systemd-sysext(8) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       systemd-sysext [OPTIONS...] COMMAND


       systemd-confext [OPTIONS...] COMMAND


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

       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

       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

       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 overlaying or 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. erofs, squashfs or ext4)

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

       System extensions are searched for in the directories
       /etc/extensions/, /run/extensions/ and /var/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. When invoked in the initrd, the additional
       directory /.extra/sysext/ is included in the directories that are
       searched for extension images. Note however, that by default a
       tighter image policy applies to images found there, though, see
       below. This directory is populated by systemd-stub(7) with
       extension images found in the system's EFI System Partition.

       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 the Portable Services[2]
       page 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 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

       Note that there is no concept of enabling/disabling installed
       system extension images: all installed extension images are
       automatically activated at boot. However, you can place an empty
       directory named like the extension (no .raw) in /etc/extensions/
       to "mask" an extension with the same name in a system folder with
       lower precedence.

       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 unless
       "_any" is set for the extension. If the extension ID= is not
       "_any", the SYSEXT_LEVEL= field (if defined) has to match. If the
       latter is not defined, the VERSION_ID= field has to match
       instead. If the extension defines the ARCHITECTURE= field and the
       value is not "_any" it has to match the kernel's architecture
       reported by uname(2) but the used architecture identifiers are
       the same as for ConditionArchitecture= described in
       systemd.unit(5).  EXTENSION_RELOAD_MANAGER= can be set to 1 if
       the extension requires a service manager reload after application
       of the extension. Note that the for the reasons mentioned
       earlier: Portable Services[2] remain the recommended way to ship
       system services. 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.

       The systemd-confext concept follows the same principle as the
       systemd-sysext(1) functionality but instead of working on /usr
       and /opt, confext will extend only /etc. Files and directories
       contained in the confext images outside of the /etc/ hierarchy
       are not merged, and hence have no effect when included in the
       image. Formats for these images are of the same as sysext images.
       The merged hierarchy will be mounted with "nosuid" and (if not
       disabled via --noexec=false) "noexec".

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

       Again, just like sysext images, the confext images will contain a
       /etc/extension-release.d/extension-release.NAME file, which must
       match the image name (with the usual escape hatch of the
       user.extension-release.strict xattr(7)), and again with content
       being one or more of ID=, VERSION_ID=, and CONFEXT_LEVEL. Confext
       images will then be checked and matched against the base OS

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

       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.

       For the confext case, the OSConfig project aims to perform
       runtime reconfiguration of OS services. Sometimes, there is a
       need to swap certain configuration parameter values or restart
       only a specific service without deployment of new code or a
       complete OS deployment. In other words, we want to be able to tie
       the most frequently configured options to runtime updateable
       flags that can be changed without a system reboot. This will help
       reduce servicing times when there is a need for changing the OS

COMMANDS         top

       The following commands are understood by both the sysext and
       confext concepts:

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

           Added in version 248.

           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. For confext,
           the merge happens into the /etc/ directory instead.

           Added in version 248.

           Unmerges all currently installed system extension images from
           /usr/ and /opt/ for sysext and /etc/, for confext, by
           unmounting the "overlayfs" file systems created by merge

           Added in version 248.

           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.

           Added in version 248.

           A brief list of installed extension images is shown.

           Added in version 248.

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

           Print a short version string and exit.

OPTIONS         top

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

           Added in version 248.

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

           Added in version 248.

           Takes an image policy string as argument, as per
           systemd.image-policy(7). The policy is enforced when
           operating on system extension disk images. If not specified
           defaults to
           for system extensions, i.e. only the root and /usr/ file
           systems in the image are used. For configuration extensions
           defaults to
           "root=verity+signed+encrypted+unprotected+absent". When run
           in the initrd and operating on a system extension image
           stored in the /.extra/sysext/ directory a slightly stricter
           policy is used by default:
           "root=signed+absent:usr=signed+absent", see above for

           Added in version 254.

           When merging configuration extensions into /etc/ the
           "MS_NOEXEC" mount flag is used by default. This option can be
           used to disable it.

           Added in version 254.

           When used with merge, unmerge or refresh, do not reload
           daemon after executing the changes even if an extension that
           is applied requires a reload via the
           EXTENSION_RELOAD_MANAGER= set to 1.

           Added in version 255.

           Do not pipe output into a pager.

           Do not print the legend, i.e. column headers and the footer
           with hints.

           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), systemd-stub(7)

NOTES         top

        1. Discoverable Partitions Specification

        2. 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
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-22.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

systemd 255                                            SYSTEMD-SYSEXT(8)

Pages that refer to this page: portablectl(1)systemd-cryptenroll(1)org.freedesktop.portable1(5)os-release(5)systemd.directives(7)systemd.image-policy(7)systemd.index(7)systemd-repart(8)