The basic configuration files, the kernel configuration file, the startup files, and the /etc/inetd.conf or /etc/xinetd.conf file are necessary for installing the TCP/IP software on a Unix system. The kernel comes configured to run TCP/IP on most systems. Some systems, such as Solaris, are designed to eliminate kernel configuration. Others, such as Linux, encourage it as a way to produce a more efficient kernel. In either case, a network administrator needs to be aware of the kernel configuration commands required for TCP/IP so that they are not accidentally removed from the kernel when it is rebuilt.
Network services are either started at boot time from a startup script or are started on demand using xinetd or inetd. BSD systems have a few startup scripts that are run in sequence for every boot. System V Unix runs a different set of startup scripts for each runlevel. Runlevels are used to start the system in different modes, e.g., single user mode or multi-user mode. Both Solaris and Linux use the System V startup scheme.
inetd and xinetd start essential network services. Most Unix systems use inetd, although some, such as Red Hat Linux, use xinetd. Reconfigure inetd or xinetd to add new services and to improve security. Security can be improved by removing unneeded services or by adding access control. Chapter 12 provides additional information on how inetd and xinetd are used to improve system security.
The kernel configuration defines the network interface. In the next chapter we configure it, calling upon the planning we did in Chapter 4.