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
  94                 <option>Service=</option> option described below.
  95                 Depending on the setting of <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 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>DirectoryMode=</varname></term>
 376                                 <listitem><para>If listening on a file
 377                                 system socket or FIFO, the parent
 378                                 directories are automatically created
 379                                 if needed. This option specifies the
 380                                 file system access mode used when
 381                                 creating these directories. Takes an
 382                                 access mode in octal
 383                                 notation. Defaults to
 384                                 0755.</para></listitem>
 385                         </varlistentry>
 386
 387                         <varlistentry>
 388                                 <term><varname>SocketMode=</varname></term>
 389                                 <listitem><para>If listening on a file
 390                                 system socket or FIFO, this option
 391                                 specifies the file system access mode
 392                                 used when creating the file
 393                                 node. Takes an access mode in octal
 394                                 notation. Defaults to
 395                                 0666.</para></listitem>
 396                         </varlistentry>
 397
 398                         <varlistentry>
 399                                 <term><varname>Accept=</varname></term>
 400                                 <listitem><para>Takes a boolean
 401                                 argument. If true, a service instance
 402                                 is spawned for each incoming
 403                                 connection and only the connection
 404                                 socket is passed to it. If false, all
 405                                 listening sockets themselves are
 406                                 passed to the started service unit,
 407                                 and only one service unit is spawned
 408                                 for all connections (also see
 409                                 above). This value is ignored for
 410                                 datagram sockets and FIFOs where a
 411                                 single service unit unconditionally
 412                                 handles all incoming traffic. Defaults
 413                                 to <option>false</option>. For
 414                                 performance reasons, it is recommended
 415                                 to write new daemons only in a way
 416                                 that is suitable for
 417                                 <option>Accept=false</option>. A
 418                                 daemon listening on an <constant>AF_UNIX</constant> socket
 419                                 may, but does not need to, call
 420                                 <citerefentry><refentrytitle>close</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 421                                 on the received socket before
 422                                 exiting. However, it must not unlink
 423                                 the socket from a file system. It
 424                                 should not invoke
 425                                 <citerefentry><refentrytitle>shutdown</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 426                                 on sockets it got with
 427                                 <varname>Accept=false</varname>, but
 428                                 it may do so for sockets it got with
 429                                 <varname>Accept=true</varname> set.
 430                                 Setting <varname>Accept=true</varname>
 431                                 is mostly useful to allow daemons
 432                                 designed for usage with
 433                                 <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 434                                 to work unmodified with systemd socket
 435                                 activation.</para></listitem>
 436                         </varlistentry>
 437
 438                         <varlistentry>
 439                                 <term><varname>MaxConnections=</varname></term>
 440                                 <listitem><para>The maximum number of
 441                                 connections to simultaneously run
 442                                 services instances for, when
 443                                 <option>Accept=true</option> is
 444                                 set. If more concurrent connections
 445                                 are coming in, they will be refused
 446                                 until at least one existing connection
 447                                 is terminated. This setting has no
 448                                 effect on sockets configured with
 449                                 <option>Accept=false</option> or datagram
 450                                 sockets. Defaults to
 451                                 64.</para></listitem>
 452                         </varlistentry>
 453
 454                         <varlistentry>
 455                                 <term><varname>KeepAlive=</varname></term>
 456                                 <listitem><para>Takes a boolean
 457                                 argument. If true, the TCP/IP stack
 458                                 will send a keep alive message after
 459                                 2h (depending on the configuration of
 460                                 <filename>/proc/sys/net/ipv4/tcp_keepalive_time</filename>)
 461                                 for all TCP streams accepted on this
 462                                 socket. This controls the SO_KEEPALIVE
 463                                 socket option (see
 464                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 465                                 and the <ulink
 466                                 url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
 467                                 Keepalive HOWTO</ulink> for details.)
 468                                 Defaults to
 469                                 <option>false</option>.</para></listitem>
 470                         </varlistentry>
 471
 472                         <varlistentry>
 473                                 <term><varname>Priority=</varname></term>
 474                                 <listitem><para>Takes an integer
 475                                 argument controlling the priority for
 476                                 all traffic sent from this
 477                                 socket. This controls the SO_PRIORITY
 478                                 socket option (see
 479                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 480                                 for details.).</para></listitem>
 481                         </varlistentry>
 482
 483                         <varlistentry>
 484                                 <term><varname>ReceiveBuffer=</varname></term>
 485                                 <term><varname>SendBuffer=</varname></term>
 486                                 <listitem><para>Takes an integer
 487                                 argument controlling the receive
 488                                 or send buffer sizes of this
 489                                 socket, respectively. This controls the SO_RCVBUF
 490                                 and SO_SNDBUF socket options (see
 491                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 492                                 for details.).</para></listitem>
 493                         </varlistentry>
 494
 495                         <varlistentry>
 496                                 <term><varname>IPTOS=</varname></term>
 497                                 <listitem><para>Takes an integer
 498                                 argument controlling the IP
 499                                 Type-Of-Service field for packets
 500                                 generated from this socket. This
 501                                 controls the IP_TOS socket option (see
 502                                 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 503                                 for details.). Either a numeric string
 504                                 or one of <option>low-delay</option>,
 505                                 <option>throughput</option>,
 506                                 <option>reliability</option> or
 507                                 <option>low-cost</option> may be
 508                                 specified.</para></listitem>
 509                         </varlistentry>
 510
 511                         <varlistentry>
 512                                 <term><varname>IPTTL=</varname></term>
 513                                 <listitem><para>Takes an integer
 514                                 argument controlling the IPv4
 515                                 Time-To-Live/IPv6 Hop-Count field for
 516                                 packets generated from this
 517                                 socket. This sets the
 518                                 IP_TTL/IPV6_UNICAST_HOPS socket
 519                                 options (see
 520                                 <citerefentry><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 521                                 and
 522                                 <citerefentry><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 523                                 for details.)</para></listitem>
 524                         </varlistentry>
 525
 526                         <varlistentry>
 527                                 <term><varname>Mark=</varname></term>
 528                                 <listitem><para>Takes an integer
 529                                 value. Controls the firewall mark of
 530                                 packets generated by this socket. This
 531                                 can be used in the firewall logic to
 532                                 filter packets from this socket. This
 533                                 sets the SO_MARK socket option. See
 534                                 <citerefentry><refentrytitle>iptables</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 535                                 for details.</para></listitem>
 536                         </varlistentry>
 537
 538                         <varlistentry>
 539                                 <term><varname>ReusePort=</varname></term>
 540                                 <listitem><para>Takes a boolean
 541                                 value. If true, allows multiple <citerefentry><refentrytitle>bind</refentrytitle><manvolnum>2</manvolnum></citerefentry>s
 542                                 to this TCP or UDP port.  This
 543                                 controls the SO_REUSEPORT socket
 544                                 option.  See
 545                                 <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 546                                 for details.</para></listitem>
 547                         </varlistentry>
 548
 549                         <varlistentry>
 550                                 <term><varname>SmackLabel=</varname></term>
 551                                 <term><varname>SmackLabelIPIn=</varname></term>
 552                                 <term><varname>SmackLabelIPOut=</varname></term>
 553                                 <listitem><para>Takes a string
 554                                 value. Controls the extended
 555                                 attributes
 556                                 <literal>security.SMACK64</literal>,
 557                                 <literal>security.SMACK64IPIN</literal>
 558                                 and
 559                                 <literal>security.SMACK64IPOUT</literal>,
 560                                 respectively, i.e. the security label
 561                                 of the FIFO, or the security label for
 562                                 the incoming or outgoing connections
 563                                 of the socket, respectively. See
 564                                 <ulink
 565                                 url="https://www.kernel.org/doc/Documentation/security/Smack.txt">Smack.txt</ulink>
 566                                 for details.</para></listitem>
 567                         </varlistentry>
 568
 569                         <varlistentry>
 570                                 <term><varname>PipeSize=</varname></term>
 571                                 <listitem><para>Takes an integer
 572                                 value. Controls the pipe buffer size
 573                                 of FIFOs configured in this socket
 574                                 unit. See
 575                                 <citerefentry><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 576                                 for details.</para></listitem>
 577                         </varlistentry>
 578
 579                         <varlistentry>
 580                                 <term><varname>MessageQueueMaxMessages=</varname>,
 581                                 <varname>MessageQueueMessageSize=</varname></term>
 582                                 <listitem><para>These two settings
 583                                 take integer values and control the
 584                                 mq_maxmsg field or the mq_msgsize field, respectively, when
 585                                 creating the message queue. Note that
 586                                 either none or both of these variables
 587                                 need to be set. See
 588                                 <citerefentry><refentrytitle>mq_setattr</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 589                                 for details.</para></listitem>
 590                         </varlistentry>
 591
 592                         <varlistentry>
 593                                 <term><varname>FreeBind=</varname></term>
 594                                 <listitem><para>Takes a boolean
 595                                 value. Controls whether the socket can
 596                                 be bound to non-local IP
 597                                 addresses. This is useful to configure
 598                                 sockets listening on specific IP
 599                                 addresses before those IP addresses
 600                                 are successfully configured on a
 601                                 network interface. This sets the
 602                                 IP_FREEBIND socket option. For
 603                                 robustness reasons it is recommended
 604                                 to use this option whenever you bind a
 605                                 socket to a specific IP
 606                                 address. Defaults to <option>false</option>.</para></listitem>
 607                         </varlistentry>
 608
 609                         <varlistentry>
 610                                 <term><varname>Transparent=</varname></term>
 611                                 <listitem><para>Takes a boolean
 612                                 value. Controls the IP_TRANSPARENT
 613                                 socket option. Defaults to
 614                                 <option>false</option>.</para></listitem>
 615                         </varlistentry>
 616
 617                         <varlistentry>
 618                                 <term><varname>Broadcast=</varname></term>
 619                                 <listitem><para>Takes a boolean
 620                                 value. This controls the SO_BROADCAST
 621                                 socket option, which allows broadcast
 622                                 datagrams to be sent from this
 623                                 socket. Defaults to
 624                                 <option>false</option>.</para></listitem>
 625                         </varlistentry>
 626
 627                         <varlistentry>
 628                                 <term><varname>PassCredentials=</varname></term>
 629                                 <listitem><para>Takes a boolean
 630                                 value. This controls the SO_PASSCRED
 631                                 socket option, which allows <constant>AF_UNIX</constant> sockets to
 632                                 receive the credentials of the sending
 633                                 process in an ancillary message.
 634                                 Defaults to
 635                                 <option>false</option>.</para></listitem>
 636                         </varlistentry>
 637
 638                         <varlistentry>
 639                                 <term><varname>PassSecurity=</varname></term>
 640                                 <listitem><para>Takes a boolean
 641                                 value. This controls the SO_PASSSEC
 642                                 socket option, which allows <constant>AF_UNIX</constant>
 643                                 sockets to receive the security
 644                                 context of the sending process in an
 645                                 ancillary message. Defaults to
 646                                 <option>false</option>.</para></listitem>
 647                         </varlistentry>
 648
 649                         <varlistentry>
 650                                 <term><varname>TCPCongestion=</varname></term>
 651                                 <listitem><para>Takes a string
 652                                 value. Controls the TCP congestion
 653                                 algorithm used by this socket. Should
 654                                 be one of "westwood", "veno", "cubic",
 655                                 "lp" or any other available algorithm
 656                                 supported by the IP stack. This
 657                                 setting applies only to stream
 658                                 sockets.</para></listitem>
 659                         </varlistentry>
 660
 661                         <varlistentry>
 662                                 <term><varname>ExecStartPre=</varname></term>
 663                                 <term><varname>ExecStartPost=</varname></term>
 664                                 <listitem><para>Takes one or more
 665                                 command lines, which are executed
 666                                 before or after the listening
 667                                 sockets/FIFOs are created and
 668                                 bound, respectively. The first token of the command
 669                                 line must be an absolute filename,
 670                                 then followed by arguments for the
 671                                 process. Multiple command lines may be
 672                                 specified following the same scheme as
 673                                 used for
 674                                 <varname>ExecStartPre=</varname> of
 675                                 service unit files.</para></listitem>
 676                         </varlistentry>
 677
 678                         <varlistentry>
 679                                 <term><varname>ExecStopPre=</varname></term>
 680                                 <term><varname>ExecStopPost=</varname></term>
 681                                 <listitem><para>Additional commands
 682                                 that are executed before or after
 683                                 the listening sockets/FIFOs are closed
 684                                 and removed, respectively. Multiple command lines
 685                                 may be specified following the same
 686                                 scheme as used for
 687                                 <varname>ExecStartPre=</varname> of
 688                                 service unit files.</para></listitem>
 689                         </varlistentry>
 690
 691                         <varlistentry>
 692                                 <term><varname>TimeoutSec=</varname></term>
 693                                 <listitem><para>Configures the time to
 694                                 wait for the commands specified in
 695                                 <varname>ExecStartPre=</varname>,
 696                                 <varname>ExecStartPost=</varname>,
 697                                 <varname>ExecStopPre=</varname> and
 698                                 <varname>ExecStopPost=</varname> to
 699                                 finish. If a command does not exit
 700                                 within the configured time, the socket
 701                                 will be considered failed and be shut
 702                                 down again. All commands still running,
 703                                 will be terminated forcibly via
 704                                 <constant>SIGTERM</constant>, and after another delay of
 705                                 this time with <constant>SIGKILL</constant>. (See
 706                                 <option>KillMode=</option> in <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>.)
 707                                 Takes a unit-less value in seconds, or
 708                                 a time span value such as "5min
 709                                 20s". Pass 0 to disable the timeout
 710                                 logic. Defaults to <varname>TimeoutStartSec=</varname> from the
 711                                 manager configuration file.</para></listitem>
 712                         </varlistentry>
 713
 714                         <varlistentry>
 715                                 <term><varname>Service=</varname></term>
 716                                 <listitem><para>Specifies the service
 717                                 unit name to activate on incoming
 718                                 traffic. This setting is only allowed
 719                                 for sockets with
 720                                 <varname>Accept=no</varname>. It
 721                                 defaults to the service that bears the
 722                                 same name as the socket (with the
 723                                 suffix replaced). In most cases, it
 724                                 should not be necessary to use this
 725                                 option.</para></listitem>
 726                         </varlistentry>
 727
 728                 </variablelist>
 729
 730                 <para>Check
 731                 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 732                 and
 733                 <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 734                 for more settings.</para>
 735
 736         </refsect1>
 737
 738         <refsect1>
 739                   <title>See Also</title>
 740                   <para>
 741                           <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 742                           <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 743                           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 744                           <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 745                           <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 746                           <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 747                           <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 748                           <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 749                   </para>
 750
 751                   <para>
 752                           For more extensive descriptions see the "systemd for Developers" series:
 753                           <ulink url="http://0pointer.de/blog/projects/socket-activation.html">Socket Activation</ulink>,
 754                           <ulink url="http://0pointer.de/blog/projects/socket-activation2.html">Socket Activation, part II</ulink>,
 755                           <ulink url="http://0pointer.de/blog/projects/inetd.html">Converting inetd Services</ulink>,
 756                           <ulink url="http://0pointer.de/blog/projects/socket-activated-containers.html">Socket Activated Internet Services and OS Containers</ulink>.
 757                   </para>
 758         </refsect1>
 759
 760 </refentry>