Chapter 17. OpenSSH

Chapter 17. OpenSSH

17.1. Features of SSH
17.2. SSH Protocol Versions
17.3. Event Sequence of an SSH Connection
17.4. Configuring an OpenSSH Server
17.5. OpenSSH Configuration Files
17.6. Configuring an OpenSSH Client
17.7. More Than a Secure Shell
17.8. Additional Resources

SSH™ (or Secure SHell) is a protocol which facilitates secure communications between two systems using a client/server architecture and allows users to log into server host systems remotely. Unlike other remote communication protocols, such as FTP or Telnet, SSH encrypts the login session, rendering the connection difficult for intruders to collect unencrypted passwords.

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.

17.1. 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.

  • 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[5] 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 encrypted communications.

17.1.1. 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 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 host.

    This attack can be mounted through techniques known as DNS poisoning[6] or IP spoofing[7].

Both techniques intercept potentially sensitive information and, if the interception is made 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 does not work, since each packet is encrypted using a key known only by the local and remote systems.



[5] X11 refers to the X11R7 windowing display system, traditionally referred to as the X Window System or X. Red Hat Enterprise Linux includes X11R7, an open source X Window System.

[6] DNS poisoning occurs when an intruder cracks a DNS server, pointing client systems to a maliciously duplicated host.

[7] IP spoofing occurs when an intruder sends network packets which falsely appear to be from a trusted host on the network.


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