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OCI systemd hook enables users to run systemd in docker and OCI compatible runtimes such as runc without requiring --privileged flag.
This project produces a C binary that can be used with runc and Docker (with minor code changes).
If you clone this branch and build/install oci-systemd-hook, a binary should be placed in
/usr/libexec/oci/hooks.d named oci-systemd-hook.
Running Docker or OCI runc containers with this executable, oci-systemd-hook is called just before a container is started and after it is provisioned. If the CMD to run inside of the container is init or systemd, this hook will configure the container image to run a systemd environment. For all other CMD's, this hook will just exit.
When oci-systemd-hook detects systemd inside of the container it does the following:
- Mounts a tmpfs on /run and /tmp
- If there is content in the container image's /run and /tmp that content will be copied onto the tmpfs.
- Creates a /etc/machine-id based on the the containers UUID
- Mounts the hosts /sys/fs/cgroups file systemd read-only into the container
- /sys/fs/cgroup/systemd will be mounted read/write into the container.
When the container stops, these file systems will be umounted.
systemd is expected to be able to run within the container without requiring
the --privileged option. However you will still need to specify a special --stop-signal. Standard docker containers sends SIGTERM to pid 1, but systemd
does not shut down properly when it recieves a SIGTERM. systemd specified that it needs to receive a RTMIN+3 signal to shutdown properly.
Usage
If you created a container image based on a Dockerfile like the following:
cat Dockerfile
FROM fedora:latest
ENV container docker
RUN yum -y update && yum -y install httpd && yum clean all
RUN systemctl mask dnf-makecache.timer && systemctl enable httpd
CMD [ "/sbin/init" ]
(The systemctl mask dnf-makecache.timer is a workaround for a container base image bug)
You should then be able to execute the following commands:
docker build -t httpd .
docker run -ti --stop-signal=RTMIN+3 httpd
If you run this hook along with oci-register-machine oci hook, you will be able to show the containers journal information on the host, using journalctl.
journalctl -M CONTAINER_UUID
Disabling oci-systemd-hook
To disable oci-systemd-hook for a particular run, which is primarily useful in an Atomic Host environment, the environment variable 'oci-systemd-hook' can be set to 'disabled'. This prevents oci-systemd-hook from being run for that invocation. A sample usage is:
docker run --env oci-systemd-hook=disabled -it --rm fedora /bin/bash
To build and install
Prior to installing oci-systemd-hook, install the following packages on your linux distro:
- autoconf
- automake
- gcc
- git
- go-md2man
- libmount-devel
- libselinux-devel
- yajl-devel
In Fedora, you can use this command:
yum -y install \
autoconf \
automake \
gcc \
git \
go-md2man \
libmount-devel \
libselinux-devel \
yajl-devel
Then clone this branch and follow these steps:
git clone https://github.com/projectatomic/oci-systemd-hook
cd oci-systemd-hook
autoreconf -i
./configure --libexecdir=/usr/libexec/oci/hooks.d
make
make install