systemd.socket

title: systemd.socket(5)

summary: Socket unit configuration

text:
# 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. Typically, for performance sensitive services, a choice of Accept=no is preferable, since that way only the first connection will have to pay the activation resource cost. On the other hand, for sporadically used services Accept=yes can be preferable as it simplifies the implementation (as the service program code only has to process a single connection instead of handling multiple) and enables stronger security (since the various sandboxing options can be used to isolate parallel connections from each other, as each is serviced by a separate service instance and process).

A service 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 in particular useful for allowing 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 number. These two variables correspond to those defined by the CGI interface for web services (see RFC 3875[2]).

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.

If Accept=yes is used, the activated service process will have set the $SO_COOKIE environment variable to the Linux socket cookie, formatted as decimal integer. The socket cookie can otherwise be acquired via getsockopt(7).

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[3] 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[3] 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[3] 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[3] 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[4] 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.

PassPIDFD=

Takes a boolean value. This controls the SO_PASSPIDFD socket option, which allows AF_UNIX sockets to receive the pidfd of the sending process in an ancillary message. Defaults to false. Added in version 258.

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.

AcceptFileDescriptors=

Takes a boolean value. This controls the SO_PASSRIGHTS socket option, which when disabled prohibits the peer from sending SCM_RIGHTS ancillary messages (aka file descriptors) via AF_UNIX sockets. Defaults to true. Added in version 258.

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.

DeferTrigger=

Takes a boolean argument, or "patient". May only be used when Accept=no. If enabled, job mode "lenient" instead of "replace" is used when triggering the service, which means currently activating/running units that conflict with the service won't be disturbed/brought down. Furthermore, if a conflict exists, the socket unit will wait for current job queue to complete and potentially defer the activation by then. An upper limit of total time to wait can be configured via DeferTriggerMaxSec=. If set to yes, the socket unit will fail if all jobs have finished or the timeout has been reached but the conflict remains. If patient, always wait until DeferTriggerMaxSec= elapses. Defaults to no. This setting is particularly useful if the socket unit should stay active across switch-root/soft-reboot operations while the triggered service is stopped.

Added in version 258.

DeferTriggerMaxSec=

Configures the maximum time to defer the triggering when DeferTrigger= is enabled. If the service cannot be activated within the specified time, the socket will be considered failed and get terminated. Takes a unit-less value in seconds, or a time span value such as "5min 20s". Pass "0" or "infinity" to disable the timeout logic (the default). Added in version 258.

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[5], Socket Activation, part II[6], Converting inetd Services[7], Socket Activated Internet Services and OS Containers[8].

## NOTES
1.

USB FunctionFS

https://docs.kernel.org/usb/functionfs.html

2.

RFC 3875

https://datatracker.ietf.org/doc/html/rfc3875

3.

TCP Keepalive HOWTO

http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/

4.

Smack

https://docs.kernel.org/admin-guide/LSM/Smack.html

5.

Socket Activation

https://0pointer.de/blog/projects/socket-activation.html

6.

Socket Activation, part II

https://0pointer.de/blog/projects/socket-activation2.html

7.

Converting inetd Services

https://0pointer.de/blog/projects/inetd.html

8.

Socket Activated Internet Services and OS Containers

https://0pointer.de/blog/projects/socket-activated-containers.html


systemd.swap

package: systemd

title: systemd.swap(5)

summary: Swap unit configuration

text:
# systemd.swap(5)

## NAME
systemd.swap - Swap unit configuration

## SYNOPSIS
swap.swap

## DESCRIPTION
A unit configuration file whose name ends in ".swap" encodes information about a swap device or file for memory paging controlled and supervised by systemd.

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 swap specific configuration options are configured in the [Swap] section.

Additional options are listed in systemd.exec(5), which define the execution environment the swapon(8) program is executed in, in systemd.kill(5), which define the way these processes are terminated, and in systemd.resource-control(5), which configure resource control settings for these processes of the unit.

Swap units must be named after the devices or files they control. Example: the swap device /dev/sda5 must be configured in a unit file dev-sda5.swap. For details about the escaping logic used to convert a file system path to a unit name, see systemd.unit(5). Note that swap units cannot be templated, nor is possible to add multiple names to a swap unit by creating additional symlinks to it.

Note that swap support on Linux is privileged, swap units are hence only available in the system service manager (and root's user service manager), but not in unprivileged user's service manager.

## AUTOMATIC DEPENDENCIES

### Implicit Dependencies
The following dependencies are implicitly added:

•All swap units automatically get the Requires= and After= dependencies on the device units or the mount units of the files they are activated from.

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:

•Swap units automatically acquire a Conflicts= and a Before= dependency on umount.target so that they are deactivated at shutdown as well as a Before=swap.target dependency.

## FSTAB
Swap units may either be configured via unit files, or via /etc/fstab (see fstab(5) for details). Swaps listed in /etc/fstab will be converted into native units dynamically at boot and when the configuration of the system manager is reloaded. See systemd-fstab-generator(8) for details about the conversion.

If a swap device or file is configured in both /etc/fstab and a unit file, the configuration in the latter takes precedence.

When reading /etc/fstab, a few special options are understood by systemd which influence how dependencies are created for swap units.

noauto, auto

With noauto, the swap unit will not be added as a dependency for swap.target. This means that it will not be activated automatically during boot, unless it is pulled in by some other unit. The auto option has the opposite meaning and is the default. Added in version 218.

nofail

With nofail, the swap unit will be only wanted, not required by swap.target. This means that the boot will continue even if this swap device is not activated successfully. Added in version 218.

x-systemd.device-timeout=

Configure how long systemd should wait for a device to show up before giving up on an entry from /etc/fstab. Specify a time in seconds or explicitly append a unit such as "s", "min", "h", "ms". Note that this option can only be used in /etc/fstab, and will be ignored when part of the Options= setting in a unit file.

Added in version 215.

x-systemd.makefs

The swap structure will be initialized on the device. If the device is not "empty", i.e. it contains any signature, the operation will be skipped. It is hence expected that this option remains set even after the device has been initialized. Note that this option can only be used in /etc/fstab, and will be ignored when part of the Options= setting in a unit file.

See systemd-mkswap@.service(8) and the discussion of wipefs(8) in systemd.mount(5).

Added in version 240.

## OPTIONS
Swap unit files may include [Unit] and [Install] sections, which are described in systemd.unit(5).

Swap unit files must include a [Swap] section, which carries information about the swap device 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 [Swap] section of swap units are the following:

What=

Takes an absolute path or a fstab-style identifier of a device node or file to use for paging. See swapon(8) for details. If this refers to a device node, a dependency on the respective device unit is automatically created. (See systemd.device(5) for more information.) If this refers to a file, a dependency on the respective mount unit is automatically created. (See systemd.mount(5) for more information.) This option is mandatory. Note that the usual specifier expansion is applied to this setting, literal percent characters should hence be written as "%%".

Priority=

Swap priority to use when activating the swap device or file. This takes an integer. This setting is optional and ignored when the priority is set by pri= in the Options= key.

Options=

May contain an option string for the swap device. This may be used for controlling discard options among other functionality, if the swap backing device supports the discard or trim operation. (See swapon(8) for more information.) Note that the usual specifier expansion is applied to this setting, literal percent characters should hence be written as "%%". Added in version 217.

TimeoutSec=

Configures the time to wait for the swapon command to finish. If a command does not exit within the configured time, the swap 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)).

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.device(5), systemd.mount(5), swapon(8), systemd-fstab-generator(8), systemd.directives(7)

branch: sisyphus


systemd.target

title: systemd.target(5)

summary: Target unit configuration

text:
# systemd.target(5)

## NAME
systemd.target - Target unit configuration

## SYNOPSIS
target.target

## DESCRIPTION
A unit configuration file whose name ends in ".target" encodes information about a target unit of systemd. Target units are used to group units and to set synchronization points for ordering dependencies with other unit files.

This unit type has no specific options. 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. A separate [Target] section does not exist, since no target-specific options may be configured.

Target units do not offer any additional functionality on top of the generic functionality provided by units. They merely group units, allowing a single target name to be used in Wants= and Requires= settings to establish a dependency on a set of units defined by the target, and in Before= and After= settings to establish ordering. Targets establish standardized names for synchronization points during boot and shutdown. Importantly, see systemd.special(7) for examples and descriptions of standard systemd targets.

Target units provide a more flexible replacement for SysV runlevels in the classic SysV init system.

Note that a target unit file must not be empty, lest it be considered a masked unit. It is recommended to provide a [Unit] section which includes informative Description= and Documentation= options.

## AUTOMATIC DEPENDENCIES

### Implicit Dependencies
There are no implicit dependencies for target units.

### Default Dependencies
The following dependencies are added unless DefaultDependencies=no is set:

•Target units will automatically complement all configured dependencies of type Wants= or Requires= with dependencies of type After= unless DefaultDependencies=no is set in the specified units. Note that the reverse is not true. For example, defining Wants=that.target in some.service will not automatically add the After=that.target ordering dependency for some.service. Instead, some.service should use the primary synchronization function of target type units, by setting a specific After=that.target or Before=that.target ordering dependency in its .service unit file.

•Target units automatically gain Conflicts= and Before= dependencies against shutdown.target.

## OPTIONS
Target unit files may include [Unit] and [Install] sections, which are described in systemd.unit(5). No options specific to this file type are supported.

## EXAMPLE
Example 1. Simple standalone target

```
# emergency-net.target
[Unit]
Description=Emergency Mode with Networking
Requires=emergency.target systemd-networkd.service
After=emergency.target systemd-networkd.service
AllowIsolate=yes
```

When adding dependencies to other units, it is important to check if they set DefaultDependencies=. Service units, unless they set DefaultDependencies=no, automatically get a dependency on sysinit.target. In this case, both emergency.target and systemd-networkd.service have DefaultDependencies=no, so they are suitable for use in this target, and do not pull in sysinit.target.

You can now switch into this emergency mode by running systemctl isolate emergency-net.target or by passing the option systemd.unit=emergency-net.target on the kernel command line.

Other units can have WantedBy=emergency-net.target in the [Install] section. After they are enabled using systemctl enable, they will be started before emergency-net.target is started. It is also possible to add arbitrary units as dependencies of emergency.target without modifying them by using systemctl add-wants.

## SEE ALSO
systemd(1), systemctl(1), systemd.unit(5), systemd.special(7), systemd.directives(7)


systemd.timer

title: systemd.timer(5)

summary: Timer unit configuration

text:
# systemd.timer(5)

## NAME
systemd.timer - Timer unit configuration

## SYNOPSIS
timer.timer

## DESCRIPTION
A unit configuration file whose name ends in ".timer" encodes information about a timer controlled and supervised by systemd, for timer-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 timer specific configuration options are configured in the [Timer] section.

For each timer file, a matching unit file must exist, describing the unit to activate when the timer elapses. By default, a service by the same name as the timer (except for the suffix) is activated. Example: a timer file foo.timer activates a matching service foo.service. The unit to activate may be controlled by Unit= (see below).

Note that in case the unit to activate is already active at the time the timer elapses it is not restarted, but simply left running. There is no concept of spawning new service instances in this case. Due to this, services with RemainAfterExit=yes set (which stay around continuously even after the service's main process exited) are usually not suitable for activation via repetitive timers, as they will only be activated once, and then stay around forever. Target units, which by default do not deactivate on their own, can be activated repeatedly by timers by setting StopWhenUnneeded=yes on them. This will cause a target unit to be stopped immediately after its activation, if it is not a dependency of another running unit.

## AUTOMATIC DEPENDENCIES

### Implicit Dependencies
The following dependencies are implicitly added:

•Timer units automatically gain a Before= dependency on the service they are supposed to activate.

### Default Dependencies
The following dependencies are added unless DefaultDependencies=no is set:

•Timer units will automatically have dependencies of type Requires= and After= on sysinit.target, a dependency of type Before= on timers.target, as well as Conflicts= and Before= on shutdown.target to ensure that they are stopped cleanly prior to system shutdown. Only timer units involved with early boot or late system shutdown should disable the DefaultDependencies= option.

•Timer units with at least one OnCalendar= directive acquire a pair of additional After= dependencies on time-set.target and time-sync.target, in order to avoid being started before the system clock has been correctly set. See systemd.special(7) for details on these two targets.

## OPTIONS
Timer unit files may include [Unit] and [Install] sections, which are described in systemd.unit(5).

Timer unit files must include a [Timer] section, which carries information about the timer it defines. The options specific to the [Timer] section of timer units are the following:

OnActiveSec=, OnBootSec=, OnStartupSec=, OnUnitActiveSec=, OnUnitInactiveSec=

Defines monotonic timers relative to different starting points: Table 1. Settings and their starting points

Setting

Meaning

OnActiveSec=

Defines a timer relative to the moment the timer unit itself is activated.

OnBootSec=

Defines a timer relative to when the machine was booted up. In containers, for the system manager instance, this is mapped to OnStartupSec=, making both equivalent.

OnStartupSec=

Defines a timer relative to when the service manager was first started. For system timer units this is very similar to OnBootSec= as the system service manager is generally started very early at boot. It's primarily useful when configured in units running in the per-user service manager, as the user service manager is generally started on first login only, not already during boot.

OnUnitActiveSec=

Defines a timer relative to when the unit the timer unit is activating was last activated.

OnUnitInactiveSec=

Defines a timer relative to when the unit the timer unit is activating was last deactivated.

Multiple directives may be combined of the same and of different types, in which case the timer unit will trigger whenever any of the specified timer expressions elapse. For example, by combining

OnBootSec= and OnUnitActiveSec=, it is possible to define a timer that elapses in regular intervals and activates a specific service each time. Moreover, both monotonic time expressions and OnCalendar= calendar expressions may be combined in the same timer unit.

The arguments to the directives are time spans configured in seconds. Example: "OnBootSec=50" means 50s after boot-up. The argument may also include time units. Example: "OnBootSec=5h 30min" means 5 hours and 30 minutes after boot-up. For details about the syntax of time spans, see systemd.time(7).

If a timer configured with OnBootSec= or OnStartupSec= is already in the past when the timer unit is activated, it will immediately elapse and the configured unit is started. This is not the case for timers defined in the other directives.

These are monotonic timers, independent of wall-clock time and timezones. If the computer is temporarily suspended, the monotonic clock generally pauses, too. Note that if WakeSystem= is used, a different monotonic clock is selected that continues to advance while the system is suspended and thus can be used as the trigger to resume the system.

If the empty string is assigned to any of these options, the list of timers is reset (both monotonic timers and OnCalendar= timers, see below), and all prior assignments will have no effect.

Note that timers do not necessarily expire at the precise time configured with these settings, as they are subject to the AccuracySec= setting below.

OnCalendar=

Defines realtime (i.e. wallclock) timers with calendar event expressions. See systemd.time(7) for more information on the syntax of calendar event expressions. Otherwise, the semantics are similar to OnActiveSec= and related settings. Note that timers do not necessarily expire at the precise time configured with this setting, as it is subject to the AccuracySec= setting below.

May be specified more than once, in which case the timer unit will trigger whenever any of the specified expressions elapse. Moreover, calendar timers and monotonic timers (see above) may be combined within the same timer unit.

If the empty string is assigned to any of these options, the list of timers is reset (both OnCalendar= timers and monotonic timers, see above), and all prior assignments will have no effect.

Note that calendar timers might be triggered at unexpected times if the system's realtime clock is not set correctly. Specifically, on systems that lack a battery-buffered Realtime Clock (RTC) it might be wise to enable systemd-time-wait-sync.service to ensure the clock is adjusted to a network time source before the timer event is set up. Timer units with at least one OnCalendar= expression are automatically ordered after time-sync.target, which systemd-time-wait-sync.service is ordered before.

When a system is temporarily put to sleep (i.e. system suspend or hibernation) the realtime clock does not pause. When a calendar timer elapses while the system is sleeping it will not be acted on immediately, but once the system is later resumed it will catch up and process all timers that triggered while the system was sleeping. Note that if a calendar timer elapsed more than once while the system was continuously sleeping the timer will only result in a single service activation. If WakeSystem= (see below) is enabled a calendar time event elapsing while the system is suspended will cause the system to wake up (under the condition the system's hardware supports time-triggered wake-up functionality).

Added in version 197.

AccuracySec=

Specify the accuracy the timer shall elapse with. Defaults to 1min. The timer is scheduled to elapse within a time window starting with the time specified in OnCalendar=, OnActiveSec=, OnBootSec=, OnStartupSec=, OnUnitActiveSec= or OnUnitInactiveSec= and ending the time configured with AccuracySec= later. Within this time window, the expiry time will be placed at a host-specific, randomized, but stable position that is synchronized between all local timer units. This is done in order to optimize power consumption to suppress unnecessary CPU wake-ups. To get best accuracy, set this option to 1us. Note that the timer is still subject to the timer slack configured via systemd-system.conf(5)'s TimerSlackNSec= setting. See prctl(2) for details. To optimize power consumption, make sure to set this value as high as possible and as low as necessary. Note that this setting is primarily a power saving option that allows coalescing CPU wake-ups. It should not be confused with RandomizedDelaySec= (see below) which adds a random value to the time the timer shall elapse next and whose purpose is the opposite: to stretch elapsing of timer events over a longer period to reduce workload spikes. For further details and explanations and how both settings play together, see below.

Added in version 209.

RandomizedDelaySec=

Delay the timer by a randomly selected, evenly distributed amount of time between 0 and the specified time value. Defaults to 0, indicating that no randomized delay shall be applied. Each timer unit will determine this delay randomly before each iteration, unless modified with FixedRandomDelay=, see below. The delay is added on top of the next determined elapsing time or the service manager's startup time, whichever is later. This setting is useful to stretch dispatching of similarly configured timer events over a certain time interval, to prevent them from firing all at the same time, possibly resulting in resource congestion on the local system.

Note the relation to AccuracySec= above: the latter allows the service manager to coalesce timer events within a specified time range in order to minimize wakeups, while this setting does the opposite: it stretches timer events over an interval, to make it unlikely that they fire simultaneously. If RandomizedDelaySec= and AccuracySec= are used in conjunction, first the randomized delay is added, and then the result is possibly further shifted to coalesce it with other timer events happening on the system. As mentioned above AccuracySec= defaults to 1 minute and RandomizedDelaySec= to 0, thus encouraging coalescing of timer events. In order to optimally stretch timer events over a certain range of time, set AccuracySec=1us and RandomizedDelaySec= to some higher value.

Added in version 229.

FixedRandomDelay=

Takes a boolean argument. When enabled, the randomized delay specified by RandomizedDelaySec= is chosen deterministically, and remains stable between all firings of the same timer, even if the manager is restarted. The delay is derived from the machine ID, the manager's user identifier, and the timer unit's name. This effectively creates a unique fixed offset for each timer, reducing the jitter in firings of an individual timer while still avoiding firing at the same time as other similarly configured timers. This setting has an effect only if RandomizedDelaySec= is not 0. Defaults to false.

Added in version 247.

RandomizedOffsetSec=

Offsets the timer by a stable, randomly-selected, and evenly distributed amount of time between 0 and the specified time value. Defaults to 0, indicating that no such offset shall be applied. The offset is chosen deterministically, and is derived the same way as FixedRandomDelay=, see above. The offset is added on top of the next determined elapsing time. This setting only has an effect on timers configured with OnCalendar=, and it can be combined with RandomizedDelaySec=. Much like RandomizedDelaySec=, this setting is for distributing timer events to prevent them from firing all at once. However, this setting is most useful to prevent resource congestion on a remote service, from a fleet of similarly-configured clients. Unlike RandomizedDelaySec=, this setting applies its offset with no regard to manager startup time. This maintains the periodicity of configured OnCalendar= events across manager restarts.

For example, let's say you're running a backup service and have a fleet of laptops that wish to make backups weekly. To distribute load on the backup service, each laptop should randomly pick a weekday to upload its backups. This could be achieved by setting OnCalendar= to "weekly", and then configuring a RandomizedDelaySec= of "5 days" with FixedRandomDelay= enabled. Let's say that some laptop randomly chooses a delay of 4 days. If this laptop is restarted more often than that, then the timer will never fire: on each fresh boot, the 4 day delay is restarted and will not be finished by the time of the next shutdown. Instead, you should use RandomizedOffsetSec=, which will maintain the configured weekly cadence of timer events, even across reboots.

Added in version 258.

DeferReactivation=

Takes a boolean argument. When enabled, the timer schedules the next elapse based on the trigger unit entering inactivity, instead of the last trigger time. This is most apparent in the case where the service unit takes longer to run than the timer interval. With this setting enabled, the timer will schedule the next elapse based on when the service finishes running, and so it will have to wait until the next realtime elapse time to trigger. Otherwise, the default behavior is for the timer unit to immediately trigger again once the service finishes running. This happens because the timer schedules the next elapse based on the previous trigger time, and since the interval is shorter than the service runtime, that elapse will be in the past, causing it to immediately trigger once done. This setting has an effect only if a realtime timer has been specified with OnCalendar=. Defaults to false.

Added in version 257.

OnClockChange=, OnTimezoneChange=

These options take boolean arguments. When true, the service unit will be triggered when the system clock (CLOCK_REALTIME) jumps relative to the monotonic clock (CLOCK_MONOTONIC), or when the local system timezone is modified. These options can be used alone or in combination with other timer expressions (see above) within the same timer unit. These options default to false. Added in version 242.

Unit=

The unit to activate when this timer elapses. The argument is a unit name, whose suffix is not ".timer". If not specified, this value defaults to a service that has the same name as the timer unit, except for the suffix. (See above.) It is recommended that the unit name that is activated and the unit name of the timer unit are named identically, except for the suffix.

Persistent=

Takes a boolean argument. If true, the time when the service unit was last triggered is stored on disk. When the timer is activated, the service unit is triggered immediately if it would have been triggered at least once during the time when the timer was inactive. Such triggering is nonetheless subject to the delay imposed by RandomizedDelaySec=. This is useful to catch up on missed runs of the service when the system was powered down. Note that this setting only has an effect on timers configured with OnCalendar=. Defaults to false. Use systemctl clean --what=state ... on the timer unit to remove the timestamp file maintained by this option from disk. In particular, use this command before uninstalling a timer unit. See systemctl(1) for details.

Added in version 212.

WakeSystem=

Takes a boolean argument. If true, an elapsing timer will cause the system to resume from suspend, should it be suspended and if the system supports this. Note that this option will only make sure the system resumes on the appropriate times, it will not take care of suspending it again after any work that is to be done is finished. Defaults to false. Note that this functionality requires privileges and is thus generally only available in the system service manager.

Note that behaviour of monotonic clock timers (as configured with OnActiveSec=, OnBootSec=, OnStartupSec=, OnUnitActiveSec=, OnUnitInactiveSec=, see above) is altered depending on this option. If false, a monotonic clock is used that is paused during system suspend (CLOCK_MONOTONIC), if true a different monotonic clock is used that continues advancing during system suspend (CLOCK_BOOTTIME), see clock_getres(2) for details.

Added in version 212.

RemainAfterElapse=

Takes a boolean argument. If true, a timer will stay loaded, and its state remains queryable even after it elapsed and the associated unit (as configured with Unit=, see above) deactivated again. If false, an elapsed timer unit that cannot elapse anymore is unloaded once its associated unit deactivated again. Turning this off is particularly useful for transient timer units. Note that this setting has an effect when repeatedly starting a timer unit: if RemainAfterElapse= is on, starting the timer a second time has no effect. However, if RemainAfterElapse= is off and the timer unit was already unloaded, it can be started again, and thus the service can be triggered multiple times. Defaults to true. Added in version 229.

Check systemd.unit(5), systemd.exec(5), and systemd.kill(5) for more settings.

## SEE ALSO
Environment variables with details on the trigger will be set for triggered units. See the "Environment Variables Set or Propagated by the Service Manager" section in systemd.exec(5) for more details.

systemd(1), systemctl(1), systemd.unit(5), systemd.service(5), systemd.time(7), systemd.directives(7), systemd-system.conf(5), prctl(2)