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.