Building, running, and managing containers
Building, running, and managing Linux containers on Red Hat Enterprise Linux 8
Abstract
Preface
Red Hat classifies container use cases into two distinct groups: single node and multi-node, with multi-node sometimes called distributed systems. OpenShift was built for multi-node systems, although single-node, all-in-one installations are supported as well. Beyond OpenShift, however, it is useful to have a small, nimble set of tools for working with containers.
The set of container tools we are referring to can be used in a single-node use case. However, you can also wire these tools into existing build systems, CI/CD environments, and even use them to tackle workload-specific use cases, such as big data. To target the single-node use case, Red Hat Enterprise Linux (RHEL) 8 offers a set of tools to find, run, build, and share individual containers.
This guide describes how to work with Linux containers on RHEL 8 systems using command-line tools such as podman, buildah, skopeo and runc. In addition to these tools, Red Hat provides base images, to act as the foundation for your own images. Some of these base images target use cases ranging from business applications (such as Node.js, PHP, Java, and Python) to infrastructure (such as logging, data collection, and authentication).
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Chapter 1. Starting with containers
Linux Containers have emerged as a key open source application packaging and delivery technology, combining lightweight application isolation with the flexibility of image-based deployment methods.
Red Hat Enterprise Linux implements Linux Containers using core technologies such as Control Groups (Cgroups) for Resource Management, Namespaces for Process Isolation, SELinux for Security, enabling secure multi-tenancy and reducing the potential for security exploits. All this is meant to provide you with an environment to producing and running enterprise-quality containers.
Red Hat OpenShift provides powerful command-line and Web UI tools for building, managing and running containers in units referred to as pods
. However, there are times when you might want to build and manage individual containers and container images outside of OpenShift. Tools provided to perform those tasks that run directly on RHEL systems are described in this guide.
Unlike other container tools implementations, tools described here do not center around the monolithic Docker container engine and docker
command. Instead, we provide a set of command-line tools that can operate without a container engine. These include:
- podman - For directly managing pods and container images (run, stop, start, ps, attach, exec, and so on)
- buildah - For building, pushing and signing container images
- skopeo - For copying, inspecting, deleting, and signing images
- runc - For providing container run and build features to podman and buildah
Because these tools are compatible with the Open Container Initiative (OCI), they can be used to manage the same Linux containers that are produced and managed by Docker and other OCI-compatible container engines. However, they are especially suited to run directly on Red Hat Enterprise Linux, in single-node use cases. For a list of supported container tools, see What container runtimes and container management tools are supported?.
For a multi-node container platform, see OpenShift. Instead of relying on the single-node, daemonless tools described in this document, OpenShift requires a daemon-based container engine. Please see Using the CRI-O Container Engine for details.
1.1. Running containers without Docker
Red Hat did not just remove the Docker container engine from OpenShift. It also removed the Docker container engine, along with the docker
command, from Red Hat Enterprise Linux 8 entirely. For RHEL 8, Docker is not included and not supported by Red Hat (although it is still available from other sources).
The removal of Docker reflects a change in Red Hat’s way of thinking about how containers are handled:
- In the enterprise, the focus is not on running individual containers from the command line. The primary venue for running containers is a Kubernetes-based platform, such as OpenShift.
- By repositioning OpenShift as the project for running containers, container engines like Docker become just another component of OpenShift with no direct access by end users.
- Because the container engine in OpenShift is not meant to be used directly, it can be implemented with a limited feature set that focuses on doing everything that OpenShift needs, without having to implement lots of standalone features.
Although Docker is gone from RHEL 8, and OpenShift’s container engine is disconnected from single-node uses, people still want to use commands to work with containers and images manually. So Red Hat set about to create a set of tools to implement most of what the docker
command does.
Tools like podman
, skopeo
, and buildah
were developed to take over those docker
command features. Each tool in this scenario can be more light-weight and focused on a subset of features. And with no need for a daemon process running to implement a container engine, these tools can run without the overhead of having to work with a daemon process.
If you feel that you still want to use Docker in RHEL 8, know that you can get Docker from different upstream projects, but that its use is unsupported in RHEL 8. Because so many docker
command-line features have been implemented exactly in podman
, you can set up an alias so that typing docker
causes podman to run.
Installing the podman-docker package sets up such an alias. So every time you run a docker
command line, it actually runs podman
for you. More on this package later.
1.2. Choosing a RHEL architecture for containers
Red Hat provides container images and container-related software for the following computer architectures:
- AMD64 and Intel 64 (base and layered images) (no support for the 32-bit AMD and Intel architecture)
- PowerPC 8 and 9 64-bit (base image and most layered images)
- IBM Z (base image and most layered images)
- ARM 64-bit (base image only)
Although not all Red Hat images were supported across all architectures at first, nearly all are now available on all listed architectures. See Universal Base Images (UBI): Images, repositories, and packages for a list of supported images.
1.3. Getting container tools
To get an environment where you can manipulate individual containers, you can install a Red Hat Enterprise Linux 8 system, then add a set of container tools to find, run, build and share containers. Here are examples of container-related tools you can install with RHEL 8:
-
podman - Client tool for managing containers. Can replace most features of the
docker
command for working with individual containers and images. - buildah - Client tool for building OCI-compliant container images.
- skopeo - Client tool for copying container images to and from container registries. Includes features for signing and authenticating images as well.
- runc - Container runtime client for running and working with Open Container Initiative (OCI) format containers.
Using the RHEL subscription model, if you want to create container images, you must properly register and entitle the host computer on which you build them. When you install packages, as part of the process of building a container, the build process automatically has access to entitlements available from the RHEL host. So it can get RPM packages from any repository enabled on that host.
- Install RHEL: If you are ready to begin, you can start by installing a Red Hat Enterprise Linux system.
Register RHEL: Once RHEL is installed, register the system. You will be prompted to enter your user name and password. Note that the user name and password are the same as your login credentials for Red Hat Customer Portal.
# subscription-manager register Registering to: subscription.rhsm.redhat.com:443/subscription Username: ******** Password: **********
Subscribe RHEL: Either auto subscribe or determine the pool ID of a subscription that includes Red Hat Enterprise Linux. Here is an example of auto-attaching a subscription:
# subscription-manager attach --auto
Install packages: To start building and working with individual containers, install the container-tools module, which pulls in the full set of container software packages:
# yum module install -y container-tools
Install podman-docker (optional): If you are comfortable with the
docker
command or use scripts that calldocker
directly, you can install the podman-docker package. That package installs a link that replaces thedocker
command-line interface with the matchingpodman
commands instead. It also links the man pages together, soman docker info
will show thepodman info
man page.# yum install -y podman-docker
1.4. Enabling container settings
No container engine (such as Docker or CRI-O) is required for you to run containers on your local system. However, configuration settings in the /etc/containers/registries.conf
file let you define access to container registries when you work with container tools such as podman
and buildah
.
Here are example settings in the /etc/containers/registries.conf
file:
[registries.search] registries = ['registry.redhat.io', 'registry.access.redhat.com', 'quay.io', 'docker.io'] [registries.insecure] registries = [] [registries.block] registries = []
By default, when you use podman search
to search for images from a container registries, based on the registries.conf
file, podman
looks for the requested image in registry.redhat.io, registry.access.redhat.com, quay.io, and docker.io, in that order.
To add access to a registry that doesn’t require authentication (an insecure registry), you must add the name of that registry under the [registries.insecure]
section. Any registries that you want to disallow from access from your local system needs to be added under the [registries.block]
section.
Here are a few other things you should know about configuring container registries:
- Make sure that each registry is surrounded by single quotes.
-
If there are multiple registries set for the
registries =
value, you must separate those registries by commas. - Registries can be identified by either IP address or hostname.
- If the registry uses a non-standard port (i.e., other than TCP ports 443 for secure and 80 for insecure), you should enter that port number with the registry name. For example: host.example.com:9999
-
Registries are searched in the order in which they appear for each section of the
registries.conf
file. -
If you are a regular user (rootless) running podman and related tools, you can create your own
registries.conf
file to override the default settings.
1.5. Running containers as root or rootless
Running the container tools such as podman
, skopeo
, or buildah
as a user with superuser privilege (root user) is the best way to ensure that your containers have full access to any feature available on your system. However, with the feature called "Rootless Containers," generally available as of RHEL 8.1, you can work with containers as a regular user.
Although container engines, such as Docker, let you run docker
commands as a regular (non-root) user, the docker daemon that carries out those requests runs as root. So, effectively, regular users can make requests through their containers that harm the system, without there being clarity about who made those requests. By setting up rootless container users, system administrators limit potentially damaging container activities from regular users, while still allowing those users to safely run many container features under their own accounts.
This section describes how to set up your system to use container tools (Podman, Skopeo, and Buildah) to work with containers as a non-root user (rootless). It also describes some of the limitations you will encounter because regular user accounts don’t have full access to all operating system features that their containers might need to run.
1.5.1. Set up for rootless containers
You need to become root user to set up your RHEL system to allow non-root user accounts to use container tools:
- Install RHEL: Install RHEL 8.1 or upgrade to RHEL 8.1 from RHEL 8.0. Earlier RHEL 7 versions are missing features needed for this procedure. If you are upgrading from RHEL 7.6 or earlier, continue to "Upgrade to rootless containers" after this procedure is done.
Install podman and slirp4netns: If not already installed, install the podman and slirp4netns packages:
# yum install slirp4netns podman -y
Increase user namespaces: To increase the number of user namespaces in the kernel, type the following:
# echo "user.max_user_namespaces=28633" > /etc/sysctl.d/userns.conf # sysctl -p /etc/sysctl.d/userns.conf
Create a new user account: To create a new user account and add a password for that account (for example, joe), type the following:
# useradd -c "Joe Jones" joe # passwd joe
The user is automatically configured to be able to use rootless podman.
Try a podman command: Log in directly as the user you just configured (don’t use
su
orsu -
to become that user because that doesn’t set the correct environment variables) and try to pull and run an image:$ podman pull registry.access.redhat.com/ubi8/ubi $ podman run registry.access.redhat.com/ubi8/ubi cat /etc/os-release NAME="Red Hat Enterprise Linux" VERSION="8.1 (Ootpa)" ...
Check rootless configuration: To check that your rootless configuration is set up properly, you can run commands inside the modified user namespace with the
podman unshare
command. As the rootless user, the following command lets you see how the uids are assigned to the user namespace:$ podman unshare cat /proc/self/uid_map 0 1001 1 1 65537 65536
1.5.2. Upgrade to rootless containers
If you have upgraded from RHEL 7, you must configure subuid and subgid values manually for any existing user you want to be able to use rootless podman.
Using an existing user name and group name (for example, jill), set the range of accessible user and group IDs that can be used for their containers. Here are a couple of warnings:
- Don’t include the rootless user’s UID and GID in these ranges
- If you set multiple rootless container users, use unique ranges for each user
- We recommend 65536 UIDs and GIDs for maximum compatibility with existing container images, but the number can be reduced
Never use UIDs or GIDs under 1000 or reuse UIDs or GIDs from existing user accounts (which, by default, start at 1000)
Here is an example:
# echo "jill:165537:65536" >> /etc/subuid # echo "jill:165537:65536" >> /etc/subgid
The user/group jill is now allocated 65535 user and group IDs, ranging from 165537-231072. That user should be able to begin running commands to work with containers now.
1.5.3. Special considerations for rootless
Here are some things to consider when running containers as a non-root user:
-
As a non-root container user, container images are stored under your home directory (
$HOME/.local/share/containers/storage/
), instead of/var/lib/containers
. - Users running rootless containers are given special permission to run as a range of user and group IDs on the host system. However, they otherwise have no root privileges to the operating system on the host.
-
If you need to configure your rootless container environment, edit configuration files in your home directory (
$HOME/.config/containers
). Configuration files includestorage.conf
(for configuring storage) andlibpod.conf
(for a variety of container settings). You could also create aregistries.conf
file to identify container registries available when you usepodman
to pull, search or run images. A container running as root in a rootless account can turn on privileged features within its own namespace. But that doesn’t provide any special privileges to access protected features on the host (beyond having extra UIDs and GIDs). Here are examples of container actions you might expect to work from a rootless account that will not work:
- Anything you want to access from a mounted directory from the host must be accessible by the UID running your container or your request to access that component will fail.
There are some system features you won’t be able to change without privilege. For example, you cannot change the system clock by simply setting a SYS_TIME capability inside a container and running the network time service (ntpd). You would have to run that container as root, bypassing your rootless container environment and using the root user’s environment, for that capability to work, such as:
$ sudo podman run -d --cap-add SYS_TIME ntpd
Note that this example allows ntpd to adjust time for the entire system, and not just within the container.
A rootless container has no ability to access a port less than 1024. Inside the rootless container’s namespace it can, for example, start a service that exposes port 80 from an httpd service from the container, but it will not be accessible outside of the namespace:
$ podman run -d httpd
However, a container would need root privilege, again using the root user’s container environment, to expose that port to the host system:
$ sudo podman run -d -p 80:80 httpd
The administrator of a workstation can configure it to allow users to expose services below 1024, but they should understand the security implications. A regular user could, for example, run a web server on the official port 80 and trick external users into believing that it was configured by the administrator. This is generally OK on a workstation, but might not be on a network-accessible development server, and definitely should not be done on production servers. To allow users to bind to ports down to port 80 run the following command:
# echo 80 > /proc/sys/net/ipv4/ip_unprivileged_port_start
-
An on-going list of shortcomings of running
podman
and related tools without root privilege is contained in Shortcomings of Rootless Podman.
Chapter 2. Working with container images
Using podman
, you can run, investigate, start, stop, investigate, and remove container images.
2.1. Pulling images from registries
To get container images from a remote registry (such as Red Hat’s own container registry) and add them to your local system, use the podman pull
command:
# podman pull <registry>[:<port>]/[<namespace>/]<name>:<tag>
The <registry> is a host that provides a container registry service on TCP <port>. Together, <namespace> and <name> identify a particular image controlled by <namespace> at that registry. Some registries also support raw <name>; for those, <namespace> is optional. When it is included, however, the additional level of hierarchy that <namespace> provides is useful to distinguish between images with the same <name>. For example:
Namespace | Examples (<namespace>/<name>) |
---|---|
organization |
|
login (user name) |
|
role |
|
The registries that Red Hat provides are registry.redhat.io (requiring authentication), registry.access.redhat.com (requires no authentication), and registry.connect.redhat.com (holds Red Hat Partner Connect program images). For details on the transition to registry.redhat.io, see Red Hat Container Registry Authentication . Before you can pull containers from registry.redhat.io, you need to authenticate. For example:
# podman login registry.redhat.io Username: myusername Password: ************ Login Succeeded!
Use the pull option to pull an image from a remote registry. To pull the rhel base image and rsyslog logging image from the Red Hat registry, type:
# podman pull registry.redhat.io/ubi8/ubi # podman pull registry.redhat.io/rhel8/rsyslog
An image is identified by a registry name (registry.redhat.io), a namespace name (ubi8) and the image name (ubi). You could also add a tag (which defaults to :latest if not entered). The repository name ubi, when passed to the podman pull command without the name of a registry preceding it, is ambiguous and could result in the retrieval of an image that originates from an untrusted registry. If there are multiple versions of the same image, adding a tag, such as latest to form a name such as ubi8/ubi:latest, lets you choose the image more explicitly.
To see the images that resulted from the above podman pull command, along with any other images on your system, type podman images:
REPOSITORY TAG IMAGE ID CREATED SIZE registry.redhat.io/ubi8/ubi latest eb205f07ce7d 2 weeks ago 214MB registry.redhat.io/rhel8/rsyslog latest 85cfba5cd49c 2 weeks ago 234MB
The ubi
and rsyslog
images are now available on your local system for you to work with.
2.2. Investigating images
Using podman images
you can see which images have been pulled to your local system. To look at the metadata associated with an image, use podman inspect
.
2.2.1. Listing images
To see which images have been pulled to your local system and are available to use, type:
# podman images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE registry.redhat.io/rhel8/support-tools latest b3d6ce4e0043 2 days ago 234MB registry.redhat.io/ubi8/ubi-init latest 779a05997856 2 days ago 225MB registry.redhat.io/ubi8/ubi latest a80dad1c1953 3 days ago 210MB
2.2.2. Inspecting local images
After you pull an image to your local system and before you run it, it is a good idea to investigate that image. Reasons for investigating an image before you run it include:
- Understanding what the image does
- Checking what software is inside the image
The podman inspect
command displays basic information about what an image does. You also have the option of mounting the image to your host system and using tools from the host to investigate what’s in the image. Here is an example of investigating what a container image does before you run it:
Inspect an image: Run
podman inspect
to see what command is executed when you run the container image, as well as other information. Here are examples of examining the ubi8/ubi and rhel8/rsyslog container images (with only snippets of information shown here):# podman pull registry.redhat.io/ubi8/ubi # podman inspect registry.redhat.io/ubi8/ubi | less ... "Cmd": [ "/bin/bash" ], "Labels": { "License": "GPLv3", "architecture": "x86_64", "authoritative-source-url": "registry.redhat.io", "build-date": "2018-10-24T16:46:08.916139", "com.redhat.build-host": "cpt-0009.osbs.prod.upshift.rdu2.redhat.com", "com.redhat.component": "rhel-server-container", "description": "The Red Hat Enterprise Linux Base image is designed to be a fully supported... ...
# podman pull registry.redhat.io/rhel8/rsyslog # podman inspect registry.redhat.io/rhel8/rsyslog "Cmd": [ "/bin/rsyslog.sh" ], "Labels": { "License": "GPLv3", "architecture": "x86_64", ... "install": "podman run --rm --privileged -v /:/host -e HOST=/host -e IMAGE=IMAGE -e NAME=NAME IMAGE /bin/install.sh", ... "run": "podman run -d --privileged --name NAME --net=host --pid=host -v /etc/pki/rsyslog:/etc/pki/rsyslog -v /etc/rsyslog.conf:/etc/rsyslog.conf -v /etc/sysconfig/rsyslog:/etc/sysconfig/rsyslog -v /etc/rsyslog.d:/etc/rsyslog.d -v /var/log:/var/log -v /var/lib/rsyslog:/var/lib/rsyslog -v /run:/run -v /etc/machine-id:/etc/machine-id -v /etc/localtime:/etc/localtime -e IMAGE=IMAGE -e NAME=NAME --restart=always IMAGE /bin/rsyslog.sh", "summary": "A containerized version of the rsyslog utility ...
The ubi8/ubi container will execute the bash shell, if no other argument is given when you start it with
podman run
. If an Entrypoint were set, its value would be used instead of the Cmd value (and the value of Cmd would be used as an argument to the Entrypoint command).In the second example, the rhel8/rsyslog container image has built-in
install
andrun
labels. Those labels give an indication of how the container is meant to be set up on the system (install) and executed (run).Mount a container: Using the
podman
command, mount an active container to further investigate its contents. This example runs and lists a runningrsyslog
container, then displays the mount point from which you can examine the contents of its file system:# podman run -d registry.redhat.io/rhel8/rsyslog # podman ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 1cc92aea398d ...rsyslog:latest /bin/rsyslog.sh 37 minutes ago Up 1 day ago myrsyslog # podman mount 1cc92aea398d /var/lib/containers/storage/overlay/65881e78.../merged # ls /var/lib/containers/storage/overlay/65881e78*/merged bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
After the podman mount, the contents of the container are accessible from the listed directory on the host. Use
ls
to explore the contents of the image.Check the image’s package list: To check the packages installed in the container, tell the
rpm
command to examine the packages installed on the container’s mount point:# rpm -qa --root=/var/lib/containers/storage/overlay/65881e78.../merged redhat-release-server-7.6-4.el7.x86_64 filesystem-3.2-25.el7.x86_64 basesystem-10.0-7.el7.noarch ncurses-base-5.9-14.20130511.el7_4.noarch glibc-common-2.17-260.el7.x86_64 nspr-4.19.0-1.el7_5.x86_64 libstdc++-4.8.5-36.el7.x86_64
2.2.3. Inspecting remote images
To inspect a container image before you pull it to your system, you can use the skopeo inspect
command. With skopeo inspect
, you can display information about an image that resides in a remote container registry.
The following command inspects the ubi8-init
image from the Red Hat registry:
# skopeo inspect docker://registry.redhat.io/ubi8/ubi8-init { "Name": "registry.redhat.io/ubi8/ubi8-init", "Digest": "sha256:53dfe24...", "RepoTags": [ "8.0.0-9", "8.0.0", "latest" ], "Created": "2019-05-13T20:50:11.437931Z", "DockerVersion": "1.13.1", "Labels": { "architecture": "x86_64", "authoritative-source-url": "registry.access.redhat.com", "build-date": "2019-05-13T20:49:44.207967", "com.redhat.build-host": "cpt-0013.osbs.prod.upshift.rdu2.redhat.com", "com.redhat.component": "ubi8-init-container", "description": "The Red Hat Enterprise Linux Init image is designed to be...
2.3. Searching for images
The podman search
command lets you search selected container registries for images. The registries searched come from those listed in the registries.conf
file.
Another, more intuitive way, to search for images maintained by Red Hat is to search the Red Hat Container Registry. Included with each image listed in the Red Hat Container Catalog are descriptions of the image, its contents, health index, and other pertinent information.
By default, podman search
searches the following registries:
- registry.redhat.io
- registry.access.redhat.com
- quay.io
- docker.io
As root user, you can edit the /etc/containers/registries.conf
file to change the default, system-wide search settings. As a regular (rootless) user of podman
, you can create your own registries.conf
file in your home directory ($HOME/.config/containers/registries.conf
) to override the system-wide settings.
The following are some podman search
command examples. The first example illustrates trying, but failing, to search all images accessible to you from quay.io. The backslash at the end says to search the whole registry for all images accessible to you:
# podman search quay.io/ ERRO[0000] error searching registry "quay.io": couldn't search registry "quay.io": unable to retrieve auth token: invalid username/password
Try again, but this time log in first:
# podman login quay.io Username: johndoe Password: *********** Login Succeeded! # podman search quay.io/ INDEX NAME DESCRIPTION STARS OFFICIAL AUTOMATED quay.io quay.io/test/myquay 0 quay.io quay.io/test/redistest 0 quay.io quay.io/johndoe/websrv21 0 quay.io quay.io/johndoe/mydbtest 0 quay.io quay.io/johndoe/newbuild-10 0
Search all available registries for postgresql (resulting in more than 40 images found):
# podman search postgresql-10 INDEX NAME DESCRIPTION STARS OFFICIAL AUTOMATED redhat.io registry.redhat.io/rhel8/postgresql-10 This container image ... 0 redhat.io registry.redhat.io/rhscl/postgresql-10-rhel7 PostgreSQL is an advanced ... 0 quay.io quay.io/mettle/postgresql-database-provisioning docker.io docker.io/centos/postgresql-10-centos7 PostgreSQL is an advanced ... 13 ...
To limit your search for postgresql to images from registry.redhat.io, type the following. Notice that by entering the registry and the image name, any repository in the registry can be matched:
# podman search registry.redhat.io/postgresql-10 INDEX NAME DESCRIPTION STARS OFFICIAL AUTOMATED redhat.io registry.redhat.io/rhel8/postgresql-10 This container image ... 0 redhat.io registry.redhat.io/rhscl/postgresql-10-rhel7 PostgreSQL is an ... 0
To get longer descriptions, add --no-trunc
:
# podman search --no-trunc registry.redhat.io/rhel8/postgresql-10 INDEX NAME DESCRIPTION STARS OFFICIAL AUTOMATED redhat.io registry.redhat.io/rhel8/postgresql-10 This container image provides a containerized packaging of the PostgreSQL postgres daemon and client application. The postgres server daemon accepts connections from clients and provides access to content from PostgreSQL databases on behalf of the clients. 0
If you want to change your search to use a registry that is not currently set up to search, edit the [registries.search]
section of the /etc/containers/registries.conf
file. One reason you might want to do this is to find a Red Hat partner-supported image (such as crunchy-postgres) instead of a public one from docker.io or quay.io. To do that, change this:
[registries.search] registries = ['registry.redhat.io', 'registry.access.redhat.com', 'quay.io', 'docker.io']
To this:
[registries.search] registries = ['registry.connect.redhat.com']
Now search for crunchy-postgres:
# podman search crunchy-postgres INDEX NAME DESCRIPTION STARS OFFICIAL AUTOMATED redhat.com registry.connect.redhat.com/crunchydata/crunchy-postgres The crunchy-postgres container executes the ... 0 redhat.com registry.connect.redhat.com/crunchydata/crunchy-postgres-gis The crunchy-postgres-gis container executes ... 0
Instead of choosing from random Crunchy postgres database images, you can see a list of images supported by an official Red Hat partner.
2.4. Tagging images
You can add names to images to make it more intuitive to understand what they contain. Tagging images can also be used to identify the target registry for which the image is intended. Using the podman tag
command, you essentially add an alias to the image that can consist of several parts. Those parts can include:
registryhost/username/NAME:tag
You can add just NAME if you like. For example:
# podman tag 474ff279782b myrhel8
In the previous example, the rhel8
image had an image ID of 474ff279782b. Using podman tag
, the name myrhel8
now also is attached to the image ID. So you could run this container by name (rhel8 or myrhel8) or by image ID. Notice that without adding a :tag to the name, it was assigned :latest as the tag. You could have set the tag to 8.0 as follows:
# podman tag 474ff279782b myrhel8:8.0
To the beginning of the name, you can optionally add a user name and/or a registry name. The user name is actually the repository on Docker.io that relates to the user account that owns the repository. Tagging an image with a registry name was shown in the "Tagging Images" section earlier in this document. Here’s an example of adding a user name:
# podman tag 474ff279782b jsmith/myrhel8 # podman images | grep 474ff279782b rhel8 latest 474ff279782b 7 days ago 139.6 MB myrhel8 latest 474ff279782b 7 months ago 139.6 MB myrhel8 7.1 474ff279782b 7 months ago 139.6 MB jsmith/myrhel8 latest 474ff279782b 7 months ago 139.6 MB
Above, you can see all the image names assigned to the single image ID.
2.5. Saving and loading images
If you want to save a container image you have stored locally, you can use podman save
to save the image to an archive file or directory and restore it later to another container environment. The archive you save can be in any of several different container image formats: docker-archive, oci-archive, oci-dir (directory with oci manifext type), or docker-dir (directory with v2s2 manifest type). After you save an image, you can store it or send it to someone else, then load
the image later to reuse it. Here is an example of saving an image as a tarball in the default docker-archive format:
# podman save -o myrsyslog.tar registry.redhat.io/rhel8/rsyslog:latest # file myrsyslog.tar myrsyslog.tar: POSIX tar archive
The myrsyslog.tar
file is now stored in your current directory. Later, when you are ready to reuse the tarball as a container image, you can import it to another podman environment as follows:
# podman load -i myrsyslog.tar # podman images REPOSITORY TAG IMAGE ID CREATED SIZE registry.redhat.io/rhel8/rsyslog latest 1f5313131bf0 7 weeks ago 235 MB
Instead of using save
and load
to store and reload an image, you can make a copy of a container instead, using podman export
and podman import
.
2.6. Removing Images
To see a list of images that are on your system, run the podman images
command. To remove images you no longer need, use the podman rmi
command, with the image ID or name as an option. (You must stop any containers run from an image before you can remove the image.) Here is an example:
# podman rmi ubi8-init 7e85c34f126351ccb9d24e492488ba7e49820be08fe53bee02301226f2773293
You can remove multiple images on the same command line:
# podman rmi registry.redhat.io/rhel8/rsyslog support-tools 46da8e23fa1461b658f9276191b4f473f366759a6c840805ed0c9ff694aa7c2f 85cfba5cd49c84786c773a9f66b8d6fca04582d5d7b921a308f04bb8ec071205
If you want to clear out all your images, you could use a command like the following to remove all images from your local registry (make sure you mean it before you do this!):
# podman rmi -a 1ca061b47bd70141d11dcb2272dee0f9ea3f76e9afd71cd121a000f3f5423731 ed904b8f2d5c1b5502dea190977e066b4f76776b98f6d5aa1e389256d5212993 83508706ef1b603e511b1b19afcb5faab565053559942db5d00415fb1ee21e96
To remove images that have multiple names (tags) associated with them, you need to add the force option to remove them. For example:
# podman rmi -a A container associated with containers/storage, i.e. via Buildah, CRI-O, etc., may be associated with this image: 1de7d7b3f531 # podman rmi -f 1de7d7b3f531 1de7d7b3f531...
Chapter 3. Working with containers
Containers represent a running or stopped process spawned from the files located in a decompressed container image. Tools for running containers and working with them are described in this section.
3.1. Running containers
When you execute a podman run
command, you essentially spin up and create a new container from a container image. The command you pass on the podman run
command line sees the inside of the container as its running environment so, by default, very little can be seen of the host system. For example, by default, the running application sees:
- The file system provided by the container image.
- A new process table from inside the container (no processes from the host can be seen).
If you want to make a directory from the host available to the container, map network ports from the container to the host, limit the amount of memory the container can use, or expand the CPU shares available to the container, you can do those things from the podman run
command line. Here are some examples of podman run
command lines that enable different features.
EXAMPLE #1 (Run a quick command): This podman command runs the cat /etc/os-release
command to see the type of operating system used as the basis for the container. After the container runs the command, the container exits and is deleted (--rm
).
# podman run --rm registry.redhat.io/ubi8/ubi cat /etc/os-release NAME="Red Hat Enterprise Linux" VERSION="8.0 (Ootpa)" ID="rhel" ID_LIKE="fedora" VERSION_ID="8.0" PLATFORM_ID="platform:el8" PRETTY_NAME="Red Hat Enterprise Linux 8.0 (Ootpa)" ANSI_COLOR="0;31" CPE_NAME="cpe:/o:redhat:enterprise_linux:8.0:latest" HOME_URL="https://www.redhat.com/" BUG_REPORT_URL="https://bugzilla.redhat.com/" REDHAT_BUGZILLA_PRODUCT="Red Hat Enterprise Linux 8" REDHAT_BUGZILLA_PRODUCT_VERSION=8.0 REDHAT_SUPPORT_PRODUCT="Red Hat Enterprise Linux" REDHAT_SUPPORT_PRODUCT_VERSION="8.0" ...
EXAMPLE #2 (View the Dockerfile in the container): This is another example of running a quick command to inspect the content of a container from the host. All layered images that Red Hat provides include the Dockerfile from which they are built in /root/buildinfo
. In this case you do not need to mount any volumes from the host.
# podman run --rm \ registry.access.redhat.com/rhel8/rsyslog \ ls /root/buildinfo Dockerfile-rhel8-rsyslog-8
Now you know what the Dockerfile is called, you can list its contents:
# podman run --rm registry.access.redhat.com/rhel8/rsyslog \ cat /root/buildinfo/Dockerfile-rhel8-rsyslog-8 FROM sha256:eb205f07ce7d0bb63bfe560... LABEL maintainer="Red Hat, Inc." RUN INSTALL_PKGS="\ rsyslog \ rsyslog-gnutls \ rsyslog-gssapi \ rsyslog-mysql \ rsyslog-pgsql \ rsyslog-relp \ " && yum -y install $INSTALL_PKGS && rpm -V --nosize --nofiledigest --nomtime --nomode $INSTALL_PKGS && yum clean all LABEL com.redhat.component="rsyslog-container" LABEL name="rhel8/rsyslog" LABEL version="8.0" ...
EXAMPLE #3 (Run a shell inside the container): Using a container to launch a bash shell lets you look inside the container and change the contents. This sets the name of the container to mybash
. The -i
creates an interactive session and -t
opens a terminal session. Without -i
, the shell would open and then exit. Without -t
, the shell would stay open, but you wouldn’t be able to type anything to the shell.
Once you run the command, you are presented with a shell prompt and you can start running commands from inside the container:
# podman run --name=mybash -it registry.redhat.io/ubi8/ubi /bin/bash [root@ed904b8f2d5c/]# yum install procps-ng [root@ed904b8f2d5c/]# ps -ef UID PID PPID C STIME TTY TIME CMD root 1 0 0 00:46 pts/0 00:00:00 /bin/bash root 35 1 0 00:51 pts/0 00:00:00 ps -ef [root@49830c4f9cc4/]# exit
Although the container is no longer running once you exit, the container still exists with the new software package still installed. Use podman ps -a
to list the container:
# podman ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES IS INFRA 1ca061b47bd7 .../ubi8/ubi:latest /bin/bash 8 minutes ago Exited 12 seconds ago musing_brown false ...
You could start that container again using podman start
with the -ai
options. For example:
# podman start -ai mybash [root@ed904b8f2d5c/]#
EXAMPLE #4 (Bind mounting log files): One way to make log messages from inside a container available to the host system is to bind mount the host’s /dev/log device inside the container. This example illustrates how to run an application in a RHEL container that is named log_test
that generates log messages (just the logger command in this case) and directs those messages to the /dev/log device that is mounted in the container from the host. The --rm
option removes the container after it runs.
# podman run --name="log_test" -v /dev/log:/dev/log --rm \ registry.redhat.io/ubi8/ubi logger "Testing logging to the host" # journalctl -b | grep Testing Nov 12 20:00:10 ubi8 root[17210]: Testing logging to the host
3.2. Investigating running and stopped containers
After you have some running containers, you can list both those containers that are still running and those that have exited or stopped with the podman ps
command. You can also use the podman inspect
to look at specific pieces of information within those containers.
3.2.1. Listing containers
Let’s say you have one or more containers running on your host. To work with containers from the host system, you can open a shell and try some of the following commands.
podman ps
: The ps option shows all containers that are currently running:
# podman run -d registry.redhat.io/rhel8/rsyslog # podman ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 74b1da000a11 rhel8/rsyslog /bin/rsyslog.sh 2 minutes ago Up About a minute musing_brown
If there are containers that are not running, but were not removed (--rm option), the containers are still hanging around and can be restarted. The podman ps -a
command shows all containers, running or stopped.
# podman ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES IS INFRA d65aecc325a4 ubi8/ubi /bin/bash 3 secs ago Exited (0) 5 secs ago peaceful_hopper false 74b1da000a11 rhel8/rsyslog rsyslog.sh 2 mins ago Up About a minute musing_brown false
3.2.2. Inspecting containers
To inspect the metadata of an existing container, use the podman inspect
command. You can show all metadata or just selected metadata for the container. For example, to show all metadata for a selected container, type:
# podman inspect 74b1da000a11 ... "ID": "74b1da000a114015886c557deec8bed9dfb80c888097aa83f30ca4074ff55fb2", "Created": "2018-11-13T10:30:31.884673073-05:00", "Path": "/bin/rsyslog.sh", "Args": [ "/bin/rsyslog.sh" ], "State": { OciVersion": "1.0.1-dev", Status": "running", Running": true, ...
You can also use inspect to pull out particular pieces of information from a container. The information is stored in a hierarchy. So to see the container’s IP address (IPAddress under NetworkSettings), use the --format
option and the identity of the container. For example:
# podman inspect --format='{{.NetworkSettings.IPAddress}}' 74b1da000a11 10.88.0.31
Examples of other pieces of information you might want to inspect include .Path (to see the command run with the container), .Args (arguments to the command), .Config.ExposedPorts (TCP or UDP ports exposed from the container), .State.Pid (to see the process id of the container) and .HostConfig.PortBindings (port mapping from container to host). Here’s an example of .State.Pid and .State.StartedAt:
# podman inspect --format='{{.State.Pid}}' 74b1da000a11 19593 # ps -ef | grep 19593 root 19593 19583 0 10:30 ? 00:00:00 /usr/sbin/rsyslogd -n # podman inspect --format='{{.State.StartedAt}}' 74b1da000a11 2018-11-13 10:30:35.358175255 -0500 EST
In the first example, you can see the process ID of the containerized executable on the host system (PID 19593). The ps -ef
command confirms that it is the rsyslogd daemon running. The second example shows the date and time that the container was run.
3.2.3. Investigating within a container
To investigate within a running container, you can use the podman exec
command. With podman exec
, you can run a command (such as /bin/bash
) to enter a running container process to investigate that container.
The reason for using podman exec
, instead of just launching the container into a bash shell, is that you can investigate the container as it is running its intended application. By attaching to the container as it is performing its intended task, you get a better view of what the container actually does, without necessarily interrupting the container’s activity.
Here is an example using podman exec
to look into a running rsyslog, then look around inside that container.
Launch a container
: Launch a container such the rsyslog container image described earlier. Typepodman ps
to make sure it is running:# podman ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 74b1da000a11 rsyslog:latest "/usr/rsyslog.sh 6 minutes ago Up 6 minutes rsyslog
Enter the container with
podman exec
: Use the container ID or name to open a bash shell to access the running container. Then you can investigate the attributes of the container as follows:# podman exec -it 74b1da000a11 /bin/bash [root@74b1da000a11 /]# cat /etc/redhat-release Red Hat Enterprise Linux release 8.0 [root@74b1da000a11 /]# yum install procps-ng [root@74b1da000a11 /]# ps -ef UID PID PPID C STIME TTY TIME CMD root 1 0 0 15:30 ? 00:00:00 /usr/sbin/rsyslogd -n root 8 0 6 16:01 pts/0 00:00:00 /bin/bash root 21 8 0 16:01 pts/0 00:00:00 ps -ef [root@74b1da000a11 /]# df -h Filesystem Size Used Avail Use% Mounted on overlay 39G 2.5G 37G 7% / tmpfs 64M 0 64M 0% /dev tmpfs 1.5G 8.7M 1.5G 1% /etc/hosts shm 63M 0 63M 0% /dev/shm tmpfs 1.5G 0 1.5G 0% /sys/fs/cgroup tmpfs 1.5G 0 1.5G 0% /proc/acpi tmpfs 1.5G 0 1.5G 0% /proc/scsi tmpfs 1.5G 0 1.5G 0% /sys/firmware [root@74b1da000a11 /]# uname -r 4.18.0-80.1.2.el8_0.x86_64 [root@74b1da000a11 /]# rpm -qa | more redhat-release-8.0-0.44.el8.x86_64 filesystem-3.8-2.el8.x86_64 basesystem-11-5.el8.noarch ncurses-base-6.1-7.20180224.el8.noarch ... bash-4.2# free -m total used free shared buff/cache available Mem: 1941 560 139 10 1241 1189 Swap: 1023 15 1008 [root@74b1da000a11 /]# exit
The commands just run from the bash shell (running inside the container) show you several things.
- The container was built from a RHEL release 8.0 image.
- The process table (ps -ef) shows that the /usr/sbin/rsyslogd command is process ID 1.
- Processes running in the host’s process table cannot be seen from within the container. Although the rsyslogd process can be seen on the host process table (it was process ID 19593 on the host).
- There is no separate kernel running in the container (uname -r shows the host system’s kernel).
- The rpm -qa command lets you see the RPM packages that are included inside the container. In other words, there is an RPM database inside of the container.
- Viewing memory (free -m) shows the available memory on the host (although what the container can actually use can be limited using cgroups).
3.3. Starting and stopping containers
If you ran a container, but didn’t remove it (--rm
), that container is stored on your local system and ready to run again. To start a previously run container that wasn’t removed, use the start
option. To stop a running container, use the stop
option.
3.3.1. Starting containers
A container that doesn’t need to run interactively can sometimes be restarted after being stopped with only the start
option and the container ID or name. For example:
# podman start myrhel_httpd myrhel_httpd
To start a container so you can work with it from the local shell, use the -a (attach) and -i (interactive) options. Once the bash shell starts, run the commands you want inside the container and type exit to kill the shell and stop the container.
# podman start -a -i agitated_hopper [root@d65aecc325a4 /]# exit
3.3.2. Stopping containers
To stop a running container that is not attached to a terminal session, use the stop option and the container ID or number. For example:
# podman stop 74b1da000a11 74b1da000a114015886c557deec8bed9dfb80c888097aa83f30ca4074ff55fb2
The stop
option sends a SIGTERM signal to terminate a running container. If the container doesn’t stop after a grace period (10 seconds by default), podman
sends a SIGKILL signal. You could also use the podman kill
command to kill a container (SIGKILL) or send a different signal to a container. Here’s an example of sending a SIGHUP signal to a container (if supported by the application, a SIGHUP causes the application to re-read its configuration files):
# podman kill --signal="SIGHUP" 74b1da000a11 74b1da000a114015886c557deec8bed9dfb80c888097aa83f30ca4074ff55fb2
3.4. Removing containers
To see a list of containers that are still hanging around your system, run the podman ps -a
command. To remove containers you no longer need, use the podman rm
command, with the container ID or name as an option. You should stop any containers that are still running before removing them. Here is an example:
# podman rm goofy_wozniak
You can remove multiple containers on the same command line:
# podman rm clever_yonath furious_shockley drunk_newton
If you want to clear out all your containers, you could use a command like the following to remove all containers (not images) from your local system (make sure you mean it before you do this!):
# podman rm -a 56c496350bd534da7620fe2fa660526a6fc7f1c57b0298291cd2210311fe723b 83ad58c17b20f9e8271171f3023ae094dbfab6ce5708344a68feb121916961ca a93b696a1f5629300382a8ce860c4ba42f664db98101e82c2dbcc2074b428faf bee71e61b53bd8b036b2e8cb8f570ef8308403502760a27ee23a4b675d92b93d
Chapter 4. Using Red Hat Universal Base Images (standard, minimal, and runtimes)
Red Hat Enterprise Linux (RHEL) base images are meant to form the foundation for the container images you build. For RHEL 8, all Red Hat base images are available as new Universal Base Images (UBI). These include versions of RHEL standard, minimal, init, and Red Hat Software Collections that are all now freely available and redistributable. Characteristics of RHEL base images include:
- Supported: Supported by Red Hat for use with your containerized applications. Contains the same secured, tested, and certified software packages you have in Red Hat Enterprise Linux.
- Cataloged: Listed in the Red Hat Container Catalog, where you can find descriptions, technical details, and a health index for each image.
- Updated: Offered with a well-defined update schedule, so you know you are getting the latest software (see Red Hat Container Image Updates).
- Tracked: Tracked by errata, to help you understand the changes that go into each update.
- Reusable: Only need to be downloaded and cached in your production environment once, where each base image can be reused by all containers that include it as their foundation.
Red Hat Universal Base Images (UBI) for RHEL 8 provide the same quality RHEL software for building container images as their non-UBI predecessors (rhel6
, rhel7
, rhel-init
, and rhel-minimal
base images), but offer more freedom in how they are used and distributed.
For a list of available Red Hat UBI images, and associated information about UBI repositories and source code, see Universal Base Images (UBI): Images, repositories, and packages.
4.1. What are Red Hat base images?
Red Hat provides multiple base images that you can use as a starting point for your own images. These images are available through the Red Hat Registry (registry.access.redhat.com and registry.redhat.io) and described in the Red Hat Container Catalog.
For RHEL 8, there are standard, minimal and init base image available. Red Hat also provides a set of language runtime images, based on Application Streams, that you can build on when you are creating containers for applications that require specific runtimes. Runtime images include python, php, ruby, nodejs, and others. All of the RHEL 8 images are UBI images, which means that you can freely obtain and redistribute them.
There is a set of RHEL 7 images as well that you can run on RHEL 8 systems. For RHEL 7, there are both UBI (redistributable) and non-UBI (require subscription access and are non-redistributable) base images. Those images include three regular base images (rhel7
, rhel-init
, and rhel-minimal
) and three UBI images (ubi7
, ubi7-init
, and ubi7-minimal
).
Although Red Hat does not offer tools for running containers on RHEL 6 systems, it does offer RHEL 6 container images you can use. There are standard (rhel6
) and Init (rhel6-init
) base image available for RHEL 6, but no minimal RHEL 6 image. Likewise, there are no RHEL 6 UBI images.
4.1.1. Using standard Red Hat base images
Standard RHEL 8 base images (ubi8
) have a robust set of software features that include the following:
- init system: All the features of the systemd initialization system you need to manage systemd services are available in the standard base images. These init systems let you install RPM packages that are pre-configured to start up services automatically, such as a Web server (httpd) or FTP server (vsftpd).
-
yum: Software needed to install software packages is included via the standard set of
yum
commands (yum
,yum-config-manager
,yumdownloader
, and so on). For the UBI base images, you have access to free yum repositories for adding and updating software. -
utilities: The standard base image includes some useful utilities for working inside the container. Utilities that are in this base image that are not in the minimal images include
tar
,dmidecode
,gzip
,getfacl
(and other acl commands),dmsetup
(and other device mapper commands), and others.
4.1.2. Using minimal Red Hat base images
The ubi8-minimal
images are stripped-down RHEL images to use when a bare-bones base image in desired. If you are looking for the smallest possible base image to use as part of the larger Red Hat ecosystem, you can start with these minimal images.
RHEL minimal images provide a base for your own container images that is less than half the size of the standard image, while still being able to draw on RHEL software repositories and maintain any compliance requirements your software has.
Here are some features of the minimal base images:
- Small size: Minimal images are about 92M on disk and 32M compressed. This makes it less than half the size of the standard images.
- Software installation (microdnf): Instead of including the full-blown yum facility for working with software repositories and RPM software packages, the minimal images includes the microdnf utility. Microdnf is a scaled-down version of dnf. It includes only what is needed to enable and disable repositories, as well as install, remove, and update packages. It also has a clean option, to clean out cache after packages have been installed.
- Based on RHEL packaging: Because minimal images incorporate regular RHEL software RPM packages, with a few features removed such as extra language files or documentation, you can continue to rely on RHEL repositories for building your images. This allows you to still maintain compliance requirements you have that are based on RHEL software. Features of minimal images make them perfect for trying out applications you want to run with RHEL, while carrying the smallest possible amount of overhead. What you don’t get with minimal images is an initialization and service management system (systemd or System V init), a Python run-time environment, and a bunch of common shell utilities.
-
Modules for
microdnf
are not supported: Modules used with thednf
command let you install multiple versions of the same software, when available. Themicrodnf
utility included with minimal images does not support modules. So if modules are required, you should use a non-minimal base images, which include yum.
If your goal, however, is just to try to run some simple binaries or pre-packaged software that doesn’t have a lot of requirements from the operating system, the minimal images might suit your needs. If your application does have dependencies on other software from RHEL, you can simply use microdnf to install the needed packages at build time.
Red Hat intends for you to always use the latest version of the minimal images, which is implied by simply requesting ubi8/ubi-minimal
or ubi8-minimal
. Red Hat does not expect to support older versions of minimal images going forward.
4.1.3. Using Init Red Hat base images
The UBI ubi8-init
images contains the systemd initialization system, making them useful for building images in which you want to run systemd services, such as a web server or file server. The Init image contents are less than what you get with the standard images, but more than what is in the minimal images.
Because the ubi8-init
image builds on top of the ubi8
image, their contents are mostly the same. There are a few critical differences, however. In ubi8-init
, the Cmd is set to /sbin/init
, instead of bash
, to start the systemd Init service by default. It includes ps
and process related commands (procps-ng package), which ubi8 does not. Also, ubi8-init
sets SIGRTMIN+3 as the StopSignal, as systemd in ubi8-init
ignores normal signals to exit (SIGTERM and SIGKILL), but will terminate if it receives SIGRTMIN+3.
Historically, Red Hat Enterprise Linux base container images were designed for Red Hat customers to run enterprise applications, but were not free to redistribute. This can create challenges for some organizations that need to redistribute their applications. That’s where the Red Hat Universal Base Images come in.
4.2. How are UBI images different?
UBI images were created so you can build your container images on a foundation of official Red Hat software that can be freely shared and deployed. From a technical perspective, they are nearly identical to legacy Red Hat Enterprise Linux images, which means they have great security, performance, and life cycles, but they are released under a different End User License Agreement. Here are some attributes of Red Hat UBI images:
- Built from a subset of RHEL content: Red Hat Universal Base images are built from a subset of normal Red Hat Enterprise Linux content. All of the content used to build selected UBI images is released in a publicly available set of yum repositories. This lets you install extra packages, as well as update any package in UBI base images.
- Redistributable: The intent of UBI images is to allow Red Hat customers, partners, ISVs, and others to standardize on one container base image, allowing users to focus on application needs instead of distribution rules. These images can be shared and run in any environment capable of running those images. As long as you follow some basic guidelines, you will be able to freely redistribute your UBI-based images.
- Base and runtime images: Besides the three types of base images, UBI versions of various runtime images are available as well. These runtime images provide a foundation for applications that can benefit from standard, supported runtimes such as python, php, nodejs, and ruby.
Enabled yum repositories: The following yum repositories are enabled within each RHEL 8 UBI image:
-
The
ubi-8-baseos
repo holds the redistributable subset of RHEL packages you can include in your container. -
The
ubi-8-appstream
repo holds Red Hat Software Collections packages that you can add to a UBI image to help you standardize the environments you use with applications that require particular runtimes.
-
The
- Licensing: You are free to use and redistribute UBI images, provided you adhere to the Red Hat Universal Base Image End User Licensing Agreement.
- Adding UBI RPMs: You can add RPM packages to UBI images from preconfigured UBI repositories. If you happen to be in a disconnected environment, you must whitelist the UBI Content Delivery Network (https://cdn-ubi.redhat.com) to use that feature. See the Connect to https://cdn-ubi.redhat.com solution for details.
Although the legacy RHEL 7 base images will continue to be supported, UBI images are recommended going forward. For that reason, examples in the rest of this chapter are done with RHEL 8 UBI images.
4.3. Get UBI images
The following table shows the UBI base images currently available. Notice that some of those images have multiple names you can use to refer to them.
Table 4.1. Available Red Hat Universal Base Images
Registry names | Namespace | Image | RHEL version |
---|---|---|---|
registry.access.redhat.com registry.redhat.io | / | ubi8 | 8.0.0 |
ubi8-minimal | 8.0.0 | ||
ubi8-init | 8.0.0 | ||
registry.access.redhat.com registry.redhat.io | ubi8/ | ubi | 8.0.0 |
ubi-init | 8.0.0 | ||
ubi-minimal | 8.0.0 | ||
s2i-core | 8.0.0 | ||
s2i-base | 8.0.0 | ||
dotnet-21-runtime | 8.0.0 | ||
dotnet-21 | 8.0.0 | ||
perl-526 | 8.0.0 | ||
php-72 | 8.0.0 | ||
nodejs-10 | 8.0.0 | ||
ruby-25 | 8.0.0 | ||
python-27 | 8.0.0 | ||
python-36 | 8.0.0 | ||
registry.redhat.io | rhel8 | s2i-core | 8.0.0 |
s2i-base | 8.0.0 | ||
dotnet-21-runtime | 8.0.0 | ||
dotnet-21 | 8.0.0 | ||
nginx-114 | 8.0.0 | ||
perl-526 | 8.0.0 | ||
php-72 | 8.0.0 | ||
nodejs-10 | 8.0.0 | ||
httpd-24 | 8.0.0 | ||
redis-5 | 8.0.0 | ||
varnish-6 | 8.0.0 | ||
ruby-25 | 8.0.0 | ||
python-27 | 8.0.0 | ||
python-36 | 8.0.0 | ||
memcached | 8.0.0 | ||
net-snmp | 8.0.0 | ||
sssd | 8.0.0 | ||
ipa-server | 8.0.0 | ||
support-tools | 8.0.0 | ||
rsyslog | 8.0.0 | ||
GCC | 8.0.0 | ||
rust-toolset | 8.0.0 | ||
go-toolset | 8.0.0 | ||
llvm-toolset | 8.0.0 | ||
postgresql-10 | 8.0.0 | ||
postgresql-96 | 8.0.0 | ||
mysql-80 | 8.0.0 | ||
mariadb-103 | 8.0.0 | ||
registry.access.redhat.com registry.redhat.io | / | ubi7 | 7.6.4 |
ubi7-minimal | 7.6.4 | ||
ubi7-init | 7.6.4 | ||
registry.access.redhat.com registry.redhat.io | ubi7/ | ubi | 7.6.4 |
ubi-init | 7.6.4 | ||
ubi-minimal | 7.6.4 | ||
php-72 | 7.6.4 | ||
nodejs-8 | 7.6.4 | ||
ruby-25 | 7.6.4 | ||
python-27 | 7.6.4 | ||
python-36 | 7.6.4 | ||
s2i-core | 7.6.4 | ||
s2i-base | 7.6.4 |
4.4. Pull UBI images
To pull UBI images to your system so you can use them with tools such as podman, buildah or skopeo, type the following:
# podman pull registry.access.redhat.com/ubi8/ubi:latest # podman pull registry.access.redhat.com/ubi8/ubi-minimal:latest
To check that the images are available on your system, type:
# podman images REPOSITORY TAG IMAGE ID CREATED SIZE registry.access.redhat.com/ubi8/ubi-minimal latest c94a444803e3 8 hours ago 80.9 MB registry.access.redhat.com/ubi8/ubi latest 40b488f87628 17 hours ago 214 MB
When pulled in this way, images are available and usable by podman
, buildah
, skopeo
and the CRI-O container image, but they are not available to the Docker service or docker
command. To use these images with Docker, you can run docker pull
instead.
4.5. Redistributing UBI images
After you pull a UBI image, you are free to push it to your own registry and share it with others. You can upgrade or add to that image from UBI yum repositories as you like. Here is an example of how to push a UBI image to your own or another third-party repository:
# podman pull registry.redhat.io/ubi8/ubi # podman tag registry.access.redhat.com/ubi8/ubi registry.example.com:5000/ubi8/ubi # podman push registry.example.com:5000/ubi8/ubi
While there are few restrictions on how you use these images, there are some restrictions about how you can refer to them. For example, you can’t call those images Red Hat certified or Red Hat supported unless you certify it through the Red Hat Partner Connect Program, either with Red Hat Container Certification or Red Hat OpenShift Operator Certification.
4.6. Run UBI images
To start a container from a UBI image and run the bash shell in that image (so you can look around inside), do the following (type exit when you are done):
# podman run --rm -it registry.access.redhat.com/ubi8/ubi-minimal:latest /bin/bash [root@da9213157c51 /]# # podman run --rm -it registry.access.redhat.com/ubi8/ubi:latest /bin/bash bash-4.2#
While in the container:
-
Run
rpm -qa
to see a list of package inside each container. -
Type
yum list available
to see packages available to add to the image from the UBI yum repos. (The yum command is not available in theubi-minimal
containers.) - Get source code, as described in the "Getting UBI Container Image Source Code," later in this chapter.
On systems that include the Docker service, you can use docker run
instead.
4.7. Add software to a running UBI container
UBI images are built from 100% Red Hat content. These UBI images also provide a subset of Red Hat Enterprise Linux packages that are freely available to install for use with UBI. To add or update software, UBI images are pre-configured to point to the freely available yum repositories that hold official Red Hat RPMs.
To add packages from UBI repos to running UBI containers:
-
On
ubi
images, the yum command is installed to let you draw packages -
On
ubi-minimal
images, themicrodnf
command (with a smaller feature set) is included instead ofyum
.
Keep in mind that installing and working with software packages directly in running containers is just for adding packages temporarily or learning about the repos. Refer to the “Build a UBI-based image” for more permanent ways of building UBI-based images.
When you add software to a UBI container, procedures differ for updating UBI images on a subscribed RHEL host or on an unsubscribed (or non-RHEL) system. Those two ways of working with UBI images are illustrated below.
4.7.1. Adding software to a UBI container (subscribed host)
If you are running a UBI container on a registered and subscribed RHEL host, the main RHEL Server repository is enabled inside the standard UBI container, along with all the UBI repos. So the full set of Red Hat packages is available. From the UBI minimal container, all UBI repos are enabled by default, but no repos are enabled from the host by default.
4.7.2. Adding software inside the standard UBI container
To ensure the containers you build can be redistributed, disable non-UBI yum repositories in the standard UBI image when you add software. If you disable all yum repositories except for UBI repositories, only packages from the freely available repos are used when you add software.
With a shell open inside a standard UBI base image container (ubi8/ubi
) from a subscribed RHEL host, run the following command to add a package to that container (for example, the bzip2 package):
# yum install --disablerepo=* --enablerepo=ubi-8-appstream --enablerepo=ubi-8-baseos bzip2
To add software inside a standard UBI container that is in the RHEL server repo, but not in UBI repos, don’t disable any repositories and just install the package:
# yum install zsh
To install a package that is in a different host repo from inside the standard UBI container, you have to explicitly enable the repo you need. For example:
# yum install --enablerepo=rhel-7-server-optional-rpms zsh-html
Installing Red Hat packages that are not inside the Red Hat UBI repos might limit how widely you can distribute the container outside of subscribed hosts.
4.7.3. Adding software inside the minimal UBI container
UBI yum repositories are enabled inside the UBI minimal image by default.
To install the same package demonstrated earlier (bzip2) from one of those UBI yum repositories on a subscribed RHEL host from the UBI minimal container, type:
# microdnf install bzip2
To install packages inside a minimal UBI container from repos available on a subscribed host that are not part of a UBI yum repo, you would have to explicitly enable those repos. For example:
# microdnf install --enablerepo=rhel-7-server-rpms zsh # microdnf install --enablerepo=rhel-7-server-rpms \ --enablerepo=rhel-7-server-optional-rpms zsh-html
Using non-UBI RHEL repositories to install packages in your UBI images could restrict your ability to share those images to run outside of subscribed RHEL systems.
4.7.4. Adding software to a UBI container (unsubscribed host)
To add software packages to a running container that is either on an unsubscribed RHEL host or some other Linux system, you don’t have to disable any yum repositories. For example:
# yum install bzip2
To install that package on an unsubscribed RHEL host from the UBI minimal container, type:
# microdnf install bzip2
As noted earlier, both of these means of adding software to a running UBI container are not intended for creating permanent UBI-based container images. For that, you should build new layers on to UBI images, as described in the following section.
4.7.5. Build a UBI-based image
You can build UBI-based container images in the same way you build other images, with one exception. You should disable all non-UBI yum repositories when you actually build the images, if you want to be sure that your image only contains Red Hat software that you can redistribute.
Here’s an example of creating a UBI-based Web server container from a Dockerfile with the buildah
utility:
For ubi8/ubi-minimal images, use microdnf instead of yum below:
RUN microdnf update -y && rm -rf /var/cache/yum RUN microdnf install httpd -y && microdnf clean all
Create a Dockerfile: Add a
Dockerfile
with the following contents to a new directory:FROM registry.access.redhat.com/ubi8/ubi USER root LABEL maintainer="John Doe" # Update image RUN yum update --disablerepo=* --enablerepo=ubi-8-appstream --enablerepo=ubi-8-baseos -y && rm -rf /var/cache/yum RUN yum install --disablerepo=* --enablerepo=ubi-8-appstream --enablerepo=ubi-8-baseos httpd -y && rm -rf /var/cache/yum # Add default Web page and expose port RUN echo "The Web Server is Running" > /var/www/html/index.html EXPOSE 80 # Start the service CMD ["-D", "FOREGROUND"] ENTRYPOINT ["/usr/sbin/httpd"]
Build the new image: While in that directory, use
buildah
to create a new UBI layered image:# buildah bud -t johndoe/webserver . STEP 1: FROM registry.access.redhat.com/ubi8/ubi:latest STEP 2: USER root STEP 3: LABEL maintainer="John Doe" STEP 4: RUN yum update --disablerepo=* --enablerepo=ubi-8-appstream --enablerepo=ubi-8-baseos -y . . . No packages marked for update STEP 5: RUN yum install --disablerepo=* --enablerepo=ubi-8-appstream --enablerepo=ubi-8-baseos httpd -y Loaded plugins: ovl, product-id, search-disabled-repos Resolving Dependencies --> Running transaction check ============================================================= Package Arch Version Repository Size ============================================================= Installing: httpd x86_64 2.4.37-10 latest-rhubi-8.0-appstream 1.4 M Installing dependencies: apr x86_64 1.6.3-9.el8 latest-rhubi-8.0-appstream 125 k apr-util x86_64 1.6.1-6.el8 latest-rhubi-8.0-appstream 105 k httpd-filesystem noarch 2.4.37-10 latest-rhubi-8.0-appstream 34 k httpd-tools x86_64 2.4.37-10. ... Transaction Summary ... Complete! STEP 6: RUN echo "The Web Server is Running" > /var/www/html/index.html STEP 7: EXPOSE 80 STEP 8: CMD ["-D", "FOREGROUND"] STEP 9: ENTRYPOINT ["/usr/sbin/httpd"] STEP 10: COMMIT ... Writing manifest to image destination Storing signatures --> 36a604cc0dd3657b46f8762d7ef69873f65e16343b54c63096e636c80f0d68c7
Test: Test the UBI layered webserver image:
# podman run -d -p 80:80 johndoe/webserver bbe98c71d18720d966e4567949888dc4fb86eec7d304e785d5177168a5965f64 # curl http://localhost/index.html The Web Server is Running
4.7.6. Using AppStream runtime images
Red Hat Enterprise Linux 8 AppStream offers another set of container images that you can use as the basis for your container builds. These images are built on RHEL standard base images, with most already updated as UBI images. Each of these images include additional software you might want to use for specific runtime environments.
So, if you expect to build multiple images that require, for example, php runtime software, you can use provide a more consistent platform for those images by starting with a PHP software collections image.
Here are a few examples of AppStream container images built on UBI base images, that are available from the Red Hat Registry (registry.access.redhat.com or registry.redhat.io):
- ubi8/php-72: PHP 7.2 platform for building and running applications
- ubi8/nodejs-10: Node.js 10 platform for building and running applications. Used by Node.js 10 Source-To-Image builds
- ubi8/ruby25: Ruby 2.5 platform for building and running applications
- ubi8/python-27: Python 2.7 platform for building and running applications
- ubi8/python-36: Python 3.6 platform for building and running applications
- ubi8/s2i-core: Base image with essential libraries and tools used as a base for builder images like perl, python, ruby, and so on
- ubi8/s2i-base: Base image for Source-to-Image builds
Because these UBI images contain the same basic software as their legacy image counterparts, you can learn about those images from the Using Red Hat Software Collections Container Images guide. Be sure to use the UBI image names to pull those images.
RHEL 8 AppStream container images are updated every time RHEL 8 base images are updated. For RHEL 7, these same images (referred to as Red Hat Software Collections images) are updated on a schedule that is separate from RHEL base image updates (as are related images for Dotnet and DevTools). Search the Red Hat Container Catalog for details on any of these images. For more information on update schedules, see Red Hat Container Image Updates.
4.7.7. Getting UBI Container Image Source Code
You can download the source code for all UBI base images (excluding the minimal images) by starting up those images with a bash shell and running the following set of commands from inside that container:
# yum install yum-utils -y # for i in `rpm -qa` do yumdownloader --source $i done
The source code RPM for each binary RPM package is downloaded to the current directory. Because the UBI minimal images include a subset of RPMs from the regular UBI images, running the yumdownloader
loop just shown will get you the minimal image packages as well.
4.7.8. Tips and tricks for using UBI images
Here are a few issues to consider when working with UBI images:
- Hundreds of RPM packages used in existing Red Hat Software Collections runtime images are stored in the yum repositories packaged with the new UBI images. Feel free to install those RPMs on your UBI images to emulate the runtime (python, php, nodejs, etc.) that interests you.
-
Because some language files and documentation have been stripped out of the minimal UBI image (
ubi8/ubi-minimal
), runningrpm -Va
inside that container will show the contents of many packages as being missing or modified. If having a complete list of files inside that container is important to you, consider using a tool such asTripwire
to record the files in the container and check it later. -
After a layered image has been created, use
podman history
to check which UBI image it was built on. For example, after completing the webserver example shown earlier, typepodman history johndoe/webserver
to see that the image it was built on includes the image ID of the UBI image you added on the FROM line of the Dockerfile.
4.7.9. How to request new features in UBI?
Red Hat partners and customers can request new features, including package requests, by filing a support ticket through standard methods. Non-Red Hat customers do not receive support, but can file requests through the standard Red Hat Bugzilla for the appropriate RHEL product. See also: Red Hat Bugzilla Queue
4.7.10. How to file a support case for UBI?
Red Hat partners and customers can file support tickets through standard methods when running UBI on a supported Red Hat platform (OpenShift/RHEL). Red Hat support staff will guide partners and customers
See also: Open a Support Case
Chapter 5. Running special container images
Once you become familiar with common ways of working with containers and container images, use this section to learn about some special types of container images you might find useful. These include:
-
Toolbox: Instead of burdening a host system by installing tools needed to debug problems or monitor features, you can run the
toolbox
command. Toolbox starts asupport-tools
container image that holds tools you can use to run reports or diagnose problems on the host. -
Runlabels: Some container images have labels built in that allow you to run those containers with preset options and arguments. When you do a
podman run
with a runlabel, the result can be a prescriptive set of features when you install, run, remove, or upgrade a container image.
5.1. Troubleshooting container hosts with toolbox
Instead of installing troubleshooting tools directly to your Red Hat Enterprise Linux 8 system, the toolbox
utility offers a way to temporarily add those tools, then easily discard them when you are done. The toolbox
utility works by:
-
Pulling the
registry.redhat.io/rhel8/support-tools
image to your local system. - Starting up a container from the image, then running a shell inside the container from which you can access the host system.
The support-tools
container allows you to:
-
Run commands that may not be installed on the host system, such as
sosreport
,strace
, ortcpdump
, in a way that lets them act on the host system. - Install more software inside the container to use on the host system.
- Discard the container when you are done.
The following illustrates a typical toolbox
session.
Procedure
Install the toolbox and podman packages, if you haven’t already. A good way to do that is to install the full set of container tools:
# yum module install container-tools -y
Run the toolbox command to pull and run the
support-tools
image (inserting your Red Hat Customer Portal credentials when prompted):# toolbox Trying to pull registry.redhat.io/rhel8/support-tools... ... Would you like to authenticate to registry: 'registry.redhat.io' and try again? [y/N] y Username: johndoe Password: ************* Login Succeeded! Trying to pull registry.redhat.io/rhel8/support-tools...Getting image source signatures ... Storing signatures 30e261462851238d38f4ef2afdaf55f1f8187775c5ca373b43e0f55722faaf97 Spawning a container 'toolbox-root' with image 'registry.redhat.io/rhel8/support-tools' Detected RUN label in the container image. Using that as the default... command: podman run -it --name toolbox-root --privileged --ipc=host --net=host --pid=host -e HOST=/host -e NAME=toolbox-root -e IMAGE=registry.redhat.io/rhel8/support-tools:latest -v /run:/run -v /var/log:/var/log -v /etc/machine-id:/etc/machine-id -v /etc/localtime:/etc/localtime -v /:/host registry.redhat.io/rhel8/support-tools:latest bash-4.4#
A bash shell opens, ready for you to run commands inside the container.
From inside the container, the root file system on the host is available from the
/host
directory. The other directories shown are all inside the container.# ls / bin dev home lib lost+found mnt proc run srv tmp var boot etc host lib64 media opt root sbin sys usr
From inside the container, you can try a command. For example, you can run
sosreport
to generate information about your system to send to Red Hat support:bash-4.4# sosreport sosreport (version 3.6) This command will collect diagnostic and configuration information from this Red Hat Enterprise Linux system and installed applications. An archive containing the collected information will be generated in /host/var/tmp/sos.u82evisb and may be provided to a Red Hat support representative. ... Press ENTER to continue, or CTRL-C to quit. <Press ENTER> ... Your sosreport has been generated and saved in: /host/var/tmp/sosreport-rhel81beta-12345678-2019-10-29-pmgjncg.tar.xz The checksum is: c4e1fd3ee45f78a17afb4e45a05842ed Please send this file to your support representative.
Notice that
sosreport
is aware that you are in a container. As a result it knows to run on the host and save the report to the host (/host/var/tmp/sosreport-…
).Install a software package inside the container, to add tools that are not already in the container. For example, to get a core dump of a running process on the host, install the
procps
andgcore
packages, useps
to get the process ID of a running daemon, then usegcore
to get a core dump:bash-4.4# yum install procps gdb -y bash-4.4# ps -ef | grep chronyd 994 809 1 0 Oct28 ? 00:00:00 /usr/sbin/chronyd bash-4.4# gcore -o /host/tmp/chronyd.core 809 Missing separate debuginfo for target:/usr/sbin/chronyd Try: dnf --enablerepo='*debug*' install /usr/lib/debug/.build-id/96/0789a8a3bf28932b093e94b816be379f16a56a.debug ... Saved corefile /host/tmp/chronyd.core.809 [Inferior 1 (process 809) detached] # exit
Once you type
exit
, you leave the container and return to the host. You can see that the file saved to/host/tmp/chronyd.core.809
is available from/tmp/chronyd.core.809
on the host.
At this point, the container is no longer running, but still exists on the system. You can choose to:
-
Start up the container again: Type
toolbox
again to restart the container (namedtoolbox-root
). Any software additions or changes made previously to the container are maintained. -
Start with a fresh container: To get rid of the old container, type
podman rm toolbox-root
. Then runtoolbox
again to start with a freshsupport-tools
container. Start with different values: You can change the registry, image, or container name used by
toolbox
by adding the following values to your host/root/.toolboxrc
file:-
REGISTRY: Change the registry from which the toolbox image is pulled. For example:
REGISTRY=registry.example.com
-
IMAGE: Change the image that is used. For example,
IMAGE=mysupport-tools
-
TOOLBOX_NAME: Change the name assigned to the running container. For example,
TOOLBOX_NAME=mytoolbox
-
REGISTRY: Change the registry from which the toolbox image is pulled. For example:
The next time you run toolbox
, the new values from the .toolboxrc
file are used.
5.1.1. Opening privileges to the host
When you run other commands from within the support-tools container (or any privileged container), they can behave differently then when run in a non-privileged container. Although sosreport
can tell when it is running in a container, other commands would have to specifically be told to act on the host system (the /host
directory). Here are examples of features that may or not be open to the host from a container:
-
Privileges: A privileged container (
--privileged
) runs applications as root user on the host by default. The container has this ability because it runs with anunconfined_t
SELinux security context. So you would be able to, for example, delete files and directories mounted from the host that were owned by the root user. -
Process tables: Unlike a regular container, that only sees the processes running inside the container, running a
ps -e
command within a privileged container (with--pid=host
set) lets you see every process running on the host. So, you can pass a process ID from the host to commands that run in the privileged container (for example,kill <PID>
). With some commands, however, permissions issues could occur when they try to access processes from the container. -
Network interfaces: By default, a container has only one external network interface and one loopback network interface. With network interfaces open to the host (
--net=host
), you can access those network interfaces directly from within the container. -
Inter-process communications: The IPC facility on the host is accessible from within the privileged container. So, you can run commands such as
ipcs
to see information about active message queues, shared memory segments, and semaphone sets on the host.
5.2. Running containers with runlabels
Some Red Hat images include labels that provide pre-set command lines for working with those images. Using the podman container runlabel <label>
command, you can tell podman
to execute the command defined in that <label> for the image. Existing runlabels include:
- install: Sets up the host system before executing the image. Typically, this results in creating files and directories on the host that the container can access when it is run later.
- run: Identifies podman command line options to use when running the container. Typically, the options will open privileges on the host and mount the host content the container needs to remain permanently on the host.
- uninstall: Cleans up the host system after you are done running the container.
Red Hat images that have one or more runlabels include the rsyslog and support-tools images. The following procedure illustrates how to use those images.
5.2.1. Running rsyslog with runlabels
The rhel8/rsyslog container image is made to run a containerized version of the rsyslogd daemon. Inside the rsyslog image are install
, run
and uninstall
runlabels. The following procedure steps you through installing, running, and uninstalling the rsyslog image:
Procedure
Pull the rsyslog image:
# podman pull registry.redhat.io/rhel8/rsyslog
Display (but do not yet run) the
install
runlabel for rsyslog:# podman container runlabel install --display rhel8/rsyslog command: podman run --rm --privileged -v /:/host -e HOST=/host -e IMAGE=registry.redhat.io/rhel8/rsyslog:latest -e NAME=rsyslog registry.redhat.io/rhel8/rsyslog:latest /bin/install.sh
This shows that the command will open privileges to the host, mount the host root filesystem on
/host
in the container, and run aninstall.sh
script.Run the
install
runlabel for rsyslog:# podman container runlabel install rhel8/rsyslog command: podman run --rm --privileged -v /:/host -e HOST=/host -e IMAGE=registry.redhat.io/rhel8/rsyslog:latest -e NAME=rsyslog registry.redhat.io/rhel8/rsyslog:latest /bin/install.sh Creating directory at /host//etc/pki/rsyslog Creating directory at /host//etc/rsyslog.d Installing file at /host//etc/rsyslog.conf Installing file at /host//etc/sysconfig/rsyslog Installing file at /host//etc/logrotate.d/syslog
This creates files on the host system that the rsyslog image will use later.
Display the
run
runlabel for rsyslog:# podman container runlabel run --display rhel8/rsyslog command: podman run -d --privileged --name rsyslog --net=host --pid=host -v /etc/pki/rsyslog:/etc/pki/rsyslog -v /etc/rsyslog.conf:/etc/rsyslog.conf -v /etc/sysconfig/rsyslog:/etc/sysconfig/rsyslog -v /etc/rsyslog.d:/etc/rsyslog.d -v /var/log:/var/log -v /var/lib/rsyslog:/var/lib/rsyslog -v /run:/run -v /etc/machine-id:/etc/machine-id -v /etc/localtime:/etc/localtime -e IMAGE=registry.redhat.io/rhel8/rsyslog:latest -e NAME=rsyslog --restart=always registry.redhat.io/rhel8/rsyslog:latest /bin/rsyslog.sh
This shows that the command opens privileges to the host and mount a bunch of files and directories from the host inside the container, when it launches the rsyslog container to run the rsyslogd daemon.
Execute the
run
runlabel for rsyslog:# podman container runlabel run rhel8/rsyslog command: podman run -d --privileged --name rsyslog --net=host --pid=host -v /etc/pki/rsyslog:/etc/pki/rsyslog -v /etc/rsyslog.conf:/etc/rsyslog.conf -v /etc/sysconfig/rsyslog:/etc/sysconfig/rsyslog -v /etc/rsyslog.d:/etc/rsyslog.d -v /var/log:/var/log -v /var/lib/rsyslog:/var/lib/rsyslog -v /run:/run -v /etc/machine-id:/etc/machine-id -v /etc/localtime:/etc/localtime -e IMAGE=registry.redhat.io/rhel8/rsyslog:latest -e NAME=rsyslog --restart=always registry.redhat.io/rhel8/rsyslog:latest /bin/rsyslog.sh 28a0d719ff179adcea81eb63cc90fcd09f1755d5edb121399068a4ea59bd0f53
The rsyslog container opens privileges, mounts what it needs from the host, and runs the rsyslogd daemon in the background (-d). The rsyslogd daemon begins gathering log messages and directing messages to files in the
/var/log
directory.Display the
uninstall
runlabel for rsyslog:# podman container runlabel uninstall --display rhel8/rsyslog command: podman run --rm --privileged -v /:/host -e HOST=/host -e IMAGE=registry.redhat.io/rhel8/rsyslog:latest -e NAME=rsyslog registry.redhat.io/rhel8/rsyslog:latest /bin/uninstall.sh
Run the
uninstall
runlabel for rsyslog:# podman container runlabel uninstall rhel8/rsyslog command: podman run --rm --privileged -v /:/host -e HOST=/host -e IMAGE=registry.redhat.io/rhel8/rsyslog:latest -e NAME=rsyslog registry.redhat.io/rhel8/rsyslog:latest /bin/uninstall.sh
In this case, the
uninstall.sh
script just removes the/etc/logrotate.d/syslog
file. Note that it doesn’t clean up the configuration files.
5.2.2. Running support-tools with runlabels
The rhel8/support-tools container image is made to run tools such as sosreport and sos-collector to help you analyze your host system. To simplify running the support-tools image, it includes a run
runlabel. The following procedure describes how to run the support-tools image:
Procedure
Pull the support-tools image:
# podman pull registry.redhat.io/rhel8/support-tools
Display (but do not yet run) the
run
runlabel for support-tools:# podman container runlabel run --display rhel8/support-tools command: podman run -it --name support-tools --privileged --ipc=host --net=host --pid=host -e HOST=/host -e NAME=support-tools -e IMAGE=registry.redhat.io/rhel8/support-tools:latest -v /run:/run -v /var/log:/var/log -v /etc/machine-id:/etc/machine-id -v /etc/localtime:/etc/localtime -v /:/host registry.redhat.io/rhel8/support-tools:latest
This shows that the command mounts directories and opens privileges and namespaces (ipc, net, and pid) to the host system. It assigns the host’s root file system to the
/host
directory in the container.Execute the
run
runlabel for support-tools:# podman container runlabel run rhel8/support-tools command: podman run -it --name support-tools --privileged --ipc=host --net=host --pid=host -e HOST=/host -e NAME=support-tools -e IMAGE=registry.redhat.io/rhel8/support-tools:latest -v /run:/run -v /var/log:/var/log -v /etc/machine-id:/etc/machine-id -v /etc/localtime:/etc/localtime -v /:/host registry.redhat.io/rhel8/support-tools:latest bash-4.4#
This opens a bash shell inside the support-tools container.
You can now run reports or debug tools against the host system (/host
). When you are done, type exit
to exit the shell and stop the container.
Chapter 6. Building container images with Buildah
The buildah
command lets you create container images from a working container, a Dockerfile, or from scratch. The resulting images are OCI compliant, so they will work on any container runtime that meets the OCI Runtime Specification (such as Docker and CRI-O).
This section describes how to use the buildah
command to create and otherwise work with containers and container images.
6.1. Understanding Buildah
Using Buildah is different from building images with the docker
command in the following ways:
- No Daemon!: Bypasses the Docker daemon! So no container runtime (Docker, CRI-O, or other) is needed to use Buildah.
- Base image or scratch: Lets you not only build an image based on another container, but also lets you start with an empty image (scratch).
Build tools external: Doesn’t include build tools within the image itself. As a result, Buildah:
- Reduces the size of images you build
- Makes the image more secure by not having the software used to build the container (like gcc, make, and dnf) within the resulting image.
- Creates images that require fewer resources to transport the images (because they are smaller).
Buildah is able to operate without Docker or other container runtimes by storing data separately and by including features that let you not only build images, but run those images as containers as well. By default, Buildah stores images in an area identified as containers-storage
(/var/lib/containers).
The containers-storage location that the buildah
command uses by default is the same place that the CRI-O container engine uses for storing local copies of images. So images pulled from a registry by either CRI-O or Buildah, or committed by the buildah
command, will be stored in the same directory structure. Currently, however, CRI-O and Buildah cannot share containers, though they can share images.
There are more than a dozen options to use with the buildah
command. Some of the main activities you can do with the buildah
command include:
-
Build a container from a Dockerfile: Use a Dockerfile to build a new container image (
buildah bud
). -
Build a container from another image or scratch: Build a new container, starting with an existing base image (
buildah from <imagename>
) or from scratch (buildah from scratch
) -
Inspecting a container or image: View metadata associated with the container or image (
buildah inspect
) -
Mount a container: Mount a container’s root filesystem to add or change content (
buildah mount
). -
Create a new container layer: Use the updated contents of a container’s root filesystem as a filesystem layer to commit content to a new image (
buildah commit
). -
Unmount a container: Unmount a mounted container (
buildah umount
). -
Delete a container or an image: Remove a container (
buildah rm
) or a container image (buildah rmi
).
For more details on Buildah, see the GitHub Buildah page. The GitHub Buildah site includes man pages and software that might be more recent than is available with the RHEL version. Here are some other articles on Buildah that might interest you:
6.1.1. Installing Buildah
The buildah package is available with the container-tools module in RHEL 8 (yum module install container-tools
). You can install the buildah package separately by typing:
# yum -y install buildah
With the buildah package installed, you can refer to the man pages included with the buildah package for details on how to use it. To see the available man pages and other documentation, then open a man page, type:
# rpm -qd buildah # man buildah buildah(1) General Commands Manual buildah(1) NAME Buildah - A command line tool that facilitates building OCI container images. ...
The following sections describe how to use buildah
to get containers, build a container from a Dockerfile, build one from scratch, and manage containers in various ways.
6.2. Getting images with Buildah
To get a container image to use with buildah
, use the buildah from
command. Here’s how to get a RHEL 8 image from the Red Hat Registry as a working container to use with the buildah
command:
# buildah from registry.redhat.io/ubi8/ubi Getting image source signatures Copying blob… Writing manifest to image destination Storing signatures ubi-working-container # buildah images IMAGE ID IMAGE NAME CREATED AT SIZE 3da40a1670b5 registry.redhat.io/ubi8/ubi:latest May 8, 2019 21:55 214 MB # buildah containers CONTAINER ID BUILDER IMAGE ID IMAGE NAME CONTAINER NAME c6c9279ecc0f * 3da40a1670b5 ...ubi8/ubi:latest ubi-working-container
Notice that the result of the buildah from
command is an image (registry.redhat.io/ubi8/ubi:latest) and a working container that is ready to run from that image (ubi-working-container). Here’s an example of how to execute a command from that container:
# podman run ubi-working-container cat /etc/redhat-release Red Hat Enterprise Linux release 8.0
The image and container are now ready for use with Buildah.
6.3. Building an image from a Dockerfile with Buildah
With the buildah
command, you can create a new image from a Dockerfile. The following steps show how to build an image that includes a simple script that is executed when the image is run.
This simple example starts with two files in the current directory: Dockerfile (which holds the instructions for building the container image) and myecho (a script that echoes a few words to the screen):
# ls Dockerfile myecho # cat Dockerfile FROM registry.redhat.io/ubi8/ubi ADD myecho /usr/local/bin ENTRYPOINT "/usr/local/bin/myecho" # cat myecho echo "This container works!" # chmod 755 myecho # ./myecho This container works!
With the Dockerfile in the current directory, build the new container as follows:
# buildah bud -t myecho . STEP 1: FROM registry.redhat.io/ubi8/ubi STEP 2: ADD myecho /usr/local/bin STEP 3: ENTRYPOINT "/usr/local/bin/myecho"
The buildah bud
command creates a new image named myecho. To run see that new image, type:
# buildah images IMAGE NAME IMAGE TAG IMAGE ID CREATED AT SIZE localhost/myecho latest a3882af49784 Jun 21, 2019 12:21 216 MB
Next, you can run the image, to make sure it is working.
6.3.1. Running the image you built
To check that the image you built previously works, you can run the image using podman run
:
# podman run myecho This container works!
6.3.2. Inspecting a container with Buildah
With buildah inspect
, you can show information about a container or image. For example, to inspect the myecho
image you created earlier, type:
# buildah inspect myecho | less { "Type": "buildah 0.0.1", "FromImage": "docker.io/library/myecho:latest", "FromImage-ID": "e2b190ac8...", "Config": "{\"created\":\"2018-11-13... "Entrypoint": [ "/usr/local/bin/myecho" ], "WorkingDir": "/", "Labels": { "architecture": "x86_64", "authoritative-source-url": "registry.access.redhat.com", "build-date": "2018-09-19T20:46:28.459833",
To inspect a container from that same image, type the following:
# buildah inspect myecho-working-container | less { "Type": "buildah 0.0.1", "FromImage": "docker.io/library/myecho:latest", "FromImage-ID": "e2b190a...", "Config": "{\"created\":\"2018-11-13T19:5... ... "Container": "myecho-working-container", "ContainerID": "c0cd2e494d...", "MountPoint": "", "ProcessLabel": "system_u:system_r:svirt_lxc_net_t:s0:c89,c921", "MountLabel": "",
Note that the container output has added information, such as the container name, container id, process label, and mount label to what was in the image.
6.4. Modifying a container to create a new image with Buildah
There are several ways you can modify an existing container with the buildah
command and commit those changes to a new container image:
- Mount a container and copy files to it
-
Use
buildah copy
andbuildah config
to modify a container
Once you have modified the container, use buildah commit
to commit the changes to a new image.
6.4.1. Using buildah mount
to modify a container
After getting an image with buildah from
, you can use that image as the basis for a new image. The following text shows how to create a new image by mounting a working container, adding files to that container, then committing the changes to a new image.
Type the following to view the working container you used earlier:
# buildah containers CONTAINER ID BUILDER IMAGE ID IMAGE NAME CONTAINER NAME dc8f21af4a47 * 1456eedf8101 registry.redhat.io/ubi8/ubi:latest ubi-working-container 6d1ffccb557d * ab230ac5aba3 docker.io/library/myecho:latest myecho-working-container
Mount the container image and set the mount point to a variable ($mymount) to make it easier to deal with:
# mymount=$(buildah mount myecho-working-container) # echo $mymount /var/lib/containers/storage/devicemapper/mnt/176c273fe28c23e5319805a2c48559305a57a706cc7ae7bec7da4cd79edd3c02/rootfs
Add content to the script created earlier in the mounted container:
# echo 'echo "We even modified it."' >> $mymount/usr/local/bin/myecho
To commit the content you added to create a new image (named myecho), type the following:
# buildah commit myecho-working-container containers-storage:myecho2
To check that the new image includes your changes, create a working container and run it:
# buildah images IMAGE ID IMAGE NAME CREATED AT SIZE a7e06d3cd0e2 docker.io/library/myecho2:latest Oct 12, 2017 15:15 3.144 KB # buildah from docker.io/library/myecho2:latest myecho2-working-container # buildah run myecho2-working-container This container works! We even modified it.
You can see that the new echo
command added to the script displays the additional text.
When you are done, you can unmount the container:
# buildah umount myecho-working-container
6.4.2. Using buildah copy
and buildah config
to modify a container
With buildah copy
, you can copy files to a container without mounting it first. Here’s an example, using the myecho-working-container
created (and unmounted) in the previous section, to copy a new script to the container and change the container’s configuration to run that script by default.
Create a script called newecho
and make it executable:
# cat newecho echo "I changed this container" # chmod 755 newecho
Create a new working container:
# buildah from myecho:latest myecho-working-container-2
Copy newecho
to /usr/local/bin inside the container:
# buildah copy myecho-working-container-2 newecho /usr/local/bin
Change the configuration to use the newecho
script as the new entrypoint:
# buildah config --entrypoint "/bin/sh -c /usr/local/bin/newecho "myecho-working-container-2
Run the new container, which should result in the newecho
command being executed:
# buildah run myecho-working-container-2 I changed this container
If the container behaved as you expected it would, you could then commit it to a new image (mynewecho):
# buildah commit myecho-working-container-2 containers-storage:mynewecho
6.5. Creating images from scratch with Buildah
Instead of starting with a base image, you can create a new container that holds no content and only a small amount of container metadata. This is referred to as a scratch
container. Here are a few issues to consider when choosing to create an image starting from a scratch container with the buildah
command:
- With a scratch container, you can simply copy executables that have no dependencies to the scratch image and make a few configuration settings to get a minimal container to work.
-
To use tools like
yum
orrpm
packages to populate the scratch container, you need to at least initialize an RPM database in the container and add a release package. The example below shows how to do that. -
If you end up adding a lot of RPM packages, consider using the
rhel
orrhel-minimal
base images instead of a scratch image. Those base images have had documentation, language packs, and other components trimmed out, which can ultimately result in your image being smaller.
This example adds a Web service (httpd) to a container and configures it to run. In the example, instead of committing the image to Buildah (containers-storage which stores locally in /var/lib/containers), we illustrate how to commit the image so it can be managed by the local Docker service (docker-daemon which stores locally in /var/lib/docker). You could just have easily committed it to Buildah, which would let you then push it to a Docker service (docker), a local OSTree repository (ostree), or other OCI-compliant storage (oci). (Type man buildah push
for details.)
To begin, create a scratch container:
# buildah from scratch working-container
This creates just an empty container (no image) that you can mount as follows:
# scratchmnt=$(buildah mount working-container) # echo $scratchmnt /var/lib/containers/storage/devicemapper/mnt/cc92011e9a2b077d03a97c0809f1f3e7fef0f29bdc6ab5e86b85430ec77b2bf6/rootfs
Initialize an RPM database within the scratch image and add the redhat-release package (which includes other files needed for RPMs to work):
# dnf install -y --releasever=8 --installroot=$scratchmnt redhat-release
Install the httpd service to the scratch directory:
# dnf install -y --setopt=reposdir=/etc/yum.repos.d \ --installroot=$scratchmnt \ --setopt=cachedir=/var/cache/dnf httpd
Add some text to an index.html file in the container, so you will be able to test it later:
# echo "Your httpd container from scratch worked." > $scratchmnt/var/www/html/index.html
Instead of running httpd as an init service, set a few buildah config
options to run the httpd daemon directly from the container:
# buildah config --cmd "/usr/sbin/httpd -DFOREGROUND" working-container # buildah config --port 80/tcp working-container # buildah commit working-container docker-daemon:myhttpd:latest
By default, the buildah commit
command adds the docker.io repository name to the image name and copies the image to the storage area for your local Docker service (/var/lib/docker). For now, you can use the Image ID to run the new image as a container with the docker
command:
# docker images REPOSITORY TAG IMAGE ID CREATED SIZE docker.io/myhttpd latest 47c0795d7b0e 9 minutes ago 665.6 MB # docker run -p 8080:80 -d --name httpd-server 47c0795d7b0e # curl localhost:8080 Your httpd container from scratch worked.
6.6. Removing images or containers with Buildah
When you are done with particular containers or images, you can remove them with buildah rm
or buildah rmi
, respectively. Here are some examples.
To remove the container created in the previous section, you could type the following to see the mounted container, unmount it and remove it:
# buildah containers CONTAINER ID BUILDER IMAGE ID IMAGE NAME CONTAINER NAME 05387e29ab93 * c37e14066ac7 docker.io/library/myecho:latest myecho-working-container # buildah mount 05387e29ab93 /var/lib/containers/storage/devicemapper/mnt/9274181773a.../rootfs # buildah umount 05387e29ab93 # buildah rm 05387e29ab93 05387e29ab93151cf52e9c85c573f3e8ab64af1592b1ff9315db8a10a77d7c22
To remove the image you created previously, you could type the following:
# buildah rmi docker.io/library/myecho:latest untagged: docker.io/library/myecho:latest ab230ac5aba3b5a0a7c3d2c5e0793280c1a1b4d2457a75a01b70a4b7a9ed415a
6.7. Using container registries with Buildah
With Buildah, you can push and pull container images between your local system and public or private container registries. The following examples show how to:
- Push containers to and pull them from a private registry with buildah.
- Push and pull container between your local system and the Docker Registry.
- Use credentials to associate your containers with a registry account when you push them.
Use the skopeo command, in tandem with the buildah
command, to query registries for information about container images.
6.7.1. Pushing containers to a private registry
Pushing containers to a private container registry with the buildah
command works much the same as pushing containers with the docker
command. You need to:
- Set up a private registry (OpenShift provides a container registry or you can set up a Red Hat Quay container registry).
- Create or acquire the container image you want to push.
-
Use
buildah push
to push the image to the registry.
To push an image from your local Buildah container storage, check the image name, then push it using the buildah push
command. Remember to identify both the local image name and a new name that includes the location. For example, a registry running on the local system that is listening on TCP port 5000 would be identified as localhost:5000.
# buildah images IMAGE ID IMAGE NAME CREATED AT SIZE cb702d492ee9 docker.io/library/myecho2:latest Nov 12, 2018 16:50 3.143 KB # buildah push --tls-verify=false myecho2:latest localhost:5000/myecho2:latest Getting image source signatures Copying blob sha256:e4efd0... ... Writing manifest to image destination Storing signatures
Use the curl
command to list the images in the registry and skopeo
to inspect metadata about the image:
# curl http://localhost:5000/v2/_catalog {"repositories":["myatomic","myecho2"]} # curl http://localhost:5000/v2/myecho2/tags/list {"name":"myecho2","tags":["latest"]} # skopeo inspect --tls-verify=false docker://localhost:5000/myecho2:latest | less { "Name": "localhost:5000/myecho2", "Digest": "sha256:8999ff6050...", "RepoTags": [ "latest" ], "Created": "2017-11-21T16:50:25.830343Z", "DockerVersion": "", "Labels": { "architecture": "x86_64", "authoritative-source-url": "registry.redhat.io",
At this point, any tool that can pull container images from a container registry can get a copy of your pushed image. For example, on a RHEL 7 system you could start the docker daemon and try to pull the image so it can be used by the docker
command as follows:
# systemctl start docker # docker pull localhost:5000/myecho2 # docker run localhost:5000/myecho2 This container works!
6.7.2. Pushing containers to the Docker Hub
You can use your Docker Hub credentials to push and pull images from the Docker Hub with the buildah
command. For this example, replace the username and password (testaccountXX:My00P@sswd) with your own Docker Hub credentials:
# buildah push --creds testaccountXX:My00P@sswd \ docker.io/library/myecho2:latest docker://testaccountXX/myecho2:latest
As with the private registry, you can then get and run the container from the Docker Hub with the podman
, buildah
or docker
command:
# podman run docker.io/textaccountXX/myecho2:latest This container works! # buildah from docker.io/textaccountXX/myecho2:latest myecho2-working-container-2 # podman run myecho2-working-container-2 This container works!
Chapter 7. Running containers as systemd services with Podman
Podman (Pod Manager) is a fully featured container engine that is a simple daemonless tool. Podman provides a Docker-CLI comparable command line that eases the transition from other container engines and allows the management of pods, containers and images. It was not originally designed to bring up an entire Linux system or manage services for such things as start-up order, dependency checking, and failed service recovery. That is the job of a full-blown initialization system like systemd.
Red Hat has become a leader in integrating containers with systemd, so that OCI and Docker-formatted containers built by Podman can be managed in the same way that other services and features are managed in a Linux system. This chapter describes how you can use the systemd initialization service to work with containers in two different ways:
- Starting Containers with systemd: By setting up a systemd unit file on your host computer, you can have the host automatically start, stop, check the status, and otherwise manage a container as a systemd service.
- Starting services within a container using systemd: Many Linux services (Web servers, file servers, database servers, and so on) are already packaged for Red Hat Enterprise Linux to run as systemd services. If you are using the latest RHEL container image, you can set the RHEL container image to start the systemd service, then automatically start selected services within the container when the container starts up.
The following two sections describe how to use systemd container in those ways.
7.1. Starting containers with systemd
When you set up a container to start as a systemd service, you can define the order in which the containerized service runs, check for dependencies (like making sure another service is running, a file is available or a resource is mounted), and even have a container start by using the runc command.
This section provides an example of a container that is configured to run directly on a RHEL system as a systemd service.
Get the image you want to run on your system. For example, to use the redis service from docker.io, run the following command:
# podman pull docker.io/redis
Run the image as a container, giving it a name you want to use in the systemd service file. For example, to name the running redis container redis_server, type the following:
# podman run -d --name redis_server -p 6379:6379 redis
Configure the container as a systemd service by creating the unit configuration file in the
/etc/systemd/system/
directory. For example, the contents of the/etc/systemd/system/redis-container.service
can look as follows (note that redis_server matches the name you set on the podman run line):[Unit] Description=Redis container [Service] Restart=always ExecStart=/usr/bin/podman start -a redis_server ExecStop=/usr/bin/podman stop -t 2 redis_server [Install] WantedBy=local.target
After creating the unit file, to start the container automatically at boot time, type the following:
# systemctl enable redis-container.service
Once the service is enabled, it will start at boot time. To start it immediately and check the status of the service, type the following:
# systemctl start redis-container.service # systemctl status redis-container.service * redis-container.service - Redis container Loaded: loaded (/etc/systemd/system/redis-container.service; enabled; vendor preset: disabled) Active: active (running) since Fri 2019-03-15 16:22:55 EDT; 6s ago Main PID: 1540 (podman) Tasks: 8 (limit: 2353) Memory: 7.7M CGroup: /system.slice/redis-container.service └─1540 /usr/bin/podman start -a redis_server Mar 15 16:22:55 localhost.localdomain systemd[1]: Started Redis container.
To learn more about configuring services with systemd, refer to the System Administrator’s Guide chapter called Managing Services with systemd.
7.2. Starting services within a container using systemd
A package with the systemd initialization system is included in the official Red Hat Enterprise Linux Init base image named registry.access.redhat.com/ubi8/ubi-init This means that applications created to be managed with systemd can be started and managed inside a container. A container running systemd will:
- Start the /sbin/init process (the systemd service) to run as PID 1 within the container.
- Start all systemd services that are installed and enabled within the container, in order of dependencies.
- Allow systemd to restart services or kill zombie processes for services started within the container.
The general steps for building a container that is ready to be used as a systemd services is:
- Install the package containing the systemd-enabled service inside the container. This can include dozens of services that come with RHEL, such as Apache Web Server (httpd), FTP server (vsftpd), Proxy server (squid), and many others. For this example, we simply install an Apache (httpd) Web server.
- Use the systemctl command to enable the service inside the container.
- Add data for the service to use in the container (in this example, we add a Web server test page). For a real deployment, you would probably connect to outside storage.
- Expose any ports needed to access the service.
In this example, we build a container by creating a Dockerfile that installs and configures a Web server (httpd) to start automatically by the systemd service (/sbin/init) when the container is run on a host system.
Create Dockerfile: In a separate directory, create a file named Dockerfile with the following contents:
FROM registry.access.redhat.com/ubi8/ubi-init RUN yum -y install httpd; yum clean all; systemctl enable httpd; RUN echo "Successful Web Server Test" > /var/www/html/index.html RUN mkdir /etc/systemd/system/httpd.service.d/; echo -e '[Service]\nRestart=always' > /etc/systemd/system/httpd.service.d/httpd.conf EXPOSE 80
The Dockerfile installs the httpd package, enables the httpd service to start at boot time (i.e. when the container starts), creates a test file (index.html), exposes the Web server to the host (port 80), and starts the systemd init service (/sbin/init) when the container starts.
Build the container: From the directory containing the Dockerfile, type the following:
# podman build --format=docker -t mysysd .
Open Selinux permission. If SELinux is enabled on your system, you must turn on the
container_manage_cgroup
boolean to run containers with systemd as shown here (see the Containers running systemd solution for details):# setsebool -P container_manage_cgroup 1
Run the container: Once the container is built and named mysysd, type the following to run the container:
# podman run -d --name=mysysd_run -p 80:80 mysysd
From this command, the mysysd image runs as the mysysd_run container as a daemon process, with port 80 from the container exposed to port 80 on the host system.
Check that the container is running: To make sure that the container is running and that the service is working, type the following commands:
# podman ps | grep mysysd_run a282b0c2ad3d localhost/mysysd:latest /sbin/init 15 seconds ago Up 14 seconds ago 0.0.0.0:80->80/tcp mysysd_run # curl localhost/index.html Successful Web Server Test
At this point, you have a container that starts up a Web server as a systemd service inside the container. Install and run any services you like in this same way by modifying the Dockerfile and configuring data and opening ports as appropriate.
Chapter 8. Container command-line reference
8.1. podman
The podman
command (which stands for Pod Manager) lets you run containers as standalone entities, without requiring that Kubernetes, the Docker runtime, or any other container runtime be involved. It is a tool that can act as a replacement for the docker
command, implementing the same command-line syntax, while it adds even more container management features. The podman
features include:
-
Based on docker interface: Because
podman
syntax mirrors thedocker
command, transitioning topodman
should be easy for those familiar withdocker
. Managing containers and images: Both Docker- and OCI-compatible container images can be used with
podman
to:- Run, stop and restart containers
- Create and manage container images (push, commit, configure, build, and so on)
-
Managing pods: Besides running individual containers,
podman
can run a set of containers grouped in a pod. A pod is the smallest container unit that Kubernetes manages. -
Working with no runtime: No runtime environment is used by
podman
to work with containers.
Here are a few implementation features of podman
you should know about:
-
Podman, Buildah, and the CRI-O container engine all use the same back-end store directory,
/var/lib/containers
, instead of using the Docker storage location (/var/lib/docker
), by default. - Although Podman, Buildah, and CRI-O share the same storage directory, they cannot interact with each other’s containers. Those tools can share images, however. Eventually those features will be able to share containers.
-
The
podman
command, like thedocker
command, can build container images from a Dockerfile. -
The
podman
command can be a useful troubleshooting tool when theCRI-O
service is unavailable. -
Options to the
docker
command that are not supported bypodman
include network, node, plugin (podman
does not support plugins), rename (use rm and create to rename containers withpodman
), secret, service, stack, and swarm (podman
does not support Docker Swarm). The container and image options are used to run subcommands that are used directly inpodman
. -
To interact programmatically with
podman
, a remote API for Podman is available using a technology called varlink. This will letpodman
listen for API requests from remote tools (such as the RHEL 8 web console or theatomic
command) and respond to them.
8.1.1. Using podman commands
If you are used to using the docker
command to work with containers, you will find most of the features and options match those of podman
. Table 1 shows a list of commands you can use with podman
(type podman -h
to see this list):
Table 8.1. Commands supported by podman
podman command | Description | podman command | Description |
attach | Attach to a running container | commit | Create new image from changed container |
build | Build an image using Dockerfile instructions | create | Create, but do not start, a container |
diff | Inspect changes on container’s filesystems | exec | Run a process in a running container |
export | Export container’s filesystem contents as a tar archive | help, h | Shows a list of commands or help for one command |
history | Show history of a specified image | images | List images in local storage |
import | Import a tarball to create a filesystem image | info | Display system information |
inspect | Display the configuration of a container or image | kill | Send a specific signal to one or more running containers |
load | Load an image from an archive | login | Login to a container registry |
logout | Logout of a container registry | logs | Fetch the logs of a container |
mount | Mount a working container’s root filesystem | pause | Pauses all the processes in one or more containers |
ps | List containers | port | List port mappings or a specific mapping for the container |
pull | Pull an image from a registry | push | Push an image to a specified destination |
restart | Restart one or more containers | rm |
Remove one or more containers from host. Add |
rmi | removes one or more images from local storage | run | run a command in a new container |
save | Save image to an archive | search | search registry for image |
start | Start one or more containers | stats | Display percentage of CPU, memory, network I/O, block I/O and PIDs for one or more containers |
stop | Stop one or more containers | tag | Add an additional name to a local image |
top | Display the running processes of a container | umount, unmount | Unmount a working container’s root filesystem |
unpause | Unpause the processes in one or more containers | version | Display podman version information |
wait | Block on one or more containers |
8.1.2. Trying basic podman commands
Because the use of podman
mirrors the features and syntax of the docker
command, you can refer to Working with Docker Formatted Container Images for examples of how to use those options to work with containers. Simply replace docker
with podman
in most cases. Here are some examples of using podman
.
8.1.3. Pull a container image to the local system
# podman pull registry.redhat.io/ubi8/ubi Trying to pull registry.redhat...Getting image source signatures Copying blob sha256:d1fe25896eb5cbcee... Writing manifest to image destination Storing signatures fd1ba0b398a82d56900bb798c...
8.1.4. List local container images
# podman images REPOSITORY TAG IMAGE ID CREATED SIZE registry.redhat.io/ubi8/ubi latest de9c26f23799 5 weeks ago 80.1MB registry.redhat.io/ubi8/ubi latest fd1ba0b398a8 5 weeks ago 211MB
8.1.5. Inspect a container image
# podman inspect registry.redhat.io/ubi8/ubi | less [ { "Id": "4bbd153adf8487a8a5114af0d6...", "Digest": "sha256:9999e735605c73f...", "RepoTags": [ "registry.access.redhat.com/ubi8/ubi:latest" ], "RepoDigests": [ "registry.access.redhat.com/ubi8/ubi/rhel@sha256:9999e7356...
8.1.6. Run a container image
Run a container that opens a shell inside the container:
# podman run -it registry.redhat.io/ubi8/ubi /bin/bash [root@8414218c04f9 /]# ps -ef UID PID PPID C STIME TTY TIME CMD root 1 0 0 13:48 pts/0 00:00:00 /bin/bash root 21 1 0 13:49 pts/0 00:00:00 ps -ef [root@8414218c04f9 /]# exit #
8.1.7. List containers that are running or have exited
# podman ps -a CONTAINER ID IMAGE COMMAND CREATED AT STATUS PORTS NAMES 440becd26893 registry.redhat.io/ubi8/ubi-minimal:latest /bin/bash 2018-05-10 09:02:52 -0400 EDT Exited (0) About an hour ago happy_hodgkin 8414218c04f9 registry.redhat.io/ubi8/ubi:latest /bin/bash 2018-05-10 09:48:07 -0400 EDT Exited (0) 14 minutes ago nostalgic_boyd
8.1.8. Remove a container or image
Remove a container by its container ID:
# podman rm 440becd26893
Remove a container image by its image ID or name (use -f to force):
# podman rmi registry.redhat.io/ubi8/ubi # podman rmi de9c26f23799 # podman rmi -f registry.redhat.io/ubi8/ubi:latest
8.1.9. Generate a Kube pod yaml or systemd unit file
Using the podman generate
command you can create a systemd unit file or a Kubernetes pod yaml file from a container or a pod. Here’s an example of generating a Kubernetes yaml file:
Start a containerized service that runs as a daemon process (mariadb, in this example):
# podman run -d -e MYSQL_USER=user -e MYSQL_PASSWORD=pass \ -e MYSQL_DATABASE=db -p 3306:3306 --name mymariadb rhscl/mariadb-102-rhel7
Get the container name or ID:
# podman ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES e421a3424ab0 registry.access.redhat.com/rhscl/mariadb-102-rhel7:latest container-entrypo... 19 seconds ago Up 16 seconds ago 0.0.0.0:3306->3306/tcp mymariadb
Use the container name or ID to generate the Kubernetes yaml and direct it into a file:
# podman generate kube mymariadb > mymariadbkube.yaml # less mymariadbkube.yaml apiVersion: v1 kind: Pod metadata: creationTimestamp: "2019-10-31T13:19:41Z" labels: app: mymariadb name: mymariadb spec: containers: - command: ... - name: MYSQL_VERSION value: "10.2" - name: CONTAINER_SCRIPTS_PATH value: /usr/share/container-scripts/mysql - name: MYSQL_PREFIX value: /opt/rh/rh-mariadb102/root/usr - name: ENABLED_COLLECTIONS value: rh-mariadb102 - name: DESCRIPTION value: MariaDB is a multi-user, multi-threaded SQL database server. The container image provides a containerized packaging of the MariaDB mysqld daemon and client application. The mysqld server daemon accepts connections from clients and provides access to content from MariaDB databases on behalf of the clients. - name: STI_SCRIPTS_PATH value: /usr/libexec/s2i - name: PROMPT_COMMAND value: . /usr/share/container-scripts/mysql/scl_enable - name: MYSQL_USER value: user - name: MYSQL_PASSWORD value: pass ...
Note that this tool does not handle volumes and PVs.
You can now use the yaml file to create a pod in your Kubernetes or OpenShift environment:
# kubectl create -f mymariadbkube.yaml
8.1.10. Build a container
# cat Dockerfile FROM registry.redhat.io/ubi8/ubi ENTRYPOINT "echo 'Podman built this container.'" # podman build -t podbuilt . STEP 1: FROM registry.access... ... Writing manifest to image destination Storing signatures 91e043c11617c08d4f8... # podman run podbuilt Podman build this container.
8.2. runc
"runC" is a lightweight, portable implementation of the Open Container Initiative (OCI) container runtime specification. runC unites a lot of the low-level features that make running containers possible. It shares a lot of low-level code with Docker but it is not dependent on any of the components of the Docker platform.
runc supports Linux namespaces, live migration, and has portable performance profiles. It also provides full support for Linux security features such as SELinux, control groups (cgroups), seccomp, and others. You can build and run images with runc, or you can run OCI-compatible images with runc.
8.2.1. Running containers with runc
With runc, containers are configured using bundles. A bundle for a container is a directory that includes a specification file named "config.json" and a root filesystem. The root filesystem contains the contents of the container.
To create a bundle, run:
$ runc spec
This command creates a config.json file that only contains a bare-bones structure that you will need to edit. Most importantly, you will need to change the "args" parameter to identify the executable to run. By default, "args" is set to "sh".
"args": [ "sh" ],
As an example, you can download the Red Hat Enterprise Linux base image (ubi8/ubi) using podman then export it, create a new bundle for it with runc, and edit the "config.json" file to point to that image. You can then create the container image and run an instance of that image with runc. Use the following commands:
# podman pull registry.redhat.io/ubi8/ubi # podman export $(podman create registry.redhat.io/ubi8/ubi) > rhel.tar # mkdir -p rhel-runc/rootfs # tar -C rhel-runc/rootfs -xf rhel.tar # runc spec -b rhel-runc # vi rhel-runc/config.json Change any setting you like # runc create -b rhel-runc/ rhel-container # runc start rhel-container sh-4.2#
In this example, the name of the container instance is "rhel-container". Running that container, by default, starts a shell, so you can begin looking around and running commands from inside that container. Type exit
when you are done.
The name of a container instance must be unique on the host. To start a new instance of a container:
# runc start <container_name>
You can provide the bundle directory using the "-b" option. By default, the value for the bundle is the current directory.
You will need root privileges to start containers with runc. To see all commands available to runc and their usage, run "runc --help".
8.3. skopeo
With the skopeo command, you can work with container images from registries without using the docker daemon or the docker
command. Registries can include the Docker Registry, your own local registries, Red Hat Quay or OpenShift registries. Activities you can do with skopeo include:
-
inspect
: The output of askopeo inspect
command is similar to what you see from adocker inspect
command: low-level information about the container image. That output can be in json format (default) or raw format (using the --raw option). -
copy
: Withskopeo copy
you can copy a container image from a registry to another registry or to a local directory. -
layers
: Theskopeo layers
command lets you download the layers associated with images so that they are stored as tarballs and associated manifest files in a local directory.
Like the buildah
command and other tools that rely on the containers/image library, the skopeo
command can work with images from container storage areas other than those associated with Docker. Available transports to other types of container storage include: containers-storage (for images stored by buildah
and CRI-O), ostree (for atomic and system containers), oci (for content stored in an OCI-compliant directory), and others. See the skopeo man page for details.
To try out skopeo, you could set up a local registry, then run the commands that follow to inspect, copy, and download image layers. If you want to follow along with the examples, start by doing the following:
- Install a local registry (such as Red Hat Quay. Container registry software available in the docker-distribution package for RHEL 7, is not available for RHEL 8.
-
Pull the latest RHEL image to your local system (
podman pull ubi8/ubi
). Retag the RHEL image and push it to your local registry as follows:
# podman tag ubi8/ubi localhost/myubi8 # podman push localhost/myubi8
The rest of this section describes how to inspect, copy and get layers from the RHEL image.
The skopeo
tool by default requires a TLS connection. It fails when trying to use an unencrypted connection. To override the default and use an http registry, prepend http:
to the <registry>/<image>
string.
8.3.1. Inspecting container images with skopeo
When you inspect a container image from a registry, you need to identify the container format (such as docker), the location of the registry (such as docker.io or localhost), and the repository/image (such as ubi8/ubi).
The following example inspects the mariadb container image from the Docker Registry:
# skopeo inspect docker://docker.io/library/mariadb { "Name": "docker.io/library/mariadb", "Tag": "latest", "Digest": "sha256:d3f56b143b62690b400ef42e876e628eb5e488d2d0d2a35d6438a4aa841d89c4", "RepoTags": [ "10.0.15", "10.0.16", "10.0.17", "10.0.19", ... "Created": "2018-06-10T01:53:48.812217692Z", "DockerVersion": "1.10.3", "Labels": {}, "Architecture": "amd64", "Os": "linux", "Layers": [ ...
Assuming you pushed a container image tagged localhost/myubi8
to a container registry running on your local system, the following command inspects that image:
# skopeo inspect docker://localhost/myubi8 { "Name": "localhost/myubi8", "Tag": "latest", "Digest": "sha256:4e09c308a9ddf56c0ff6e321d135136eb04152456f73786a16166ce7cba7c904", "RepoTags": [ "latest" ], "Created": "2018-06-16T17:27:13Z", "DockerVersion": "1.7.0", "Labels": { "Architecture": "x86_64", "Authoritative_Registry": "registry.access.redhat.com", "BZComponent": "rhel-server-docker", "Build_Host": "rcm-img01.build.eng.bos.redhat.com", "Name": "myubi8", "Release": "75", "Vendor": "Red Hat, Inc.", "Version": "8.0" }, "Architecture": "amd64", "Os": "linux", "Layers": [ "sha256:16dc1f96e3a1bb628be2e00518fec2bb97bd5933859de592a00e2eb7774b6ecf" ] }
8.3.2. Copying container images with skopeo
This command copies the myubi8 container image from a local registry into a directory on the local system:
# skopeo copy docker://localhost/myubi8 dir:/root/test/ INFO[0000] Downloading myubi8/blobs/sha256:16dc1f96e3a1bb628be2e00518fec2bb97bd5933859de592a00e2eb7774b6ecf # ls /root/test 16dc1f96e3a1bb628be2e00518fec2bb97bd5933859de592a00e2eb7774b6ecf.tar manifest.json
The result of the skopeo copy
command is a tarball (16d*.tar) and a manifest.json file representing the image being copied to the directory you identified. If there were multiple layers, there would be multiple tarballs. The skopeo copy
command can also copy images to another registry. If you need to provide a signature to write to the destination registry, you can do that by adding a --sign-by=
option to the command line, followed by the required key-id.
8.3.3. Getting image layers with skopeo
The skopeo layers
command is similar to skopeo copy
, with the difference being that the copy
option can copy an image to another registry or to a local directory, while the layers
option just drops the layers (tarballs and manifest.jason file) in the current directory. For example
# skopeo layers docker://localhost/myubi8 INFO[0000] Downloading myubi8/blobs/sha256:16dc1f96e3a1bb628be2e00518fec2bb97bd5933859de592a00e2eb7774b6ecf # find . ./layers-myubi8-latest-698503105 ./layers-myubi-latest-698503105/manifest.json ./layers-myubi8-latest-698503105/16dc1f96e3a1bb628be2e00518fec2bb97bd5933859de592a00e2eb7774b6ecf.tar
As you can see from this example, a new directory is created (layers-myubi8-latest-698503105) and, in this case, a single layer tarball and a manifest.json file are copied to that directory.