# Docker

Related articles

- systemd-nspawn
- Linux Containers
- Lxc-systemd
- Vagrant

Docker is a utility to pack, ship and run any application as a lightweight container.

## Installation

Note: Docker doesn't support i686. [1]

Install the docker package or, for the development version, the docker-gitAUR package. You may need to reboot. Next start and enable `docker.service` and verify operation:

    # docker info

If you want to be able to run docker as a regular user, add yourself to the docker group:

Warning: Anyone added to the 'docker' group is root equivalent. More information here and here.

    # gpasswd -a user docker

Then re-login or to make your current user session aware of this new group, you can use:

    $ newgrp docker

## Configuration

### Opening remote API

To open the Remote API to port `4243` manually, run:

     # docker daemon -H tcp://0.0.0.0:4243 -H unix:///var/run/docker.sock

`-H tcp://0.0.0.0:4243` part is for opening the Remote API.

`-H unix:///var/run/docker.sock` part for host machine access via terminal.

### Daemon socket configuration

The docker daemon listens to a Unix socket by default. To listen on a specified port instead, edit `/etc/systemd/system/docker.socket`, where `ListenStream` is the used port:

    [Socket]
    ListenStream=0.0.0.0:2375

### Proxies

Proxy configuration is broken down into two. First is the host configuration of the Docker daemon, second is the configuration required for your container to see your proxy.

#### Proxy configuration

First, create a systemd drop-in directory for the docker service: `mkdir /etc/systemd/system/docker.service.d`

Now create a file called `/etc/systemd/system/docker.service.d/http-proxy.conf` that adds the `HTTP_PROXY` environment variable:

    [Service]
    Environment="HTTP_PROXY=192.168.1.1"

Note: This assumes `192.168.1.1` is your proxy server, do not use `127.0.0.1`.

Flush changes: `sudo systemctl daemon-reload`

Verify that the configuration has been loaded:

    sudo systemctl show docker --property Environment
    Environment=HTTP_PROXY=192.168.1.1

Restart Docker: `sudo systemctl restart docker`

#### Container configuration

The settings in the `docker.service` file will not translate into containers. To achieve this you must set `ENV` variables in your `Dockerfile` thus:

     FROM base/archlinux
     ENV http_proxy="http://192.168.1.1:3128"
     ENV https_proxy="https://192.168.1.1:3128"

Docker provide detailed information on configuration via `ENV` within a Dockerfile.

### Configuring DNS

By default, docker will make resolv.conf in the container match resolv.conf on the host machine, filtering out local addresses (e.g. `127.0.0.1`). If this yields an empty file, then googles DNS servers are defaulted. If you are using a service like dnsmasq to provide name resolution, you will need to add an entry to your resolv.conf for docker's network interface so that it isn't filtered out.

### Images location

By default, docker images are located at `/var/lib/docker`. They can be moved to other partitions. First, stop the `docker.service`.

If you have run the docker images, you need to make sure the images are unmounted totally. Once that is completed, you may move the images from `/var/lib/docker` to the target destination.

Then add a Drop-in snippet for the `docker.service`, adding the `-g` parameter to the `ExecStart`:

    /etc/systemd/system/docker.service.d/imagelocation.conf

    [Service]
    ExecStart=
    ExecStart=/usr/bin/dockerd -g /path/to/new/location/docker -H fd://

Finally, reload configuration and start `docker.service` again.

## Docker 0.9.0 -- 1.2.x and LXC

Since version 0.9.0 Docker provides a new way to start containers without relying on a LXC library called libcontainer.

The lxc exec driver and the -lxc-conf option may also be removed in the near future, [2]

Hence, you will not be able to use `lxc-attach` with containers managed by Docker 0.9.0+ by default. It is required to make Docker daemon run with `-e lxc` as an argument.

You can create a file named `lxc.conf` under `/etc/systemd/system/docker.service.d/` with the following contents:

    [Service]
    ExecStart=
    ExecStart=/usr/bin/docker -d -e lxc

## Images

### Arch Linux

#### x86_64

The following command pulls the base/archlinux x86_64 image.

    # docker pull base/archlinux

#### i686

The default Arch Linux image in Docker Registry is for x86_64 only. i686 image must be built manually.

#### Build Image

Instead, check docker base/archlinux registry and click the `mkimage-arch.sh` link to download `mkimage-arch.sh` and `mkimage-arch-pacman.conf` to the same directory as raw files. Next, make the script executable and run it:

    $ chmod +x mkimage-arch.sh
    $ cp /etc/pacman.conf ./mkimage-arch-pacman.conf # or get a pacman.conf from somewhere else
    $ ./mkimage-arch.sh
    # docker run -t -i --rm archlinux /bin/bash  # try it

For slow network connections or CPU, the build timeout can be extended:

    $ sed -i 's/timeout 60/timeout 120/' mkimage-arch.sh

### Debian

Build Debian image with debootstrap:

    # mkdir jessie-chroot
    # debootstrap jessie ./jessie-chroot http://http.debian.net/debian/
    # cd jessie-chroot
    # tar cpf - . | docker import - debian
    # docker run -t -i --rm debian /bin/bash

## Arch Linux image with snapshot repository

Arch Linux on Docker can become problematic when multiple images are created and updated each having different package versions. To keep Docker containers with consistent package versions, a Docker image with a snapshot repository is available. This allows installing new packages from the official repository as it was on the day that the snapshot was created.

    $ docker pull pritunl/archlinux:latest
    $ docker run --rm -t -i pritunl/archlinux:latest /bin/bash

Alternatively, you could use Arch Linux Archive by freezing `/etc/pacman.d/mirrorlist`

     Server=https://archive.archlinux.org/repos/2020/01/02/$repo/os/$arch

## Clean Remove Docker + Images

In case you want to remove Docker entirely you can do this by following the steps below:

Note: Don't just copy paste those commands without making sure you know what you are doing!

Check for running containers:

    # docker ps

Killing still running containers:

    # docker kill <CONTAINER ID>

List all containers running on the host for deletion:

    # docker ps -a

Delete all containers listed by ID:

    # docker rm <CONTAINER ID>

List all Docker images:

    # docker images

Delete all images by ID:

    # docker rmi <IMAGE ID>

Disable Docker:

    # systemctl disable docker
    # systemctl stop docker

Remove Docker/Compose from the system:

    # pacman -Rs docker docker-compose

Remove users from docker group:

    # gpasswd -d <user> docker

Delete docker group from system:

    # groupdel docker

Delete all Docker data (purge directory):

    # rm -R /var/lib/docker

## Useful tips

To grab the IP address of a running container:

    $ docker inspect --format '{{ .NetworkSettings.IPAddress }}' <container-name OR id>

    172.17.0.37

## Troubleshooting

### Docker info errors out

If running `docker info` gives an error that looks like this:

     FATA[0000] Get http:///var/run/docker.sock/v1.17/info: read unix /var/run/docker.sock: connection reset by peer. Are you trying to connect to a TLS-enabled daemon without TLS?

then you might not have the `bridge` module loaded. You can check for it by running `lsmod` . If it is not loaded, you can try to load it with `modprobe` or simply reboot (a reboot might be required if you have upgraded your kernel recently without rebooting and the bridge module was built for the more recent kernel.)

See this issue on GitHub for more information.

### Deleting Docker Images in a BTRFS Filesystem

Deleting docker images in a btrfs filesystem leaves the images in `/var/lib/docker/btrfs/subvolumes/` with a size of 0. When you try to delete this you get a permission error.

     # docker rm bab4ff309870
     # rm -Rf /var/lib/docker/btrfs/subvolumes/*
     rm: cannot remove '/var/lib/docker/btrfs/subvolumes/85122f1472a76b7519ed0095637d8501f1d456787be1a87f2e9e02792c4200ab': Operation not permitted

This is caused by btrfs which created subvolumes for the docker images. So the correct command to delete them is:

     # btrfs subvolume delete /var/lib/docker/btrfs/subvolumes/85122f1472a76b7519ed0095637d8501f1d456787be1a87f2e9e02792c4200ab

### docker0 Bridge gets no IP / no internet access in containers

Docker enables IP forwarding by itself, but by default systemd overrides the respective sysctl setting. The following disables this override (for all interfaces):

    # cat > /etc/systemd/network/ipforward.network <<EOF
    [Network]
    IPForward=ipv4
    EOF

    # cat > /etc/sysctl.d/99-docker.conf <<EOF
    net.ipv4.ip_forward = 1
    EOF

    # sysctl -w net.ipv4.ip_forward=1

Finally restart the `systemd-networkd` and `docker` services.

### docker complains about no loopback devices

If starting the docker service fails and `journalctl` says that no loopback device can be found, try following the steps outlined in TrueCrypt's troubleshooting section. In particular, if you've upgraded the kernel since last rebooting, you just need to reboot.

### Default number of allowed processes/threads too low

If you run into error messages like

    # e.g. Java
    java.lang.OutOfMemoryError: unable to create new native thread
    # e.g. C, bash, ...
    fork failed: Resource temporarily unavailable

then you might need to adjust the number of processes allowed by systemd. Default (see system.conf) is 500, which is pretty small for running several docker containers. You need to create a drop-in service file for this:

    # mkdir /etc/systemd/system/docker.service.d
    # cat > /etc/systemd/system/docker.service.d/tasks.conf <<EOF
    [Service]
    TasksMax=infinity
    EOF
    # systemctl daemon-reload
    # systemctl restart docker.service

## See also

- Arch Linux on docs.docker.com
- Are Docker containers really secure? — opensource.com

Category:

- Virtualization