Many servers provide users with access to very limited and specialized parts of the server computer. For instance, a time server (see Chapter 10, Maintaining Consistent Time: Time Servers) allows clients to read the server's clock, and a font server (see Chapter 15, Providing Consistent Fonts with Font Servers) delivers font bitmaps to clients . There is a class of server, though, that provides more-or-less complete access to the server computer: remote login servers. These allow an individual to log in to the computer and run arbitrary programs on the system, much as can be done by sitting at the console. Remote login servers also allow many people to use a single computer simultaneously .
There are several different types of remote login server that can be useful in different situations. This chapter covers a broad class of remote login server that provides text-mode access to the system. Using these programs, you can run text-based tools like the pine or mutt mail clients, development tools like gcc , and text-based editors like Vi or Emacs. By themselves , these tools don't allow you to run X-based programs like KMail or Nedit, though; that's the job of GUI access tools like those discussed in Chapter 14, Handling GUI Access with X and VNC Servers. (Sometimes you'll use a text-based login tool as part of the process of establishing GUI access, though.)
This chapter covers three specific text-based remote login tools: rlogind , Telnet, and SSH. Each of these has its unique characteristics, and is therefore best suited for particular types of uses. These differences relate to security and number of extra features, with rlogind providing the least security and features and SSH the most. If you run Kerberized versions of rlogind or Telnet, though, these servers can provide security on a par with that available in SSH. Chapter 6, Authenticating Users via Kerberos, covers this matter.