Provided by: systemd_234-2ubuntu12_amd64 bug

NAME

       systemd.exec - Execution environment configuration

SYNOPSIS

       service.service, socket.socket, mount.mount, swap.swap

DESCRIPTION

       Unit configuration files for services, sockets, mount points, and swap devices share a
       subset of configuration options which define the execution environment of spawned
       processes.

       This man page lists the configuration options shared by these four unit types. See
       systemd.unit(5) for the common options of all unit configuration files, and
       systemd.service(5), systemd.socket(5), systemd.swap(5), and systemd.mount(5) for more
       information on the specific unit configuration files. The execution specific configuration
       options are configured in the [Service], [Socket], [Mount], or [Swap] sections, depending
       on the unit type.

       In addition, options which control resources through Linux Control Groups (cgroups) are
       listed in systemd.resource-control(5). Those options complement options listed here.

AUTOMATIC DEPENDENCIES

       A few execution parameters result in additional, automatic dependencies to be added.

       Units with WorkingDirectory=, RootDirectory= or RootImage= set automatically gain
       dependencies of type Requires= and After= on all mount units required to access the
       specified paths. This is equivalent to having them listed explicitly in
       RequiresMountsFor=.

       Similar, units with PrivateTmp= enabled automatically get mount unit dependencies for all
       mounts required to access /tmp and /var/tmp. They will also gain an automatic After=
       dependency on systemd-tmpfiles-setup.service(8).

       Units whose standard output or error output is connected to journal, syslog or kmsg (or
       their combinations with console output, see below) automatically acquire dependencies of
       type After= on systemd-journald.socket.

OPTIONS

       WorkingDirectory=
           Takes a directory path relative to the service's root directory specified by
           RootDirectory=, or the special value "~". Sets the working directory for executed
           processes. If set to "~", the home directory of the user specified in User= is used.
           If not set, defaults to the root directory when systemd is running as a system
           instance and the respective user's home directory if run as user. If the setting is
           prefixed with the "-" character, a missing working directory is not considered fatal.
           If RootDirectory=/RootImage= is not set, then WorkingDirectory= is relative to the
           root of the system running the service manager. Note that setting this parameter might
           result in additional dependencies to be added to the unit (see above).

       RootDirectory=
           Takes a directory path relative to the host's root directory (i.e. the root of the
           system running the service manager). Sets the root directory for executed processes,
           with the chroot(2) system call. If this is used, it must be ensured that the process
           binary and all its auxiliary files are available in the chroot() jail. Note that
           setting this parameter might result in additional dependencies to be added to the unit
           (see above).

           The MountAPIVFS= and PrivateUsers= settings are particularly useful in conjunction
           with RootDirectory=. For details, see below.

       RootImage=
           Takes a path to a block device node or regular file as argument. This call is similar
           to RootDirectory= however mounts a file system hierarchy from a block device node or
           loopback file instead of a directory. The device node or file system image file needs
           to contain a file system without a partition table, or a file system within an
           MBR/MS-DOS or GPT partition table with only a single Linux-compatible partition, or a
           set of file systems within a GPT partition table that follows the Discoverable
           Partitions Specification[1].

       MountAPIVFS=
           Takes a boolean argument. If on, a private mount namespace for the unit's processes is
           created and the API file systems /proc, /sys, and /dev are mounted inside of it,
           unless they are already mounted. Note that this option has no effect unless used in
           conjunction with RootDirectory=/RootImage= as these three mounts are generally mounted
           in the host anyway, and unless the root directory is changed, the private mount
           namespace will be a 1:1 copy of the host's, and include these three mounts. Note that
           the /dev file system of the host is bind mounted if this option is used without
           PrivateDevices=. To run the service with a private, minimal version of /dev/, combine
           this option with PrivateDevices=.

       User=, Group=
           Set the UNIX user or group that the processes are executed as, respectively. Takes a
           single user or group name, or a numeric ID as argument. For system services (services
           run by the system service manager, i.e. managed by PID 1) and for user services of the
           root user (services managed by root's instance of systemd --user), the default is
           "root", but User= may be used to specify a different user. For user services of any
           other user, switching user identity is not permitted, hence the only valid setting is
           the same user the user's service manager is running as. If no group is set, the
           default group of the user is used. This setting does not affect commands whose command
           line is prefixed with "+".

           Note that restrictions on the user/group name syntax are enforced: the specified name
           must consist only of the characters a-z, A-Z, 0-9, "_" and "-", except for the first
           character which must be one of a-z, A-Z or "_" (i.e. numbers and "-" are not permitted
           as first character). The user/group name must have at least one character, and at most
           31. These restrictions are enforced in order to avoid ambiguities and to ensure
           user/group names and unit files remain portable among Linux systems.

           When used in conjunction with DynamicUser= the user/group name specified is
           dynamically allocated at the time the service is started, and released at the time the
           service is stopped — unless it is already allocated statically (see below). If
           DynamicUser= is not used the specified user and group must have been created
           statically in the user database no later than the moment the service is started, for
           example using the sysusers.d(5) facility, which is applied at boot or package install
           time.

       DynamicUser=
           Takes a boolean parameter. If set, a UNIX user and group pair is allocated dynamically
           when the unit is started, and released as soon as it is stopped. The user and group
           will not be added to /etc/passwd or /etc/group, but are managed transiently during
           runtime. The nss-systemd(8) glibc NSS module provides integration of these dynamic
           users/groups into the system's user and group databases. The user and group name to
           use may be configured via User= and Group= (see above). If these options are not used
           and dynamic user/group allocation is enabled for a unit, the name of the dynamic
           user/group is implicitly derived from the unit name. If the unit name without the type
           suffix qualifies as valid user name it is used directly, otherwise a name
           incorporating a hash of it is used. If a statically allocated user or group of the
           configured name already exists, it is used and no dynamic user/group is allocated.
           Dynamic users/groups are allocated from the UID/GID range 61184...65519. It is
           recommended to avoid this range for regular system or login users. At any point in
           time each UID/GID from this range is only assigned to zero or one dynamically
           allocated users/groups in use. However, UID/GIDs are recycled after a unit is
           terminated. Care should be taken that any processes running as part of a unit for
           which dynamic users/groups are enabled do not leave files or directories owned by
           these users/groups around, as a different unit might get the same UID/GID assigned
           later on, and thus gain access to these files or directories. If DynamicUser= is
           enabled, RemoveIPC=, PrivateTmp= are implied. This ensures that the lifetime of IPC
           objects and temporary files created by the executed processes is bound to the runtime
           of the service, and hence the lifetime of the dynamic user/group. Since /tmp and
           /var/tmp are usually the only world-writable directories on a system this ensures that
           a unit making use of dynamic user/group allocation cannot leave files around after
           unit termination. Moreover ProtectSystem=strict and ProtectHome=read-only are implied,
           thus prohibiting the service to write to arbitrary file system locations. In order to
           allow the service to write to certain directories, they have to be whitelisted using
           ReadWritePaths=, but care must be taken so that UID/GID recycling doesn't create
           security issues involving files created by the service. Use RuntimeDirectory= (see
           below) in order to assign a writable runtime directory to a service, owned by the
           dynamic user/group and removed automatically when the unit is terminated. Defaults to
           off.

       SupplementaryGroups=
           Sets the supplementary Unix groups the processes are executed as. This takes a
           space-separated list of group names or IDs. This option may be specified more than
           once, in which case all listed groups are set as supplementary groups. When the empty
           string is assigned, the list of supplementary groups is reset, and all assignments
           prior to this one will have no effect. In any way, this option does not override, but
           extends the list of supplementary groups configured in the system group database for
           the user. This does not affect commands prefixed with "+".

       RemoveIPC=
           Takes a boolean parameter. If set, all System V and POSIX IPC objects owned by the
           user and group the processes of this unit are run as are removed when the unit is
           stopped. This setting only has an effect if at least one of User=, Group= and
           DynamicUser= are used. It has no effect on IPC objects owned by the root user.
           Specifically, this removes System V semaphores, as well as System V and POSIX shared
           memory segments and message queues. If multiple units use the same user or group the
           IPC objects are removed when the last of these units is stopped. This setting is
           implied if DynamicUser= is set.

       Nice=
           Sets the default nice level (scheduling priority) for executed processes. Takes an
           integer between -20 (highest priority) and 19 (lowest priority). See setpriority(2)
           for details.

       OOMScoreAdjust=
           Sets the adjustment level for the Out-Of-Memory killer for executed processes. Takes
           an integer between -1000 (to disable OOM killing for this process) and 1000 (to make
           killing of this process under memory pressure very likely). See proc.txt[2] for
           details.

       IOSchedulingClass=
           Sets the I/O scheduling class for executed processes. Takes an integer between 0 and 3
           or one of the strings none, realtime, best-effort or idle. See ioprio_set(2) for
           details.

       IOSchedulingPriority=
           Sets the I/O scheduling priority for executed processes. Takes an integer between 0
           (highest priority) and 7 (lowest priority). The available priorities depend on the
           selected I/O scheduling class (see above). See ioprio_set(2) for details.

       CPUSchedulingPolicy=
           Sets the CPU scheduling policy for executed processes. Takes one of other, batch,
           idle, fifo or rr. See sched_setscheduler(2) for details.

       CPUSchedulingPriority=
           Sets the CPU scheduling priority for executed processes. The available priority range
           depends on the selected CPU scheduling policy (see above). For real-time scheduling
           policies an integer between 1 (lowest priority) and 99 (highest priority) can be used.
           See sched_setscheduler(2) for details.

       CPUSchedulingResetOnFork=
           Takes a boolean argument. If true, elevated CPU scheduling priorities and policies
           will be reset when the executed processes fork, and can hence not leak into child
           processes. See sched_setscheduler(2) for details. Defaults to false.

       CPUAffinity=
           Controls the CPU affinity of the executed processes. Takes a list of CPU indices or
           ranges separated by either whitespace or commas. CPU ranges are specified by the lower
           and upper CPU indices separated by a dash. This option may be specified more than
           once, in which case the specified CPU affinity masks are merged. If the empty string
           is assigned, the mask is reset, all assignments prior to this will have no effect. See
           sched_setaffinity(2) for details.

       UMask=
           Controls the file mode creation mask. Takes an access mode in octal notation. See
           umask(2) for details. Defaults to 0022.

       Environment=
           Sets environment variables for executed processes. Takes a space-separated list of
           variable assignments. This option may be specified more than once, in which case all
           listed variables will be set. If the same variable is set twice, the later setting
           will override the earlier setting. If the empty string is assigned to this option, the
           list of environment variables is reset, all prior assignments have no effect. Variable
           expansion is not performed inside the strings, however, specifier expansion is
           possible. The $ character has no special meaning. If you need to assign a value
           containing spaces or the equals sign to a variable, use double quotes (") for the
           assignment.

           Example:

               Environment="VAR1=word1 word2" VAR2=word3 "VAR3=$word 5 6"

           gives three variables "VAR1", "VAR2", "VAR3" with the values "word1 word2", "word3",
           "$word 5 6".

           See environ(7) for details about environment variables.

       EnvironmentFile=
           Similar to Environment= but reads the environment variables from a text file. The text
           file should contain new-line-separated variable assignments. Empty lines, lines
           without an "=" separator, or lines starting with ; or # will be ignored, which may be
           used for commenting. A line ending with a backslash will be concatenated with the
           following one, allowing multiline variable definitions. The parser strips leading and
           trailing whitespace from the values of assignments, unless you use double quotes (").

           The argument passed should be an absolute filename or wildcard expression, optionally
           prefixed with "-", which indicates that if the file does not exist, it will not be
           read and no error or warning message is logged. This option may be specified more than
           once in which case all specified files are read. If the empty string is assigned to
           this option, the list of file to read is reset, all prior assignments have no effect.

           The files listed with this directive will be read shortly before the process is
           executed (more specifically, after all processes from a previous unit state
           terminated. This means you can generate these files in one unit state, and read it
           with this option in the next).

           Settings from these files override settings made with Environment=. If the same
           variable is set twice from these files, the files will be read in the order they are
           specified and the later setting will override the earlier setting.

       PassEnvironment=
           Pass environment variables from the systemd system manager to executed processes.
           Takes a space-separated list of variable names. This option may be specified more than
           once, in which case all listed variables will be set. If the empty string is assigned
           to this option, the list of environment variables is reset, all prior assignments have
           no effect. Variables that are not set in the system manager will not be passed and
           will be silently ignored.

           Variables passed from this setting are overridden by those passed from Environment= or
           EnvironmentFile=.

           Example:

               PassEnvironment=VAR1 VAR2 VAR3

           passes three variables "VAR1", "VAR2", "VAR3" with the values set for those variables
           in PID1.

           See environ(7) for details about environment variables.

       StandardInput=
           Controls where file descriptor 0 (STDIN) of the executed processes is connected to.
           Takes one of null, tty, tty-force, tty-fail, socket or fd.

           If null is selected, standard input will be connected to /dev/null, i.e. all read
           attempts by the process will result in immediate EOF.

           If tty is selected, standard input is connected to a TTY (as configured by TTYPath=,
           see below) and the executed process becomes the controlling process of the terminal.
           If the terminal is already being controlled by another process, the executed process
           waits until the current controlling process releases the terminal.

           tty-force is similar to tty, but the executed process is forcefully and immediately
           made the controlling process of the terminal, potentially removing previous
           controlling processes from the terminal.

           tty-fail is similar to tty but if the terminal already has a controlling process
           start-up of the executed process fails.

           The socket option is only valid in socket-activated services, and only when the socket
           configuration file (see systemd.socket(5) for details) specifies a single socket only.
           If this option is set, standard input will be connected to the socket the service was
           activated from, which is primarily useful for compatibility with daemons designed for
           use with the traditional inetd(8) daemon.

           The fd option connects the input stream to a single file descriptor provided by a
           socket unit. A custom named file descriptor can be specified as part of this option,
           after a ":" (e.g.  "fd:foobar"). If no name is specified, "stdin" is assumed (i.e.
           "fd" is equivalent to "fd:stdin"). At least one socket unit defining such name must be
           explicitly provided via the Sockets= option, and file descriptor name may differ from
           the name of its containing socket unit. If multiple matches are found, the first one
           will be used. See FileDescriptorName= in systemd.socket(5) for more details about
           named descriptors and ordering.

           This setting defaults to null.

       StandardOutput=
           Controls where file descriptor 1 (STDOUT) of the executed processes is connected to.
           Takes one of inherit, null, tty, journal, syslog, kmsg, journal+console,
           syslog+console, kmsg+console, socket or fd.

           inherit duplicates the file descriptor of standard input for standard output.

           null connects standard output to /dev/null, i.e. everything written to it will be
           lost.

           tty connects standard output to a tty (as configured via TTYPath=, see below). If the
           TTY is used for output only, the executed process will not become the controlling
           process of the terminal, and will not fail or wait for other processes to release the
           terminal.

           journal connects standard output with the journal which is accessible via
           journalctl(1). Note that everything that is written to syslog or kmsg (see below) is
           implicitly stored in the journal as well, the specific two options listed below are
           hence supersets of this one.

           syslog connects standard output to the syslog(3) system syslog service, in addition to
           the journal. Note that the journal daemon is usually configured to forward everything
           it receives to syslog anyway, in which case this option is no different from journal.

           kmsg connects standard output with the kernel log buffer which is accessible via
           dmesg(1), in addition to the journal. The journal daemon might be configured to send
           all logs to kmsg anyway, in which case this option is no different from journal.

           journal+console, syslog+console and kmsg+console work in a similar way as the three
           options above but copy the output to the system console as well.

           socket connects standard output to a socket acquired via socket activation. The
           semantics are similar to the same option of StandardInput=.

           The fd option connects the output stream to a single file descriptor provided by a
           socket unit. A custom named file descriptor can be specified as part of this option,
           after a ":" (e.g.  "fd:foobar"). If no name is specified, "stdout" is assumed (i.e.
           "fd" is equivalent to "fd:stdout"). At least one socket unit defining such name must
           be explicitly provided via the Sockets= option, and file descriptor name may differ
           from the name of its containing socket unit. If multiple matches are found, the first
           one will be used. See FileDescriptorName= in systemd.socket(5) for more details about
           named descriptors and ordering.

           If the standard output (or error output, see below) of a unit is connected to the
           journal, syslog or the kernel log buffer, the unit will implicitly gain a dependency
           of type After= on systemd-journald.socket (also see the automatic dependencies section
           above).

           This setting defaults to the value set with DefaultStandardOutput= in systemd-
           system.conf(5), which defaults to journal. Note that setting this parameter might
           result in additional dependencies to be added to the unit (see above).

       StandardError=
           Controls where file descriptor 2 (STDERR) of the executed processes is connected to.
           The available options are identical to those of StandardOutput=, with some exceptions:
           if set to inherit the file descriptor used for standard output is duplicated for
           standard error, while fd operates on the error stream and will look by default for a
           descriptor named "stderr".

           This setting defaults to the value set with DefaultStandardError= in systemd-
           system.conf(5), which defaults to inherit. Note that setting this parameter might
           result in additional dependencies to be added to the unit (see above).

       TTYPath=
           Sets the terminal device node to use if standard input, output, or error are connected
           to a TTY (see above). Defaults to /dev/console.

       TTYReset=
           Reset the terminal device specified with TTYPath= before and after execution. Defaults
           to "no".

       TTYVHangup=
           Disconnect all clients which have opened the terminal device specified with TTYPath=
           before and after execution. Defaults to "no".

       TTYVTDisallocate=
           If the terminal device specified with TTYPath= is a virtual console terminal, try to
           deallocate the TTY before and after execution. This ensures that the screen and
           scrollback buffer is cleared. Defaults to "no".

       SyslogIdentifier=
           Sets the process name to prefix log lines sent to the logging system or the kernel log
           buffer with. If not set, defaults to the process name of the executed process. This
           option is only useful when StandardOutput= or StandardError= are set to syslog,
           journal or kmsg (or to the same settings in combination with +console).

       SyslogFacility=
           Sets the syslog facility to use when logging to syslog. One of kern, user, mail,
           daemon, auth, syslog, lpr, news, uucp, cron, authpriv, ftp, local0, local1, local2,
           local3, local4, local5, local6 or local7. See syslog(3) for details. This option is
           only useful when StandardOutput= or StandardError= are set to syslog. Defaults to
           daemon.

       SyslogLevel=
           The default syslog level to use when logging to syslog or the kernel log buffer. One
           of emerg, alert, crit, err, warning, notice, info, debug. See syslog(3) for details.
           This option is only useful when StandardOutput= or StandardError= are set to syslog or
           kmsg. Note that individual lines output by the daemon might be prefixed with a
           different log level which can be used to override the default log level specified
           here. The interpretation of these prefixes may be disabled with SyslogLevelPrefix=,
           see below. For details, see sd-daemon(3). Defaults to info.

       SyslogLevelPrefix=
           Takes a boolean argument. If true and StandardOutput= or StandardError= are set to
           syslog, kmsg or journal, log lines written by the executed process that are prefixed
           with a log level will be passed on to syslog with this log level set but the prefix
           removed. If set to false, the interpretation of these prefixes is disabled and the
           logged lines are passed on as-is. For details about this prefixing see sd-daemon(3).
           Defaults to true.

       TimerSlackNSec=
           Sets the timer slack in nanoseconds for the executed processes. The timer slack
           controls the accuracy of wake-ups triggered by timers. See prctl(2) for more
           information. Note that in contrast to most other time span definitions this parameter
           takes an integer value in nano-seconds if no unit is specified. The usual time units
           are understood too.

       LimitCPU=, LimitFSIZE=, LimitDATA=, LimitSTACK=, LimitCORE=, LimitRSS=, LimitNOFILE=,
       LimitAS=, LimitNPROC=, LimitMEMLOCK=, LimitLOCKS=, LimitSIGPENDING=, LimitMSGQUEUE=,
       LimitNICE=, LimitRTPRIO=, LimitRTTIME=
           Set soft and hard limits on various resources for executed processes. See setrlimit(2)
           for details on the resource limit concept. Resource limits may be specified in two
           formats: either as single value to set a specific soft and hard limit to the same
           value, or as colon-separated pair soft:hard to set both limits individually (e.g.
           "LimitAS=4G:16G"). Use the string infinity to configure no limit on a specific
           resource. The multiplicative suffixes K, M, G, T, P and E (to the base 1024) may be
           used for resource limits measured in bytes (e.g. LimitAS=16G). For the limits
           referring to time values, the usual time units ms, s, min, h and so on may be used
           (see systemd.time(7) for details). Note that if no time unit is specified for
           LimitCPU= the default unit of seconds is implied, while for LimitRTTIME= the default
           unit of microseconds is implied. Also, note that the effective granularity of the
           limits might influence their enforcement. For example, time limits specified for
           LimitCPU= will be rounded up implicitly to multiples of 1s. For LimitNICE= the value
           may be specified in two syntaxes: if prefixed with "+" or "-", the value is understood
           as regular Linux nice value in the range -20..19. If not prefixed like this the value
           is understood as raw resource limit parameter in the range 0..40 (with 0 being
           equivalent to 1).

           Note that most process resource limits configured with these options are per-process,
           and processes may fork in order to acquire a new set of resources that are accounted
           independently of the original process, and may thus escape limits set. Also note that
           LimitRSS= is not implemented on Linux, and setting it has no effect. Often it is
           advisable to prefer the resource controls listed in systemd.resource-control(5) over
           these per-process limits, as they apply to services as a whole, may be altered
           dynamically at runtime, and are generally more expressive. For example, MemoryLimit=
           is a more powerful (and working) replacement for LimitRSS=.

           For system units these resource limits may be chosen freely. For user units however
           (i.e. units run by a per-user instance of systemd(1)), these limits are bound by
           (possibly more restrictive) per-user limits enforced by the OS.

           Resource limits not configured explicitly for a unit default to the value configured
           in the various DefaultLimitCPU=, DefaultLimitFSIZE=, ... options available in systemd-
           system.conf(5), and – if not configured there – the kernel or per-user defaults, as
           defined by the OS (the latter only for user services, see above).

           Table 1. Resource limit directives, their equivalent ulimit shell commands and the
           unit used
           ┌─────────────────┬───────────────────┬──────────────────────────┐
           │Directiveulimit equivalent │ Unit                     │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitCPU=        │ ulimit -t         │ Seconds                  │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitFSIZE=      │ ulimit -f         │ Bytes                    │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitDATA=       │ ulimit -d         │ Bytes                    │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitSTACK=      │ ulimit -s         │ Bytes                    │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitCORE=       │ ulimit -c         │ Bytes                    │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitRSS=        │ ulimit -m         │ Bytes                    │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitNOFILE=     │ ulimit -n         │ Number of File           │
           │                 │                   │ Descriptors              │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitAS=         │ ulimit -v         │ Bytes                    │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitNPROC=      │ ulimit -u         │ Number of Processes      │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitMEMLOCK=    │ ulimit -l         │ Bytes                    │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitLOCKS=      │ ulimit -x         │ Number of Locks          │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitSIGPENDING= │ ulimit -i         │ Number of Queued Signals │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitMSGQUEUE=   │ ulimit -q         │ Bytes                    │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitNICE=       │ ulimit -e         │ Nice Level               │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitRTPRIO=     │ ulimit -r         │ Realtime Priority        │
           ├─────────────────┼───────────────────┼──────────────────────────┤
           │LimitRTTIME=     │ No equivalent     │ Microseconds             │
           └─────────────────┴───────────────────┴──────────────────────────┘

       PAMName=
           Sets the PAM service name to set up a session as. If set, the executed process will be
           registered as a PAM session under the specified service name. This is only useful in
           conjunction with the User= setting, and is otherwise ignored. If not set, no PAM
           session will be opened for the executed processes. See pam(8) for details.

           Note that for each unit making use of this option a PAM session handler process will
           be maintained as part of the unit and stays around as long as the unit is active, to
           ensure that appropriate actions can be taken when the unit and hence the PAM session
           terminates. This process is named "(sd-pam)" and is an immediate child process of the
           unit's main process.

       CapabilityBoundingSet=
           Controls which capabilities to include in the capability bounding set for the executed
           process. See capabilities(7) for details. Takes a whitespace-separated list of
           capability names, e.g.  CAP_SYS_ADMIN, CAP_DAC_OVERRIDE, CAP_SYS_PTRACE. Capabilities
           listed will be included in the bounding set, all others are removed. If the list of
           capabilities is prefixed with "~", all but the listed capabilities will be included,
           the effect of the assignment inverted. Note that this option also affects the
           respective capabilities in the effective, permitted and inheritable capability sets.
           If this option is not used, the capability bounding set is not modified on process
           execution, hence no limits on the capabilities of the process are enforced. This
           option may appear more than once, in which case the bounding sets are merged. If the
           empty string is assigned to this option, the bounding set is reset to the empty
           capability set, and all prior settings have no effect. If set to "~" (without any
           further argument), the bounding set is reset to the full set of available
           capabilities, also undoing any previous settings. This does not affect commands
           prefixed with "+".

       AmbientCapabilities=
           Controls which capabilities to include in the ambient capability set for the executed
           process. Takes a whitespace-separated list of capability names, e.g.  CAP_SYS_ADMIN,
           CAP_DAC_OVERRIDE, CAP_SYS_PTRACE. This option may appear more than once in which case
           the ambient capability sets are merged. If the list of capabilities is prefixed with
           "~", all but the listed capabilities will be included, the effect of the assignment
           inverted. If the empty string is assigned to this option, the ambient capability set
           is reset to the empty capability set, and all prior settings have no effect. If set to
           "~" (without any further argument), the ambient capability set is reset to the full
           set of available capabilities, also undoing any previous settings. Note that adding
           capabilities to ambient capability set adds them to the process's inherited capability
           set.

           Ambient capability sets are useful if you want to execute a process as a
           non-privileged user but still want to give it some capabilities. Note that in this
           case option keep-caps is automatically added to SecureBits= to retain the capabilities
           over the user change.  AmbientCapabilities= does not affect commands prefixed with
           "+".

       SecureBits=
           Controls the secure bits set for the executed process. Takes a space-separated
           combination of options from the following list: keep-caps, keep-caps-locked,
           no-setuid-fixup, no-setuid-fixup-locked, noroot, and noroot-locked. This option may
           appear more than once, in which case the secure bits are ORed. If the empty string is
           assigned to this option, the bits are reset to 0. This does not affect commands
           prefixed with "+". See capabilities(7) for details.

       ReadWritePaths=, ReadOnlyPaths=, InaccessiblePaths=
           Sets up a new file system namespace for executed processes. These options may be used
           to limit access a process might have to the file system hierarchy. Each setting takes
           a space-separated list of paths relative to the host's root directory (i.e. the system
           running the service manager). Note that if paths contain symlinks, they are resolved
           relative to the root directory set with RootDirectory=/RootImage=.

           Paths listed in ReadWritePaths= are accessible from within the namespace with the same
           access modes as from outside of it. Paths listed in ReadOnlyPaths= are accessible for
           reading only, writing will be refused even if the usual file access controls would
           permit this. Nest ReadWritePaths= inside of ReadOnlyPaths= in order to provide
           writable subdirectories within read-only directories. Use ReadWritePaths= in order to
           whitelist specific paths for write access if ProtectSystem=strict is used. Paths
           listed in InaccessiblePaths= will be made inaccessible for processes inside the
           namespace (along with everything below them in the file system hierarchy).

           Note that restricting access with these options does not extend to submounts of a
           directory that are created later on. Non-directory paths may be specified as well.
           These options may be specified more than once, in which case all paths listed will
           have limited access from within the namespace. If the empty string is assigned to this
           option, the specific list is reset, and all prior assignments have no effect.

           Paths in ReadWritePaths=, ReadOnlyPaths= and InaccessiblePaths= may be prefixed with
           "-", in which case they will be ignored when they do not exist. If prefixed with "+"
           the paths are taken relative to the root directory of the unit, as configured with
           RootDirectory=/RootImage=, instead of relative to the root directory of the host (see
           above). When combining "-" and "+" on the same path make sure to specify "-" first,
           and "+" second.

           Note that using this setting will disconnect propagation of mounts from the service to
           the host (propagation in the opposite direction continues to work). This means that
           this setting may not be used for services which shall be able to install mount points
           in the main mount namespace. Note that the effect of these settings may be undone by
           privileged processes. In order to set up an effective sandboxed environment for a unit
           it is thus recommended to combine these settings with either
           CapabilityBoundingSet=~CAP_SYS_ADMIN or SystemCallFilter=~@mount.

       BindPaths=, BindReadOnlyPaths=
           Configures unit-specific bind mounts. A bind mount makes a particular file or
           directory available at an additional place in the unit's view of the file system. Any
           bind mounts created with this option are specific to the unit, and are not visible in
           the host's mount table. This option expects a whitespace separated list of bind mount
           definitions. Each definition consists of a colon-separated triple of source path,
           destination path and option string, where the latter two are optional. If only a
           source path is specified the source and destination is taken to be the same. The
           option string may be either "rbind" or "norbind" for configuring a recursive or
           non-recursive bind mount. If the destination path is omitted, the option string must
           be omitted too.

           BindPaths= creates regular writable bind mounts (unless the source file system mount
           is already marked read-only), while BindReadOnlyPaths= creates read-only bind mounts.
           These settings may be used more than once, each usage appends to the unit's list of
           bind mounts. If the empty string is assigned to either of these two options the entire
           list of bind mounts defined prior to this is reset. Note that in this case both
           read-only and regular bind mounts are reset, regardless which of the two settings is
           used.

           This option is particularly useful when RootDirectory=/RootImage= is used. In this
           case the source path refers to a path on the host file system, while the destination
           path refers to a path below the root directory of the unit.

       PrivateTmp=
           Takes a boolean argument. If true, sets up a new file system namespace for the
           executed processes and mounts private /tmp and /var/tmp directories inside it that is
           not shared by processes outside of the namespace. This is useful to secure access to
           temporary files of the process, but makes sharing between processes via /tmp or
           /var/tmp impossible. If this is enabled, all temporary files created by a service in
           these directories will be removed after the service is stopped. Defaults to false. It
           is possible to run two or more units within the same private /tmp and /var/tmp
           namespace by using the JoinsNamespaceOf= directive, see systemd.unit(5) for details.
           This setting is implied if DynamicUser= is set. For this setting the same restrictions
           regarding mount propagation and privileges apply as for ReadOnlyPaths= and related
           calls, see above. Enabling this setting has the side effect of adding Requires= and
           After= dependencies on all mount units necessary to access /tmp and /var/tmp. Moreover
           an implicitly After= ordering on systemd-tmpfiles-setup.service(8) is added.

           Note that the implementation of this setting might be impossible (for example if mount
           namespaces are not available), and the unit should be written in a way that does not
           solely rely on this setting for security.

       PrivateDevices=
           Takes a boolean argument. If true, sets up a new /dev mount for the executed processes
           and only adds API pseudo devices such as /dev/null, /dev/zero or /dev/random (as well
           as the pseudo TTY subsystem) to it, but no physical devices such as /dev/sda, system
           memory /dev/mem, system ports /dev/port and others. This is useful to securely turn
           off physical device access by the executed process. Defaults to false. Enabling this
           option will install a system call filter to block low-level I/O system calls that are
           grouped in the @raw-io set, will also remove CAP_MKNOD and CAP_SYS_RAWIO from the
           capability bounding set for the unit (see above), and set DevicePolicy=closed (see
           systemd.resource-control(5) for details). Note that using this setting will disconnect
           propagation of mounts from the service to the host (propagation in the opposite
           direction continues to work). This means that this setting may not be used for
           services which shall be able to install mount points in the main mount namespace. The
           new /dev will be mounted read-only and 'noexec'. The latter may break old programs
           which try to set up executable memory by using mmap(2) of /dev/zero instead of using
           MAP_ANON. This setting is implied if DynamicUser= is set. For this setting the same
           restrictions regarding mount propagation and privileges apply as for ReadOnlyPaths=
           and related calls, see above. If turned on and if running in user mode, or in system
           mode, but without the CAP_SYS_ADMIN capability (e.g. setting User=),
           NoNewPrivileges=yes is implied.

           Note that the implementation of this setting might be impossible (for example if mount
           namespaces are not available), and the unit should be written in a way that does not
           solely rely on this setting for security.

       PrivateNetwork=
           Takes a boolean argument. If true, sets up a new network namespace for the executed
           processes and configures only the loopback network device "lo" inside it. No other
           network devices will be available to the executed process. This is useful to turn off
           network access by the executed process. Defaults to false. It is possible to run two
           or more units within the same private network namespace by using the JoinsNamespaceOf=
           directive, see systemd.unit(5) for details. Note that this option will disconnect all
           socket families from the host, this includes AF_NETLINK and AF_UNIX. The latter has
           the effect that AF_UNIX sockets in the abstract socket namespace will become
           unavailable to the processes (however, those located in the file system will continue
           to be accessible).

           Note that the implementation of this setting might be impossible (for example if
           network namespaces are not available), and the unit should be written in a way that
           does not solely rely on this setting for security.

       PrivateUsers=
           Takes a boolean argument. If true, sets up a new user namespace for the executed
           processes and configures a minimal user and group mapping, that maps the "root" user
           and group as well as the unit's own user and group to themselves and everything else
           to the "nobody" user and group. This is useful to securely detach the user and group
           databases used by the unit from the rest of the system, and thus to create an
           effective sandbox environment. All files, directories, processes, IPC objects and
           other resources owned by users/groups not equaling "root" or the unit's own will stay
           visible from within the unit but appear owned by the "nobody" user and group. If this
           mode is enabled, all unit processes are run without privileges in the host user
           namespace (regardless if the unit's own user/group is "root" or not). Specifically
           this means that the process will have zero process capabilities on the host's user
           namespace, but full capabilities within the service's user namespace. Settings such as
           CapabilityBoundingSet= will affect only the latter, and there's no way to acquire
           additional capabilities in the host's user namespace. Defaults to off.

           This setting is particularly useful in conjunction with RootDirectory=/RootImage=, as
           the need to synchronize the user and group databases in the root directory and on the
           host is reduced, as the only users and groups who need to be matched are "root",
           "nobody" and the unit's own user and group.

           Note that the implementation of this setting might be impossible (for example if user
           namespaces are not available), and the unit should be written in a way that does not
           solely rely on this setting for security.

       ProtectSystem=
           Takes a boolean argument or the special values "full" or "strict". If true, mounts the
           /usr and /boot directories read-only for processes invoked by this unit. If set to
           "full", the /etc directory is mounted read-only, too. If set to "strict" the entire
           file system hierarchy is mounted read-only, except for the API file system subtrees
           /dev, /proc and /sys (protect these directories using PrivateDevices=,
           ProtectKernelTunables=, ProtectControlGroups=). This setting ensures that any
           modification of the vendor-supplied operating system (and optionally its
           configuration, and local mounts) is prohibited for the service. It is recommended to
           enable this setting for all long-running services, unless they are involved with
           system updates or need to modify the operating system in other ways. If this option is
           used, ReadWritePaths= may be used to exclude specific directories from being made
           read-only. This setting is implied if DynamicUser= is set. For this setting the same
           restrictions regarding mount propagation and privileges apply as for ReadOnlyPaths=
           and related calls, see above. Defaults to off.

       ProtectHome=
           Takes a boolean argument or "read-only". If true, the directories /home, /root and
           /run/user are made inaccessible and empty for processes invoked by this unit. If set
           to "read-only", the three directories are made read-only instead. It is recommended to
           enable this setting for all long-running services (in particular network-facing ones),
           to ensure they cannot get access to private user data, unless the services actually
           require access to the user's private data. This setting is implied if DynamicUser= is
           set. For this setting the same restrictions regarding mount propagation and privileges
           apply as for ReadOnlyPaths= and related calls, see above.

       ProtectKernelTunables=
           Takes a boolean argument. If true, kernel variables accessible through /proc/sys,
           /sys, /proc/sysrq-trigger, /proc/latency_stats, /proc/acpi, /proc/timer_stats,
           /proc/fs and /proc/irq will be made read-only to all processes of the unit. Usually,
           tunable kernel variables should be initialized only at boot-time, for example with the
           sysctl.d(5) mechanism. Few services need to write to these at runtime; it is hence
           recommended to turn this on for most services. For this setting the same restrictions
           regarding mount propagation and privileges apply as for ReadOnlyPaths= and related
           calls, see above. Defaults to off. If turned on and if running in user mode, or in
           system mode, but without the CAP_SYS_ADMIN capability (e.g. services for which User=
           is set), NoNewPrivileges=yes is implied. Note that this option does not prevent
           indirect changes to kernel tunables effected by IPC calls to other processes. However,
           InaccessiblePaths= may be used to make relevant IPC file system objects inaccessible.
           If ProtectKernelTunables= is set, MountAPIVFS=yes is implied.

       ProtectKernelModules=
           Takes a boolean argument. If true, explicit module loading will be denied. This allows
           to turn off module load and unload operations on modular kernels. It is recommended to
           turn this on for most services that do not need special file systems or extra kernel
           modules to work. Default to off. Enabling this option removes CAP_SYS_MODULE from the
           capability bounding set for the unit, and installs a system call filter to block
           module system calls, also /usr/lib/modules is made inaccessible. For this setting the
           same restrictions regarding mount propagation and privileges apply as for
           ReadOnlyPaths= and related calls, see above. Note that limited automatic module
           loading due to user configuration or kernel mapping tables might still happen as side
           effect of requested user operations, both privileged and unprivileged. To disable
           module auto-load feature please see sysctl.d(5) kernel.modules_disabled mechanism and
           /proc/sys/kernel/modules_disabled documentation. If turned on and if running in user
           mode, or in system mode, but without the CAP_SYS_ADMIN capability (e.g. setting
           User=), NoNewPrivileges=yes is implied.

       ProtectControlGroups=
           Takes a boolean argument. If true, the Linux Control Groups (cgroups(7)) hierarchies
           accessible through /sys/fs/cgroup will be made read-only to all processes of the unit.
           Except for container managers no services should require write access to the control
           groups hierarchies; it is hence recommended to turn this on for most services. For
           this setting the same restrictions regarding mount propagation and privileges apply as
           for ReadOnlyPaths= and related calls, see above. Defaults to off. If
           ProtectControlGroups= is set, MountAPIVFS=yes is implied.

       MountFlags=
           Takes a mount propagation flag: shared, slave or private, which control whether mounts
           in the file system namespace set up for this unit's processes will receive or
           propagate mounts and unmounts. See mount(2) for details. Defaults to shared. Use
           shared to ensure that mounts and unmounts are propagated from systemd's namespace to
           the service's namespace and vice versa. Use slave to run processes so that none of
           their mounts and unmounts will propagate to the host. Use private to also ensure that
           no mounts and unmounts from the host will propagate into the unit processes'
           namespace. If this is set to slave or private, any mounts created by spawned processes
           will be unmounted after the completion of the current command line of ExecStartPre=,
           ExecStartPost=, ExecStart=, and ExecStopPost=. Note that slave means that file systems
           mounted on the host might stay mounted continuously in the unit's namespace, and thus
           keep the device busy. Note that the file system namespace related options
           (PrivateTmp=, PrivateDevices=, ProtectSystem=, ProtectHome=, ProtectKernelTunables=,
           ProtectControlGroups=, ReadOnlyPaths=, InaccessiblePaths=, ReadWritePaths=) require
           that mount and unmount propagation from the unit's file system namespace is disabled,
           and hence downgrade shared to slave.

       UtmpIdentifier=
           Takes a four character identifier string for an utmp(5) and wtmp entry for this
           service. This should only be set for services such as getty implementations (such as
           agetty(8)) where utmp/wtmp entries must be created and cleared before and after
           execution, or for services that shall be executed as if they were run by a getty
           process (see below). If the configured string is longer than four characters, it is
           truncated and the terminal four characters are used. This setting interprets %I style
           string replacements. This setting is unset by default, i.e. no utmp/wtmp entries are
           created or cleaned up for this service.

       UtmpMode=
           Takes one of "init", "login" or "user". If UtmpIdentifier= is set, controls which type
           of utmp(5)/wtmp entries for this service are generated. This setting has no effect
           unless UtmpIdentifier= is set too. If "init" is set, only an INIT_PROCESS entry is
           generated and the invoked process must implement a getty-compatible utmp/wtmp logic.
           If "login" is set, first an INIT_PROCESS entry, followed by a LOGIN_PROCESS entry is
           generated. In this case, the invoked process must implement a login(1)-compatible
           utmp/wtmp logic. If "user" is set, first an INIT_PROCESS entry, then a LOGIN_PROCESS
           entry and finally a USER_PROCESS entry is generated. In this case, the invoked process
           may be any process that is suitable to be run as session leader. Defaults to "init".

       SELinuxContext=
           Set the SELinux security context of the executed process. If set, this will override
           the automated domain transition. However, the policy still needs to authorize the
           transition. This directive is ignored if SELinux is disabled. If prefixed by "-", all
           errors will be ignored. This does not affect commands prefixed with "+". See
           setexeccon(3) for details.

       AppArmorProfile=
           Takes a profile name as argument. The process executed by the unit will switch to this
           profile when started. Profiles must already be loaded in the kernel, or the unit will
           fail. This result in a non operation if AppArmor is not enabled. If prefixed by "-",
           all errors will be ignored. This does not affect commands prefixed with "+".

       SmackProcessLabel=
           Takes a SMACK64 security label as argument. The process executed by the unit will be
           started under this label and SMACK will decide whether the process is allowed to run
           or not, based on it. The process will continue to run under the label specified here
           unless the executable has its own SMACK64EXEC label, in which case the process will
           transition to run under that label. When not specified, the label that systemd is
           running under is used. This directive is ignored if SMACK is disabled.

           The value may be prefixed by "-", in which case all errors will be ignored. An empty
           value may be specified to unset previous assignments. This does not affect commands
           prefixed with "+".

       IgnoreSIGPIPE=
           Takes a boolean argument. If true, causes SIGPIPE to be ignored in the executed
           process. Defaults to true because SIGPIPE generally is useful only in shell pipelines.

       NoNewPrivileges=
           Takes a boolean argument. If true, ensures that the service process and all its
           children can never gain new privileges through execve() (e.g. via setuid or setgid
           bits, or filesystem capabilities). This is the simplest and most effective way to
           ensure that a process and its children can never elevate privileges again. Defaults to
           false, but certain settings force NoNewPrivileges=yes, ignoring the value of this
           setting. This is the case when SystemCallFilter=, SystemCallArchitectures=,
           RestrictAddressFamilies=, RestrictNamespaces=, PrivateDevices=,
           ProtectKernelTunables=, ProtectKernelModules=, MemoryDenyWriteExecute=, or
           RestrictRealtime= are specified.

       SystemCallFilter=
           Takes a space-separated list of system call names. If this setting is used, all system
           calls executed by the unit processes except for the listed ones will result in
           immediate process termination with the SIGSYS signal (whitelisting). If the first
           character of the list is "~", the effect is inverted: only the listed system calls
           will result in immediate process termination (blacklisting). If running in user mode,
           or in system mode, but without the CAP_SYS_ADMIN capability (e.g. setting
           User=nobody), NoNewPrivileges=yes is implied. This feature makes use of the Secure
           Computing Mode 2 interfaces of the kernel ('seccomp filtering') and is useful for
           enforcing a minimal sandboxing environment. Note that the execve, exit, exit_group,
           getrlimit, rt_sigreturn, sigreturn system calls and the system calls for querying time
           and sleeping are implicitly whitelisted and do not need to be listed explicitly. This
           option may be specified more than once, in which case the filter masks are merged. If
           the empty string is assigned, the filter is reset, all prior assignments will have no
           effect. This does not affect commands prefixed with "+".

           Note that on systems supporting multiple ABIs (such as x86/x86-64) it is recommended
           to turn off alternative ABIs for services, so that they cannot be used to circumvent
           the restrictions of this option. Specifically, it is recommended to combine this
           option with SystemCallArchitectures=native or similar.

           Note that strict system call filters may impact execution and error handling code
           paths of the service invocation. Specifically, access to the execve system call is
           required for the execution of the service binary — if it is blocked service invocation
           will necessarily fail. Also, if execution of the service binary fails for some reason
           (for example: missing service executable), the error handling logic might require
           access to an additional set of system calls in order to process and log this failure
           correctly. It might be necessary to temporarily disable system call filters in order
           to simplify debugging of such failures.

           If you specify both types of this option (i.e. whitelisting and blacklisting), the
           first encountered will take precedence and will dictate the default action
           (termination or approval of a system call). Then the next occurrences of this option
           will add or delete the listed system calls from the set of the filtered system calls,
           depending of its type and the default action. (For example, if you have started with a
           whitelisting of read and write, and right after it add a blacklisting of write, then
           write will be removed from the set.)

           As the number of possible system calls is large, predefined sets of system calls are
           provided. A set starts with "@" character, followed by name of the set.

           Table 2. Currently predefined system call sets
           ┌───────────────┬──────────────────────────────────┐
           │SetDescription                      │
           ├───────────────┼──────────────────────────────────┤
           │@basic-io      │ System calls for basic I/O:      │
           │               │ reading, writing, seeking, file  │
           │               │ descriptor duplication and       │
           │               │ closing (read(2), write(2), and  │
           │               │ related calls)                   │
           ├───────────────┼──────────────────────────────────┤
           │@clock         │ System calls for changing the    │
           │               │ system clock (adjtimex(2),       │
           │               │ settimeofday(2), and related     │
           │               │ calls)                           │
           ├───────────────┼──────────────────────────────────┤
           │@cpu-emulation │ System calls for CPU emulation   │
           │               │ functionality (vm86(2) and       │
           │               │ related calls)                   │
           ├───────────────┼──────────────────────────────────┤
           │@debug         │ Debugging, performance           │
           │               │ monitoring and tracing           │
           │               │ functionality (ptrace(2),        │
           │               │ perf_event_open(2) and related   │
           │               │ calls)                           │
           ├───────────────┼──────────────────────────────────┤
           │@file-system   │ File system operations: opening, │
           │               │ creating files and directories   │
           │               │ for read and write, renaming and │
           │               │ removing them, reading file      │
           │               │ properties, or creating hard and │
           │               │ symbolic links.                  │
           ├───────────────┼──────────────────────────────────┤
           │@io-event      │ Event loop system calls          │
           │               │ (poll(2), select(2), epoll(7),   │
           │               │ eventfd(2) and related calls)    │
           ├───────────────┼──────────────────────────────────┤
           │@ipc           │ Pipes, SysV IPC, POSIX Message   │
           │               │ Queues and other IPC             │
           │               │ (mq_overview(7), svipc(7))       │
           ├───────────────┼──────────────────────────────────┤
           │@keyring       │ Kernel keyring access (keyctl(2) │
           │               │ and related calls)               │
           ├───────────────┼──────────────────────────────────┤
           │@module        │ Loading and unloading of kernel  │
           │               │ modules (init_module(2),         │
           │               │ delete_module(2) and related     │
           │               │ calls)                           │
           ├───────────────┼──────────────────────────────────┤
           │@mount         │ Mounting and unmounting of file  │
           │               │ systems (mount(2), chroot(2),    │
           │               │ and related calls)               │
           ├───────────────┼──────────────────────────────────┤
           │@network-io    │ Socket I/O (including local      │
           │               │ AF_UNIX): socket(7), unix(7)     │
           ├───────────────┼──────────────────────────────────┤
           │@obsolete      │ Unusual, obsolete or             │
           │               │ unimplemented (create_module(2), │
           │               │ gtty(2), ...)                    │
           ├───────────────┼──────────────────────────────────┤
           │@privileged    │ All system calls which need      │
           │               │ super-user capabilities          │
           │               │ (capabilities(7))                │
           ├───────────────┼──────────────────────────────────┤
           │@process       │ Process control, execution,      │
           │               │ namespaceing operations          │
           │               │ (clone(2), kill(2),              │
           │               │ namespaces(7), ...               │
           ├───────────────┼──────────────────────────────────┤
           │@raw-io        │ Raw I/O port access (ioperm(2),  │
           │               │ iopl(2), pciconfig_read(), ...)  │
           ├───────────────┼──────────────────────────────────┤
           │@reboot        │ System calls for rebooting and   │
           │               │ reboot preparation (reboot(2),   │
           │               │ kexec(), ...)                    │
           ├───────────────┼──────────────────────────────────┤
           │@resources     │ System calls for changing        │
           │               │ resource limits, memory and      │
           │               │ scheduling parameters            │
           │               │ (setrlimit(2), setpriority(2),   │
           │               │ ...)                             │
           ├───────────────┼──────────────────────────────────┤
           │@swap          │ System calls for                 │
           │               │ enabling/disabling swap devices  │
           │               │ (swapon(2), swapoff(2))          │
           └───────────────┴──────────────────────────────────┘
           Note, that as new system calls are added to the kernel, additional system calls might
           be added to the groups above. Contents of the sets may also change between systemd
           versions. In addition, the list of system calls depends on the kernel version and
           architecture for which systemd was compiled. Use systemd-analyze syscall-filter to
           list the actual list of system calls in each filter.

           It is recommended to combine the file system namespacing related options with
           SystemCallFilter=~@mount, in order to prohibit the unit's processes to undo the
           mappings. Specifically these are the options PrivateTmp=, PrivateDevices=,
           ProtectSystem=, ProtectHome=, ProtectKernelTunables=, ProtectControlGroups=,
           ReadOnlyPaths=, InaccessiblePaths= and ReadWritePaths=.

       SystemCallErrorNumber=
           Takes an "errno" error number name to return when the system call filter configured
           with SystemCallFilter= is triggered, instead of terminating the process immediately.
           Takes an error name such as EPERM, EACCES or EUCLEAN. When this setting is not used,
           or when the empty string is assigned, the process will be terminated immediately when
           the filter is triggered.

       SystemCallArchitectures=
           Takes a space-separated list of architecture identifiers to include in the system call
           filter. The known architecture identifiers are the same as for ConditionArchitecture=
           described in systemd.unit(5), as well as x32, mips64-n32, mips64-le-n32, and the
           special identifier native. Only system calls of the specified architectures will be
           permitted to processes of this unit. This is an effective way to disable compatibility
           with non-native architectures for processes, for example to prohibit execution of
           32-bit x86 binaries on 64-bit x86-64 systems. The special native identifier implicitly
           maps to the native architecture of the system (or more strictly: to the architecture
           the system manager is compiled for). If running in user mode, or in system mode, but
           without the CAP_SYS_ADMIN capability (e.g. setting User=nobody), NoNewPrivileges=yes
           is implied. Note that setting this option to a non-empty list implies that native is
           included too. By default, this option is set to the empty list, i.e. no system call
           architecture filtering is applied.

           Note that system call filtering is not equally effective on all architectures. For
           example, on x86 filtering of network socket-related calls is not possible, due to ABI
           limitations — a limitation that x86-64 does not have, however. On systems supporting
           multiple ABIs at the same time — such as x86/x86-64 — it is hence recommended to limit
           the set of permitted system call architectures so that secondary ABIs may not be used
           to circumvent the restrictions applied to the native ABI of the system. In particular,
           setting SystemCallFilter=native is a good choice for disabling non-native ABIs.

           System call architectures may also be restricted system-wide via the
           SystemCallArchitectures= option in the global configuration. See systemd-
           system.conf(5) for details.

       RestrictAddressFamilies=
           Restricts the set of socket address families accessible to the processes of this unit.
           Takes a space-separated list of address family names to whitelist, such as AF_UNIX,
           AF_INET or AF_INET6. When prefixed with ~ the listed address families will be applied
           as blacklist, otherwise as whitelist. Note that this restricts access to the socket(2)
           system call only. Sockets passed into the process by other means (for example, by
           using socket activation with socket units, see systemd.socket(5)) are unaffected.
           Also, sockets created with socketpair() (which creates connected AF_UNIX sockets only)
           are unaffected. Note that this option has no effect on 32-bit x86, s390, s390x, mips,
           mips-le, ppc, ppc-le, pcc64, ppc64-le and is ignored (but works correctly on other
           ABIs, including x86-64). Note that on systems supporting multiple ABIs (such as
           x86/x86-64) it is recommended to turn off alternative ABIs for services, so that they
           cannot be used to circumvent the restrictions of this option. Specifically, it is
           recommended to combine this option with SystemCallArchitectures=native or similar. If
           running in user mode, or in system mode, but without the CAP_SYS_ADMIN capability
           (e.g. setting User=nobody), NoNewPrivileges=yes is implied. By default, no
           restrictions apply, all address families are accessible to processes. If assigned the
           empty string, any previous address familiy restriction changes are undone. This
           setting does not affect commands prefixed with "+".

           Use this option to limit exposure of processes to remote access, in particular via
           exotic and sensitive network protocols, such as AF_PACKET. Note that in most cases,
           the local AF_UNIX address family should be included in the configured whitelist as it
           is frequently used for local communication, including for syslog(2) logging.

       RestrictNamespaces=
           Restricts access to Linux namespace functionality for the processes of this unit. For
           details about Linux namespaces, see namespaces(7). Either takes a boolean argument, or
           a space-separated list of namespace type identifiers. If false (the default), no
           restrictions on namespace creation and switching are made. If true, access to any kind
           of namespacing is prohibited. Otherwise, a space-separated list of namespace type
           identifiers must be specified, consisting of any combination of: cgroup, ipc, net,
           mnt, pid, user and uts. Any namespace type listed is made accessible to the unit's
           processes, access to namespace types not listed is prohibited (whitelisting). By
           prepending the list with a single tilda character ("~") the effect may be inverted:
           only the listed namespace types will be made inaccessible, all unlisted ones are
           permitted (blacklisting). If the empty string is assigned, the default namespace
           restrictions are applied, which is equivalent to false. Internally, this setting
           limits access to the unshare(2), clone(2) and setns(2) system calls, taking the
           specified flags parameters into account. Note that — if this option is used — in
           addition to restricting creation and switching of the specified types of namespaces
           (or all of them, if true) access to the setns() system call with a zero flags
           parameter is prohibited. This setting is only supported on x86, x86-64, mips, mips-le,
           mips64, mips64-le, mips64-n32, mips64-le-n32, ppc64, ppc64-le, s390 and s390x, and
           enforces no restrictions on other architectures. If running in user mode, or in system
           mode, but without the CAP_SYS_ADMIN capability (e.g. setting User=),
           NoNewPrivileges=yes is implied.

       Personality=
           Controls which kernel architecture uname(2) shall report, when invoked by unit
           processes. Takes one of the architecture identifiers x86, x86-64, ppc, ppc-le, ppc64,
           ppc64-le, s390 or s390x. Which personality architectures are supported depends on the
           system architecture. Usually the 64bit versions of the various system architectures
           support their immediate 32bit personality architecture counterpart, but no others. For
           example, x86-64 systems support the x86-64 and x86 personalities but no others. The
           personality feature is useful when running 32-bit services on a 64-bit host system. If
           not specified, the personality is left unmodified and thus reflects the personality of
           the host system's kernel.

       RuntimeDirectory=
           Takes a list of directory names. If set, one or more directories by the specified
           names will be created below /run (for system services) or below $XDG_RUNTIME_DIR (for
           user services) when the unit is started, and removed when the unit is stopped. The
           directories will have the access mode specified in RuntimeDirectoryMode=, and will be
           owned by the user and group specified in User= and Group=. Use this to manage one or
           more runtime directories of the unit and bind their lifetime to the daemon runtime.
           The specified directory names must be relative, and may not include a "/", i.e. must
           refer to simple directories to create or remove. This is particularly useful for
           unprivileged daemons that cannot create runtime directories in /run due to lack of
           privileges, and to make sure the runtime directory is cleaned up automatically after
           use. For runtime directories that require more complex or different configuration or
           lifetime guarantees, please consider using tmpfiles.d(5).

       RuntimeDirectoryMode=
           Specifies the access mode of the directories specified in RuntimeDirectory= as an
           octal number. Defaults to 0755. See "Permissions" in path_resolution(7) for a
           discussion of the meaning of permission bits.

       MemoryDenyWriteExecute=
           Takes a boolean argument. If set, attempts to create memory mappings that are writable
           and executable at the same time, or to change existing memory mappings to become
           executable, or mapping shared memory segments as executable are prohibited.
           Specifically, a system call filter is added that rejects mmap(2) system calls with
           both PROT_EXEC and PROT_WRITE set, mprotect(2) system calls with PROT_EXEC set and
           shmat(2) system calls with SHM_EXEC set. Note that this option is incompatible with
           programs and libraries that generate program code dynamically at runtime, including
           JIT execution engines, executable stacks, and code "trampoline" feature of various C
           compilers. This option improves service security, as it makes harder for software
           exploits to change running code dynamically. Note that this feature is fully available
           on x86-64, and partially on x86. Specifically, the shmat() protection is not available
           on x86. Note that on systems supporting multiple ABIs (such as x86/x86-64) it is
           recommended to turn off alternative ABIs for services, so that they cannot be used to
           circumvent the restrictions of this option. Specifically, it is recommended to combine
           this option with SystemCallArchitectures=native or similar. If running in user mode,
           or in system mode, but without the CAP_SYS_ADMIN capability (e.g. setting User=),
           NoNewPrivileges=yes is implied.

       RestrictRealtime=
           Takes a boolean argument. If set, any attempts to enable realtime scheduling in a
           process of the unit are refused. This restricts access to realtime task scheduling
           policies such as SCHED_FIFO, SCHED_RR or SCHED_DEADLINE. See sched(7) for details
           about these scheduling policies. If running in user mode, or in system mode, but
           without the CAP_SYS_ADMIN capability (e.g. setting User=), NoNewPrivileges=yes is
           implied. Realtime scheduling policies may be used to monopolize CPU time for longer
           periods of time, and may hence be used to lock up or otherwise trigger
           Denial-of-Service situations on the system. It is hence recommended to restrict access
           to realtime scheduling to the few programs that actually require them. Defaults to
           off.

ENVIRONMENT VARIABLES IN SPAWNED PROCESSES

       Processes started by the system are executed in a clean environment in which select
       variables listed below are set. System processes started by systemd do not inherit
       variables from PID 1, but processes started by user systemd instances inherit all
       environment variables from the user systemd instance.

       $PATH
           Colon-separated list of directories to use when launching executables. Systemd uses a
           fixed value of /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin.

       $LANG
           Locale. Can be set in locale.conf(5) or on the kernel command line (see systemd(1) and
           kernel-command-line(7)).

       $USER, $LOGNAME, $HOME, $SHELL
           User name (twice), home directory, and the login shell. The variables are set for the
           units that have User= set, which includes user systemd instances. See passwd(5).

       $INVOCATION_ID
           Contains a randomized, unique 128bit ID identifying each runtime cycle of the unit,
           formatted as 32 character hexadecimal string. A new ID is assigned each time the unit
           changes from an inactive state into an activating or active state, and may be used to
           identify this specific runtime cycle, in particular in data stored offline, such as
           the journal. The same ID is passed to all processes run as part of the unit.

       $XDG_RUNTIME_DIR
           The directory for volatile state. Set for the user systemd instance, and also in user
           sessions. See pam_systemd(8).

       $XDG_SESSION_ID, $XDG_SEAT, $XDG_VTNR
           The identifier of the session, the seat name, and virtual terminal of the session. Set
           by pam_systemd(8) for login sessions.  $XDG_SEAT and $XDG_VTNR will only be set when
           attached to a seat and a tty.

       $MAINPID
           The PID of the unit's main process if it is known. This is only set for control
           processes as invoked by ExecReload= and similar.

       $MANAGERPID
           The PID of the user systemd instance, set for processes spawned by it.

       $LISTEN_FDS, $LISTEN_PID, $LISTEN_FDNAMES
           Information about file descriptors passed to a service for socket activation. See
           sd_listen_fds(3).

       $NOTIFY_SOCKET
           The socket sd_notify() talks to. See sd_notify(3).

       $WATCHDOG_PID, $WATCHDOG_USEC
           Information about watchdog keep-alive notifications. See sd_watchdog_enabled(3).

       $TERM
           Terminal type, set only for units connected to a terminal (StandardInput=tty,
           StandardOutput=tty, or StandardError=tty). See termcap(5).

       $JOURNAL_STREAM
           If the standard output or standard error output of the executed processes are
           connected to the journal (for example, by setting StandardError=journal)
           $JOURNAL_STREAM contains the device and inode numbers of the connection file
           descriptor, formatted in decimal, separated by a colon (":"). This permits invoked
           processes to safely detect whether their standard output or standard error output are
           connected to the journal. The device and inode numbers of the file descriptors should
           be compared with the values set in the environment variable to determine whether the
           process output is still connected to the journal. Note that it is generally not
           sufficient to only check whether $JOURNAL_STREAM is set at all as services might
           invoke external processes replacing their standard output or standard error output,
           without unsetting the environment variable.

           This environment variable is primarily useful to allow services to optionally upgrade
           their used log protocol to the native journal protocol (using sd_journal_print(3) and
           other functions) if their standard output or standard error output is connected to the
           journal anyway, thus enabling delivery of structured metadata along with logged
           messages.

       $SERVICE_RESULT
           Only defined for the service unit type, this environment variable is passed to all
           ExecStop= and ExecStopPost= processes, and encodes the service "result". Currently,
           the following values are defined: "protocol" (in case of a protocol violation; if a
           service did not take the steps required by its unit configuration), "timeout" (in case
           of an operation timeout), "exit-code" (if a service process exited with a non-zero
           exit code; see $EXIT_CODE below for the actual exit code returned), "signal" (if a
           service process was terminated abnormally by a signal; see $EXIT_CODE below for the
           actual signal used for the termination), "core-dump" (if a service process terminated
           abnormally and dumped core), "watchdog" (if the watchdog keep-alive ping was enabled
           for the service but it missed the deadline), or "resources" (a catch-all condition in
           case a system operation failed).

           This environment variable is useful to monitor failure or successful termination of a
           service. Even though this variable is available in both ExecStop= and ExecStopPost=,
           it is usually a better choice to place monitoring tools in the latter, as the former
           is only invoked for services that managed to start up correctly, and the latter covers
           both services that failed during their start-up and those which failed during their
           runtime.

       $EXIT_CODE, $EXIT_STATUS
           Only defined for the service unit type, these environment variables are passed to all
           ExecStop=, ExecStopPost= processes and contain exit status/code information of the
           main process of the service. For the precise definition of the exit code and status,
           see wait(2).  $EXIT_CODE is one of "exited", "killed", "dumped".  $EXIT_STATUS
           contains the numeric exit code formatted as string if $EXIT_CODE is "exited", and the
           signal name in all other cases. Note that these environment variables are only set if
           the service manager succeeded to start and identify the main process of the service.

           Table 3. Summary of possible service result variable values
           ┌──────────────────────┬────────────────────────┬──────────────────────────┐
           │$SERVICE_RESULT$EXIT_CODE$EXIT_STATUS             │
           ├──────────────────────┼────────────────────────┼──────────────────────────┤
           │"protocol"            │ not set                │ not set                  │
           │                      ├────────────────────────┼──────────────────────────┤
           │                      │ "exited"               │ "0"                      │
           ├──────────────────────┼────────────────────────┼──────────────────────────┤
           │"timeout"             │ "killed"               │ "TERM", "KILL"           │
           │                      ├────────────────────────┼──────────────────────────┤
           │                      │ "exited"               │ "0", "1", "2", "3", ..., │
           │                      │                        │ "255"                    │
           ├──────────────────────┼────────────────────────┼──────────────────────────┤
           │"exit-code"           │ "exited"               │ "0", "1", "2", "3", ..., │
           │                      │                        │ "255"                    │
           ├──────────────────────┼────────────────────────┼──────────────────────────┤
           │"signal"              │ "killed"               │ "HUP", "INT", "KILL",    │
           │                      │                        │ ...                      │
           ├──────────────────────┼────────────────────────┼──────────────────────────┤
           │"core-dump"           │ "dumped"               │ "ABRT", "SEGV", "QUIT",  │
           │                      │                        │ ...                      │
           ├──────────────────────┼────────────────────────┼──────────────────────────┤
           │"watchdog"            │ "dumped"               │ "ABRT"                   │
           │                      ├────────────────────────┼──────────────────────────┤
           │                      │ "killed"               │ "TERM", "KILL"           │
           │                      ├────────────────────────┼──────────────────────────┤
           │                      │ "exited"               │ "0", "1", "2", "3", ..., │
           │                      │                        │ "255"                    │
           ├──────────────────────┼────────────────────────┼──────────────────────────┤
           │"resources"           │ any of the above       │ any of the above         │
           ├──────────────────────┴────────────────────────┼──────────────────────────┤
           │Note: the process may be also terminated by a  │                          │
           │signal not sent by systemd. In particular the  │                          │
           │process may send an arbitrary signal to itself │                          │
           │in a handler for any of the non-maskable       │                          │
           │signals. Nevertheless, in the "timeout" and    │                          │
           │"watchdog" rows above only the signals that    │                          │
           │systemd sends have been included.              │                          │
           └───────────────────────────────────────────────┴──────────────────────────┘

       Additional variables may be configured by the following means: for processes spawned in
       specific units, use the Environment=, EnvironmentFile= and PassEnvironment= options above;
       to specify variables globally, use DefaultEnvironment= (see systemd-system.conf(5)) or the
       kernel option systemd.setenv= (see systemd(1)). Additional variables may also be set
       through PAM, cf. pam_env(8).

SEE ALSO

       systemd(1), systemctl(1), systemd-analyze(1), journalctl(8), systemd.unit(5),
       systemd.service(5), systemd.socket(5), systemd.swap(5), systemd.mount(5), systemd.kill(5),
       systemd.resource-control(5), systemd.time(7), systemd.directives(7), tmpfiles.d(5),
       exec(3)

NOTES

        1. Discoverable Partitions Specification
           https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/

        2. proc.txt
           https://www.kernel.org/doc/Documentation/filesystems/proc.txt