What is a daemon?
A daemon, or system service, is a background process usually started during the initial boot sequence by ?init. Daemons typically run independent of users, waiting for system events to occur and providing services in response. Some common daemons include:
sshd - listens for and manages incoming ?SSH connections
acpid - listens for power management events and executes scripts based on them
apache - provides a local HTTP web server
Managing daemons in Debian
Debian makes use of System V-style init scripts for daemon management. This allows daemons to operate conditionally, based on the current RunLevel of the computer. For example, a daemon can be configured to run only when the computer is in single-user mode (runlevel 1) or, more commonly, when in multi-user mode (runlevels 2-5). For more information, see ?init and RunLevel.
Note: Many daemons provide essential services to a working Debian installation; others can pose unnecessary security risks when enabled carelessly. Caution should be used whenever managing daemons. When in doubt, refer to the documentation of the daemon itself.
Most system services accept a common set of options, which can be passed directly to the daemon script at the command line, for example:
# /etc/init.d/ssh restart Restarting OpenBSD Secure Shell server: sshd.
A brief description of each option follows:
start: start a service
stop: stop a service
restart: restart a service without reloading its job config file
reload: send a SIGHUP signal to running process
status: return the status of a service
Stopping and starting daemons in this manner is temporary and will not survive a reboot. Refer to the next section to enable/disable daemons on a more permanent basis.
The terms add and remove are a bit of a misnomer because we're not actually adding or removing the services from Debian, rather we're enabling or disabling them.
To disable a service, run the following in a terminal as root:
# update-rc.d -f <servicename> remove
To (re)enable a service, using the default settings, run the following:
# update-rc.d <servicename> defaults
For detailed information on what these commands do, refer to the update-rc.d manpage.