Chapter 11. SSH Protocol

This chapter covers the benefits of the SSH™ protocol, the sequence of events that occur when a secure connection is made to a remote system, the different layers of SSH, and methods to ensure SSH is used by users connecting to your system.

Common methods for remotely logging into another system through a shell (telnet, rlogin, or rsh) or copying files between hosts (ftp or rcp) are insecure and should be avoided. Instead, you should only connect to a remote host using a secure shell or an encrypted virtual private network. Using secure methods to remotely log in to other systems will decrease the security risks for both your system and the remote system.


SSH (or Secure SHell) is a protocol for creating a secure connection between two systems. Using SSH, the client machine initiates a connection with a server machine. The following safeguards are provided by SSH:

The server benefits from SSH, as well, especially if it is running a number of services. If you use port forwarding, otherwise insecure protocols (for example, POP) can be encrypted for secure communication with remote machines. SSH makes it relatively simple to encrypt different types of communication normally sent insecurely over public networks.

Red Hat Linux 7.1 includes the OpenSSH server (openssh-server) and client (openssh-clients) packages, as well as the general OpenSSH package (openssh) which must be installed for either of them to work. Please see the Official Red Hat Linux Customization Guide for instructions on installing and deploying OpenSSH on your Red Hat Linux system.

The OpenSSH packages require the OpenSSL package (openssl). OpenSSL installs several important cryptographic libraries that help OpenSSH provide encrypted communications. You must install the openssl package before installing any OpenSSH packages.

A large number of client and server programs can use the SSH protocol, including many open source and freely available applications. 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.

Why Use SSH?

Threats to network traffic include packet sniffing, DNS and IP spoofing[2] and the promulgation of fake routing information. In general terms, these threats can be categorized as follows:

  • Interception of communication between two systems — In this scenario, a third party exists somewhere on the network between communicating entities and makes a copy of the information being passed between them. The intercepting party may intercept and keep the information, or it may alter the information and send it on to the intended recipient.

  • Impersonation of a particular host — Using this strategy, an intercepting system pretends to be the intended recipient of a message. If the strategy works, the client remains unaware of the deception and continues to communicate with the interceptor as if its traffic had successfully reached its destination.

Both techniques cause information to be intercepted, possibly for hostile reasons. The results can be disastrous, whether that goal is achieved by listening for all packets on a LAN or a hacked DNS server pointing to a maliciously duplicated host.

If SSH is used for remote shell logins and file copying, these security threats can be greatly diminished. A server's digital signature provides verification for its identity. The entire communication between client and server systems cannot be used if intercepted, because each of the packets 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.



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.


Spoofing commonly means appearing to others to be a particular system when you are actually not that system.