nextcloud-docs/centos/content.md

CentOS

CentOS Linux is a community-supported distribution derived from sources freely provided to the public by Red Hat for Red Hat Enterprise Linux (RHEL). As such, CentOS Linux aims to be functionally compatible with RHEL. The CentOS Project mainly changes packages to remove upstream vendor branding and artwork. CentOS Linux is no-cost and free to redistribute. Each CentOS Linux version is maintained for up to 10 years (by means of security updates -- the duration of the support interval by Red Hat has varied over time with respect to Sources released). A new CentOS Linux version is released approximately every 2 years and each CentOS Linux version is periodically updated (roughly every 6 months) to support newer hardware. This results in a secure, low-maintenance, reliable, predictable, and reproducible Linux environment.

wiki.centos.org

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CentOS image documentation

The centos:latest tag is always the most recent version currently available.

The CentOS Project offers regularly updated images for all active releases. These images will be updated monthly or as needed for emergency fixes. These rolling updates are tagged with the major version number only. For example: docker pull centos:6 or docker pull centos:7

Additionally, images that correspond to install media are also offered. These images DO NOT recieve updates as they are intended to match installation iso contents. If you choose to use these images it is highly recommended that you include RUN yum -y update && yum clean all in your Dockerfile, or otherwise address any potential security concerns. To use these images, please specify the minor version tag:

For example: docker pull centos:5.11 or docker pull centos:6.6

Systemd integration

Currently, systemd in CentOS 7 has been removed and replaced with a fakesystemd package for dependency resolution. This is due to systemd requiring the CAP_SYS_ADMIN capability, as well as being able to read the host's cgroups. If you wish to replace the fakesystemd package and use systemd normally, please follow the steps below.

Dockerfile for systemd base image

FROM centos:7
MAINTAINER "you" <[email protected]>
ENV container docker
RUN yum -y swap -- remove fakesystemd -- install systemd systemd-libs
RUN yum -y update; yum clean all; \
(cd /lib/systemd/system/sysinit.target.wants/; for i in *; do [ $i ==
systemd-tmpfiles-setup.service ] || rm -f $i; done); \
rm -f /lib/systemd/system/multi-user.target.wants/*;\
rm -f /etc/systemd/system/*.wants/*;\
rm -f /lib/systemd/system/local-fs.target.wants/*; \
rm -f /lib/systemd/system/sockets.target.wants/*udev*; \
rm -f /lib/systemd/system/sockets.target.wants/*initctl*; \
rm -f /lib/systemd/system/basic.target.wants/*;\
rm -f /lib/systemd/system/anaconda.target.wants/*;
VOLUME [ "/sys/fs/cgroup" ]
CMD ["/usr/sbin/init"]

This Dockerfile swaps out fakesystemd for the real package, but deletes a number of unit files which might cause issues. From here, you are ready to build your base image.

docker build --rm -t local/c7-systemd .

Example systemd enabled app container

In order to use the systemd enabled base container created above, you will need to create your Dockerfile similar to the one below.

FROM local/c7-systemd
RUN yum -y install httpd; yum clean all; systemctl enable httpd.service
EXPOSE 80
CMD ["/usr/sbin/init"]

Build this image:

docker build --rm -t local/c7-systemd-httpd

Running a systemd enabled app container

In order to run a container with systemd, you will need to use the --privileged option mentioned earlier, as well as mounting the cgroups volumes from the host. Below is an example command that will run the systemd enabled httpd container created earlier.

docker run --privileged -ti -v /sys/fs/cgroup:/sys/fs/cgroup:ro -p 80:80 local/c7-systemd-httpd

This container is running with systemd in a limited context, but it must always be run as a privileged container with the cgroups filesystem mounted.