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Reproducing a repro image locally

The repro image provides a bit for bit reproducible build of the base image.

Note that, to ensure reproducibility, the pacman keys are stripped from this image, so you're expected to run pacman-key --init && pacman-key --populate archlinux before being able to update the system and install packages via pacman when using this image.

To reproduce the repro image locally, follow the below instructions.

Disclaimer

Reproducible builds expect the same build environment across builds.

While it should be fine in most cases, this means we cannot guarantee that you will always be able to successfully reproduce a specific image locally over time.

Technically speaking, the older the image you're trying to reproduce is, the more chance there is to have more or less significant differences between your build environment and the one used to build the original image (for instance in terms of packages versions).
Such differences can affect the build (and the resulting artifacts). Please note that failing to reproduce an image locally does not necessarily mean that it isn't reproducible per se, but can just be the result of significant enough differences between your build environment and the one used to build the original image.

You can avoid (or mitigate) eventual issues due to such differences by restoring all packages of your build environment to the build date of the original image (see the related instructions from the Arch Wiki).

Dependencies

Install the following Arch Linux packages:

  • make
  • devtools
  • git
  • podman
  • fakechroot
  • fakeroot
  • diffoscope (to optionally check the reproducibility of the rootFS)
  • diffoci

Prepare the build environment

Prepare the build environment by setting the following environment variables:

  • BUILD_VERSION: The build version of the repro image you want to reproduce. For instance, if you're aiming to reproduce the repro-20260331.0.508794 image:
export BUILD_VERSION="20260331.0.508794"
  • ARCHIVE_SNAPSHOT: The date of the Arch Linux repository archive snaphot to build the image against. This is based on the date included in the image's BUILD_VERSION:
export ARCHIVE_SNAPSHOT=$(date -d "${BUILD_VERSION%%.*} -1 day" +"%Y/%m/%d")
  • SOURCE_DATE_EPOCH: The value to normalize timestamps with during the build. This is based on the date included in the image's BUILD_VERSION:
export SOURCE_DATE_EPOCH=$(date -u -d "${BUILD_VERSION%%.*} 00:00:00" +"%s")

Then clone the archlinux-docker repository and move into it:

git clone https://gitlab.archlinux.org/archlinux/archlinux-docker.git
cd archlinux-docker

Note that all the following instructions assume that you are at the root of the archlinux-docker repository cloned above.

Build the rootFS and generate the Dockerfile

Build the rootFS with the required parameters:

make \
    ARCHIVE_SNAPSHOT="$ARCHIVE_SNAPSHOT" \
    SOURCE_DATE_EPOCH="$SOURCE_DATE_EPOCH" \
    $PWD/output/Dockerfile.repro

The following resulting artifacts will be located in $PWD/output:

  • repro.tar.zst (the rootFS)
  • repro.tar.zst.SHA256 (sha256 hash of the rootFS)
  • Dockerfile.repro (the generated Dockerfile)

Optional - Check the rootFS reproducibility

At that point, if the artifacts built for the image you're aiming to reproduce are still available for download from the rootfs stage of the corresponding archlinux-docker pipeline , you can optionally compare the content of the repro.tar.zst.SHA256 file from the pipeline to the one generated during your local build (which should be the same, indicating that the rootFS has been successfully reproduced).

Additionally, you can check differences between the repro.tar.zst tarball from the pipeline and the one built during your local build with diffoscope (where /tmp/repro.tar.zst is the rootFS tarball downloaded from the pipeline and $PWD/output/repro.tar.zst is the rootFS tarball built during your local build in the following example):

diffoscope /tmp/repro.tar.zst $PWD/output/repro.tar.zst

This should return no difference, acting as additional indicator that the rootFS has been successfully reproduced.

Build the image

You can now (re)build the image against the rootFS and the Dockerfile generated in the previous step.
To do so, build the image with the required parameters:

podman build \
    --no-cache \
    --source-date-epoch=$SOURCE_DATE_EPOCH \
    --rewrite-timestamp \
    -f "$PWD/output/Dockerfile.repro" \
    -t "archlinux:repro-$BUILD_VERSION" \
    "$PWD/output"

The built image will be accessible in your local podman container storage under the name: localhost/archlinux:repro-$BUILD_VERSION.

Check the image reproducibility

Pull the image you're aiming to reproduce from Docker Hub:

podman pull docker.io/archlinux/archlinux:repro-$BUILD_VERSION

Compare the digest of the image pulled from Docker Hub to the digest of the image you built locally:

podman inspect --format '{{.Digest}}' docker.io/archlinux/archlinux:repro-$BUILD_VERSION
podman inspect --format '{{.Digest}}' localhost/archlinux:repro-$BUILD_VERSION

Both digests should be identical, indicating that the image has been successfully reproduced.

Additionally, you can check difference between the image pulled from Docker Hub and the image you built locally with diffoci:

diffoci diff --ignore-image-name --verbose podman://docker.io/archlinux/archlinux:repro-$BUILD_VERSION podman://localhost/archlinux:repro-$BUILD_VERSION

This should show no difference, acting as additional indicator that the image has been successfully reproduced (see the following section about the --ignore-image-name flag requirement).

Note about the necessity of the --ignore-image-name flag with diffoci

Docker / Podman does not allow to have two images with the same name & tag combination stored locally, making it impossible to compare two images with the same name with diffoci. To work around this limitation, one of the two image has to be named differently, whether by setting a different name / tag combination at build time (as done in this guide) or by renaming it post-build with e.g. podman tag.

However, the image name & tag combination is automatically reported (and updated in the case of a renaming) in the image annotations and it's not possible to fully overwrite it during build or update it post-build in a straightforward way.
This unavoidably introduces non-deterministic data in the image name annotations that diffoci will systematically report by default.
See for instance the following diffoci output reporting a difference in the image name annotation:

Event: "DescriptorMismatch" (field "Annotations")
  map[string]string{
        "io.containerd.image.name": strings.Join({
-               "docker.io/archlinux/archlinux:repro-20260331.0.508794",
+               "localhost/archlinux:repro-20260331.0.508794",
        }, ""),

Given that it's currently not possible to have two images with the same name & tag combination stored locally and that it's also not possible to "normalize" the related annotations metadata during (or after) the build, we are currently forced to ignore those with the --ignore-image-name flag to workaround this technical constraint.

Regardless, we can attest that:

  • This limitation is specific to metadata handling in container tooling and does not affect the actual filesystem contents or runtime behavior of the image.
  • The reported difference in the image name annotation when running diffoci without the --ignore-image-name flag is (or is supposed to be, at least) the only difference being reported when comparing the two images.
  • The image name annotation metadata are not part of the hashed object when generating the image digest, meaning that this difference does not prevent digest equality between the two images (allowing us to claim bit for bit reproducibility regardless).