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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="systemd-sysext"
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>systemd-sysext</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>systemd-sysext</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd-sysext</refname>
<refname>systemd-sysext.service</refname>
<refpurpose>Activates System Extension Images</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>systemd-sysext</command>
<arg choice="opt" rep="repeat">OPTIONS</arg>
<arg choice="plain">COMMAND</arg>
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</cmdsynopsis>
<para><literallayout><filename>systemd-sysext.service</filename></literallayout></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><command>systemd-sysext</command> activates/deactivates system extension images. System extension
images may dynamically at runtime — extend the <filename>/usr/</filename> and
<filename>/opt/</filename> directory hierarchies with additional files. This is particularly useful on
immutable system images where a <filename>/usr/</filename> and/or <filename>/opt/</filename> hierarchy
residing on a read-only file system shall be extended temporarily at runtime without making any
persistent modifications.</para>
<para>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
<filename>/usr/</filename> and <filename>/opt/</filename> hierarchies are combined via
<literal>overlayfs</literal> with the same hierarchies of the host OS, and the host
<filename>/usr/</filename> and <filename>/opt/</filename> overmounted with it ("merging"). When they are
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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
<filename>/usr/</filename> and <filename>/opt/</filename> 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.</para>
<para>Files and directories contained in the extension images outside of the <filename>/usr/</filename>
and <filename>/opt/</filename> hierarchies are <emphasis>not</emphasis> merged, and hence have no effect
when included in a system extension image. In particular, files in the <filename>/etc/</filename> and
<filename>/var/</filename> included in a system extension image will <emphasis>not</emphasis> appear in
the respective hierarchies after activation.</para>
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<para>System extension images are strictly read-only, and the host <filename>/usr/</filename> and
<filename>/opt/</filename> hierarchies become read-only too while they are activated.</para>
<para>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.</para>
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<para>System extension images may be provided in the following formats:</para>
<orderedlist>
<listitem><para>Plain directories or btrfs subvolumes containing the OS tree</para></listitem>
<listitem><para>Disk images with a GPT disk label, following the <ulink
url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink></para></listitem>
<listitem><para>Disk images lacking a partition table, with a naked Linux file system (e.g. erofs,
squashfs or ext4)</para></listitem>
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</orderedlist>
<para>These image formats are the same ones that
<citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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supports via its <option>--directory=</option>/<option>--image=</option> switches and those that the
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service manager supports via <option>RootDirectory=</option>/<option>RootImage=</option>. Similar to
them they may optionally carry Verity authentication information.</para>
<para>System extensions are automatically looked for in the directories
<filename>/etc/extensions/</filename>, <filename>/run/extensions/</filename>,
<filename>/var/lib/extensions/</filename>, <filename>/usr/lib/extensions/</filename> and
<filename>/usr/local/lib/extensions/</filename>. 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 <filename>/var/lib/extensions/</filename>. Any directories found in
these search directories are considered directory based extension images, any files with the
<filename>.raw</filename> suffix are considered disk image based extension images.</para>
<para>During boot OS extension images are activated automatically, if the
<filename>systemd-sysext.service</filename> 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
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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
<citerefentry><refentrytitle>systemd-sysusers</refentrytitle><manvolnum>8</manvolnum></citerefentry>
man: fix issues reported by the manpage-l10n project Fixes #25780. > Man page: crypttab.5 > Issue 1: Missing fullstop > Issue 2: I<cipher=>, I<hash=>, I<size=> → B<cipher=>, B<hash=>, B<size=> > > "Force LUKS mode\\&. When this mode is used, the following options are " > "ignored since they are provided by the LUKS header on the device: " > "I<cipher=>, I<hash=>, I<size=>" Seems OK to me. The full stop is there and has been for at least a few years. And we use <option> for the markup, which is appropriate here. > Man page: crypttab.5 > Issue 1: Missing fullstop > Issue 2: I<cipher=>, I<hash=>, I<keyfile-offset=>, I<keyfile-size=>, I<size=> → B<cipher=>, B<hash=>, B<keyfile-offset=>, B<keyfile-size=>, B<size=> > > "Use TrueCrypt encryption mode\\&. When this mode is used, the following " > "options are ignored since they are provided by the TrueCrypt header on the " > "device or do not apply: I<cipher=>, I<hash=>, I<keyfile-offset=>, I<keyfile-" > "size=>, I<size=>" Same. > Man page: journalctl.1 > Issue 1: make be → may be Fixed. > Issue 2: below\\&. → below: Fixed. > Man page: journalctl.1 > Issue: Colon at the end? > > "The following commands are understood\\&. If none is specified the default " > "is to display journal records\\&." > msgstr "" > "Die folgenden Befehle werden verstanden\\&. Falls keiner festgelegt ist, ist " > "die Anzeige von Journal-Datensätzen die Vorgabe\\&." This is a bit awkward, but I'm not sure how to fix it. > Man page: kernel-install.8 > Issue: methods a fallback → methods fallback It was correct, but I added a comma to make the sense clearer. > Man page: loader.conf.5 > Issue 1: secure boot variables → Secure Boot variables > Issue 2: one → one for (multiple times) > > "Supported secure boot variables are one database for authorized images, one " > "key exchange key (KEK) and one platform key (PK)\\&. For more information, " > "refer to the \\m[blue]B<UEFI specification>\\m[]\\&\\s-2\\u[2]\\d\\s+2, " > "under Secure Boot and Driver Signing\\&. Another resource that describe the " > "interplay of the different variables is the \\m[blue]B<EDK2 " > "documentation>\\m[]\\&\\s-2\\u[3]\\d\\s+2\\&." "one of" would sound strange. "One this and one that" is OK. > Man page: loader.conf.5 > Issue: systemd-boot → B<systemd-boot>(7) Fixed. > Man page: logind.conf.5 > Issue: systemd-logind → B<systemd-logind>(8) We use <filename>systemd-logind</> on subsequent references… I think that's good enough. > Man page: nss-myhostname.8 > Issue: B<getent> → B<getent>(1) Fixed. > Man page: nss-resolve.8 > Issue: B<systemd-resolved> → B<systemd-resolved>(8) The first reference does this, subsequent are shorter. > Man page: os-release.5 > Issue: Portable Services → Portable Services Documentation? Updated. > Man page: pam_systemd_home.8 > Issue: auth and account use "reason", while session and password do not? Reworded. > Man page: portablectl.1 > Issue: In systemd-portabled.service(8): Portable Services Documentation Updated. > Man page: repart.d.5 > Issue: The partition → the partition Fixed. > Man page: repart.d.5 > Issue: B<systemd-repart> → B<systemd-repart>(8) The first reference does this. I also change this one, because it's pretty far down in the text. > Man page: systemd.1 > Issue: kernel command line twice? > > "Takes a boolean argument\\&. If false disables importing credentials from " > "the kernel command line, qemu_fw_cfg subsystem or the kernel command line\\&." Apparently this was fixed already. > Man page: systemd-boot.7 > Issue: enrollement → enrollment Fixed. > Man page: systemd-cryptenroll.1 > Issue: multiple cases: any specified → the specified Reworded. > Man page: systemd-cryptenroll.1 > Issue: If this this → If this Fixed tree-wide. > Man page: systemd-cryptsetup-generator.8 > Issue: and the initrd → and in the initrd "Is honoured by the initrd" is OK, because we often speak about the initrd as a single unit. But in the same paragraph we also used "in the initrd", which makes the other use look sloppy. I changed it to "in the initrd" everywhere in that file. > Man page: systemd.directives.7 > Issue: Why are these two quoted (but not others)? > > "B<\\*(Aqh\\*(Aq>" > > B<\\*(Aqs\\*(Aq>" > > "B<\\*(Aqy\\*(Aq>" This is autogenerated from files… We use slightly different markup in different files, and it's just too hard to make it consistent. We gave up on this. > Man page: systemd.exec.5 > Issue 1: B<at>(1p) → B<at>(1) > Issue 2: B<crontab>(1p) → B<crontab>(1) Fixed. > Man page: systemd.exec.5 > Issue: B<select()> → B<select>(2) Fixed. > Man page: systemd.exec.5 > Issue: qemu → B<qemu>(1) The man page doesn't seem to be in any of the canonical places on the web. I added a link to online docs. > Man page: systemd.exec.5 > Issue: variable → variables Seems to be fixed already. > Man page: systemd-integritysetup-generator.8 > Issue: systemd-integritysetup-generator → B<systemd-integritysetup-generator> I changed <filename> to <command>. > Man page: systemd-integritysetup-generator.8 > Issue: superfluous comma at the end Already fixed. > Man page: systemd-measure.1 > Issue: (see B<--pcr-bank=>) below → (see B<--pcr-bank=> below) Reworded. > Man page: systemd-measure.1 > Issue: =PATH> → =>I<PATH> Fixed. > Man page: systemd-measure.1.po > Issue: B<--bank=DIGEST> → B<--bank=>I<DIGEST> Fixed. > Man page: systemd.netdev.5 > Issue: os the → on the Appears to have been fixed already. > Man page: systemd.netdev.5 > Issue: Onboard → On-board (as in previous string) Updated. > Man page: systemd.network.5 > Issue: B<systemd-networkd> -> B<systemd-networkd>(8) First reference does this, subsequent do not. > Man page: systemd.network.5 > Issue: B<netlabelctl> → B<netlabelctl>(8) First reference does this, subsequent do not. > Man page: systemd.network.5 > Issue: Missing verb (aquired? configured?) in the half sentence starting with "or by a " I dropped the comma. > Man page: systemd-nspawn.1 > Issue: All host users outside of that range → All other host users Reworded. > # FIXME no effect → no effect\\&. > #. type: Plain text > #: archlinux debian-unstable fedora-rawhide mageia-cauldron opensuse-tumbleweed > msgid "" > "Whichever ID mapping option is used, the same mapping will be used for users " > "and groups IDs\\&. If B<rootidmap> is used, the group owning the bind " > "mounted directory will have no effect" A period is added. Not sure if there's some other issue. > Man page: systemd-oomd.service.8 > Issue: B<systemd> → B<systemd>(1) Done. > Man page: systemd.path.5 > Issue 1: B<systemd.exec>(1) → B<systemd.exec>(5) > Issue 2: This section does not (yet?) exist Fixed. > Man page: systemd-pcrphase.service.8 > Issue 1: indicate phases into TPM2 PCR 11 ?? > Issue 2: Colon at the end of the paragraph? Fixed. > Man page: systemd-pcrphase.service.8 > Issue: final boot phase → final shutdown phase? Updated. > Man page: systemd-pcrphase.service.8 > Issue: for the the → for the Fixed tree-wide. > Man page: systemd-portabled.service.8 > Issue: In systemd-portabled.service(8): Portable Services Documentation Updated. > Man page: systemd-pstore.service.8 > Issue: Here and the following paragraphs: . → \\&. // Upstream: What does this comment mean? // You normally write \\&. for a full dot (full stop etc.); here you write only "." (i.e. a plain dot). > > "and we look up \"localhost\", nss-dns will send the following queries to " > "systemd-resolved listening on 127.0.0.53:53: first \"localhost.foobar.com\", " > "then \"localhost.barbar.com\", and finally \"localhost\". If (hopefully) the " > "first two queries fail, systemd-resolved will synthesize an answer for the " > "third query." Looks all OK to me. > Man page: systemd.resource-control.5 > Issue: Missing closing bracket after link to Control Groups version 1 Fixed. > Man page: systemd-sysext.8 > Issue: In systemd-portabled.service(8): Portable Services Documentation Updated. > Man page: systemd.timer.5 > Issue 1: B<systemd.exec>(1) → B<systemd.exec>(5) > Issue 2: This section does not (yet?) exist Fixed. > Man page: systemd.unit.5 > Issue: that is → that are Fixed. > Man page: systemd-veritysetup-generator.8 > Issue: systemd-veritysetup-generator → B<systemd-veritysetup-generator> > > "systemd-veritysetup-generator implements B<systemd.generator>(7)\\&." > > "systemd-veritysetup-generator understands the following kernel command line " > "parameters:" Updated. > Man page: systemd-volatile-root.service.8 > Issue: initrdyes → Initrd Fixed. > Man page: sysupdate.d.5 > Issue: : → \\&. (As above in TRANSFER) Updated. > Man page: sysupdate.d.5 > Issue: some → certain Updated. > Man page: sysupdate.d.5 > Issue 1: i\\&.e\\& → I\\&.e\\& Fixed. > Issue 2: the image → the system "image" seems correct. > Man page: tmpfiles.d.5 > Issue: systemd-tmpfiles → B<systemd-tmpfiles>(8) Updated.
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definitions. See the <ulink url="https://systemd.io/PORTABLE_SERVICES">Portable Services Documentation</ulink>
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 <filename>systemd-sysext.service</filename> service is
guaranteed to finish start-up before <filename>basic.target</filename> is reached; i.e. at the time
regular services initialize (those which do not use <varname>DefaultDependencies=no</varname>), the files
and directories system extensions provide are available in <filename>/usr/</filename> and
<filename>/opt/</filename> and may be accessed.</para>
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<para>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 <filename>.raw</filename>) in <filename>/etc/extensions/</filename> to "mask"
an extension with the same name in a system folder with lower precedence.</para>
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<para>A simple mechanism for version compatibility is enforced: a system extension image must carry a
<filename>/usr/lib/extension-release.d/extension-release.<replaceable>$name</replaceable></filename>
file, which must match its image name, that is compared with the host <filename>os-release</filename>
file: the contained <varname>ID=</varname> fields have to match unless <literal>_any</literal> is set
for the extension. If the extension <varname>ID=</varname> is not <literal>_any</literal>, the
<varname>SYSEXT_LEVEL=</varname> field (if defined) has to match. If the latter is not defined, the
<varname>VERSION_ID=</varname> field has to match instead. If the extension defines the
<varname>ARCHITECTURE=</varname> field and the value is not <literal>_any</literal> it has to match the kernel's
architecture reported by <citerefentry><refentrytitle>uname</refentrytitle><manvolnum>2</manvolnum></citerefentry>
but the used architecture identifiers are the same as for <varname>ConditionArchitecture=</varname>
described in <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
System extensions should not ship a <filename>/usr/lib/os-release</filename> file (as that would be merged
into the host <filename>/usr/</filename> tree, overriding the host OS version data, which is not desirable).
The <filename>extension-release</filename> file follows the same format and semantics, and carries the same
content, as the <filename>os-release</filename> file of the OS, but it describes the resources carried
in the extension image.</para>
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</refsect1>
<refsect1>
<title>Uses</title>
<para>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.
<citerefentry project='man-pages'><refentrytitle>strace</refentrytitle><manvolnum>1</manvolnum></citerefentry>
and
<citerefentry project='man-pages'><refentrytitle>gdb</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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.</para>
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<para>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 <command>DESTDIR=/var/lib/extensions/mytest
make install &amp;&amp; systemd-sysext refresh</command>, making it available in
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<filename>/usr/</filename> as if it was installed in the OS image itself.) This case works regardless if
the underlying host <filename>/usr/</filename> is managed as immutable disk image or is a traditional
package manager controlled (i.e. writable) tree.</para>
</refsect1>
<refsect1>
<title>Commands</title>
<para>The following commands are understood:</para>
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<variablelist>
<varlistentry>
<term><option>status</option></term>
<listitem><para>When invoked without any command verb, or when <option>status</option> is specified
the current merge status is shown, separately for both <filename>/usr/</filename> and
<filename>/opt/</filename>.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>merge</option></term>
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<listitem><para>Merges all currently installed system extension images into
<filename>/usr/</filename> and <filename>/opt/</filename>, by overmounting these hierarchies with an
<literal>overlayfs</literal> file system combining the underlying hierarchies with those included in
the extension images. This command will fail if the hierarchies are already merged.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>unmerge</option></term>
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<listitem><para>Unmerges all currently installed system extension images from
<filename>/usr/</filename> and <filename>/opt/</filename>, by unmounting the
<literal>overlayfs</literal> file systems created by <option>merge</option>
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prior.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>refresh</option></term>
<listitem><para>A combination of <option>unmerge</option> and <option>merge</option>: if already
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mounted the existing <literal>overlayfs</literal> 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 <literal>overlayfs</literal> file system accordingly. If no system extensions
are installed when this command is executed, the equivalent of <option>unmerge</option> is
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executed, without establishing any new <literal>overlayfs</literal> instance. Note that currently
there's a brief moment where neither the old nor the new <literal>overlayfs</literal> 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.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>list</option></term>
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<listitem><para>A brief list of installed extension images is shown.</para></listitem>
</varlistentry>
<xi:include href="standard-options.xml" xpointer="help" />
<xi:include href="standard-options.xml" xpointer="version" />
</variablelist>
</refsect1>
<refsect1>
<title>Options</title>
<variablelist>
<varlistentry>
<term><option>--root=</option></term>
<listitem><para>Operate relative to the specified root directory, i.e. establish the
<literal>overlayfs</literal> mount not on the top-level host <filename>/usr/</filename> and
<filename>/opt/</filename> hierarchies, but below some specified root directory.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>--force</option></term>
<listitem><para>When merging system extensions into <filename>/usr/</filename> and
<filename>/opt/</filename>, ignore version incompatibilities, i.e. force merging regardless of
whether the version information included in the extension images matches the host or
not.</para></listitem>
</varlistentry>
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<xi:include href="standard-options.xml" xpointer="no-pager" />
<xi:include href="standard-options.xml" xpointer="no-legend" />
<xi:include href="standard-options.xml" xpointer="json" />
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</variablelist>
</refsect1>
<refsect1>
<title>Exit status</title>
<para>On success, 0 is returned.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>