Remote login servers allow your users to run programs and perform other ordinary command shell tasks from just about any computer that can reach your system via a network connection. This is an extremely useful feature, but also one that presents many security challenges.
Common login servers for Linux include rlogind , Telnet, and SSH. Of the three, SSH is the most sophisticated and secure, and is the best choice for a login server to be used over the Internet. (Secure Telnet variants may be an exception in some cases, but these are very rare.) Telnet and rlogind are acceptable for use on trusted private networks, but you should take great care if you must use one of them on the Internet because their security models are inadequate to the task of protecting a system in today's hostile Internet environment. All three servers are fairly simple to configure for basic operation, but you can add options to modify their configurations in various ways. This is particularly true of SSH, which supports several security and other options.