Docker Setup and Configuration

Owned by Swaminathan Mathivanan
Last updated: Sept 01, 2020
10 min read

Install using the repository

Before you install Docker Engine for the first time on a new host machine, you need to set up the Docker repository. Afterward, you can install and update Docker from the repository.

SET UP THE REPOSITORY

  1. Update the apt package index and install packages to allow apt to use a repository over HTTPS:

    $ sudo apt-get update $ sudo apt-get install \ apt-transport-https \ ca-certificates \ curl \ gnupg-agent \ software-properties-common

  2. Add Docker’s official GPG key:

    $ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

    Verify that you now have the key with the fingerprint 9DC8 5822 9FC7 DD38 854A  E2D8 8D81 803C 0EBF CD88, by searching for the last 8 characters of the fingerprint.

    $ sudo apt-key fingerprint 0EBFCD88 pub rsa4096 2017-02-22 [SCEA] 9DC8 5822 9FC7 DD38 854A E2D8 8D81 803C 0EBF CD88 uid [ unknown] Docker Release (CE deb) <docker@docker.com> sub rsa4096 2017-02-22 [S]

  3. Use the following command to set up the stable repository. To add the nightly or test repository, add the word nightly or test (or both) after the word stable in the commands below.

    Note: The lsb_release -cs sub-command below returns the name of your Ubuntu distribution, such as xenial. Sometimes, in a distribution like Linux Mint, you might need to change $(lsb_release -cs) to your parent Ubuntu distribution. For example, if you are using Linux Mint Tessa, you could use bionic. Docker does not offer any guarantees on untested and unsupported Ubuntu distributions.

  4. x86_64 / amd64

INSTALL DOCKER ENGINE

  1. Update the apt package index, and install the latest version of Docker Engine and containerd, or go to the next step to install a specific version:

    If you have multiple Docker repositories enabled, installing or updating without specifying a version in the apt-get install or apt-get update command always installs the highest possible version, which may not be appropriate for your stability needs.

  2. To install a specific version of Docker Engine, list the available versions in the repo, then select and install:

    a. List the versions available in your repo:

    b. Install a specific version using the version string from the second column, for example, 5:18.09.1~3-0~ubuntu-xenial.

  3. Verify that Docker Engine is installed correctly by running the hello-world image.

    This command downloads a test image and runs it in a container. When the container runs, it prints an informational message and exits.

Docker Engine is installed and running. The docker group is created but no users are added to it. You need to use sudo to run Docker commands.

Manage Docker as a non-root user

The Docker daemon binds to a Unix socket instead of a TCP port. By default that Unix socket is owned by the user root and other users can only access it using sudo. The Docker daemon always runs as the root user.

If you don’t want to preface the docker command with sudo, create a Unix group called docker and add users to it. When the Docker daemon starts, it creates a Unix socket accessible by members of the docker group.

Warning

The docker group grants privileges equivalent to the root user. For details on how this impacts security in your system.

Note:

To run Docker without root privileges.

Rootless mode is currently available as an experimental feature.

To create the docker group and add your user:

  1. Create the docker group.

  2. Add your user to the docker group.

  3. Log out and log back in so that your group membership is re-evaluated.

    If testing on a virtual machine, it may be necessary to restart the virtual machine for changes to take effect.

    On a desktop Linux environment such as X Windows, log out of your session completely and then log back in.

    On Linux, you can also run the following command to activate the changes to groups:

  4. Verify that you can run docker commands without sudo.

    This command downloads a test image and runs it in a container. When the container runs, it prints an informational message and exits.

    If you initially ran Docker CLI commands using sudo before adding your user to the docker group, you may see the following error, which indicates that your ~/.docker/ directory was created with incorrect permissions due to the sudo commands.

    To fix this problem, either remove the ~/.docker/ directory (it is recreated automatically, but any custom settings are lost), or change its ownership and permissions using the following commands:

Configure Docker to start on boot

Most current Linux distributions (RHEL, CentOS, Fedora, Ubuntu 16.04 and higher) use systemd to manage which services start when the system boots. Ubuntu 14.10 and below use upstart.

systemd

To disable this behavior, use disable instead.

upstart

Docker is automatically configured to start on boot using upstart. To disable this behavior, use the following command:

chkconfig

Use a different storage engine

For information about the different storage engines. The default storage engine and the list of supported storage engines depend on your host’s Linux distribution and available kernel drivers.

Configure default logging driver

Docker provides the capability to collect and view log data from all containers running on a host via a series of logging drivers. The default logging driver, json-file, writes log data to JSON-formatted files on the host filesystem. Over time, these log files expand in size, leading to potential exhaustion of disk resources.

Configure where the Docker daemon listens for connections

By default, the Docker daemon listens for connections on a UNIX socket to accept requests from local clients. It is possible to allow Docker to accept requests from remote hosts by configuring it to listen on an IP address and port as well as the UNIX socket.

Secure your connection

Before configuring Docker to accept connections from remote hosts it is critically important that you understand the security implications of opening docker to the network. If steps are not taken to secure the connection, it is possible for remote non-root users to gain root access on the host. For more information on how to use TLS certificates to secure this connection

Configuring Docker to accept remote connections can be done with the docker.service systemd unit file for Linux distributions using systemd, such as recent versions of RedHat, CentOS, Ubuntu and SLES, or with the daemon.json file which is recommended for Linux distributions that do not use systemd.

systemd vs daemon.json

Configuring Docker to listen for connections using both the systemd unit file and the daemon.json file causes a conflict that prevents Docker from starting.

Configuring remote access with systemd unit file

  1. Use the command sudo systemctl edit docker.service to open an override file for docker.service in a text editor.

  2. Add or modify the following lines, substituting your own values.

  3. Save the file.

  4. Reload the systemctl configuration.

  5. Restart Docker.

  6. Check to see whether the change was honored by reviewing the output of netstat to confirm dockerd is listening on the configured port.

Configuring remote access with daemon.json

  1. Set the hosts array in the /etc/docker/daemon.json to connect to the UNIX socket and an IP address, as follows:

  2. Restart Docker.

  3. Check to see whether the change was honored by reviewing the output of netstat to confirm dockerd is listening on the configured port.

Enable IPv6 on the Docker daemon

To enable IPv6 on the Docker daemon, see Enable IPv6 support.

Troubleshooting

Kernel compatibility

Docker cannot run correctly if your kernel is older than version 3.10 or if it is missing some modules. To check kernel compatibility, you can download and run the check-config.sh script.

The script only works on Linux, not macOS.

Cannot connect to the Docker daemon

If you see an error such as the following, your Docker client may be configured to connect to a Docker daemon on a different host, and that host may not be reachable.

To see which host your client is configured to connect to, check the value of the DOCKER_HOST variable in your environment.

If this command returns a value, the Docker client is set to connect to a Docker daemon running on that host. If it is unset, the Docker client is set to connect to the Docker daemon running on the local host. If it is set in error, use the following command to unset it:

You may need to edit your environment in files such as ~/.bashrc or ~/.profile to prevent the DOCKER_HOST variable from being set erroneously.

If DOCKER_HOST is set as intended, verify that the Docker daemon is running on the remote host and that a firewall or network outage is not preventing you from connecting.

IP forwarding problems

If you manually configure your network using systemd-network with systemd version 219 or higher, Docker containers may not be able to access your network. Beginning with systemd version 220, the forwarding setting for a given network (net.ipv4.conf.<interface>.forwarding) defaults to off. This setting prevents IP forwarding. It also conflicts with Docker’s behavior of enabling the net.ipv4.conf.all.forwarding setting within containers.

To work around this on RHEL, CentOS, or Fedora, edit the <interface>.network file in /usr/lib/systemd/network/ on your Docker host (ex: /usr/lib/systemd/network/80-container-host0.network) and add the following block within the [Network] section.

This configuration allows IP forwarding from the container as expected.

DNS resolver found in resolv.conf and containers can't use it

Linux systems which use a GUI often have a network manager running, which uses a dnsmasq instance running on a loopback address such as 127.0.0.1 or 127.0.1.1 to cache DNS requests, and adds this entry to /etc/resolv.conf. The dnsmasq service speeds up DNS look-ups and also provides DHCP services. This configuration does not work within a Docker container which has its own network namespace, because the Docker container resolves loopback addresses such as 127.0.0.1 to itself, and it is very unlikely to be running a DNS server on its own loopback address.

If Docker detects that no DNS server referenced in /etc/resolv.conf is a fully functional DNS server, the following warning occurs and Docker uses the public DNS servers provided by Google at 8.8.8.8 and 8.8.4.4 for DNS resolution.

If you see this warning, first check to see if you use dnsmasq:

If your container needs to resolve hosts which are internal to your network, the public nameservers are not adequate. You have two choices:

  • You can specify a DNS server for Docker to use, or

  • You can disable dnsmasq in NetworkManager. If you do this, NetworkManager adds your true DNS nameserver to /etc/resolv.conf, but you lose the possible benefits of dnsmasq.

You only need to use one of these methods.

Specify DNS servers for Docker

The default location of the configuration file is /etc/docker/daemon.json. You can change the location of the configuration file using the --config-file daemon flag. The documentation below assumes the configuration file is located at /etc/docker/daemon.json.

  1. Create or edit the Docker daemon configuration file, which defaults to /etc/docker/daemon.json file, which controls the Docker daemon configuration.

  2. Add a dns key with one or more IP addresses as values. If the file has existing contents, you only need to add or edit the dns line.

    If your internal DNS server cannot resolve public IP addresses, include at least one DNS server which can, so that you can connect to Docker Hub and so that your containers can resolve internet domain names.

    Save and close the file.

  3. Restart the Docker daemon.

  4. Verify that Docker can resolve external IP addresses by trying to pull an image:

  5. If necessary, verify that Docker containers can resolve an internal hostname by pinging it.

DISABLE DNSMASQ

Ubuntu

If you prefer not to change the Docker daemon’s configuration to use a specific IP address, follow these instructions to disable dnsmasq in NetworkManager.

  1. Edit the /etc/NetworkManager/NetworkManager.conf file.

  2. Comment out the dns=dnsmasq line by adding a # character to the beginning of the line.

    Save and close the file.

  3. Restart both NetworkManager and Docker. As an alternative, you can reboot your system.

Allow access to the remote API through a firewall

If you run a firewall on the same host as you run Docker and you want to access the Docker Remote API from another host and remote access is enabled, you need to configure your firewall to allow incoming connections on the Docker port, which defaults to 2376 if TLS encrypted transport is enabled or 2375 otherwise.

Two common firewall daemons are UFW (often used for Ubuntu systems) and firewalld (often used for RPM-based systems). Consult the documentation for your OS and firewall, but the following information might help you get started. These options are fairly permissive and you may want to use a different configuration that locks your system down more.

  • UFW: Set DEFAULT_FORWARD_POLICY="ACCEPT" in your configuration.

  • firewalld: Add rules similar to the following to your policy (one for incoming requests and one for outgoing requests). Be sure the interface names and chain names are correct.

Your kernel does not support cgroup swap limit capabilities

On Ubuntu or Debian hosts, You may see messages similar to the following when working with an image.

This warning does not occur on RPM-based systems, which enable these capabilities by default.

If you don’t need these capabilities, you can ignore the warning. You can enable these capabilities on Ubuntu or Debian by following these instructions. Memory and swap accounting incur an overhead of about 1% of the total available memory and a 10% overall performance degradation, even if Docker is not running.

  1. Log into the Ubuntu or Debian host as a user with sudo privileges.

  2. Edit the /etc/default/grub file. Add or edit the GRUB_CMDLINE_LINUX line to add the following two key-value pairs:

    Save and close the file.

  3. Update GRUB.

    If your GRUB configuration file has incorrect syntax, an error occurs. In this case, repeat steps 2 and 3.

    The changes take effect when the system is rebooted.