Appendix D. Terminal server configuration

Terminal servers were originally designed for connecting terminals to minicomputers. Each terminal would have an RS-232 port. The connection to the minicomputer usually used an ethernet port. Connecting terminals would be connected to a command line interface where they could select from a list of predefined machines. A Telnet session would then be started to that machine.

Over time terminal servers gained more features. For example, modems could be connected. These initially allowed people to dial in to the minicomputer but grew in features until most terminal servers became routers with a great number of serial ports.

As well as allowing the connection of many console to a single terminal, the terminal server can be configured with user accounts and passwords, preventing unauthenticated access to the console whilst still allowing the console to be reached from any modem.

Internet Service Providers have been large users of terminal servers in the past. Each modem would be connected to a terminal server port and incoming users would be permitted to send IP packets anywhere, not just to some predefined minicomputer. Manufacturers renamed the equipment to ‘access server’ or ‘modem server’ to reflect this new use.

These devices have been superseded by a new generation of access server that allows telephone trunks to be plugged directly into the ISP's router. There are no discreet modems; the modem tones are decoded by digital signal processing chips within the router.

As a result terminal servers are currently readily available on the second-hand market.

Most old terminal servers will not support Secure Shell. In this is the case accessing the terminal server by its ethernet port is a poor idea: when you login to the console you password will travel across the Internet in clear text. Either dial in to the terminal server or use a one-time password system such as the RADIUS protocol with S/Key authentication.

An alternative to using a terminal server is to use a multiport serial card in another Linux system.

This remainder of this section lists the cabling pinouts and basic software configuration needed for differing types of terminal servers.

Further contributions are welcome and should be e-mailed to the maintainer of this HOWTO.