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+<?xml version='1.0'?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="systemd.socket" xmlns:xi="http://www.w3.org/2001/XInclude">
+ <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 unit, a matching service unit 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 units). 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
+ <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 about the precise protocol used and the order in which the file descriptors are passed) or via
+ 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
+ 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 <constant>AF_UNIX</constant>
+ 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 unit files may include [Unit] and [Install] sections, which are described in
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+ </para>
+
+ <para>Socket unit 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
+ <literal><replaceable>v.w.x.y</replaceable>:<replaceable>z</replaceable></literal>, it is interpreted
+ as IPv4 address <replaceable>v.w.x.y</replaceable> and port <replaceable>z</replaceable>.</para>
+
+ <para>If the address string is a string in the format
+ <literal>[<replaceable>x</replaceable>]:<replaceable>y</replaceable></literal>, it is interpreted as
+ IPv6 address <replaceable>x</replaceable> and port <replaceable>y</replaceable>. An optional
+ interface scope (interface name or number) may be specified after a <literal>%</literal> symbol:
+ <literal>[<replaceable>x</replaceable>]:<replaceable>y</replaceable>%<replaceable>dev</replaceable></literal>.
+ Interface scopes are only useful with link-local addresses, because the kernel ignores them in other
+ cases. Note that if an address is specified as IPv6, it might still 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:<replaceable>x</replaceable>:<replaceable>y</replaceable></literal>, it is read as CID
+ <replaceable>x</replaceable> on a port <replaceable>y</replaceable> 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 (see <citerefentry
+ project='man-pages'><refentrytitle>fifo</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
+ details) 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 (see <citerefentry
+ project='man-pages'><refentrytitle>mq_overview</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ for details). This expects a valid message queue name (i.e. beginning with
+ <literal>/</literal>). 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://docs.kernel.org/usb/functionfs.html">USB
+ FunctionFS</ulink> endpoints location to listen on, for
+ implementation of USB gadget functions. This expects an
+ absolute file system path of a 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>. The socket will use the UDP-Lite
+ (<constant>IPPROTO_UDPLITE</constant>) or SCTP
+ (<constant>IPPROTO_SCTP</constant>) protocol, 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 32bit 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. Note that this value is silently capped by the <literal>net.core.somaxconn</literal> sysctl,
+ which typically defaults to 4096. By default this is set to 4294967295, so that the sysctl takes full
+ effect.</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
+ <constant>SO_BINDTODEVICE</constant> 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 is created
+ (see <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
+ 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 <constant>AF_UNIX</constant>
+ 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 yes, a service instance is spawned for each incoming
+ connection and only the connection socket is passed to it. If no, 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>no</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>Note that depending on this setting the services activated by units of this type are either
+ regular services (in case of <varname>Accept=</varname><option>no</option>) or instances of templated
+ services (in case of <varname>Accept=</varname><option>yes</option>). See the Description section
+ above for a more detailed discussion of the naming rules of triggered services.</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 <constant>SOCK_RAW</constant>, the port is the IP
+ protocol.</para>
+
+ <para>It is recommended to set <varname>CollectMode=inactive-or-failed</varname> for service
+ instances activated via <varname>Accept=yes</varname>, to ensure that failed connection services are
+ cleaned up and released from memory, and do not accumulate.</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>FlushPending=</varname></term>
+ <listitem><para>Takes a boolean argument. May only be used when
+ <option>Accept=no</option>. If yes, the socket's buffers are cleared after the
+ triggered service exited. This causes any pending data to be
+ flushed and any pending incoming connections to be rejected. If no, the
+ socket's buffers won't be cleared, permitting the service to handle any
+ pending connections after restart, which is the usually expected behaviour.
+ Defaults to <option>no</option>.
+ </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 <constant>SO_KEEPALIVE</constant> 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 <constant>SO_KEEPALIVE</constant> has been set on this socket. This controls the
+ <constant>TCP_KEEPINTVL</constant> 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 <constant>SO_PRIORITY</constant> 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 <constant>SO_RCVBUF</constant> and
+ <constant>SO_SNDBUF</constant> 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 <constant>IP_TOS</constant> 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
+ <constant>IP_TTL</constant>/<constant>IPV6_UNICAST_HOPS</constant> 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
+ <constant>SO_MARK</constant> 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 <constant>SO_REUSEPORT</constant> 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://docs.kernel.org/admin-guide/LSM/Smack.html">Smack</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 applies only when activated service
+ is passed in single socket file descriptor, i.e. service
+ instances that have standard input connected to a socket or
+ services triggered by exactly one socket unit. 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
+ <constant>IP_FREEBIND</constant>/<constant>IPV6_FREEBIND</constant> 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
+ <constant>IP_TRANSPARENT</constant>/<constant>IPV6_TRANSPARENT</constant> 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 <constant>SO_BROADCAST</constant> 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 <constant>SO_PASSCRED</constant> 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 <constant>SO_PASSSEC</constant> 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>PassPacketInfo=</varname></term>
+ <listitem><para>Takes a boolean value. This controls the <constant>IP_PKTINFO</constant>,
+ <constant>IPV6_RECVPKTINFO</constant>, <constant>NETLINK_PKTINFO</constant> or
+ <constant>PACKET_AUXDATA</constant> socket options, which enable reception of additional per-packet
+ metadata as ancillary message, on <constant>AF_INET</constant>, <constant>AF_INET6</constant>,
+ <constant>AF_UNIX</constant> and <constant>AF_PACKET</constant> sockets. Defaults to
+ <option>false</option>.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>Timestamping=</varname></term>
+ <listitem><para>Takes one of <literal>off</literal>, <literal>us</literal> (alias:
+ <literal>usec</literal>, <literal>µs</literal>) or <literal>ns</literal> (alias:
+ <literal>nsec</literal>). This controls the <constant>SO_TIMESTAMP</constant> or
+ <constant>SO_TIMESTAMPNS</constant> socket options, and enables whether ingress network traffic shall
+ carry timestamping metadata. Defaults to <option>off</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 <literal>westwood</literal>, <literal>veno</literal>,
+ <literal>cubic</literal>, <literal>lp</literal> 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 <constant>AF_UNIX</constant> 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 may 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>
+
+ <xi:include href="systemd.service.xml" xpointer="shared-unit-options" />
+ </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="https://0pointer.de/blog/projects/socket-activation.html">Socket Activation</ulink>,
+ <ulink url="https://0pointer.de/blog/projects/socket-activation2.html">Socket Activation, part II</ulink>,
+ <ulink url="https://0pointer.de/blog/projects/inetd.html">Converting inetd Services</ulink>,
+ <ulink url="https://0pointer.de/blog/projects/socket-activated-containers.html">Socket Activated Internet Services and OS Containers</ulink>.
+ </para>
+ </refsect1>
+
+</refentry>