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

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

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

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.

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.

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 "+".

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 "+".

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

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

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

Table 1. Resource limit directives, their equivalent ulimit shell commands and the unit used

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

It is recommended to enable this setting for all long-running services (in particular network-facing ones), to ensure they cannot get access to private user data, unless the services actually require access to the user's private data. This setting is implied if DynamicUser= is set. For this setting the same restrictions regarding mount propagation and privileges apply as for ReadOnlyPaths= and related calls, see below.

Table 2. Automatic directory creation and environment variables

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

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

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

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

Use RuntimeDirectory= to manage one or more runtime directories for the unit and bind their lifetime to the daemon runtime. This is particularly useful for unprivileged daemons that cannot create runtime directories in /run due to lack of privileges, and to make sure the runtime directory is cleaned up automatically after use. For runtime directories that require more complex or different configuration or lifetime guarantees, please consider using tmpfiles.d(5).

Example: if a system service unit has the following,

RuntimeDirectory=foo/bar baz

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

Example: if a system service unit has the following,

RuntimeDirectory=foo/bar
StateDirectory=aaa/bbb ccc

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

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

Paths listed in InaccessiblePaths= will be made inaccessible for processes inside the namespace along with everything below them in the file system hierarchy. This may be more restrictive than desired, because it is not possible to nest ReadWritePaths=, ReadOnlyPaths=, BindPaths=, or BindReadOnlyPaths= inside it. For a more flexible option, see TemporaryFileSystem=.

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

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

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

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

Example: if a unit has the following,

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

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

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.

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.

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.

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.

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

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.

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 setting only controls the final propagation setting in effect on all mount points of the file system namespace created for each process of this unit. Other file system namespacing unit settings (see the discussion in PrivateMounts= above) will implicitly disable mount and unmount propagation from the unit's processes towards the host by changing the propagation setting of all mount points in the unit's file system namepace to slave first. Setting this option to shared does not reestablish propagation in that case. Conversely, if this option is set, but no other file system namespace setting is used, then new file system namespaces will be created for the unit's processes and this propagation flag will be applied right away to all mounts within it, without the intermediary application of slave.

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

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

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

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

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

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

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

Table 3. Currently predefined system call sets

Note, that as new system calls are added to the kernel, additional system calls might be added to the groups above. Contents of the sets may also change between systemd versions. In addition, the list of system calls depends on the kernel version and architecture for which systemd was compiled. Use
systemd-analyze syscall-filter to list the actual list of system calls in each filter.

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

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

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

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.

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.

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

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

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

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

Example:

PassEnvironment=VAR1 VAR2 VAR3

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

See environ(7) for details about environment variables.

See environ(7) for details about environment variables.

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.

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

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

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

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

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

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

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

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

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

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

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

If the standard output (or error output, see below) of a unit is connected to the journal, syslog or the kernel log buffer, the unit will implicitly gain a dependency of type After= on systemd-journald.socket (also see the "Implicit Dependencies" section above). Also note that in this case stdout (or stderr, see below) will be an AF_UNIX stream socket, and not a pipe or FIFO that can be re-opened. This means when executing shell scripts the construct echo "hello" > /dev/stderr for writing text to stderr will not work. To mitigate this use the construct echo "hello" >&2 instead, which is mostly equivalent and avoids this pitfall.

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

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

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

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

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

...
StandardInput=data
StandardInputData=SWNrIHNpdHplIGRhIHVuJyBlc3NlIEtsb3BzLAp1ZmYgZWVtYWwga2xvcHAncy4KSWNrIGtpZWtl \


                  LCBzdGF1bmUsIHd1bmRyZSBtaXIsCnVmZiBlZW1hbCBqZWh0IHNlIHVmZiBkaWUgVMO8ci4KTmFu \


                  dSwgZGVuayBpY2ssIGljayBkZW5rIG5hbnUhCkpldHogaXNzZSB1ZmYsIGVyc2NodCB3YXIgc2Ug \


                  enUhCkljayBqZWhlIHJhdXMgdW5kIGJsaWNrZSDigJQKdW5kIHdlciBzdGVodCBkcmF1w59lbj8g \


                  SWNrZSEK
...

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.

Table 4. Defined $SERVICE_RESULT values

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

Table 5. Summary of possible service result variable values