Overview

The primary goal of any network is to provide a mechanism for the sharing of resources and/or data. The sharing of resources such as printers is a subject in its own right and will typically be dependant upon the operating systems in use at both the host and client sites. The sharing of data however, generally falls into two distinct groups, namely, the wholesale copying of files, and on-line access. Files may be copied in their entirety through the implementation of the File Transfer Protocol (FTP), or the Trivial File Transfer Protocol (TFTP). These are discussed in detail in Chapters 17, and 19 respectively. On-line access to host resources though is the subject of this chapter.

The Telnet and Rlogin protocols provide a general bi-directional communications path that interfaces terminal devices to a terminal oriented process on a host machine, thus providing the illusion that the user has a local connection. Rlogin (available with Berkeley BSD UNIX systems) is built upon the ideas of Telnet but introduces an idea of trusted hosts thus eliminating the necessity for the user to identify himself when accessing a remote system. With these ideas in mind, we shall look at the Telnet protocol, and then examine how Rlogin modifies this to create what is a much simpler user interface.

16.1 The Telnet Protocol

Telnet (defined by RFC 854) uses a TCP connection (using port 23 ₁₀) over which it transmits data interspersed (where required) with control characters. The protocol itself assumes that...