man/daemon.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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   8   Copyright 2010 Lennart Poettering
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  23
  24 <refentry id="daemon">
  25
  26         <refentryinfo>
  27                 <title>daemon</title>
  28                 <productname>systemd</productname>
  29
  30                 <authorgroup>
  31                         <author>
  32                                 <contrib>Developer</contrib>
  33                                 <firstname>Lennart</firstname>
  34                                 <surname>Poettering</surname>
  35                                 <email>lennart@poettering.net</email>
  36                         </author>
  37                 </authorgroup>
  38         </refentryinfo>
  39
  40         <refmeta>
  41                 <refentrytitle>daemon</refentrytitle>
  42                 <manvolnum>7</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>daemon</refname>
  47                 <refpurpose>Writing and packaging system daemons</refpurpose>
  48         </refnamediv>
  49
  50         <refsect1>
  51                 <title>Description</title>
  52
  53                 <para>A daemon is a service process that runs in the
  54                 background and supervises the system or provides
  55                 functionality to other processes. Traditionally,
  56                 daemons are implemented following a scheme originating
  57                 in SysV Unix. Modern daemons should follow a simpler
  58                 yet more powerful scheme (here called "new-style"
  59                 daemons), as implemented by
  60                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>. This
  61                 manual page covers both schemes, and in
  62                 particular includes recommendations for daemons that
  63                 shall be included in the systemd init system.</para>
  64
  65                 <refsect2>
  66                         <title>SysV Daemons</title>
  67
  68                         <para>When a traditional SysV daemon
  69                         starts, it should execute the following steps
  70                         as part of the initialization. Note that these
  71                         steps are unnecessary for new-style daemons (see below),
  72                         and should only be implemented if compatibility
  73                         with SysV is essential.</para>
  74
  75                         <orderedlist>
  76                                 <listitem><para>Close all open file
  77                                 descriptors except stdin, stdout,
  78                                 stderr (i.e. the first three file
  79                                 descriptors 0, 1, 2). This ensures
  80                                 that no accidentally passed file
  81                                 descriptor stays around in the daemon
  82                                 process. On Linux, this is best
  83                                 implemented by iterating through
  84                                 <filename>/proc/self/fd</filename>,
  85                                 with a fallback of iterating from file
  86                                 descriptor 3 to the value returned by
  87                                 <function>getrlimit()</function> for
  88                                 RLIMIT_NOFILE.</para></listitem>
  89
  90                                 <listitem><para>Reset all signal
  91                                 handlers to their default. This is
  92                                 best done by iterating through the
  93                                 available signals up to the limit of
  94                                 _NSIG and resetting them to
  95                                 <constant>SIG_DFL</constant>.</para></listitem>
  96
  97                                 <listitem><para>Reset the signal mask
  98                                 using
  99                                 <function>sigprocmask()</function>.</para></listitem>
 100
 101                                 <listitem><para>Sanitize the
 102                                 environment block, removing or
 103                                 resetting environment variables that
 104                                 might negatively impact daemon
 105                                 runtime.</para></listitem>
 106
 107                                 <listitem><para>Call <function>fork()</function>,
 108                                 to create a background
 109                                 process.</para></listitem>
 110
 111                                 <listitem><para>In the child, call
 112                                 <function>setsid()</function> to
 113                                 detach from any terminal and create an
 114                                 independent session.</para></listitem>
 115
 116                                 <listitem><para>In the child, call
 117                                 <function>fork()</function> again, to
 118                                 ensure that the daemon can never re-acquire
 119                                 a terminal again.</para></listitem>
 120
 121                                 <listitem><para>Call <function>exit()</function> in the
 122                                 first child, so that only the second
 123                                 child (the actual daemon process)
 124                                 stays around. This ensures that the
 125                                 daemon process is re-parented to
 126                                 init/PID 1, as all daemons should
 127                                 be.</para></listitem>
 128
 129                                 <listitem><para>In the daemon process,
 130                                 connect <filename>/dev/null</filename>
 131                                 to standard input, output, and error.
 132                                 </para></listitem>
 133
 134                                 <listitem><para>In the daemon process,
 135                                 reset the umask to 0, so that the file
 136                                 modes passed to <function>open()</function>, <function>mkdir()</function> and
 137                                 suchlike directly control the access
 138                                 mode of the created files and
 139                                 directories.</para></listitem>
 140
 141                                 <listitem><para>In the daemon process,
 142                                 change the current directory to the
 143                                 root directory (/), in order to avoid
 144                                 that the daemon involuntarily
 145                                 blocks mount points from being
 146                                 unmounted.</para></listitem>
 147
 148                                 <listitem><para>In the daemon process,
 149                                 write the daemon PID (as returned by
 150                                 <function>getpid()</function>) to a
 151                                 PID file, for example
 152                                 <filename>/run/foobar.pid</filename>
 153                                 (for a hypothetical daemon "foobar")
 154                                 to ensure that the daemon cannot be
 155                                 started more than once. This must be
 156                                 implemented in race-free fashion so
 157                                 that the PID file is only updated when
 158                                 it is verified at the same time that
 159                                 the PID previously stored in the PID
 160                                 file no longer exists or belongs to a
 161                                 foreign process. Commonly, some kind of
 162                                 file locking is employed to implement
 163                                 this logic.</para></listitem>
 164
 165                                 <listitem><para>In the daemon process,
 166                                 drop privileges, if possible and
 167                                 applicable.</para></listitem>
 168
 169                                 <listitem><para>From the daemon
 170                                 process, notify the original process
 171                                 started that initialization is
 172                                 complete. This can be implemented via
 173                                 an unnamed pipe or similar
 174                                 communication channel that is created
 175                                 before the first
 176                                 <function>fork()</function> and hence
 177                                 available in both the original and the
 178                                 daemon process.</para></listitem>
 179
 180                                 <listitem><para>Call
 181                                 <function>exit()</function> in the
 182                                 original process. The process that
 183                                 invoked the daemon must be able to
 184                                 rely on that this
 185                                 <function>exit()</function> happens
 186                                 after initialization is complete and
 187                                 all external communication channels
 188                                 are established and
 189                                 accessible.</para></listitem>
 190                         </orderedlist>
 191
 192                         <para>The BSD <function>daemon()</function> function should not be
 193                         used, as it implements only a subset of these steps.</para>
 194
 195                         <para>A daemon that needs to provide
 196                         compatibility with SysV systems should
 197                         implement the scheme pointed out
 198                         above. However, it is recommended to make this
 199                         behavior optional and configurable via a
 200                         command line argument to ease debugging as
 201                         well as to simplify integration into systems
 202                         using systemd.</para>
 203                 </refsect2>
 204
 205                 <refsect2>
 206                         <title>New-Style Daemons</title>
 207
 208                         <para>Modern services for Linux should be
 209                         implemented as new-style daemons. This makes it
 210                         easier to supervise and control them at
 211                         runtime and simplifies their
 212                         implementation.</para>
 213
 214                         <para>For developing a new-style daemon, none
 215                         of the initialization steps recommended for
 216                         SysV daemons need to be implemented. New-style
 217                         init systems such as systemd make all of them
 218                         redundant. Moreover, since some of these steps
 219                         interfere with process monitoring, file
 220                         descriptor passing and other functionality of
 221                         the init system, it is recommended not to
 222                         execute them when run as new-style
 223                         service.</para>
 224
 225                         <para>Note that new-style init systems
 226                         guarantee execution of daemon processes in a
 227                         clean process context: it is guaranteed that
 228                         the environment block is sanitized, that the
 229                         signal handlers and mask is reset and that no
 230                         left-over file descriptors are passed. Daemons
 231                         will be executed in their own session, with
 232                         standard input/output/error connected to
 233                         <filename>/dev/null</filename> unless
 234                         otherwise configured. The umask is reset.
 235                         </para>
 236
 237                         <para>It is recommended for new-style daemons
 238                         to implement the following:</para>
 239
 240                         <orderedlist>
 241                                 <listitem><para>If <constant>SIGTERM</constant> is
 242                                 received, shut down the daemon and
 243                                 exit cleanly.</para></listitem>
 244
 245                                 <listitem><para>If <constant>SIGHUP</constant> is received,
 246                                 reload the configuration files, if
 247                                 this applies.</para></listitem>
 248
 249                                 <listitem><para>Provide a correct exit
 250                                 code from the main daemon process, as
 251                                 this is used by the init system to
 252                                 detect service errors and problems. It
 253                                 is recommended to follow the exit code
 254                                 scheme as defined in the <ulink
 255                                 url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
 256                                 recommendations for SysV init
 257                                 scripts</ulink>.</para></listitem>
 258
 259                                 <listitem><para>If possible and
 260                                 applicable, expose the daemon's control
 261                                 interface via the D-Bus IPC system and
 262                                 grab a bus name as last step of
 263                                 initialization.</para></listitem>
 264
 265                                 <listitem><para>For integration in
 266                                 systemd, provide a
 267                                 <filename>.service</filename> unit
 268                                 file that carries information about
 269                                 starting, stopping and otherwise
 270                                 maintaining the daemon. See
 271                                 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 272                                 for details.</para></listitem>
 273
 274                                 <listitem><para>As much as possible,
 275                                 rely on the init system's
 276                                 functionality to limit the access of
 277                                 the daemon to files, services and
 278                                 other resources, i.e. in the case of
 279                                 systemd, rely on systemd's resource
 280                                 limit control instead of implementing
 281                                 your own, rely on systemd's privilege
 282                                 dropping code instead of implementing
 283                                 it in the daemon, and similar. See
 284                                 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 285                                 for the available
 286                                 controls.</para></listitem>
 287
 288                                 <listitem><para>If D-Bus is used, make
 289                                 your daemon bus-activatable by
 290                                 supplying a D-Bus service activation
 291                                 configuration file. This has multiple
 292                                 advantages: your daemon may be started
 293                                 lazily on-demand; it may be started in
 294                                 parallel to other daemons requiring it
 295                                 -- which maximizes parallelization and
 296                                 boot-up speed; your daemon can be
 297                                 restarted on failure without losing
 298                                 any bus requests, as the bus queues
 299                                 requests for activatable services. See
 300                                 below for details.</para></listitem>
 301
 302                                 <listitem><para>If your daemon
 303                                 provides services to other local
 304                                 processes or remote clients via a
 305                                 socket, it should be made
 306                                 socket-activatable following the
 307                                 scheme pointed out below. Like D-Bus
 308                                 activation, this enables on-demand
 309                                 starting of services as well as it
 310                                 allows improved parallelization of
 311                                 service start-up. Also, for state-less
 312                                 protocols (such as syslog, DNS), a
 313                                 daemon implementing socket-based
 314                                 activation can be restarted without
 315                                 losing a single request. See below for
 316                                 details.</para></listitem>
 317
 318                                 <listitem><para>If applicable, a daemon
 319                                 should notify the init system about
 320                                 startup completion or status updates
 321                                 via the
 322                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 323                                 interface.</para></listitem>
 324
 325                                 <listitem><para>Instead of using the
 326                                 <function>syslog()</function> call to log directly to the
 327                                 system syslog service, a new-style daemon may
 328                                 choose to simply log to standard error via
 329                                 <function>fprintf()</function>, which is then forwarded to
 330                                 syslog by the init system. If log
 331                                 priorities are necessary, these can be
 332                                 encoded by prefixing individual log
 333                                 lines with strings like "&lt;4&gt;"
 334                                 (for log priority 4 "WARNING" in the
 335                                 syslog priority scheme), following a
 336                                 similar style as the Linux kernel's
 337                                 <function>printk()</function> priority system. In fact,
 338                                 using this style of logging also
 339                                 enables the init system to optionally
 340                                 direct all application logging to the
 341                                 kernel log buffer (kmsg), as
 342                                 accessible via
 343                                 <citerefentry><refentrytitle>dmesg</refentrytitle><manvolnum>1</manvolnum></citerefentry>. This
 344                                 kind of logging may be enabled by
 345                                 setting
 346                                 <varname>StandardError=syslog</varname>
 347                                 in the service unit file. For details,
 348                                 see
 349                                 <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 350                                 and
 351                                 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 352
 353                         </orderedlist>
 354
 355                         <para>These recommendations are similar but
 356                         not identical to the <ulink
 357                         url="http://developer.apple.com/mac/library/documentation/MacOSX/Conceptual/BPSystemStartup/Articles/LaunchOnDemandDaemons.html#//apple_ref/doc/uid/TP40001762-104738">Apple
 358                         MacOS X Daemon Requirements</ulink>.</para>
 359                 </refsect2>
 360
 361         </refsect1>
 362         <refsect1>
 363                 <title>Activation</title>
 364
 365                 <para>New-style init systems provide multiple
 366                 additional mechanisms to activate services, as
 367                 detailed below. It is common that services are
 368                 configured to be activated via more than one mechanism
 369                 at the same time. An example for systemd:
 370                 <filename>bluetoothd.service</filename> might get
 371                 activated either when Bluetooth hardware is plugged
 372                 in, or when an application accesses its programming
 373                 interfaces via D-Bus. Or, a print server daemon might
 374                 get activated when traffic arrives at an IPP port, or
 375                 when a printer is plugged in, or when a file is queued
 376                 in the printer spool directory. Even for services that
 377                 are intended to be started on system bootup
 378                 unconditionally, it is a good idea to implement some of
 379                 the various activation schemes outlined below, in
 380                 order to maximize parallelization. If a daemon
 381                 implements a D-Bus service or listening socket,
 382                 implementing the full bus and socket activation scheme
 383                 allows starting of the daemon with its clients in
 384                 parallel (which speeds up boot-up), since all its
 385                 communication channels are established already, and no
 386                 request is lost because client requests will be queued
 387                 by the bus system (in case of D-Bus) or the kernel (in
 388                 case of sockets) until the activation is
 389                 completed.</para>
 390
 391                 <refsect2>
 392                         <title>Activation on Boot</title>
 393
 394                         <para>Old-style daemons are usually activated
 395                         exclusively on boot (and manually by the
 396                         administrator) via SysV init scripts, as
 397                         detailed in the <ulink
 398                         url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
 399                         Linux Standard Base Core
 400                         Specification</ulink>. This method of
 401                         activation is supported ubiquitously on Linux
 402                         init systems, both old-style and new-style
 403                         systems. Among other issues, SysV init scripts
 404                         have the disadvantage of involving shell
 405                         scripts in the boot process. New-style init
 406                         systems generally employ updated versions of
 407                         activation, both during boot-up and during
 408                         runtime and using more minimal service
 409                         description files.</para>
 410
 411                         <para>In systemd, if the developer or
 412                         administrator wants to make sure that a service or
 413                         other unit is activated automatically on boot,
 414                         it is recommended to place a symlink to the
 415                         unit file in the <filename>.wants/</filename>
 416                         directory of either
 417                         <filename>multi-user.target</filename> or
 418                         <filename>graphical.target</filename>, which
 419                         are normally used as boot targets at system
 420                         startup. See
 421                         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 422                         for details about the
 423                         <filename>.wants/</filename> directories, and
 424                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 425                         for details about the two boot targets.</para>
 426
 427                 </refsect2>
 428
 429                 <refsect2>
 430                         <title>Socket-Based Activation</title>
 431
 432                         <para>In order to maximize the possible
 433                         parallelization and robustness and simplify
 434                         configuration and development, it is
 435                         recommended for all new-style daemons that
 436                         communicate via listening sockets to employ
 437                         socket-based activation. In a socket-based
 438                         activation scheme, the creation and binding of
 439                         the listening socket as primary communication
 440                         channel of daemons to local (and sometimes
 441                         remote) clients is moved out of the daemon
 442                         code and into the init system. Based on
 443                         per-daemon configuration, the init system
 444                         installs the sockets and then hands them off
 445                         to the spawned process as soon as the
 446                         respective daemon is to be started.
 447                         Optionally, activation of the service can be
 448                         delayed until the first inbound traffic
 449                         arrives at the socket to implement on-demand
 450                         activation of daemons. However, the primary
 451                         advantage of this scheme is that all providers
 452                         and all consumers of the sockets can be
 453                         started in parallel as soon as all sockets
 454                         are established. In addition to that, daemons
 455                         can be restarted with losing only a minimal
 456                         number of client transactions, or even any
 457                         client request at all (the latter is
 458                         particularly true for state-less protocols,
 459                         such as DNS or syslog), because the socket
 460                         stays bound and accessible during the restart,
 461                         and all requests are queued while the daemon
 462                         cannot process them.</para>
 463
 464                         <para>New-style daemons which support socket
 465                         activation must be able to receive their
 466                         sockets from the init system instead of
 467                         creating and binding them themselves. For
 468                         details about the programming interfaces for
 469                         this scheme provided by systemd, see
 470                         <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 471                         and
 472                         <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>. For
 473                         details about porting existing daemons to
 474                         socket-based activation, see below. With
 475                         minimal effort, it is possible to implement
 476                         socket-based activation in addition to
 477                         traditional internal socket creation in the
 478                         same codebase in order to support both
 479                         new-style and old-style init systems from the
 480                         same daemon binary.</para>
 481
 482                         <para>systemd implements socket-based
 483                         activation via <filename>.socket</filename>
 484                         units, which are described in
 485                         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>. When
 486                         configuring socket units for socket-based
 487                         activation, it is essential that all listening
 488                         sockets are pulled in by the special target
 489                         unit <filename>sockets.target</filename>. It
 490                         is recommended to place a
 491                         <varname>WantedBy=sockets.target</varname>
 492                         directive in the <literal>[Install]</literal>
 493                         section to automatically add such a
 494                         dependency on installation of a socket
 495                         unit. Unless
 496                         <varname>DefaultDependencies=no</varname> is
 497                         set, the necessary ordering dependencies are
 498                         implicitly created for all socket units. For
 499                         more information about
 500                         <filename>sockets.target</filename>, see
 501                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>. It
 502                         is not necessary or recommended to place any
 503                         additional dependencies on socket units (for
 504                         example from
 505                         <filename>multi-user.target</filename> or
 506                         suchlike) when one is installed in
 507                         <filename>sockets.target</filename>.</para>
 508                 </refsect2>
 509
 510                 <refsect2>
 511                         <title>Bus-Based Activation</title>
 512
 513                         <para>When the D-Bus IPC system is used for
 514                         communication with clients, new-style daemons
 515                         should employ bus activation so that they are
 516                         automatically activated when a client
 517                         application accesses their IPC
 518                         interfaces. This is configured in D-Bus
 519                         service files (not to be confused with systemd
 520                         service unit files!). To ensure that D-Bus
 521                         uses systemd to start-up and maintain the
 522                         daemon, use the
 523                         <varname>SystemdService=</varname> directive
 524                         in these service files to configure the
 525                         matching systemd service for a D-Bus
 526                         service. e.g.: For a D-Bus service whose D-Bus
 527                         activation file is named
 528                         <filename>org.freedesktop.RealtimeKit.service</filename>,
 529                         make sure to set
 530                         <varname>SystemdService=rtkit-daemon.service</varname>
 531                         in that file to bind it to the systemd
 532                         service
 533                         <filename>rtkit-daemon.service</filename>. This
 534                         is needed to make sure that the daemon is
 535                         started in a race-free fashion when activated
 536                         via multiple mechanisms simultaneously.</para>
 537                 </refsect2>
 538
 539                 <refsect2>
 540                         <title>Device-Based Activation</title>
 541
 542                         <para>Often, daemons that manage a particular
 543                         type of hardware should be activated only when
 544                         the hardware of the respective kind is plugged
 545                         in or otherwise becomes available. In a
 546                         new-style init system, it is possible to bind
 547                         activation to hardware plug/unplug events. In
 548                         systemd, kernel devices appearing in the
 549                         sysfs/udev device tree can be exposed as units
 550                         if they are tagged with the string
 551                         <literal>systemd</literal>. Like any other
 552                         kind of unit, they may then pull in other units
 553                         when activated (i.e. plugged in) and thus
 554                         implement device-based activation. systemd
 555                         dependencies may be encoded in the udev
 556                         database via the
 557                         <varname>SYSTEMD_WANTS=</varname>
 558                         property. See
 559                         <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 560                         for details. Often, it is nicer to pull in
 561                         services from devices only indirectly via
 562                         dedicated targets. Example: Instead of pulling
 563                         in <filename>bluetoothd.service</filename>
 564                         from all the various bluetooth dongles and
 565                         other hardware available, pull in
 566                         bluetooth.target from them and
 567                         <filename>bluetoothd.service</filename> from
 568                         that target. This provides for nicer
 569                         abstraction and gives administrators the
 570                         option to enable
 571                         <filename>bluetoothd.service</filename> via
 572                         controlling a
 573                         <filename>bluetooth.target.wants/</filename>
 574                         symlink uniformly with a command like
 575                         <command>enable</command> of
 576                         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 577                         instead of manipulating the udev
 578                         ruleset.</para>
 579                 </refsect2>
 580
 581                 <refsect2>
 582                         <title>Path-Based Activation</title>
 583
 584                         <para>Often, runtime of daemons processing
 585                         spool files or directories (such as a printing
 586                         system) can be delayed until these file system
 587                         objects change state, or become
 588                         non-empty. New-style init systems provide a
 589                         way to bind service activation to file system
 590                         changes. systemd implements this scheme via
 591                         path-based activation configured in
 592                         <filename>.path</filename> units, as outlined
 593                         in
 594                         <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 595                 </refsect2>
 596
 597                 <refsect2>
 598                         <title>Timer-Based Activation</title>
 599
 600                         <para>Some daemons that implement clean-up
 601                         jobs that are intended to be executed in
 602                         regular intervals benefit from timer-based
 603                         activation. In systemd, this is implemented
 604                         via <filename>.timer</filename> units, as
 605                         described in
 606                         <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 607                 </refsect2>
 608
 609                 <refsect2>
 610                         <title>Other Forms of Activation</title>
 611
 612                         <para>Other forms of activation have been
 613                         suggested and implemented in some
 614                         systems. However, there are often simpler or
 615                         better alternatives, or they can be put
 616                         together of combinations of the schemes
 617                         above. Example: Sometimes, it appears useful to
 618                         start daemons or <filename>.socket</filename>
 619                         units when a specific IP address is configured
 620                         on a network interface, because network
 621                         sockets shall be bound to the
 622                         address. However, an alternative to implement
 623                         this is by utilizing the Linux IP_FREEBIND
 624                         socket option, as accessible via
 625                         <varname>FreeBind=yes</varname> in systemd
 626                         socket files (see
 627                         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 628                         for details). This option, when enabled,
 629                         allows sockets to be bound to a non-local, not
 630                         configured IP address, and hence allows
 631                         bindings to a particular IP address before it
 632                         actually becomes available, making such an
 633                         explicit dependency to the configured address
 634                         redundant. Another often suggested trigger for
 635                         service activation is low system
 636                         load. However, here too, a more convincing
 637                         approach might be to make proper use of
 638                         features of the operating system, in
 639                         particular, the CPU or IO scheduler of
 640                         Linux. Instead of scheduling jobs from
 641                         userspace based on monitoring the OS
 642                         scheduler, it is advisable to leave the
 643                         scheduling of processes to the OS scheduler
 644                         itself. systemd provides fine-grained access
 645                         to the CPU and IO schedulers. If a process
 646                         executed by the init system shall not
 647                         negatively impact the amount of CPU or IO
 648                         bandwidth available to other processes, it
 649                         should be configured with
 650                         <varname>CPUSchedulingPolicy=idle</varname>
 651                         and/or
 652                         <varname>IOSchedulingClass=idle</varname>. Optionally,
 653                         this may be combined with timer-based
 654                         activation to schedule background jobs during
 655                         runtime and with minimal impact on the system,
 656                         and remove it from the boot phase
 657                         itself.</para>
 658                 </refsect2>
 659
 660         </refsect1>
 661         <refsect1>
 662                 <title>Integration with Systemd</title>
 663
 664                 <refsect2>
 665                         <title>Writing Systemd Unit Files</title>
 666
 667                         <para>When writing systemd unit files, it is
 668                         recommended to consider the following
 669                         suggestions:</para>
 670
 671                         <orderedlist>
 672                                 <listitem><para>If possible, do not use
 673                                 the <varname>Type=forking</varname>
 674                                 setting in service files. But if you
 675                                 do, make sure to set the PID file path
 676                                 using <varname>PIDFile=</varname>. See
 677                                 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 678                                 for details.</para></listitem>
 679
 680                                 <listitem><para>If your daemon
 681                                 registers a D-Bus name on the bus,
 682                                 make sure to use
 683                                 <varname>Type=dbus</varname> in the
 684                                 service file if
 685                                 possible.</para></listitem>
 686
 687                                 <listitem><para>Make sure to set a
 688                                 good human-readable description string
 689                                 with
 690                                 <varname>Description=</varname>.</para></listitem>
 691
 692                                 <listitem><para>Do not disable
 693                                 <varname>DefaultDependencies=</varname>,
 694                                 unless you really know what you do and
 695                                 your unit is involved in early boot or
 696                                 late system shutdown.</para></listitem>
 697
 698                                 <listitem><para>Normally, little if
 699                                 any dependencies should need to
 700                                 be defined explicitly. However, if you
 701                                 do configure explicit dependencies, only refer to
 702                                 unit names listed on
 703                                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 704                                 or names introduced by your own
 705                                 package to keep the unit file
 706                                 operating
 707                                 system-independent.</para></listitem>
 708
 709                                 <listitem><para>Make sure to include
 710                                 an <literal>[Install]</literal>
 711                                 section including installation
 712                                 information for the unit file. See
 713                                 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 714                                 for details. To activate your service
 715                                 on boot, make sure to add a
 716                                 <varname>WantedBy=multi-user.target</varname>
 717                                 or
 718                                 <varname>WantedBy=graphical.target</varname>
 719                                 directive. To activate your socket on
 720                                 boot, make sure to add
 721                                 <varname>WantedBy=sockets.target</varname>. Usually,
 722                                 you also want to make sure that when
 723                                 your service is installed, your socket
 724                                 is installed too, hence add
 725                                 <varname>Also=foo.socket</varname> in
 726                                 your service file
 727                                 <filename>foo.service</filename>, for
 728                                 a hypothetical program
 729                                 <filename>foo</filename>.</para></listitem>
 730
 731                         </orderedlist>
 732                 </refsect2>
 733
 734                 <refsect2>
 735                         <title>Installing Systemd Service Files</title>
 736
 737                         <para>At the build installation time
 738                         (e.g. <command>make install</command> during
 739                         package build), packages are recommended to
 740                         install their systemd unit files in the
 741                         directory returned by <command>pkg-config
 742                         systemd
 743                         --variable=systemdsystemunitdir</command> (for
 744                         system services) or <command>pkg-config
 745                         systemd
 746                         --variable=systemduserunitdir</command>
 747                         (for user services). This will make the
 748                         services available in the system on explicit
 749                         request but not activate them automatically
 750                         during boot. Optionally, during package
 751                         installation (e.g. <command>rpm -i</command>
 752                         by the administrator), symlinks should be
 753                         created in the systemd configuration
 754                         directories via the <command>enable</command>
 755                         command of the
 756                         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 757                         tool to activate them automatically on
 758                         boot.</para>
 759
 760                         <para>Packages using
 761                         <citerefentry><refentrytitle>autoconf</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 762                         are recommended to use a configure script
 763                         excerpt like the following to determine the
 764                         unit installation path during source
 765                         configuration:</para>
 766
 767                         <programlisting>PKG_PROG_PKG_CONFIG
 768 AC_ARG_WITH([systemdsystemunitdir],
 769      [AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],,
 770      [with_systemdsystemunitdir=auto])
 771 AS_IF([test "x$with_systemdsystemunitdir" = "xyes" -o "x$with_systemdsystemunitdir" = "xauto"], [
 772      def_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)
 773
 774      AS_IF([test "x$def_systemdsystemunitdir" = "x"],
 775          [AS_IF([test "x$with_systemdsystemunitdir" = "xyes"],
 776                 [AC_MSG_ERROR([systemd support requested but pkg-config unable to query systemd package])])
 777           with_systemdsystemunitdir=no],
 778          [with_systemdsystemunitdir="$def_systemdsystemunitdir"])])
 779 AS_IF([test "x$with_systemdsystemunitdir" != "xno"],
 780       [AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])])
 781 AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])</programlisting>
 782
 783                         <para>This snippet allows automatic
 784                         installation of the unit files on systemd
 785                         machines, and optionally allows their
 786                         installation even on machines lacking
 787                         systemd. (Modification of this snippet for the
 788                         user unit directory is left as an exercise for the
 789                         reader.)</para>
 790
 791                         <para>Additionally, to ensure that
 792                         <command>make distcheck</command> continues to
 793                         work, it is recommended to add the following
 794                         to the top-level <filename>Makefile.am</filename>
 795                         file in
 796                         <citerefentry><refentrytitle>automake</refentrytitle><manvolnum>1</manvolnum></citerefentry>-based
 797                         projects:</para>
 798
 799                         <programlisting>DISTCHECK_CONFIGURE_FLAGS = \
 800         --with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)</programlisting>
 801
 802                         <para>Finally, unit files should be installed in the system with an automake excerpt like the following:</para>
 803
 804                         <programlisting>if HAVE_SYSTEMD
 805 systemdsystemunit_DATA = \
 806         foobar.socket \
 807         foobar.service
 808 endif</programlisting>
 809
 810                         <para>In the
 811                         <citerefentry><refentrytitle>rpm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 812                         <filename>.spec</filename> file, use snippets
 813                         like the following to enable/disable the
 814                         service during
 815                         installation/deinstallation. This makes use of
 816                         the RPM macros shipped along systemd. Consult
 817                         the packaging guidelines of your distribution
 818                         for details and the equivalent for other
 819                         package managers.</para>
 820
 821                         <para>At the top of the file:</para>
 822
 823                         <programlisting>BuildRequires: systemd
 824 %{?systemd_requires}</programlisting>
 825
 826                         <para>And as scriptlets, further down:</para>
 827
 828                         <programlisting>%post
 829 %systemd_post foobar.service foobar.socket
 830
 831 %preun
 832 %systemd_preun foobar.service foobar.socket
 833
 834 %postun
 835 %systemd_postun</programlisting>
 836
 837                         <para>If the service shall be restarted during
 838                         upgrades, replace the
 839                         <literal>%postun</literal> scriptlet above
 840                         with the following:</para>
 841
 842                         <programlisting>%postun
 843 %systemd_postun_with_restart foobar.service</programlisting>
 844
 845                         <para>Note that
 846                         <literal>%systemd_post</literal> and
 847                         <literal>%systemd_preun</literal> expect the
 848                         names of all units that are installed/removed
 849                         as arguments, separated by
 850                         spaces. <literal>%systemd_postun</literal>
 851                         expects no
 852                         arguments. <literal>%systemd_postun_with_restart</literal>
 853                         expects the units to restart as
 854                         arguments.</para>
 855
 856                         <para>To facilitate upgrades from a package
 857                         version that shipped only SysV init scripts to
 858                         a package version that ships both a SysV init
 859                         script and a native systemd service file, use
 860                         a fragment like the following:</para>
 861
 862                         <programlisting>%triggerun -- foobar &lt; 0.47.11-1
 863 if /sbin/chkconfig --level 5 foobar ; then
 864         /bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&amp;1 || :
 865 fi</programlisting>
 866
 867                         <para>Where 0.47.11-1 is the first package
 868                         version that includes the native unit
 869                         file. This fragment will ensure that the first
 870                         time the unit file is installed, it will be
 871                         enabled if and only if the SysV init script is
 872                         enabled, thus making sure that the enable
 873                         status is not changed. Note that
 874                         <command>chkconfig</command> is a command
 875                         specific to Fedora which can be used to check
 876                         whether a SysV init script is enabled. Other
 877                         operating systems will have to use different
 878                         commands here.</para>
 879                 </refsect2>
 880         </refsect1>
 881
 882         <refsect1>
 883                 <title>Porting Existing Daemons</title>
 884
 885                 <para>Since new-style init systems such as systemd are
 886                 compatible with traditional SysV init systems, it is
 887                 not strictly necessary to port existing daemons to the
 888                 new style. However, doing so offers additional
 889                 functionality to the daemons as well as simplifying
 890                 integration into new-style init systems.</para>
 891
 892                 <para>To port an existing SysV compatible daemon, the
 893                 following steps are recommended:</para>
 894
 895                 <orderedlist>
 896                         <listitem><para>If not already implemented,
 897                         add an optional command line switch to the
 898                         daemon to disable daemonization. This is
 899                         useful not only for using the daemon in
 900                         new-style init systems, but also to ease
 901                         debugging.</para></listitem>
 902
 903                         <listitem><para>If the daemon offers
 904                         interfaces to other software running on the
 905                         local system via local <constant>AF_UNIX</constant> sockets,
 906                         consider implementing socket-based activation
 907                         (see above). Usually, a minimal patch is
 908                         sufficient to implement this: Extend the
 909                         socket creation in the daemon code so that
 910                         <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 911                         is checked for already passed sockets
 912                         first. If sockets are passed (i.e. when
 913                         <function>sd_listen_fds()</function> returns a
 914                         positive value), skip the socket creation step
 915                         and use the passed sockets. Secondly, ensure
 916                         that the file system socket nodes for local
 917                         <constant>AF_UNIX</constant> sockets used in the socket-based
 918                         activation are not removed when the daemon
 919                         shuts down, if sockets have been
 920                         passed. Third, if the daemon normally closes
 921                         all remaining open file descriptors as part of
 922                         its initialization, the sockets passed from
 923                         the init system must be spared. Since
 924                         new-style init systems guarantee that no
 925                         left-over file descriptors are passed to
 926                         executed processes, it might be a good choice
 927                         to simply skip the closing of all remaining
 928                         open file descriptors if sockets are
 929                         passed.</para></listitem>
 930
 931                         <listitem><para>Write and install a systemd
 932                         unit file for the service (and the sockets if
 933                         socket-based activation is used, as well as a
 934                         path unit file, if the daemon processes a
 935                         spool directory), see above for
 936                         details.</para></listitem>
 937
 938                         <listitem><para>If the daemon exposes
 939                         interfaces via D-Bus, write and install a
 940                         D-Bus activation file for the service, see
 941                         above for details.</para></listitem>
 942                 </orderedlist>
 943         </refsect1>
 944
 945         <refsect1>
 946                 <title>See Also</title>
 947                 <para>
 948                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 949                         <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 950                         <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 951                         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 952                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 953                         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 954                 </para>
 955         </refsect1>
 956
 957 </refentry>