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7 This file is part of systemd.
9 Copyright 2010 Lennart Poettering
11 systemd is free software; you can redistribute it and/or modify it
12 under the terms of the GNU Lesser General Public License as published by
13 the Free Software Foundation; either version 2.1 of the License, or
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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>Socket unit configuration</refpurpose>
51 <para><filename><replaceable>socket</replaceable>.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>ExecStopPost=</option> commands are executed
80 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
81 which define the way the processes are
84 <para>For each socket file a matching service file
86 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
87 for details) must exist, describing the service to
88 start on incoming traffic on the socket. Depending on
89 the setting of <option>Accept=</option> (see below),
90 this must either be named like the socket unit, but
91 with the suffix replaced; or it must be a template
92 file named the same way. Example: a socket file
93 <filename>foo.socket</filename> needs a matching
94 service <filename>foo.service</filename> if
95 <option>Accept=false</option> is set. If
96 <option>Accept=true</option> is set a service template
97 file <filename>foo@.service</filename> must exist from
98 which services are instantiated for each incoming
101 <para>Unless <varname>DefaultDependencies=</varname>
102 is set to <option>false</option>, socket units will
103 implicitly have dependencies of type
104 <varname>Requires=</varname> and
105 <varname>After=</varname> on
106 <filename>sysinit.target</filename> as well as
107 dependencies of type <varname>Conflicts=</varname> and
108 <varname>Before=</varname> on
109 <filename>shutdown.target</filename>. These ensure
110 that socket units pull in basic system
111 initialization, and are terminated cleanly prior to
112 system shutdown. Only sockets involved with early
113 boot or late system shutdown should disable this
116 <para>Socket units may be used to implement on-demand
117 starting of services, as well as parallelized starting
120 <para>Note that the daemon software configured for
121 socket activation with socket units needs to be able
122 to accept sockets from systemd, either via systemd's
123 native socket passing interface (see
124 <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
125 for details) or via the traditional
126 <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>-style
127 socket passing (i.e. sockets passed in via STDIN and
128 STDOUT, using <varname>StandardInput=socket</varname>
129 in the service file).</para>
133 <title>Options</title>
135 <para>Socket files must include a [Socket] section,
136 which carries information about the socket or FIFO it
137 supervises. A number of options that may be used in
138 this section are shared with other unit types. These
139 options are documented in
140 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
142 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>. The
143 options specific to the [Socket] section of socket
144 units are the following:</para>
146 <variablelist class='unit-directives'>
148 <term><varname>ListenStream=</varname></term>
149 <term><varname>ListenDatagram=</varname></term>
150 <term><varname>ListenSequentialPacket=</varname></term>
151 <listitem><para>Specifies an address
152 to listen on for a stream
153 (SOCK_STREAM), datagram (SOCK_DGRAM),
155 (SOCK_SEQPACKET) socket, respectively. The address
156 can be written in various formats:</para>
158 <para>If the address starts with a
159 slash (/), it is read as file system
160 socket in the AF_UNIX socket
163 <para>If the address starts with an
164 at symbol (@) it is read as abstract
165 namespace socket in the AF_UNIX
166 family. The @ is replaced with a NUL
167 character before binding. For details
169 <citerefentry><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
171 <para>If the address string is a
172 single number it is read as port
173 number to listen on via
174 IPv6. Depending on the value of
175 <varname>BindIPv6Only=</varname> (see below) this
176 might result in the service being
177 available via both IPv6 and IPv4 (default) or
181 <para>If the address string is a
182 string in the format v.w.x.y:z it is
183 read as IPv4 specifier for listening
184 on an address v.w.x.y on a port
187 <para>If the address string is a
188 string in the format [x]:y it is read
189 as IPv6 address x on a port y. Note
190 that this might make the service
191 available via IPv4, too, depending on
192 the <varname>BindIPv6Only=</varname>
196 <para>Note that SOCK_SEQPACKET
197 (i.e. <varname>ListenSequentialPacket=</varname>)
198 is only available for AF_UNIX
200 (i.e. <varname>ListenStream=</varname>)
201 when used for IP sockets refers to TCP
203 (i.e. <varname>ListenDatagram=</varname>)
206 <para>These options may be specified
207 more than once in which case incoming
208 traffic on any of the sockets will
209 trigger service activation, and all
210 listed sockets will be passed to the
211 service, regardless whether there is
212 incoming traffic on them or not. If
213 the empty string is assigned to any of
214 these options, the list of addresses
215 to listen on is reset, all prior uses
216 of any of these options will have no
219 <para>If an IP address is used here,
220 it is often desirable to listen on it
221 before the interface it is configured
222 on is up and running, and even
223 regardless whether it will be up and
224 running ever at all. To deal with this
225 it is recommended to set the
226 <varname>FreeBind=</varname> option
227 described below.</para></listitem>
231 <term><varname>ListenFIFO=</varname></term>
232 <listitem><para>Specifies a file
233 system FIFO to listen on. This expects
234 an absolute file system path as
235 argument. Behavior otherwise is very
237 <varname>ListenDatagram=</varname>
238 directive above.</para></listitem>
242 <term><varname>ListenSpecial=</varname></term>
243 <listitem><para>Specifies a special
244 file in the file system to listen
245 on. This expects an absolute file
246 system path as argument. Behavior
247 otherwise is very similar to the
248 <varname>ListenFIFO=</varname>
249 directive above. Use this to open
250 character device nodes as well as
252 <filename>/proc</filename> and
253 <filename>/sys</filename>.</para></listitem>
257 <term><varname>ListenNetlink=</varname></term>
258 <listitem><para>Specifies a Netlink
259 family to create a socket for to
260 listen on. This expects a short string
261 referring to the AF_NETLINK family
262 name (such as <varname>audit</varname>
263 or <varname>kobject-uevent</varname>)
264 as argument, optionally suffixed by a
265 whitespace followed by a multicast
266 group integer. Behavior otherwise is
268 <varname>ListenDatagram=</varname>
269 directive above.</para></listitem>
273 <term><varname>ListenMessageQueue=</varname></term>
274 <listitem><para>Specifies a POSIX
275 message queue name to listen on. This
276 expects a valid message queue name
277 (i.e. beginning with /). Behavior
278 otherwise is very similar to the
279 <varname>ListenFIFO=</varname>
280 directive above. On Linux message
281 queue descriptors are actually file
282 descriptors and can be inherited
283 between processes.</para></listitem>
287 <term><varname>BindIPv6Only=</varname></term>
288 <listitem><para>Takes a one of
289 <option>default</option>,
290 <option>both</option> or
291 <option>ipv6-only</option>. Controls
292 the IPV6_V6ONLY socket option (see
293 <citerefentry><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
295 <option>both</option>, IPv6 sockets
296 bound will be accessible via both IPv4
298 <option>ipv6-only</option>, they will
299 be accessible via IPv6 only. If
300 <option>default</option> (which is the
301 default, surprise!) the system wide
302 default setting is used, as controlled
304 <filename>/proc/sys/net/ipv6/bindv6only</filename>,
305 which in turn defaults to the
307 <option>both</option>.</para>
312 <term><varname>Backlog=</varname></term>
313 <listitem><para>Takes an unsigned
314 integer argument. Specifies the number
315 of connections to queue that have not
316 been accepted yet. This setting
317 matters only for stream and sequential
319 <citerefentry><refentrytitle>listen</refentrytitle><manvolnum>2</manvolnum></citerefentry>
320 for details. Defaults to SOMAXCONN
321 (128).</para></listitem>
325 <term><varname>BindToDevice=</varname></term>
326 <listitem><para>Specifies a network
327 interface name to bind this socket
328 to. If set traffic will only be
329 accepted from the specified network
330 interfaces. This controls the
331 SO_BINDTODEVICE socket option (see
332 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
333 for details). If this option is used,
334 an automatic dependency from this
335 socket unit on the network interface
337 (<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
338 is created.</para></listitem>
342 <term><varname>DirectoryMode=</varname></term>
343 <listitem><para>If listening on a file
344 system socket or FIFO, the parent
345 directories are automatically created
346 if needed. This option specifies the
347 file system access mode used when
348 creating these directories. Takes an
350 notation. Defaults to
351 0755.</para></listitem>
355 <term><varname>SocketMode=</varname></term>
356 <listitem><para>If listening on a file
357 system socket or FIFO, this option
358 specifies the file system access mode
359 used when creating the file
360 node. Takes an access mode in octal
361 notation. Defaults to
362 0666.</para></listitem>
366 <term><varname>Accept=</varname></term>
367 <listitem><para>Takes a boolean
368 argument. If true, a service instance
369 is spawned for each incoming
370 connection and only the connection
371 socket is passed to it. If false, all
372 listening sockets themselves are
373 passed to the started service unit,
374 and only one service unit is spawned
375 for all connections (also see
376 above). This value is ignored for
377 datagram sockets and FIFOs where
378 a single service unit unconditionally
379 handles all incoming traffic. Defaults
380 to <option>false</option>. For
381 performance reasons, it is recommended
382 to write new daemons only in a way
384 <option>Accept=false</option>. This
385 option is mostly useful to allow
386 daemons designed for usage with
387 <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
388 to work unmodified with systemd socket
389 activation.</para></listitem>
393 <term><varname>MaxConnections=</varname></term>
394 <listitem><para>The maximum number of
395 connections to simultaneously run
396 services instances for, when
397 <option>Accept=true</option> is
398 set. If more concurrent connections
399 are coming in, they will be refused
400 until at least one existing connection
401 is terminated. This setting has no
402 effect for sockets configured with
403 <option>Accept=false</option> or datagram
405 64.</para></listitem>
409 <term><varname>KeepAlive=</varname></term>
410 <listitem><para>Takes a boolean
411 argument. If true, the TCP/IP stack
412 will send a keep alive message after
413 2h (depending on the configuration of
414 <filename>/proc/sys/net/ipv4/tcp_keepalive_time</filename>)
415 for all TCP streams accepted on this
416 socket. This controls the SO_KEEPALIVE
418 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
420 url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
421 Keepalive HOWTO</ulink> for details.)
423 <option>false</option>.</para></listitem>
427 <term><varname>Priority=</varname></term>
428 <listitem><para>Takes an integer
429 argument controlling the priority for
430 all traffic sent from this
431 socket. This controls the SO_PRIORITY
433 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
434 for details.).</para></listitem>
438 <term><varname>ReceiveBuffer=</varname></term>
439 <term><varname>SendBuffer=</varname></term>
440 <listitem><para>Takes an integer
441 argument controlling the receive
442 or send buffer sizes of this
443 socket, respectively. This controls the SO_RCVBUF
444 and SO_SNDBUF socket options (see
445 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
446 for details.).</para></listitem>
450 <term><varname>IPTOS=</varname></term>
451 <listitem><para>Takes an integer
452 argument controlling the IP
453 Type-Of-Service field for packets
454 generated from this socket. This
455 controls the IP_TOS socket option (see
456 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
457 for details.). Either a numeric string
458 or one of <option>low-delay</option>,
459 <option>throughput</option>,
460 <option>reliability</option> or
461 <option>low-cost</option> may be
462 specified.</para></listitem>
466 <term><varname>IPTTL=</varname></term>
467 <listitem><para>Takes an integer
468 argument controlling the IPv4
469 Time-To-Live/IPv6 Hop-Count field for
470 packets generated from this
471 socket. This sets the
472 IP_TTL/IPV6_UNICAST_HOPS socket
474 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
476 <citerefentry><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
477 for details.)</para></listitem>
481 <term><varname>Mark=</varname></term>
482 <listitem><para>Takes an integer
483 value. Controls the firewall mark of
484 packets generated by this socket. This
485 can be used in the firewall logic to
486 filter packets from this socket. This
487 sets the SO_MARK socket option. See
488 <citerefentry><refentrytitle>iptables</refentrytitle><manvolnum>8</manvolnum></citerefentry>
489 for details.</para></listitem>
493 <term><varname>SmackLabel=</varname></term>
494 <term><varname>SmackLabelIPIn=</varname></term>
495 <term><varname>SmackLabelIPOut=</varname></term>
496 <listitem><para>Takes a string
497 value. Controls the extended
499 <literal>security.SMACK64</literal>,
500 <literal>security.SMACK64IPIN</literal>
502 <literal>security.SMACK64IPOUT</literal>,
503 respectively, i.e. the security label
504 of the FIFO, or the security label for
505 the incoming or outgoing connections
506 of the socket, respectively. See
508 url="https://www.kernel.org/doc/Documentation/security/Smack.txt">Smack.txt</ulink>
509 for details.</para></listitem>
513 <term><varname>PipeSize=</varname></term>
514 <listitem><para>Takes an integer
515 value. Controls the pipe buffer size
516 of FIFOs configured in this socket
518 <citerefentry><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
519 for details.</para></listitem>
523 <term><varname>MessageQueueMaxMessages=</varname>,
524 <varname>MessageQueueMessageSize=</varname></term>
525 <listitem><para>These two settings
526 take integer values and control the
527 mq_maxmsg field or the mq_msgsize field, respectively, when
528 creating the message queue. Note that
529 either none or both of these variables
531 <citerefentry><refentrytitle>mq_setattr</refentrytitle><manvolnum>3</manvolnum></citerefentry>
532 for details.</para></listitem>
536 <term><varname>FreeBind=</varname></term>
537 <listitem><para>Takes a boolean
538 value. Controls whether the socket can
539 be bound to non-local IP
540 addresses. This is useful to configure
541 sockets listening on specific IP
542 addresses before those IP addresses
543 are successfully configured on a
544 network interface. This sets the
545 IP_FREEBIND socket option. For
546 robustness reasons it is recommended
547 to use this option whenever you bind a
548 socket to a specific IP
549 address. Defaults to <option>false</option>.</para></listitem>
553 <term><varname>Transparent=</varname></term>
554 <listitem><para>Takes a boolean
555 value. Controls the IP_TRANSPARENT
556 socket option. Defaults to
557 <option>false</option>.</para></listitem>
561 <term><varname>Broadcast=</varname></term>
562 <listitem><para>Takes a boolean
563 value. This controls the SO_BROADCAST
564 socket option, which allows broadcast
565 datagrams to be sent from this
567 <option>false</option>.</para></listitem>
571 <term><varname>PassCredentials=</varname></term>
572 <listitem><para>Takes a boolean
573 value. This controls the SO_PASSCRED
574 socket option, which allows AF_UNIX sockets to
575 receive the credentials of the sending
576 process in an ancillary message.
578 <option>false</option>.</para></listitem>
582 <term><varname>PassSecurity=</varname></term>
583 <listitem><para>Takes a boolean
584 value. This controls the SO_PASSSEC
585 socket option, which allows AF_UNIX
586 sockets to receive the security
587 context of the sending process in an
588 ancillary message. Defaults to
589 <option>false</option>.</para></listitem>
593 <term><varname>TCPCongestion=</varname></term>
594 <listitem><para>Takes a string
595 value. Controls the TCP congestion
596 algorithm used by this socket. Should
597 be one of "westwood", "veno", "cubic",
598 "lp" or any other available algorithm
599 supported by the IP stack. This
600 setting applies only to stream
601 sockets.</para></listitem>
605 <term><varname>ExecStartPre=</varname></term>
606 <term><varname>ExecStartPost=</varname></term>
607 <listitem><para>Takes one or more
608 command lines, which are executed
609 before or after the listening
610 sockets/FIFOs are created and
611 bound, respectively. The first token of the command
612 line must be an absolute file name,
613 then followed by arguments for the
614 process. Multiple command lines may be
615 specified following the same scheme as
617 <varname>ExecStartPre=</varname> of
618 service unit files.</para></listitem>
622 <term><varname>ExecStopPre=</varname></term>
623 <term><varname>ExecStopPost=</varname></term>
624 <listitem><para>Additional commands
625 that are executed before or after
626 the listening sockets/FIFOs are closed
627 and removed, respectively. Multiple command lines
628 may be specified following the same
630 <varname>ExecStartPre=</varname> of
631 service unit files.</para></listitem>
635 <term><varname>TimeoutSec=</varname></term>
636 <listitem><para>Configures the time to
637 wait for the commands specified in
638 <varname>ExecStartPre=</varname>,
639 <varname>ExecStartPost=</varname>,
640 <varname>ExecStopPre=</varname> and
641 <varname>ExecStopPost=</varname> to
642 finish. If a command does not exit
643 within the configured time, the socket
644 will be considered failed and be shut
645 down again. All commands still running,
646 will be terminated forcibly via
647 SIGTERM, and after another delay of
648 this time with SIGKILL. (See
649 <option>KillMode=</option> in <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>.)
650 Takes a unit-less value in seconds, or
651 a time span value such as "5min
652 20s". Pass 0 to disable the timeout
654 90s.</para></listitem>
658 <term><varname>Service=</varname></term>
659 <listitem><para>Specifies the service
660 unit name to activate on incoming
661 traffic. This defaults to the service
662 that bears the same name as the socket
663 (ignoring the different suffixes). In
664 most cases it should not be necessary
665 to use this option.</para></listitem>
671 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
673 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
674 for more settings.</para>
679 <title>See Also</title>
681 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
682 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
683 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
684 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
685 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
686 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
687 <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>