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7 This file is part of systemd.
9 Copyright 2010 Lennart Poettering
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25 <refentry id="systemd.socket">
27 <title>systemd.socket</title>
28 <productname>systemd</productname>
32 <contrib>Developer</contrib>
33 <firstname>Lennart</firstname>
34 <surname>Poettering</surname>
35 <email>lennart@poettering.net</email>
41 <refentrytitle>systemd.socket</refentrytitle>
42 <manvolnum>5</manvolnum>
46 <refname>systemd.socket</refname>
47 <refpurpose>systemd socket configuration files</refpurpose>
51 <para><filename>systemd.socket</filename></para>
55 <title>Description</title>
57 <para>A unit configuration file whose name ends in
58 <filename>.socket</filename> encodes information about
59 an IPC or network socket or a file system FIFO
60 controlled and supervised by systemd, for socket-based
63 <para>This man page lists the configuration options
64 specific to this unit type. See
65 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
66 for the common options of all unit configuration
67 files. The common configuration items are configured
68 in the generic [Unit] and [Install] sections. The
69 socket specific configuration options are configured
70 in the [Socket] section.</para>
72 <para>Additional options are listed in
73 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
74 which define the execution environment the
75 <option>ExecStartPre=</option>,
76 <option>ExecStartPost=</option>,
77 <option>ExecStopPre=</option> and
78 <option>ExecStoptPost=</option> commands are executed
81 <para>For each socket file a matching service file
83 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
84 for details) must exist, describing the service to
85 start on incoming traffic on the socket. Depending on
86 the setting of <option>Accept=</option> (see below),
87 this must either be named like the socket unit, but
88 with the suffix replaced; or it must be a template
89 file named the same way. Example: a socket file
90 <filename>foo.socket</filename> needs a matching
91 service <filename>foo.service</filename> if
92 <option>Accept=false</option> is set. If
93 <option>Accept=true</option> is set a service template
94 file <filename>foo@.service</filename> must exist from
95 which services are instantiated for each incoming
98 <para>Unless <varname>DefaultDependencies=</varname>
99 is set to <option>false</option>, socket units will
100 implicitly have dependencies of type
101 <varname>Requires=</varname> and
102 <varname>After=</varname> on
103 <filename>sysinit.target</filename> as well as
104 dependencies of type <varname>Conflicts=</varname> and
105 <varname>Before=</varname> on
106 <filename>shutdown.target</filename>. These ensure
107 that socket units pull in basic system
108 initialization, and are terminated cleanly prior to
109 system shutdown. Only sockets involved with early
110 boot or late system shutdown should disable this
113 <para>Socket units may be used to implement on-demand
114 starting of services, as well as parallelized starting
117 <para>Note that the daemon software configured for
118 socket activation with socket units needs to be able
119 to accept sockets from systemd, either via systemd's
120 native socket passing interface (see
121 <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
122 for details) or via the traditional
123 <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>-style
124 socket passing (i.e. sockets passed in via STDIN and
125 STDOUT, using <varname>StandardInput=socket</varname>
126 in the service file).</para>
130 <title>Options</title>
132 <para>Socket files must include a [Socket] section,
133 which carries information about the socket or FIFO it
134 supervises. A number of options that may be used in
135 this section are shared with other unit types. These
136 options are documented in
137 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>. The
138 options specific to the [Socket] section of socket
139 units are the following:</para>
143 <term><varname>ListenStream=</varname></term>
144 <term><varname>ListenDatagram=</varname></term>
145 <term><varname>ListenSequentialPacket=</varname></term>
146 <listitem><para>Specifies an address
147 to listen on for a stream
148 (SOCK_STREAM), datagram (SOCK_DGRAM)
149 resp. sequential packet
150 (SOCK_SEQPACKET) socket. The address
151 can be written in various formats:</para>
153 <para>If the address starts with a
154 slash (/), it is read as file system
155 socket in the AF_UNIX socket
158 <para>If the address starts with an
159 at symbol (@) it is read as abstract
160 namespace socket in the AF_UNIX
161 family. The @ is replaced with a NUL
162 character before binding. For details
164 <citerefentry><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
166 <para>If the address string is a
167 single number it is read as port
168 number to listen on for both IPv4 and
171 <para>If the address string is a
172 string in the format v.w.x.y:z it is
173 read as IPv4 specifier for listening
174 on an address v.w.x.y on a port
177 <para>If the address string is a
178 string in the format [x]:y it is read
179 as IPv6 address x on a port y.</para>
181 <para>Note that SOCK_SEQPACKET
182 (i.e. <varname>ListenSequentialPacket=</varname>)
183 is only available for AF_UNIX
185 (i.e. <varname>ListenStream=</varname>)
186 when used for IP sockets refers to TCP
188 (i.e. <varname>ListenDatagram=</varname>)
191 <para>These options may be specified
192 more than once in which case incoming
193 traffic on any of the sockets will trigger
194 service activation, and all listed
195 sockets will be passed to the service,
196 regardless whether there is incoming
197 traffic on them or not.</para>
199 <para>If an IP address is used here, it
200 is often desirable to listen on it
201 before the interface it is configured
202 on is up and running, and even
203 regardless whether it will be up and
204 running ever at all. To deal with this it is
205 recommended to set the
206 <varname>FreeBind=</varname> option
207 described below.</para></listitem>
211 <term><varname>ListenFIFO=</varname></term>
212 <listitem><para>Specifies a file
213 system FIFO to listen on. This expects
214 an absolute file system path as
215 argument. Behaviour otherwise is very
217 <varname>ListenDatagram=</varname>
218 directive above.</para></listitem>
222 <term><varname>ListenSpecial=</varname></term>
223 <listitem><para>Specifies a special
224 file in the file system to listen
225 on. This expects an absolute file
226 system path as argument. Behaviour
227 otherwise is very similar to the
228 <varname>ListenFIFO=</varname>
229 directive above. Use this to open
230 character device nodes as well as
232 <filename>/proc</filename> and
233 <filename>/sys</filename>.</para></listitem>
237 <term><varname>ListenNetlink=</varname></term>
238 <listitem><para>Specifies a Netlink
239 family to create a socket for to
240 listen on. This expects a short string
241 referring to the AF_NETLINK family
242 name (such as <varname>audit</varname>
243 or <varname>kobject-uevent</varname>)
244 as argument, optionally suffixed by a
245 whitespace followed by a multicast
246 group integer. Behaviour otherwise is
248 <varname>ListenDatagram=</varname>
249 directive above.</para></listitem>
253 <term><varname>ListenMessageQueue=</varname></term>
254 <listitem><para>Specifies a POSIX
255 message queue name to listen on. This
256 expects a valid message queue name
257 (i.e. beginning with /). Behaviour
258 otherwise is very similar to the
259 <varname>ListenFIFO=</varname>
260 directive above. On Linux message
261 queue descriptors are actually file
262 descriptors and can be inherited
263 between processes.</para></listitem>
267 <term><varname>BindIPv6Only=</varname></term>
268 <listitem><para>Takes a one of
269 <option>default</option>,
270 <option>both</option> or
271 <option>ipv6-only</option>. Controls
272 the IPV6_V6ONLY socket option (see
273 <citerefentry><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
275 <option>both</option>, IPv6 sockets
276 bound will be accessible via both IPv4
278 <option>ipv6-only</option>, they will
279 be accessible via IPv6 only. If
280 <option>default</option> (which is the
281 default, surprise!) the system wide
282 default setting is used, as controlled
284 <filename>/proc/sys/net/ipv6/bindv6only</filename>.</para>
289 <term><varname>Backlog=</varname></term>
290 <listitem><para>Takes an unsigned
291 integer argument. Specifies the number
292 of connections to queue that have not
293 been accepted yet. This setting
294 matters only for stream and sequential
296 <citerefentry><refentrytitle>listen</refentrytitle><manvolnum>2</manvolnum></citerefentry>
297 for details. Defaults to SOMAXCONN
298 (128).</para></listitem>
302 <term><varname>BindToDevice=</varname></term>
303 <listitem><para>Specifies a network
304 interface name to bind this socket
305 to. If set traffic will only be
306 accepted from the specified network
307 interfaces. This controls the
308 SO_BINDTODEVICE socket option (see
309 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
310 for details). If this option is used,
311 an automatic dependency from this
312 socket unit on the network interface
314 (<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
315 is created.</para></listitem>
319 <term><varname>DirectoryMode=</varname></term>
320 <listitem><para>If listening on a file
321 system socket of FIFO, the parent
322 directories are automatically created
323 if needed. This option specifies the
324 file system access mode used when
325 creating these directories. Takes an
327 notation. Defaults to
328 0755.</para></listitem>
332 <term><varname>SocketMode=</varname></term>
333 <listitem><para>If listening on a file
334 system socket of FIFO, this option
335 specifies the file system access mode
336 used when creating the file
337 node. Takes an access mode in octal
338 notation. Defaults to
339 0666.</para></listitem>
343 <term><varname>Accept=</varname></term>
344 <listitem><para>Takes a boolean
345 argument. If true, a service instance
346 is spawned for each incoming
347 connection and only the connection
348 socket is passed to it. If false, all
349 listening sockets themselves are
350 passed to the started service unit,
351 and only one service unit is spawned
352 for all connections (also see
353 above). This value is ignored for
354 datagram sockets and FIFOs where
355 a single service unit unconditionally
356 handles all incoming traffic. Defaults
357 to <option>false</option>. For
358 performance reasons, it is recommended
359 to write new daemons only in a way
361 <option>Accept=false</option>. This
362 option is mostly useful to allow
363 daemons designed for usage with
364 <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
365 to work unmodified with systemd socket
366 activation.</para></listitem>
370 <term><varname>MaxConnections=</varname></term>
371 <listitem><para>The maximum number of
372 connections to simultaneously run
373 services instances for, when
374 <option>Accept=true</option> is
375 set. If more concurrent connections
376 are coming in, they will be refused
377 until at least one existing connection
378 is terminated. This setting has no
379 effect for sockets configured with
380 <option>Accept=no</option> or datagram
382 64.</para></listitem>
386 <term><varname>KeepAlive=</varname></term>
387 <listitem><para>Takes a boolean
388 argument. If true, the TCP/IP stack
389 will send a keep alive message after
390 2h (depending on the configuration of
391 <filename>/proc/sys/net/ipv4/tcp_keepalive_time</filename>)
392 for all TCP streams accepted on this
393 socket. This controls the SO_KEEPALIVE
395 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
397 url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
398 Keepalive HOWTO</ulink> for details.)
400 <option>false</option>.</para></listitem>
404 <term><varname>Priority=</varname></term>
405 <listitem><para>Takes an integer
406 argument controlling the priority for
407 all traffic sent from this
408 socket. This controls the SO_PRIORITY
410 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
411 for details.).</para></listitem>
415 <term><varname>ReceiveBuffer=</varname></term>
416 <term><varname>SendBuffer=</varname></term>
417 <listitem><para>Takes an integer
418 argument controlling the receive
419 resp. send buffer sizes of this
420 socket. This controls the SO_RCVBUF
421 resp. SO_SNDBUF socket options (see
422 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
423 for details.).</para></listitem>
427 <term><varname>IPTOS=</varname></term>
428 <listitem><para>Takes an integer
429 argument controlling the IP
430 Type-Of-Service field for packets
431 generated from this socket. This
432 controls the IP_TOS socket option (see
433 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
434 for details.). Either a numeric string
435 or one of <option>low-delay</option>,
436 <option>throughput</option>,
437 <option>reliability</option> or
438 <option>low-cost</option> may be
439 specified.</para></listitem>
443 <term><varname>IPTTL=</varname></term>
444 <listitem><para>Takes an integer
445 argument controlling the IPv4
446 Time-To-Live/IPv6 Hop-Count field for
447 packets generated from this
448 socket. This sets the
449 IP_TTL/IPV6_UNICAST_HOPS socket
451 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
453 <citerefentry><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
454 for details.)</para></listitem>
458 <term><varname>Mark=</varname></term>
459 <listitem><para>Takes an integer
460 value. Controls the firewall mark of
461 packets generated by this socket. This
462 can be used in the firewall logic to
463 filter packets from this socket. This
464 sets the SO_MARK socket option. See
465 <citerefentry><refentrytitle>iptables</refentrytitle><manvolnum>8</manvolnum></citerefentry>
466 for details.</para></listitem>
470 <term><varname>PipeSize=</varname></term>
471 <listitem><para>Takes an integer
472 value. Controls the pipe buffer size
473 of FIFOs configured in this socket
475 <citerefentry><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
476 for details.</para></listitem>
480 <term><varname>MessageQueueMaxMessages=</varname>,
481 <varname>MessageQueueMessageSize=</varname></term>
482 <listitem><para>These two settings
483 take integer values and control the
484 mq_maxmsg resp. mq_msgsize field when
485 creating the message queue. Note that
486 either none or both of these variables
488 <citerefentry><refentrytitle>mq_setattr</refentrytitle><manvolnum>3</manvolnum></citerefentry>
489 for details.</para></listitem>
493 <term><varname>FreeBind=</varname></term>
494 <listitem><para>Takes a boolean
495 value. Controls whether the socket can
496 be bound to non-local IP
497 addresses. This is useful to configure
498 sockets listening on specific IP
499 addresses before those IP addresses
500 are successfully configured on a
501 network interface. This sets the
502 IP_FREEBIND socket option. For
503 robustness reasons it is recommended
504 to use this option whenever you bind a
505 socket to a specific IP
506 address. Defaults to <option>false</option>.</para></listitem>
510 <term><varname>Transparent=</varname></term>
511 <listitem><para>Takes a boolean
512 value. Controls the IP_TRANSPARENT
513 socket option. Defaults to
514 <option>false</option>.</para></listitem>
518 <term><varname>Broadcast=</varname></term>
519 <listitem><para>Takes a boolean
520 value. This controls the SO_BROADCAST
521 socket option, which allows broadcast
522 datagrams to be sent from this
524 <option>false</option>.</para></listitem>
528 <term><varname>PassCredentials=</varname></term>
529 <listitem><para>Takes a boolean
530 value. This controls the SO_PASSCRED
531 socket option, which allows UNIX sockets to
532 receive the credentials of the sending
533 process in an ancillary message.
535 <option>false</option>.</para></listitem>
539 <term><varname>TCPCongestion=</varname></term>
540 <listitem><para>Takes a string
541 value. Controls the TCP congestion
542 algorithm used by this socket. Should
543 be one of "westwood", "veno", "cubic",
544 "lp" or any other available algorithm
545 supported by the IP stack. This
546 setting applies only to stream
547 sockets.</para></listitem>
551 <term><varname>ExecStartPre=</varname></term>
552 <term><varname>ExecStartPost=</varname></term>
553 <listitem><para>Takes one or more
554 command lines, which are executed
555 before (resp. after) the listening
556 sockets/FIFOs are created and
557 bound. The first token of the command
558 line must be an absolute file name,
559 then followed by arguments for the
560 process. Multiple command lines may be
561 specified following the same scheme as
563 <varname>ExecStartPre=</varname> of
564 service unit files.</para></listitem>
568 <term><varname>ExecStopPre=</varname></term>
569 <term><varname>ExecStopPost=</varname></term>
570 <listitem><para>Additional commands
571 that are executed before (resp. after)
572 the listening sockets/FIFOs are closed
573 and removed. Multiple command lines
574 may be specified following the same
576 <varname>ExecStartPre=</varname> of
577 service unit files.</para></listitem>
581 <term><varname>TimeoutSec=</varname></term>
582 <listitem><para>Configures the time to
583 wait for the commands specified in
584 <varname>ExecStartPre=</varname>,
585 <varname>ExecStartPost=</varname>,
586 <varname>ExecStopPre=</varname> and
587 <varname>ExecStopPost=</varname> to
588 finish. If a command does not exit
589 within the configured time, the socket
590 will be considered failed and be shut
591 down again. All commands still running,
592 will be terminated forcibly via
593 SIGTERM, and after another delay of
594 this time with SIGKILL. (See
595 <option>KillMode=</option> below.)
596 Takes a unit-less value in seconds, or
597 a time span value such as "5min
598 20s". Pass 0 to disable the timeout
600 90s.</para></listitem>
604 <term><varname>KillMode=</varname></term>
605 <listitem><para>Specifies how
606 processes of this socket unit shall be
608 <option>control-group</option>,
609 <option>process</option>,
610 <option>none</option>.</para>
612 <para>This option is mostly equivalent
613 to the <option>KillMode=</option>
614 option of service files. See
615 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
616 for details.</para></listitem>
620 <term><varname>KillSignal=</varname></term>
621 <listitem><para>Specifies which signal
622 to use when killing a process of this
623 socket. Defaults to SIGTERM.
628 <term><varname>SendSIGKILL=</varname></term>
629 <listitem><para>Specifies whether to
630 send SIGKILL to remaining processes
631 after a timeout, if the normal
632 shutdown procedure left processes of
633 the socket around. Takes a boolean
634 value. Defaults to "yes".
639 <term><varname>Service=</varname></term>
640 <listitem><para>Specifies the service
641 unit name to activate on incoming
642 traffic. This defaults to the service
643 that bears the same name as the socket
644 (ignoring the different suffixes). In
645 most cases it should not be necessary
646 to use this option.</para></listitem>
653 <title>See Also</title>
655 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
656 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
657 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
658 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
659 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>