<title>Description</title>
<para>A unit configuration file whose name ends in
- <filename>.socket</filename> encodes information about
+ <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>
<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.</para>
+ <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
- (see
+ must exist, describing the service to start on
+ incoming traffic on the socket (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
+ 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
+ <option>Service=</option> option described below.
+ Depending on the setting of <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>
+ <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
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.</para>
+ of services. See the blog stories linked at the end
+ for introduction.</para>
<para>Note that the daemon software configured for
socket activation with socket units needs to be able
<term><varname>ListenSequentialPacket=</varname></term>
<listitem><para>Specifies an address
to listen on for a stream
- (SOCK_STREAM), datagram (SOCK_DGRAM),
+ (<constant>SOCK_STREAM</constant>), datagram (<constant>SOCK_DGRAM</constant>),
or sequential packet
- (SOCK_SEQPACKET) socket, respectively. The address
+ (<constant>SOCK_SEQPACKET</constant>) socket, respectively. 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
+ 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 (@) it is read as abstract
- namespace socket in the AF_UNIX
- family. The @ is replaced with a NUL
- character before binding. For details
- see
+ <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><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
<para>If the address string is a
- single number it is read as port
+ single number, it is read as port
number to listen on via
IPv6. Depending on the value of
<varname>BindIPv6Only=</varname> (see below) this
</para>
<para>If the address string is a
- string in the format v.w.x.y:z it is
+ 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
+ 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
setting (see below).
</para>
- <para>Note that SOCK_SEQPACKET
+ <para>Note that <constant>SOCK_SEQPACKET</constant>
(i.e. <varname>ListenSequentialPacket=</varname>)
- is only available for AF_UNIX
- sockets. SOCK_STREAM
+ 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, SOCK_DGRAM
+ sockets, <constant>SOCK_DGRAM</constant>
(i.e. <varname>ListenDatagram=</varname>)
to UDP.</para>
traffic on any of the sockets will
trigger service activation, and all
listed sockets will be passed to the
- service, regardless whether there is
+ 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
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 whether it will be up and
- running ever at all. To deal with this
+ 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>
<listitem><para>Specifies a Netlink
family to create a socket for to
listen on. This expects a short string
- referring to the AF_NETLINK family
+ 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
<option>ipv6-only</option>, they will
be accessible via IPv6 only. If
<option>default</option> (which is the
- default, surprise!) the system wide
+ default, surprise!), the system wide
default setting is used, as controlled
by
<filename>/proc/sys/net/ipv6/bindv6only</filename>,
<term><varname>BindToDevice=</varname></term>
<listitem><para>Specifies a network
interface name to bind this socket
- to. If set traffic will only be
+ to. If set, traffic will only be
accepted from the specified network
interfaces. This controls the
SO_BINDTODEVICE socket option (see
to write new daemons only in a way
that is suitable for
<option>Accept=false</option>. A
- daemon listening on an AF_UNIX socket
+ 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 filesystem. It
- should note invoke
+ 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
are coming in, they will be refused
until at least one existing connection
is terminated. This setting has no
- effect for sockets configured with
+ effect on sockets configured with
<option>Accept=false</option> or datagram
sockets. Defaults to
64.</para></listitem>
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>
respectively, i.e. the security label
of the FIFO, or the security label for
the incoming or outgoing connections
- of the socket, respectively. See
+ of the socket, respectively. See
<ulink
url="https://www.kernel.org/doc/Documentation/security/Smack.txt">Smack.txt</ulink>
for details.</para></listitem>
<listitem><para>Takes an integer
value. Controls the pipe buffer size
of FIFOs configured in this socket
- unit. See
+ unit. See
<citerefentry><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
for details.</para></listitem>
</varlistentry>
<term><varname>PassCredentials=</varname></term>
<listitem><para>Takes a boolean
value. This controls the SO_PASSCRED
- socket option, which allows AF_UNIX sockets to
+ socket option, which allows <constant>AF_UNIX</constant> sockets to
receive the credentials of the sending
process in an ancillary message.
Defaults to
<term><varname>PassSecurity=</varname></term>
<listitem><para>Takes a boolean
value. This controls the SO_PASSSEC
- socket option, which allows AF_UNIX
+ socket option, which allows <constant>AF_UNIX</constant>
sockets to receive the security
context of the sending process in an
- ancillary message. Defaults to
+ ancillary message. Defaults to
<option>false</option>.</para></listitem>
</varlistentry>
before or after the listening
sockets/FIFOs are created and
bound, respectively. The first token of the command
- line must be an absolute file name,
+ line must be an absolute filename,
then followed by arguments for the
process. Multiple command lines may be
specified following the same scheme as
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
+ <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 0 to disable the timeout
- logic. Defaults to
- 90s.</para></listitem>
+ logic. Defaults to <varname>TimeoutStartSec=</varname> from the
+ manager configuration file.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>Service=</varname></term>
<listitem><para>Specifies the service
unit name to activate on incoming
- traffic. This defaults to the service
- that bears the same name as the socket
- (ignoring the different suffixes). In
- most cases it should not be necessary
- to use this option.</para></listitem>
+ 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>
</variablelist>
<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:
+ 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>,