SSH is designed to replace older, less secure terminal applications
used to log into remote hosts, such as telnet or rsh. A related program called scp replaces older programs designed to
copy files between hosts, such as rcp. Because these older applications do
not encrypt passwords transmitted 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.
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.
All data sent and received during a session is transferred
using 128-bit encryption, making intercepted transmissions
extremely difficult to decrypt and read.
The client can forward X11 applications from the server. 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
securing otherwise insecure protocols, like POP, and increasing
overall system and data security.
The OpenSSH server and client can also be configured to create a
tunnel similar to a virtual private network for traffic between
server and client machines.
Finally, OpenSSH servers and clients can be configured to
authenticate using the GSSAPI implementation of the Kerberos
network authentication protocol. For more information on
configuring Kerberos authentication services, refer to Section 42.6, “Kerberos”.
Red Hat Enterprise Linux includes the general OpenSSH package
(openssh) as well as the
OpenSSH server (openssh-server) and client (openssh-clients) packages. Note, the
OpenSSH packages require the OpenSSL package (openssl) which installs several
important cryptographic libraries, enabling OpenSSH to provide
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 somewhere on the network between the communicating
parties, copying any information passed between 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.
Impersonation of a particular host
— Using this strategy, an attacker's system is
configured to pose as the intended recipient of a
transmission. If this strategy works, the user's system
remains unaware that it is communicating with the wrong
This attack can be mounted through techniques known as DNS
poisoning or IP spoofing.
Both techniques intercept potentially sensitive information and,
if the interception is made for hostile reasons, the results can
If SSH is used for remote shell login and file copying, these
security threats can be greatly diminished. This is because the
SSH client and server use digital signatures to verify their
identity. Additionally, all communication between the client and
server systems is encrypted. Attempts to spoof the identity of
either side of a communication does not work, since each packet
is encrypted using a key known only by the local and remote