DAEMON(7) daemon DAEMON(7)
NAME
daemon - Writing and packaging system daemons
DESCRIPTION
A daemon is a service process that runs in the background and supervises the system or provides functionality
to other processes. Traditionally, daemons are implemented following a scheme originating in SysV Unix. Modern
daemons should follow a simpler yet more powerful scheme (here called "new-style" daemons), as implemented by
systemd(1). This manual page covers both schemes, and in particular includes recommendations for daemons that
shall be included in the systemd init system.
SysV Daemons
When a traditional SysV daemon starts, it should execute the following steps as part of the initialization.
Note that these steps are unnecessary for new-style daemons (see below), and should only be implemented if
compatibility with SysV is essential.
1. Close all open file descriptors except standard input, output, and error (i.e. the first three file
descriptors 0, 1, 2). This ensures that no accidentally passed file descriptor stays around in the daemon
process. On Linux, this is best implemented by iterating through /proc/self/fd, with a fallback of
iterating from file descriptor 3 to the value returned by getrlimit() for RLIMIT_NOFILE.
2. Reset all signal handlers to their default. This is best done by iterating through the available signals
up to the limit of _NSIG and resetting them to SIG_DFL.
3. Reset the signal mask using sigprocmask().
4. Sanitize the environment block, removing or resetting environment variables that might negatively impact
daemon runtime.
5. Call fork(), to create a background process.
6. In the child, call setsid() to detach from any terminal and create an independent session.
7. In the child, call fork() again, to ensure that the daemon can never re-acquire a terminal again.
8. Call exit() in the first child, so that only the second child (the actual daemon process) stays around.
This ensures that the daemon process is re-parented to init/PID 1, as all daemons should be.
9. In the daemon process, connect /dev/null to standard input, output, and error.
10. In the daemon process, reset the umask to 0, so that the file modes passed to open(), mkdir() and suchlike
directly control the access mode of the created files and directories.
11. In the daemon process, change the current directory to the root directory (/), in order to avoid that the
daemon involuntarily blocks mount points from being unmounted.
12. In the daemon process, write the daemon PID (as returned by getpid()) to a PID file, for example
/run/foobar.pid (for a hypothetical daemon "foobar") to ensure that the daemon cannot be started more than
once. This must be implemented in race-free fashion so that the PID file is only updated when it is
verified at the same time that the PID previously stored in the PID file no longer exists or belongs to a
foreign process.
13. In the daemon process, drop privileges, if possible and applicable.
14. From the daemon process, notify the original process started that initialization is complete. This can be
implemented via an unnamed pipe or similar communication channel that is created before the first fork()
and hence available in both the original and the daemon process.
Modern services for Linux should be implemented as new-style daemons. This makes it easier to supervise and
control them at runtime and simplifies their implementation.
For developing a new-style daemon, none of the initialization steps recommended for SysV daemons need to be
implemented. New-style init systems such as systemd make all of them redundant. Moreover, since some of these
steps interfere with process monitoring, file descriptor passing and other functionality of the init system,
it is recommended not to execute them when run as new-style service.
Note that new-style init systems guarantee execution of daemon processes in a clean process context: it is
guaranteed that the environment block is sanitized, that the signal handlers and mask is reset and that no
left-over file descriptors are passed. Daemons will be executed in their own session, with standard
input/output/error connected to /dev/null unless otherwise configured. The umask is reset.
It is recommended for new-style daemons to implement the following:
1. If SIGTERM is received, shut down the daemon and exit cleanly.
2. If SIGHUP is received, reload the configuration files, if this applies.
3. Provide a correct exit code from the main daemon process, as this is used by the init system to detect
service errors and problems. It is recommended to follow the exit code scheme as defined in the LSB
recommendations for SysV init scripts[1].
4. If possible and applicable, expose the daemon's control interface via the D-Bus IPC system and grab a bus
name as last step of initialization.
5. For integration in systemd, provide a .service unit file that carries information about starting, stopping
and otherwise maintaining the daemon. See systemd.service(5) for details.
6. As much as possible, rely on the init system's functionality to limit the access of the daemon to files,
services and other resources, i.e. in the case of systemd, rely on systemd's resource limit control
instead of implementing your own, rely on systemd's privilege dropping code instead of implementing it in
the daemon, and similar. See systemd.exec(5) for the available controls.
7. If D-Bus is used, make your daemon bus-activatable by supplying a D-Bus service activation configuration
file. This has multiple advantages: your daemon may be started lazily on-demand; it may be started in
parallel to other daemons requiring it -- which maximizes parallelization and boot-up speed; your daemon
can be restarted on failure without losing any bus requests, as the bus queues requests for activatable
services. See below for details.
8. If your daemon provides services to other local processes or remote clients via a socket, it should be
made socket-activatable following the scheme pointed out below. Like D-Bus activation, this enables
on-demand starting of services as well as it allows improved parallelization of service start-up. Also,
for state-less protocols (such as syslog, DNS), a daemon implementing socket-based activation can be
restarted without losing a single request. See below for details.
9. If applicable, a daemon should notify the init system about startup completion or status updates via the
sd_notify(3) interface.
10. Instead of using the syslog() call to log directly to the system syslog service, a new-style daemon may
choose to simply log to standard error via fprintf(), which is then forwarded to syslog by the init
system. If log levels are necessary, these can be encoded by prefixing individual log lines with strings
like "<4>" (for log level 4 "WARNING" in the syslog priority scheme), following a similar style as the
Linux kernel's printk() level system. For details, see sd-daemon(3) and systemd.exec(5).
scheme allows starting of the daemon with its clients in parallel (which speeds up boot-up), since all its
communication channels are established already, and no request is lost because client requests will be queued
by the bus system (in case of D-Bus) or the kernel (in case of sockets) until the activation is completed.
Activation on Boot
Old-style daemons are usually activated exclusively on boot (and manually by the administrator) via SysV init
scripts, as detailed in the LSB Linux Standard Base Core Specification[1]. This method of activation is
supported ubiquitously on Linux init systems, both old-style and new-style systems. Among other issues, SysV
init scripts have the disadvantage of involving shell scripts in the boot process. New-style init systems
generally employ updated versions of activation, both during boot-up and during runtime and using more minimal
service description files.
In systemd, if the developer or administrator wants to make sure that a service or other unit is activated
automatically on boot, it is recommended to place a symlink to the unit file in the .wants/ directory of
either multi-user.target or graphical.target, which are normally used as boot targets at system startup. See
systemd.unit(5) for details about the .wants/ directories, and systemd.special(7) for details about the two
boot targets.
Socket-Based Activation
In order to maximize the possible parallelization and robustness and simplify configuration and development,
it is recommended for all new-style daemons that communicate via listening sockets to employ socket-based
activation. In a socket-based activation scheme, the creation and binding of the listening socket as primary
communication channel of daemons to local (and sometimes remote) clients is moved out of the daemon code and
into the init system. Based on per-daemon configuration, the init system installs the sockets and then hands
them off to the spawned process as soon as the respective daemon is to be started. Optionally, activation of
the service can be delayed until the first inbound traffic arrives at the socket to implement on-demand
activation of daemons. However, the primary advantage of this scheme is that all providers and all consumers
of the sockets can be started in parallel as soon as all sockets are established. In addition to that, daemons
can be restarted with losing only a minimal number of client transactions, or even any client request at all
(the latter is particularly true for state-less protocols, such as DNS or syslog), because the socket stays
bound and accessible during the restart, and all requests are queued while the daemon cannot process them.
New-style daemons which support socket activation must be able to receive their sockets from the init system
instead of creating and binding them themselves. For details about the programming interfaces for this scheme
provided by systemd, see sd_listen_fds(3) and sd-daemon(3). For details about porting existing daemons to
socket-based activation, see below. With minimal effort, it is possible to implement socket-based activation
in addition to traditional internal socket creation in the same codebase in order to support both new-style
and old-style init systems from the same daemon binary.
systemd implements socket-based activation via .socket units, which are described in systemd.socket(5). When
configuring socket units for socket-based activation, it is essential that all listening sockets are pulled in
by the special target unit sockets.target. It is recommended to place a WantedBy=sockets.target directive in
the "[Install]" section to automatically add such a dependency on installation of a socket unit. Unless
DefaultDependencies=no is set, the necessary ordering dependencies are implicitly created for all socket
units. For more information about sockets.target, see systemd.special(7). It is not necessary or recommended
to place any additional dependencies on socket units (for example from multi-user.target or suchlike) when one
is installed in sockets.target.
Bus-Based Activation
When the D-Bus IPC system is used for communication with clients, new-style daemons should employ bus
activation so that they are automatically activated when a client application accesses their IPC interfaces.
This is configured in D-Bus service files (not to be confused with systemd service unit files!). To ensure
that D-Bus uses systemd to start-up and maintain the daemon, use the SystemdService= directive in these
service files to configure the matching systemd service for a D-Bus service. e.g.: For a D-Bus service whose
systemd.device(5) for details. Often, it is nicer to pull in services from devices only indirectly via
dedicated targets. Example: Instead of pulling in bluetoothd.service from all the various bluetooth dongles
and other hardware available, pull in bluetooth.target from them and bluetoothd.service from that target. This
provides for nicer abstraction and gives administrators the option to enable bluetoothd.service via
controlling a bluetooth.target.wants/ symlink uniformly with a command like enable of systemctl(1) instead of
manipulating the udev ruleset.
Path-Based Activation
Often, runtime of daemons processing spool files or directories (such as a printing system) can be delayed
until these file system objects change state, or become non-empty. New-style init systems provide a way to
bind service activation to file system changes. systemd implements this scheme via path-based activation
configured in .path units, as outlined in systemd.path(5).
Timer-Based Activation
Some daemons that implement clean-up jobs that are intended to be executed in regular intervals benefit from
timer-based activation. In systemd, this is implemented via .timer units, as described in systemd.timer(5).
Other Forms of Activation
Other forms of activation have been suggested and implemented in some systems. However, there are often
simpler or better alternatives, or they can be put together of combinations of the schemes above. Example:
Sometimes, it appears useful to start daemons or .socket units when a specific IP address is configured on a
network interface, because network sockets shall be bound to the address. However, an alternative to implement
this is by utilizing the Linux IP_FREEBIND socket option, as accessible via FreeBind=yes in systemd socket
files (see systemd.socket(5) for details). This option, when enabled, allows sockets to be bound to a
non-local, not configured IP address, and hence allows bindings to a particular IP address before it actually
becomes available, making such an explicit dependency to the configured address redundant. Another often
suggested trigger for service activation is low system load. However, here too, a more convincing approach
might be to make proper use of features of the operating system, in particular, the CPU or IO scheduler of
Linux. Instead of scheduling jobs from userspace based on monitoring the OS scheduler, it is advisable to
leave the scheduling of processes to the OS scheduler itself. systemd provides fine-grained access to the CPU
and IO schedulers. If a process executed by the init system shall not negatively impact the amount of CPU or
IO bandwidth available to other processes, it should be configured with CPUSchedulingPolicy=idle and/or
IOSchedulingClass=idle. Optionally, this may be combined with timer-based activation to schedule background
jobs during runtime and with minimal impact on the system, and remove it from the boot phase itself.
INTEGRATION WITH SYSTEMD
Writing Systemd Unit Files
When writing systemd unit files, it is recommended to consider the following suggestions:
1. If possible, do not use the Type=forking setting in service files. But if you do, make sure to set the PID
file path using PIDFile=. See systemd.service(5) for details.
2. If your daemon registers a D-Bus name on the bus, make sure to use Type=dbus in the service file if
possible.
3. Make sure to set a good human-readable description string with Description=.
4. Do not disable DefaultDependencies=, unless you really know what you do and your unit is involved in early
boot or late system shutdown.
5. Normally, little if any dependencies should need to be defined explicitly. However, if you do configure
explicit dependencies, only refer to unit names listed on systemd.special(7) or names introduced by your
own package to keep the unit file operating system-independent.
Optionally, during package installation (e.g. rpm -i by the administrator), symlinks should be created in the
systemd configuration directories via the enable command of the systemctl(1) tool to activate them
automatically on boot.
Packages using autoconf(1) are recommended to use a configure script excerpt like the following to determine
the unit installation path during source configuration:
PKG_PROG_PKG_CONFIG
AC_ARG_WITH([systemdsystemunitdir],
[AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],,
[with_systemdsystemunitdir=auto])
AS_IF([test "x$with_systemdsystemunitdir" = "xyes" -o "x$with_systemdsystemunitdir" = "xauto"], [
def_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)
AS_IF([test "x$def_systemdsystemunitdir" = "x"],
[AS_IF([test "x$with_systemdsystemunitdir" = "xyes"],
[AC_MSG_ERROR([systemd support requested but pkg-config unable to query systemd package])])
with_systemdsystemunitdir=no],
[with_systemdsystemunitdir="$def_systemdsystemunitdir"])])
AS_IF([test "x$with_systemdsystemunitdir" != "xno"],
[AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])])
AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])
This snippet allows automatic installation of the unit files on systemd machines, and optionally allows their
installation even on machines lacking systemd. (Modification of this snippet for the user unit directory is
left as an exercise for the reader.)
Additionally, to ensure that make distcheck continues to work, it is recommended to add the following to the
top-level Makefile.am file in automake(1)-based projects:
DISTCHECK_CONFIGURE_FLAGS = \
--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
Finally, unit files should be installed in the system with an automake excerpt like the following:
if HAVE_SYSTEMD
systemdsystemunit_DATA = \
foobar.socket \
foobar.service
endif
In the rpm(8).spec file, use snippets like the following to enable/disable the service during
installation/deinstallation. This makes use of the RPM macros shipped along systemd. Consult the packaging
guidelines of your distribution for details and the equivalent for other package managers.
At the top of the file:
BuildRequires: systemd
%{?systemd_requires}
And as scriptlets, further down:
%post
%systemd_post foobar.service foobar.socket
Note that "%systemd_post" and "%systemd_preun" expect the names of all units that are installed/removed as
arguments, separated by spaces. "%systemd_postun" expects no arguments. "%systemd_postun_with_restart"
expects the units to restart as arguments.
To facilitate upgrades from a package version that shipped only SysV init scripts to a package version that
ships both a SysV init script and a native systemd service file, use a fragment like the following:
%triggerun -- foobar < 0.47.11-1
if /sbin/chkconfig --level 5 foobar ; then
/bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&1 || :
fi
Where 0.47.11-1 is the first package version that includes the native unit file. This fragment will ensure
that the first time the unit file is installed, it will be enabled if and only if the SysV init script is
enabled, thus making sure that the enable status is not changed. Note that chkconfig is a command specific to
Fedora which can be used to check whether a SysV init script is enabled. Other operating systems will have to
use different commands here.
PORTING EXISTING DAEMONS
Since new-style init systems such as systemd are compatible with traditional SysV init systems, it is not
strictly necessary to port existing daemons to the new style. However, doing so offers additional
functionality to the daemons as well as simplifying integration into new-style init systems.
To port an existing SysV compatible daemon, the following steps are recommended:
1. If not already implemented, add an optional command line switch to the daemon to disable daemonization.
This is useful not only for using the daemon in new-style init systems, but also to ease debugging.
2. If the daemon offers interfaces to other software running on the local system via local AF_UNIX sockets,
consider implementing socket-based activation (see above). Usually, a minimal patch is sufficient to
implement this: Extend the socket creation in the daemon code so that sd_listen_fds(3) is checked for
already passed sockets first. If sockets are passed (i.e. when sd_listen_fds() returns a positive value),
skip the socket creation step and use the passed sockets. Secondly, ensure that the file system socket
nodes for local AF_UNIX sockets used in the socket-based activation are not removed when the daemon shuts
down, if sockets have been passed. Third, if the daemon normally closes all remaining open file
descriptors as part of its initialization, the sockets passed from the init system must be spared. Since
new-style init systems guarantee that no left-over file descriptors are passed to executed processes, it
might be a good choice to simply skip the closing of all remaining open file descriptors if sockets are
passed.
3. Write and install a systemd unit file for the service (and the sockets if socket-based activation is used,
as well as a path unit file, if the daemon processes a spool directory), see above for details.
4. If the daemon exposes interfaces via D-Bus, write and install a D-Bus activation file for the service, see
above for details.
PLACING DAEMON DATA
It is recommended to follow the general guidelines for placing package files, as discussed in file-
hierarchy(7).
SEE ALSO
systemd(1), sd-daemon(3), sd_listen_fds(3), sd_notify(3), daemon(3), systemd.service(5), file-hierarchy(7)
NOTES
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