Ubuntu Manpage: daemon - Writing and packaging system daemons

Provided by: systemd_204-5ubuntu20.31_amd64

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 STDIN, STDOUT, STDERR (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 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 STDIN, STDOUT, STDERR.

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 /var/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 at the same time it is verified that
the PID previously stored in the PID file no longer exists or belongs to a foreign
process. Commonly some kind of file locking is employed to implement this logic.

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.

15. 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.

New-Style Daemons
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 clean process
contexts: 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, and STDIN/STDOUT/STDERR 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, via 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 STDERR via fprintf(), which is then
forwarded to syslog by the init system. If log priorities are necessary these can be
encoded by prefixing individual log lines with strings like "<4>" (for log priority 4
"WARNING" in the syslog priority scheme), following a similar style as the Linux
kernel's printk() priority system. In fact, using this style of logging also enables
the init system to optionally direct all application logging to the kernel log buffer
(kmsg), as accessible via dmesg(1). This kind of logging may be enabled by setting
StandardError=syslog in the service unit file. For details see sd-daemon(3) and
systemd.exec(5).

These recommendations are similar but not identical to the Apple MacOS X Daemon
Requirements[2].

ACTIVATION

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.

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 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 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.

Device-Based Activation
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.

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,
often there are 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.

        6. 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.

   Installing Systemd Service Files
       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=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)])
           if test "x$with_systemdsystemunitdir" != xno; then
                   AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])
           fi
           AM_CONDITIONAL(HAVE_SYSTEMD, [test -n "$with_systemdsystemunitdir" -a "x$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

           %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 || :
           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.

NOTES