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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>systemd.socket</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><style>
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</style><a href="index.html">Index </a>·
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<a href="../libudev/index.html">gudev </a><span style="float:right">systemd 219</span><hr><div class="refentry"><a name="systemd.socket"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>systemd.socket — Socket unit configuration</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><p><code class="filename"><em class="replaceable"><code>socket</code></em>.socket</code></p></div><div class="refsect1"><a name="idm140227022440160"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description"></a></h2><p>A unit configuration file whose name ends in
"<code class="literal">.socket</code>" encodes information about an IPC or
network socket or a file system FIFO controlled and supervised by
systemd, for socket-based activation.</p><p>This man page lists the configuration options specific to
this unit type. See
<a href="systemd.unit.html"><span class="citerefentry"><span class="refentrytitle">systemd.unit</span>(5)</span></a>
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.</p><p>Additional options are listed in
<a href="systemd.exec.html"><span class="citerefentry"><span class="refentrytitle">systemd.exec</span>(5)</span></a>,
which define the execution environment the
<code class="option">ExecStartPre=</code>, <code class="option">ExecStartPost=</code>,
<code class="option">ExecStopPre=</code> and <code class="option">ExecStopPost=</code>
commands are executed in, and in
<a href="systemd.kill.html"><span class="citerefentry"><span class="refentrytitle">systemd.kill</span>(5)</span></a>,
which define the way the processes are terminated, and in
<a href="systemd.resource-control.html"><span class="citerefentry"><span class="refentrytitle">systemd.resource-control</span>(5)</span></a>,
which configure resource control settings for the processes of the
socket.</p><p>For each socket file, a matching service file must exist,
describing the service to start on incoming traffic on the socket
(see
<a href="systemd.service.html"><span class="citerefentry"><span class="refentrytitle">systemd.service</span>(5)</span></a>
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 <code class="option">Service=</code> option
described below. Depending on the setting of the
<code class="option">Accept=</code> option described below, this .service
unit must either be named like the .socket unit, but with the
suffix replaced, unless overridden with <code class="option">Service=</code>;
or it must be a template unit named the same way. Example: a
socket file <code class="filename">foo.socket</code> needs a matching
service <code class="filename">foo.service</code> if
<code class="option">Accept=false</code> is set. If
<code class="option">Accept=true</code> is set, a service template file
<code class="filename">foo@.service</code> must exist from which services
are instantiated for each incoming connection.</p><p>Unless <code class="varname">DefaultDependencies=</code> is set to
<code class="option">false</code>, socket units will implicitly have
dependencies of type <code class="varname">Requires=</code> and
<code class="varname">After=</code> on <code class="filename">sysinit.target</code>
as well as dependencies of type <code class="varname">Conflicts=</code> and
<code class="varname">Before=</code> on
<code class="filename">shutdown.target</code>. 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.</p><p>Socket units will have a <code class="varname">Before=</code>
dependency on the service which they trigger added implicitly. No
implicit <code class="varname">WantedBy=</code> or
<code class="varname">RequiredBy=</code> 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
<code class="varname">Requires=</code> dependency may be added.</p><p>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.</p><p>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
<a href="sd_listen_fds.html"><span class="citerefentry"><span class="refentrytitle">sd_listen_fds</span>(3)</span></a>
for details) or via the traditional
<a href="inetd.html"><span class="citerefentry"><span class="refentrytitle">inetd</span>(8)</span></a>-style
socket passing (i.e. sockets passed in via standard input and
output, using <code class="varname">StandardInput=socket</code> in the
service file).</p></div><div class="refsect1"><a name="idm140227026313728"></a><h2 id="Options">Options<a class="headerlink" title="Permalink to this headline" href="#Options"></a></h2><p>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
<a href="systemd.exec.html"><span class="citerefentry"><span class="refentrytitle">systemd.exec</span>(5)</span></a>
and
<a href="systemd.kill.html"><span class="citerefentry"><span class="refentrytitle">systemd.kill</span>(5)</span></a>.
The options specific to the [Socket] section of socket units are
the following:</p><div class="variablelist"><dl class="variablelist"><dt id="ListenStream="><span class="term"><code class="varname">ListenStream=</code>, </span><span class="term"><code class="varname">ListenDatagram=</code>, </span><span class="term"><code class="varname">ListenSequentialPacket=</code></span><a class="headerlink" title="Permalink to this term" href="#ListenStream="></a></dt><dd><p>Specifies an address to listen on for a stream
(<code class="constant">SOCK_STREAM</code>), datagram
(<code class="constant">SOCK_DGRAM</code>), or sequential packet
(<code class="constant">SOCK_SEQPACKET</code>) socket, respectively.
The address can be written in various formats:</p><p>If the address starts with a slash
("<code class="literal">/</code>"), it is read as file system socket in
the <code class="constant">AF_UNIX</code> socket family.</p><p>If the address starts with an at symbol
("<code class="literal">@</code>"), it is read as abstract namespace
socket in the <code class="constant">AF_UNIX</code> family. The
"<code class="literal">@</code>" is replaced with a
<code class="constant">NUL</code> character before binding. For
details, see
<a href="http://man7.org/linux/man-pages/man7/unix.7.html"><span class="citerefentry"><span class="refentrytitle">unix</span>(7)</span></a>.</p><p>If the address string is a single number, it is read as
port number to listen on via IPv6. Depending on the value of
<code class="varname">BindIPv6Only=</code> (see below) this might result
in the service being available via both IPv6 and IPv4
(default) or just via IPv6.
</p><p>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.</p><p>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
<code class="varname">BindIPv6Only=</code> setting (see below).
</p><p>Note that <code class="constant">SOCK_SEQPACKET</code> (i.e.
<code class="varname">ListenSequentialPacket=</code>) is only available
for <code class="constant">AF_UNIX</code> sockets.
<code class="constant">SOCK_STREAM</code> (i.e.
<code class="varname">ListenStream=</code>) when used for IP sockets
refers to TCP sockets, <code class="constant">SOCK_DGRAM</code> (i.e.
<code class="varname">ListenDatagram=</code>) to UDP.</p><p>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.</p><p>It is also possible to have more than one socket unit
for the same service when using <code class="varname">Service=</code>,
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.</p><p>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 <code class="varname">FreeBind=</code> option described
below.</p></dd><dt id="ListenFIFO="><span class="term"><code class="varname">ListenFIFO=</code></span><a class="headerlink" title="Permalink to this term" href="#ListenFIFO="></a></dt><dd><p>Specifies a file system FIFO to listen on.
This expects an absolute file system path as argument.
Behavior otherwise is very similar to the
<code class="varname">ListenDatagram=</code> directive
above.</p></dd><dt id="ListenSpecial="><span class="term"><code class="varname">ListenSpecial=</code></span><a class="headerlink" title="Permalink to this term" href="#ListenSpecial="></a></dt><dd><p>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
<code class="varname">ListenFIFO=</code> directive above. Use this to
open character device nodes as well as special files in
<code class="filename">/proc</code> and
<code class="filename">/sys</code>.</p></dd><dt id="ListenNetlink="><span class="term"><code class="varname">ListenNetlink=</code></span><a class="headerlink" title="Permalink to this term" href="#ListenNetlink="></a></dt><dd><p>Specifies a Netlink family to create a socket
for to listen on. This expects a short string referring to the
<code class="constant">AF_NETLINK</code> family name (such as
<code class="varname">audit</code> or <code class="varname">kobject-uevent</code>)
as argument, optionally suffixed by a whitespace followed by a
multicast group integer. Behavior otherwise is very similar to
the <code class="varname">ListenDatagram=</code> directive
above.</p></dd><dt id="ListenMessageQueue="><span class="term"><code class="varname">ListenMessageQueue=</code></span><a class="headerlink" title="Permalink to this term" href="#ListenMessageQueue="></a></dt><dd><p>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
<code class="varname">ListenFIFO=</code> directive above. On Linux
message queue descriptors are actually file descriptors and
can be inherited between processes.</p></dd><dt id="BindIPv6Only="><span class="term"><code class="varname">BindIPv6Only=</code></span><a class="headerlink" title="Permalink to this term" href="#BindIPv6Only="></a></dt><dd><p>Takes a one of <code class="option">default</code>,
<code class="option">both</code> or <code class="option">ipv6-only</code>. Controls
the IPV6_V6ONLY socket option (see
<a href="ipv6.html"><span class="citerefentry"><span class="refentrytitle">ipv6</span>(7)</span></a>
for details). If <code class="option">both</code>, IPv6 sockets bound
will be accessible via both IPv4 and IPv6. If
<code class="option">ipv6-only</code>, they will be accessible via IPv6
only. If <code class="option">default</code> (which is the default,
surprise!), the system wide default setting is used, as
controlled by
<code class="filename">/proc/sys/net/ipv6/bindv6only</code>, which in
turn defaults to the equivalent of
<code class="option">both</code>.</p></dd><dt id="Backlog="><span class="term"><code class="varname">Backlog=</code></span><a class="headerlink" title="Permalink to this term" href="#Backlog="></a></dt><dd><p>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
<a href="http://man7.org/linux/man-pages/man2/listen.2.html"><span class="citerefentry"><span class="refentrytitle">listen</span>(2)</span></a>
for details. Defaults to SOMAXCONN (128).</p></dd><dt id="BindToDevice="><span class="term"><code class="varname">BindToDevice=</code></span><a class="headerlink" title="Permalink to this term" href="#BindToDevice="></a></dt><dd><p>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
<a href="socket.html"><span class="citerefentry"><span class="refentrytitle">socket</span>(7)</span></a>
for details). If this option is used, an automatic dependency
from this socket unit on the network interface device unit
(<a href="systemd.device.html"><span class="citerefentry"><span class="refentrytitle">systemd.device</span>(5)</span></a>
is created.</p></dd><dt id="SocketUser="><span class="term"><code class="varname">SocketUser=</code>, </span><span class="term"><code class="varname">SocketGroup=</code></span><a class="headerlink" title="Permalink to this term" href="#SocketUser="></a></dt><dd><p>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.</p></dd><dt id="SocketMode="><span class="term"><code class="varname">SocketMode=</code></span><a class="headerlink" title="Permalink to this term" href="#SocketMode="></a></dt><dd><p>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.</p></dd><dt id="DirectoryMode="><span class="term"><code class="varname">DirectoryMode=</code></span><a class="headerlink" title="Permalink to this term" href="#DirectoryMode="></a></dt><dd><p>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.</p></dd><dt id="Accept="><span class="term"><code class="varname">Accept=</code></span><a class="headerlink" title="Permalink to this term" href="#Accept="></a></dt><dd><p>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 <code class="option">false</code>. For
performance reasons, it is recommended to write new daemons
only in a way that is suitable for
<code class="option">Accept=false</code>. A daemon listening on an
<code class="constant">AF_UNIX</code> socket may, but does not need to,
call
<a href="http://man7.org/linux/man-pages/man2/close.2.html"><span class="citerefentry"><span class="refentrytitle">close</span>(2)</span></a>
on the received socket before exiting. However, it must not
unlink the socket from a file system. It should not invoke
<a href="http://man7.org/linux/man-pages/man2/shutdown.2.html"><span class="citerefentry"><span class="refentrytitle">shutdown</span>(2)</span></a>
on sockets it got with <code class="varname">Accept=false</code>, but it
may do so for sockets it got with
<code class="varname">Accept=true</code> set. Setting
<code class="varname">Accept=true</code> is mostly useful to allow
daemons designed for usage with
<a href="inetd.html"><span class="citerefentry"><span class="refentrytitle">inetd</span>(8)</span></a>
to work unmodified with systemd socket
activation.</p></dd><dt id="MaxConnections="><span class="term"><code class="varname">MaxConnections=</code></span><a class="headerlink" title="Permalink to this term" href="#MaxConnections="></a></dt><dd><p>The maximum number of connections to
simultaneously run services instances for, when
<code class="option">Accept=true</code> 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
<code class="option">Accept=false</code> or datagram sockets. Defaults to
64.</p></dd><dt id="KeepAlive="><span class="term"><code class="varname">KeepAlive=</code></span><a class="headerlink" title="Permalink to this term" href="#KeepAlive="></a></dt><dd><p>Takes a boolean argument. If true, the TCP/IP
stack will send a keep alive message after 2h (depending on
the configuration of
<code class="filename">/proc/sys/net/ipv4/tcp_keepalive_time</code>)
for all TCP streams accepted on this socket. This controls the
SO_KEEPALIVE socket option (see
<a href="socket.html"><span class="citerefentry"><span class="refentrytitle">socket</span>(7)</span></a>
and the <a class="ulink" href="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/" target="_top">TCP
Keepalive HOWTO</a> for details.) Defaults to
<code class="option">false</code>.</p></dd><dt id="KeepAliveTimeSec="><span class="term"><code class="varname">KeepAliveTimeSec=</code></span><a class="headerlink" title="Permalink to this term" href="#KeepAliveTimeSec="></a></dt><dd><p>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
<a href="socket.html"><span class="citerefentry"><span class="refentrytitle">socket</span>(7)</span></a>
and the <a class="ulink" href="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/" target="_top">TCP
Keepalive HOWTO</a> for details.)
Defaults value is 7200 seconds (2 hours).</p></dd><dt id="KeepAliveIntervalSec="><span class="term"><code class="varname">KeepAliveIntervalSec=</code></span><a class="headerlink" title="Permalink to this term" href="#KeepAliveIntervalSec="></a></dt><dd><p>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
<a href="socket.html"><span class="citerefentry"><span class="refentrytitle">socket</span>(7)</span></a>
and the <a class="ulink" href="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/" target="_top">TCP
Keepalive HOWTO</a> for details.) Defaults value is 75
seconds.</p></dd><dt id="KeepAliveProbes="><span class="term"><code class="varname">KeepAliveProbes=</code></span><a class="headerlink" title="Permalink to this term" href="#KeepAliveProbes="></a></dt><dd><p>Takes integer 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
<a href="socket.html"><span class="citerefentry"><span class="refentrytitle">socket</span>(7)</span></a>
and the <a class="ulink" href="http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/" target="_top">TCP
Keepalive HOWTO</a> for details.) Defaults value is
9.</p></dd><dt id="NoDelay="><span class="term"><code class="varname">NoDelay=</code></span><a class="headerlink" title="Permalink to this term" href="#NoDelay="></a></dt><dd><p>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
<a href="tcp.html"><span class="citerefentry"><span class="refentrytitle">tcp</span>(7)</span></a>
Defaults to <code class="option">false</code>.</p></dd><dt id="Priority="><span class="term"><code class="varname">Priority=</code></span><a class="headerlink" title="Permalink to this term" href="#Priority="></a></dt><dd><p>Takes an integer argument controlling the
priority for all traffic sent from this socket. This controls
the SO_PRIORITY socket option (see
<a href="socket.html"><span class="citerefentry"><span class="refentrytitle">socket</span>(7)</span></a>
for details.).</p></dd><dt id="DeferAcceptSec="><span class="term"><code class="varname">DeferAcceptSec=</code></span><a class="headerlink" title="Permalink to this term" href="#DeferAcceptSec="></a></dt><dd><p>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
<code class="constant">TCP_DEFER_ACCEPT</code> socket option will be
used (see
<a href="tcp.html"><span class="citerefentry"><span class="refentrytitle">tcp</span>(7)</span></a>),
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.
</p><p>If the client also uses the
<code class="constant">TCP_DEFER_ACCEPT</code> 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").</p><p>Disabled by default.</p></dd><dt id="ReceiveBuffer="><span class="term"><code class="varname">ReceiveBuffer=</code>, </span><span class="term"><code class="varname">SendBuffer=</code></span><a class="headerlink" title="Permalink to this term" href="#ReceiveBuffer="></a></dt><dd><p>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
<a href="socket.html"><span class="citerefentry"><span class="refentrytitle">socket</span>(7)</span></a>
for details.). The usual suffixes K, M, G are supported and
are understood to the base of 1024.</p></dd><dt id="IPTOS="><span class="term"><code class="varname">IPTOS=</code></span><a class="headerlink" title="Permalink to this term" href="#IPTOS="></a></dt><dd><p>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
<a href="ip.html"><span class="citerefentry"><span class="refentrytitle">ip</span>(7)</span></a>
for details.). Either a numeric string or one of
<code class="option">low-delay</code>, <code class="option">throughput</code>,
<code class="option">reliability</code> or <code class="option">low-cost</code> may
be specified.</p></dd><dt id="IPTTL="><span class="term"><code class="varname">IPTTL=</code></span><a class="headerlink" title="Permalink to this term" href="#IPTTL="></a></dt><dd><p>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
<a href="ip.html"><span class="citerefentry"><span class="refentrytitle">ip</span>(7)</span></a>
and
<a href="ipv6.html"><span class="citerefentry"><span class="refentrytitle">ipv6</span>(7)</span></a>
for details.)</p></dd><dt id="Mark="><span class="term"><code class="varname">Mark=</code></span><a class="headerlink" title="Permalink to this term" href="#Mark="></a></dt><dd><p>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
<a href="iptables.html"><span class="citerefentry"><span class="refentrytitle">iptables</span>(8)</span></a>
for details.</p></dd><dt id="ReusePort="><span class="term"><code class="varname">ReusePort=</code></span><a class="headerlink" title="Permalink to this term" href="#ReusePort="></a></dt><dd><p>Takes a boolean value. If true, allows
multiple
<a href="http://man7.org/linux/man-pages/man2/bind.2.html"><span class="citerefentry"><span class="refentrytitle">bind</span>(2)</span></a>s
to this TCP or UDP port. This controls the SO_REUSEPORT socket
option. See
<a href="socket.html"><span class="citerefentry"><span class="refentrytitle">socket</span>(7)</span></a>
for details.</p></dd><dt id="SmackLabel="><span class="term"><code class="varname">SmackLabel=</code>, </span><span class="term"><code class="varname">SmackLabelIPIn=</code>, </span><span class="term"><code class="varname">SmackLabelIPOut=</code></span><a class="headerlink" title="Permalink to this term" href="#SmackLabel="></a></dt><dd><p>Takes a string value. Controls the extended
attributes "<code class="literal">security.SMACK64</code>",
"<code class="literal">security.SMACK64IPIN</code>" and
"<code class="literal">security.SMACK64IPOUT</code>", respectively, i.e.
the security label of the FIFO, or the security label for the
incoming or outgoing connections of the socket, respectively.
See <a class="ulink" href="https://www.kernel.org/doc/Documentation/security/Smack.txt" target="_top">Smack.txt</a>
for details.</p></dd><dt id="SELinuxContextFromNet="><span class="term"><code class="varname">SELinuxContextFromNet=</code></span><a class="headerlink" title="Permalink to this term" href="#SELinuxContextFromNet="></a></dt><dd><p>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 <code class="varname">SELinuxContext=</code> option.
This configuration option only affects sockets with
<code class="varname">Accept=</code> mode set to
"<code class="literal">true</code>". Also note that this option is useful
only when MLS/MCS SELinux policy is deployed. Defaults to
"<code class="literal">false</code>". </p></dd><dt id="PipeSize="><span class="term"><code class="varname">PipeSize=</code></span><a class="headerlink" title="Permalink to this term" href="#PipeSize="></a></dt><dd><p>Takes a size in bytes. Controls the pipe
buffer size of FIFOs configured in this socket unit. See
<a href="http://man7.org/linux/man-pages/man2/fcntl.2.html"><span class="citerefentry"><span class="refentrytitle">fcntl</span>(2)</span></a>
for details. The usual suffixes K, M, G are supported and are
understood to the base of 1024.</p></dd><dt id="MessageQueueMaxMessages=,
MessageQueueMessageSize="><span class="term"><code class="varname">MessageQueueMaxMessages=</code>,
<code class="varname">MessageQueueMessageSize=</code></span><a class="headerlink" title="Permalink to this term" href="#MessageQueueMaxMessages=,%0A%20%20%20%20%20%20%20%20MessageQueueMessageSize="></a></dt><dd><p>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
<a href="mq_setattr.html"><span class="citerefentry"><span class="refentrytitle">mq_setattr</span>(3)</span></a>
for details.</p></dd><dt id="FreeBind="><span class="term"><code class="varname">FreeBind=</code></span><a class="headerlink" title="Permalink to this term" href="#FreeBind="></a></dt><dd><p>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 <code class="option">false</code>.</p></dd><dt id="Transparent="><span class="term"><code class="varname">Transparent=</code></span><a class="headerlink" title="Permalink to this term" href="#Transparent="></a></dt><dd><p>Takes a boolean value. Controls the
IP_TRANSPARENT socket option. Defaults to
<code class="option">false</code>.</p></dd><dt id="Broadcast="><span class="term"><code class="varname">Broadcast=</code></span><a class="headerlink" title="Permalink to this term" href="#Broadcast="></a></dt><dd><p>Takes a boolean value. This controls the
SO_BROADCAST socket option, which allows broadcast datagrams
to be sent from this socket. Defaults to
<code class="option">false</code>.</p></dd><dt id="PassCredentials="><span class="term"><code class="varname">PassCredentials=</code></span><a class="headerlink" title="Permalink to this term" href="#PassCredentials="></a></dt><dd><p>Takes a boolean value. This controls the
SO_PASSCRED socket option, which allows
<code class="constant">AF_UNIX</code> sockets to receive the
credentials of the sending process in an ancillary message.
Defaults to <code class="option">false</code>.</p></dd><dt id="PassSecurity="><span class="term"><code class="varname">PassSecurity=</code></span><a class="headerlink" title="Permalink to this term" href="#PassSecurity="></a></dt><dd><p>Takes a boolean value. This controls the
SO_PASSSEC socket option, which allows
<code class="constant">AF_UNIX</code> sockets to receive the security
context of the sending process in an ancillary message.
Defaults to <code class="option">false</code>.</p></dd><dt id="TCPCongestion="><span class="term"><code class="varname">TCPCongestion=</code></span><a class="headerlink" title="Permalink to this term" href="#TCPCongestion="></a></dt><dd><p>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.</p></dd><dt id="ExecStartPre="><span class="term"><code class="varname">ExecStartPre=</code>, </span><span class="term"><code class="varname">ExecStartPost=</code></span><a class="headerlink" title="Permalink to this term" href="#ExecStartPre="></a></dt><dd><p>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
<code class="varname">ExecStartPre=</code> of service unit
files.</p></dd><dt id="ExecStopPre="><span class="term"><code class="varname">ExecStopPre=</code>, </span><span class="term"><code class="varname">ExecStopPost=</code></span><a class="headerlink" title="Permalink to this term" href="#ExecStopPre="></a></dt><dd><p>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
<code class="varname">ExecStartPre=</code> of service unit
files.</p></dd><dt id="TimeoutSec="><span class="term"><code class="varname">TimeoutSec=</code></span><a class="headerlink" title="Permalink to this term" href="#TimeoutSec="></a></dt><dd><p>Configures the time to wait for the commands
specified in <code class="varname">ExecStartPre=</code>,
<code class="varname">ExecStartPost=</code>,
<code class="varname">ExecStopPre=</code> and
<code class="varname">ExecStopPost=</code> 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
<code class="constant">SIGTERM</code>, and after another delay of this
time with <code class="constant">SIGKILL</code>. (See
<code class="option">KillMode=</code> in
<a href="systemd.kill.html"><span class="citerefentry"><span class="refentrytitle">systemd.kill</span>(5)</span></a>.)
Takes a unit-less value in seconds, or a time span value such
as "5min 20s". Pass "<code class="literal">0</code>" to disable the
timeout logic. Defaults to
<code class="varname">DefaultTimeoutStartSec=</code> from the manager
configuration file (see
<a href="systemd-system.conf.html"><span class="citerefentry"><span class="refentrytitle">systemd-system.conf</span>(5)</span></a>).
</p></dd><dt id="Service="><span class="term"><code class="varname">Service=</code></span><a class="headerlink" title="Permalink to this term" href="#Service="></a></dt><dd><p>Specifies the service unit name to activate on
incoming traffic. This setting is only allowed for sockets
with <code class="varname">Accept=no</code>. 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.</p></dd><dt id="RemoveOnStop="><span class="term"><code class="varname">RemoveOnStop=</code></span><a class="headerlink" title="Permalink to this term" href="#RemoveOnStop="></a></dt><dd><p>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 <code class="varname">Symlinks=</code>. 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.</p></dd><dt id="Symlinks="><span class="term"><code class="varname">Symlinks=</code></span><a class="headerlink" title="Permalink to this term" href="#Symlinks="></a></dt><dd><p>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.</p></dd></dl></div><p>Check
<a href="systemd.exec.html"><span class="citerefentry"><span class="refentrytitle">systemd.exec</span>(5)</span></a>
and
<a href="systemd.kill.html"><span class="citerefentry"><span class="refentrytitle">systemd.kill</span>(5)</span></a>
for more settings.</p></div><div class="refsect1"><a name="idm140227021246720"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also"></a></h2><p>
<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
<a href="systemctl.html"><span class="citerefentry"><span class="refentrytitle">systemctl</span>(1)</span></a>,
<a href="systemd.unit.html"><span class="citerefentry"><span class="refentrytitle">systemd.unit</span>(5)</span></a>,
<a href="systemd.exec.html"><span class="citerefentry"><span class="refentrytitle">systemd.exec</span>(5)</span></a>,
<a href="systemd.kill.html"><span class="citerefentry"><span class="refentrytitle">systemd.kill</span>(5)</span></a>,
<a href="systemd.resource-control.html"><span class="citerefentry"><span class="refentrytitle">systemd.resource-control</span>(5)</span></a>,
<a href="systemd.service.html"><span class="citerefentry"><span class="refentrytitle">systemd.service</span>(5)</span></a>,
<a href="systemd.directives.html"><span class="citerefentry"><span class="refentrytitle">systemd.directives</span>(7)</span></a>
</p><p>
For more extensive descriptions see the "systemd for Developers" series:
<a class="ulink" href="http://0pointer.de/blog/projects/socket-activation.html" target="_top">Socket Activation</a>,
<a class="ulink" href="http://0pointer.de/blog/projects/socket-activation2.html" target="_top">Socket Activation, part II</a>,
<a class="ulink" href="http://0pointer.de/blog/projects/inetd.html" target="_top">Converting inetd Services</a>,
<a class="ulink" href="http://0pointer.de/blog/projects/socket-activated-containers.html" target="_top">Socket Activated Internet Services and OS Containers</a>.
</p></div></div></body></html>
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