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 sligtly 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, but many other existing tools will do too.

Portable services may also be constructed from layers, similarly to container environments. See 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}, [email protected]{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.

  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.

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(s) image(s) 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(s) image(s) must at least contain one matching unit file each, with the right name prefix and suffix (see above).

# portablectl attach --extension foobar_0.7.23.raw debian-runtime_11.1.raw foobar
# portablectl attach --extension barbaz_7.0.23.raw 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 [email protected], so that instantiation is as simple as:

# portablectl attach foobar_0.7.23.raw
# systemctl enable --now [email protected]
# systemctl enable --now [email protected]

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.