Network Services In the Service Configuration window, there are a few important things to note. First, the status line immediately below the toolbar icons shows us that the system is running in Runlevel 5, and that this is the runlevel that is being edited. (A runlevel is a particular state of the operating system, characterized by a set of processes that are started. If we haven’t changed the system runlevel, then we’ll be editing the default, which is just what we want. For more information on runlevels, see the man pages for inittab and telinit. Be careful if you experiment with these, as mistakes could render your system unbootable, forcing you to boot from CD-ROM to correct the problem.) 4. If you click on the name of a service, the information panels on the right of the window will show a brief description of what the service is for, together with its status. Note Not all services represented here represent external services, some are internal daemons vital for the smooth running of your computer. It’s a good idea to take a moment to read through all the descriptions here and start to familiarize yourself with what’s available. 5. Now scroll through the list of services, and clear the check boxes next to lpd and portmapper, as these are two services that do allow remote connections to our system and that are enabled by default but probably aren’t needed in the majority of cases. When we make a change to any of the services, the Save button is no longer grayed out. So, when you’re happy with the changes you’ve made, click on save to update the configuration files. Note that this does not actually start or stop any services; all it does is update configuration files so that next time the runlevel that is being edited is entered, the right services are started, and when it is being left, the right services are stopped. If you want to make the change effective immediately, highlight the service and click the Start, Stop or Restart buttons on the toolbar according to what you want to do. xinetd You may have noticed in the table of services and their daemons presented earlier that several services (such as echo, daytime, and chargen) appear to be provided by the same service, namely xinetd. This isn’t a typo - xinetd does provide these, and it also looks after a whole lot more. xinetd is an enhanced version of the standard Unix inetd (internet daemon) program, which manages incoming connections to many different services (hence it is sometimes called a “super server”). It is a long-running service (usually started when the system boots) that listens for incoming TCP/IP connections on ports that it has been told about in its configuration file. When a connection request arrives, xinetd checks that the 418

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