systemd-sysext(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | USES | COMMANDS | OPTIONS | EXIT STATUS | SEE ALSO | NOTES | COLOPHON

SYSTEMD-SYSEXT(8)            systemd-sysext            SYSTEMD-SYSEXT(8)

NAME         top

       systemd-sysext, systemd-sysext.service - Activates System
       Extension Images

SYNOPSIS         top

       systemd-sysext [OPTIONS...]

       systemd-sysext.service

DESCRIPTION         top

       systemd-sysext activates/deactivates system extension images.
       System extension images may – dynamically at runtime — extend the
       /usr/ and /opt/ directory hierarchies with additional files. This
       is particularly useful on immutable system images where a /usr/
       and/or /opt/ hierarchy residing on a read-only file system shall
       be extended temporarily at runtime without making any persistent
       modifications.

       System extension images should contain files and directories
       similar in fashion to regular operating system tree. When one or
       more system extension images are activated, their /usr/ and /opt/
       hierarchies are combined via "overlayfs" with the same
       hierarchies of the host OS, and the host /usr/ and /opt/
       overmounted with it ("merging"). When they are deactivated, the
       mount point is disassembled — again revealing the unmodified
       original host version of the hierarchy ("unmerging"). Merging
       thus makes the extension's resources suddenly appear below the
       /usr/ and /opt/ hierarchies as if they were included in the base
       OS image itself. Unmerging makes them disappear again, leaving in
       place only the files that were shipped with the base OS image
       itself.

       Files and directories contained in the extension images outside
       of the /usr/ and /opt/ hierarchies are not merged, and hence have
       no effect when included in a system extension image. In
       particular, files in the /etc/ and /var/ included in a system
       extension image will not appear in the respective hierarchies
       after activation.

       System extension images are strictly read-only, and the host
       /usr/ and /opt/ hierarchies become read-only too while they are
       activated.

       System extensions are supposed to be purely additive, i.e. they
       are supposed to include only files that do not exist in the
       underlying basic OS image. However, the underlying mechanism
       (overlayfs) also allows removing files, but it is recommended not
       to make use of this.

       System extension images may be provided in the following formats:

        1. Plain directories or btrfs subvolumes containing the OS tree

        2. Disk images with a GPT disk label, following the Discoverable
           Partitions Specification[1]

        3. Disk images lacking a partition table, with a naked Linux
           file system (e.g. squashfs or ext4)

       These image formats are the same ones that systemd-nspawn(1)
       supports via it's --directory=/--image= switches and those that
       the service manager supports via RootDirectory=/RootImage=.
       Similar to them they may optionally carry Verity authentication
       information.

       System extensions are automatically looked for in the directories
       /etc/extensions/, /run/extensions/, /var/lib/extensions/,
       /usr/lib/extensions/ and /usr/local/lib/extensions/. The first
       two listed directories are not suitable for carrying large binary
       images, however are still useful for carrying symlinks to them.
       The primary place for installing system extensions is
       /var/lib/extensions/. Any directories found in these search
       directories are considered directory based extension images, any
       files with the .raw suffix are considered disk image based
       extension images.

       During boot OS extension images are activated automatically, if
       the systemd-sysext.service is enabled. Note that this service
       runs only after the underlying file systems where system
       extensions may be located have been mounted. This means they are
       not suitable for shipping resources that are processed by
       subsystems running in earliest boot. Specifically, OS extension
       images are not suitable for shipping system services or
       systemd-sysusers(8) definitions. See Portable Services[2] for a
       simple mechanism for shipping system services in disk images, in
       a similar fashion to OS extensions. Note the different isolation
       on these two mechanisms: while system extension directly extend
       the underlying OS image with additional files that appear in a
       way very similar to as if they were shipped in the OS image
       itself and thus imply no security isolation, portable services
       imply service level sandboxing in one way or another. The
       systemd-sysext.service service is guaranteed to finish start-up
       before basic.target is reached; i.e. at the time regular services
       initialize (those which do not use DefaultDependencies=no), the
       files and directories system extensions provide are available in
       /usr/ and /opt/ and may be accessed.

       Note that there is no concept of enabling/disabling installed
       system extension images: all installed extension images are
       automatically activated at boot.

       A simple mechanism for version compatibility is enforced: a
       system extension image must carry a
       /usr/lib/extension-release.d/extension-release.$name file, which
       must match its image name, that is compared with the host
       os-release file: the contained ID= fields have to match, as well
       as the SYSEXT_LEVEL= field (if defined). If the latter is not
       defined, the VERSION_ID= field has to match instead. System
       extensions should not ship a /usr/lib/os-release file (as that
       would be merged into the host /usr/ tree, overriding the host OS
       version data, which is not desirable). The extension-release file
       follows the same format and semantics, and carries the same
       content, as the os-release file of the OS, but it describes the
       resources carried in the extension image.

USES         top

       The primary use case for system images are immutable environments
       where debugging and development tools shall optionally be made
       available, but not included in the immutable base OS image itself
       (e.g.  strace(1) and gdb(1) shall be an optionally installable
       addition in order to make debugging/development easier). System
       extension images should not be misunderstood as a generic
       software packaging framework, as no dependency scheme is
       available: system extensions should carry all files they need
       themselves, except for those already shipped in the underlying
       host system image. Typically, system extension images are built
       at the same time as the base OS image — within the same build
       system.

       Another use case for the system extension concept is temporarily
       overriding OS supplied resources with newer ones, for example to
       install a locally compiled development version of some low-level
       component over the immutable OS image without doing a full OS
       rebuild or modifying the nominally immutable image. (e.g.
       "install" a locally built package with
       DESTDIR=/var/lib/extensions/mytest make install && systemd-sysext
       refresh, making it available in /usr/ as if it was installed in
       the OS image itself.) This case works regardless if the
       underlying host /usr/ is managed as immutable disk image or is a
       traditional package manager controlled (i.e. writable) tree.

COMMANDS         top

       The following commands are understood:

       status
           When invoked without any command verb, or when status is
           specified the current merge status is shown, separately for
           both /usr/ and /opt/.

       merge
           Merges all currently installed system extension images into
           /usr/ and /opt/, by overmounting these hierarchies with an
           "overlayfs" file system combining the underlying hierarchies
           with those included in the extension images. This command
           will fail if the hierarchies are already merged.

       unmerge
           Unmerges all currently installed system extension images from
           /usr/ and /opt/, by unmounting the "overlayfs" file systems
           created by merge prior.

       refresh
           A combination of unmerge and merge: if already mounted the
           existing "overlayfs" instance is unmounted temporarily, and
           then replaced by a new version. This command is useful after
           installing/removing system extension images, in order to
           update the "overlayfs" file system accordingly. If no system
           extensions are installed when this command is executed, the
           equivalent of unmerge is executed, without establishing any
           new "overlayfs" instance. Note that currently there's a brief
           moment where neither the old nor the new "overlayfs" file
           system is mounted. This implies that all resources supplied
           by a system extension will briefly disappear — even if it
           exists continuously during the refresh operation.

       list
           A brief list of installed extension images is shown.

       -h, --help
           Print a short help text and exit.

       --version
           Print a short version string and exit.

OPTIONS         top

       --root=
           Operate relative to the specified root directory, i.e.
           establish the "overlayfs" mount not on the top-level host
           /usr/ and /opt/ hierarchies, but below some specified root
           directory.

       --force
           When merging system extensions into /usr/ and /opt/, ignore
           version incompatibilities, i.e. force merging regardless of
           whether the version information included in the extension
           images matches the host or not.

       --no-pager
           Do not pipe output into a pager.

       --no-legend
           Do not print the legend, i.e. column headers and the footer
           with hints.

       --json=MODE
           Shows output formatted as JSON. Expects one of "short" (for
           the shortest possible output without any redundant whitespace
           or line breaks), "pretty" (for a pretty version of the same,
           with indentation and line breaks) or "off" (to turn off JSON
           output, the default).

EXIT STATUS         top

       On success, 0 is returned.

SEE ALSO         top

       systemd(1), systemd-nspawn(1)

NOTES         top

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

        2. Portable Services
           https://systemd.io/PORTABLE_SERVICES

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have
       a bug report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
       This page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2021-08-27.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2021-08-27.)  If you discover any rendering
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       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
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systemd 249                                            SYSTEMD-SYSEXT(8)

Pages that refer to this page: org.freedesktop.portable1(5)os-release(5)