path: root/man/systemd.socket.xml

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<refentry id="systemd.socket">
        <refentryinfo>
                <title>systemd.socket</title>
                <productname>systemd</productname>

                <authorgroup>
                        <author>
                                <contrib>Developer</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </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=false</option> is set. If
                <option>Accept=true</option> is set, a service
                template file <filename>foo@.service</filename> must
                exist from which services are instantiated for each
                incoming connection.</para>

                <para>Unless <varname>DefaultDependencies=</varname>
                is set to <option>false</option>, socket units will
                implicitly have dependencies of type
                <varname>Requires=</varname> and
                <varname>After=</varname> on
                <filename>sysinit.target</filename> as well as
                dependencies of type <varname>Conflicts=</varname> and
                <varname>Before=</varname> on
                <filename>shutdown.target</filename>. These ensure
                that socket units pull in basic system
                initialization, and are terminated cleanly prior to
                system shutdown. Only sockets involved with early
                boot or late system shutdown should disable this
                option.</para>

                <para>Socket units will have a
                <varname>Before=</varname> dependency on the service
                which they trigger added implicitly. 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><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>
        </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>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>BindIPv6Only=</varname></term>
                                <listitem><para>Takes a one of
                                <option>default</option>,
                                <option>both</option> or
                                <option>ipv6-only</option>. Controls
                                the IPV6_V6ONLY socket option (see
                                <citerefentry><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><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                for details). If this option is used,
                                an automatic dependency from this
                                socket unit on the network interface
                                device unit
                                (<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                                is created.</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=false</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=false</varname>, but
                                it may do so for sockets it got with
                                <varname>Accept=true</varname> set.
                                Setting <varname>Accept=true</varname>
                                is mostly useful to allow daemons
                                designed for usage with
                                <citerefentry><refentrytitle>inetd</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                to work unmodified with systemd socket
                                activation.</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=true</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=false</option> or datagram
                                sockets. Defaults to
                                64.</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><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>KeepAliveTime=</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><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>KeepAliveInterval=</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 seconds as argument.
                                This controls the TCP_KEEPINTVL socket option (see
                                <citerefentry><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 interger as argument. It's 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><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><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><refentrytitle>socket</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                for details.).</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>DeferAccept=</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><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 behaviour of honouring
                                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><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><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><refentrytitle>ip</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                and
                                <citerefentry><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><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><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>SELinuxLabelViaNet=</varname></term>
                          <listitem><para>Takes a boolean
                          value. Controls whether systemd attempts to figure out
                          SELinux label used for instantiated service from
                          information handed by peer over the
                          network. Configuration option has effect only
                          on sockets with <literal>Accept=</literal>
                          mode set to <literal>yes</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><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-systemd.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.</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 AF_UNIX
                                socket path or FIFO path of this
                                socket unit.  If this setting is used,
                                only one AF_UNIX 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. Defaults to the
                                empty list.</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>8</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>
                  </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>