NAME

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

SYNOPSIS

       systemd-sysext [OPTIONS...]

       systemd-sysext.service

DESCRIPTION

       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

       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

       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

       --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

       On success, 0 is returned.

SEE ALSO

       systemd(1), systemd-nspawn(1)

NOTES

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

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