Project Atomic is now sunset

The Atomic Host platform is now replaced by CoreOS. Users of Atomic Host are encouraged to join the CoreOS community on the Fedora CoreOS communication channels.

The documentation contained below and throughout this site has been retained for historical purposes, but can no longer be guaranteed to be accurate.

Compose Your Own Atomic Updates

Project Atomic hosts are built from standard RPM packages which have been composed into filesystem trees using rpm-ostree. This guide provides a method for customizing existing filesystem trees or creating new trees.


  • a machine running CentOS or Fedora for composing filesystem trees
  • a web server for hosting these tree repositories


  • The rpm-ostree program takes as input a manifest file that describes the target system, and commits the result to an OSTree repository.
  • This tree is made available via web server, for Atomic hosts to consume.

Example Dockerfile

The tree compose and hosting functions can both be performed in a container, if you choose. The simple Dockerfile below will suffice, or you may use a similarly-configured Fedora or CentOS machine:

FROM fedora:25

# install needed packages

RUN dnf install -y rpm-ostree git python; \
dnf clean all

# create working dir, clone fedora atomic definitions

RUN mkdir -p /srv; \
cd /srv; \
git clone; \

# create and initialize repo directory

mkdir -p /srv/repo && \
cd /srv/ && \
ostree --repo=repo init --mode=archive-z2; \

# make a cache dir

mkdir -p /srv/cache

# expose default SimpleHTTPServer port, set working dir


# start SimpleHTTPServer

CMD python -m SimpleHTTPServer

Build, Run and Enter the Container

docker build --rm -t $USER/atomicrepo .

docker run --privileged -d -p 8000:8000 --name atomicrepo $USER/atomicrepo

docker exec -it atomicrepo bash 

Compose Your Custom Tree

The Dockerfile above pulls in the definition files for Atomic CentOS and Atomic Fedora, which may be modified to produce a custom tree. The tree manifest syntax is documented here.

For example, here’s how to produce a version of the Atomic Fedora 25 tree that adds the fortune command:

cd fedora-atomic

git checkout f25

vi fedora-atomic-docker-host.json

Now, in the "packages": section of fedora-atomic-docker-host.json, and insert a line like this: "fortune-mod",.

Next, compose the new tree with the command:

rpm-ostree compose tree  --cachedir=/srv/cache  --repo=/srv/repo fedora-atomic-docker-host.json

Configure Your Atomic Host with the New Repository

To configure an Atomic host to receive updates from your build machine, run a pair of commands like the following to add a new withfortune repo definition to your host, and then rebase to that tree:

sudo ostree remote add withfortune http://$YOUR_IP:8000/repo --no-gpg-verify

sudo rpm-ostree rebase withfortune:fedora-atomic/25/x86_64/docker-host

Once the rebase operation is complete, run sudo systemctl reboot to reboot into your updated tree.