1SYSTEMD.SOCKET(5) systemd.socket SYSTEMD.SOCKET(5)
2
6 systemd.socket - Socket unit configuration
7
9 socket.socket
10
12 A unit configuration file whose name ends in ".socket" encodes
13 information about an IPC or network socket or a file system FIFO
14 controlled and supervised by systemd, for socket-based activation.
15 This man page lists the configuration options specific to this unit
17 type. See systemd.unit(5) for the common options of all unit
18 configuration files. The common configuration items are configured in
19 the generic [Unit] and [Install] sections. The socket specific
20 configuration options are configured in the [Socket] section.
21 Additional options are listed in systemd.exec(5), which define the
23 execution environment the ExecStartPre=, ExecStartPost=, ExecStopPre=
24 and ExecStopPost= commands are executed in, and in systemd.kill(5),
25 which define the way the processes are terminated, and in
26 systemd.resource-control(5), which configure resource control settings
27 for the processes of the socket.
28 For each socket unit, a matching service unit must exist, describing
30 the service to start on incoming traffic on the socket (see
31 systemd.service(5) for more information about .service units). The name
32 of the .service unit is by default the same as the name of the .socket
33 unit, but can be altered with the Service= option described below.
34 Depending on the setting of the Accept= option described below, this
35 .service unit must either be named like the .socket unit, but with the
36 suffix replaced, unless overridden with Service=; or it must be a
37 template unit named the same way. Example: a socket file foo.socket
38 needs a matching service foo.service if Accept=no is set. If Accept=yes
39 is set, a service template foo@.service must exist from which services
40 are instantiated for each incoming connection.
41 No implicit WantedBy= or RequiredBy= dependency from the socket to the
43 service is added. This means that the service may be started without
44 the socket, in which case it must be able to open sockets by itself. To
45 prevent this, an explicit Requires= dependency may be added.
46 Socket units may be used to implement on-demand starting of services,
48 as well as parallelized starting of services. See the blog stories
49 linked at the end for an introduction.
50 Note that the daemon software configured for socket activation with
52 socket units needs to be able to accept sockets from systemd, either
53 via systemd's native socket passing interface (see sd_listen_fds(3) for
54 details about the precise protocol used and the order in which the file
55 descriptors are passed) or via traditional inetd(8)-style socket
56 passing (i.e. sockets passed in via standard input and output, using
57 StandardInput=socket in the service file).
58 All network sockets allocated through .socket units are allocated in
60 the host's network namespace (see network_namespaces(7)). This does not
61 mean however that the service activated by a configured socket unit has
62 to be part of the host's network namespace as well. It is supported and
63 even good practice to run services in their own network namespace (for
64 example through PrivateNetwork=, see systemd.exec(5)), receiving only
65 the sockets configured through socket-activation from the host's
66 namespace. In such a set-up communication within the host's network
67 namespace is only permitted through the activation sockets passed in
68 while all sockets allocated from the service code itself will be
69 associated with the service's own namespace, and thus possibly subject
70 to a restrictive configuration.
71
73 Implicit Dependencies
74 The following dependencies are implicitly added:
75 • Socket units automatically gain a Before= dependency on the service
77 units they activate.
78 • Socket units referring to file system paths (such as AF_UNIX
80 sockets or FIFOs) implicitly gain Requires= and After= dependencies
81 on all mount units necessary to access those paths.
82 • Socket units using the BindToDevice= setting automatically gain a
84 BindsTo= and After= dependency on the device unit encapsulating the
85 specified network interface.
86 Additional implicit dependencies may be added as result of execution
88 and resource control parameters as documented in systemd.exec(5) and
89 systemd.resource-control(5).
90 Default Dependencies
92 The following dependencies are added unless DefaultDependencies=no is
93 set:
94 • Socket units automatically gain a Before= dependency on
96 sockets.target.
97 • Socket units automatically gain a pair of After= and Requires=
99 dependency on sysinit.target, and a pair of Before= and Conflicts=
100 dependencies on shutdown.target. These dependencies ensure that the
101 socket unit is started before normal services at boot, and is
102 stopped on shutdown. Only sockets involved with early boot or late
103 system shutdown should disable DefaultDependencies= option.
104
106 Socket unit files may include [Unit] and [Install] sections, which are
107 described in systemd.unit(5).
108 Socket unit files must include a [Socket] section, which carries
110 information about the socket or FIFO it supervises. A number of options
111 that may be used in this section are shared with other unit types.
112 These options are documented in systemd.exec(5) and systemd.kill(5).
113 The options specific to the [Socket] section of socket units are the
114 following:
115 ListenStream=, ListenDatagram=, ListenSequentialPacket=
117 Specifies an address to listen on for a stream (SOCK_STREAM),
118 datagram (SOCK_DGRAM), or sequential packet (SOCK_SEQPACKET)
119 socket, respectively. The address can be written in various
120 formats:
121 If the address starts with a slash ("/"), it is read as file system
123 socket in the AF_UNIX socket family.
124 If the address starts with an at symbol ("@"), it is read as
126 abstract namespace socket in the AF_UNIX family. The "@" is
127 replaced with a NUL character before binding. For details, see
128 unix(7).
129 If the address string is a single number, it is read as port number
131 to listen on via IPv6. Depending on the value of BindIPv6Only= (see
132 below) this might result in the service being available via both
133 IPv6 and IPv4 (default) or just via IPv6.
134 If the address string is a string in the format "v.w.x.y:z", it is
136 interpreted as IPv4 address v.w.x.y and port z.
137 If the address string is a string in the format "[x]:y", it is
139 interpreted as IPv6 address x and port y. An optional interface
140 scope (interface name or number) may be specified after a "%"
141 symbol: "[x]:y%dev". Interface scopes are only useful with
142 link-local addresses, because the kernel ignores them in other
143 cases. Note that if an address is specified as IPv6, it might still
144 make the service available via IPv4 too, depending on the
145 BindIPv6Only= setting (see below).
146 If the address string is a string in the format "vsock:x:y", it is
148 read as CID x on a port y address in the AF_VSOCK family. The CID
149 is a unique 32-bit integer identifier in AF_VSOCK analogous to an
150 IP address. Specifying the CID is optional, and may be set to the
151 empty string.
152 Note that SOCK_SEQPACKET (i.e. ListenSequentialPacket=) is only
154 available for AF_UNIX sockets. SOCK_STREAM (i.e. ListenStream=)
155 when used for IP sockets refers to TCP sockets, SOCK_DGRAM (i.e.
156 ListenDatagram=) to UDP.
157 These options may be specified more than once, in which case
159 incoming traffic on any of the sockets will trigger service
160 activation, and all listed sockets will be passed to the service,
161 regardless of whether there is incoming traffic on them or not. If
162 the empty string is assigned to any of these options, the list of
163 addresses to listen on is reset, all prior uses of any of these
164 options will have no effect.
165 It is also possible to have more than one socket unit for the same
167 service when using Service=, and the service will receive all the
168 sockets configured in all the socket units. Sockets configured in
169 one unit are passed in the order of configuration, but no ordering
170 between socket units is specified.
171 If an IP address is used here, it is often desirable to listen on
173 it before the interface it is configured on is up and running, and
174 even regardless of whether it will be up and running at any point.
175 To deal with this, it is recommended to set the FreeBind= option
176 described below.
177 ListenFIFO=
179 Specifies a file system FIFO (see fifo(7) for details) to listen
180 on. This expects an absolute file system path as argument. Behavior
181 otherwise is very similar to the ListenDatagram= directive above.
182 ListenSpecial=
184 Specifies a special file in the file system to listen on. This
185 expects an absolute file system path as argument. Behavior
186 otherwise is very similar to the ListenFIFO= directive above. Use
187 this to open character device nodes as well as special files in
188 /proc/ and /sys/.
189 ListenNetlink=
191 Specifies a Netlink family to create a socket for to listen on.
192 This expects a short string referring to the AF_NETLINK family name
193 (such as audit or kobject-uevent) as argument, optionally suffixed
194 by a whitespace followed by a multicast group integer. Behavior
195 otherwise is very similar to the ListenDatagram= directive above.
196 ListenMessageQueue=
198 Specifies a POSIX message queue name to listen on (see
199 mq_overview(7) for details). This expects a valid message queue
200 name (i.e. beginning with "/"). Behavior otherwise is very similar
201 to the ListenFIFO= directive above. On Linux message queue
202 descriptors are actually file descriptors and can be inherited
203 between processes.
204 ListenUSBFunction=
206 Specifies a USB FunctionFS[1] endpoints location to listen on, for
207 implementation of USB gadget functions. This expects an absolute
208 file system path of a FunctionFS mount point as the argument.
209 Behavior otherwise is very similar to the ListenFIFO= directive
210 above. Use this to open the FunctionFS endpoint ep0. When using
211 this option, the activated service has to have the
212 USBFunctionDescriptors= and USBFunctionStrings= options set.
213 SocketProtocol=
215 Takes one of udplite or sctp. The socket will use the UDP-Lite
216 (IPPROTO_UDPLITE) or SCTP (IPPROTO_SCTP) protocol, respectively.
217 BindIPv6Only=
219 Takes one of default, both or ipv6-only. Controls the IPV6_V6ONLY
220 socket option (see ipv6(7) for details). If both, IPv6 sockets
221 bound will be accessible via both IPv4 and IPv6. If ipv6-only, they
222 will be accessible via IPv6 only. If default (which is the default,
223 surprise!), the system wide default setting is used, as controlled
224 by /proc/sys/net/ipv6/bindv6only, which in turn defaults to the
225 equivalent of both.
226 Backlog=
228 Takes an unsigned integer argument. Specifies the number of
229 connections to queue that have not been accepted yet. This setting
230 matters only for stream and sequential packet sockets. See
231 listen(2) for details. Defaults to SOMAXCONN (128).
232 BindToDevice=
234 Specifies a network interface name to bind this socket to. If set,
235 traffic will only be accepted from the specified network
236 interfaces. This controls the SO_BINDTODEVICE socket option (see
237 socket(7) for details). If this option is used, an implicit
238 dependency from this socket unit on the network interface device
239 unit is created (see systemd.device(5)). Note that setting this
240 parameter might result in additional dependencies to be added to
241 the unit (see above).
242 SocketUser=, SocketGroup=
244 Takes a UNIX user/group name. When specified, all AF_UNIX sockets
245 and FIFO nodes in the file system are owned by the specified user
246 and group. If unset (the default), the nodes are owned by the root
247 user/group (if run in system context) or the invoking user/group
248 (if run in user context). If only a user is specified but no group,
249 then the group is derived from the user's default group.
250 SocketMode=
252 If listening on a file system socket or FIFO, this option specifies
253 the file system access mode used when creating the file node. Takes
254 an access mode in octal notation. Defaults to 0666.
255 DirectoryMode=
257 If listening on a file system socket or FIFO, the parent
258 directories are automatically created if needed. This option
259 specifies the file system access mode used when creating these
260 directories. Takes an access mode in octal notation. Defaults to
261 0755.
262 Accept=
264 Takes a boolean argument. If yes, a service instance is spawned for
265 each incoming connection and only the connection socket is passed
266 to it. If no, all listening sockets themselves are passed to the
267 started service unit, and only one service unit is spawned for all
268 connections (also see above). This value is ignored for datagram
269 sockets and FIFOs where a single service unit unconditionally
270 handles all incoming traffic. Defaults to no. For performance
271 reasons, it is recommended to write new daemons only in a way that
272 is suitable for Accept=no. A daemon listening on an AF_UNIX socket
273 may, but does not need to, call close(2) on the received socket
274 before exiting. However, it must not unlink the socket from a file
275 system. It should not invoke shutdown(2) on sockets it got with
276 Accept=no, but it may do so for sockets it got with Accept=yes set.
277 Setting Accept=yes is mostly useful to allow daemons designed for
278 usage with inetd(8) to work unmodified with systemd socket
279 activation.
280 For IPv4 and IPv6 connections, the REMOTE_ADDR environment variable
282 will contain the remote IP address, and REMOTE_PORT will contain
283 the remote port. This is the same as the format used by CGI. For
284 SOCK_RAW, the port is the IP protocol.
285 It is recommended to set CollectMode=inactive-or-failed for service
287 instances activated via Accept=yes, to ensure that failed
288 connection services are cleaned up and released from memory, and do
289 not accumulate.
290 Writable=
292 Takes a boolean argument. May only be used in conjunction with
293 ListenSpecial=. If true, the specified special file is opened in
294 read-write mode, if false, in read-only mode. Defaults to false.
295 FlushPending=
297 Takes a boolean argument. May only be used when Accept=no. If yes,
298 the socket's buffers are cleared after the triggered service
299 exited. This causes any pending data to be flushed and any pending
300 incoming connections to be rejected. If no, the socket's buffers
301 won't be cleared, permitting the service to handle any pending
302 connections after restart, which is the usually expected behaviour.
303 Defaults to no.
304 MaxConnections=
306 The maximum number of connections to simultaneously run services
307 instances for, when Accept=yes is set. If more concurrent
308 connections are coming in, they will be refused until at least one
309 existing connection is terminated. This setting has no effect on
310 sockets configured with Accept=no or datagram sockets. Defaults to
311 64.
312 MaxConnectionsPerSource=
314 The maximum number of connections for a service per source IP
315 address. This is very similar to the MaxConnections= directive
316 above. Disabled by default.
317 KeepAlive=
319 Takes a boolean argument. If true, the TCP/IP stack will send a
320 keep alive message after 2h (depending on the configuration of
321 /proc/sys/net/ipv4/tcp_keepalive_time) for all TCP streams accepted
322 on this socket. This controls the SO_KEEPALIVE socket option (see
323 socket(7) and the TCP Keepalive HOWTO[2] for details.) Defaults to
324 false.
325 KeepAliveTimeSec=
327 Takes time (in seconds) as argument. The connection needs to remain
328 idle before TCP starts sending keepalive probes. This controls the
329 TCP_KEEPIDLE socket option (see socket(7) and the TCP Keepalive
330 HOWTO[2] for details.) Defaults value is 7200 seconds (2 hours).
331 KeepAliveIntervalSec=
333 Takes time (in seconds) as argument between individual keepalive
334 probes, if the socket option SO_KEEPALIVE has been set on this
335 socket. This controls the TCP_KEEPINTVL socket option (see
336 socket(7) and the TCP Keepalive HOWTO[2] for details.) Defaults
337 value is 75 seconds.
338 KeepAliveProbes=
340 Takes an integer as argument. It is the number of unacknowledged
341 probes to send before considering the connection dead and notifying
342 the application layer. This controls the TCP_KEEPCNT socket option
343 (see socket(7) and the TCP Keepalive HOWTO[2] for details.)
344 Defaults value is 9.
345 NoDelay=
347 Takes a boolean argument. TCP Nagle's algorithm works by combining
348 a number of small outgoing messages, and sending them all at once.
349 This controls the TCP_NODELAY socket option (see tcp(7)). Defaults
350 to false.
351 Priority=
353 Takes an integer argument controlling the priority for all traffic
354 sent from this socket. This controls the SO_PRIORITY socket option
355 (see socket(7) for details.).
356 DeferAcceptSec=
358 Takes time (in seconds) as argument. If set, the listening process
359 will be awakened only when data arrives on the socket, and not
360 immediately when connection is established. When this option is
361 set, the TCP_DEFER_ACCEPT socket option will be used (see tcp(7)),
362 and the kernel will ignore initial ACK packets without any data.
363 The argument specifies the approximate amount of time the kernel
364 should wait for incoming data before falling back to the normal
365 behavior of honoring empty ACK packets. This option is beneficial
366 for protocols where the client sends the data first (e.g. HTTP, in
367 contrast to SMTP), because the server process will not be woken up
368 unnecessarily before it can take any action.
369 If the client also uses the TCP_DEFER_ACCEPT option, the latency of
371 the initial connection may be reduced, because the kernel will send
372 data in the final packet establishing the connection (the third
373 packet in the "three-way handshake").
374 Disabled by default.
376 ReceiveBuffer=, SendBuffer=
378 Takes an integer argument controlling the receive or send buffer
379 sizes of this socket, respectively. This controls the SO_RCVBUF and
380 SO_SNDBUF socket options (see socket(7) for details.). The usual
381 suffixes K, M, G are supported and are understood to the base of
382 1024.
383 IPTOS=
385 Takes an integer argument controlling the IP Type-Of-Service field
386 for packets generated from this socket. This controls the IP_TOS
387 socket option (see ip(7) for details.). Either a numeric string or
388 one of low-delay, throughput, reliability or low-cost may be
389 specified.
390 IPTTL=
392 Takes an integer argument controlling the IPv4 Time-To-Live/IPv6
393 Hop-Count field for packets generated from this socket. This sets
394 the IP_TTL/IPV6_UNICAST_HOPS socket options (see ip(7) and ipv6(7)
395 for details.)
396 Mark=
398 Takes an integer value. Controls the firewall mark of packets
399 generated by this socket. This can be used in the firewall logic to
400 filter packets from this socket. This sets the SO_MARK socket
401 option. See iptables(8) for details.
402 ReusePort=
404 Takes a boolean value. If true, allows multiple bind(2)s to this
405 TCP or UDP port. This controls the SO_REUSEPORT socket option. See
406 socket(7) for details.
407 SmackLabel=, SmackLabelIPIn=, SmackLabelIPOut=
409 Takes a string value. Controls the extended attributes
410 "security.SMACK64", "security.SMACK64IPIN" and
411 "security.SMACK64IPOUT", respectively, i.e. the security label of
412 the FIFO, or the security label for the incoming or outgoing
413 connections of the socket, respectively. See Smack.txt[3] for
414 details.
415 SELinuxContextFromNet=
417 Takes a boolean argument. When true, systemd will attempt to figure
418 out the SELinux label used for the instantiated service from the
419 information handed by the peer over the network. Note that only the
420 security level is used from the information provided by the peer.
421 Other parts of the resulting SELinux context originate from either
422 the target binary that is effectively triggered by socket unit or
423 from the value of the SELinuxContext= option. This configuration
424 option applies only when activated service is passed in single
425 socket file descriptor, i.e. service instances that have standard
426 input connected to a socket or services triggered by exactly one
427 socket unit. Also note that this option is useful only when MLS/MCS
428 SELinux policy is deployed. Defaults to "false".
429 PipeSize=
431 Takes a size in bytes. Controls the pipe buffer size of FIFOs
432 configured in this socket unit. See fcntl(2) for details. The usual
433 suffixes K, M, G are supported and are understood to the base of
434 1024.
435 MessageQueueMaxMessages=, MessageQueueMessageSize=
437 These two settings take integer values and control the mq_maxmsg
438 field or the mq_msgsize field, respectively, when creating the
439 message queue. Note that either none or both of these variables
440 need to be set. See mq_setattr(3) for details.
441 FreeBind=
443 Takes a boolean value. Controls whether the socket can be bound to
444 non-local IP addresses. This is useful to configure sockets
445 listening on specific IP addresses before those IP addresses are
446 successfully configured on a network interface. This sets the
447 IP_FREEBIND/IPV6_FREEBIND socket option. For robustness reasons it
448 is recommended to use this option whenever you bind a socket to a
449 specific IP address. Defaults to false.
450 Transparent=
452 Takes a boolean value. Controls the IP_TRANSPARENT/IPV6_TRANSPARENT
453 socket option. Defaults to false.
454 Broadcast=
456 Takes a boolean value. This controls the SO_BROADCAST socket
457 option, which allows broadcast datagrams to be sent from this
458 socket. Defaults to false.
459 PassCredentials=
461 Takes a boolean value. This controls the SO_PASSCRED socket option,
462 which allows AF_UNIX sockets to receive the credentials of the
463 sending process in an ancillary message. Defaults to false.
464 PassSecurity=
466 Takes a boolean value. This controls the SO_PASSSEC socket option,
467 which allows AF_UNIX sockets to receive the security context of the
468 sending process in an ancillary message. Defaults to false.
469 PassPacketInfo=
471 Takes a boolean value. This controls the IP_PKTINFO,
472 IPV6_RECVPKTINFO, NETLINK_PKTINFO or PACKET_AUXDATA socket options,
473 which enable reception of additional per-packet metadata as
474 ancillary message, on AF_INET, AF_INET6, AF_UNIX and AF_PACKET
475 sockets. Defaults to false.
476 Timestamping=
478 Takes one of "off", "us" (alias: "usec", "µs") or "ns" (alias:
479 "nsec"). This controls the SO_TIMESTAMP or SO_TIMESTAMPNS socket
480 options, and enables whether ingress network traffic shall carry
481 timestamping metadata. Defaults to off.
482 TCPCongestion=
484 Takes a string value. Controls the TCP congestion algorithm used by
485 this socket. Should be one of "westwood", "veno", "cubic", "lp" or
486 any other available algorithm supported by the IP stack. This
487 setting applies only to stream sockets.
488 ExecStartPre=, ExecStartPost=
490 Takes one or more command lines, which are executed before or after
491 the listening sockets/FIFOs are created and bound, respectively.
492 The first token of the command line must be an absolute filename,
493 then followed by arguments for the process. Multiple command lines
494 may be specified following the same scheme as used for
495 ExecStartPre= of service unit files.
496 ExecStopPre=, ExecStopPost=
498 Additional commands that are executed before or after the listening
499 sockets/FIFOs are closed and removed, respectively. Multiple
500 command lines may be specified following the same scheme as used
501 for ExecStartPre= of service unit files.
502 TimeoutSec=
504 Configures the time to wait for the commands specified in
505 ExecStartPre=, ExecStartPost=, ExecStopPre= and ExecStopPost= to
506 finish. If a command does not exit within the configured time, the
507 socket will be considered failed and be shut down again. All
508 commands still running will be terminated forcibly via SIGTERM, and
509 after another delay of this time with SIGKILL. (See KillMode= in
510 systemd.kill(5).) Takes a unit-less value in seconds, or a time
511 span value such as "5min 20s". Pass "0" to disable the timeout
512 logic. Defaults to DefaultTimeoutStartSec= from the manager
513 configuration file (see systemd-system.conf(5)).
514 Service=
516 Specifies the service unit name to activate on incoming traffic.
517 This setting is only allowed for sockets with Accept=no. It
518 defaults to the service that bears the same name as the socket
519 (with the suffix replaced). In most cases, it should not be
520 necessary to use this option. Note that setting this parameter
521 might result in additional dependencies to be added to the unit
522 (see above).
523 RemoveOnStop=
525 Takes a boolean argument. If enabled, any file nodes created by
526 this socket unit are removed when it is stopped. This applies to
527 AF_UNIX sockets in the file system, POSIX message queues, FIFOs, as
528 well as any symlinks to them configured with Symlinks=. Normally,
529 it should not be necessary to use this option, and is not
530 recommended as services might continue to run after the socket unit
531 has been terminated and it should still be possible to communicate
532 with them via their file system node. Defaults to off.
533 Symlinks=
535 Takes a list of file system paths. The specified paths will be
536 created as symlinks to the AF_UNIX socket path or FIFO path of this
537 socket unit. If this setting is used, only one AF_UNIX socket in
538 the file system or one FIFO may be configured for the socket unit.
539 Use this option to manage one or more symlinked alias names for a
540 socket, binding their lifecycle together. Note that if creation of
541 a symlink fails this is not considered fatal for the socket unit,
542 and the socket unit may still start. If an empty string is
543 assigned, the list of paths is reset. Defaults to an empty list.
544 FileDescriptorName=
546 Assigns a name to all file descriptors this socket unit
547 encapsulates. This is useful to help activated services identify
548 specific file descriptors, if multiple fds are passed. Services may
549 use the sd_listen_fds_with_names(3) call to acquire the names
550 configured for the received file descriptors. Names may contain any
551 ASCII character, but must exclude control characters and ":", and
552 must be at most 255 characters in length. If this setting is not
553 used, the file descriptor name defaults to the name of the socket
554 unit, including its .socket suffix.
555 TriggerLimitIntervalSec=, TriggerLimitBurst=
557 Configures a limit on how often this socket unit may be activated
558 within a specific time interval. The TriggerLimitIntervalSec= may
559 be used to configure the length of the time interval in the usual
560 time units "us", "ms", "s", "min", "h", ... and defaults to 2s (See
561 systemd.time(7) for details on the various time units understood).
562 The TriggerLimitBurst= setting takes a positive integer value and
563 specifies the number of permitted activations per time interval,
564 and defaults to 200 for Accept=yes sockets (thus by default
565 permitting 200 activations per 2s), and 20 otherwise (20
566 activations per 2s). Set either to 0 to disable any form of trigger
567 rate limiting. If the limit is hit, the socket unit is placed into
568 a failure mode, and will not be connectible anymore until
569 restarted. Note that this limit is enforced before the service
570 activation is enqueued.
571 Check systemd.unit(5), systemd.exec(5), and systemd.kill(5) for more
573 settings.
574
576 systemd(1), systemctl(1), systemd-system.conf(5), systemd.unit(5),
577 systemd.exec(5), systemd.kill(5), systemd.resource-control(5),
578 systemd.service(5), systemd.directives(7), sd_listen_fds(3),
579 sd_listen_fds_with_names(3)
580 For more extensive descriptions see the "systemd for Developers"
582 series: Socket Activation[4], Socket Activation, part II[5], Converting
583 inetd Services[6], Socket Activated Internet Services and OS
584 Containers[7].
585
587 1. USB FunctionFS
588 https://www.kernel.org/doc/Documentation/usb/functionfs.txt
589 2. TCP Keepalive HOWTO
591 http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/
592 3. Smack.txt
594 https://www.kernel.org/doc/Documentation/security/Smack.txt
595 4. Socket Activation
597 http://0pointer.de/blog/projects/socket-activation.html
598 5. Socket Activation, part II
600 http://0pointer.de/blog/projects/socket-activation2.html
601 6. Converting inetd Services
603 http://0pointer.de/blog/projects/inetd.html
604 7. Socket Activated Internet Services and OS Containers
606 http://0pointer.de/blog/projects/socket-activated-containers.html
607systemd 251 SYSTEMD.SOCKET(5)