for instructions on installing and
deploying OpenSSH. Also note that the OpenSSH packages require the OpenSSL package
(
openssl
). OpenSSL installs several important cryptographic libraries that help OpenSSH provide
encrypted communications.
A large number of client and server programs can use the SSH protocol. Several different SSH client
versions are available for almost every major operating system in use today. Even if the users con 
necting to your system are not running Red Hat Linux, they can still find and use an SSH client native
for their operating system.
1. X11 refers to the X11R6 windowing display system, traditionally referred to as X. Red Hat Linux includes
XFree86, a widely used, open source X Window System, which is based on X11R6.






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