Let's create a Docker container that can connect to CentOS 8 with the minimum configuration by SSH

(2020/09/25) The troubleshooting part has been separated into the following articles. Cannot connect to VM with SSH connectable Docker container


I've been a little interested in Docker lately, but I haven't had a chance to come into contact with Docker at work so far, so I decided to study for myself. Google teachers and ancestors will introduce a lot of articles to build around CentOS 7, but since CentOS 8 has been released for a while (quite?), It's a new one, so it's a trial and error process. Let's play a little with the knowledge. [^ 1] This time, the goal is to create a container that can be connected by SSH, and to enable SSH access from the host to the container.

[^ 1]: CentOS 8 seems to come with a Docker-compatible system called Podman as standard equipment, so you may not need to use Docker.


--Windows10 Home (1909) (host)


VM boot

It may be easy to install by default, but for the time being, I decided to try how far it can be built with Minimal (minimum configuration). CentOS 8 uses a full-size image, and during installation there will be a place to select the configuration, so select the minimum configuration there. The construction procedure in VirtualBox is omitted. I referred to the following articles.

Reference: Build CentOS8 in Virtualbox environment Reference: CentOS8 at first glance

SSH connection related settings

After installing the OS, bring it from TeraTerm on the host side to a place where SSH connection is possible.

First, set up port forwarding on the VirtualBox side. It would be nice if SSH access was possible, but with the image below, I would like to be able to access the guest VM by specifying port 2222 and the container by specifying port 22222 when connecting.

Host --- Guest --- Container 2222 ----- 22 22222 --- 2222 --- 22

Therefore, in VirtualBox, set the association between 2222 and 22 and 22222 and 2222. The association between guest VM No. 2222 and container No. 22 will be performed later in the Docker-related settings. キャプチャ.PNG

Move to the settings on the guest VM side. First, set the firewall to allow SSH connections.

# firewall-cmd --add-service=ssh --permanent
# firewall-cmd --reload

Enable network interface.

# nmcli connection up enp0s3
# nmcli connection modify enp0s3 connection.autoconnect yes
# nmcli connection show enp0s3

Disable SELinux and restart.

# vi /etc/selinux/config
# reboot

Now you can SSH from the host side TeraTerm to the guest VM. When connecting, specify the IP address for the host name ( in our environment), and specify the port as set for port forwarding in Virtual Box (2222 this time).

Introducing Docker

~~ This time, I will try it on the assumption that it will be built in an offline environment. ~~ </ font> I was thinking of building it in an offline environment until the middle of the process, but I ended up working in an online environment because external communication was absolutely necessary when starting the Docker container in the latter stage. Therefore, please forgive me though I am taking unnecessary steps for offline to the middle. (Maybe all the steps are rewritten online)

In addition, the following article was helpful for the offline introduction procedure.

Reference: Docker environment construction-think about how to use it in an offline environment-

First, download the necessary materials to the host side.

Docker 19.03.8 https://download.docker.com/linux/static/stable/x86_64/docker-19.03.8.tgz Docker Compose 1.25.4 https://github.com/docker/compose/releases/download/1.25.4/docker-compose-Linux-x86_64

Send materials to the guest VM with WinSCP etc. and deploy with the following command.

$ tar zxvf docker-19.03.8.tgz
-bash: tar:Command not found

…… I see, there is no tar command in the minimum configuration installation. I introduced the command and tried again, wondering why the tar was not included.

Reference: List of ridiculous things after installing CentOS8 (Minimum)

$ sudo yum install tar
$ tar zxvf docker-19.03.8.tgz

Since the download material itself is an executable file for Docker Compose, you can rename it, put it in a predetermined position, and make it executable.

$ sudo mv docker-compose-Linux-x86_64 /usr/local/bin/docker-compose
$ sudo chmod +x /usr/local/bin/docker-compose
$ sudo ln -s /usr/local/bin/docker-compose /usr/bin/docker-compose

From now on, it is troublesome to add sudo and type the docker command, so grant authority.

$ sudo usermod -aG docker $USER

Container startup

Create Dockerfile and docker-compose.yml in the guest VM's home directory to run ssh in the container. Alpine Linux, a lightweight OS, is used for the image file of the container. In addition, I referred to the following article for how to make ssh resident in the container.

Reference: SSH without systemd in Docker container


FROM alpine

#Introduced ssh and others
RUN set -x && apk update && apk upgrade \
  && apk add --no-cache openssh openrc \
  && apk add --no-cache bash \
  && apk add --update busybox-suid
RUN rc-update add sshd && rc-status
RUN mkdir -p /run/openrc/ && touch /run/openrc/softlevel

#sshd settings
RUN sed -i 's/^AllowTcpForwarding no/AllowTcpForwarding yes/g' /etc/ssh/sshd_config
RUN sed -i 's/^#PermitRootLogin prohibit-password/PermitRootLogin yes/g' /etc/ssh/sshd_config

#Created by general user
RUN adduser -D test

#Password setting
RUN echo 'root:root' | chpasswd
RUN echo "test:test" | chpasswd

#Hide banner when logging in
RUN echo "" > /etc/motd

#Start sshd and leave it as it is
CMD /etc/init.d/sshd start && tail -f /dev/null


version: '3.8'

    build: .
    container_name: test
    hostname: test
      - "2222:22" #Setting to associate guest VM with container port
    tty: true

After creating the above file, finally start the container.

[centos@dockertest ~]$ docker-compose build
[centos@dockertest ~]$ docker-compose up -d

When the container starts up, try accessing the container with SSH from the host.


With the above, I thought that I was able to create a Docker container with SSH access.

(Continued to Problem Occurrence)


While groping, I was able to run a Docker container that can be connected to SSH for the time being. Even so, I was surprised that Minimal's CentOS 8 doesn't include tar. How was it decided that it was unnecessary?

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