daemon — Writing and packaging system daemons
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.
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.
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.
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.
Reset the signal mask
using
sigprocmask().
Sanitize the environment block, removing or resetting environment variables that might negatively impact daemon runtime.
Call fork(),
to create a background
process.
In the child, call
setsid() to
detach from any terminal and create an
independent session.
In the child, call
fork() again, to
ensure that the daemon can never re-acquire
a terminal again.
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.
In the daemon process,
connect /dev/null
to standard input, output, and error.
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.
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.
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.
In the daemon process, drop privileges, if possible and applicable.
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.
Call
exit() in the
original process. The process that
invoked the daemon must be able to
rely on that this
exit() happens
after initialization is complete and
all external communication channels
are established and
accessible.
The BSD daemon() function should not be
used, as it implements only a subset of these steps.
A daemon that needs to provide compatibility with SysV systems should implement the scheme pointed out above. However, it is recommended to make this behavior optional and configurable via a command line argument to ease debugging as well as to simplify integration into systems using systemd.
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:
If SIGTERM is
received, shut down the daemon and
exit cleanly.
If SIGHUP is received,
reload the configuration files, if
this applies.
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.
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.
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.
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.
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.
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.
If applicable, a daemon should notify the init system about startup completion or status updates via the sd_notify(3) interface.
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).
These recommendations are similar but not identical to the Apple MacOS X Daemon Requirements.
New-style init systems provide multiple
additional mechanisms to activate services, as
detailed below. It is common that services are
configured to be activated via more than one mechanism
at the same time. An example for systemd:
bluetoothd.service might get
activated either when Bluetooth hardware is plugged
in, or when an application accesses its programming
interfaces via D-Bus. Or, a print server daemon might
get activated when traffic arrives at an IPP port, or
when a printer is plugged in, or when a file is queued
in the printer spool directory. Even for services that
are intended to be started on system bootup
unconditionally, it is a good idea to implement some of
the various activation schemes outlined below, in
order to maximize parallelization. If a daemon
implements a D-Bus service or listening socket,
implementing the full bus and socket activation 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.
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. 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.
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.
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 D-Bus
activation file is named
org.freedesktop.RealtimeKit.service,
make sure to set
SystemdService=rtkit-daemon.service
in that file to bind it to the systemd
service
rtkit-daemon.service. This
is needed to make sure that the daemon is
started in a race-free fashion when activated
via multiple mechanisms simultaneously.
Often, daemons that manage a particular
type of hardware should be activated only when
the hardware of the respective kind is plugged
in or otherwise becomes available. In a
new-style init system, it is possible to bind
activation to hardware plug/unplug events. In
systemd, kernel devices appearing in the
sysfs/udev device tree can be exposed as units
if they are tagged with the string
"systemd". Like any other
kind of unit, they may then pull in other units
when activated (i.e. plugged in) and thus
implement device-based activation. systemd
dependencies may be encoded in the udev
database via the
SYSTEMD_WANTS=
property. See
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.
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).
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 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.
When writing systemd unit files, it is recommended to consider the following suggestions:
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.
If your daemon
registers a D-Bus name on the bus,
make sure to use
Type=dbus in the
service file if
possible.
Make sure to set a
good human-readable description string
with
Description=.
Do not disable
DefaultDependencies=,
unless you really know what you do and
your unit is involved in early boot or
late system shutdown.
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.
Make sure to include
an "[Install]"
section including installation
information for the unit file. See
systemd.unit(5)
for details. To activate your service
on boot, make sure to add a
WantedBy=multi-user.target
or
WantedBy=graphical.target
directive. To activate your socket on
boot, make sure to add
WantedBy=sockets.target. Usually,
you also want to make sure that when
your service is installed, your socket
is installed too, hence add
Also=foo.socket in
your service file
foo.service, for
a hypothetical program
foo.
At the build installation time (e.g. make install during package build), packages are recommended to install their systemd unit files in the directory returned by pkg-config systemd --variable=systemdsystemunitdir (for system services) or pkg-config systemd --variable=systemduserunitdir (for user services). This will make the services available in the system on explicit request but not activate them automatically during boot. 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
endifIn 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 %preun %systemd_preun foobar.service foobar.socket %postun %systemd_postun
If the service shall be restarted during
upgrades, replace the
"%postun" scriptlet above
with the following:
%postun %systemd_postun_with_restart foobar.service
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 || :
fiWhere 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.
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:
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.
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.
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.
If the daemon exposes interfaces via D-Bus, write and install a D-Bus activation file for the service, see above for details.
It is recommended to follow the general guidelines for placing package files, as discussed in file-hierarchy(7).