systemd.socket — Socket unit configuration
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
which define the execution environment the
commands are executed in, and in
which define the way the processes are terminated, and in
which configure resource control settings for the processes of the
For each socket file, a matching service file must exist,
describing the service to start on incoming traffic on the socket
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
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
or it must be a template unit named the same way. Example: a
foo.socket needs a matching
Accept=no is set. If
Accept=yes is set, a service template file
foo@.service must exist from which services
are instantiated for each incoming connection.
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
for details) or via the traditional
socket passing (i.e. sockets passed in via standard input and
StandardInput=socket in the
All network sockets allocated through
.socket units are allocated in the host's network
namespace (see network_namespaces(7)). This
does not mean however that the service activated by a configured socket unit has to be part of the host's network
namespace as well. It is supported and even good practice to run services in their own network namespace (for
systemd.exec(5)), receiving only
the sockets configured through socket-activation from the host's namespace. In such a set-up communication within
the host's network namespace is only permitted through the activation sockets passed in while all sockets allocated
from the service code itself will be associated with the service's own namespace, and thus possibly subject to a a
much more restrictive configuration.
The following dependencies are implicitly added:
Socket units automatically gain a
dependency on the service units they activate.
Socket units referring to file system paths (such as AF_UNIX
sockets or FIFOs) implicitly gain
After= dependencies on all mount units
necessary to access those paths.
Socket units using the
setting automatically gain a
After= dependency on the device unit
encapsulating the specified network interface.
The following dependencies are added unless
DefaultDependencies=no is set:
Socket units automatically gain a
Before= dependency on
Socket units automatically gain a pair of
sysinit.target, and a pair of
dependencies ensure that the socket unit is started before normal
services at boot, and is stopped on shutdown. Only sockets
involved with early boot or late system shutdown should disable
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:
Specifies an address to listen on for a stream
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
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
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 "
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
ListenSequentialPacket=) is only available
ListenStream=) when used for IP sockets
refers to TCP sockets,
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
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
FreeBind= option described
Specifies a file system FIFO to listen on.
This expects an absolute file system path as argument.
Behavior otherwise is very similar to the
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
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
as argument, optionally suffixed by a whitespace followed by a
multicast group integer. Behavior otherwise is very similar to
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.
Specifies a USB
FunctionFS 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
directive above. Use this to open the FunctionFS endpoint
ep0. When using this option, the
activated service has to have the
USBFunctionStrings= options set.
Takes one of
sctp. Specifies a socket protocol
IPPROTO_SCTP) SCTP socket respectively.
Takes one of
the IPV6_V6ONLY socket option (see
for details). If
both, IPv6 sockets bound
will be accessible via both IPv4 and IPv6. If
ipv6-only, they will be accessible via IPv6
default (which is the default,
surprise!), the system wide default setting is used, as
/proc/sys/net/ipv6/bindv6only, which in
turn defaults to the equivalent of
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).
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 implicit 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).
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.
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.
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.
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
performance reasons, it is recommended to write new daemons
only in a way that is suitable for
Accept=no. A daemon listening on an
AF_UNIX socket may, but does not need to,
on the received socket before exiting. However, it must not
unlink the socket from a file system. It should not invoke
on sockets it got with
Accept=no, but it
may do so for sockets it got with
Accept=yes set. Setting
Accept=yes is mostly useful to allow
daemons designed for usage with
to work unmodified with systemd socket
For IPv4 and IPv6 connections, the
environment variable will contain the remote IP address, and
will contain the remote port. This is the same as the format used by CGI.
For SOCK_RAW, the port is the IP protocol.
Takes a boolean argument. May only be used in
ListenSpecial=. If true,
the specified special file is opened in read-write mode, if
false, in read-only mode. Defaults to false.
The maximum number of connections to
simultaneously run services instances for, when
Accept=yes 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=no or datagram sockets. Defaults to
The maximum number of connections for a service per source IP address.
This is very similar to the
above. Disabled by default.
Takes a boolean argument. If true, the TCP/IP
stack will send a keep alive message after 2h (depending on
the configuration of
for all TCP streams accepted on this socket. This controls the
SO_KEEPALIVE socket option (see
and the TCP
Keepalive HOWTO for details.) Defaults to
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 for details.) Defaults value is 7200 seconds (2 hours).
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 for details.) Defaults value is 75 seconds.
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 for details.) Defaults value is 9.
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
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.).
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
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.
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.
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
for details.). Either a numeric string or one of
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.)
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.
Takes a string value. Controls the extended
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.
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
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
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.
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.
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
Takes a boolean value. Controls the
IP_TRANSPARENT socket option. Defaults to
Takes a boolean value. This controls the
SO_BROADCAST socket option, which allows broadcast datagrams
to be sent from this socket. Defaults to
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.
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.
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.
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
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
Configures the time to wait for the commands
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
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
Specifies the service unit name to activate on
incoming traffic. This setting is only allowed for sockets
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
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
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
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. Note
that if creation of a symlink fails this is not considered fatal for the socket unit, and the socket unit may
still start. If an empty string is assigned, the list of paths is reset. Defaults to an empty
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
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
Configures a limit on how often this socket unit my be activated within a specific time
TriggerLimitIntervalSec= may be used to configure the length of the time
interval in the usual time units "
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.
systemd(1), systemctl(1), systemd-system.conf(5), 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, Socket Activation, part II, Converting inetd Services, Socket Activated Internet Services and OS Containers.