OCI Containers

This document shows how to build Fossil into OCI compatible containers and how to use those containers in interesting ways. We start off using the original and still most popular container development and runtime platform, Docker, but since you have more options than that, we will show some of these options later on.

1. Quick Start

Fossil ships a Dockerfile at the top of its source tree, here, which you can build like so:

  $ docker build -t fossil .

If the image built successfully, you can create a container from it and test that it runs:

  $ docker run --name fossil -p 9999:8080/tcp fossil

This shows us remapping the internal TCP listening port as 9999 on the host. This feature of OCI runtimes means there’s little point to using the “fossil server --port” feature inside the container. We can let Fossil default to 8080 internally, then remap it to wherever we want it on the host instead.

For debugging the live container while it runs, you can get an interactive shell like so:

  $ docker exec -it -u fossil fossil sh

Our stock Dockerfile configures Fossil with the default feature set, so you may wish to modify the Dockerfile to add configuration options, add APK packages to support those options, and so forth.

The Fossil Makefile provides two convenience targets, “make container-image” and “make container-run”. The first creates a versioned container image, and the second does that and then launches a fresh container based on that image. You can pass extra arguments to the first command via the Makefile’s DBFLAGS variable and to the second with the DCFLAGS variable. (DB is short for “docker build”, and DC is short for “docker create”, a sub-step of the “run” target.) To get the custom port setting as in second command above, say:

  $ make container-run DCFLAGS='-p 9999:8080/tcp'

Contrast the raw “docker” commands above, which create an unversioned image called fossil:latest and from that a container simply called fossil. The unversioned names are more convenient for interactive use, while the versioned ones are good for CI/CD type applications since they avoid a conflict with past versions; it lets you keep old containers around for quick roll-backs while replacing them with fresh ones.

2. Repository Storage Options

If you want the container to serve an existing repository, there are at least two right ways to do it.

The wrong way is to use the Dockerfile COPY command, because by baking the repo into the image at build time, it will become one of the image’s base layers. The end result is that each time you build a container from that image, the repo will be reset to its build-time state. Worse, restarting the container will do the same thing, since the base image layers are immutable in Docker. This is almost certainly not what you want.

The correct ways put the repo into the container created from the image, not in the image itself.

2.1 Storing the Repo Inside the Container

The simplest method is to stop the container if it was running, then say:

  $ docker cp /path/to/my-project.fossil fossil:/jail/museum/repo.fossil
  $ docker start fossil
  $ docker exec fossil chown -R 499 /jail/museum

That copies the local Fossil repo into the container where the server expects to find it, so that the “start” command causes it to serve from that copied-in file instead. Since it lives atop the immutable base layers, it persists as part of the container proper, surviving restarts.

Notice that the copy command changes the name of the repository database. The container configuration expects it to be called repo.fossil, which it almost certainly was not out on the host system. This is because there is only one repository inside this container, so we don’t have to name it after the project it contains, as is traditional. A generic name lets us hard-code the server start command.

If you skip the “chown” command above and put “http://localhost:9999/” into your browser, expecting to see the copied-in repo’s home page, you will get an opaque “Not Found” error. This is because the user and group ID of the file will be that of your local user on the container’s host machine, which is unlikely to map to anything in the container’s /etc/passwd and /etc/group files, effectively preventing the server from reading the copied-in repository file. 499 is the default “fossil” user ID inside the container, causing Fossil to run with that user’s privileges after it enters the chroot. (See below for how to change this default.) You don’t have to restart the server after fixing this with chmod: simply reload the browser, and Fossil will try again.

2.2 Storing the Repo Outside the Container

The simple storage method above has a problem: Docker containers are designed to be killed off at the slightest cause, rebuilt, and redeployed. If you do that with the repo inside the container, it gets destroyed, too. The solution is to replace the “run” command above with the following:

  $ docker run \
    --publish 9999:8080 \
    --name fossil-bind-mount \
    --volume ~/museum:/jail/museum \

Because this bind mount maps a host-side directory (~/museum) into the container, you don’t need to docker cp the repo into the container at all. It still expects to find the repository as repo.fossil under that directory, but now both the host and the container can see that repo DB.

Instead of a bind mount, you could instead set up a separate Docker volume, at which point you would need to docker cp the repo file into the container.

Either way, files in these mounted directories have a lifetime independent of the container(s) they’re mounted into. When you need to rebuild the container or its underlying image — such as to upgrade to a newer version of Fossil — the external directory remains behind and gets remapped into the new container when you recreate it with --volume/-v.

2.2.1 WAL Mode Interactions

You might be aware that OCI containers allow mapping a single file into the repository rather than a whole directory. Since Fossil repositories are specially-formatted SQLite databases, you might be wondering why we don’t say things like:

  --volume ~/museum/my-project.fossil:/jail/museum/repo.fossil

That lets us have a convenient file name for the project outside the container while letting the configuration inside the container refer to the generic “/museum/repo.fossil” name. Why should we have to name the repo generically on the outside merely to placate the container?

The reason is, you might be serving that repo with WAL mode enabled. If you map the repo DB alone into the container, the Fossil instance inside the container will write the -journal and -wal files alongside the mapped-in repository inside the container. That’s fine as far as it goes, but if you then try using the same repo DB from outside the container while there’s an active WAL, the Fossil instance outside won’t know about it. It will think it needs to write its own -journal and -wal files outside the container, creating a high risk of database corruption.

If we map a whole directory, both sides see the same set of WAL files. Testing gives us a reasonable level of confidence that using WAL across a container boundary is safe when used in this manner.

3. Security

3.1 Why Chroot?

A potentially surprising feature of this container is that it runs Fossil as root. Since that causes the chroot jail feature to kick in, and a Docker container is a type of über-jail already, you may be wondering why we bother. Instead, why not either:

The reason is, although this container is quite stripped-down by today’s standards, it’s based on the surprisingly powerful Busybox project. (This author made a living for years in the early 1990s using Unix systems that were less powerful than this container.) If someone ever figured out how to make a Fossil binary execute arbitrary commands on the host or to open up a remote shell, the power available to them at that point would make it likely that they’d be able to island-hop from there into the rest of your network. That power is there for you as the system administrator alone, to let you inspect the container’s runtime behavior, change things on the fly, and so forth. Fossil proper doesn’t need that power; if we take it away via this cute double-jail dance, we keep any potential attacker from making use of it should they ever get in.

Having said this, know that we deem this risk low since a) it’s never happened, that we know of; and b) we haven’t enabled any of the risky features of Fossil such as TH1 docs. Nevertheless, we believe defense-in-depth strategies are wise.

If you say something like “docker exec fossil ps” while the system is idle, it’s likely to report a single fossil process running as root even though the chroot feature is documented as causing Fossil to drop its privileges in favor of the owner of the repository database or its containing folder. If the repo file is owned by the in-container user “fossil”, why is the server still running as root?

It’s because you’re seeing only the parent process, which assumes it’s running on bare metal or a VM and thus may need to do rootly things like listening on port 80 or 443 before forking off any children to handle HTTP hits. Fossil’s chroot feature only takes effect in these child processes. This is why you can fix broken permissions with chown after the container is already running, without restarting it: each hit reevaluates the repository file permissions when deciding what user to become when dropping root privileges.

3.2 Dropping Unnecessary Capabilities

The example commands above create the container with a default set of Linux kernel capabilities. Although Docker strips away almost all of the traditional root capabilities by default, and Fossil doesn’t need any of those it does take away, Docker does leave some enabled that Fossil doesn’t actually need. You can tighten the scope of capabilities by adding “--cap-drop” options to your container creation commands.


All together, we recommend adding the following options to your “docker run” commands, as well as to any “docker create” command that will be followed by “docker start”:

  --cap-drop AUDIT_WRITE \
  --cap-drop CHOWN \
  --cap-drop FSETID \
  --cap-drop KILL \
  --cap-drop MKNOD \
  --cap-drop NET_BIND_SERVICE \
  --cap-drop NET_RAW \
  --cap-drop SETFCAP \
  --cap-drop SETPCAP

In the next section, we’ll show a case where you create a container without ever running it, making these options pointless.

4. Extracting a Static Binary

Our 2-stage build process uses Alpine Linux only as a build host. Once we’ve got everything reduced to the two key static binaries — Fossil and BusyBox — we throw all the rest of it away.

A secondary benefit falls out of this process for free: it’s arguably the easiest way to build a purely static Fossil binary for Linux. Most modern Linux distros make this surprisingly difficult, but Alpine’s back-to-basics nature makes static builds work the way they used to, back in the day. If that’s all you’re after, you can do so as easily as this:

  $ docker build -t fossil .
  $ docker create --name fossil-static-tmp fossil
  $ docker cp fossil-static-tmp:/jail/bin/fossil .
  $ docker container rm fossil-static-tmp

The resulting binary is the single largest file inside that container, at about 6 MiB. (It’s built stripped.)

5. Container Build Arguments

5.1 Package Versions

You can override the default versions of Fossil and BusyBox that get fetched in the build step. To get the latest-and-greatest of everything, you could say:

  $ docker build -t fossil \
    --build-arg FSLVER=trunk \
    --build-arg BBXVER=master .

(But don’t, for reasons we will get to.)

Because the BusyBox configuration file we ship was created with and tested against a specific stable release, that’s the version we pull by default. It does try to merge the defaults for any new configuration settings into the stock set, but since it’s possible this will fail, we don’t blindly update the BusyBox version merely because a new release came out. Someone needs to get around to vetting it against our stock configuration first.

As for Fossil, it defaults to fetching the same version as the checkout you’re running the build command from, based on checkin ID. You could use this to get a release build, for instance:

  $ docker build -t fossil \
    --build-arg FSLVER=version-2.20 .

Or equivalently, using Fossil’s Makefile convenience target:

  $ make container-image \
    DBFLAGS='--build-arg FSLVER=version-2.20'

While you could instead use the generic “release” tag here, it’s better to use a specific version number since Docker caches downloaded files and tries to reuse them across builds. If you ask for “release” before a new version is tagged and then immediately after, you might expect to get two different tarballs, but because the underlying source tarball URL remains the same when you do that, you’ll end up reusing the old tarball from your Docker cache. This will occur even if you pass the “docker build --no-cache” option.

This is why we default to pulling the Fossil tarball by checkin ID rather than let it default to the generic “trunk” tag: so the URL will change each time you update your Fossil source tree, forcing Docker to pull a fresh tarball.

5.2 User & Group IDs

The “fossil” user and group IDs inside the container default to 499. Why? Regular user IDs start at 500 or 1000 on most Unix type systems, leaving those below it for system users like this Fossil daemon owner. Since it’s typical for these to start at 0 and go upward, we started at 500 and went down one instead to reduce the chance of a conflict to as close to zero as we can manage.

To change it to something else, say:

  $ make container-image \
    DBFLAGS='--build-arg UID=501'

This is particularly useful if you’re putting your repository on a Docker volume since the IDs “leak” out into the host environment via file permissions. You may therefore wish them to mean something on both sides of the container barrier rather than have “499” appear on the host in “ls -l” output.

5.3 Fossil Configuration Options

You can use this same mechanism to enable non-default Fossil configuration options in your build. For instance, to turn on the JSON API and the TH1 docs extension:

  $ make container-image \
    DBFLAGS='--build-arg FSLCFG="--json --with-th1-docs"'

If you also wanted the Tcl evaluation extension, that’s trickier because it requires the tcl-dev package to be installed atop Alpine Linux in the first image build stage. We don’t currently have a way to do that because it brings you to a new problem: Alpine provides only a dynamic Tcl library, which conflicts with our wish for a static Fossil binary. For those who want such a “batteries included” container, we recommend taking a look at this alternative; needless to say, it’s inherently less secure than our stock container, but you may find the tradeoff worthwhile.

6. Lightweight Alternatives to Docker

Those afflicted with sticker shock at seeing the size of a Docker Desktop installation — 1.65 GB here — might’ve immediately “noped” out of the whole concept of containers. The first thing to realize is that when it comes to actually serving simple containers like the ones shown above is that Docker Engine suffices, at about a quarter of the size.

Yet on a small server — say, a $4/month 10 GiB Digital Ocean droplet — that’s still a big chunk of your storage budget. It takes 100:1 overhead just to run a 4 MiB Fossil server container? Once again, I wouldn’t blame you if you noped right on out of here, but if you will be patient, you will find that there are ways to run Fossil inside a container even on entry-level cloud VPSes. These are well-suited to running Fossil; you don’t have to resort to raw Fossil service to succeed, leaving the benefits of containerization to those with bigger budgets.

For the sake of simple examples in this section, we’ll assume you’re integrating Fossil into a larger web site, such as with our Debian + nginx + TLS plan. This is why all of the examples below create the container with this option:


The assumption is that there’s a reverse proxy running somewhere that redirects public web hits to localhost port 9999, which in turn goes to port 8080 inside the container. This use of Docker/Podman port publishing effectively replaces the use of the “fossil server --localhost” option.

For the nginx case, you need to add --scgi to these commands, and you might also need to specify --baseurl.

Containers are a fine addition to such a scheme as they isolate the Fossil sections of the site from the rest of the back-end resources, thus greatly reducing the chance that they’ll ever be used to break into the host as a whole.

(If you wanted to be double-safe, you could put the web server into another container, restricting it to reading from the static web site directory and connecting across localhost to back-end dynamic content servers such as Fossil. That’s way outside the scope of this document, but you can find ready advice for that elsewhere. Seeing how we do this with Fossil should help you bridge the gap in extending this idea to the rest of your site.)

6.1 Stripping Docker Engine Down

The core of Docker Engine is its containerd daemon and the runc container runner. Add to this the out-of-core CLI program nerdctl and you have enough of the engine to run Fossil containers. The big things you’re missing are:

In exchange, you get a runtime that’s about half the size of Docker Engine. The commands are essentially the same as above, but you say “nerdctl” instead of “docker”. You might alias one to the other, because you’re still going to be using Docker to build and ship your container images.

6.2 Podman

A lighter-weight alternative to either of the prior options that doesn’t give up the image builder is Podman. Initially created by Red Hat and thus popular on that family of OSes, it will run on any flavor of Linux. It can even be made to run on macOS via Homebrew or on Windows via WSL2.

On Ubuntu 22.04, the installation size is about 38 MiB, roughly a tenth the size of Docker Engine.

Although Podman bills itself as a drop-in replacement for the docker command and everything that sits behind it, some of the tool’s design decisions affect how our Fossil containers run, as compared to using Docker. The most important of these is that, by default, Podman wants to run your container “rootless,” meaning that it runs as a regular user. This is generally better for security, but we dealt with that risk differently above already. Since neither choice is unassailably correct in all conditions, we’ll document both options here.

6.2.1 Fossil in a Rootless Podman Container

If you build the stock Fossil container under podman, it will fail at two key steps:

  1. The mknod calls in the second stage, which create the /jail/dev nodes. For a rootless container, we want it to use the “real” /dev tree mounted into the container’s root filesystem instead.

  2. Anything that depends on the /jail directory and the fact that it becomes the file system’s root once the Fossil server is up and running.

The changes to fix this aren’t complicated. Simply apply that patch to our stock Dockerfile and rebuild:

  $ patch -p0 < containers/Dockerfile-nojail.patch
  $ podman build -t fossil:nojail .
  $ podman create \
    --name fossil-nojail \
    --publish \
    --volume ~/museum:/museum \

Do realize that by doing this, if an attacker ever managed to get shell access on your container, they’d have a BusyBox installation to play around in. That shouldn’t be enough to let them break out of the container entirely, but they’ll have powerful tools like wget, and they’ll be connected to the network the container runs on. Once the bad guy is inside the house, he doesn’t necessarily have to go after the residents directly to cause problems for them.

6.2.2 Fossil in a Rootful Podman Container

Simple Method

Fortunately, it’s easy enough to have it both ways. Simply run your podman commands as root:

  $ sudo podman build -t fossil --cap-add MKNOD .
  $ sudo podman create \
    --name fossil \
    --cap-drop CHOWN \
    --cap-drop FSETID \
    --cap-drop KILL \
    --cap-drop NET_BIND_SERVICE \
    --cap-drop SETFCAP \
    --cap-drop SETPCAP \
    --publish \
  $ sudo podman start fossil

It’s obvious why we have to start the container as root, but why create and build it as root, too? Isn’t that a regression from the modern practice of doing as much as possible with a normal user?

We have to do the build under sudo in part because we’re doing rootly things with the file system image layers we’re building up. Just because it’s done inside a container runtime’s build environment doesn’t mean we can get away without root privileges to do things like create the /jail/dev/null node.

The other reason we need “sudo podman build” is because it puts the result into root’s Podman image registry, where the next steps look for it.

That in turn explains why we need “sudo podman create:” because it’s creating a container based on an image that was created by root. If you ran that step without sudo, it wouldn’t be able to find the image.

If Docker is looking better and better to you as a result of all this, realize that it’s doing the same thing. It just hides it better by creating the docker group, so that when your user gets added to that group, you get silent root privilege escalation on your build machine. This is why Podman defaults to rootless containers. If you can get away with it, it’s a better way to work. We would not be recommending running podman under sudo if it didn’t buy us something we wanted badly.

Notice that we had to add the ability to run mknod(8) during the build. Podman sensibly denies this by default, which lets us leave off the corresponding --cap-drop option. Podman also denies CAP_NET_RAW and CAP_AUDIT_WRITE by default, which we don’t need, so we’ve simply removed them from the --cap-drop list relative to the commands for Docker above.

Building Under Docker, Running Under Podman

If you have a remote host where the Fossil instance needs to run, it’s possible to get around this need to build the image as root on the remote system. You still have to build as root on the local system, but as I said above, Docker already does this. What we’re doing is shifting the risk of running as root from the public host to the local one.

Once you have the image built on the local machine, create a “fossil” repository on your container repository of choice such as Docker Hub, then say:

  $ docker login
  $ docker tag fossil:latest mydockername/fossil:latest
  $ docker image push mydockername/fossil:latest

That will push the image up to your account, so that you can then switch to the remote machine and say:

  $ sudo podman create \
    --any-options-you-like \

This round-trip through the public image registry has another side benefit: your local system might be a lot faster than your remote one, as when the remote is a small VPS. Even with the overhead of schlepping container images across the Internet, it can be a net win in terms of build time.

6.3 systemd-container

If even the Podman stack is too big for you, the next-best option I’m aware of is the systemd-container infrastructure on modern Linuxes, available since version 239 or so. Its runtime tooling requires only about 1.4 MiB of disk space:

  $ sudo apt install systemd-container btrfs-tools

That command assumes the primary test environment for this guide, Ubuntu 22.04 LTS with systemd 249. For best results, /var/lib/machines should be a btrfs volume, because $REASONS. For CentOS Stream 9 and other Red Hattish systems, you will have to make several adjustments, which we’ve collected below to keep these examples clear.

We’ll assume your Fossil repository stores something called “myproject” within ~/museum/myproject/repo.fossil, named according to the reasons given above. We’ll make consistent use of this naming scheme in the examples below so that you will be able to replace the “myproject” element of the various file and path names.

If you use the stock Dockerfile to generate your base image, nspawn won’t recognize it as containing an OS unless you change the “FROM scratch AS os” line at the top of the second stage to something like this:

  FROM AS os

Using that as a base image provides all the files nspawn checks for to determine whether the container is sufficiently close to a Linux VM for the following step to proceed:

  $ make container
  $ docker container export $(make container-version) |
    machinectl import-tar - myproject

Next, create /etc/systemd/nspawn/myproject.nspawn:

Parameters=bin/fossil server                \
    --baseurl \
    --chroot /jail                          \
    --create                                \
    --jsmode bundled                        \
    --localhost                             \
    --port 9000                             \
    --scgi                                  \
    --user admin                            \
DropCapability=          \
    CAP_AUDIT_WRITE      \
    CAP_CHOWN            \
    CAP_FSETID           \
    CAP_KILL             \
    CAP_MKNOD            \
    CAP_NET_RAW          \
    CAP_SETFCAP          \



If you recognize most of that from the Dockerfile discussion above, congratulations, you’ve been paying attention. The rest should also be clear from context.

Some of this is expected to vary:

That being done, we also need a generic systemd unit file called /etc/systemd/system/fossil@.service, containing:

Description=Fossil %i Repo Service
Wants=modprobe@tun.service modprobe@loop.service systemd-resolved.service modprobe@tun.service modprobe@loop.service

ExecStart=systemd-nspawn --settings=override --read-only --machine=%i bin/fossil


You shouldn’t have to change any of this because we’ve given the --setting=override flag, meaning any setting in the nspawn file overrides the setting passed to systemd-nspawn. This arrangement not only keeps the unit file simple, it allows multiple services to share the base configuration, varying on a per-repo level through adjustments to their individual *.nspawn files.

You may then start the service in the normal way:

  $ sudo systemctl enable fossil@myproject
  $ sudo systemctl start  fossil@myproject

You should then find it running on localhost port 9000 per the nspawn configuration file above, suitable for proxying Fossil out to the public using nginx via SCGI. If you aren’t using a front-end proxy and want Fossil exposed to the world via HTTPS, you might say this instead in the *.nspawn file:

Parameters=bin/fossil server \
    --cert /path/to/cert.pem \
    --chroot /jail           \
    --create                 \
    --jsmode bundled         \
    --port 443               \
    --user admin             \

You would also need to un-drop the CAP_NET_BIND_SERVICE capability to allow Fossil to bind to this low-numbered port.

We use systemd’s template file feature to allow multiple Fossil servers running on a single machine, each on a different TCP port, as when proxying them out as subdirectories of a larger site. To add another project, you must first clone the base “machine” layer:

  $ sudo machinectl clone myproject otherthing

That will not only create a clone of /var/lib/machines/myproject as ../otherthing, it will create a matching otherthing.nspawn file for you as a copy of the first one. Adjust its contents to suit, then enable and start it as above.

6.3.1 Getting It Working on a RHEL Clone

The biggest difference between doing this on OSes like CentOS versus Ubuntu is that RHEL (thus also its clones) doesn’t ship btrfs in its kernel, thus ships with no package repositories containing mkfs.btrfs, which machinectl depends on for achieving its various purposes.

Fortunately, there are workarounds.

First, the apt install command above becomes:

  $ sudo dnf install systemd-container

Second, you have to hack around the lack of machinectl import-tar:

  $ rootfs=/var/lib/machines/fossil
  $ sudo mkdir -p $rootfs
  $ docker container export fossil | sudo tar -xf -C $rootfs -

The parent directory path in the rootfs variable is important, because although we aren’t able to use machinectl on such systems, the systemd-nspawn developers assume you’re using them together; when you give --machine, it assumes the machinectl directory scheme. You could instead use --directory, allowing you to store the rootfs wherever you like, but why make things difficult? It’s a perfectly sensible default, consistent with the LHS rules.

The final element — the machine name — can be anything you like so long as it matches the nspawn file’s base name.

Finally, since you can’t use machinectl clone, you have to make a wasteful copy of /var/lib/machines/myproject when standing up multiple Fossil repo services on a single machine. (This is one of the reasons machinectl depends on btrfs: cheap copy-on-write subvolumes.) Because we give the --read-only flag, you can simply cp -r one machine to a new name rather than go through the export-and-import dance you used to create the first one.

6.3.2 What Am I Missing Out On?

For all the runtime size savings in this method, you may be wondering what you’re missing out on relative to Podman, which takes up roughly 27× more disk space. Short answer: lots. Long answer:

  1. Build system. You’ll have to build and test your containers some other way. This method is only suitable for running them once they’re built.

  2. Orchestration. All of the higher-level things like “compose” files, Docker Swarm mode, and Kubernetes are unavailable to you at this level. You can run multiple instances of Fossil, but on a single machine only and with a static configuration.

  3. Image layer sharing. When you update an image using one of the above methods, Docker and Podman are smart enough to copy only changed layers. Furthermore, when you base multiple containers on a single image, they don’t make copies of the base layers; they can share them, because base layers are immutable, thus cannot cross-contaminate.

    Because we use systemd-nspawn --read-only, we get some of this benefit, particularly when using machinectl with /var/lib/machines as a btrfs volume. Even so, the disk space and network I/O optimizations go deeper in the Docker and Podman worlds.

  4. Tooling. Hand-creating and modifying those systemd files sucks compared to “podman container create ...” This is but one of many affordances you will find in the runtimes aimed at daily-use devops warriors.

  5. Network virtualization. In the scheme above, we turn off the systemd private networking support because in its default mode, it wants to hide containerized services entirely. While there are ways to expose Fossil’s single network service port under that scheme, it adds a lot of administration complexity. In the big-boy container runtimes, docker create --publish fixes all this up in a single option, whereas systemd-nspawn --port does approximately none of that despite the command’s superficial similarity.

    From a purely functional point of view, this isn’t a huge problem if you consider the inbound service direction only, being external connections to the Fossil service we’re providing. Since we do want this Fossil service to be exposed — else why are we running it? — we get all the control we need via fossil server --localhost and similar options.

    The complexity of the systemd networking infrastructure’s interactions with containers make more sense when you consider the outbound path. Consider what happens if you enable Fossil’s optional TH1 docs feature plus its Tcl evaluation feature. That would enable anyone with the rights to commit to your repository the ability to make arbitrary network connections on the Fossil host. Then, let us say you have a client-server DBMS server on that same host, bound to localhost for private use by other services on the machine. Now that DBMS is open to access by a rogue Fossil committer because the host’s loopback interface is mapped directly into the container’s network namespace.

    Proper network virtualization would protect you in this instance.

This author expects that the set of considerations is broader than presented here, but that it suffices to make our case as it is: if you can afford the space of Podman or Docker, we strongly recommend using either of them over the much lower-level systemd-container infrastructure. You’re getting a considerable amount of value for the higher runtime cost; it isn’t pointless overhead.

(Incidentally, these are essentially the same reasons why we no longer talk about the crun tool underpinning Podman in this document. It’s even more limited than nspawn, making it even more difficult to administer while providing no runtime size advantage. The runc tool underpinning Docker is even worse on this score, being scarcely easier to use than crun while having a much larger footprint.)

6.3.3 Violated Assumptions

The systemd-container infrastructure has a bunch of hard-coded assumptions baked into it. We papered over these problems above, but if you’re using these tools for other purposes on the machine you’re serving Fossil from, you may need to know which assumptions our container violates and the resulting consequences.

Some of it we discussed above already, but there’s one big class of problems we haven’t covered yet. It stems from the fact that our stock container starts a single static executable inside a barebones container rather than “boot” an OS image. That causes a bunch of commands to fail:

If these are problems for you, you may wish to build a fatter container using debootstrap or similar. (External tutorial.)