SYSTEMD.SOCKET(5) systemd.socket SYSTEMD.SOCKET(5)
NAME
systemd.socket - Socket unit configuration
SYNOPSIS
socket.socket
DESCRIPTION
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 unit, a matching service unit must exist, describing the
service to start on incoming traffic on the socket (see
systemd.service(5) for more information about .service units). 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=no is set. If Accept=yes
is set, a service template foo@.service must exist from which services
are instantiated for each incoming connection.
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 about the precise protocol used and the order in which the file
descriptors are passed) or via traditional inetd(8)-style socket passing
(i.e. sockets passed in via standard input and output, using
StandardInput=socket in the service file).
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
example through PrivateNetwork=, see 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 restrictive configuration.
AUTOMATIC DEPENDENCIES
Implicit Dependencies
The following dependencies are implicitly added:
• 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.
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).
Default Dependencies
The following dependencies are added unless DefaultDependencies=no is
set:
• Socket units automatically gain a Before= dependency on
sockets.target.
• Socket units automatically 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. Only sockets involved with early boot or late
system shutdown should disable DefaultDependencies= option.
OPTIONS
Socket unit files may include [Unit] and [Install] sections, which are
described in systemd.unit(5).
Socket unit 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), systemd.kill(5) and
systemd.resource-control(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
interpreted as IPv4 address v.w.x.y and port z.
If the address string is a string in the format "[x]:y", it is
interpreted as IPv6 address x and port y. An optional interface
scope (interface name or number) may be specified after a "%"
symbol: "[x]:y%dev". Interface scopes are only useful with
link-local addresses, because the kernel ignores them in other
cases. Note that if an address is specified as IPv6, it might still
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. "vsock" may be replaced with "vsock-stream", "vsock-dgram"
or "vsock-seqpacket" to force usage of the corresponding socket
type.
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 (see fifo(7) for details) 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 (see
mq_overview(7) for details). 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 a 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.
Added in version 227.
SocketProtocol=
Takes one of udplite, sctp or mptcp. The socket will use the
UDP-Lite (IPPROTO_UDPLITE), SCTP (IPPROTO_SCTP) or MPTCP
(IPPROTO_MPTCP) protocol, respectively.
Added in version 229.
BindIPv6Only=
Takes 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 32-bit 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 4294967295. Note that this value is
silently capped by the "net.core.somaxconn" sysctl, which typically
defaults to 4096, so typically the sysctl is the setting that
actually matters.
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 implicit dependency from this
socket unit on the network interface device unit is created (see
systemd.device(5)). 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,
FIFO nodes, and message queues 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.
Added in version 214.
SocketMode=
If listening on a file system socket, FIFO, or message queue, 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 yes, a service instance is spawned for
each incoming connection and only the connection socket is passed to
it. If no, 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 no. For 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, 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=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 inetd(8) to work unmodified with systemd socket
activation.
Note that depending on this setting the services activated by units
of this type are either regular services (in case of Accept=no) or
instances of templated services (in case of Accept=yes). See the
Description section above for a more detailed discussion of the
naming rules of triggered services.
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 AF_UNIX socket connections, the $REMOTE_ADDR environment
variable will contain either the remote socket's file system path
starting with a slash ("/") or its address in the abstract namespace
starting with an at symbol ("@"). If the socket is unnamed,
$REMOTE_ADDR will not be set.
It is recommended to set CollectMode=inactive-or-failed for service
instances activated via Accept=yes, to ensure that failed connection
services are cleaned up and released from memory, and do not
accumulate.
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.
Added in version 227.
FlushPending=
Takes a boolean argument. May only be used when Accept=no. If yes,
the socket's buffers are cleared after the triggered service exited.
This causes any pending data to be flushed and any pending incoming
connections to be rejected. If no, the socket's buffers will not be
cleared, permitting the service to handle any pending connections
after restart, which is the usually expected behaviour. Defaults to
no.
Added in version 247.
MaxConnections=
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
64.
MaxConnectionsPerSource=
The maximum number of connections for a service per source IP
address (in case of IPv4/IPv6), per source CID (in case of
AF_VSOCK), or source UID (in case of AF_UNIX). This is very similar
to the MaxConnections= directive above. Defaults to 0, i.e.
disabled.
Added in version 232.
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.) Default value is 7200 seconds (2 hours).
Added in version 216.
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.) Default value is 75
seconds.
Added in version 216.
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.) Default
value is 9.
Added in version 216.
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.
Added in version 216.
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.
Added in version 216.
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.
Added in version 206.
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[3] for details.
Added in version 196.
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 applies only when activated service is passed in single
socket file descriptor, i.e. service instances that have standard
input connected to a socket or services triggered by exactly one
socket unit. Also note that this option is useful only when MLS/MCS
SELinux policy is deployed. Defaults to "false".
Added in version 217.
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/IPV6_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/IPV6_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.
PassPacketInfo=
Takes a boolean value. This controls the IP_PKTINFO,
IPV6_RECVPKTINFO, NETLINK_PKTINFO or PACKET_AUXDATA socket options,
which enable reception of additional per-packet metadata as
ancillary message, on AF_INET, AF_INET6, AF_UNIX and AF_PACKET
sockets. Defaults to false.
Added in version 246.
Timestamping=
Takes one of "off", "us" (alias: "usec", "μs") or "ns" (alias:
"nsec"). This controls the SO_TIMESTAMP or SO_TIMESTAMPNS socket
options, and enables whether ingress network traffic shall carry
timestamping metadata. Defaults to off.
Added in version 247.
TCPCongestion=
Takes a string value. Controls the TCP congestion algorithm used by
this socket. Should be one of "westwood", "reno", "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.
Added in version 214.
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. 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 list.
Added in version 214.
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) when Accept=no, "connection"
otherwise.
Added in version 227.
TriggerLimitIntervalSec=, TriggerLimitBurst=
Configures a limit on how often this socket unit may be activated
within a specific time interval. The TriggerLimitIntervalSec=
setting 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.
Compare with PollLimitIntervalSec=/PollLimitBurst= described below,
which implements a temporary slowdown if a socket unit is flooded
with incoming traffic, as opposed to the permanent failure state
TriggerLimitIntervalSec=/TriggerLimitBurst= results in.
Added in version 230.
PollLimitIntervalSec=, PollLimitBurst=
Configures a limit on how often polling events on the file
descriptors backing this socket unit will be considered. This pair
of settings is similar to
TriggerLimitIntervalSec=/TriggerLimitBurst= but instead of putting a
(fatal) limit on the activation frequency puts a (transient) limit
on the polling frequency. The expected parameter syntax and range
are identical to that of the aforementioned options, and can be
disabled the same way.
If the polling limit is hit polling is temporarily disabled on it
until the specified time window passes. The polling limit hence
slows down connection attempts if hit, but unlike the trigger limit
will not cause permanent failures. It's the recommended mechanism to
deal with DoS attempts through packet flooding.
The polling limit is enforced per file descriptor to listen on, as
opposed to the trigger limit which is enforced for the entire socket
unit. This distinction matters for socket units that listen on
multiple file descriptors (i.e. have multiple ListenXYZ= stanzas).
These setting defaults to 150 (in case of Accept=yes) and 15
(otherwise) polling events per 2s. This is considerably lower than
the default values for the trigger limit (see above) and means that
the polling limit should typically ensure the trigger limit is never
hit, unless one of them is reconfigured or disabled.
Added in version 255.
PassFileDescriptorsToExec=
Takes a boolean argument. Defaults to off. If enabled, file
descriptors created by the socket unit are passed to ExecStartPost=,
ExecStopPre=, and ExecStopPost= commands from the socket unit. The
passed file descriptors can be accessed with sd_listen_fds(3) as if
the commands were invoked from the associated service units. Note
that ExecStartPre= command cannot access socket file descriptors.
Added in version 256.
Check systemd.unit(5), systemd.exec(5), and systemd.kill(5) for more
settings.
SEE ALSO
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[4], Socket Activation, part II[5], Converting inetd
Services[6], Socket Activated Internet Services and OS Containers[7].
NOTES
1. USB FunctionFS
https://docs.kernel.org/usb/functionfs.html
2. TCP Keepalive HOWTO
http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/
3. Smack
https://docs.kernel.org/admin-guide/LSM/Smack.html
4. Socket Activation
https://0pointer.de/blog/projects/socket-activation.html
5. Socket Activation, part II
https://0pointer.de/blog/projects/socket-activation2.html
6. Converting inetd Services
https://0pointer.de/blog/projects/inetd.html
7. Socket Activated Internet Services and OS Containers
https://0pointer.de/blog/projects/socket-activated-containers.html
systemd 257.9 SYSTEMD.SOCKET(5)
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