man/systemd.socket.xml

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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                 <literal>.socket</literal> 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 terminated, and
  82                 in
  83                 <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
  84                 which configure resource control settings for the
  85                 processes of the socket.</para>
  86
  87                 <para>For each socket file, a matching service file
  88                 must exist, describing the service to start on
  89                 incoming traffic on the socket (see
  90                 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  91                 for more information about .service files). The name
  92                 of the .service unit is by default the same as the
  93                 name of the .socket unit, but can be altered with the
  94                 <option>Service=</option> option described below.
  95                 Depending on the setting of the <option>Accept=</option>
  96                 option described below, this .service unit must either
  97                 be named like the .socket unit, but with the suffix
  98                 replaced, unless overridden with
  99                 <option>Service=</option>; or it must be a template
 100                 unit named the same way. Example: a socket file
 101                 <filename>foo.socket</filename> needs a matching
 102                 service <filename>foo.service</filename> if
 103                 <option>Accept=false</option> is set. If
 104                 <option>Accept=true</option> is set, a service
 105                 template file <filename>foo@.service</filename> must
 106                 exist from which services are instantiated for each
 107                 incoming connection.</para>
 108
 109                 <para>Unless <varname>DefaultDependencies=</varname>
 110                 is set to <option>false</option>, socket units will
 111                 implicitly have dependencies of type
 112                 <varname>Requires=</varname> and
 113                 <varname>After=</varname> on
 114                 <filename>sysinit.target</filename> as well as
 115                 dependencies of type <varname>Conflicts=</varname> and
 116                 <varname>Before=</varname> on
 117                 <filename>shutdown.target</filename>. These ensure
 118                 that socket units pull in basic system
 119                 initialization, and are terminated cleanly prior to
 120                 system shutdown. Only sockets involved with early
 121                 boot or late system shutdown should disable this
 122                 option.</para>
 123
 124                 <para>Socket units will have a
 125                 <varname>Before=</varname> dependency on the service
 126                 which they trigger added implicitly. No implicit
 127                 <varname>WantedBy=</varname> or
 128                 <varname>RequiredBy=</varname> dependency from the
 129                 socket to the service is added. This means that the
 130                 service may be started without the socket, in which
 131                 case it must be able to open sockets by itself. To
 132                 prevent this, an explicit <varname>Requires=</varname>
 133                 dependency may be added.</para>
 134
 135                 <para>Socket units may be used to implement on-demand
 136                 starting of services, as well as parallelized starting
 137                 of services. See the blog stories linked at the end
 138                 for an introduction.</para>
 139
 140                 <para>Note that the daemon software configured for
 141                 socket activation with socket units needs to be able
 142                 to accept sockets from systemd, either via systemd's
 143                 native socket passing interface (see
 144                 <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 145                 for details) or via the traditional
 146                 <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>-style
 147                 socket passing (i.e. sockets passed in via standard input and
 148                 output, using <varname>StandardInput=socket</varname>
 149                 in the service file).</para>
 150         </refsect1>
 151
 152         <refsect1>
 153                 <title>Options</title>
 154
 155                 <para>Socket files must include a [Socket] section,
 156                 which carries information about the socket or FIFO it
 157                 supervises. A number of options that may be used in
 158                 this section are shared with other unit types. These
 159                 options are documented in
 160                 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 161                 and
 162                 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>. The
 163                 options specific to the [Socket] section of socket
 164                 units are the following:</para>
 165
 166                 <variablelist class='unit-directives'>
 167                         <varlistentry>
 168                                 <term><varname>ListenStream=</varname></term>
 169                                 <term><varname>ListenDatagram=</varname></term>
 170                                 <term><varname>ListenSequentialPacket=</varname></term>
 171                                 <listitem><para>Specifies an address
 172                                 to listen on for a stream
 173                                 (<constant>SOCK_STREAM</constant>), datagram (<constant>SOCK_DGRAM</constant>),
 174                                 or sequential packet
 175                                 (<constant>SOCK_SEQPACKET</constant>) socket, respectively. The address
 176                                 can be written in various formats:</para>
 177
 178                                 <para>If the address starts with a
 179                                 slash (<literal>/</literal>), it is read as file system
 180                                 socket in the <constant>AF_UNIX</constant> socket
 181                                 family.</para>
 182
 183                                 <para>If the address starts with an at
 184                                 symbol (<literal>@</literal>), it is read as abstract
 185                                 namespace socket in the
 186                                 <constant>AF_UNIX</constant>
 187                                 family. The <literal>@</literal> is
 188                                 replaced with a
 189                                 <constant>NUL</constant> character
 190                                 before binding. For details, see
 191                                 <citerefentry><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 192
 193                                 <para>If the address string is a
 194                                 single number, it is read as port
 195                                 number to listen on via
 196                                 IPv6. Depending on the value of
 197                                 <varname>BindIPv6Only=</varname> (see below) this
 198                                 might result in the service being
 199                                 available via both IPv6 and IPv4 (default) or
 200                                 just via IPv6.
 201                                 </para>
 202
 203                                 <para>If the address string is a
 204                                 string in the format v.w.x.y:z, it is
 205                                 read as IPv4 specifier for listening
 206                                 on an address v.w.x.y on a port
 207                                 z.</para>
 208
 209                                 <para>If the address string is a
 210                                 string in the format [x]:y, it is read
 211                                 as IPv6 address x on a port y. Note
 212                                 that this might make the service
 213                                 available via IPv4, too, depending on
 214                                 the <varname>BindIPv6Only=</varname>
 215                                 setting (see below).
 216                                 </para>
 217
 218                                 <para>Note that <constant>SOCK_SEQPACKET</constant>
 219                                 (i.e. <varname>ListenSequentialPacket=</varname>)
 220                                 is only available for <constant>AF_UNIX</constant>
 221                                 sockets. <constant>SOCK_STREAM</constant>
 222                                 (i.e. <varname>ListenStream=</varname>)
 223                                 when used for IP sockets refers to TCP
 224                                 sockets, <constant>SOCK_DGRAM</constant>
 225                                 (i.e. <varname>ListenDatagram=</varname>)
 226                                 to UDP.</para>
 227
 228                                 <para>These options may be specified
 229                                 more than once in which case incoming
 230                                 traffic on any of the sockets will
 231                                 trigger service activation, and all
 232                                 listed sockets will be passed to the
 233                                 service, regardless of whether there is
 234                                 incoming traffic on them or not. If
 235                                 the empty string is assigned to any of
 236                                 these options, the list of addresses
 237                                 to listen on is reset, all prior uses
 238                                 of any of these options will have no
 239                                 effect.</para>
 240
 241                                 <para>It is also possible to have more
 242                                 than one socket unit for the same
 243                                 service when using
 244                                 <varname>Service=</varname>, and the
 245                                 service will receive all the sockets
 246                                 configured in all the socket units.
 247                                 Sockets configured in one unit are
 248                                 passed in the order of configuration,
 249                                 but no ordering between socket units
 250                                 is specified.</para>
 251
 252                                 <para>If an IP address is used here,
 253                                 it is often desirable to listen on it
 254                                 before the interface it is configured
 255                                 on is up and running, and even
 256                                 regardless of whether it will be up and
 257                                 running at any point. To deal with this,
 258                                 it is recommended to set the
 259                                 <varname>FreeBind=</varname> option
 260                                 described below.</para></listitem>
 261                         </varlistentry>
 262
 263                         <varlistentry>
 264                                 <term><varname>ListenFIFO=</varname></term>
 265                                 <listitem><para>Specifies a file
 266                                 system FIFO to listen on. This expects
 267                                 an absolute file system path as
 268                                 argument. Behavior otherwise is very
 269                                 similar to the
 270                                 <varname>ListenDatagram=</varname>
 271                                 directive above.</para></listitem>
 272                         </varlistentry>
 273
 274                         <varlistentry>
 275                                 <term><varname>ListenSpecial=</varname></term>
 276                                 <listitem><para>Specifies a special
 277                                 file in the file system to listen
 278                                 on. This expects an absolute file
 279                                 system path as argument. Behavior
 280                                 otherwise is very similar to the
 281                                 <varname>ListenFIFO=</varname>
 282                                 directive above. Use this to open
 283                                 character device nodes as well as
 284                                 special files in
 285                                 <filename>/proc</filename> and
 286                                 <filename>/sys</filename>.</para></listitem>
 287                         </varlistentry>
 288
 289                         <varlistentry>
 290                                 <term><varname>ListenNetlink=</varname></term>
 291                                 <listitem><para>Specifies a Netlink
 292                                 family to create a socket for to
 293                                 listen on. This expects a short string
 294                                 referring to the <constant>AF_NETLINK</constant> family
 295                                 name (such as <varname>audit</varname>
 296                                 or <varname>kobject-uevent</varname>)
 297                                 as argument, optionally suffixed by a
 298                                 whitespace followed by a multicast
 299                                 group integer. Behavior otherwise is
 300                                 very similar to the
 301                                 <varname>ListenDatagram=</varname>
 302                                 directive above.</para></listitem>
 303                         </varlistentry>
 304
 305                         <varlistentry>
 306                                 <term><varname>ListenMessageQueue=</varname></term>
 307                                 <listitem><para>Specifies a POSIX
 308                                 message queue name to listen on. This
 309                                 expects a valid message queue name
 310                                 (i.e. beginning with /). Behavior
 311                                 otherwise is very similar to the
 312                                 <varname>ListenFIFO=</varname>
 313                                 directive above. On Linux message
 314                                 queue descriptors are actually file
 315                                 descriptors and can be inherited
 316                                 between processes.</para></listitem>
 317                         </varlistentry>
 318
 319                         <varlistentry>
 320                                 <term><varname>BindIPv6Only=</varname></term>
 321                                 <listitem><para>Takes a one of
 322                                 <option>default</option>,
 323                                 <option>both</option> or
 324                                 <option>ipv6-only</option>. Controls
 325                                 the IPV6_V6ONLY socket option (see
 326                                 <citerefentry><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 327                                 for details). If
 328                                 <option>both</option>, IPv6 sockets
 329                                 bound will be accessible via both IPv4
 330                                 and IPv6. If
 331                                 <option>ipv6-only</option>, they will
 332                                 be accessible via IPv6 only. If
 333                                 <option>default</option> (which is the
 334                                 default, surprise!), the system wide
 335                                 default setting is used, as controlled
 336                                 by
 337                                 <filename>/proc/sys/net/ipv6/bindv6only</filename>,
 338                                 which in turn defaults to the
 339                                 equivalent of
 340                                 <option>both</option>.</para>
 341                                 </listitem>
 342                         </varlistentry>
 343
 344                         <varlistentry>
 345                                 <term><varname>Backlog=</varname></term>
 346                                 <listitem><para>Takes an unsigned
 347                                 integer argument. Specifies the number
 348                                 of connections to queue that have not
 349                                 been accepted yet. This setting
 350                                 matters only for stream and sequential
 351                                 packet sockets. See
 352                                 <citerefentry><refentrytitle>listen</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 353                                 for details. Defaults to SOMAXCONN
 354                                 (128).</para></listitem>
 355                         </varlistentry>
 356
 357                         <varlistentry>
 358                                 <term><varname>BindToDevice=</varname></term>
 359                                 <listitem><para>Specifies a network
 360                                 interface name to bind this socket
 361                                 to. If set, traffic will only be
 362                                 accepted from the specified network
 363                                 interfaces. This controls the
 364                                 SO_BINDTODEVICE socket option (see
 365                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 366                                 for details). If this option is used,
 367                                 an automatic dependency from this
 368                                 socket unit on the network interface
 369                                 device unit
 370                                 (<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 371                                 is created.</para></listitem>
 372                         </varlistentry>
 373
 374                         <varlistentry>
 375                                 <term><varname>SocketUser=</varname></term>
 376                                 <term><varname>SocketGroup=</varname></term>
 377
 378                                 <listitem><para>Takes a UNIX
 379                                 user/group name. When specified,
 380                                 all AF_UNIX sockets and FIFO nodes in
 381                                 the file system are owned by the
 382                                 specified user and group. If unset
 383                                 (the default), the nodes are owned by
 384                                 the root user/group (if run in system
 385                                 context) or the invoking user/group
 386                                 (if run in user context). If only a
 387                                 user is specified but no group, then
 388                                 the group is derived from the user's
 389                                 default group.</para></listitem>
 390                         </varlistentry>
 391
 392                         <varlistentry>
 393                                 <term><varname>SocketMode=</varname></term>
 394                                 <listitem><para>If listening on a file
 395                                 system socket or FIFO, this option
 396                                 specifies the file system access mode
 397                                 used when creating the file
 398                                 node. Takes an access mode in octal
 399                                 notation. Defaults to
 400                                 0666.</para></listitem>
 401                         </varlistentry>
 402
 403                         <varlistentry>
 404                                 <term><varname>DirectoryMode=</varname></term>
 405                                 <listitem><para>If listening on a file
 406                                 system socket or FIFO, the parent
 407                                 directories are automatically created
 408                                 if needed. This option specifies the
 409                                 file system access mode used when
 410                                 creating these directories. Takes an
 411                                 access mode in octal
 412                                 notation. Defaults to
 413                                 0755.</para></listitem>
 414                         </varlistentry>
 415
 416                         <varlistentry>
 417                                 <term><varname>Accept=</varname></term>
 418                                 <listitem><para>Takes a boolean
 419                                 argument. If true, a service instance
 420                                 is spawned for each incoming
 421                                 connection and only the connection
 422                                 socket is passed to it. If false, all
 423                                 listening sockets themselves are
 424                                 passed to the started service unit,
 425                                 and only one service unit is spawned
 426                                 for all connections (also see
 427                                 above). This value is ignored for
 428                                 datagram sockets and FIFOs where a
 429                                 single service unit unconditionally
 430                                 handles all incoming traffic. Defaults
 431                                 to <option>false</option>. For
 432                                 performance reasons, it is recommended
 433                                 to write new daemons only in a way
 434                                 that is suitable for
 435                                 <option>Accept=false</option>. A
 436                                 daemon listening on an <constant>AF_UNIX</constant> socket
 437                                 may, but does not need to, call
 438                                 <citerefentry><refentrytitle>close</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 439                                 on the received socket before
 440                                 exiting. However, it must not unlink
 441                                 the socket from a file system. It
 442                                 should not invoke
 443                                 <citerefentry><refentrytitle>shutdown</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 444                                 on sockets it got with
 445                                 <varname>Accept=false</varname>, but
 446                                 it may do so for sockets it got with
 447                                 <varname>Accept=true</varname> set.
 448                                 Setting <varname>Accept=true</varname>
 449                                 is mostly useful to allow daemons
 450                                 designed for usage with
 451                                 <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 452                                 to work unmodified with systemd socket
 453                                 activation.</para></listitem>
 454                         </varlistentry>
 455
 456                         <varlistentry>
 457                                 <term><varname>MaxConnections=</varname></term>
 458                                 <listitem><para>The maximum number of
 459                                 connections to simultaneously run
 460                                 services instances for, when
 461                                 <option>Accept=true</option> is
 462                                 set. If more concurrent connections
 463                                 are coming in, they will be refused
 464                                 until at least one existing connection
 465                                 is terminated. This setting has no
 466                                 effect on sockets configured with
 467                                 <option>Accept=false</option> or datagram
 468                                 sockets. Defaults to
 469                                 64.</para></listitem>
 470                         </varlistentry>
 471
 472                         <varlistentry>
 473                                 <term><varname>KeepAlive=</varname></term>
 474                                 <listitem><para>Takes a boolean
 475                                 argument. If true, the TCP/IP stack
 476                                 will send a keep alive message after
 477                                 2h (depending on the configuration of
 478                                 <filename>/proc/sys/net/ipv4/tcp_keepalive_time</filename>)
 479                                 for all TCP streams accepted on this
 480                                 socket. This controls the SO_KEEPALIVE
 481                                 socket option (see
 482                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 483                                 and the <ulink
 484                                 url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
 485                                 Keepalive HOWTO</ulink> for details.)
 486                                 Defaults to
 487                                 <option>false</option>.</para></listitem>
 488                         </varlistentry>
 489
 490                         <varlistentry>
 491                                 <term><varname>Priority=</varname></term>
 492                                 <listitem><para>Takes an integer
 493                                 argument controlling the priority for
 494                                 all traffic sent from this
 495                                 socket. This controls the SO_PRIORITY
 496                                 socket option (see
 497                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 498                                 for details.).</para></listitem>
 499                         </varlistentry>
 500
 501                         <varlistentry>
 502                                 <term><varname>ReceiveBuffer=</varname></term>
 503                                 <term><varname>SendBuffer=</varname></term>
 504                                 <listitem><para>Takes an integer
 505                                 argument controlling the receive or
 506                                 send buffer sizes of this socket,
 507                                 respectively. This controls the
 508                                 SO_RCVBUF and SO_SNDBUF socket options
 509                                 (see
 510                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 511                                 for details.). The usual suffixes K,
 512                                 M, G are supported and are understood
 513                                 to the base of 1024.</para></listitem>
 514                         </varlistentry>
 515
 516                         <varlistentry>
 517                                 <term><varname>IPTOS=</varname></term>
 518                                 <listitem><para>Takes an integer
 519                                 argument controlling the IP
 520                                 Type-Of-Service field for packets
 521                                 generated from this socket. This
 522                                 controls the IP_TOS socket option (see
 523                                 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 524                                 for details.). Either a numeric string
 525                                 or one of <option>low-delay</option>,
 526                                 <option>throughput</option>,
 527                                 <option>reliability</option> or
 528                                 <option>low-cost</option> may be
 529                                 specified.</para></listitem>
 530                         </varlistentry>
 531
 532                         <varlistentry>
 533                                 <term><varname>IPTTL=</varname></term>
 534                                 <listitem><para>Takes an integer
 535                                 argument controlling the IPv4
 536                                 Time-To-Live/IPv6 Hop-Count field for
 537                                 packets generated from this
 538                                 socket. This sets the
 539                                 IP_TTL/IPV6_UNICAST_HOPS socket
 540                                 options (see
 541                                 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 542                                 and
 543                                 <citerefentry><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 544                                 for details.)</para></listitem>
 545                         </varlistentry>
 546
 547                         <varlistentry>
 548                                 <term><varname>Mark=</varname></term>
 549                                 <listitem><para>Takes an integer
 550                                 value. Controls the firewall mark of
 551                                 packets generated by this socket. This
 552                                 can be used in the firewall logic to
 553                                 filter packets from this socket. This
 554                                 sets the SO_MARK socket option. See
 555                                 <citerefentry><refentrytitle>iptables</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 556                                 for details.</para></listitem>
 557                         </varlistentry>
 558
 559                         <varlistentry>
 560                                 <term><varname>ReusePort=</varname></term>
 561                                 <listitem><para>Takes a boolean
 562                                 value. If true, allows multiple <citerefentry><refentrytitle>bind</refentrytitle><manvolnum>2</manvolnum></citerefentry>s
 563                                 to this TCP or UDP port.  This
 564                                 controls the SO_REUSEPORT socket
 565                                 option.  See
 566                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 567                                 for details.</para></listitem>
 568                         </varlistentry>
 569
 570                         <varlistentry>
 571                                 <term><varname>SmackLabel=</varname></term>
 572                                 <term><varname>SmackLabelIPIn=</varname></term>
 573                                 <term><varname>SmackLabelIPOut=</varname></term>
 574                                 <listitem><para>Takes a string
 575                                 value. Controls the extended
 576                                 attributes
 577                                 <literal>security.SMACK64</literal>,
 578                                 <literal>security.SMACK64IPIN</literal>
 579                                 and
 580                                 <literal>security.SMACK64IPOUT</literal>,
 581                                 respectively, i.e. the security label
 582                                 of the FIFO, or the security label for
 583                                 the incoming or outgoing connections
 584                                 of the socket, respectively. See
 585                                 <ulink
 586                                 url="https://www.kernel.org/doc/Documentation/security/Smack.txt">Smack.txt</ulink>
 587                                 for details.</para></listitem>
 588                         </varlistentry>
 589
 590                         <varlistentry>
 591                                 <term><varname>PipeSize=</varname></term>
 592                                 <listitem><para>Takes a size in
 593                                 bytes. Controls the pipe buffer size
 594                                 of FIFOs configured in this socket
 595                                 unit. See
 596                                 <citerefentry><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 597                                 for details. The usual suffixes K, M,
 598                                 G are supported and are understood to
 599                                 the base of 1024.</para></listitem>
 600                         </varlistentry>
 601
 602                         <varlistentry>
 603                                 <term><varname>MessageQueueMaxMessages=</varname>,
 604                                 <varname>MessageQueueMessageSize=</varname></term>
 605                                 <listitem><para>These two settings
 606                                 take integer values and control the
 607                                 mq_maxmsg field or the mq_msgsize field, respectively, when
 608                                 creating the message queue. Note that
 609                                 either none or both of these variables
 610                                 need to be set. See
 611                                 <citerefentry><refentrytitle>mq_setattr</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 612                                 for details.</para></listitem>
 613                         </varlistentry>
 614
 615                         <varlistentry>
 616                                 <term><varname>FreeBind=</varname></term>
 617                                 <listitem><para>Takes a boolean
 618                                 value. Controls whether the socket can
 619                                 be bound to non-local IP
 620                                 addresses. This is useful to configure
 621                                 sockets listening on specific IP
 622                                 addresses before those IP addresses
 623                                 are successfully configured on a
 624                                 network interface. This sets the
 625                                 IP_FREEBIND socket option. For
 626                                 robustness reasons it is recommended
 627                                 to use this option whenever you bind a
 628                                 socket to a specific IP
 629                                 address. Defaults to <option>false</option>.</para></listitem>
 630                         </varlistentry>
 631
 632                         <varlistentry>
 633                                 <term><varname>Transparent=</varname></term>
 634                                 <listitem><para>Takes a boolean
 635                                 value. Controls the IP_TRANSPARENT
 636                                 socket option. Defaults to
 637                                 <option>false</option>.</para></listitem>
 638                         </varlistentry>
 639
 640                         <varlistentry>
 641                                 <term><varname>Broadcast=</varname></term>
 642                                 <listitem><para>Takes a boolean
 643                                 value. This controls the SO_BROADCAST
 644                                 socket option, which allows broadcast
 645                                 datagrams to be sent from this
 646                                 socket. Defaults to
 647                                 <option>false</option>.</para></listitem>
 648                         </varlistentry>
 649
 650                         <varlistentry>
 651                                 <term><varname>PassCredentials=</varname></term>
 652                                 <listitem><para>Takes a boolean
 653                                 value. This controls the SO_PASSCRED
 654                                 socket option, which allows <constant>AF_UNIX</constant> sockets to
 655                                 receive the credentials of the sending
 656                                 process in an ancillary message.
 657                                 Defaults to
 658                                 <option>false</option>.</para></listitem>
 659                         </varlistentry>
 660
 661                         <varlistentry>
 662                                 <term><varname>PassSecurity=</varname></term>
 663                                 <listitem><para>Takes a boolean
 664                                 value. This controls the SO_PASSSEC
 665                                 socket option, which allows <constant>AF_UNIX</constant>
 666                                 sockets to receive the security
 667                                 context of the sending process in an
 668                                 ancillary message. Defaults to
 669                                 <option>false</option>.</para></listitem>
 670                         </varlistentry>
 671
 672                         <varlistentry>
 673                                 <term><varname>TCPCongestion=</varname></term>
 674                                 <listitem><para>Takes a string
 675                                 value. Controls the TCP congestion
 676                                 algorithm used by this socket. Should
 677                                 be one of "westwood", "veno", "cubic",
 678                                 "lp" or any other available algorithm
 679                                 supported by the IP stack. This
 680                                 setting applies only to stream
 681                                 sockets.</para></listitem>
 682                         </varlistentry>
 683
 684                         <varlistentry>
 685                                 <term><varname>ExecStartPre=</varname></term>
 686                                 <term><varname>ExecStartPost=</varname></term>
 687                                 <listitem><para>Takes one or more
 688                                 command lines, which are executed
 689                                 before or after the listening
 690                                 sockets/FIFOs are created and
 691                                 bound, respectively. The first token of the command
 692                                 line must be an absolute filename,
 693                                 then followed by arguments for the
 694                                 process. Multiple command lines may be
 695                                 specified following the same scheme as
 696                                 used for
 697                                 <varname>ExecStartPre=</varname> of
 698                                 service unit files.</para></listitem>
 699                         </varlistentry>
 700
 701                         <varlistentry>
 702                                 <term><varname>ExecStopPre=</varname></term>
 703                                 <term><varname>ExecStopPost=</varname></term>
 704                                 <listitem><para>Additional commands
 705                                 that are executed before or after
 706                                 the listening sockets/FIFOs are closed
 707                                 and removed, respectively. Multiple command lines
 708                                 may be specified following the same
 709                                 scheme as used for
 710                                 <varname>ExecStartPre=</varname> of
 711                                 service unit files.</para></listitem>
 712                         </varlistentry>
 713
 714                         <varlistentry>
 715                                 <term><varname>TimeoutSec=</varname></term>
 716                                 <listitem><para>Configures the time to
 717                                 wait for the commands specified in
 718                                 <varname>ExecStartPre=</varname>,
 719                                 <varname>ExecStartPost=</varname>,
 720                                 <varname>ExecStopPre=</varname> and
 721                                 <varname>ExecStopPost=</varname> to
 722                                 finish. If a command does not exit
 723                                 within the configured time, the socket
 724                                 will be considered failed and be shut
 725                                 down again. All commands still running,
 726                                 will be terminated forcibly via
 727                                 <constant>SIGTERM</constant>, and after another delay of
 728                                 this time with <constant>SIGKILL</constant>. (See
 729                                 <option>KillMode=</option> in <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>.)
 730                                 Takes a unit-less value in seconds, or
 731                                 a time span value such as "5min
 732                                 20s". Pass 0 to disable the timeout
 733                                 logic. Defaults to <varname>TimeoutStartSec=</varname> from the
 734                                 manager configuration file.</para></listitem>
 735                         </varlistentry>
 736
 737                         <varlistentry>
 738                                 <term><varname>Service=</varname></term>
 739                                 <listitem><para>Specifies the service
 740                                 unit name to activate on incoming
 741                                 traffic. This setting is only allowed
 742                                 for sockets with
 743                                 <varname>Accept=no</varname>. It
 744                                 defaults to the service that bears the
 745                                 same name as the socket (with the
 746                                 suffix replaced). In most cases, it
 747                                 should not be necessary to use this
 748                                 option.</para></listitem>
 749                         </varlistentry>
 750
 751                         <varlistentry>
 752                                 <term><varname>RemoveOnStop=</varname></term>
 753                                 <listitem><para>Takes a boolean
 754                                 argument. If enabled, any file nodes
 755                                 created by this socket unit are
 756                                 removed when it is stopped. This
 757                                 applies to AF_UNIX sockets in the file
 758                                 system, POSIX message queues, FIFOs,
 759                                 as well as any symlinks to
 760                                 them configured with
 761                                 <varname>Symlinks=</varname>. Normally,
 762                                 it should not be necessary to use this
 763                                 option, and is not recommended as
 764                                 services might continue to run after
 765                                 the socket unit has been terminated
 766                                 and it should still be possible to
 767                                 communicate with them via their file
 768                                 system node. Defaults to
 769                                 off.</para></listitem>
 770                         </varlistentry>
 771
 772                         <varlistentry>
 773                                 <term><varname>Symlinks=</varname></term>
 774                                 <listitem><para>Takes a list of file
 775                                 system paths. The specified paths will
 776                                 be created as symlinks to the AF_UNIX
 777                                 socket path or FIFO path of this
 778                                 socket unit.  If this setting is used,
 779                                 only one AF_UNIX socket in the file
 780                                 system or one FIFO may be configured
 781                                 for the socket unit. Use this option
 782                                 to manage one or more symlinked alias
 783                                 names for a socket, binding their
 784                                 lifecycle together. Defaults to the
 785                                 empty list.</para></listitem>
 786                         </varlistentry>
 787
 788                 </variablelist>
 789
 790                 <para>Check
 791                 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 792                 and
 793                 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 794                 for more settings.</para>
 795
 796         </refsect1>
 797
 798         <refsect1>
 799                   <title>See Also</title>
 800                   <para>
 801                           <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 802                           <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 803                           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 804                           <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 805                           <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 806                           <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 807                           <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 808                           <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 809                   </para>
 810
 811                   <para>
 812                           For more extensive descriptions see the "systemd for Developers" series:
 813                           <ulink url="http://0pointer.de/blog/projects/socket-activation.html">Socket Activation</ulink>,
 814                           <ulink url="http://0pointer.de/blog/projects/socket-activation2.html">Socket Activation, part II</ulink>,
 815                           <ulink url="http://0pointer.de/blog/projects/inetd.html">Converting inetd Services</ulink>,
 816                           <ulink url="http://0pointer.de/blog/projects/socket-activated-containers.html">Socket Activated Internet Services and OS Containers</ulink>.
 817                   </para>
 818         </refsect1>
 819
 820 </refentry>