SYSTEMD.EXEC(5)

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.

IMPLICIT DEPENDENCIES

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

    • Units with WorkingDirectory=, RootDirectory=, RootImage=, RuntimeDirectory=, StateDirectory=, CacheDirectory=, LogsDirectory= or ConfigurationDirectory= 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.

PATHS

The following settings may be used to change a service's view of the filesystem. Please note that the paths must be absolute and must not contain a ".." path component.

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].

    When DevicePolicy= is set to "closed" or "strict", or set to "auto" and DeviceAllow= is set, then this setting adds /dev/loop-control with rw mode, "block-loop" and "block-blkext" with rwm mode to DeviceAllow=. See systemd.resource-control(5) for the details about DevicePolicy= or DeviceAllow=. Also, see PrivateDevices= below, as it may change the setting of DevicePolicy=.

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=.

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. Each bind mount definition may be prefixed with "-", in which case it will be ignored when its source path does not exist.

    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.

CREDENTIALS

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. Note that if User= is specified and the static group with the name exists, then it is required that the static user with the name already exists. Similarly, if Group= is specified and the static user with the name exists, then it is required that the static group with the name already exists. 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. Use StateDirectory=, CacheDirectory= and LogsDirectory= in order to assign a set of writable directories for specific purposes to the service in a way that they are protected from vulnerabilities due to UID reuse (see below). 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 "+".

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.

    Note that when this option is used for a unit it is very likely (depending on PAM configuration) that the main unit process will be migrated to its own session scope unit when it is activated. This process will hence be associated with two units: the unit it was originally started from (and for which PAMName= was configured), and the session scope unit. Any child processes of that process will however be associated with the session scope unit only. This has implications when used in combination with NotifyAccess=all, as these child processes will not be able to affect changes in the original unit through notification messages. These messages will be considered belonging to the session scope unit and not the original unit. It is hence not recommended to use PAMName= in combination with NotifyAccess=all.

CAPABILITIES

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 by OR, or by AND if the lines are prefixed with "~" (see below). 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 "+".

    Example: if a unit has the following,

        CapabilityBoundingSet=CAP_A CAP_B
        CapabilityBoundingSet=CAP_B CAP_C

    then CAP_A, CAP_B, and CAP_C are set. If the second line is prefixed with "~", e.g.,

        CapabilityBoundingSet=CAP_A CAP_B
        CapabilityBoundingSet=~CAP_B CAP_C

    then, only CAP_A is set.

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 (see the above examples in CapabilityBoundingSet=). 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 "+".

SECURITY

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 override this and ignore the value of this setting. This is the case when SystemCallFilter=, SystemCallArchitectures=, RestrictAddressFamilies=, RestrictNamespaces=, PrivateDevices=, ProtectKernelTunables=, ProtectKernelModules=, MemoryDenyWriteExecute=, RestrictRealtime=, or LockPersonality= are specified. Note that even if this setting is overridden by them, systemctl show shows the original value of this setting. Also see No New Privileges Flag[2].

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.

MANDATORY ACCESS CONTROL

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 "+".

PROCESS PROPERTIES

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
    Directive | ulimit 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

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

KeyringMode=
    Controls how the kernel session keyring is set up for the service (see session-keyring(7) for details on the session keyring). Takes one of inherit, private, shared. If set to inherit no special keyring setup is done, and the kernel's default behaviour is applied. If private is used a new session keyring is allocated when a service process is invoked, and it is not linked up with any user keyring. This is the recommended setting for system services, as this ensures that multiple services running under the same system user ID (in particular the root user) do not share their key material among each other. If shared is used a new session keyring is allocated as for private, but the user keyring of the user configured with User= is linked into it, so that keys assigned to the user may be requested by the unit's processes. In this modes multiple units running processes under the same user ID may share key material. Unless inherit is selected the unique invocation ID for the unit (see below) is added as a protected key by the name "invocation_id" to the newly created session keyring. Defaults to private for services of the system service manager and to inherit for non-service units and for services of the user service manager.

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[3] for details.

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.

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.

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.

SCHEDULING

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.

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.

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. If the empty string is assigned to this option, all prior assignments to both IOSchedulingClass= and IOSchedulingPriority= have no effect. 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). If the empty string is assigned to this option, all prior assignments to both IOSchedulingClass= and IOSchedulingPriority= have no effect. See ioprio_set(2) for details.

SANDBOXING

The following sandboxing options are an effective way to limit the exposure of the system towards the unit's processes. It is recommended to turn on as many of these options for each unit as is possible without negatively affecting the process' ability to operate. Note that many of these sandboxing features are gracefully turned off on systems where the underlying security mechanism is not available. For example, ProtectSystem= has no effect if the kernel is built without file system namespacing or if the service manager runs in a container manager that makes file system namespacing unavailable to its payload. Similar, RestrictRealtime= has no effect on systems that lack support for SECCOMP system call filtering, or in containers where support for this is turned off.

Also note that some sandboxing functionality is generally not available in user services (i.e. services run by the per-user service manager). Specifically, the various settings requiring file system namespacing support (such as ProtectSystem=) are not available, as the underlying kernel functionality is only accessible to privileged processes.

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. This setting cannot ensure protection in all cases. In general it has the same limitations as ReadOnlyPaths=, see below. Defaults to off.

ProtectHome=
    Takes a boolean argument or the special values "read-only" or "tmpfs". 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. If set to "tmpfs", temporary file systems are mounted on the three directories in read-only mode. The value "tmpfs" is useful to hide home directories not relevant to the processes invoked by the unit, while necessary directories are still visible by combining with BindPaths= or BindReadOnlyPaths=.

    Setting this to "yes" is mostly equivalent to set the three directories in InaccessiblePaths=. Similarly, "read-only" is mostly equivalent to ReadOnlyPaths=, and "tmpfs" is mostly equivalent to TemporaryFileSystem=.

    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. This setting cannot ensure protection in all cases. In general it has the same limitations as ReadOnlyPaths=, see below.

RuntimeDirectory=, StateDirectory=, CacheDirectory=, LogsDirectory=, ConfigurationDirectory=
    These options take a whitespace-separated list of directory names. The specified directory names must be relative, and may not include "..". If set, one or more directories by the specified names will be created (including their parents) below the locations defined in the following table, when the unit is started. Also, the corresponding environment variable is defined with the full path of directories. If multiple directories are set, then in the environment variable the paths are concatenated with colon (":").

    Table 2. Automatic directory creation and environment variables
    Locations | for system | for users | Environment variable
    RuntimeDirectory= | /run | $XDG_RUNTIME_DIR | $RUNTIME_DIRECTORY
    StateDirectory= | /var/lib | $XDG_CONFIG_HOME | $STATE_DIRECTORY
    CacheDirectory= | /var/cache | $XDG_CACHE_HOME | $CACHE_DIRECTORY
    LogsDirectory= | /var/log | $XDG_CONFIG_HOME/log | $LOGS_DIRECTORY
    ConfigurationDirectory= | /etc | $XDG_CONFIG_HOME | $CONFIGURATION_DIRECTORY

    In case of RuntimeDirectory= the lowest subdirectories are removed when the unit is stopped. It is possible to preserve the specified directories in this case if RuntimeDirectoryPreserve= is configured to restart or yes (see below). The directories specified with StateDirectory=, CacheDirectory=, LogsDirectory=, ConfigurationDirectory= are not removed when the unit is stopped.

    Except in case of ConfigurationDirectory=, the innermost specified directories will be owned by the user and group specified in User= and Group=. If the specified directories already exist and their owning user or group do not match the configured ones, all files and directories below the specified directories as well as the directories themselves will have their file ownership recursively changed to match what is configured. As an optimization, if the specified directories are already owned by the right user and group, files and directories below of them are left as-is, even if they do not match what is requested. The innermost specified directories will have their access mode adjusted to the what is specified in RuntimeDirectoryMode=, StateDirectoryMode=, CacheDirectoryMode=, LogsDirectoryMode= and ConfigurationDirectoryMode=.

    These options imply BindPaths= for the specified paths. When combined with RootDirectory= or RootImage= these paths always reside on the host and are mounted from there into the unit's file system namespace.

    If DynamicUser= is used in conjunction with StateDirectory=, CacheDirectory= and LogsDirectory= is slightly altered: the directories are created below /var/lib/private, /var/cache/private and /var/log/private, respectively, which are host directories made inaccessible to unprivileged users, which ensures that access to these directories cannot be gained through dynamic user ID recycling. Symbolic links are created to hide this difference in behaviour. Both from perspective of the host and from inside the unit, the relevant directories hence always appear directly below /var/lib, /var/cache and /var/log.

    Use RuntimeDirectory= to manage one or more runtime directories for the unit and bind their lifetime to the daemon runtime. 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).

    Example: if a system service unit has the following,

        RuntimeDirectory=foo/bar baz

    the service manager creates /run/foo (if it does not exist), /run/foo/bar, and /run/baz. The directories /run/foo/bar and /run/baz except /run/foo are owned by the user and group specified in User= and Group=, and removed when the service is stopped.

    Example: if a system service unit has the following,

        RuntimeDirectory=foo/bar
        StateDirectory=aaa/bbb ccc

    then the environment variable "RUNTIME_DIRECTORY" is set with "/run/foo/bar", and "STATE_DIRECTORY" is set with "/var/lib/aaa/bbb:/var/lib/ccc".

RuntimeDirectoryMode=, StateDirectoryMode=, CacheDirectoryMode=, LogsDirectoryMode=, ConfigurationDirectoryMode=
    Specifies the access mode of the directories specified in RuntimeDirectory=, StateDirectory=, CacheDirectory=, LogsDirectory=, or ConfigurationDirectory=, respectively, as an octal number. Defaults to 0755. See "Permissions" in path_resolution(7) for a discussion of the meaning of permission bits.

RuntimeDirectoryPreserve=
    Takes a boolean argument or restart. If set to no (the default), the directories specified in RuntimeDirectory= are always removed when the service stops. If set to restart the directories are preserved when the service is both automatically and manually restarted. Here, the automatic restart means the operation specified in Restart=, and manual restart means the one triggered by systemctl restart foo.service. If set to yes, then the directories are not removed when the service is stopped. Note that since the runtime directory /run is a mount point of "tmpfs", then for system services the directories specified in RuntimeDirectory= are removed when the system is rebooted.

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. This may be more restrictive than desired, because it is not possible to nest ReadWritePaths=, ReadOnlyPaths=, BindPaths=, or BindReadOnlyPaths= inside it. For a more flexible option, see TemporaryFileSystem=.

    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 these settings will disconnect propagation of mounts from the unit's processes to the host. This means that this setting may not be used for services which shall be able to install mount points in the main mount namespace. For ReadWritePaths= and ReadOnlyPaths= propagation in the other direction is not affected, i.e. mounts created on the host generally appear in the unit processes' namespace, and mounts removed on the host also disappear there too. In particular, note that mount propagation from host to unit will result in unmodified mounts to be created in the unit's namespace, i.e. writable mounts appearing on the host will be writable in the unit's namespace too, even when propagated below a path marked with ReadOnlyPaths=! Restricting access with these options hence does not extend to submounts of a directory that are created later on. This means the lock-down offered by that setting is not complete, and does not offer full protection.

    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.

TemporaryFileSystem=
    Takes a space-separated list of mount points for temporary file systems (tmpfs). If set, a new file system namespace is set up for executed processes, and a temporary file system is mounted on each mount point. This option may be specified more than once, in which case temporary file systems are mounted on all listed mount points. If the empty string is assigned to this option, the list is reset, and all prior assignments have no effect. Each mount point may optionally be suffixed with a colon (":") and mount options such as "size=10%" or "ro". By default, each temporary file system is mounted with "nodev,strictatime,mode=0755". These can be disabled by explicitly specifying the corresponding mount options, e.g., "dev" or "nostrictatime".

    This is useful to hide files or directories not relevant to the processes invoked by the unit, while necessary files or directories can be still accessed by combining with BindPaths= or BindReadOnlyPaths=. See the example below.

    Example: if a unit has the following,

        TemporaryFileSystem=/var:ro
        BindReadOnlyPaths=/var/lib/systemd

    then the invoked processes by the unit cannot see any files or directories under /var except for /var/lib/systemd or its contents.

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. 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, including AF_NETLINK and AF_UNIX. Effectively, for AF_NETLINK this means that device configuration events received from systemd-udevd.service(8) are not delivered to the unit's processes. And for AF_UNIX this has the effect that AF_UNIX sockets in the abstract socket namespace of the host will become unavailable to the unit's 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.

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 module load and unload operations to be turned off 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. Defaults 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.

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 tilde 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. This option may appear more than once, in which case the namespace types are merged by OR, or by AND if the lines are prefixed with "~" (see examples below). 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.

    Example: if a unit has the following,

        RestrictNamespaces=cgroup ipc
        RestrictNamespaces=cgroup net

    then cgroup, ipc, and net are set. If the second line is prefixed with "~", e.g.,

        RestrictNamespaces=cgroup ipc
        RestrictNamespaces=~cgroup net

    then, only ipc is set.

LockPersonality=
    Takes a boolean argument. If set, locks down the personality(2) system call so that the kernel execution domain may not be changed from the default or the personality selected with Personality= directive. This may be useful to improve security, because odd personality emulations may be poorly tested and source of vulnerabilities. If running in user mode, or in system mode, but without the CAP_SYS_ADMIN capability (e.g. setting User=), NoNewPrivileges=yes is implied.

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) or pkey_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. However, the protection can be circumvented, if the service can write to a filesystem, which is not mounted with noexec (such as /dev/shm), or it can use memfd_create(). This can be prevented by making such file systems inaccessible to the service (e.g. InaccessiblePaths=/dev/shm) and installing further system call filters (SystemCallFilter=~memfd_create). 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.

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.

PrivateMounts=
    Takes a boolean parameter. If set, the processes of this unit will be run in their own private file system (mount) namespace with all mount propagation from the processes towards the host's main file system namespace turned off. This means any file system mount points established or removed by the unit's processes will be private to them and not be visible to the host. However, file system mount points established or removed on the host will be propagated to the unit's processes. See mount_namespaces(7) for details on file system namespaces. Defaults to off.

    When turned on, this executes three operations for each invoked process: a new CLONE_NEWNS namespace is created, after which all existing mounts are remounted to MS_SLAVE to disable propagation from the unit's processes to the host (but leaving propagation in the opposite direction in effect). Finally, the mounts are remounted again to the propagation mode configured with MountFlags=, see below.

    File system namespaces are set up individually for each process forked off by the service manager. Mounts established in the namespace of the process created by ExecStartPre= will hence be cleaned up automatically as soon as that process exits and will not be available to subsequent processes forked off for ExecStart= (and similar applies to the various other commands configured for units). Similarly, JoinsNamespaceOf= does not permit sharing kernel mount namespaces between units, it only enables sharing of the /tmp/ and /var/tmp/ directories.

    Other file system namespace unit settings — PrivateMounts=, PrivateTmp=, PrivateDevices=, ProtectSystem=, ProtectHome=, ReadOnlyPaths=, InaccessiblePaths=, ReadWritePaths=, ... — also enable file system namespacing in a fashion equivalent to this option. Hence it is primarily useful to explicitly request this behaviour if none of the other settings are used.

MountFlags=
    Takes a mount propagation setting: shared, slave or private, which controls whether file system mount points in the file system namespaces set up for this unit's processes will receive or propagate mounts and unmounts from other file system namespaces. See mount(2) for details on mount propagation, and the three propagation flags in particular.

    This setting only controls the final propagation setting in effect on all mount points of the file system namespace created for each process of this unit. Other file system namespacing unit settings (see the discussion in PrivateMounts= above) will implicitly disable mount and unmount propagation from the unit's processes towards the host by changing the propagation setting of all mount points in the unit's file system namepace to slave first. Setting this option to shared does not reestablish propagation in that case.

    If not set – but file system namespaces are enabled through another file system namespace unit setting – shared mount propagation is used, but — as mentioned — as slave is applied first, propagation from the unit's processes to the host is still turned off.

    It is not recommended to to use private mount propagation for units, as this means temporary mounts (such as removable media) of the host will stay mounted and thus indefinitely busy in forked off processes, as unmount propagation events won't be received by the file system namespace of the unit.

    Usually, it is best to leave this setting unmodified, and use higher level file system namespacing options instead, in particular PrivateMounts=, see above.

SYSTEM CALL FILTERING

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). Blacklisted system calls and system call groups may optionally be suffixed with a colon (":") and "errno" error number (between 0 and 4095) or errno name such as EPERM, EACCES or EUCLEAN. This value will be returned when a blacklisted system call is triggered, instead of terminating the processes immediately. This value takes precedence over the one given in SystemCallErrorNumber=. 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 3. Currently predefined system call sets
    Set | Description
    @aio | Asynchronous I/O (io_setup(2), io_submit(2), and related calls)
    @basic-io | System calls for basic I/O: reading, writing, seeking, file descriptor duplication and closing (read(2), write(2), and related calls)
    @chown | Changing file ownership (chown(2), fchownat(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)
    @memlock | Locking of memory into RAM (mlock(2), mlockall(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), ...)
    @setuid | System calls for changing user ID and group ID credentials, (setuid(2), setgid(2), setresuid(2), ...)
    @signal | System calls for manipulating and handling process signals (signal(2), sigprocmask(2), ...)
    @swap | System calls for enabling/disabling swap devices (swapon(2), swapoff(2))
    @sync | Synchronizing files and memory to disk: (fsync(2), msync(2), and related calls)
    @system-service | A reasonable set of system calls used by common system services, excluding any special purpose calls. This is the recommended starting point for whitelisting system calls for system services, as it contains what is typically needed by system services, but excludes overly specific interfaces. For example, the following APIs are excluded: "@clock", "@mount", "@swap", "@reboot".
    @timer | System calls for scheduling operations by time (alarm(2), timer_create(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.

    Generally, whitelisting system calls (rather than blacklisting) is the safer mode of operation. It is recommended to enforce system call whitelists for all long-running system services. Specifically, the following lines are a relatively safe basic choice for the majority of system services:

        [Service]
        SystemCallFilter=@system-service
        SystemCallErrorNumber=EPERM

    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 (between 1 and 4095) or errno name such as EPERM, EACCES or EUCLEAN, to return when the system call filter configured with SystemCallFilter= is triggered, instead of terminating the process immediately. 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. The special identifier native implicitly maps to the native architecture of the system (or more precisely: 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. By default, this option is set to the empty list, i.e. no system call architecture filtering is applied.

    If this setting is used, processes of this unit will only be permitted to call native system calls, and system calls of the specified architectures. For the purposes of this option, the x32 architecture is treated as including x86-64 system calls. However, this setting still fulfills its purpose, as explained below, on x32.

    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 SystemCallArchitectures=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.

ENVIRONMENT

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.

    Note that environment variables are not suitable for passing secrets (such as passwords, key material, ...) to service processes. Environment variables set for a unit are exposed to unprivileged clients via D-Bus IPC, and generally not understood as being data that requires protection. Moreover, environment variables are propagated down the process tree, including across security boundaries (such as setuid/setgid executables), and hence might leak to processes that should not have access to the secret data.

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 set for the system service 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 passed. If the empty string is assigned to this option, the list of environment variables to pass is reset, all prior assignments have no effect. Variables specified that are not set for the system manager will not be passed and will be silently ignored. Note that this option is only relevant for the system service manager, as system services by default do not automatically inherit any environment variables set for the service manager itself. However, in case of the user service manager all environment variables are passed to the executed processes anyway, hence this option is without effect for the user service manager.

    Variables set for invoked processes due to this setting are subject to being overridden by those configured with 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.

UnsetEnvironment=
    Explicitly unset environment variable assignments that would normally be passed from the service manager to invoked processes of this unit. Takes a space-separated list of variable names or variable assignments. This option may be specified more than once, in which case all listed variables/assignments will be unset. If the empty string is assigned to this option, the list of environment variables/assignments to unset is reset. If a variable assignment is specified (that is: a variable name, followed by "=", followed by its value), then any environment variable matching this precise assignment is removed. If a variable name is specified (that is a variable name without any following "=" or value), then any assignment matching the variable name, regardless of its value is removed. Note that the effect of UnsetEnvironment= is applied as final step when the environment list passed to executed processes is compiled. That means it may undo assignments from any configuration source, including assignments made through Environment= or EnvironmentFile=, inherited from the system manager's global set of environment variables, inherited via PassEnvironment=, set by the service manager itself (such as $NOTIFY_SOCKET and such), or set by a PAM module (in case PAMName= is used).

    See environ(7) for details about environment variables.

LOGGING AND STANDARD INPUT/OUTPUT

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

    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 data option may be used to configure arbitrary textual or binary data to pass via standard input to the executed process. The data to pass is configured via StandardInputText=/StandardInputData= (see below). Note that the actual file descriptor type passed (memory file, regular file, UNIX pipe, ...) might depend on the kernel and available privileges. In any case, the file descriptor is read-only, and when read returns the specified data followed by EOF.

    The file:path option may be used to connect a specific file system object to standard input. An absolute path following the ":" character is expected, which may refer to a regular file, a FIFO or special file. If an AF_UNIX socket in the file system is specified, a stream socket is connected to it. The latter is useful for connecting standard input of processes to arbitrary system services.

    The socket option is valid in socket-activated services only, and requires the relevant socket unit file (see systemd.socket(5) for details) to have Accept=yes set, or to specify 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) socket activation daemon.

    The fd:name option connects standard input to a specific, named file descriptor provided by a socket unit. The name may be specified as part of this option, following a ":" character (e.g. "fd:foobar"). If no name is specified, the name "stdin" is implied (i.e. "fd" is equivalent to "fd:stdin"). At least one socket unit defining the specified name must be provided via the Sockets= option, and the 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 file descriptors and their ordering.

    This setting defaults to null.

    Note that services which specify DefaultDependencies=no and use StandardInput= or StandardOutput= with tty/tty-force/tty-fail, should specify After=systemd-vconsole-setup.service, to make sure that the tty intialization is finished before they start.

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, file:path, append:path, socket or fd:name.

    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.

    The file:path option may be used to connect a specific file system object to standard output. The semantics are similar to the same option of StandardInput=, see above. If path refers to a regular file on the filesystem, it is opened (created if it doesn't exist yet) for writing at the beginning of the file, but without truncating it. If standard input and output are directed to the same file path, it is opened only once, for reading as well as writing and duplicated. This is particularly useful when the specified path refers to an AF_UNIX socket in the file system, as in that case only a single stream connection is created for both input and output.

    append:path is similar to file:path above, but it opens the file in append mode.

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

    The fd:name option connects standard output to a specific, named file descriptor provided by a socket unit. A name may be specified as part of this option, following a ":" character (e.g. "fd:foobar"). If no name is specified, the name "stdout" is implied (i.e. "fd" is equivalent to "fd:stdout"). At least one socket unit defining the specified name must be provided via the Sockets= option, and the 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 their 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 "Implicit Dependencies" section above). Also note that in this case stdout (or stderr, see below) will be an AF_UNIX stream socket, and not a pipe or FIFO that can be re-opened. This means when executing shell scripts the construct echo "hello" > /dev/stderr for writing text to stderr will not work. To mitigate this use the construct echo "hello" >&2 instead, which is mostly equivalent and avoids this pitfall.

    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:name will use a default file descriptor name of "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).

StandardInputText=, StandardInputData=
    Configures arbitrary textual or binary data to pass via file descriptor 0 (STDIN) to the executed processes. These settings have no effect unless StandardInput= is set to data. Use this option to embed process input data directly in the unit file.

    StandardInputText= accepts arbitrary textual data. C-style escapes for special characters as well as the usual "%"-specifiers are resolved. Each time this setting is used the specified text is appended to the per-unit data buffer, followed by a newline character (thus every use appends a new line to the end of the buffer). Note that leading and trailing whitespace of lines configured with this option is removed. If an empty line is specified the buffer is cleared (hence, in order to insert an empty line, add an additional "\n" to the end or beginning of a line).

    StandardInputData= accepts arbitrary binary data, encoded in Base64[4]. No escape sequences or specifiers are resolved. Any whitespace in the encoded version is ignored during decoding.

    Note that StandardInputText= and StandardInputData= operate on the same data buffer, and may be mixed in order to configure both binary and textual data for the same input stream. The textual or binary data is joined strictly in the order the settings appear in the unit file. Assigning an empty string to either will reset the data buffer.

    Please keep in mind that in order to maintain readability long unit file settings may be split into multiple lines, by suffixing each line (except for the last) with a "\" character (see systemd.unit(5) for details). This is particularly useful for large data configured with these two options. Example:

        ...
        StandardInput=data
        StandardInputData=SWNrIHNpdHplIGRhIHVuJyBlc3NlIEtsb3BzLAp1ZmYgZWVtYWwga2xvcHAncy4KSWNrIGtpZWtl \
                          LCBzdGF1bmUsIHd1bmRyZSBtaXIsCnVmZiBlZW1hbCBqZWh0IHNlIHVmZiBkaWUgVMO8ci4KTmFu \
                          dSwgZGVuayBpY2ssIGljayBkZW5rIG5hbnUhCkpldHogaXNzZSB1ZmYsIGVyc2NodCB3YXIgc2Ug \
                          enUhCkljayBqZWhlIHJhdXMgdW5kIGJsaWNrZSDigJQKdW5kIHdlciBzdGVodCBkcmF1w59lbj8g \
                          SWNrZSEK
        ...

LogLevelMax=
    Configures filtering by log level of log messages generated by this unit. Takes a syslog log level, one of emerg (lowest log level, only highest priority messages), alert, crit, err, warning, notice, info, debug (highest log level, also lowest priority messages). See syslog(3) for details. By default no filtering is applied (i.e. the default maximum log level is debug). Use this option to configure the logging system to drop log messages of a specific service above the specified level. For example, set LogLevelMax=info in order to turn off debug logging of a particularly chatty unit. Note that the configured level is applied to any log messages written by any of the processes belonging to this unit, sent via any supported logging protocol. The filtering is applied early in the logging pipeline, before any kind of further processing is done. Moreover, messages which pass through this filter successfully might still be dropped by filters applied at a later stage in the logging subsystem. For example, MaxLevelStore= configured in journald.conf(5) might prohibit messages of higher log levels to be stored on disk, even though the per-unit LogLevelMax= permitted it to be processed.

LogExtraFields=
    Configures additional log metadata fields to include in all log records generated by processes associated with this unit. This setting takes one or more journal field assignments in the format "FIELD=VALUE" separated by whitespace. See systemd.journal-fields(7) for details on the journal field concept. Even though the underlying journal implementation permits binary field values, this setting accepts only valid UTF-8 values. To include space characters in a journal field value, enclose the assignment in double quotes ("). The usual specifiers are expanded in all assignments (see below). Note that this setting is not only useful for attaching additional metadata to log records of a unit, but given that all fields and values are indexed may also be used to implement cross-unit log record matching. Assign an empty string to reset the list.

LogRateLimitIntervalSec=, LogRateLimitBurst=
    Configures the rate limiting that is applied to messages generated by this unit. If, in the time interval defined by LogRateLimitIntervalSec=, more messages than specified in LogRateLimitBurst= are logged by a service, all further messages within the interval are dropped until the interval is over. A message about the number of dropped messages is generated. The time specification for LogRateLimitIntervalSec= may be specified in the following units: "s", "min", "h", "ms", "us" (see systemd.time(7) for details). The default settings are set by RateLimitIntervalSec= and RateLimitBurst= configured in journald.conf(5).

SyslogIdentifier=
    Sets the process name ("syslog tag") 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 journal, syslog or kmsg (or to the same settings in combination with +console) and only applies to log messages written to stdout or stderr.

SyslogFacility=
    Sets the syslog facility identifier to use when logging. 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 journal, syslog or kmsg (or to the same settings in combination with +console), and only applies to log messages written to stdout or stderr. Defaults to daemon.

SyslogLevel=
    The default syslog log level to use when logging to the logging system 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 journal, syslog or kmsg (or to the same settings in combination with +console), and only applies to log messages written to stdout or stderr. Note that individual lines output by executed processes may 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 journal, syslog or kmsg (or to the same settings in combination with +console), log lines written by the executed process that are prefixed with a log level will be processed 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. This only applies to log messages written to stdout or stderr. For details about this prefixing see sd-daemon(3). Defaults to true.

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".

SYSTEM V COMPATIBILITY

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".

ENVIRONMENT VARIABLES IN SPAWNED PROCESSES

Processes started by the service manager are executed with an environment variable block assembled from multiple sources. Processes started by the system service manager generally do not inherit environment variables set for the service manager itself (but this may be altered via PassEnvironment=), but processes started by the user service manager instances generally do inherit all environment variables set for the service manager itself.

For each invoked process the list of environment variables set is compiled from the following sources:

    • Variables globally configured for the service manager, using the DefaultEnvironment= setting in systemd-system.conf(5), the kernel command line option systemd.setenv= (see systemd(1)) or via systemctl set-environment (see systemctl(1)).

    • Variables defined by the service manager itself (see the list below)

    • Variables set in the service manager's own environment variable block (subject to PassEnvironment= for the system service manager)

    • Variables set via Environment= in the unit file

    • Variables read from files specified via EnvironmentFile= in the unit file

    • Variables set by any PAM modules in case PAMName= is in effect, cf. pam_env(8)

If the same environment variables are set by multiple of these sources, the later source — according to the order of the list above — wins. Note that as final step all variables listed in UnsetEnvironment= are removed again from the compiled environment variable list, immediately before it is passed to the executed process.

The following select environment variables are set or propagated by the service manager for each invoked process:

$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 to use for runtime objects (such as IPC objects) and volatile state. Set for all services run by the user systemd instance, as well as any system services that use PAMName= with a PAM stack that includes pam_systemd. See below and pam_systemd(8) for more information.

$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.

    If both standard output and standard error of the executed processes are connected to the journal via a stream socket, this environment variable will contain information about the standard error stream, as that's usually the preferred destination for log data. (Note that typically the same stream is used for both standard output and standard error, hence very likely the environment variable contains device and inode information matching both stream file descriptors.)

    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:

    Table 4. Defined $SERVICE_RESULT values
    Value | Meaning
    "success" | The service ran successfully and exited cleanly.
    "protocol" | A protocol violation occurred: the service did not take the steps required by its unit configuration (specifically what is configured in its Type= setting).
    "timeout" | One of the steps timed out.
    "exit-code" | Service process exited with a non-zero exit code; see $EXIT_CODE below for the actual exit code returned.
    "signal" | A service process was terminated abnormally by a signal, without dumping core. See $EXIT_CODE below for the actual signal causing the termination.
    "core-dump" | A service process terminated abnormally with a signal and dumped core. See $EXIT_CODE below for the signal causing the termination.
    "watchdog" | Watchdog keep-alive ping was enabled for the service, but the deadline was missed.
    "start-limit-hit" | A start limit was defined for the unit and it was hit, causing the unit to fail to start. See systemd.unit(5)'s StartLimitIntervalSec= and StartLimitBurst= for details.
    "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 5. Summary of possible service result variable values
    $SERVICE_RESULT | $EXIT_CODE | $EXIT_STATUS
    "success" | "exited" | "0"
    "protocol" | not set | not set
     | "exited" | "0"
    "timeout" | "killed" | "TERM", "KILL"
     | "exited" | "0", "1", "2", "3", ..., "255"
    "exit-code" | "exited" | "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"
    "start-limit-hit" | not set | not set
    "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. Moreover, using SuccessExitStatus= additional exit statuses may be declared to indicate clean termination, which is not reflected by this table.

For system services, when PAMName= is enabled and pam_systemd is part of the selected PAM stack, additional environment variables defined by systemd may be set for services. Specifically, these are $XDG_SEAT, $XDG_VTNR, see pam_systemd(8) for details.

PROCESS EXIT CODES

When invoking a unit process the service manager possibly fails to apply the execution parameters configured with the settings above. In that case the already created service process will exit with a non-zero exit code before the configured command line is executed. (Or in other words, the child process possibly exits with these error codes, after having been created by the fork(2) system call, but before the matching execve(2) system call is called.) Specifically, exit codes defined by the C library, by the LSB specification and by the systemd service manager itself are used.

The following basic service exit codes are defined by the C library.

Table 6. Basic C library exit codes
Exit Code | Symbolic Name | Description
0 | EXIT_SUCCESS | Generic success code.
1 | EXIT_FAILURE | Generic failure or unspecified error.

The following service exit codes are defined by the LSB specification[5].

Table 7. LSB service exit codes
Exit Code | Symbolic Name | Description
2 | EXIT_INVALIDARGUMENT | Invalid or excess arguments.
3 | EXIT_NOTIMPLEMENTED | Unimplemented feature.
4 | EXIT_NOPERMISSION | The user has insufficient privileges.
5 | EXIT_NOTINSTALLED | The program is not installed.
6 | EXIT_NOTCONFIGURED | The program is not configured.
7 | EXIT_NOTRUNNING | The program is not running.

The LSB specification suggests that error codes 200 and above are reserved for implementations. Some of them are used by the service manager to indicate problems during process invocation:

Table 8. systemd-specific exit codes
Exit Code | Symbolic Name | Description
200 | EXIT_CHDIR | Changing to the requested working directory failed. See WorkingDirectory= above.
201 | EXIT_NICE | Failed to set up process scheduling priority (nice level). See Nice= above.
202 | EXIT_FDS | Failed to close unwanted file descriptors, or to adjust passed file descriptors.
203 | EXIT_EXEC | The actual process execution failed (specifically, the execve(2) system call). Most likely this is caused by a missing or non-accessible executable file.
204 | EXIT_MEMORY | Failed to perform an action due to memory shortage.
205 | EXIT_LIMITS | Failed to adjust resource limits. See LimitCPU= and related settings above.
206 | EXIT_OOM_ADJUST | Failed to adjust the OOM setting. See OOMScoreAdjust= above.
207 | EXIT_SIGNAL_MASK | Failed to set process signal mask.
208 | EXIT_STDIN | Failed to set up standard input. See StandardInput= above.
209 | EXIT_STDOUT | Failed to set up standard output. See StandardOutput= above.
210 | EXIT_CHROOT | Failed to change root directory (chroot(2)). See RootDirectory=/RootImage= above.
211 | EXIT_IOPRIO | Failed to set up IO scheduling priority. See IOSchedulingClass=/IOSchedulingPriority= above.
212 | EXIT_TIMERSLACK | Failed to set up timer slack. See TimerSlackNSec= above.
213 | EXIT_SECUREBITS | Failed to set process secure bits. See SecureBits= above.
214 | EXIT_SETSCHEDULER | Failed to set up CPU scheduling. See CPUSchedulingPolicy=/CPUSchedulingPriority= above.
215 | EXIT_CPUAFFINITY | Failed to set up CPU affinity. See CPUAffinity= above.
216 | EXIT_GROUP | Failed to determine or change group credentials. See Group=/SupplementaryGroups= above.
217 | EXIT_USER | Failed to determine or change user credentials, or to set up user namespacing. See User=/PrivateUsers= above.
218 | EXIT_CAPABILITIES | Failed to drop capabilities, or apply ambient capabilities. See CapabilityBoundingSet=/AmbientCapabilities= above.
219 | EXIT_CGROUP | Setting up the service control group failed.
220 | EXIT_SETSID | Failed to create new process session.
221 | EXIT_CONFIRM | Execution has been cancelled by the user. See the systemd.confirm_spawn= kernel command line setting on kernel-command-line(7) for details.
222 | EXIT_STDERR | Failed to set up standard error output. See StandardError= above.
224 | EXIT_PAM | Failed to set up PAM session. See PAMName= above.
225 | EXIT_NETWORK | Failed to set up network namespacing. See PrivateNetwork= above.
226 | EXIT_NAMESPACE | Failed to set up mount namespacing. See ReadOnlyPaths= and related settings above.
227 | EXIT_NO_NEW_PRIVILEGES | Failed to disable new privileges. See NoNewPrivileges=yes above.
228 | EXIT_SECCOMP | Failed to apply system call filters. See SystemCallFilter= and related settings above.
229 | EXIT_SELINUX_CONTEXT | Determining or changing SELinux context failed. See SELinuxContext= above.
230 | EXIT_PERSONALITY | Failed to set up an execution domain (personality). See Personality= above.
231 | EXIT_APPARMOR_PROFILE | Failed to prepare changing AppArmor profile. See AppArmorProfile= above.
232 | EXIT_ADDRESS_FAMILIES | Failed to restrict address families. See RestrictAddressFamilies= above.
233 | EXIT_RUNTIME_DIRECTORY | Setting up runtime directory failed. See RuntimeDirectory= and related settings above.
235 | EXIT_CHOWN | Failed to adjust socket ownership. Used for socket units only.
236 | EXIT_SMACK_PROCESS_LABEL | Failed to set SMACK label. See SmackProcessLabel= above.
237 | EXIT_KEYRING | Failed to set up kernel keyring.
238 | EXIT_STATE_DIRECTORY | Failed to set up unit's state directory. See StateDirectory= above.
239 | EXIT_CACHE_DIRECTORY | Failed to set up unit's cache directory. See CacheDirectory= above.
240 | EXIT_LOGS_DIRECTORY | Failed to set up unit's logging directory. See LogsDirectory= above.
241 | EXIT_CONFIGURATION_DIRECTORY | Failed to set up unit's configuration directory. See ConfigurationDirectory= above.

Finally, the BSD operating systems define a set of exit codes, typically defined on Linux systems too:

Table 9. BSD exit codes
Exit Code | Symbolic Name | Description
64 | EX_USAGE | Command line usage error
65 | EX_DATAERR | Data format error
66 | EX_NOINPUT | Cannot open input
67 | EX_NOUSER | Addressee unknown
68 | EX_NOHOST | Host name unknown
69 | EX_UNAVAILABLE | Service unavailable
70 | EX_SOFTWARE | internal software error
71 | EX_OSERR | System error (e.g., can't fork)
72 | EX_OSFILE | Critical OS file missing
73 | EX_CANTCREAT | Can't create (user) output file
74 | EX_IOERR | Input/output error
75 | EX_TEMPFAIL | Temporary failure; user is invited to retry
76 | EX_PROTOCOL | Remote error in protocol
77 | EX_NOPERM | Permission denied
78 | EX_CONFIG | Configuration error

SEE ALSO

systemd(1), systemctl(1), systemd-analyze(1), journalctl(1), systemd-system.conf(5), 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.  No New Privileges Flag
    https://www.kernel.org/doc/html/latest/userspace-api/no_new_privs.html

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

4.  Base64
    https://tools.ietf.org/html/rfc2045#section-6.8

5.  LSB specification
    https://refspecs.linuxbase.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/iniscrptact.html