9.3. Network Logins

The main (physical) difference between logging in to a system through a serial terminal and logging in to a system through a network is that the connection between the terminal and the computer isn't point-to-point. In this case, login is simply a service available, just like any other network service, such as FTP or SMTP.

With the terminal logins that we described in the previous section, init knows which terminal devices are enabled for logins and spawns a getty process for each device. In the case of network logins, however, all the logins come through the kernel's network interface drivers (e.g., the Ethernet driver), and we don't know ahead of time how many of these will occur. Instead of having a process waiting for each possible login, we now have to wait for a network connection request to arrive.

To allow the same software to process logins over both terminal logins and network logins, a software driver called a pseudo terminal is used to emulate the behavior of a serial terminal and map terminal operations to network operations, and vice versa. (In Chapter 19, we'll talk about pseudo terminals in detail.)

BSD Network Logins

In BSD, a single process waits for most network connections: the inetd process, sometimes called the Internet superserver. In this section, we'll look at the sequence of processes involved in network logins for a BSD system. We are not interested in the detailed network programming aspects of these processes; refer to Stevens, Fenner, and Rudoff [2004] for all the details.

As part of the system start-up, init invokes a shell that executes the shell script /etc/rc. One of the daemons that is started by this shell script is inetd. Once the shell script terminates, the parent process of inetd becomes init; inetd waits for TCP/IP connection requests to arrive at the host. When a connection request arrives for it to handle, inetd does a fork and exec of the appropriate program.

Let's assume that a TCP connection request arrives for the TELNET server. TELNET is a remote login application that uses the TCP protocol. A user on another host (that is connected to the server's host through a network of some form) or on the same host initiates the login by starting the TELNET client:

   telnet hostname

The client opens a TCP connection to hostname, and the program that's started on hostname is called the TELNET server. The client and the server then exchange data across the TCP connection using the TELNET application protocol. What has happened is that the user who started the client program is now logged in to the server's host. (This assumes, of course, that the user has a valid account on the server's host.) Figure 9.4 shows the sequence of processes involved in executing the TELNET server, called telnetd.

The telnetd process then opens a pseudo-terminal device and splits into two processes using fork. The parent handles the communication across the network connection, and the child does an exec of the login program. The parent and the child are connected through the pseudo terminal. Before doing the exec, the child sets up file descriptors 0, 1, and 2 to the pseudo terminal. If we log in correctly, login performs the same steps we described in Section 9.2: it changes to our home directory and sets our group IDs, user ID, and our initial environment. Then login replaces itself with our login shell by calling exec. Figure 9.5 shows the arrangement of the processes at this point.

Obviously, a lot is going on between the pseudo-terminal device driver and the actual user at the terminal. We'll show all the processes involved in this type of arrangement in Chapter 19 when we talk about pseudo terminals in more detail.

The important thing to understand is that whether we log in through a terminal (Figure 9.3) or a network (Figure 9.5), we have a login shell with its standard input, standard output, and standard error connected to either a terminal device or a pseudo-terminal device. We'll see in the coming sections that this login shell is the start of a POSIX.1 session, and that the terminal or pseudo terminal is the controlling terminal for the session.

Mac OS X Network Logins

Logging in to a Mac OS X system over a network is identical to a BSD system, because Mac OS X is based partially on FreeBSD.

Linux Network Logins

Network logins under Linux are the same as under BSD, except that an alternate inetd process is used, called the extended Internet services daemon, xinetd. The xinetd process provides a finer level of control over services it starts than does inetd.

Solaris Network Logins

The scenario for network logins under Solaris is almost identical to the steps under BSD and Linux. An inetd server is used similar to the BSD version. The Solaris version has the additional ability to run under the service access facility framework, although it is not configured to do so. Instead, the inetd server is started by init. Either way, we end up with the same overall picture as in Figure 9.5.