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NAME | SYNOPSIS | DESCRIPTION | AUTOMATIC DEPENDENCIES | OPTIONS | SEE ALSO | NOTES | COLOPHON |
SYSTEMD.SOCKET(5) systemd.socket SYSTEMD.SOCKET(5)
systemd.socket - Socket unit configuration
socket.socket
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.
This man page lists the configuration options specific to this unit
type. See systemd.unit(5) 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.
Additional options are listed in systemd.exec(5), which define the
execution environment the ExecStartPre=, ExecStartPost=, ExecStopPre=
and ExecStopPost= commands are executed in, and in systemd.kill(5),
which define the way the processes are terminated, and in
systemd.resource-control(5), which configure resource control
settings for the processes of the socket.
For each socket file, a matching service file must exist, describing
the service to start on incoming traffic on the socket (see
systemd.service(5) 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 Service= option described
below. Depending on the setting of the Accept= option described
below, this .service unit must either be named like the .socket unit,
but with the suffix replaced, unless overridden with Service=; or it
must be a template unit named the same way. Example: a socket file
foo.socket needs a matching service foo.service if Accept=false is
set. If Accept=true is set, a service template file foo@.service must
exist from which services are instantiated for each incoming
connection.
Unless DefaultDependencies= in the "[Unit]" section is set to false,
socket units will implicitly have dependencies of type Requires= and
After= on sysinit.target as well as dependencies of type Conflicts=
and Before= on shutdown.target. 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.
Socket units will have a Before= dependency on the service which they
trigger added implicitly. No implicit WantedBy= or RequiredBy=
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
Requires= dependency may be added.
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.
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 sd_listen_fds(3)
for details) or via the traditional inetd(8)-style socket passing
(i.e. sockets passed in via standard input and output, using
StandardInput=socket in the service file).
Socket units automatically gain a Before= dependency on the service
units they activate.
Socket units referring to file system paths (such as AF_UNIX sockets
or FIFOs) implicitly gain Requires= and After= dependencies on all
mount units necessary to access those paths.
Socket units using the BindToDevice= setting automatically gain a
BindsTo= and After= dependency on the device unit encapsulating the
specified network interface.
If DefaultDependencies=yes is set (the default), socket units
automatically gain a Before= dependency on sockets.target. They also
gain a pair of After= and Requires= dependency on sysinit.target, and
a pair of Before= and Conflicts= dependencies on shutdown.target.
These dependencies ensure that the socket unit is started before
normal services at boot, and is stopped on shutdown.
Additional implicit dependencies may be added as result of execution
and resource control parameters as documented in systemd.exec(5) and
systemd.resource-control(5).
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 systemd.exec(5) and
systemd.kill(5). The options specific to the [Socket] section of
socket units are the following:
ListenStream=, ListenDatagram=, ListenSequentialPacket=
Specifies an address to listen on for a stream (SOCK_STREAM),
datagram (SOCK_DGRAM), or sequential packet (SOCK_SEQPACKET)
socket, respectively. The address can be written in various
formats:
If the address starts with a slash ("/"), it is read as file
system socket in the AF_UNIX socket family.
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
unix(7).
If the address string is a single number, it is read as port
number to listen on via IPv6. Depending on the value of
BindIPv6Only= (see below) this might result in the service being
available via both IPv6 and IPv4 (default) or just via IPv6.
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.
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 BindIPv6Only=
setting (see below).
If the address string is a string in the format "vsock:x:y", it
is read as CID "x" on a port "y" address in the AF_VSOCK family.
The CID is a unique 32-bit integer identifier in AF_VSOCK
analogous to an IP address. Specifying the CID is optional, and
may be set to the empty string.
Note that SOCK_SEQPACKET (i.e. ListenSequentialPacket=) is only
available for AF_UNIX sockets. SOCK_STREAM (i.e. ListenStream=)
when used for IP sockets refers to TCP sockets, SOCK_DGRAM (i.e.
ListenDatagram=) to UDP.
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.
It is also possible to have more than one socket unit for the
same service when using Service=, 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.
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 FreeBind=
option described below.
ListenFIFO=
Specifies a file system FIFO to listen on. This expects an
absolute file system path as argument. Behavior otherwise is very
similar to the ListenDatagram= directive above.
ListenSpecial=
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 ListenFIFO= directive above. Use
this to open character device nodes as well as special files in
/proc and /sys.
ListenNetlink=
Specifies a Netlink family to create a socket for to listen on.
This expects a short string referring to the AF_NETLINK family
name (such as audit or kobject-uevent) as argument, optionally
suffixed by a whitespace followed by a multicast group integer.
Behavior otherwise is very similar to the ListenDatagram=
directive above.
ListenMessageQueue=
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 ListenFIFO= directive above. On
Linux message queue descriptors are actually file descriptors and
can be inherited between processes.
ListenUSBFunction=
Specifies a USB FunctionFS[1] endpoints location to listen on,
for implementation of USB gadget functions. This expects an
absolute file system path of functionfs mount point as the
argument. Behavior otherwise is very similar to the ListenFIFO=
directive above. Use this to open the FunctionFS endpoint ep0.
When using this option, the activated service has to have the
USBFunctionDescriptors= and USBFunctionStrings= options set.
SocketProtocol=
Takes a one of udplite or sctp. Specifies a socket protocol
(IPPROTO_UDPLITE) UDP-Lite (IPPROTO_SCTP) SCTP socket
respectively.
BindIPv6Only=
Takes a one of default, both or ipv6-only. Controls the
IPV6_V6ONLY socket option (see ipv6(7) for details). If both,
IPv6 sockets bound will be accessible via both IPv4 and IPv6. If
ipv6-only, they will be accessible via IPv6 only. If default
(which is the default, surprise!), the system wide default
setting is used, as controlled by /proc/sys/net/ipv6/bindv6only,
which in turn defaults to the equivalent of both.
Backlog=
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 listen(2) for details. Defaults to SOMAXCONN (128).
BindToDevice=
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
socket(7) for details). If this option is used, an automatic
dependency from this socket unit on the network interface device
unit (systemd.device(5) is created. Note that setting this
parameter might result in additional dependencies to be added to
the unit (see above).
SocketUser=, SocketGroup=
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.
SocketMode=
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.
DirectoryMode=
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.
Accept=
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
false. For performance reasons, it is recommended to write new
daemons only in a way that is suitable for Accept=false. A daemon
listening on an AF_UNIX socket may, but does not need to, call
close(2) on the received socket before exiting. However, it must
not unlink the socket from a file system. It should not invoke
shutdown(2) on sockets it got with Accept=false, but it may do so
for sockets it got with Accept=true set. Setting Accept=true is
mostly useful to allow daemons designed for usage with inetd(8)
to work unmodified with systemd socket activation.
For IPv4 and IPv6 connections, the REMOTE_ADDR environment
variable will contain the remote IP address, and REMOTE_PORT will
contain the remote port. This is the same as the format used by
CGI. For SOCK_RAW, the port is the IP protocol.
Writable=
Takes a boolean argument. May only be used in conjunction with
ListenSpecial=. If true, the specified special file is opened in
read-write mode, if false, in read-only mode. Defaults to false.
MaxConnections=
The maximum number of connections to simultaneously run services
instances for, when Accept=true 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 Accept=false or datagram sockets.
Defaults to 64.
MaxConnectionsPerSource=
The maximum number of connections for a service per source IP
address. This is very similar to the MaxConnections= directive
above. Disabled by default.
KeepAlive=
Takes a boolean argument. If true, the TCP/IP stack will send a
keep alive message after 2h (depending on the configuration of
/proc/sys/net/ipv4/tcp_keepalive_time) for all TCP streams
accepted on this socket. This controls the SO_KEEPALIVE socket
option (see socket(7) and the TCP Keepalive HOWTO[2] for
details.) Defaults to false.
KeepAliveTimeSec=
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 socket(7) and the
TCP Keepalive HOWTO[2] for details.) Defaults value is 7200
seconds (2 hours).
KeepAliveIntervalSec=
Takes time (in seconds) as argument between individual keepalive
probes, if the socket option SO_KEEPALIVE has been set on this
socket. This controls the TCP_KEEPINTVL socket option (see
socket(7) and the TCP Keepalive HOWTO[2] for details.) Defaults
value is 75 seconds.
KeepAliveProbes=
Takes an integer as argument. It is the number of unacknowledged
probes to send before considering the connection dead and
notifying the application layer. This controls the TCP_KEEPCNT
socket option (see socket(7) and the TCP Keepalive HOWTO[2] for
details.) Defaults value is 9.
NoDelay=
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
tcp(7) Defaults to false.
Priority=
Takes an integer argument controlling the priority for all
traffic sent from this socket. This controls the SO_PRIORITY
socket option (see socket(7) for details.).
DeferAcceptSec=
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 TCP_DEFER_ACCEPT socket option will be used
(see tcp(7)), and the kernel will ignore initial ACK packets
without any data. The argument specifies the approximate amount
of time the kernel should wait for incoming data before falling
back to the normal behavior of honoring empty ACK packets. This
option is beneficial for protocols where the client sends the
data first (e.g. HTTP, in contrast to SMTP), because the server
process will not be woken up unnecessarily before it can take any
action.
If the client also uses the TCP_DEFER_ACCEPT 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").
Disabled by default.
ReceiveBuffer=, SendBuffer=
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 socket(7) for details.). The
usual suffixes K, M, G are supported and are understood to the
base of 1024.
IPTOS=
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 ip(7) for details.). Either a numeric
string or one of low-delay, throughput, reliability or low-cost
may be specified.
IPTTL=
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 ip(7) and
ipv6(7) for details.)
Mark=
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 iptables(8) for details.
ReusePort=
Takes a boolean value. If true, allows multiple bind(2)s to this
TCP or UDP port. This controls the SO_REUSEPORT socket option.
See socket(7) for details.
SmackLabel=, SmackLabelIPIn=, SmackLabelIPOut=
Takes a string value. Controls the extended attributes
"security.SMACK64", "security.SMACK64IPIN" and
"security.SMACK64IPOUT", respectively, i.e. the security label of
the FIFO, or the security label for the incoming or outgoing
connections of the socket, respectively. See Smack.txt[3] for
details.
SELinuxContextFromNet=
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
SELinuxContext= option. This configuration option only affects
sockets with Accept= mode set to "true". Also note that this
option is useful only when MLS/MCS SELinux policy is deployed.
Defaults to "false".
PipeSize=
Takes a size in bytes. Controls the pipe buffer size of FIFOs
configured in this socket unit. See fcntl(2) for details. The
usual suffixes K, M, G are supported and are understood to the
base of 1024.
MessageQueueMaxMessages=, MessageQueueMessageSize=
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 mq_setattr(3) for details.
FreeBind=
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 false.
Transparent=
Takes a boolean value. Controls the IP_TRANSPARENT socket option.
Defaults to false.
Broadcast=
Takes a boolean value. This controls the SO_BROADCAST socket
option, which allows broadcast datagrams to be sent from this
socket. Defaults to false.
PassCredentials=
Takes a boolean value. This controls the SO_PASSCRED socket
option, which allows AF_UNIX sockets to receive the credentials
of the sending process in an ancillary message. Defaults to
false.
PassSecurity=
Takes a boolean value. This controls the SO_PASSSEC socket
option, which allows AF_UNIX sockets to receive the security
context of the sending process in an ancillary message. Defaults
to false.
TCPCongestion=
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.
ExecStartPre=, ExecStartPost=
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 ExecStartPre= of service unit files.
ExecStopPre=, ExecStopPost=
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 ExecStartPre= of service unit files.
TimeoutSec=
Configures the time to wait for the commands specified in
ExecStartPre=, ExecStartPost=, ExecStopPre= and ExecStopPost= 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 SIGTERM,
and after another delay of this time with SIGKILL. (See KillMode=
in systemd.kill(5).) 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 DefaultTimeoutStartSec= from the
manager configuration file (see systemd-system.conf(5)).
Service=
Specifies the service unit name to activate on incoming traffic.
This setting is only allowed for sockets with Accept=no. It
defaults to the service that bears the same name as the socket
(with the suffix replaced). In most cases, it should not be
necessary to use this option. Note that setting this parameter
might result in additional dependencies to be added to the unit
(see above).
RemoveOnStop=
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 Symlinks=.
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.
Symlinks=
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.
FileDescriptorName=
Assigns a name to all file descriptors this socket unit
encapsulates. This is useful to help activated services identify
specific file descriptors, if multiple fds are passed. Services
may use the sd_listen_fds_with_names(3) call to acquire the names
configured for the received file descriptors. Names may contain
any ASCII character, but must exclude control characters and ":",
and must be at most 255 characters in length. If this setting is
not used, the file descriptor name defaults to the name of the
socket unit, including its .socket suffix.
TriggerLimitIntervalSec=, TriggerLimitBurst=
Configures a limit on how often this socket unit my be activated
within a specific time interval. The TriggerLimitIntervalSec= may
be used to configure the length of the time interval in the usual
time units "us", "ms", "s", "min", "h", ... and defaults to 2s
(See systemd.time(7) for details on the various time units
understood). The TriggerLimitBurst= setting takes a positive
integer value and specifies the number of permitted activations
per time interval, and defaults to 200 for Accept=yes sockets
(thus by default permitting 200 activations per 2s), and 20
otherwise (20 activations per 2s). Set either to 0 to disable any
form of trigger rate limiting. If the limit is hit, the socket
unit is placed into a failure mode, and will not be connectible
anymore until restarted. Note that this limit is enforced before
the service activation is enqueued.
Check systemd.exec(5) and systemd.kill(5) for more settings.
systemd(1), systemctl(1), systemd.unit(5), systemd.exec(5),
systemd.kill(5), systemd.resource-control(5), systemd.service(5),
systemd.directives(7), sd_listen_fds(3), sd_listen_fds_with_names(3)
For more extensive descriptions see the "systemd for Developers"
series: Socket Activation[4], Socket Activation, part II[5],
Converting inetd Services[6], Socket Activated Internet Services and
OS Containers[7].
1. USB FunctionFS
https://www.kernel.org/doc/Documentation/usb/functionfs.txt
2. TCP Keepalive HOWTO
http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/
3. Smack.txt
https://www.kernel.org/doc/Documentation/security/Smack.txt
4. Socket Activation
http://0pointer.de/blog/projects/socket-activation.html
5. Socket Activation, part II
http://0pointer.de/blog/projects/socket-activation2.html
6. Converting inetd Services
http://0pointer.de/blog/projects/inetd.html
7. Socket Activated Internet Services and OS Containers
http://0pointer.de/blog/projects/socket-activated-containers.html
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2017-07-05. If you dis‐
cover any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
systemd 234 SYSTEMD.SOCKET(5)
Pages that refer to this page: systemctl(1), systemd(1), systemd-socket-activate(1), sd-daemon(3), sd_is_fifo(3), sd_listen_fds(3), systemd.exec(5), systemd.kill(5), systemd.resource-control(5), systemd.service(5), systemd.unit(5), daemon(7), systemd.directives(7), systemd.index(7), systemd.special(7), systemd-activate(8), systemd-socket-proxyd(8)