Ssh Protocol; Features Of Ssh - Red Hat ENTERPRISE LINUX 3 Reference Manual

SSH™ (or Secure SHell) is a protocol which facilitates secure communications between two systems
using a client/server architecture and allowing users to log into server host systems remotely. But
unlike other remote communication protocols such as FTP or Telnet, SSH encrypts the login session,
making it impossible for intruders to collect unencrypted passwords.
SSH is designed to replace older, less secure terminal applications used to log into remote hosts,
such as or
telnet rsh
between hosts, such as
between the client and the server, avoid them whenever possible. Using secure methods to log into
remote systems decreases the risks for both the client system and the remote host.

19.1. Features of SSH

The SSH protocol provides the following safeguards:
After an initial connection, the client can verify that it is connecting to the same server it had
•
connected to previously.
The client transmits its authentication information to the server using strong, 128-bit encryption.
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All data sent and received during a session is transferred using 128-bit encryption, making inter-
•
cepted transmissions extremely difficult to decrypt and read.
The client can forward X11
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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 otherwise insecure protocols, like POP, and increasing overall system and data security.
Red Hat Enterprise Linux includes the general OpenSSH package (
OpenSSH server (
openssh-server
chapter titled OpenSSH in the Red Hat Enterprise Linux System Administration Guide for
instructions on installing and deploying OpenSSH. Also note that the OpenSSH packages require the
OpenSSL package (
OpenSSH to provide encrypted communications.
19.1.1. Why Use SSH?
Nefarious computer users have a variety of tools at their disposal enabling them to disrupt, intercept,
and re-route network traffic in an effort to gain access to a system. In general terms, these threats can
be categorized as follows:
Interception of communication between two systems — In this scenario, the attacker can be some-
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where on the network between the communicating entities, copying any information passed be-
tween them. The attacker may intercept and keep the information, or alter the information and send
it on to the intended recipient.
This attack can be mounted through the use of a packet sniffer — a common network utility.
1. X11 refers to the X11R6 windowing display system, traditionally referred to as the X Window System or X.
Red Hat Enterprise Linux includes XFree86, an open source X Window System.
. A related program called
. Because these older applications do not encrypt passwords transmitted
rcp
1 applications from the server. This technique, called X11 forwarding,
) and client (
) which installs several important cryptographic libraries, enabling
openssl
replaces older programs designed to copy files
scp
openssh-clients
Chapter 19.

SSH Protocol

) as well as the
openssh
) packages. Refer to the