# 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.

[Docker]: https://www.docker.com/
[OCI]:    https://opencontainers.org/


## 1. Quick Start

Fossil ships a `Dockerfile` at the top of its source tree,
[here][DF], 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.

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.

[DF]: /file/Dockerfile


## 2. <a id="storage"></a>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. 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.


### <a id="repo-inside"></a> 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:/museum/repo.fossil
  $ docker start fossil
  $ docker exec fossil chown -R 499 /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](#args) 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 <a id="bind-mount"></a>Storing the Repo Outside the Container

The simple storage method above has a problem: 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:/museum \
    fossil
```

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
[volume](https://docs.docker.com/storage/volumes/), 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 <a id="wal-mode"></a>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:/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][wal]
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][dbcorr].

If we map a whole directory, both sides see the same set of WAL files.
[Testing](https://tangentsoft.com/sqlite/dir/walbanger?ci=trunk)
gives us a reasonable level of confidence that using WAL across a
container boundary is safe when used in this manner.

[dbcorr]: https://www.sqlite.org/howtocorrupt.html#_deleting_a_hot_journal
[wal]:    https://www.sqlite.org/wal.html


## 3. <a id="security"></a>Security

### 3.1 <a id="chroot"></a>Why Not Chroot?

Prior to 2023.03.26, the stock Fossil container relied on [the chroot
jail feature](./chroot.md) to wall away the shell and other tools
provided by [BusyBox]. It included that as a bare-bones operating system
inside the container on the off chance that someone might need it for
debugging, but the thing is, Fossil is self-contained, needing none of
that power in the main-line use cases.

Our weak “you might need it” justification collapsed when we realized
you could restore this basic shell environment with a one-line change to
the `Dockerfile`, as shown [below](#run).

[BusyBox]: https://www.busybox.net/BusyBox.html


### 3.2 <a id="caps"></a>Dropping Unnecessary Capabilities

The example commands above create the container with [a default set of
Linux kernel capabilities][defcap]. 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.

Specifically:

*   **`AUDIT_WRITE`**: Fossil doesn’t write to the kernel’s auditing
    log, and we can’t see any reason you’d want to be able to do that as
    an administrator shelled into the container, either. Auditing is
    something done on the host, not from inside each individual
    container.

*   **`CHOWN`**: The Fossil server never even calls `chown(2)`, and our
    image build process sets up all file ownership properly, to the
    extent that this is possible under the limitations of our
    automation.

    Curiously, stripping this capability doesn’t affect your ability to
    run commands like “`chown -R fossil:fossil /museum`” when
    you’re using bind mounts or external volumes — as we recommend
    [above](#bind-mount) — because it’s the host OS’s kernel
    capabilities that affect the underlying `chown(2)` call in that
    case, not those of the container.

    If for some reason you did have to change file ownership of
    in-container files, it’s best to do that by changing the
    `Dockerfile` to suit, then rebuilding the container, since that
    bakes the need for the change into your reproducible build process.
    If you had to do it without rebuilding the container, [there’s a
    workaround][capchg] for the fact that capabilities are a create-time
    change, baked semi-indelibly into the container configuration.

*    **`FSETID`**: Fossil doesn’t use the SUID and SGID bits itself, and
    our build process doesn’t set those flags on any of the files.
    Although the second fact means we can’t see any harm from leaving
    this enabled, we also can’t see any good reason to allow it, so we
    strip it.

*    **`KILL`**: The only place Fossil calls `kill(2)` is in the
    [backoffice], and then only for processes it created on earlier
    runs; it doesn’t need the ability to kill processes created by other
    users. You might wish for this ability as an administrator shelled
    into the container, but you can pass the “`docker exec --user`”
    option to run commands within your container as the legitimate owner
    of the process, removing the need for this capability.

*   **`MKNOD`**: As of 2023.03.26, the stock container uses the
    runtime’s default `/dev` node tree. Prior to this, we had to create
    `/dev/null` and `/dev/urandom` inside [the chroot jail](#chroot),
    but even then, these device nodes were created at build time and
    were never changed at run time, so we didn’t need this run-time
    capability even then.

*    **`NET_BIND_SERVICE`**: With containerized deployment, Fossil never
    needs the ability to bind the server to low-numbered TCP ports, not
    even if you’re running the server in production with TLS enabled and
    want the service bound to port 443. It’s perfectly fine to let the
    Fossil instance inside the container bind to its default port (8080)
    because you can rebind it on the host with the
    “`docker create --publish 443:8080`” option. It’s the container’s
    _host_ that needs this ability, not the container itself.

    (Even the container runtime might not need that capability if you’re
    [terminating TLS with a front-end proxy](./ssl.wiki#server). You’re
    more likely to say something like “`-p localhost:12345:8080`” and then
    configure the reverse proxy to translate external HTTPS calls into
    HTTP directed at this internal port 12345.)

*   **`NET_RAW`**: Fossil itself doesn’t use raw sockets, and while
    you could [swap out the run layer](#run) for something more
    functional that *does* make use of raw sockets, there’s little call
    for it. The best reason I can come up with is to be able to run
    utilities like `ping` and `traceroute`, but since we aren’t doing
    anything clever with the networking configuration, there’s no
    particularly compelling reason to run these from inside the
    container. If you need to ping something, do it on the host.

    If we did not take this hard-line stance, an attacker that broke
    into the container and gained root privileges might use raw sockets
    to do a wide array of bad things to any network the container is
    bound to.

*    **`SETFCAP, SETPCAP`**: There isn’t much call for file permission
    granularity beyond the classic Unix ones inside the container, so we
    drop root’s ability to change them.

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.

[backoffice]: ./backoffice.md
[defcap]:     https://docs.docker.com/engine/security/#linux-kernel-capabilities
[capchg]:     https://stackoverflow.com/a/45752205/142454



## 4. <a id="static"></a>Extracting a Static Binary

Our 2-stage build process uses Alpine Linux only as a build host. Once
we’ve got everything reduced to a single static Fossil binary,
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][lsl], 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:/bin/fossil .
  $ docker container rm fossil-static-tmp
```

The result is six or seven megs, depending on the CPU architecture you
build for. It’s built stripped.

[lsl]: https://stackoverflow.com/questions/3430400/linux-static-linking-is-dead


## 5. <a id="custom" name="args"></a>Customization Points

### <a id="pkg-vers"></a> 5.1 Fossil Version

The default version of Fossil fetched in the build is the version in the
checkout directory at the time you run it.  You could override it to get
a release build like so:

```
  $ 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 container builders cache downloaded files, hoping 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 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 the builder to
pull a fresh tarball.


### 5.2 <a id="uids"></a>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
separate 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 <a id="cengine"></a>Container Engine

Although the Fossil container build system defaults to Docker, we allow
for use of any OCI container system that implements the same interfaces.
We go into more details about this [below](#light), but
for now, it suffices to point out that you can switch to Podman while
using our `Makefile` convenience targets unchanged by saying:

```
    $ make CENGINE=podman container-run
```


### 5.4 <a id="config"></a>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](./th1.md#tclEval),
that brings us to [the next point](#run).


### 5.5 <a id="run"></a>Elaborating the Run Layer

If you want a basic shell environment for temporary debugging of the
running container, that’s easily added. Simply change this line in the
`Dockerfile`…

      FROM scratch AS run

…to this:

      FROM busybox AS run

Rebuild and redeploy to give your Fossil container a [BusyBox]-based
shell environment that you can get into via:

      $ docker exec -it -u fossil $(make container-version) sh

That command assumes you built it via “`make container`” and are
therefore using its versioning scheme.

You will likely want to remove the `PATH` override in the “RUN” stage
when doing this since it’s written for the case where everything is in
`/bin`, and that will no longer be the case with a more full-featured
“`run`” layer. As long as the parent layer’s `PATH` value contains
`/bin`, delegating to it is more likely the correct thing.

Another useful case to consider is that you’ve installed a [server
extension](./serverext.wiki) and you need an interpreter for that
script. The first option above won’t work except in the unlikely case that
it’s written for one of the bare-bones script interpreters that BusyBox
ships.(^[BusyBox]’s `/bin/sh` is based on the old 4.4BSD Lite Almquist
shell, implementing little more than what POSIX specified in 1989, plus
equally stripped-down versions of `awk` and `sed`.)

Let’s say the extension is written in Python. Because this is one of the
most popular programming languages in the world, we have many options
for achieving this. For instance, there is a whole class of
“[distroless]” images that will do this efficiently by changing
“`STAGE 2`” in the `Dockefile` to this:

```
    ## ---------------------------------------------------------------------
    ## STAGE 2: Pare that back to the bare essentials, plus Python.
    ## ---------------------------------------------------------------------
    FROM cgr.dev/chainguard/python:latest
    USER root
    ARG UID=499
    ENV PATH "/sbin:/usr/sbin:/bin:/usr/bin"
    COPY --from=builder /tmp/fossil /bin/
    COPY --from=builder /bin/busybox.static /bin/busybox
    RUN [ "/bin/busybox", "--install", "/bin" ]
    RUN set -x                                                              \
        && echo "fossil:x:${UID}:${UID}:User:/museum:/false" >> /etc/passwd \
        && echo "fossil:x:${UID}:fossil"                     >> /etc/group  \
        && install -d -m 700 -o fossil -g fossil log museum
```

You will also have to add `busybox-static` to the APK package list in
STAGE 1 for the `RUN` script at the end of that stage to work, since the
[Chainguard Python image][cgimgs] lacks a shell, on purpose. The need to
install root-level binaries is why we change `USER` temporarily here.

Build it and test that it works like so:

```
    $ make container-run &&
      docker exec -i $(make container-version) python --version 
    3.11.2
```

The compensation for the hassle of using Chainguard over something more
general purpose like changing the `run` layer to Alpine and then adding
a “`apk add python`” command to the `Dockerfile`
is huge: we no longer leave a package manager sitting around inside the
container, waiting for some malefactor to figure out how to abuse it.

Beware that there’s a limit to this über-jail’s ability to save you when
you go and provide a more capable runtime layer like this. The container
layer should stop an attacker from accessing any files out on the host
that you haven’t explicitly mounted into the container’s namespace, but
it can’t stop them from making outbound network connections or modifying
the repo DB inside the container.

[cgimgs]:     https://github.com/chainguard-images/images/tree/main/images
[distroless]: https://www.chainguard.dev/unchained/minimal-container-images-towards-a-more-secure-future
[MTA]:        https://en.wikipedia.org/wiki/Message_transfer_agent


### 5.6 <a id="alerts"></a>Email Alerts

The nature of our single static binary container precludes two of the
options for [sending email alerts](./alerts.md) from Fossil:

*   pipe to a command
*   SMTP relay host

There is no `/usr/sbin/sendmail` inside the container, and the container
cannot connect out to a TCP service on the host by default.

While it is possible to get around the first lack by [elaborating the
run layer](#run), to inject a full-blown Sendmail setup into the
container would go against the whole idea of containerization.
Forwarding an SMTP relay port into the container isn’t nearly as bad,
but it’s still bending the intent behind containers out of shape.

A far better option in this case is the “store emails in database”
method since the containerized Fossil binary knows perfectly well how to
write SQLite DB files without relying on any external code. Using the
paths in the configuration recommended above, the database path should
be set to something like `/museum/mail.db`. This, along with the use of
[bind mounts](#bind-mount) means you can have a process running outside
the container that passes the emails along to the host-side MTA.

The included [`email-sender.tcl`](/file/tools/email-sender.tcl) script
works reasonably well for this, though in my own usage, I had to make
two changes to it:

1.  The shebang line at the top has to be `#!/usr/bin/tclsh` on my server.
2.  I parameterized the `DBFILE` variable at the top thus:

        set DBFILE [lindex $argv 0]

I then wanted a way to start this Tcl script on startup and keep it
running, which made me reach for systemd. My server is set to allow user
services to run at boot(^”Desktop” class Linuxes tend to disable that by
default under the theory that you don’t want those services to run until
you’ve logged into the GUI as that user. If you find yourself running
into this, [enable linger
mode](https://www.freedesktop.org/software/systemd/man/loginctl.html).)
so I was able to create a unit file called
`~/.local/share/systemd/user/alert-sender@.service` with these contents:

```
    [Unit]
    Description=Fossil email alert sender for %I

    [Service]
    WorkingDirectory=/home/fossil/museum
    ExecStart=/home/fossil/bin/alert-sender %I/mail.db
    Restart=always
    RestartSec=3

    [Install]
    WantedBy=default.target
```

I was then able to enable email alert forwarding for select repositories
after configuring them per [the docs](./alerts.md) by saying:

```
    $ systemctl --user daemon-reload
    $ systemctl --user enable alert-sender@myproject
    $ systemctl --user start  alert-sender@myproject
```

Because this is a parameterized script and we’ve set our repository
paths predictably, you can do this for as many repositories as you need
to by passing their names after the “`@`” sign in the commands above.


## 6. <a id="light"></a>Lightweight Alternatives to Docker

Those afflicted with sticker shock at seeing the size of a [Docker
Desktop][DD] 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][DE] suffices, at about a
quarter of the size.

Yet on a small server — say, a $4/month ten gig Digital Ocean droplet —
that’s still a big chunk of your storage budget. It takes ~60:1 overhead
merely to run a 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][srv] 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][DNT] plan. This is why all of the examples below create
the container with this option:

```
  --publish 127.0.0.1:9999:8080
```

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 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.)

[DD]:  https://www.docker.com/products/docker-desktop/
[DE]:  https://docs.docker.com/engine/
[DNT]: ./server/debian/nginx.md
[srv]: ./server/


### 6.1 <a id="nerdctl" name="containerd"></a>Stripping Docker Engine Down

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

*   **BuildKit**: The container build engine, which doesn’t matter if
    you’re building elsewhere and shipping the images to the target.
    A good example is using a container registry as an
    intermediary between the build and deployment hosts.

*   **SwarmKit**: A powerful yet simple orchestrator for Docker that you
    probably aren’t using with Fossil anyway.

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.

[ctrd]:    https://containerd.io/
[nerdctl]: https://github.com/containerd/nerdctl
[runc]:    https://github.com/opencontainers/runc


### 6.2 <a id="podman"></a>Podman

A lighter-weight [rootless][rl] [drop-in replacement][whatis] 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][pmmac]
or [on Windows via WSL2][pmwin].

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

For our purposes here, the only thing that changes relative to the
examples at the top of this document are the initial command:

```
  $ podman build -t fossil .
  $ podman run --name fossil -p 9999:8080/tcp fossil
```

Your Linux package repo may have a `podman-docker` package which
provides a “`docker`” script that calls “`podman`” for you, eliminating
even the command name difference. With that installed, the `make`
commands above will work with Podman as-is.

The only difference that matters here is that Podman doesn’t have the
same [default Linux kernel capability set](#caps) as Docker, which
affects the `--cap-drop` flags recommended above to:

```
  $ 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 127.0.0.1:9999:8080 \
    localhost/fossil
  $ podman start fossil
```

[pmmac]:  https://podman.io/getting-started/installation.html#macos
[pmwin]:  https://github.com/containers/podman/blob/main/docs/tutorials/podman-for-windows.md
[Podman]: https://podman.io/
[rl]:     https://github.com/containers/podman/blob/main/docs/tutorials/rootless_tutorial.md
[whatis]: https://podman.io/whatis.html


### 6.3 <a id="nspawn"></a>`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`][mcfad]. For CentOS Stream 9 and other Red Hattish
systems, you will have to make several adjustments, which we’ve
collected [below](#nspawn-centos) 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](#repo-inside). 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`][DF] 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 gcr.io/distroless/static-debian11 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`:

----

```
[Exec]
WorkingDirectory=/
Parameters=bin/fossil server                \
    --baseurl https://example.com/myproject \
    --create                                \
    --jsmode bundled                        \
    --localhost                             \
    --port 9000                             \
    --scgi                                  \
    --user admin                            \
    museum/repo.fossil
DropCapability=          \
    CAP_AUDIT_WRITE      \
    CAP_CHOWN            \
    CAP_FSETID           \
    CAP_KILL             \
    CAP_MKNOD            \
    CAP_NET_BIND_SERVICE \
    CAP_NET_RAW          \
    CAP_SETFCAP          \
    CAP_SETPCAP
ProcessTwo=yes
LinkJournal=no
Timezone=no

[Files]
Bind=/home/fossil/museum/myproject:/museum

[Network]
VirtualEthernet=no
```

----

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:

*   The references to `example.com` and `myproject` are stand-ins for
    your actual web site and repository name.

*   The command given in the `Parameters` directive assumes you’re
    setting up [SCGI proxying via nginx][DNT], but with adjustment,
    it’ll work with the other repository service methods we’ve
    [documented][srv].

*   The path in the host-side part of the `Bind` value must point at the
    directory containing the `repo.fossil` file referenced in said
    command so that `/museum/repo.fossil` refers to your repo out
    on the host for the reasons given [above](#bind-mount).

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

----

```
[Unit]
Description=Fossil %i Repo Service
Wants=modprobe@tun.service modprobe@loop.service
After=network.target systemd-resolved.service modprobe@tun.service modprobe@loop.service

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

[Install]
WantedBy=multi-user.target
```

----

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 \
    --create                 \
    --jsmode bundled         \
    --port 443               \
    --user admin             \
    museum/repo.fossil
```

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 the `systemd` 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.

[mcfad]: https://www.freedesktop.org/software/systemd/man/machinectl.html#Files%20and%20Directories


### 6.3.1 <a id="nspawn-rhel"></a>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`][mctl] 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 &mdash; the machine name &mdash; 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.

[LHS]:  https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html
[mctl]: https://www.freedesktop.org/software/systemd/man/machinectl.html


### 6.3.2 <a id="nspawn-weaknesses"></a>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][ndcmp] 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.)

[ndcmp]:  https://wiki.archlinux.org/title/systemd-networkd#Usage_with_containers


### 6.3.3 <a id="nspawn-assumptions"></a>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 bare-bones container
rather than “boot” an OS image. That causes a bunch of commands to fail:

*   **`machinectl poweroff`** will fail because the container
    isn’t running dbus.

*   **`machinectl start`** will try to find an `/sbin/init`
    program in the rootfs, which we haven’t got.  We could
    rename `/bin/fossil` to `/sbin/init` and then hack
    the chroot scheme to match, but ick.  (This, incidentally,
    is why we set `ProcessTwo=yes` above even though Fossil is
    perfectly capable of running as PID 1, a fact we depend on
    in the other methods above.)

*   **`machinectl shell`** will fail because there is no login
    daemon running, which we purposefully avoided adding by
    creating a “`FROM scratch`” container. (If you need a
    shell, say: `sudo systemd-nspawn --machine=myproject /bin/sh`)

*   **`machinectl status`** won’t give you the container logs
    because we disabled the shared journal, which was in turn
    necessary because we don’t run `systemd` *inside* the
    container, just outside.

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

[medtut]: https://medium.com/@huljar/setting-up-containers-with-systemd-nspawn-b719cff0fb8d

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