Post-installation steps for Linux
Estimated reading time: 15 minutesThis section contains optional procedures for configuring Linux hosts to work better with Docker.
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 theroot
user. For details on how this impacts security in your system, see Docker Daemon Attack Surface.
To create the docker
group and add your user:
-
Create the
docker
group.$ sudo groupadd docker
-
Add your user to the
docker
group.$ sudo usermod -aG docker $USER
-
Log out and log back in so that your group membership is re-evaluated.
If testing on a virtual machine, it may be necessary to restart the virtual machine for changes to take effect.
On a desktop Linux environment such as X Windows, log out of your session completely and then log back in.
-
Verify that you can run
docker
commands withoutsudo
.$ docker run hello-world
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 thedocker
group, you may see the following error, which indicates that your~/.docker/
directory was created with incorrect permissions due to thesudo
commands.WARNING: Error loading config file: /home/user/.docker/config.json - stat /home/user/.docker/config.json: permission denied
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:$ sudo chown "$USER":"$USER" /home/"$USER"/.docker -R $ sudo chmod g+rwx "$HOME/.docker" -R
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
$ sudo systemctl enable docker
To disable this behavior, use disable
instead.
$ sudo systemctl disable docker
If you need to add an HTTP Proxy, set a different directory or partition for the Docker runtime files, or make other customizations, see customize your systemd Docker daemon options.
upstart
Docker is automatically configured to start on boot using
upstart
. To disable this behavior, use the following command:
$ echo manual | sudo tee /etc/init/docker.override
chkconfig
$ sudo chkconfig docker on
Use a different storage engine
For information about the different storage engines, see Storage drivers. The default storage engine and the list of supported storage engines depend on your host’s Linux distribution and available kernel drivers.
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. For more detailed information on this configuration option take a look at “Bind Docker to another host/port or a unix socket” section of the Docker CLI Reference article.
Docker EE customers
Docker EE customers can get remote CLI access to UCP with the UCP client bundle. A UCP Client Bundle is generated by UCP and secured by mutual TLS. See the document on CLI access for UCP for more information.
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, check this article on how to protect the Docker daemon socket.
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 thedaemon.json
file causes a conflict that prevents Docker from starting.
Configuring remote access with systemd
unit file
-
Use the command
sudo systemctl edit docker.service
to open an override file fordocker.service
in a text editor. -
Add or modify the following lines, substituting your own values.
[Service] ExecStart= ExecStart=/usr/bin/dockerd -H fd:// -H tcp://127.0.0.1:2375
-
Save the file.
-
Reload the
systemctl
configuration.$ sudo systemctl daemon-reload
-
Restart Docker.
$ sudo systemctl restart docker.service
-
Check to see whether the change was honored by reviewing the output of
netstat
to confirmdockerd
is listening on the configured port.$ sudo netstat -lntp | grep dockerd tcp 0 0 127.0.0.1:2375 0.0.0.0:* LISTEN 3758/dockerd
Configuring remote access with daemon.json
-
Set the
hosts
array in the/etc/docker/daemon.json
to connect to the UNIX socket and an IP address, as follows:{ "hosts": ["unix:///var/run/docker.sock", "tcp://127.0.0.1:2375"] }
-
Restart Docker.
-
Check to see whether the change was honored by reviewing the output of
netstat
to confirmdockerd
is listening on the configured port.$ sudo netstat -lntp | grep dockerd tcp 0 0 127.0.0.1:2375 0.0.0.0:* LISTEN 3758/dockerd
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.
$ curl https://raw.githubusercontent.com/docker/docker/master/contrib/check-config.sh > check-config.sh
$ bash ./check-config.sh
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.
Cannot connect to the Docker daemon. Is 'docker daemon' running on this host?
To see which host your client is configured to connect to, check the value of
the DOCKER_HOST
variable in your environment.
$ env | grep DOCKER_HOST
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:
$ unset DOCKER_HOST
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.
[Network]
...
IPForward=kernel
# OR
IPForward=true
...
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.
WARNING: Local (127.0.0.1) DNS resolver found in resolv.conf and containers
can't use it. Using default external servers : [8.8.8.8 8.8.4.4]
If you see this warning, first check to see if you use dnsmasq
:
$ ps aux |grep 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 ofdnsmasq
.
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
.
-
Create or edit the Docker daemon configuration file, which defaults to
/etc/docker/daemon.json
file, which controls the Docker daemon configuration.$ sudo nano /etc/docker/daemon.json
-
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 thedns
line.{ "dns": ["8.8.8.8", "8.8.4.4"] }
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.
-
Restart the Docker daemon.
$ sudo service docker restart
-
Verify that Docker can resolve external IP addresses by trying to pull an image:
$ docker pull hello-world
-
If necessary, verify that Docker containers can resolve an internal hostname by pinging it.
$ docker run --rm -it alpine ping -c4 <my_internal_host> PING google.com (192.168.1.2): 56 data bytes 64 bytes from 192.168.1.2: seq=0 ttl=41 time=7.597 ms 64 bytes from 192.168.1.2: seq=1 ttl=41 time=7.635 ms 64 bytes from 192.168.1.2: seq=2 ttl=41 time=7.660 ms 64 bytes from 192.168.1.2: seq=3 ttl=41 time=7.677 ms
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.
-
Edit the
/etc/NetworkManager/NetworkManager.conf
file. -
Comment out the
dns=dnsmasq
line by adding a#
character to the beginning of the line.# dns=dnsmasq
Save and close the file.
-
Restart both NetworkManager and Docker. As an alternative, you can reboot your system.
$ sudo restart network-manager $ sudo restart docker
RHEL, CentOS, or Fedora
To disable dnsmasq
on RHEL, CentOS, or Fedora:
-
Disable the
dnsmasq
service:$ sudo service dnsmasq stop $ sudo systemctl disable dnsmasq
-
Configure the DNS servers manually using the Red Hat documentation.
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 (Uncomplicated Firewall) (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.
<direct> [ <rule ipv="ipv6" table="filter" chain="FORWARD_direct" priority="0"> -i zt0 -j ACCEPT </rule> ] [ <rule ipv="ipv6" table="filter" chain="FORWARD_direct" priority="0"> -o zt0 -j ACCEPT </rule> ] </direct>
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.
WARNING: Your kernel does not support swap limit capabilities. Limitation discarded.
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.
-
Log into the Ubuntu or Debian host as a user with
sudo
privileges. -
Edit the
/etc/default/grub
file. Add or edit theGRUB_CMDLINE_LINUX
line to add the following two key-value pairs:GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount=1"
Save and close the file.
-
Update GRUB.
$ sudo 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.
Next steps
- Continue with the User Guide.