Adding upstream version 241.upstream/241upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 875 insertions, 0 deletions

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+<?xml version='1.0'?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  SPDX-License-Identifier: LGPL-2.1+
+-->
+
+<refentry id="systemd.socket">
+  <refentryinfo>
+    <title>systemd.socket</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd.socket</refentrytitle>
+    <manvolnum>5</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd.socket</refname>
+    <refpurpose>Socket unit configuration</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <para><filename><replaceable>socket</replaceable>.socket</filename></para>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para>A unit configuration file whose name ends in
+    <literal>.socket</literal> encodes information about an IPC or
+    network socket or a file system FIFO controlled and supervised by
+    systemd, for socket-based activation.</para>
+
+    <para>This man page lists the configuration options specific to
+    this unit type. See
+    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    for the common options of all unit configuration files. The common
+    configuration items are configured in the generic [Unit] and
+    [Install] sections. The socket specific configuration options are
+    configured in the [Socket] section.</para>
+
+    <para>Additional options are listed in
+    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+    which define the execution environment the
+    <option>ExecStartPre=</option>, <option>ExecStartPost=</option>,
+    <option>ExecStopPre=</option> and <option>ExecStopPost=</option>
+    commands are executed in, and in
+    <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+    which define the way the processes are terminated, and in
+    <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+    which configure resource control settings for the processes of the
+    socket.</para>
+
+    <para>For each socket file, a matching service file must exist,
+    describing the service to start on incoming traffic on the socket
+    (see
+    <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    for more information about .service files). The name of the
+    .service unit is by default the same as the name of the .socket
+    unit, but can be altered with the <option>Service=</option> option
+    described below. Depending on the setting of the
+    <option>Accept=</option> option described below, this .service
+    unit must either be named like the .socket unit, but with the
+    suffix replaced, unless overridden with <option>Service=</option>;
+    or it must be a template unit named the same way. Example: a
+    socket file <filename>foo.socket</filename> needs a matching
+    service <filename>foo.service</filename> if
+    <option>Accept=no</option> is set. If
+    <option>Accept=yes</option> is set, a service template file
+    <filename>foo@.service</filename> must exist from which services
+    are instantiated for each incoming connection.</para>
+
+    <para>No implicit <varname>WantedBy=</varname> or
+    <varname>RequiredBy=</varname> dependency from the socket to the
+    service is added. This means that the service may be started
+    without the socket, in which case it must be able to open sockets
+    by itself. To prevent this, an explicit
+    <varname>Requires=</varname> dependency may be added.</para>
+
+    <para>Socket units may be used to implement on-demand starting of
+    services, as well as parallelized starting of services. See the
+    blog stories linked at the end for an introduction.</para>
+
+    <para>Note that the daemon software configured for socket
+    activation with socket units needs to be able to accept sockets
+    from systemd, either via systemd's native socket passing interface
+    (see
+    <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    for details) or via the traditional
+    <citerefentry project='freebsd'><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>-style
+    socket passing (i.e. sockets passed in via standard input and
+    output, using <varname>StandardInput=socket</varname> in the
+    service file).</para>
+
+    <para>All network sockets allocated through <filename>.socket</filename> units are allocated in the host's network
+    namespace (see <citerefentry
+    project='man-pages'><refentrytitle>network_namespaces</refentrytitle><manvolnum>7</manvolnum></citerefentry>). This
+    does not mean however that the service activated by a configured socket unit has to be part of the host's network
+    namespace as well.  It is supported and even good practice to run services in their own network namespace (for
+    example through <varname>PrivateNetwork=</varname>, see
+    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>), receiving only
+    the sockets configured through socket-activation from the host's namespace. In such a set-up communication within
+    the host's network namespace is only permitted through the activation sockets passed in while all sockets allocated
+    from the service code itself will be associated with the service's own namespace, and thus possibly subject to a a
+    much more restrictive configuration.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Automatic Dependencies</title>
+
+    <refsect2>
+      <title>Implicit Dependencies</title>
+
+      <para>The following dependencies are implicitly added:</para>
+
+      <itemizedlist>
+        <listitem><para>Socket units automatically gain a <varname>Before=</varname>
+        dependency on the service units they activate.</para></listitem>
+
+        <listitem><para>Socket units referring to file system paths (such as AF_UNIX
+        sockets or FIFOs) implicitly gain <varname>Requires=</varname> and
+        <varname>After=</varname> dependencies on all mount units
+        necessary to access those paths.</para></listitem>
+
+        <listitem><para>Socket units using the <varname>BindToDevice=</varname>
+        setting automatically gain a <varname>BindsTo=</varname> and
+        <varname>After=</varname> dependency on the device unit
+        encapsulating the specified network interface.</para></listitem>
+      </itemizedlist>
+
+      <para>Additional implicit dependencies may be added as result of
+      execution and resource control parameters as documented in
+      <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      and
+      <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+    </refsect2>
+
+    <refsect2>
+      <title>Default Dependencies</title>
+
+      <para>The following dependencies are added unless
+      <varname>DefaultDependencies=no</varname> is set:</para>
+
+      <itemizedlist>
+        <listitem><para>Socket units automatically gain a
+        <varname>Before=</varname> dependency on
+        <filename>sockets.target</filename>.</para></listitem>
+
+        <listitem><para>Socket units automatically gain a pair of
+        <varname>After=</varname> and <varname>Requires=</varname>
+        dependency on <filename>sysinit.target</filename>, and a pair of
+        <varname>Before=</varname> and <varname>Conflicts=</varname>
+        dependencies on <filename>shutdown.target</filename>. These
+        dependencies ensure that the socket unit is started before normal
+        services at boot, and is stopped on shutdown. Only sockets
+        involved with early boot or late system shutdown should disable
+        <varname>DefaultDependencies=</varname> option.</para></listitem>
+      </itemizedlist>
+    </refsect2>
+  </refsect1>
+
+  <refsect1>
+    <title>Options</title>
+
+    <para>Socket files must include a [Socket] section, which carries
+    information about the socket or FIFO it supervises. A number of
+    options that may be used in this section are shared with other
+    unit types. These options are documented in
+    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    and
+    <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+    The options specific to the [Socket] section of socket units are
+    the following:</para>
+
+    <variablelist class='unit-directives'>
+      <varlistentry>
+        <term><varname>ListenStream=</varname></term>
+        <term><varname>ListenDatagram=</varname></term>
+        <term><varname>ListenSequentialPacket=</varname></term>
+        <listitem><para>Specifies an address to listen on for a stream
+        (<constant>SOCK_STREAM</constant>), datagram
+        (<constant>SOCK_DGRAM</constant>), or sequential packet
+        (<constant>SOCK_SEQPACKET</constant>) socket, respectively.
+        The address can be written in various formats:</para>
+
+        <para>If the address starts with a slash
+        (<literal>/</literal>), it is read as file system socket in
+        the <constant>AF_UNIX</constant> socket family.</para>
+
+        <para>If the address starts with an at symbol
+        (<literal>@</literal>), it is read as abstract namespace
+        socket in the <constant>AF_UNIX</constant> family. The
+        <literal>@</literal> is replaced with a
+        <constant>NUL</constant> character before binding. For
+        details, see
+        <citerefentry project='man-pages'><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
+
+        <para>If the address string is a single number, it is read as
+        port number to listen on via IPv6. Depending on the value of
+        <varname>BindIPv6Only=</varname> (see below) this might result
+        in the service being available via both IPv6 and IPv4
+        (default) or just via IPv6.
+        </para>
+
+        <para>If the address string is a string in the format
+        v.w.x.y:z, it is read as IPv4 specifier for listening on an
+        address v.w.x.y on a port z.</para>
+
+        <para>If the address string is a string in the format [x]:y,
+        it is read as IPv6 address x on a port y. Note that this might
+        make the service available via IPv4, too, depending on the
+        <varname>BindIPv6Only=</varname> setting (see below).
+        </para>
+
+        <para>If the address string is a string in the format
+        <literal>vsock:x:y</literal>, it is read as CID <literal>x</literal> on
+        a port <literal>y</literal> address in the
+        <constant>AF_VSOCK</constant> family.  The CID is a unique 32-bit
+        integer identifier in <constant>AF_VSOCK</constant> analogous to an IP
+        address.  Specifying the CID is optional, and may be set to the empty
+        string.</para>
+
+        <para>Note that <constant>SOCK_SEQPACKET</constant> (i.e.
+        <varname>ListenSequentialPacket=</varname>) is only available
+        for <constant>AF_UNIX</constant> sockets.
+        <constant>SOCK_STREAM</constant> (i.e.
+        <varname>ListenStream=</varname>) when used for IP sockets
+        refers to TCP sockets, <constant>SOCK_DGRAM</constant> (i.e.
+        <varname>ListenDatagram=</varname>) to UDP.</para>
+
+        <para>These options may be specified more than once, in which
+        case incoming traffic on any of the sockets will trigger
+        service activation, and all listed sockets will be passed to
+        the service, regardless of whether there is incoming traffic
+        on them or not. If the empty string is assigned to any of
+        these options, the list of addresses to listen on is reset,
+        all prior uses of any of these options will have no
+        effect.</para>
+
+        <para>It is also possible to have more than one socket unit
+        for the same service when using <varname>Service=</varname>,
+        and the service will receive all the sockets configured in all
+        the socket units. Sockets configured in one unit are passed in
+        the order of configuration, but no ordering between socket
+        units is specified.</para>
+
+        <para>If an IP address is used here, it is often desirable to
+        listen on it before the interface it is configured on is up
+        and running, and even regardless of whether it will be up and
+        running at any point. To deal with this, it is recommended to
+        set the <varname>FreeBind=</varname> option described
+        below.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ListenFIFO=</varname></term>
+        <listitem><para>Specifies a file system FIFO to listen on.
+        This expects an absolute file system path as argument.
+        Behavior otherwise is very similar to the
+        <varname>ListenDatagram=</varname> directive
+        above.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ListenSpecial=</varname></term>
+        <listitem><para>Specifies a special file in the file system to
+        listen on. This expects an absolute file system path as
+        argument. Behavior otherwise is very similar to the
+        <varname>ListenFIFO=</varname> directive above. Use this to
+        open character device nodes as well as special files in
+        <filename>/proc</filename> and
+        <filename>/sys</filename>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ListenNetlink=</varname></term>
+        <listitem><para>Specifies a Netlink family to create a socket
+        for to listen on. This expects a short string referring to the
+        <constant>AF_NETLINK</constant> family name (such as
+        <varname>audit</varname> or <varname>kobject-uevent</varname>)
+        as argument, optionally suffixed by a whitespace followed by a
+        multicast group integer. Behavior otherwise is very similar to
+        the <varname>ListenDatagram=</varname> directive
+        above.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ListenMessageQueue=</varname></term>
+        <listitem><para>Specifies a POSIX message queue name to listen
+        on. This expects a valid message queue name (i.e. beginning
+        with /). Behavior otherwise is very similar to the
+        <varname>ListenFIFO=</varname> directive above. On Linux
+        message queue descriptors are actually file descriptors and
+        can be inherited between processes.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ListenUSBFunction=</varname></term>
+        <listitem><para>Specifies a <ulink
+        url="https://www.kernel.org/doc/Documentation/usb/functionfs.txt">USB
+        FunctionFS</ulink> endpoints location to listen on, for
+        implementation of USB gadget functions. This expects an
+        absolute file system path of functionfs mount point as the argument.
+        Behavior otherwise is very similar to the <varname>ListenFIFO=</varname>
+        directive above. Use this to open the FunctionFS endpoint
+        <filename>ep0</filename>. When using this option, the
+        activated service has to have the
+        <varname>USBFunctionDescriptors=</varname> and
+        <varname>USBFunctionStrings=</varname> options set.
+        </para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>SocketProtocol=</varname></term>
+        <listitem><para>Takes one of <option>udplite</option>
+        or <option>sctp</option>. Specifies a socket protocol
+        (<constant>IPPROTO_UDPLITE</constant>) UDP-Lite
+        (<constant>IPPROTO_SCTP</constant>) SCTP socket respectively. </para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>BindIPv6Only=</varname></term>
+        <listitem><para>Takes one of <option>default</option>,
+        <option>both</option> or <option>ipv6-only</option>. Controls
+        the IPV6_V6ONLY socket option (see
+        <citerefentry project='die-net'><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        for details). If <option>both</option>, IPv6 sockets bound
+        will be accessible via both IPv4 and IPv6. If
+        <option>ipv6-only</option>, they will be accessible via IPv6
+        only. If <option>default</option> (which is the default,
+        surprise!), the system wide default setting is used, as
+        controlled by
+        <filename>/proc/sys/net/ipv6/bindv6only</filename>, which in
+        turn defaults to the equivalent of
+        <option>both</option>.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Backlog=</varname></term>
+        <listitem><para>Takes an unsigned integer argument. Specifies
+        the number of connections to queue that have not been accepted
+        yet. This setting matters only for stream and sequential
+        packet sockets. See
+        <citerefentry><refentrytitle>listen</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+        for details. Defaults to SOMAXCONN (128).</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>BindToDevice=</varname></term>
+        <listitem><para>Specifies a network interface name to bind
+        this socket to. If set, traffic will only be accepted from the
+        specified network interfaces. This controls the
+        SO_BINDTODEVICE socket option (see <citerefentry
+        project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        for details). If this option is used, an implicit dependency
+        from this socket unit on the network interface device unit
+        (<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        is created. Note that setting this parameter might result in
+        additional dependencies to be added to the unit (see
+        above).</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>SocketUser=</varname></term>
+        <term><varname>SocketGroup=</varname></term>
+
+        <listitem><para>Takes a UNIX user/group name. When specified,
+        all AF_UNIX sockets and FIFO nodes in the file system are
+        owned by the specified user and group. If unset (the default),
+        the nodes are owned by the root user/group (if run in system
+        context) or the invoking user/group (if run in user context).
+        If only a user is specified but no group, then the group is
+        derived from the user's default group.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>SocketMode=</varname></term>
+        <listitem><para>If listening on a file system socket or FIFO,
+        this option specifies the file system access mode used when
+        creating the file node. Takes an access mode in octal
+        notation. Defaults to 0666.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>DirectoryMode=</varname></term>
+        <listitem><para>If listening on a file system socket or FIFO,
+        the parent directories are automatically created if needed.
+        This option specifies the file system access mode used when
+        creating these directories. Takes an access mode in octal
+        notation. Defaults to 0755.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Accept=</varname></term>
+        <listitem><para>Takes a boolean argument. If true, a service
+        instance is spawned for each incoming connection and only the
+        connection socket is passed to it. If false, all listening
+        sockets themselves are passed to the started service unit, and
+        only one service unit is spawned for all connections (also see
+        above). This value is ignored for datagram sockets and FIFOs
+        where a single service unit unconditionally handles all
+        incoming traffic. Defaults to <option>false</option>. For
+        performance reasons, it is recommended to write new daemons
+        only in a way that is suitable for
+        <option>Accept=no</option>. A daemon listening on an
+        <constant>AF_UNIX</constant> socket may, but does not need to,
+        call
+        <citerefentry><refentrytitle>close</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+        on the received socket before exiting. However, it must not
+        unlink the socket from a file system. It should not invoke
+        <citerefentry><refentrytitle>shutdown</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+        on sockets it got with <varname>Accept=no</varname>, but it
+        may do so for sockets it got with
+        <varname>Accept=yes</varname> set. Setting
+        <varname>Accept=yes</varname> is mostly useful to allow
+        daemons designed for usage with
+        <citerefentry project='freebsd'><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+        to work unmodified with systemd socket
+        activation.</para>
+
+        <para>For IPv4 and IPv6 connections, the <varname>REMOTE_ADDR</varname>
+        environment variable will contain the remote IP address, and <varname>REMOTE_PORT</varname>
+        will contain the remote port. This is the same as the format used by CGI.
+        For SOCK_RAW, the port is the IP protocol.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Writable=</varname></term>
+        <listitem><para>Takes a boolean argument. May only be used in
+        conjunction with <varname>ListenSpecial=</varname>. If true,
+        the specified special file is opened in read-write mode, if
+        false, in read-only mode. Defaults to false.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>MaxConnections=</varname></term>
+        <listitem><para>The maximum number of connections to
+        simultaneously run services instances for, when
+        <option>Accept=yes</option> is set. If more concurrent
+        connections are coming in, they will be refused until at least
+        one existing connection is terminated. This setting has no
+        effect on sockets configured with
+        <option>Accept=no</option> or datagram sockets. Defaults to
+        64.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>MaxConnectionsPerSource=</varname></term>
+        <listitem><para>The maximum number of connections for a service per source IP address.
+        This is very similar to the <varname>MaxConnections=</varname> directive
+        above. Disabled by default.</para>
+        </listitem>
+      </varlistentry>
+
+       <varlistentry>
+        <term><varname>KeepAlive=</varname></term>
+        <listitem><para>Takes a boolean argument. If true, the TCP/IP
+        stack will send a keep alive message after 2h (depending on
+        the configuration of
+        <filename>/proc/sys/net/ipv4/tcp_keepalive_time</filename>)
+        for all TCP streams accepted on this socket. This controls the
+        SO_KEEPALIVE socket option (see
+        <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        and the <ulink
+        url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
+        Keepalive HOWTO</ulink> for details.) Defaults to
+        <option>false</option>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>KeepAliveTimeSec=</varname></term>
+        <listitem><para>Takes time (in seconds) as argument. The connection needs to remain
+        idle before TCP starts sending keepalive probes. This controls the TCP_KEEPIDLE
+        socket option (see
+        <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        and the <ulink
+        url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
+        Keepalive HOWTO</ulink> for details.)
+        Defaults value is 7200 seconds (2 hours).</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>KeepAliveIntervalSec=</varname></term>
+        <listitem><para>Takes time (in seconds) as argument between
+        individual keepalive probes, if the socket option SO_KEEPALIVE
+        has been set on this socket. This controls
+        the TCP_KEEPINTVL socket option (see
+        <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        and the <ulink
+        url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
+        Keepalive HOWTO</ulink> for details.) Defaults value is 75
+        seconds.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>KeepAliveProbes=</varname></term>
+        <listitem><para>Takes an integer as argument. It is the number of
+        unacknowledged probes to send before considering the
+        connection dead and notifying the application layer. This
+        controls the TCP_KEEPCNT socket option (see
+        <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        and the <ulink
+        url="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/">TCP
+        Keepalive HOWTO</ulink> for details.) Defaults value is
+        9.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>NoDelay=</varname></term>
+        <listitem><para>Takes a boolean argument. TCP Nagle's
+        algorithm works by combining a number of small outgoing
+        messages, and sending them all at once. This controls the
+        TCP_NODELAY socket option (see
+        <citerefentry project='die-net'><refentrytitle>tcp</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        Defaults to <option>false</option>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Priority=</varname></term>
+        <listitem><para>Takes an integer argument controlling the
+        priority for all traffic sent from this socket. This controls
+        the SO_PRIORITY socket option (see
+        <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        for details.).</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>DeferAcceptSec=</varname></term>
+
+        <listitem><para>Takes time (in seconds) as argument. If set,
+        the listening process will be awakened only when data arrives
+        on the socket, and not immediately when connection is
+        established. When this option is set, the
+        <constant>TCP_DEFER_ACCEPT</constant> socket option will be
+        used (see
+        <citerefentry project='die-net'><refentrytitle>tcp</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
+        and the kernel will ignore initial ACK packets without any
+        data. The argument specifies the approximate amount of time
+        the kernel should wait for incoming data before falling back
+        to the normal behavior of honoring empty ACK packets. This
+        option is beneficial for protocols where the client sends the
+        data first (e.g. HTTP, in contrast to SMTP), because the
+        server process will not be woken up unnecessarily before it
+        can take any action.
+        </para>
+
+        <para>If the client also uses the
+        <constant>TCP_DEFER_ACCEPT</constant> option, the latency of
+        the initial connection may be reduced, because the kernel will
+        send data in the final packet establishing the connection (the
+        third packet in the "three-way handshake").</para>
+
+        <para>Disabled by default.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ReceiveBuffer=</varname></term>
+        <term><varname>SendBuffer=</varname></term>
+        <listitem><para>Takes an integer argument controlling the
+        receive or send buffer sizes of this socket, respectively.
+        This controls the SO_RCVBUF and SO_SNDBUF socket options (see
+        <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        for details.). The usual suffixes K, M, G are supported and
+        are understood to the base of 1024.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>IPTOS=</varname></term>
+        <listitem><para>Takes an integer argument controlling the IP
+        Type-Of-Service field for packets generated from this socket.
+        This controls the IP_TOS socket option (see
+        <citerefentry project='die-net'><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        for details.). Either a numeric string or one of
+        <option>low-delay</option>, <option>throughput</option>,
+        <option>reliability</option> or <option>low-cost</option> may
+        be specified.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>IPTTL=</varname></term>
+        <listitem><para>Takes an integer argument controlling the IPv4
+        Time-To-Live/IPv6 Hop-Count field for packets generated from
+        this socket. This sets the IP_TTL/IPV6_UNICAST_HOPS socket
+        options (see
+        <citerefentry project='die-net'><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        and
+        <citerefentry project='die-net'><refentrytitle>ipv6</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        for details.)</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Mark=</varname></term>
+        <listitem><para>Takes an integer value. Controls the firewall
+        mark of packets generated by this socket. This can be used in
+        the firewall logic to filter packets from this socket. This
+        sets the SO_MARK socket option. See
+        <citerefentry project='die-net'><refentrytitle>iptables</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+        for details.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ReusePort=</varname></term>
+        <listitem><para>Takes a boolean value. If true, allows
+        multiple
+        <citerefentry><refentrytitle>bind</refentrytitle><manvolnum>2</manvolnum></citerefentry>s
+        to this TCP or UDP port. This controls the SO_REUSEPORT socket
+        option. See
+        <citerefentry project='man-pages'><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        for details.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>SmackLabel=</varname></term>
+        <term><varname>SmackLabelIPIn=</varname></term>
+        <term><varname>SmackLabelIPOut=</varname></term>
+        <listitem><para>Takes a string value. Controls the extended
+        attributes <literal>security.SMACK64</literal>,
+        <literal>security.SMACK64IPIN</literal> and
+        <literal>security.SMACK64IPOUT</literal>, respectively, i.e.
+        the security label of the FIFO, or the security label for the
+        incoming or outgoing connections of the socket, respectively.
+        See <ulink
+        url="https://www.kernel.org/doc/Documentation/security/Smack.txt">Smack.txt</ulink>
+        for details.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>SELinuxContextFromNet=</varname></term>
+         <listitem><para>Takes a boolean argument. When true, systemd
+         will attempt to figure out the SELinux label used for the
+         instantiated service from the information handed by the peer
+         over the network. Note that only the security level is used
+         from the information provided by the peer. Other parts of the
+         resulting SELinux context originate from either the target
+         binary that is effectively triggered by socket unit or from
+         the value of the <varname>SELinuxContext=</varname> option.
+         This configuration option only affects sockets with
+         <varname>Accept=</varname> mode set to
+         <literal>true</literal>. Also note that this option is useful
+         only when MLS/MCS SELinux policy is deployed. Defaults to
+         <literal>false</literal>. </para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>PipeSize=</varname></term>
+        <listitem><para>Takes a size in bytes. Controls the pipe
+        buffer size of FIFOs configured in this socket unit. See
+        <citerefentry><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+        for details. The usual suffixes K, M, G are supported and are
+        understood to the base of 1024.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>MessageQueueMaxMessages=</varname>,
+        <varname>MessageQueueMessageSize=</varname></term>
+        <listitem><para>These two settings take integer values and
+        control the mq_maxmsg field or the mq_msgsize field,
+        respectively, when creating the message queue. Note that
+        either none or both of these variables need to be set. See
+        <citerefentry project='die-net'><refentrytitle>mq_setattr</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+        for details.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>FreeBind=</varname></term>
+        <listitem><para>Takes a boolean value. Controls whether the
+        socket can be bound to non-local IP addresses. This is useful
+        to configure sockets listening on specific IP addresses before
+        those IP addresses are successfully configured on a network
+        interface. This sets the IP_FREEBIND socket option. For
+        robustness reasons it is recommended to use this option
+        whenever you bind a socket to a specific IP address. Defaults
+        to <option>false</option>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Transparent=</varname></term>
+        <listitem><para>Takes a boolean value. Controls the
+        IP_TRANSPARENT socket option. Defaults to
+        <option>false</option>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Broadcast=</varname></term>
+        <listitem><para>Takes a boolean value. This controls the
+        SO_BROADCAST socket option, which allows broadcast datagrams
+        to be sent from this socket. Defaults to
+        <option>false</option>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>PassCredentials=</varname></term>
+        <listitem><para>Takes a boolean value. This controls the
+        SO_PASSCRED socket option, which allows
+        <constant>AF_UNIX</constant> sockets to receive the
+        credentials of the sending process in an ancillary message.
+        Defaults to <option>false</option>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>PassSecurity=</varname></term>
+        <listitem><para>Takes a boolean value. This controls the
+        SO_PASSSEC socket option, which allows
+        <constant>AF_UNIX</constant> sockets to receive the security
+        context of the sending process in an ancillary message.
+        Defaults to <option>false</option>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>TCPCongestion=</varname></term>
+        <listitem><para>Takes a string value. Controls the TCP
+        congestion algorithm used by this socket. Should be one of
+        "westwood", "veno", "cubic", "lp" or any other available
+        algorithm supported by the IP stack. This setting applies only
+        to stream sockets.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ExecStartPre=</varname></term>
+        <term><varname>ExecStartPost=</varname></term>
+        <listitem><para>Takes one or more command lines, which are
+        executed before or after the listening sockets/FIFOs are
+        created and bound, respectively. The first token of the
+        command line must be an absolute filename, then followed by
+        arguments for the process. Multiple command lines may be
+        specified following the same scheme as used for
+        <varname>ExecStartPre=</varname> of service unit
+        files.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ExecStopPre=</varname></term>
+        <term><varname>ExecStopPost=</varname></term>
+        <listitem><para>Additional commands that are executed before
+        or after the listening sockets/FIFOs are closed and removed,
+        respectively. Multiple command lines may be specified
+        following the same scheme as used for
+        <varname>ExecStartPre=</varname> of service unit
+        files.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>TimeoutSec=</varname></term>
+        <listitem><para>Configures the time to wait for the commands
+        specified in <varname>ExecStartPre=</varname>,
+        <varname>ExecStartPost=</varname>,
+        <varname>ExecStopPre=</varname> and
+        <varname>ExecStopPost=</varname> to finish. If a command does
+        not exit within the configured time, the socket will be
+        considered failed and be shut down again. All commands still
+        running will be terminated forcibly via
+        <constant>SIGTERM</constant>, and after another delay of this
+        time with <constant>SIGKILL</constant>. (See
+        <option>KillMode=</option> in
+        <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>.)
+        Takes a unit-less value in seconds, or a time span value such
+        as "5min 20s". Pass <literal>0</literal> to disable the
+        timeout logic. Defaults to
+        <varname>DefaultTimeoutStartSec=</varname> from the manager
+        configuration file (see
+        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
+        </para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Service=</varname></term>
+        <listitem><para>Specifies the service unit name to activate on
+        incoming traffic. This setting is only allowed for sockets
+        with <varname>Accept=no</varname>. It defaults to the service
+        that bears the same name as the socket (with the suffix
+        replaced). In most cases, it should not be necessary to use
+        this option. Note that setting this parameter might result in
+        additional dependencies to be added to the unit (see
+        above).</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>RemoveOnStop=</varname></term>
+        <listitem><para>Takes a boolean argument. If enabled, any file
+        nodes created by this socket unit are removed when it is
+        stopped. This applies to AF_UNIX sockets in the file system,
+        POSIX message queues, FIFOs, as well as any symlinks to them
+        configured with <varname>Symlinks=</varname>. Normally, it
+        should not be necessary to use this option, and is not
+        recommended as services might continue to run after the socket
+        unit has been terminated and it should still be possible to
+        communicate with them via their file system node. Defaults to
+        off.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>Symlinks=</varname></term>
+        <listitem><para>Takes a list of file system paths. The specified paths will be created as symlinks to the
+        <constant>AF_UNIX</constant> socket path or FIFO path of this socket unit. If this setting is used, only one
+        <constant>AF_UNIX</constant> socket in the file system or one FIFO may be configured for the socket unit. Use
+        this option to manage one or more symlinked alias names for a socket, binding their lifecycle together. Note
+        that if creation of a symlink fails this is not considered fatal for the socket unit, and the socket unit may
+        still start. If an empty string is assigned, the list of paths is reset. Defaults to an empty
+        list.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>FileDescriptorName=</varname></term>
+        <listitem><para>Assigns a name to all file descriptors this
+        socket unit encapsulates. This is useful to help activated
+        services identify specific file descriptors, if multiple fds
+        are passed. Services may use the
+        <citerefentry><refentrytitle>sd_listen_fds_with_names</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+        call to acquire the names configured for the received file
+        descriptors. Names may contain any ASCII character, but must
+        exclude control characters and <literal>:</literal>, and must
+        be at most 255 characters in length. If this setting is not
+        used, the file descriptor name defaults to the name of the
+        socket unit, including its <filename>.socket</filename>
+        suffix.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>TriggerLimitIntervalSec=</varname></term>
+        <term><varname>TriggerLimitBurst=</varname></term>
+
+        <listitem><para>Configures a limit on how often this socket unit my be activated within a specific time
+        interval. The <varname>TriggerLimitIntervalSec=</varname> may be used to configure the length of the time
+        interval in the usual time units <literal>us</literal>, <literal>ms</literal>, <literal>s</literal>,
+        <literal>min</literal>, <literal>h</literal>, … and defaults to 2s (See
+        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details on
+        the various time units understood). The <varname>TriggerLimitBurst=</varname> setting takes a positive integer
+        value and specifies the number of permitted activations per time interval, and defaults to 200 for
+        <varname>Accept=yes</varname> sockets (thus by default permitting 200 activations per 2s), and 20 otherwise (20
+        activations per 2s). Set either to 0 to disable any form of trigger rate limiting. If the limit is hit, the
+        socket unit is placed into a failure mode, and will not be connectible anymore until restarted. Note that this
+        limit is enforced before the service activation is enqueued.</para></listitem>
+      </varlistentry>
+
+    </variablelist>
+
+    <para>Check
+    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    and
+    <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    for more settings.</para>
+
+  </refsect1>
+
+  <refsect1>
+      <title>See Also</title>
+      <para>
+        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd.kill</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>sd_listen_fds_with_names</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+      </para>
+      <para>
+        For more extensive descriptions see the "systemd for Developers" series:
+        <ulink url="http://0pointer.de/blog/projects/socket-activation.html">Socket Activation</ulink>,
+        <ulink url="http://0pointer.de/blog/projects/socket-activation2.html">Socket Activation, part II</ulink>,
+        <ulink url="http://0pointer.de/blog/projects/inetd.html">Converting inetd Services</ulink>,
+        <ulink url="http://0pointer.de/blog/projects/socket-activated-containers.html">Socket Activated Internet Services and OS Containers</ulink>.
+      </para>
+  </refsect1>
+
+</refentry>