systemd.exec(5) - phpMan

SYSTEMD.EXEC(5) systemd.exec 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:

o 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=.

o Similarly, 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).

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

o Units using LogNamespace= will automatically gain ordering and requirement
dependencies on the two socket units associated with systemd-journald@.service
instances.

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.

If RootDirectory=/RootImage= are used together with NotifyAccess= the notification
socket is automatically mounted from the host into the root environment, to ensure the
notification interface can work correctly.

Note that services using RootDirectory=/RootImage= will not be able to log via the
syslog or journal protocols to the host logging infrastructure, unless the relevant
sockets are mounted from the host, specifically:

Example 1. Mounting logging sockets into root environment

BindReadOnlyPaths=/dev/log /run/systemd/journal/socket /run/systemd/journal/stdout

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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=.

Units making use of RootImage= automatically gain an After= dependency on
systemd-udevd.service.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

RootImageOptions=
Takes a comma-separated list of mount options that will be used on disk images
specified by RootImage=. Optionally a partition name can be prefixed, followed by
colon, in case the image has multiple partitions, otherwise partition name "root" is
implied. Options for multiple partitions can be specified in a single line with space
separators. Assigning an empty string removes previous assignments. Duplicated options
are ignored. For a list of valid mount options, please refer to mount(8).

Valid partition names follow the Discoverable Partitions Specification[1]: root, usr,
home, srv, esp, xbootldr, tmp, var.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

RootHash=
Takes a data integrity (dm-verity) root hash specified in hexadecimal, or the path to
a file containing a root hash in ASCII hexadecimal format. This option enables data
integrity checks using dm-verity, if the used image contains the appropriate integrity
data (see above) or if RootVerity= is used. The specified hash must match the root
hash of integrity data, and is usually at least 256 bits (and hence 64 formatted
hexadecimal characters) long (in case of SHA256 for example). If this option is not
specified, but the image file carries the "user.verity.roothash" extended file
attribute (see xattr(7)), then the root hash is read from it, also as formatted
hexadecimal characters. If the extended file attribute is not found (or is not
supported by the underlying file system), but a file with the .roothash suffix is
found next to the image file, bearing otherwise the same name (except if the image has
the .raw suffix, in which case the root hash file must not have it in its name), the
root hash is read from it and automatically used, also as formatted hexadecimal
characters.

If the disk image contains a separate /usr/ partition it may also be Verity protected,
in which case the root hash may configured via an extended attribute
"user.verity.usrhash" or a .usrhash file adjacent to the disk image. There's currently
no option to configure the root hash for the /usr/ file system via the unit file
directly.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

RootHashSignature=
Takes a PKCS7 signature of the RootHash= option as a path to a DER-encoded signature
file, or as an ASCII base64 string encoding of a DER-encoded signature prefixed by
"base64:". The dm-verity volume will only be opened if the signature of the root hash
is valid and signed by a public key present in the kernel keyring. If this option is
not specified, but a file with the .roothash.p7s suffix is found next to the image
file, bearing otherwise the same name (except if the image has the .raw suffix, in
which case the signature file must not have it in its name), the signature is read
from it and automatically used.

If the disk image contains a separate /usr/ partition it may also be Verity protected,
in which case the signature for the root hash may configured via a .usrhash.p7s file
adjacent to the disk image. There's currently no option to configure the root hash
signature for the /usr/ via the unit file directly.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

RootVerity=
Takes the path to a data integrity (dm-verity) file. This option enables data
integrity checks using dm-verity, if RootImage= is used and a root-hash is passed and
if the used image itself does not contains the integrity data. The integrity data must
be matched by the root hash. If this option is not specified, but a file with the
.verity suffix is found next to the image file, bearing otherwise the same name
(except if the image has the .raw suffix, in which case the verity data file must not
have it in its name), the verity data is read from it and automatically used.

This option is supported only for disk images that contain a single file system,
without an enveloping partition table. Images that contain a GPT partition table
should instead include both root file system and matching Verity data in the same
image, implementing the Discoverable Partitions Specification[1].

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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/, /dev/ and /run/ (as an empty "tmpfs")
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 four
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 four 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=.

In order to allow propagating mounts at runtime in a safe manner,
/run/systemd/propagate on the host will be used to set up new mounts, and
/run/host/incoming/ in the private namespace will be used as an intermediate step to
store them before being moved to the final mount point.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

ProtectProc=
Takes one of "noaccess", "invisible", "ptraceable" or "default" (which it defaults
to). When set, this controls the "hidepid=" mount option of the "procfs" instance for
the unit that controls which directories with process metainformation (/proc/PID) are
visible and accessible: when set to "noaccess" the ability to access most of other
users' process metadata in /proc/ is taken away for processes of the service. When set
to "invisible" processes owned by other users are hidden from /proc/. If "ptraceable"
all processes that cannot be ptrace()'ed by a process are hidden to it. If "default"
no restrictions on /proc/ access or visibility are made. For further details see The
/proc Filesystem[2]. It is generally recommended to run most system services with this
option set to "invisible". This option is implemented via file system namespacing, and
thus cannot be used with services that shall be able to install mount points in the
host file system hierarchy. Note that the root user is unaffected by this option, so
to be effective it has to be used together with User= or DynamicUser=yes, and also
without the "CAP_SYS_PTRACE" capability, which also allows a process to bypass this
feature. It cannot be used for services that need to access metainformation about
other users' processes. This option implies MountAPIVFS=.

If the kernel doesn't support per-mount point hidepid= mount options this setting
remains without effect, and the unit's processes will be able to access and see other
process as if the option was not used.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

ProcSubset=
Takes one of "all" (the default) and "pid". If "pid", all files and directories not
directly associated with process management and introspection are made invisible in
the /proc/ file system configured for the unit's processes. This controls the
"subset=" mount option of the "procfs" instance for the unit. For further details see
The /proc Filesystem[2]. Note that Linux exposes various kernel APIs via /proc/, which
are made unavailable with this setting. Since these APIs are used frequently this
option is useful only in a few, specific cases, and is not suitable for most
non-trivial programs.

Much like ProtectProc= above, this is implemented via file system mount namespacing,
and hence the same restrictions apply: it is only available to system services, it
disables mount propagation to the host mount table, and it implies MountAPIVFS=. Also,
like ProtectProc= this setting is gracefully disabled if the used kernel does not
support the "subset=" mount option of "procfs".

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.

Note that the destination directory must exist or systemd must be able to create it.
Thus, it is not possible to use those options for mount points nested underneath paths
specified in InaccessiblePaths=, or under /home/ and other protected directories if
ProtectHome=yes is specified. TemporaryFileSystem= with ":ro" or ProtectHome=tmpfs
should be used instead.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

MountImages=
This setting is similar to RootImage= in that it mounts a file system hierarchy from a
block device node or loopback file, but the destination directory can be specified as
well as mount options. This option expects a whitespace separated list of mount
definitions. Each definition consists of a colon-separated tuple of source path and
destination definitions, optionally followed by another colon and a list of mount
options.

Mount options may be defined as a single comma-separated list of options, in which
case they will be implicitly applied to the root partition on the image, or a series
of colon-separated tuples of partition name and mount options. Valid partition names
and mount options are the same as for RootImageOptions= setting described above.

Each mount definition may be prefixed with "-", in which case it will be ignored when
its source path does not exist. The source argument is a path to a block device node
or regular file. If source or destination contain a ":", it needs to be escaped as
"\:". The device node or file system image file needs to follow the same rules as
specified for RootImage=. Any mounts created with this option are specific to the
unit, and are not visible in the host's mount table.

These settings may be used more than once, each usage appends to the unit's list of
mount paths. If the empty string is assigned, the entire list of mount paths defined
prior to this is reset.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

ExtensionImages=
This setting is similar to MountImages= in that it mounts a file system hierarchy from
a block device node or loopback file, but instead of providing a destination path, an
overlay will be set up. This option expects a whitespace separated list of mount
definitions. Each definition consists of a source path, optionally followed by a colon
and a list of mount options.

A read-only OverlayFS will be set up on top of /usr/ and /opt/ hierarchies. The order
in which the images are listed will determine the order in which the overlay is laid
down: images specified first to last will result in overlayfs layers bottom to top.

Each mount definition may be prefixed with "-", in which case it will be ignored when
its source path does not exist. The source argument is a path to a block device node
or regular file. If the source path contains a ":", it needs to be escaped as "\:".
The device node or file system image file needs to follow the same rules as specified
for RootImage=. Any mounts created with this option are specific to the unit, and are
not visible in the host's mount table.

These settings may be used more than once, each usage appends to the unit's list of
image paths. If the empty string is assigned, the entire list of mount paths defined
prior to this is reset.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

USER/GROUP IDENTITY
These options are only available for system services and are not supported for services
running in per-user instances of the service manager.

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 this enforces only weak restrictions on the user/group name syntax, but will
generate warnings in many cases where user/group names do not adhere to the following
rules: the specified name should 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 and "_" (i.e. digits
and "-" are not permitted as first character). The user/group name must have at least
one character, and at most 31. These restrictions are made in order to avoid
ambiguities and to ensure user/group names and unit files remain portable among Linux
systems. For further details on the names accepted and the names warned about see
User/Group Name Syntax[3].

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. If the user does not exist by then program invocation will fail.

If the User= setting is used the supplementary group list is initialized from the
specified user's default group list, as defined in the system's user and group
database. Additional groups may be configured through the SupplementaryGroups= setting
(see below).

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= and PrivateTmp= are implied (and cannot be turned off). 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. Furthermore
NoNewPrivileges= and RestrictSUIDSGID= are implicitly enabled (and cannot be
disabled), to ensure that processes invoked cannot take benefit or create SUID/SGID
files or directories. 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
allow-listed 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). If this option is
enabled, care should be taken that the unit's processes do not get access to
directories outside of these explicitly configured and managed ones. Specifically, do
not use BindPaths= and be careful with AF_UNIX file descriptor passing for directory
file descriptors, as this would permit processes to create files or directories owned
by the dynamic user/group that are not subject to the lifecycle and access guarantees
of the service. 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
These options are only available for system services and are not supported for services
running in per-user instances of the service manager.

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 "+".

Use systemd-analyze(1)'s capability command to retrieve a list of capabilities defined
on the local system.

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 DynamicUser=, LockPersonality=, MemoryDenyWriteExecute=,
PrivateDevices=, ProtectClock=, ProtectHostname=, ProtectKernelLogs=,
ProtectKernelModules=, ProtectKernelTunables=, RestrictAddressFamilies=,
RestrictNamespaces=, RestrictRealtime=, RestrictSUIDSGID=, SystemCallArchitectures=,
SystemCallFilter=, or SystemCallLog= are specified. Note that even if this setting is
overridden by them, systemctl show shows the original value of this setting. In case
the service will be run in a new mount namespace anyway and SELinux is disabled, all
file systems are mounted with MS_NOSUID flag. Also see No New Privileges Flag[4].

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
These options are only available for system services and are not supported for services
running in per-user instances of the service manager.

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. If prefixed by "-", all errors will be ignored. This setting has no effect if
AppArmor is not enabled. This setting 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, MemoryMax= is
a more powerful (and working) replacement for LimitRSS=.

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 below).

For system units these resource limits may be chosen freely. When these settings are
configured in a user service (i.e. a service run by the per-user instance of the
service manager) they cannot be used to raise the limits above those set for the user
manager itself when it was first invoked, as the user's service manager generally
lacks the privileges to do so. In user context these configuration options are hence
only useful to lower the limits passed in or to raise the soft limit to the maximum of
the hard limit as configured for the user. To raise the user's limits further, the
available configuration mechanisms differ between operating systems, but typically
require privileges. In most cases it is possible to configure higher per-user resource
limits via PAM or by setting limits on the system service encapsulating the user's
service manager, i.e. the user's instance of user@.service. After making such changes,
make sure to restart the user's service manager.

UMask=
Controls the file mode creation mask. Takes an access mode in octal notation. See
umask(2) for details. Defaults to 0022 for system units. For user units the default
value is inherited from the per-user service manager (whose default is in turn
inherited from the system service manager, and thus typically also is 0022 -- unless
overridden by a PAM module). In order to change the per-user mask for all user
services, consider setting the UMask= setting of the user's user@.service system
service instance. The per-user umask may also be set via the umask field of a user's
JSON User Record[5] (for users managed by systemd-homed.service(8) this field may be
controlled via homectl --umask=). It may also be set via a PAM module, such as
pam_umask(8).

CoredumpFilter=
Controls which types of memory mappings will be saved if the process dumps core (using
the /proc/pid/coredump_filter file). Takes a whitespace-separated combination of
mapping type names or numbers (with the default base 16). Mapping type names are
private-anonymous, shared-anonymous, private-file-backed, shared-file-backed,
elf-headers, private-huge, shared-huge, private-dax, shared-dax, and the special
values all (all types) and default (the kernel default of "private-anonymous
shared-anonymous elf-headers private-huge"). See core(5) for the meaning of the
mapping types. When specified multiple times, all specified masks are ORed. When not
set, or if the empty value is assigned, the inherited value is not changed.

Example 2. Add DAX pages to the dump filter

CoredumpFilter=default private-dax shared-dax

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 value for the Linux kernel's Out-Of-Memory (OOM) killer score for
executed processes. Takes an integer between -1000 (to disable OOM killing of
processes of this unit) and 1000 (to make killing of processes of this unit under
memory pressure very likely). See proc.txt[6] for details. If not specified defaults
to the OOM score adjustment level of the service manager itself, which is normally at
0.

Use the OOMPolicy= setting of service units to configure how the service manager shall
react to the kernel OOM killer terminating a process of the service. See
systemd.service(5) 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). In case of resource
contention, smaller values mean more resources will be made available to the unit's
processes, larger values mean less resources will be made available. 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.
In case of CPU resource contention, smaller values mean less CPU time is made
available to the service, larger values mean more. 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 call fork(2), 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. Alternatively, takes a special "numa"
value in which case systemd automatically derives allowed CPU range based on the value
of NUMAMask= option. 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.

NUMAPolicy=
Controls the NUMA memory policy of the executed processes. Takes a policy type, one
of: default, preferred, bind, interleave and local. A list of NUMA nodes that should
be associated with the policy must be specified in NUMAMask=. For more details on each
policy please see, set_mempolicy(2). For overall overview of NUMA support in Linux
see, numa(7).

NUMAMask=
Controls the NUMA node list which will be applied alongside with selected NUMA policy.
Takes a list of NUMA nodes and has the same syntax as a list of CPUs for CPUAffinity=
option or special "all" value which will include all available NUMA nodes in the mask.
Note that the list of NUMA nodes is not required for default and local policies and
for preferred policy we expect a single NUMA node.

IOSchedulingClass=
Sets the I/O scheduling class for executed processes. Takes one of the strings
realtime, best-effort or idle. The kernel's default scheduling class is best-effort at
a priority of 4. 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). In case of I/O contention, smaller values
mean more I/O bandwidth is made available to the unit's processes, larger values mean
less bandwidth. 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. For the kernel's
default scheduling class (best-effort) this defaults to 4. 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. However,
most namespacing settings, that will not work on their own in user services, will work
when used in conjunction with PrivateUsers=true.

ProtectSystem=
Takes a boolean argument or the special values "full" or "strict". If true, mounts the
/usr/ and the boot loader directories (/boot and /efi) 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 still allowing necessary
directories to be made visible when listed in 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= with ":ro".

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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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, when the unit is
started, one or more directories by the specified names will be created (including
their parents) below the locations defined in the following table. Also, the
corresponding environment variable will be defined with the full paths of the
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
+------------------------+----------------+-----------------------+--------------------------+
|Directory | Below path for | Below path for user | Environment |
| | system units | units | variable set |
+------------------------+----------------+-----------------------+--------------------------+
|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 innermost 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, the logic for CacheDirectory=, LogsDirectory= and
StateDirectory= is slightly altered: the directories are created below
/var/cache/private, /var/log/private and /var/lib/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/cache, /var/log and /var/lib.

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).

The directories defined by these options are always created under the standard paths
used by systemd (/var/, /run/, /etc/, ...). If the service needs directories in a
different location, a different mechanism has to be used to create them.

tmpfiles.d(5) provides functionality that overlaps with these options. Using these
options is recommended, because the lifetime of the directories is tied directly to
the lifetime of the unit, and it is not necessary to ensure that the tmpfiles.d
configuration is executed before the unit is started.

To remove any of the directories created by these settings, use the systemctl clean
... command on the relevant units, see systemctl(1) for details.

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.

TimeoutCleanSec=
Configures a timeout on the clean-up operation requested through systemctl clean ...,
see systemctl(1) for details. Takes the usual time values and defaults to infinity,
i.e. by default no timeout is applied. If a timeout is configured the clean operation
will be aborted forcibly when the timeout is reached, potentially leaving resources on
disk.

ReadWritePaths=, ReadOnlyPaths=, InaccessiblePaths=, ExecPaths=, NoExecPaths=
Sets up a new file system namespace for executed processes. These options may be used
to limit access a process has to the file system. 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
allow-list 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=.

Content in paths listed in NoExecPaths= are not executable even if the usual file
access controls would permit this. Nest ExecPaths= inside of NoExecPaths= in order to
provide executable content within non-executable directories.

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=, InaccessiblePaths=, ExecPaths= and
NoExecPaths= 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.

Simple allow-list example using these directives:

[Service]
ReadOnlyPaths=/
ReadWritePaths=/var /run
InaccessiblePaths=-/lost+found
NoExecPaths=/
ExecPaths=/usr/sbin/my_daemon /lib /lib64

These options are only available for system services and are not supported for
services running in per-user instances of the service manager.

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=:

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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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
are 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 true, 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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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.

When this option is used on a socket unit any sockets bound on behalf of this unit
will be bound within a private network namespace. This may be combined with
JoinsNamespaceOf= to listen on sockets inside of network namespaces of other services.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

NetworkNamespacePath=
Takes an absolute file system path refererring to a Linux network namespace
pseudo-file (i.e. a file like /proc/$PID/ns/net or a bind mount or symlink to one).
When set the invoked processes are added to the network namespace referenced by that
path. The path has to point to a valid namespace file at the moment the processes are
forked off. If this option is used PrivateNetwork= has no effect. If this option is
used together with JoinsNamespaceOf= then it only has an effect if this unit is
started before any of the listed units that have PrivateNetwork= or
NetworkNamespacePath= configured, as otherwise the network namespace of those units is
reused.

When this option is used on a socket unit any sockets bound on behalf of this unit
will be bound within the specified network namespace.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

PrivateIPC=
Takes a boolean argument. If true, sets up a new IPC namespace for the executed
processes. Each IPC namespace has its own set of System V IPC identifiers and its own
POSIX message queue file system. This is useful to avoid name clash of IPC
identifiers. Defaults to false. It is possible to run two or more units within the
same private IPC namespace by using the JoinsNamespaceOf= directive, see
systemd.unit(5) for details.

Note that IPC namespacing does not have an effect on AF_UNIX sockets, which are the
most common form of IPC used on Linux. Instead, AF_UNIX sockets in the file system are
subject to mount namespacing, and those in the abstract namespace are subject to
network namespacing. IPC namespacing only has an effect on SysV IPC (which is mostly
legacy) as well as POSIX message queues (for which AF_UNIX/SOCK_SEQPACKET sockets are
typically a better replacement). IPC namespacing also has no effect on POSIX shared
memory (which is subject to mount namespacing) either. See ipc_namespaces(7) for the
details.

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

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

IPCNamespacePath=
Takes an absolute file system path refererring to a Linux IPC namespace pseudo-file
(i.e. a file like /proc/$PID/ns/ipc or a bind mount or symlink to one). When set the
invoked processes are added to the network namespace referenced by that path. The path
has to point to a valid namespace file at the moment the processes are forked off. If
this option is used PrivateIPC= has no effect. If this option is used together with
JoinsNamespaceOf= then it only has an effect if this unit is started before any of the
listed units that have PrivateIPC= or IPCNamespacePath= configured, as otherwise the
network namespace of those units is reused.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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.

When this setting is set up by a per-user instance of the service manager, the mapping
of the "root" user and group to itself is omitted (unless the user manager is root).
Additionally, in the per-user instance manager case, the user namespace will be set up
before most other namespaces. This means that combining PrivateUsers=true with other
namespaces will enable use of features not normally supported by the per-user
instances of the service manager.

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.

ProtectHostname=
Takes a boolean argument. When set, sets up a new UTS namespace for the executed
processes. In addition, changing hostname or domainname is prevented. Defaults to off.

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

Note that when this option is enabled for a service hostname changes no longer
propagate from the system into the service, it is hence not suitable for services that
need to take notice of system hostname changes dynamically.

If this setting is on, but the unit doesn't have the CAP_SYS_ADMIN capability (e.g.
services for which User= is set), NoNewPrivileges=yes is implied.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

ProtectClock=
Takes a boolean argument. If set, writes to the hardware clock or system clock will be
denied. It is recommended to turn this on for most services that do not need modify
the clock. Defaults to off. Enabling this option removes CAP_SYS_TIME and
CAP_WAKE_ALARM from the capability bounding set for this unit, installs a system call
filter to block calls that can set the clock, and DeviceAllow=char-rtc r is implied.
This ensures /dev/rtc0, /dev/rtc1, etc. are made read-only to the service. See
systemd.resource-control(5) for the details about DeviceAllow=. If this setting is on,
but the unit doesn't have the CAP_SYS_ADMIN capability (e.g. services for which User=
is set), NoNewPrivileges=yes is implied.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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 this setting is on, but the unit doesn't have
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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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 this setting is on,
but the unit doesn't have the CAP_SYS_ADMIN capability (e.g. services for which User=
is set), NoNewPrivileges=yes is implied.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

ProtectKernelLogs=
Takes a boolean argument. If true, access to the kernel log ring buffer will be
denied. It is recommended to turn this on for most services that do not need to read
from or write to the kernel log ring buffer. Enabling this option removes CAP_SYSLOG
from the capability bounding set for this unit, and installs a system call filter to
block the syslog(2) system call (not to be confused with the libc API syslog(3) for
userspace logging). The kernel exposes its log buffer to userspace via /dev/kmsg and
/proc/kmsg. If enabled, these are made inaccessible to all the processes in the unit.
If this setting is on, but the unit doesn't have the CAP_SYS_ADMIN capability (e.g.
services for which User= is set), NoNewPrivileges=yes is implied.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

RestrictAddressFamilies=
Restricts the set of socket address families accessible to the processes of this unit.
Takes "none", or a space-separated list of address family names to allow-list, such as
AF_UNIX, AF_INET or AF_INET6. When "none" is specified, then all address families will
be denied. When prefixed with "~" the listed address families will be applied as deny
list, otherwise as allow list. 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, ppc64, 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=), NoNewPrivileges=yes is implied. By default, no restrictions
apply, all address families are accessible to processes. If assigned the empty string,
any previous address family 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 allow list 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 (allow-listing). 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 (deny-listing). 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.

RestrictSUIDSGID=
Takes a boolean argument. If set, any attempts to set the set-user-ID (SUID) or
set-group-ID (SGID) bits on files or directories will be denied (for details on these
bits see inode(7)). If running in user mode, or in system mode, but without the
CAP_SYS_ADMIN capability (e.g. setting User=), NoNewPrivileges=yes is implied. As the
SUID/SGID bits are mechanisms to elevate privileges, and allows users to acquire the
identity of other users, it is recommended to restrict creation of SUID/SGID files to
the few programs that actually require them. Note that this restricts marking of any
type of file system object with these bits, including both regular files and
directories (where the SGID is a different meaning than for files, see documentation).
This option is implied if DynamicUser= is enabled. 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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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
namespace 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 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.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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 (allow-listing). (See
SystemCallErrorNumber= below for changing the default action). If the first character
of the list is "~", the effect is inverted: only the listed system calls will result
in immediate process termination (deny-listing). Deny-listed 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 (see errno(3) for
a full list). This value will be returned when a deny-listed system call is triggered,
instead of terminating the processes immediately. Special setting "kill" can be used
to explicitly specify killing. This value takes precedence over the one given in
SystemCallErrorNumber=, see below. If running in user mode, or in system mode, but
without the CAP_SYS_ADMIN capability (e.g. setting User=), 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 allow-listed 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. allow-listing and deny-listing), 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
an allow list rule for read() and write(), and right after it add a deny list rule for
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.

Generally, allow-listing system calls (rather than deny-listing) is the safer mode of
operation. It is recommended to enforce system call allow lists 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

Note that various kernel system calls are defined redundantly: there are multiple
system calls for executing the same operation. For example, the pidfd_send_signal()
system call may be used to execute operations similar to what can be done with the
older kill() system call, hence blocking the latter without the former only provides
weak protection. Since new system calls are added regularly to the kernel as
development progresses, keeping system call deny lists comprehensive requires constant
work. It is thus recommended to use allow-listing instead, which offers the benefit
that new system calls are by default implicitly blocked until the allow list is
updated.

Also note that a number of system calls are required to be accessible for the dynamic
linker to work. The dynamic linker is required for running most regular programs
(specifically: all dynamic ELF binaries, which is how most distributions build
packaged programs). This means that blocking these system calls (which include open(),
openat() or mmap()) will make most programs typically shipped with generic
distributions unusable.

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=,
ProtectKernelLogs=, ProtectClock=, 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. See errno(3) for a full
list of error codes. When this setting is not used, or when the empty string or the
special setting "kill" 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=), NoNewPrivileges=yes is implied. By
default, this option is set to the empty list, i.e. no 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.

SystemCallLog=
Takes a space-separated list of system call names. If this setting is used, all system
calls executed by the unit processes for the listed ones will be logged. If the first
character of the list is "~", the effect is inverted: all system calls except the
listed system calls will be logged. If running in user mode, or in system mode, but
without the CAP_SYS_ADMIN capability (e.g. setting User=), NoNewPrivileges=yes is
implied. This feature makes use of the Secure Computing Mode 2 interfaces of the
kernel ('seccomp filtering') and is useful for auditing or setting up a minimal
sandboxing environment. 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
"+".

ENVIRONMENT
Environment=
Sets environment variables for executed processes. Each line is unquoted using the
rules described in "Quoting" section in systemd.syntax(7) and becomes a list of
variable assignments. If you need to assign a value containing spaces or the equals
sign to a variable, put quotes around the whole assignment. Variable expansion is not
performed inside the strings and the "$" character has no special meaning. Specifier
expansion is performed, see the "Specifiers" section in systemd.unit(5).

This option may be specified more than once, in which case all listed variables will
be set. If the same variable is listed 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.

The names of the variables can contain ASCII letters, digits, and the underscore
character. Variable names cannot be empty or start with a digit. In variable values,
most characters are allowed, but non-printable characters are currently rejected.

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. Use LoadCredential= (see below) to pass data to unit processes securely.

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 (").

C escapes[7] are supported, but not most control characters[8]. "\t" and "\n" can be
used to insert tabs and newlines within EnvironmentFile=.

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. The files are read from the file system of the service
manager, before any file system changes like bind mounts take place).

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=.

C escapes[7] are supported, but not most control characters[8]. "\t" and "\n" can be
used to insert tabs and newlines within 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 "Environment Variables in Spawned Processes" below for a description of how those
settings combine to form the inherited environment. See environ(7) for general
information 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, unless StandardInputText=/StandardInputData= are set,
in which case it defaults to data.

StandardOutput=
Controls where file descriptor 1 (stdout) of the executed processes is connected to.
Takes one of inherit, null, tty, journal, kmsg, journal+console, kmsg+console,
file:path, append:path, truncate: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 kmsg (see below) is implicitly
stored in the journal as well, the specific option listed below is hence a superset of
this one. (Also note that any external, additional syslog daemons receive their log
data from the journal, too, hence this is the option to use when logging shall be
processed with such a daemon.)

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 and kmsg+console work in a similar way as the two 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.

truncate:path is similar to file:path above, but it truncates the file when opening
it. For units with multiple command lines, e.g. Type=oneshot services with multiple
ExecStart=, or services with ExecCondition=, ExecStartPre= or ExecStartPost=, the
output file is reopened and therefore re-truncated for each command line. If the
output file is truncated while another process still has the file open, e.g. by an
ExecReload= running concurrently with an ExecStart=, and the other process continues
writing to the file without adjusting its offset, then the space between the file
pointers of the two processes may be filled with NUL bytes, producing a sparse file.
Thus, truncate:path is typically only useful for units where only one process runs at
a time, such as services with a single ExecStart= and no ExecStartPost=, ExecReload=,
ExecStop= or similar.

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 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 (which is the default if StandardInput= is not set otherwise, but
StandardInputText=/StandardInputData= is). 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[9]. 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, as well as any log messages written by
the system manager process (PID 1) in reference 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).

LogNamespace=
Run the unit's processes in the specified journal namespace. Expects a short
user-defined string identifying the namespace. If not used the processes of the
service are run in the default journal namespace, i.e. their log stream is collected
and processed by systemd-journald.service. If this option is used any log data
generated by processes of this unit (regardless if via the syslog(), journal native
logging or stdout/stderr logging) is collected and processed by an instance of the
systemd-journald@.service template unit, which manages the specified namespace. The
log data is stored in a data store independent from the default log namespace's data
store. See systemd-journald.service(8) for details about journal namespaces.

Internally, journal namespaces are implemented through Linux mount namespacing and
over-mounting the directory that contains the relevant AF_UNIX sockets used for
logging in the unit's mount namespace. Since mount namespaces are used this setting
disconnects propagation of mounts from the unit's processes to the host, similar to
how ReadOnlyPaths= and similar settings (see above) work. Journal namespaces may hence
not be used for services that need to establish mount points on the host.

When this option is used the unit will automatically gain ordering and requirement
dependencies on the two socket units associated with the systemd-journald@.service
instance so that they are automatically established prior to the unit starting up.
Note that when this option is used log output of this service does not appear in the
regular journalctl(1) output, unless the --namespace= option is used.

This option is only available for system services and is not supported for services
running in per-user instances of the service manager.

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 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 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 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 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".

CREDENTIALS
LoadCredential=ID[:PATH]
Pass a credential to the unit. Credentials are limited-size binary or textual objects
that may be passed to unit processes. They are primarily used for passing
cryptographic keys (both public and private) or certificates, user account information
or identity information from host to services. The data is accessible from the unit's
processes via the file system, at a read-only location that (if possible and
permitted) is backed by non-swappable memory. The data is only accessible to the user
associated with the unit, via the User=/DynamicUser= settings (as well as the
superuser). When available, the location of credentials is exported as the
$CREDENTIALS_DIRECTORY environment variable to the unit's processes.

The LoadCredential= setting takes a textual ID to use as name for a credential plus a
file system path, separated by a colon. The ID must be a short ASCII string suitable
as filename in the filesystem, and may be chosen freely by the user. If the specified
path is absolute it is opened as regular file and the credential data is read from it.
If the absolute path refers to an AF_UNIX stream socket in the file system a
connection is made to it (only once at unit start-up) and the credential data read
from the connection, providing an easy IPC integration point for dynamically providing
credentials from other services. If the specified path is not absolute and itself
qualifies as valid credential identifier it is understood to refer to a credential
that the service manager itself received via the $CREDENTIALS_DIRECTORY environment
variable, which may be used to propagate credentials from an invoking environment
(e.g. a container manager that invoked the service manager) into a service. The
contents of the file/socket may be arbitrary binary or textual data, including newline
characters and NUL bytes. If the file system path is omitted it is chosen identical to
the credential name, i.e. this is a terse way do declare credentials to inherit from
the service manager into a service. This option may be used multiple times, each time
defining an additional credential to pass to the unit.

The credential files/IPC sockets must be accessible to the service manager, but don't
have to be directly accessible to the unit's processes: the credential data is read
and copied into separate, read-only copies for the unit that are accessible to
appropriately privileged processes. This is particularly useful in combination with
DynamicUser= as this way privileged data can be made available to processes running
under a dynamic UID (i.e. not a previously known one) without having to open up access
to all users.

In order to reference the path a credential may be read from within a ExecStart=
command line use "${CREDENTIALS_DIRECTORY}/mycred", e.g. "ExecStart=cat
${CREDENTIALS_DIRECTORY}/mycred".

Currently, an accumulated credential size limit of 1 MB per unit is enforced.

If referencing an AF_UNIX stream socket to connect to, the connection will originate
from an abstract namespace socket, that includes information about the unit and the
credential ID in its socket name. Use getpeername(2) to query this information. The
returned socket name is formatted as NUL RANDOM "/unit/" UNIT "/" ID, i.e. a NUL byte
(as required for abstract namespace socket names), followed by a random string
(consisting of alphadecimal characters), followed by the literal string "/unit/",
followed by the requesting unit name, followed by the literal character "/", followed
by the textual credential ID requested. Example:
"\0adf9d86b6eda275e/unit/foobar.service/credx" in case the credential "credx" is
requested for a unit "foobar.service". This functionality is useful for using a single
listening socket to serve credentials to multiple consumers.

SetCredential=ID:VALUE
The SetCredential= setting is similar to LoadCredential= but accepts a literal value
to use as data for the credential, instead of a file system path to read the data
from. Do not use this option for data that is supposed to be secret, as it is
accessible to unprivileged processes via IPC. It's only safe to use this for user IDs,
public key material and similar non-sensitive data. For everything else use
LoadCredential=. In order to embed binary data into the credential data use C-style
escaping (i.e. "\n" to embed a newline, or "\x00" to embed a NUL byte).

If a credential of the same ID is listed in both LoadCredential= and SetCredential=,
the latter will act as default if the former cannot be retrieved. In this case not
being able to retrieve the credential from the path specified in LoadCredential= is
not considered fatal.

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:

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

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

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

o Variables set via Environment= in the unit file.

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

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

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

The general philosophy is to expose a small curated list of environment variables to
processes. Services started by the system manager (PID 1) will be started, without
additional service-specific configuration, with just a few environment variables. The user
manager inherits environment variables as any other system service, but in addition may
receive additional environment variables from PAM, and, typically, additional imported
variables when the user starts a graphical session. It is recommended to keep the
environment blocks in both the system and user managers managers lean. Importing all
variables inherited by the graphical session or by one of the user shells is strongly
discouraged.

Hint: systemd-run -P env and systemd-run --user -P env print the effective system and user
service environment blocks.

Environment Variables Set or Propagated by the Service Manager
The following environment variables are propagated by the service manager or generated
internally 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" in the system
manager. When compiled for systems with "unmerged /usr/" (/bin is not a symlink to
/usr/bin), ":/sbin:/bin" is appended. In case of the user manager, a different path
may be configured by the distribution. It is recommended to not rely on the order of
entries, and have only one program with a given name in $PATH.

$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.

$RUNTIME_DIRECTORY, $STATE_DIRECTORY, $CACHE_DIRECTORY, $LOGS_DIRECTORY,
$CONFIGURATION_DIRECTORY
Absolute paths to the directories defined with RuntimeDirectory=, StateDirectory=,
CacheDirectory=, LogsDirectory=, and ConfigurationDirectory= when those settings are
used.

$CREDENTIALS_DIRECTORY
An absolute path to the per-unit directory with credentials configured via
LoadCredential=/SetCredential=. The directory is marked read-only and is placed in
unswappable memory (if supported and permitted), and is only accessible to the UID
associated with the unit via User= or DynamicUser= (and the superuser).

$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).

$SYSTEMD_EXEC_PID
The PID of the unit process (e.g. process invoked by ExecStart=). The child process
can use this information to determine whether the process is directly invoked by the
service manager or indirectly as a child of another process by comparing this value
with the current PID (as similar to the scheme used in sd_listen_fds(3) with
$LISTEN_PID and $LISTEN_FDS).

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

$LOG_NAMESPACE
Contains the name of the selected logging namespace when the LogNamespace= service
setting is used.

$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:

$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.

$PIDFILE
The path to the configured PID file, in case the process is forked off on behalf of a
service that uses the PIDFile= setting, see systemd.service(5) for details. Service
code may use this environment variable to automatically generate a PID file at the
location configured in the unit file. This field is set to an absolute path in the
file system.

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.

The following service exit codes are defined by the LSB specification[10].

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:

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), fork(2)

NOTES
1. Discoverable Partitions Specification
https://systemd.io/DISCOVERABLE_PARTITIONS

2. The /proc Filesystem
https://www.kernel.org/doc/html/latest/filesystems/proc.html#mount-options

3. User/Group Name Syntax
https://systemd.io/USER_NAMES

4. No New Privileges Flag
https://www.kernel.org/doc/html/latest/userspace-api/no_new_privs.html

5. JSON User Record
https://systemd.io/USER_RECORD

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

7. C escapes
https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences

8. most control characters
https://en.wikipedia.org/wiki/Control_character#In_ASCII

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

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

systemd 249 SYSTEMD.EXEC(5)

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