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.
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 connecting to your system are not running Red Hat Linux, they can still find and use an SSH client native for their operating system.
Nefarious computer users have a variety of tools at their disposal to disrupt, intercept, and re-route network traffic in an effort to gain access to your 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 entities, 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 can be 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 will remain unaware it is communicating with the wrong host.
This can be attack can be mounted through techniques known as DNS poisoning [2] or IP spoofing [3].
Both techniques intercept potentially sensitive information, and if the interception is for hostile reasons, the results can be disastrous.
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 will not work, since each packet is encrypted using a key known only by the local and remote systems.
| [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] | DNS poisoning occurs when an intruder cracks a DNS server, pointing client systems to a maliciously duplicated host. |
| [3] | IP spoofing occurs when an intruder sends network packets which falsely appear to be from a trusted host on the network. |