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+---
+title: Portable Services Introduction
+category: Concepts
+layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
+---
+
+# Portable Services Introduction
+
+systemd (since version 239) supports a concept of "Portable Services".
+"Portable Services" are a delivery method for system services that uses
+two specific features of container management:
+
+1. Applications are bundled. I.e. multiple services, their binaries and all
+   their dependencies are packaged in an image, and are run directly from it.
+
+2. Stricter default security policies, i.e. sand-boxing of applications.
+
+The primary tool for interacting with Portable Services is `portablectl`,
+and they are managed by the `systemd-portabled` service.
+
+Portable services don't bring anything inherently new to the table. All they do
+is put together known concepts to cover a specific set of use-cases in a
+slightly nicer way.
+
+## So, what *is* a "Portable Service"?
+
+A portable service is ultimately just an OS tree, either inside of a directory,
+or inside a raw disk image containing a Linux file system. This tree is called
+the "image". It can be "attached" or "detached" from the system. When
+"attached", specific systemd units from the image are made available on the
+host system, then behaving pretty much exactly like locally installed system
+services. When "detached", these units are removed again from the host, leaving
+no artifacts around (except maybe messages they might have logged).
+
+The OS tree/image can be created with any tool of your choice. For example, you
+can use `dnf --installroot=` if you like, or `debootstrap`, the image format is
+entirely generic, and doesn't have to carry any specific metadata beyond what
+distribution images carry anyway. Or to say this differently: the image format
+doesn't define any new metadata as unit files and OS tree directories or disk
+images are already sufficient, and pretty universally available these days. One
+particularly nice tool for creating suitable images is
+[mkosi](https://github.com/systemd/mkosi), but many other existing tools will
+do too.
+
+Portable services may also be constructed from layers, similarly to container
+environments. See [Extension Images](#extension-images) below.
+
+If you so will, "Portable Services" are a nicer way to manage chroot()
+environments, with better security, tooling and behavior.
+
+## Where's the difference to a "Container"?
+
+"Container" is a very vague term, after all it is used for
+systemd-nspawn/LXC-type OS containers, for Docker/rkt-like micro service
+containers, and even certain 'lightweight' VM runtimes.
+
+"Portable services" do not provide a fully isolated environment to the payload,
+like containers mostly intend to. Instead, they are more like regular system
+services, can be controlled with the same tools, are exposed the same way in
+all infrastructure, and so on. The main difference is that they use a different
+root directory than the rest of the system. Hence, the intent is not to run
+code in a different, isolated environment from the host — like most containers
+would — but to run it in the same environment, but with stricter access
+controls on what the service can see and do.
+
+One point of differentiation: since programs running as "portable services" are
+pretty much regular system services, they won't run as PID 1 (like they would
+under Docker), but as normal processes. A corollary of that is that they aren't
+supposed to manage anything in their own environment (such as the network) as
+the execution environment is mostly shared with the rest of the system.
+
+The primary focus use-case of "portable services" is to extend the host system
+with encapsulated extensions, but provide almost full integration with the rest
+of the system, though possibly restricted by security knobs. This focus
+includes system extensions otherwise sometimes called "super-privileged
+containers".
+
+Note that portable services are only available for system services, not for
+user services (i.e. the functionality cannot be used for the stuff
+bubblewrap/flatpak is focusing on).
+
+## Mode of Operation
+
+If you have a portable service image, maybe in a raw disk image called
+`foobar_0.7.23.raw`, then attaching the services to the host is as easy as:
+
+```
+# portablectl attach foobar_0.7.23.raw
+```
+
+This command does the following:
+
+1. It dissects the image, checks and validates the `os-release` file of the
+   image, and looks for all included unit files.
+
+2. It copies out all unit files with a suffix of `.service`, `.socket`,
+   `.target`, `.timer` and `.path`. whose name begins with the image's name
+   (with `.raw` removed), truncated at the first underscore if there is one.
+   This prefix name generated from the image name must be followed by a ".",
+   "-" or "@" character in the unit name. Or in other words, given the image
+   name of `foobar_0.7.23.raw` all unit files matching
+   `foobar-*.{service|socket|target|timer|path}`,
+   `foobar@.{service|socket|target|timer|path}` as well as
+   `foobar.*.{service|socket|target|timer|path}` and
+   `foobar.{service|socket|target|timer|path}` are copied out. These unit files
+   are placed in `/etc/systemd/system.attached/` (which is part of the normal
+   unit file search path of PID 1, and thus loaded exactly like regular unit
+   files). Within the images the unit files are looked for at the usual
+   locations, i.e. in `/usr/lib/systemd/system/` and `/etc/systemd/system/` and
+   so on, relative to the image's root.
+
+3. For each such unit file a drop-in file is created. Let's say
+   `foobar-waldo.service` was one of the unit files copied to
+   `/etc/systemd/system.attached/`, then a drop-in file
+   `/etc/systemd/system.attached/foobar-waldo.service.d/20-portable.conf` is
+   created, containing a few lines of additional configuration:
+
+   ```
+   [Service]
+   RootImage=/path/to/foobar.raw
+   Environment=PORTABLE=foobar
+   LogExtraFields=PORTABLE=foobar
+   ```
+
+4. For each such unit a "profile" drop-in is linked in. This "profile" drop-in
+   generally contains security options that lock down the service. By default
+   the `default` profile is used, which provides a medium level of security.
+   There's also `trusted`, which runs the service with no restrictions, i.e. in
+   the host file system root and with full privileges. The `strict` profile
+   comes with the toughest security restrictions. Finally, `nonetwork` is like
+   `default` but without network access. Users may define their own profiles
+   too (or modify the existing ones).
+
+And that's already it.
+
+Note that the images need to stay around (and in the same location) as long as the
+portable service is attached. If an image is moved, the `RootImage=` line
+written to the unit drop-in would point to an non-existent path, and break
+access to the image.
+
+The `portablectl detach` command executes the reverse operation: it looks for
+the drop-ins and the unit files associated with the image, and removes them.
+
+Note that `portablectl attach` won't enable or start any of the units it copies
+out by default, but `--enable` and `--now` parameter are available as shortcuts.
+The same is true for the opposite `detach` operation.
+
+The `portablectl reattach` command combines a `detach` with an `attach`. It is
+useful in case an image gets upgraded, as it allows performing a `restart`
+operation on the units instead of `stop` plus `start`, thus providing lower
+downtime and avoiding losing runtime state associated with the unit such as the
+file descriptor store.
+
+## Requirements on Images
+
+Note that portable services don't introduce any new image format, but most OS
+images should just work the way they are. Specifically, the following
+requirements are made for an image that can be attached/detached with
+`portablectl`.
+
+1. It must contain an executable that shall be invoked, along with all its
+   dependencies. Any binary code needs to be compiled for an architecture
+   compatible with the host.
+
+2. The image must either be a plain sub-directory (or btrfs subvolume)
+   containing the binaries and its dependencies in a classic Linux OS tree, or
+   must be a raw disk image either containing only one, naked file system, or
+   an image with a partition table understood by the Linux kernel with only a
+   single partition defined, or alternatively, a GPT partition table with a set
+   of properly marked partitions following the
+   [Discoverable Partitions Specification](DISCOVERABLE_PARTITIONS.md).
+
+3. The image must at least contain one matching unit file, with the right name
+   prefix and suffix (see above). The unit file is searched in the usual paths,
+   i.e. primarily /etc/systemd/system/ and /usr/lib/systemd/system/ within the
+   image. (The implementation will check a couple of other paths too, but it's
+   recommended to use these two paths.)
+
+4. The image must contain an os-release file, either in `/etc/os-release` or
+   `/usr/lib/os-release`. The file should follow the standard format.
+
+5. The image must contain the files `/etc/resolv.conf` and `/etc/machine-id`
+   (empty files are ok), they will be bind mounted from the host at runtime.
+
+6. The image must contain directories `/proc/`, `/sys/`, `/dev/`, `/run/`,
+   `/tmp/`, `/var/tmp/` that can be mounted over with the corresponding version
+   from the host.
+
+7. The OS might require other files or directories to be in place. For example,
+   if the image is built based on glibc, the dynamic loader needs to be
+   available in `/lib/ld-linux.so.2` or `/lib64/ld-linux-x86-64.so.2` (or
+   similar, depending on architecture), and if the distribution implements a
+   merged `/usr/` tree, this means `/lib` and/or `/lib64` need to be symlinks
+   to their respective counterparts below `/usr/`. For details see your
+   distribution's documentation.
+
+Note that images created by tools such as `debootstrap`, `dnf --installroot=`
+or `mkosi` generally satisfy all of the above. If you wonder what the most
+minimal image would be that complies with the requirements above, it could
+consist of this:
+
+```
+/usr/bin/minimald                            # a statically compiled binary
+/usr/lib/systemd/system/minimal-test.service # the unit file for the service, with ExecStart=/usr/bin/minimald
+/usr/lib/os-release                          # an os-release file explaining what this is
+/etc/resolv.conf                             # empty file to mount over with host's version
+/etc/machine-id                              # ditto
+/proc/                                       # empty directory to use as mount point for host's API fs
+/sys/                                        # ditto
+/dev/                                        # ditto
+/run/                                        # ditto
+/tmp/                                        # ditto
+/var/tmp/                                    # ditto
+```
+
+And that's it.
+
+Note that qualifying images do not have to contain an init system of their
+own. If they do, it's fine, it will be ignored by the portable service logic,
+but they generally don't have to, and it might make sense to avoid any, to keep
+images minimal.
+
+If the image is writable, and some of the files or directories that are
+overmounted from the host do not exist yet they will be automatically created.
+On read-only, immutable images (e.g. squashfs images) all files and directories
+to over-mount must exist already.
+
+Note that as no new image format or metadata is defined, it's very
+straightforward to define images than can be made use of in a number of
+different ways. For example, by using `mkosi -b` you can trivially build a
+single, unified image that:
+
+1. Can be attached as portable service, to run any container services natively
+   on the host.
+
+2. Can be run as OS container, using `systemd-nspawn`, by booting the image
+   with `systemd-nspawn -i -b`.
+
+3. Can be booted directly as VM image, using a generic VM executor such as
+   `virtualbox`/`qemu`/`kvm`
+
+4. Can be booted directly on bare-metal systems.
+
+Of course, to facilitate 2, 3 and 4 you need to include an init system in the
+image. To facilitate 3 and 4 you also need to include a boot loader in the
+image. As mentioned, `mkosi -b` takes care of all of that for you, but any
+other image generator should work too.
+
+The
+[os-release(5)](https://www.freedesktop.org/software/systemd/man/os-release.html)
+file may optionally be extended with a `PORTABLE_PREFIXES=` field listing all
+supported portable service prefixes for the image (see above). This is useful
+for informational purposes (as it allows recognizing portable service images
+from their contents as such), but is also useful to protect the image from
+being used under a wrong name and prefix. This is particularly relevant if the
+images are cryptographically authenticated (via Verity or a similar mechanism)
+as this way the (not necessarily authenticated) image file name can be
+validated against the (authenticated) image contents. If the field is not
+specified the image will work fine, but is not necessarily recognizable as
+portable service image, and any set of units included in the image may be
+attached, there are no restrictions enforced.
+
+## Extension Images
+
+Portable services can be delivered as one or multiple images that extend the base
+image, and are combined with OverlayFS at runtime, when they are attached. This
+enables a workflow that splits the base 'runtime' from the daemon, so that multiple
+portable services can share the same 'runtime' image (libraries, tools) without
+having to include everything each time, with the layering happening only at runtime.
+The `--extension` parameter of `portablectl` can be used to specify as many upper
+layers as desired. On top of the requirements listed in the previous section, the
+following must be also be observed:
+
+1. The base/OS image must contain an `os-release file`, either in `/etc/os-release`
+   or `/usr/lib/os-release`, in the standard format.
+
+2. The upper extension images must contain an extension-release file in
+   `/usr/lib/extension-release.d/`, with an `ID=` and `SYSEXT_LEVEL=`/`VERSION_ID=`
+   matching the base image.
+
+3. The base/OS image does not need to have any unit files.
+
+4. The upper extension images must contain at least one matching unit file
+   each, with the right name prefix and suffix (see above).
+
+5. As with the base/OS image, each upper extension image must be a plain
+   sub-directory, btrfs subvolume, or a raw disk image.
+
+```
+# portablectl attach --extension foobar_0.7.23.raw debian-runtime_11.1.raw foobar
+# portablectl attach --extension barbaz_7.0.23/ debian-runtime_11.1.raw barbaz
+```
+
+## Execution Environment
+
+Note that the code in portable service images is run exactly like regular
+services. Hence there's no new execution environment to consider. And, unlike
+Docker would do it, as these are regular system services they aren't run as PID
+1 either, but with regular PID values.
+
+## Access to host resources
+
+If services shipped with this mechanism shall be able to access host resources
+(such as files or AF_UNIX sockets for IPC), use the normal `BindPaths=` and
+`BindReadOnlyPaths=` settings in unit files to mount them in. In fact the
+`default` profile mentioned above makes use of this to ensure
+`/etc/resolv.conf`, the D-Bus system bus socket or write access to the logging
+subsystem are available to the service.
+
+## Instantiation
+
+Sometimes it makes sense to instantiate the same set of services multiple
+times. The portable service concept does not introduce a new logic for this. It
+is recommended to use the regular systemd unit templating for this, i.e. to
+include template units such as `foobar@.service`, so that instantiation is as
+simple as:
+
+```
+# portablectl attach foobar_0.7.23.raw
+# systemctl enable --now foobar@instancea.service
+# systemctl enable --now foobar@instanceb.service
+…
+```
+
+The benefit of this approach is that templating works exactly the same for
+units shipped with the OS itself as for attached portable services.
+
+## Immutable images with local data
+
+It's a good idea to keep portable service images read-only during normal
+operation. In fact all but the `trusted` profile will default to this kind of
+behaviour, by setting the `ProtectSystem=strict` option. In this case writable
+service data may be placed on the host file system. Use `StateDirectory=` in
+the unit files to enable such behaviour and add a local data directory to the
+services copied onto the host.
+
+## Links
+
+[`portablectl(1)`](https://www.freedesktop.org/software/systemd/man/portablectl.html)<br>
+[`systemd-portabled.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-portabled.service.html)<br>
+[Walkthrough for Portable Services](https://0pointer.net/blog/walkthrough-for-portable-services.html)<br>
+[Repo with examples](https://github.com/systemd/portable-walkthrough)