A daemon is a background process that performs a specific function or system task. In keeping with the UNIX and Linux philosophy of modularity, daemons are  pro-grams rather than parts of the kernel. Many daemons start at boot time and continue to run as long as the system is up. Other daemons are started when needed and run only as long as they are useful.

Daemons  made their way from Multics to UNIX to Linux, where they are so popular that they need a superdaemon ( xinetd  or  inetd) to manage them.

Before  inetd was written, all daemons started at boot time and ran continuously (or more accurately, they blocked waiting for work to do). Over time, more and more daemons  were added to the system. The daemon population became so large that it began to cause performance problems. In response, the Berkeley gurus developed inetd, a daemon that starts other daemons as they are needed.  inetd  successfully popularized this superdaemon model, which remains a common way to minimize the number of processes running on a server. Most versions of UNIX and Linux now use a combination of inetd and always-running daemons.

INIT
init  is the first process to run after the system boots, and in many ways it is the most important daemon. It always has a PID of 1 and is an ancestor of all user processes and all but a few system processes. At startup, init  either places the system in single-user mode or begins to execute the scripts needed to bring the system to multiuser mode. When you boot the system into single-user mode, init  runs  the startup scripts after you terminate the single-user shell by typing   exit  or .

In multiuser mode,  init  is responsible for making sure that processes are available to handle logins on every login-enabled device. Logins on serial ports are generally handled by some variant of  getty (e.g.,  agetty ,  mgetty , or  mingetty.  init  also supervises a graphical login procedure that allows users to log directly in to X Windows.

In addition to its login management duties ,  init  also has the responsibility to exor-cise undead zombie processes that would otherwise accumulate on the system. init  defines several “run levels” that determine what set  of system resources should be enabled. There are seven levels, numbered 0 to 6. The name “s” is recognized as a  synonym for level 1 (single-user mode). The characteristics of each run level are defined  in the  /etc/inittab  file.

init  usually reads its initial run level from the  /etc/inittab  file, but the run level can also be passed in as an argument from the boot loader. If “s” is specified,  init  enters single-user mode. Otherwise, it scans  /etc/inittab  for entries that apply to the re-quested run level and executes their corresponding commands.


The  telinit  command changes  init ’s run level once the system is up. For example, telinit   4  forces  init  to go to run level 4 (which is unused on our example systems). telinit ’s most useful argument is  q, which causes  init  to reread the  /etc/inittab  file.

CRON
The  cron  daemon (known as crond  on Red Hat) is responsible for running com-mands at preset times. It accepts schedule files (“crontabs”) from both users and administrators. cron  is frequently employed for administrative purposes, including management of log files and daily cleanup of the filesystem.

The  atd daemon runs commands scheduled with the  at  command. Most versions of Linux also include the  anacron scheduler, which executes jobs at time intervals rather than at specific times.  anacron is particularly useful on systems that are not always turned on, such as laptops.


XINETD
xinetd  and  inetd are daemons that manage other daemons. They start up their client daemons when there is work for them to do and allow the clients to die grace-fully once their tasks have been completed. The traditional version of inetd comes to us from the UNIX world, but most Linux distributions have migrated to xinetd , a souped-up alternative that incorporates security features similar to those formerly achieved through the use of tcpd , the “TCP wrappers” package. xinetd  also provides better protection against denial of service attacks, better log management features, and a more flexi-ble configuration language.
 
xinetd  and  inetd only work with daemons that provide services over the network. To find out when someone is trying to access one of their clients,  xinetd  and  inetd attach themselves to the network ports that would normally be managed by the qui-escent daemons. When  a connection occurs,  xinetd/inetd starts up the appropriate daemon and connects its standard  I/O channels to the network port. Daemons must be written with this convention in mind if they are to be compatible.

To change the daemons that loads at boot on Red Hat Linux, in a console, become root (type su -) and type setup, then in the menu select System Services. Various daemons are