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Section 1.2: How do SSH, Telnet and Rlogin differ?

This list summarises some of the differences between SSH, Telnet and Rlogin.

• SSH is a recently designed, high-security protocol. It uses strong cryptography to protect your connection against eavesdropping, hijacking and other attacks. Telnet and Rlogin are both older protocols offering minimal security.
• Telnet allows you to pass some settings on to the server, such as environment variables. (These control various aspects of the server's behaviour. You can usually set them by entering commands into the server once you're connected, but it's easier to have Telnet do it automatically.) SSH and Rlogin do not support this. However, most modern Telnet servers don't allow it either, because it has been a constant source of security problems.
• SSH and Rlogin both allow you to log in to the server without having to type a password. (Rlogin's method of doing this is insecure, and can allow an attacker to access your account on the server. SSH's method is much more secure, and typically breaking the security requires the attacker to have gained access to your actual client machine.)
• SSH allows you to connect to the server and automatically send a command, so that the server will run that command and then disconnect. So you can use it in automated processing.

The Internet is a hostile environment and security is everybody's responsibility. If you are connecting across the open Internet, then we recommend you use SSH. If the server you want to connect to doesn't support SSH, it might be worth trying to persuade the administrator to install it.

If you are behind a good firewall, it is more likely to be safe to use Telnet or Rlogin, but we still recommend you use SSH.

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