Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Scaricare il pdf (Pagina 131)

Chapter 10.

SSH Protocol

SSH™ allows users to log into host systems remotely. Unlike rlogin or telnet SSH encrypts the

SSH is designed to replace common methods for remotely logging into another system through a

command shell. 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 avoid them whenever possible. Using secure methods to remotely log in to

other systems will decrease the security risks for both your system and the remote system.

10.1. Introduction

SSH (or Secure SHell) is a protocol for creating a secure connection between two systems. In the SSH

protocol, the client machine initiates a connection with a server machine.

The following safeguards are provided by SSH:

• 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 it extremely difﬁcult to decrypt and read.

• The client has the ability to use X11

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 7.3 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.

10.1.1. Why Use SSH?

Threats to network trafﬁc include packet snifﬁng, DNS and IP spooﬁng

and the proliferation of fake

routing information. In general terms, these threats can be categorized as follows:

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.

2. Spooﬁng means an intruder sends network packets that falsely appear to be from a trusted host on the

network.

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 131