Docker
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Docker is a utility to pack, ship and run any application as a lightweight container.
Contents
Installation
Install the docker package or, for the development version, the docker-gitAUR package. 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.
Then re-login or to make your current user session aware of this new group, you can use:
$ newgrp docker
Configuration
Storage driver
The docker storage driver (or graph driver) has huge impact on performance. Its job is to store layers of container images efficiently, that is when several images share a layer, only one layer uses disk space. The compatible option, `devicemapper` offers suboptimal performance, which is outright terrible on rotating disks. Additionally, `devicemappper` is not recommended in production.
As Arch linux ships new kernels, there is no point using the compatibility option. A good, modern choice is overlay2
.
To see current storage driver, run # docker info | head
, modern docker installation should already use overlay2
by default.
To set your own choice of storage driver, create a Drop-in snippet and use -s
option to dockerd
:
/etc/systemd/system/docker.service.d/override.conf
[Service] ExecStart= ExecStart=/usr/bin/dockerd -H fd:// -s overlay2
Recall that ExecStart=
line is needed to drop inherited ExecStart
.
Further information on options is available on the user guide.
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.
Remote API with systemd
To start the remote API with the docker daemon, create a Drop-in snippet with the following content:
/etc/systemd/system/docker.service.d/override.conf
[Service] ExecStart= ExecStart=/usr/bin/dockerd -H tcp://0.0.0.0:4243 -H unix:///var/run/docker.sock
Daemon socket configuration
The docker daemon listens to a Unix socket by default. To listen on a specified port instead, create a Drop-in snippet with the following content:
/etc/systemd/system/docker.socket.d/socket.conf
[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
Create a Drop-in snippet with the following content:
/etc/systemd/system/docker.service.d/proxy.conf
[Service] Environment="HTTP_PROXY=192.168.1.1:8080" Environment="HTTPS_PROXY=192.168.1.1:8080"
Verify that the configuration has been loaded:
# systemctl show docker --property Environment Environment=HTTP_PROXY=192.168.1.1:8080 HTTPS_PROXY=192.168.1.1:8080
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 /etc/resolv.conf
on the host machine, filtering out local addresses (e.g. 127.0.0.1
). If this yields an empty file, then Google DNS servers are used. If you are using a service like dnsmasq to provide name resolution, you may need to add an entry to the /etc/resolv.conf
for docker's network interface so that it is not filtered out.
Running Docker with a manually-defined network
If you manually configure your network using systemd-network version 220 or higher, containers you start with Docker may be unable to access your network. Beginning with 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 which enables the net.ipv4.conf.all.forwarding
setting within a container.
To work around this, edit the <interface>.network
file in /etc/systemd/network/
on your Docker host add the following block:
/etc/systemd/network/<interface>.network
[Network] ... IPForward=kernel ...
This configuration allows IP forwarding from the container as expected.
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 --data-root
parameter to the ExecStart
:
/etc/systemd/system/docker.service.d/docker-storage.conf
[Service] ExecStart= ExecStart=/usr/bin/dockerd --data-root /path/to/new/location/docker -H fd://
Images
Arch Linux
The following command pulls the base/archlinux x86_64 image.
# docker pull base/archlinux
Manually
Check moby/contrib repo and 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
The following command pulls the debian x86_64 image.
# docker pull debian
Manually
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:
Check for running containers:
# docker ps
List all containers running on the host for deletion:
# docker ps -a
Stop a running container:
# docker stop <CONTAINER ID>
Killing still running containers:
# docker kill <CONTAINER ID>
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>
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
Cannot start a container with systemd 232
Append systemd.legacy_systemd_cgroup_controller=yes
as kernel parameter, see bug report for details.
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=kernel 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.
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
Error initializing graphdriver: devmapper
If systemctl
fails to start docker and provides an error:
Error starting daemon: error initializing graphdriver: devmapper: Device docker-8:2-915035-pool is not a thin pool
Then, try the following steps to resolve the error. Stop the service, back up /var/lib/docker/
(if desired), remove the contents of /var/lib/docker/
, and try to start the service. See the open GitHub issue for details.
See also
- Arch Linux on docs.docker.com
- Are Docker containers really secure? — opensource.com