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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+---
+title: Portable Services Introduction
+---
+
+# Portable Services Introduction
+
+This systemd version includes a preview of the "portable service"
+concept. "Portable Services" are supposed to be an incremental improvement over
+traditional system services, making two specific facets of container management
+available to system services more readily. Specifically:
+
+1. The bundling of applications, i.e. packing up multiple services, their
+   binaries and all their dependencies in a single image, and running them
+   directly from it.
+
+2. Stricter default security policies, i.e. sand-boxing of applications.
+
+The primary tool for interfacing with "portable services" is the new
+"portablectl" program. It's currently shipped in /usr/lib/systemd/portablectl
+(i.e. not in the `$PATH`), since it's not yet considered part of the officially
+supported systemd interfaces — it's a preview still after all.
+
+Portable services don't bring anything inherently new to the table. All they do
+is put together known concepts in a slightly nicer way to cover a specific set
+of use-cases in a nicer way.
+
+## So, what *is* a "Portable Service"?
+
+A portable service is ultimately just an OS tree, either inside of a directory
+tree, 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.
+
+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.
+
+The "portable service" concept ultimately will not provide a fully isolated
+environment to the payload, like containers mostly intend to. Instead they are
+from the beginning more alike regular system services, can be controlled with
+the same tools, are exposed the same way in all infrastructure and so on. Their
+main difference is that the use a different root directory than the rest of the
+system. Hence, the intention is not to run code in a different, isolated world
+from the host — like most containers would do it —, but to run it in the same
+world, but with stricter access controls on what the service can see and do.
+
+As one point of differentiation: as programs run as "portable services" are
+pretty much regular system services, they won't run as PID 1 (like Docker would
+do it), but as normal process. 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 effective 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 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:
+
+```
+# /usr/lib/systemd/portablectl attach foobar_0.7.23.raw
+```
+
+This command does the following:
+
+1. It dissects the image, checks and validates the `/etc/os-release`
+   (or `/usr/lib/os-release`, see below)  data 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 the .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 at the highest
+   privileges, i.e. host's root and everything. 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 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-existing place, and break the
+logic.
+
+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
+again.
+
+Note that `portable attach` won't enable or start any of the units it copies
+out. This still has to take place in a second, separate step. (That said We
+might add options to do this automatically later on.).
+
+## 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 a binary (and its dependencies) that shall be invoked,
+   including all its dependencies. If binary code, the 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](https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/).
+
+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.
+
+Note that generally images created by tools such as `debootstrap`, `dnf
+--installroot=` or `mkosi` qualify for all of the above in one way or
+another. 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/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
+```
+
+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.
+
+Note that as no new image format or metadata is defined, it's very
+straight-forward to define images than can be made use of it 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 facility 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.
+
+## 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. Oh, 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 unit templating of systemd for this, i.e. to
+include template units such as `foobar@.service`, so that instantiation is as
+simple as:
+
+```
+# /usr/lib/systemd/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.