man/systemd.socket.xml

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   7   This file is part of systemd.
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   9   Copyright 2010 Lennart Poettering
  10
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  24
  25 <refentry id="systemd.socket">
  26         <refentryinfo>
  27                 <title>systemd.socket</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>systemd.socket</refentrytitle>
  42                 <manvolnum>5</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>systemd.socket</refname>
  47                 <refpurpose>Socket unit configuration</refpurpose>
  48         </refnamediv>
  49
  50         <refsynopsisdiv>
  51                 <para><filename><replaceable>socket</replaceable>.socket</filename></para>
  52         </refsynopsisdiv>
  53
  54         <refsect1>
  55                 <title>Description</title>
  56
  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
  61                 activation.</para>
  62
  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>
  71
  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
  79                 in, and in
  80                 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  81                 which define the way the processes are
  82                 terminated.</para>
  83
  84                 <para>For each socket file a matching service file
  85                 (see
  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
  99                 connection.</para>
 100
 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
 114                 option.</para>
 115
 116                 <para>Socket units may be used to implement on-demand
 117                 starting of services, as well as parallelized starting
 118                 of services.</para>
 119
 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>
 130         </refsect1>
 131
 132         <refsect1>
 133                 <title>Options</title>
 134
 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>
 141                 and
 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>
 145
 146                 <variablelist class='unit-directives'>
 147                         <varlistentry>
 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                                 (<constant>SOCK_STREAM</constant>), datagram (<constant>SOCK_DGRAM</constant>),
 154                                 or sequential packet
 155                                 (<constant>SOCK_SEQPACKET</constant>) socket, respectively. The address
 156                                 can be written in various formats:</para>
 157
 158                                 <para>If the address starts with a
 159                                 slash (/), it is read as file system
 160                                 socket in the <constant>AF_UNIX</constant> socket
 161                                 family.</para>
 162
 163                                 <para>If the address starts with an
 164                                 at symbol (@) it is read as abstract
 165                                 namespace socket in the <constant>AF_UNIX</constant>
 166                                 family. The @ is replaced with a NUL
 167                                 character before binding. For details
 168                                 see
 169                                 <citerefentry><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 170
 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
 178                                 just via IPv6.
 179                                 </para>
 180
 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
 185                                 z.</para>
 186
 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>
 193                                 setting (see below).
 194                                 </para>
 195
 196                                 <para>Note that <constant>SOCK_SEQPACKET</constant>
 197                                 (i.e. <varname>ListenSequentialPacket=</varname>)
 198                                 is only available for <constant>AF_UNIX</constant>
 199                                 sockets. <constant>SOCK_STREAM</constant>
 200                                 (i.e. <varname>ListenStream=</varname>)
 201                                 when used for IP sockets refers to TCP
 202                                 sockets, <constant>SOCK_DGRAM</constant>
 203                                 (i.e. <varname>ListenDatagram=</varname>)
 204                                 to UDP.</para>
 205
 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
 217                                 effect.</para>
 218
 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>
 228                         </varlistentry>
 229
 230                         <varlistentry>
 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
 236                                 similar to the
 237                                 <varname>ListenDatagram=</varname>
 238                                 directive above.</para></listitem>
 239                         </varlistentry>
 240
 241                         <varlistentry>
 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
 251                                 special files in
 252                                 <filename>/proc</filename> and
 253                                 <filename>/sys</filename>.</para></listitem>
 254                         </varlistentry>
 255
 256                         <varlistentry>
 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 <constant>AF_NETLINK</constant> 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
 267                                 very similar to the
 268                                 <varname>ListenDatagram=</varname>
 269                                 directive above.</para></listitem>
 270                         </varlistentry>
 271
 272                         <varlistentry>
 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>
 284                         </varlistentry>
 285
 286                         <varlistentry>
 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>
 294                                 for details). If
 295                                 <option>both</option>, IPv6 sockets
 296                                 bound will be accessible via both IPv4
 297                                 and IPv6. If
 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
 303                                 by
 304                                 <filename>/proc/sys/net/ipv6/bindv6only</filename>,
 305                                 which in turn defaults to the
 306                                 equivalent of
 307                                 <option>both</option>.</para>
 308                                 </listitem>
 309                         </varlistentry>
 310
 311                         <varlistentry>
 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
 318                                 packet sockets. See
 319                                 <citerefentry><refentrytitle>listen</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 320                                 for details. Defaults to SOMAXCONN
 321                                 (128).</para></listitem>
 322                         </varlistentry>
 323
 324                         <varlistentry>
 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
 336                                 device unit
 337                                 (<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 338                                 is created.</para></listitem>
 339                         </varlistentry>
 340
 341                         <varlistentry>
 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
 349                                 access mode in octal
 350                                 notation. Defaults to
 351                                 0755.</para></listitem>
 352                         </varlistentry>
 353
 354                         <varlistentry>
 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>
 363                         </varlistentry>
 364
 365                         <varlistentry>
 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 a
 378                                 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
 383                                 that is suitable for
 384                                 <option>Accept=false</option>. A
 385                                 daemon listening on an <constant>AF_UNIX</constant> socket
 386                                 may, but does not need to, call
 387                                 <citerefentry><refentrytitle>close</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 388                                 on the received socket before
 389                                 exiting. However, it must not unlink
 390                                 the socket from a filesystem. It
 391                                 should note invoke
 392                                 <citerefentry><refentrytitle>shutdown</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 393                                 on sockets it got with
 394                                 <varname>Accept=false</varname>, but
 395                                 it may do so for sockets it got with
 396                                 <varname>Accept=true</varname> set.
 397                                 Setting <varname>Accept=true</varname>
 398                                 is mostly useful to allow daemons
 399                                 designed for usage with
 400                                 <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 401                                 to work unmodified with systemd socket
 402                                 activation.</para></listitem>
 403                         </varlistentry>
 404
 405                         <varlistentry>
 406                                 <term><varname>MaxConnections=</varname></term>
 407                                 <listitem><para>The maximum number of
 408                                 connections to simultaneously run
 409                                 services instances for, when
 410                                 <option>Accept=true</option> is
 411                                 set. If more concurrent connections
 412                                 are coming in, they will be refused
 413                                 until at least one existing connection
 414                                 is terminated. This setting has no
 415                                 effect for sockets configured with
 416                                 <option>Accept=false</option> or datagram
 417                                 sockets. Defaults to
 418                                 64.</para></listitem>
 419                         </varlistentry>
 420
 421                         <varlistentry>
 422                                 <term><varname>KeepAlive=</varname></term>
 423                                 <listitem><para>Takes a boolean
 424                                 argument. If true, the TCP/IP stack
 425                                 will send a keep alive message after
 426                                 2h (depending on the configuration of
 427                                 <filename>/proc/sys/net/ipv4/tcp_keepalive_time</filename>)
 428                                 for all TCP streams accepted on this
 429                                 socket. This controls the SO_KEEPALIVE
 430                                 socket option (see
 431                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 432                                 and the <ulink
 433                                 url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
 434                                 Keepalive HOWTO</ulink> for details.)
 435                                 Defaults to
 436                                 <option>false</option>.</para></listitem>
 437                         </varlistentry>
 438
 439                         <varlistentry>
 440                                 <term><varname>Priority=</varname></term>
 441                                 <listitem><para>Takes an integer
 442                                 argument controlling the priority for
 443                                 all traffic sent from this
 444                                 socket. This controls the SO_PRIORITY
 445                                 socket option (see
 446                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 447                                 for details.).</para></listitem>
 448                         </varlistentry>
 449
 450                         <varlistentry>
 451                                 <term><varname>ReceiveBuffer=</varname></term>
 452                                 <term><varname>SendBuffer=</varname></term>
 453                                 <listitem><para>Takes an integer
 454                                 argument controlling the receive
 455                                 or send buffer sizes of this
 456                                 socket, respectively. This controls the SO_RCVBUF
 457                                 and SO_SNDBUF socket options (see
 458                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 459                                 for details.).</para></listitem>
 460                         </varlistentry>
 461
 462                         <varlistentry>
 463                                 <term><varname>IPTOS=</varname></term>
 464                                 <listitem><para>Takes an integer
 465                                 argument controlling the IP
 466                                 Type-Of-Service field for packets
 467                                 generated from this socket. This
 468                                 controls the IP_TOS socket option (see
 469                                 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 470                                 for details.). Either a numeric string
 471                                 or one of <option>low-delay</option>,
 472                                 <option>throughput</option>,
 473                                 <option>reliability</option> or
 474                                 <option>low-cost</option> may be
 475                                 specified.</para></listitem>
 476                         </varlistentry>
 477
 478                         <varlistentry>
 479                                 <term><varname>IPTTL=</varname></term>
 480                                 <listitem><para>Takes an integer
 481                                 argument controlling the IPv4
 482                                 Time-To-Live/IPv6 Hop-Count field for
 483                                 packets generated from this
 484                                 socket. This sets the
 485                                 IP_TTL/IPV6_UNICAST_HOPS socket
 486                                 options (see
 487                                 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 488                                 and
 489                                 <citerefentry><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 490                                 for details.)</para></listitem>
 491                         </varlistentry>
 492
 493                         <varlistentry>
 494                                 <term><varname>Mark=</varname></term>
 495                                 <listitem><para>Takes an integer
 496                                 value. Controls the firewall mark of
 497                                 packets generated by this socket. This
 498                                 can be used in the firewall logic to
 499                                 filter packets from this socket. This
 500                                 sets the SO_MARK socket option. See
 501                                 <citerefentry><refentrytitle>iptables</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 502                                 for details.</para></listitem>
 503                         </varlistentry>
 504
 505                         <varlistentry>
 506                                 <term><varname>SmackLabel=</varname></term>
 507                                 <term><varname>SmackLabelIPIn=</varname></term>
 508                                 <term><varname>SmackLabelIPOut=</varname></term>
 509                                 <listitem><para>Takes a string
 510                                 value. Controls the extended
 511                                 attributes
 512                                 <literal>security.SMACK64</literal>,
 513                                 <literal>security.SMACK64IPIN</literal>
 514                                 and
 515                                 <literal>security.SMACK64IPOUT</literal>,
 516                                 respectively, i.e. the security label
 517                                 of the FIFO, or the security label for
 518                                 the incoming or outgoing connections
 519                                 of the socket, respectively.  See
 520                                 <ulink
 521                                 url="https://www.kernel.org/doc/Documentation/security/Smack.txt">Smack.txt</ulink>
 522                                 for details.</para></listitem>
 523                         </varlistentry>
 524
 525                         <varlistentry>
 526                                 <term><varname>PipeSize=</varname></term>
 527                                 <listitem><para>Takes an integer
 528                                 value. Controls the pipe buffer size
 529                                 of FIFOs configured in this socket
 530                                 unit.  See
 531                                 <citerefentry><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 532                                 for details.</para></listitem>
 533                         </varlistentry>
 534
 535                         <varlistentry>
 536                                 <term><varname>MessageQueueMaxMessages=</varname>,
 537                                 <varname>MessageQueueMessageSize=</varname></term>
 538                                 <listitem><para>These two settings
 539                                 take integer values and control the
 540                                 mq_maxmsg field or the mq_msgsize field, respectively, when
 541                                 creating the message queue. Note that
 542                                 either none or both of these variables
 543                                 need to be set. See
 544                                 <citerefentry><refentrytitle>mq_setattr</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 545                                 for details.</para></listitem>
 546                         </varlistentry>
 547
 548                         <varlistentry>
 549                                 <term><varname>FreeBind=</varname></term>
 550                                 <listitem><para>Takes a boolean
 551                                 value. Controls whether the socket can
 552                                 be bound to non-local IP
 553                                 addresses. This is useful to configure
 554                                 sockets listening on specific IP
 555                                 addresses before those IP addresses
 556                                 are successfully configured on a
 557                                 network interface. This sets the
 558                                 IP_FREEBIND socket option. For
 559                                 robustness reasons it is recommended
 560                                 to use this option whenever you bind a
 561                                 socket to a specific IP
 562                                 address. Defaults to <option>false</option>.</para></listitem>
 563                         </varlistentry>
 564
 565                         <varlistentry>
 566                                 <term><varname>Transparent=</varname></term>
 567                                 <listitem><para>Takes a boolean
 568                                 value. Controls the IP_TRANSPARENT
 569                                 socket option. Defaults to
 570                                 <option>false</option>.</para></listitem>
 571                         </varlistentry>
 572
 573                         <varlistentry>
 574                                 <term><varname>Broadcast=</varname></term>
 575                                 <listitem><para>Takes a boolean
 576                                 value. This controls the SO_BROADCAST
 577                                 socket option, which allows broadcast
 578                                 datagrams to be sent from this
 579                                 socket. Defaults to
 580                                 <option>false</option>.</para></listitem>
 581                         </varlistentry>
 582
 583                         <varlistentry>
 584                                 <term><varname>PassCredentials=</varname></term>
 585                                 <listitem><para>Takes a boolean
 586                                 value. This controls the SO_PASSCRED
 587                                 socket option, which allows <constant>AF_UNIX</constant> sockets to
 588                                 receive the credentials of the sending
 589                                 process in an ancillary message.
 590                                 Defaults to
 591                                 <option>false</option>.</para></listitem>
 592                         </varlistentry>
 593
 594                         <varlistentry>
 595                                 <term><varname>PassSecurity=</varname></term>
 596                                 <listitem><para>Takes a boolean
 597                                 value. This controls the SO_PASSSEC
 598                                 socket option, which allows <constant>AF_UNIX</constant>
 599                                 sockets to receive the security
 600                                 context of the sending process in an
 601                                 ancillary message.  Defaults to
 602                                 <option>false</option>.</para></listitem>
 603                         </varlistentry>
 604
 605                         <varlistentry>
 606                                 <term><varname>TCPCongestion=</varname></term>
 607                                 <listitem><para>Takes a string
 608                                 value. Controls the TCP congestion
 609                                 algorithm used by this socket. Should
 610                                 be one of "westwood", "veno", "cubic",
 611                                 "lp" or any other available algorithm
 612                                 supported by the IP stack. This
 613                                 setting applies only to stream
 614                                 sockets.</para></listitem>
 615                         </varlistentry>
 616
 617                         <varlistentry>
 618                                 <term><varname>ExecStartPre=</varname></term>
 619                                 <term><varname>ExecStartPost=</varname></term>
 620                                 <listitem><para>Takes one or more
 621                                 command lines, which are executed
 622                                 before or after the listening
 623                                 sockets/FIFOs are created and
 624                                 bound, respectively. The first token of the command
 625                                 line must be an absolute file name,
 626                                 then followed by arguments for the
 627                                 process. Multiple command lines may be
 628                                 specified following the same scheme as
 629                                 used for
 630                                 <varname>ExecStartPre=</varname> of
 631                                 service unit files.</para></listitem>
 632                         </varlistentry>
 633
 634                         <varlistentry>
 635                                 <term><varname>ExecStopPre=</varname></term>
 636                                 <term><varname>ExecStopPost=</varname></term>
 637                                 <listitem><para>Additional commands
 638                                 that are executed before or after
 639                                 the listening sockets/FIFOs are closed
 640                                 and removed, respectively. Multiple command lines
 641                                 may be specified following the same
 642                                 scheme as used for
 643                                 <varname>ExecStartPre=</varname> of
 644                                 service unit files.</para></listitem>
 645                         </varlistentry>
 646
 647                         <varlistentry>
 648                                 <term><varname>TimeoutSec=</varname></term>
 649                                 <listitem><para>Configures the time to
 650                                 wait for the commands specified in
 651                                 <varname>ExecStartPre=</varname>,
 652                                 <varname>ExecStartPost=</varname>,
 653                                 <varname>ExecStopPre=</varname> and
 654                                 <varname>ExecStopPost=</varname> to
 655                                 finish. If a command does not exit
 656                                 within the configured time, the socket
 657                                 will be considered failed and be shut
 658                                 down again. All commands still running,
 659                                 will be terminated forcibly via
 660                                 SIGTERM, and after another delay of
 661                                 this time with SIGKILL. (See
 662                                 <option>KillMode=</option> in <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>.)
 663                                 Takes a unit-less value in seconds, or
 664                                 a time span value such as "5min
 665                                 20s". Pass 0 to disable the timeout
 666                                 logic. Defaults to
 667                                 90s.</para></listitem>
 668                         </varlistentry>
 669
 670                         <varlistentry>
 671                                 <term><varname>Service=</varname></term>
 672                                 <listitem><para>Specifies the service
 673                                 unit name to activate on incoming
 674                                 traffic. This defaults to the service
 675                                 that bears the same name as the socket
 676                                 (ignoring the different suffixes). In
 677                                 most cases it should not be necessary
 678                                 to use this option.</para></listitem>
 679                         </varlistentry>
 680
 681                 </variablelist>
 682
 683                 <para>Check
 684                 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 685                 and
 686                 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 687                 for more settings.</para>
 688
 689         </refsect1>
 690
 691         <refsect1>
 692                   <title>See Also</title>
 693                   <para>
 694                           <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 695                           <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 696                           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 697                           <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 698                           <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 699                           <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 700                           <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 701                   </para>
 702
 703                   <para>
 704                           For more extensive descriptions see the "Systemd for Developers" series:
 705                           <ulink url="http://0pointer.de/blog/projects/socket-activation.html">Socket Activation</ulink>,
 706                           <ulink url="http://0pointer.de/blog/projects/socket-activation2.html">Socket Activation, part II</ulink>,
 707                           <ulink url="http://0pointer.de/blog/projects/inetd.html">Converting inetd Services</ulink>,
 708                           <ulink url="http://0pointer.de/blog/projects/socket-activated-containers.html">Socket Activated Internet Services and OS Containers</ulink>.
 709                   </para>
 710         </refsect1>
 711
 712 </refentry>