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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 > systemd socket configuration files</refpurpose>
</refnamediv>
<refsynopsisdiv >
<para > <filename > systemd.socket</filename> </para>
</refsynopsisdiv>
<refsect1 >
<title > Description</title>
<para > A unit configuration file whose name ends in .socket
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
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socket specific configuration options are configured
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in the [Socket] section.</para>
<para > Additional options are listed in
<citerefentry > <refentrytitle > systemd.exec</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry> .</para>
<para > For each socket file a matching service file (see <citerefentry > <refentrytitle > systemd.service</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry> for details)
must exist, describing the service to start on
incoming traffic on the socket. Depending on the
setting of <option > Accept=</option> (see below) this
must either be named like the socket unit, but with
the suffix replaced; or it must be a template file
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>
</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> . The
options specific to the [Socket] section of service
units are the following:</para>
<variablelist >
<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
(SOCK_STREAM), datagram (SOCK_DGRAM)
resp. sequential packet
(SOCK_SEQPACKET) socket. The address
can be written in various formats:</para>
<para > If the address starts with a
slash (/), it is read as file system
socket in the AF_UNIX socket
family.</para>
<para > If the address starts with an
ampersand (@) it is read as abstract
namespace socket in the AF_UNIX
family. The @ is replaced with a NUL
character before binding. For details
see
<citerefentry > <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 for both IPv4 and
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.</para>
<para > Note that SOCK_SEQPACKET
(i.e. <varname > ListenSequentialPacket=</varname> )
is only available for AF_UNIX
sockets. SOCK_STREAM
(i.e. <varname > ListenStream=</varname> )
when used for IP sockets refers to TCP
sockets, SOCK_DGRAM
(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 whether there is incoming
traffic on them or not.</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 whether it will be up and
running ever at all. 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. Behaviour otherwise is very
similar to the
<varname > ListenDatagram=</varname>
directive above.</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> .</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 > DirectoryMode=</varname> </term>
<listitem > <para > If listening on a file
system socket of FIFO the parent
directories are automatically created
if needed. This option specifies the
file system access mode used when
creating these directories. Defaults
to 0755.</para> </listitem>
</varlistentry>
<varlistentry >
<term > <varname > SocketMode=</varname> </term>
<listitem > <para > If listening on a file
system socket of FIFO this option
specifies the file system access mode
used when creating the file
node. Defaults to
0666.</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
unconditionally a single service unit
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> . This
option is mostly useful to allow
daemons designed for usage with
<citerefentry > <refentrytitle > inetd</refentrytitle> <manvolnum > 8</manvolnum> </citerefentry>
to work unmodified with system 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 for sockets configured with
<option > Accept=no</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 > 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 > ReceiveBuffer=</varname> </term>
<term > <varname > SendBuffer=</varname> </term>
<listitem > <para > Takes an integer
argument controlling the receive
resp. send buffer sizes of this
socket. This controls the SO_RCVBUF
resp. SO_SNDBUF socket options (see
<citerefentry > <refentrytitle > socket</refentrytitle> <manvolnum > 7</manvolnum> </citerefentry>
for details.).</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 > PipeSize=</varname> </term>
<listitem > <para > Takes an integer
value. Controls the pipe buffer size
of FIFOs configured in this socket
unit. See
<citerefentry > <refentrytitle > fcntl</refentrytitle> <manvolnum > 2</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 > ExecStartPre=</varname> </term>
<term > <varname > ExecStartPost=</varname> </term>
<listitem > <para > Takes a command line
that is executed before (resp. after)
the listening sockets/FIFOs are created and
bound. The first token of the command
line must be an absolute file name,
then followed by arguments for the
process. If specified more than once,
all commands are executed one after
the other, serially. Use of these
settings is optional.</para> </listitem>
</varlistentry>
<varlistentry >
<term > <varname > ExecStopPre=</varname> </term>
<term > <varname > ExecStopPost=</varname> </term>
<listitem > <para > Additional commands
that are executed before (resp. after)
the listening sockets/FIFOs are closed
and removed. If specified more than
once, all commands are executed one
after the other, serially. Use of
these settings is
optional.</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 comand 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
SIGTERM, and after another delay of
this time with SIGKILL. (See
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<option > KillMode=</option> below.)
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Takes a unit-less value in seconds, or
a time span value such as "5min
20s". Pass 0 to disable the timeout
logic. Defaults to
60s.</para> </listitem>
</varlistentry>
<varlistentry >
<term > <varname > KillMode=</varname> </term>
<listitem > <para > Specifies how
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processes of this socket unit shall be
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killed. One of
<option > control-group</option> ,
<option > process-group</option> ,
<option > process</option> ,
<option > none</option> .</para>
<para > This option is mostly equivalent
to the <option > KillMode=</option>
option of service files. See
<citerefentry > <refentrytitle > systemd.service</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry>
for details.</para> </listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 >
<title > See Also</title>
<para >
<citerefentry > <refentrytitle > systemd</refentrytitle> <manvolnum > 8</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.service</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry>
</para>
</refsect1>
</refentry>
