Getting started

This gets you from a clone of the repo to a built image. It uses the shipped turing-rk1/forky recipe as the running example; other boards build the same way with their own recipe name. Flashing and board-specific notes live on each board’s page — for the RK1, see Turing RK1.

Which track are you on? This is the shipped-recipe track — doctor then build, for a recipe that already ships a committed lock (like turing-rk1/forky). To change what a shipped recipe builds, continue with Adapting a shipped recipe. Bringing up a new board, or authoring a patch, is the longer bring-up track: see Adding a board and Adding a patch.

The build is rootless: it uses no sudo and no loop devices. You only need root to install host packages and (on some hosts) to enable unprivileged user namespaces once.

What you need

• A Linux host, x86_64 or arm64. An x86_64 desktop building the arm64 image is the common case and fully supported — it cross-builds under qemu-user. Debian and Ubuntu are the primary targets; Fedora and Arch work too (doctor knows their package names). macOS can run the read-only commands but cannot build.

• A recent stable Rust toolchain, installed via rustup.

• boot2deb on your PATH. From a clone of this repo:

  cd boot2deb
  cargo install --path crates/cli    # the crate is boot2deb-cli; the binary is boot2deb
  

  Every command on this site is written as boot2deb …, which is also how the tool writes its own hints — so anything it suggests can be pasted back. Developing from a checkout without installing? Prefix each command with cargo run -p boot2deb-cli --. Either way, run it from inside boot2deb/ (or pass --root <dir>), since the config root defaults to the current directory.

• Disk and time. A cold build bootstraps a Debian rootfs, and — for a board that needs one — compiles a kernel and a bootloader. Budget a few GB of scratch space and tens of minutes the first time; later builds reuse cached trees. A board that compiles nothing (the C201) is much cheaper: it is a rootfs bootstrap and an image assembly.

Let doctor find what’s missing

Rather than hand-installing a package list, run doctor. It probes for every tool the build needs and, for anything absent, prints the exact install command for your distro — so you never guess a package name. doctor itself needs nothing but Rust, so it is the first thing to run after cloning:

boot2deb doctor turing-rk1/forky

Run it bare — boot2deb doctor — and it checks the requirements every board shares (user namespaces and the vendored apt trust anchors) without needing a recipe chosen yet.

It reports your host arch, the two cross answers, and one line per requirement:

host arch : x86_64
target    : turing-rk1/forky (arch arm64)
toolchain : cross — the build root carries a toolchain emitting arm64
execution : emulated — needs qemu-user binfmt for arm64 maintainer scripts and sandbox compiles

  ok      git                          /usr/bin/git
  ok      unprivileged user namespaces unshare --map-root-user --map-auto works
  MISSING qemu-aarch64-static          run target binaries under binfmt — sudo apt install qemu-user-static
  ...

result    : all required host tools present

Run the install lines it reports, then re-run doctor until it prints all required host tools present. Because the list is generated from the build’s own requirements, it is always current — this page does not repeat the package names, so there is nothing here to drift out of date.

The list is short, and that is the design rather than an omission. Every compiler, packaging tool and build dependency a build runs is a package of a provisioned Debian root, resolved from your build’s own mirror list and sha256-pinned in that root’s manifest — so it is an input your lock names, not a fact about your machine. There is no host gcc, no make, no dpkg, no fakeroot in the list, because none of them is what compiles or archives anything. What remains is the handful of things no root can carry:

doctor asks only for what your recipe will actually invoke, so the table above is a superset. doctor turing-rk1/media-accel-forky on an x86_64 host wants every row; doctor asus-c201/forky wants no git and no overlay at all, because that board installs Debian’s kernel and boots its own firmware and so compiles nothing. That is deliberate: a requirement you do not need is somewhere a requirement you do need can hide.

Note that the last row’s two conditions are both real, and each one alone would over-ask. Emulation is about running target binaries, which only the image path does — the rootfs runs the target’s maintainer scripts, and the media-accel packages compile in a target-arch sandbox. A bootloader-only build like rk3576-generic/loader compiles in a host-arch root and executes nothing foreign, so it needs no qemu even though it builds for arm64. And an arm64 host runs armhf binaries directly (its kernel is built with CONFIG_COMPAT=y), so building the armhf C201 image there needs no qemu-arm either. Any x86_64 host building an arm64 or armhf image needs both.

The target-arch sandbox is not a cross-only concern. Packages like ffmpeg-rk and librga2 are built inside a userland bootstrapped for the target suite, never on your host, even when your host arch already matches the target. Their runtime Depends are derived from the libraries present at build time, so building them against your host’s libraries would stamp your host’s package names and versions into a .deb bound for a Debian forky image. That sandbox runs entirely in-process through unprivileged user namespaces — it needs no external sandbox tool — but it does run on every host, same-arch included, so those namespaces are a hard requirement even when nothing is cross.

The roots a build provisions

A build stands up as many as four Debian roots, each for one job, each bootstrapped and entered in-process through unprivileged user namespaces. They live in your work dir and boot2deb clean --sandbox reclaims them.

The two host-arch roots are host-arch on purpose. Neither compiling a freestanding kernel nor archiving a staged tree needs to link against the target’s libraries, so both run natively — which is what keeps a multi-minute kernel build and a hundred-megabyte xz off qemu-user entirely. The target-arch sandbox is the one that genuinely cannot be: dpkg-shlibdeps derives each media-accel .deb’s runtime Depends from the libraries present at build time.

Each root is provisioned for your build’s suite — the image’s, or for a bootloader-only build the board’s declared default — and each publishes a sha256-pinned manifest of its own packages beside your image, so “what compiled this” and “what archived this” are answered by name and version rather than by a --version line off your PATH. See Reproducibility.

There is no fakeroot anywhere in boot2deb, in any root or on your host. Every root maps you to uid 0, and uid 0 is what a Debian packaging tool actually wants: your staged tree is already root:root where dpkg-deb archives it, and dpkg-buildpackage picks no gain-root command at all. Nothing is faked because nothing needs to be.

The user-namespace check (common blocker on Ubuntu 24.04)

The rootless rootfs bootstrap, the sandbox, and the ext4 image staging all need unprivileged user namespaces with a subuid/subgid range for your user, which some hosts disable by default. doctor tests this by actually creating them (with the subuid mapping), and if it fails it prints the fix for your host. The usual cases:

• Ubuntu 24.04+ ships an AppArmor restriction on by default:

  sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0
  

• Debian with namespaces disabled:

  sudo sysctl -w kernel.unprivileged_userns_clone=1
  

• Either way, kernel.max_user_namespaces (or user.max_user_namespaces) must be at least 2. Every launch holds two nested user namespaces — the sandbox’s own, and the one its command enters so the kernel locks the sandbox’s mount flags — and each is charged against that ceiling. A host set to 1 creates one namespace perfectly well and fails every build at launch, so doctor probes both.
• Your user needs a subuid/subgid range (usually present by default):

  sudo usermod --add-subuids 100000-165535 --add-subgids 100000-165535 $USER
  

sysctl -w lasts until reboot; drop the same line in /etc/sysctl.d/ to make it persist.

On a build that assembles an image for an architecture your host cannot execute, doctor also checks that the qemu-<arch> binfmt handler is registered and enabled with the F (fix-binary) flag — the rootfs bootstrap relies on it. Installing qemu-user-static (with binfmt-support / systemd’s binfmt) normally registers this; doctor warns if the flag is missing.

The overlay check

Every stage that compiles gets a build root: the shared base plus that stage’s own build-dependencies, layered on with an unprivileged overlay and discarded afterwards. doctor probes whether your host can establish one, and prints the directory it probed.

That is why the overlay is a requirement of compiling rather than of every build. A board that installs Debian’s kernel and boots its own firmware layers nothing and needs only user namespaces — and so does a rebuild whose artifacts all restore from the cache, which never stands a build root up at all.

The probe is pointed at the work dir’s filesystem, not at /tmp, because that is where the overlay’s upper layer goes and the two can answer differently. An unprivileged overlay records its whiteouts in user.* extended attributes: every on-disk filesystem (ext4, xfs, btrfs) holds them, but tmpfs only gained them in Linux 6.6. So a host with a tmpfs /tmp on an older kernel would fail a /tmp probe and still build fine with its work dir on disk. If you build with --work-dir, pass the same path to doctor so it asks about the filesystem you will actually use.

The mount itself needs Linux 5.11 or later, which is where overlay in a user namespace arrived.

Build

With doctor green:

boot2deb build turing-rk1/forky

This resolves the recipe’s committed lockfile and runs the pipeline end to end. For turing-rk1/forky that is: compile the kernel and u-boot, bootstrap the Debian rootfs, and assemble a bootable disk image. A recipe runs only the stages it has — turing-rk1/media-accel-forky adds the Rockchip media userspace and ffmpeg on top of those, while build asus-c201/forky compiles nothing at all, so it is a rootfs bootstrap and an image assembly and nothing else.

The build reads only the lock, so it consults no network for its pins and is reproducible from what is committed. A patch series, where a recipe has one, is fetched automatically at its pinned commit if the config root’s sibling ../patches checkout is not already present — you do not need to clone it separately. That holds for both patch axes: the repo comes from the lock’s own pin, so a u-boot-only recipe such as rk3576-generic/loader fetches its series the same way a kernel recipe does.

The rootfs bootstrap is content-cached, so a rebuild whose solved package set is unchanged skips the multi-minute bootstrap. To force a clean rootfs, add --refresh-rootfs. To build a single stage, pass --stage (kernel, dtb, kmod, uboot, userspace, ffmpeg, rootfs, image) — see the CLI reference.

What you get

Artifacts land under the recipe’s work dir, build/turing-rk1/forky/artifacts/:

• turing-rk1-forky.img.xz — the compressed bootable image.
• turing-rk1-forky.provenance.toml — exactly what went into the image: the resolved pins, package count, toolchain identity, the archives the rootfs resolved against, and the first-boot credential.
• turing-rk1-forky.plan — the package set as an installable document: every version and sha256, plus the state of each archive they came from. boot2deb reproduce replays it to rebuild this exact userland later. See Reproducibility.

Every artifact is named for the whole build point — device and recipe (turing-rk1/forky → turing-rk1-forky) — so several recipes can share one --out-dir, and an image copied to a flashing host still says what it is.

The build prints the exact paths on its final lines, including the credential:

compressed    : .../build/turing-rk1/forky/artifacts/turing-rk1-forky.img.xz
first-boot pw : <generated>  (user debian, expired — change at first login)
provenance    : .../build/turing-rk1/forky/artifacts/turing-rk1-forky.provenance.toml

Note the first-boot password down. It is unique per image, shown once here, and stored only in the provenance file — it exists nowhere on the running system in recoverable form. It is also expired, so the first login has to replace it.

If you would rather not transcribe a password at a console, authorize your SSH key in the recipe instead and ssh debian@<board> works on the first boot. That, and the sudo policy the image ships with, are in The account, sudo, and SSH keys.

Next: flash the image. That step is board-specific — for the RK1, see Turing RK1.