Troubleshoot Docker Engine installation
This page contains instructions for troubleshooting and diagnosing the Docker Engine installation.
Kernel compatibility
Docker can't run correctly if your kernel is older than version 3.10, or if it's
missing kernel modules. To check kernel compatibility, you can download and run
the
check-config.sh
open_in_new
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.
Unable to connect to the Docker daemon
Cannot connect to the Docker daemon. Is 'docker daemon' running on this host?
This error may indicate:
- The Docker daemon isn't running on your system. Start the daemon and try running the command again.
- Your Docker client is attempting to connect to a Docker daemon on a different host, and that host is unreachable.
To see which host your client is connecting 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's unset, the Docker client is set to connect to the Docker daemon running on the local host. If it's 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 isn't preventing you from
connecting.
IP forwarding problems
If you manually configure your network using systemd-network
with systemd
version 219 or later, 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, for example,
/usr/lib/systemd/network/80-container-host0.network
.
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 issues
DNS resolver found in resolv.conf and containers can't use it
Linux desktop environments often have a network manager program running, that
uses dnsmasq
to cache DNS requests by adding them to /etc/resolv.conf
. The
dnsmasq
instance runs on a loopback address such as 127.0.0.1
or
127.0.1.1
. It speeds up DNS look-ups and provides DHCP services. Such a
configuration doesn't work within a Docker container. The Docker container uses
its own network namespace, and resolves loopback addresses such as 127.0.0.1
to itself, and it's 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:
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 aren't adequate. You have two choices:
Specify DNS servers for Docker to use.
Turn off
dnsmasq
.Turning off
dnsmasq
adds the IP addresses of actual DNS nameserver to/etc/resolv.conf
, and you lose the 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 following instruction assumes that the location of the
configuration file is /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 DNS server IP addresses as values.{ "dns": ["8.8.8.8", "8.8.4.4"] }
If the file has existing contents, you only need to add or edit the
dns
line. If your internal DNS server can't resolve public IP addresses, include at least one DNS server that can. Doing so allows you to connect to Docker Hub, and your containers to 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
Turn off dnsmasq
Ubuntu
If you prefer not to change the Docker daemon's configuration to use a specific
IP address, follow these instructions to turn off 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 systemctl restart network-manager $ sudo systemctl restart docker
RHEL, CentOS, or Fedora
To turn off dnsmasq
on RHEL, CentOS, or Fedora:
Turn off the
dnsmasq
service:$ sudo systemctl stop dnsmasq $ sudo systemctl disable dnsmasq
Configure the DNS servers manually using the Red Hat documentationopen_in_new.
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 remote host, you must configure your firewall
to allow incoming connections on the Docker port. The default port is 2376
if
you're using TLS encrypted transport, or 2375
otherwise.
Two common firewall daemons are:
- Uncomplicated Firewall (UFW)open_in_new, often used for Ubuntu systems.
- firewalldopen_in_new, often used for RPM-based systems.
Consult the documentation for your OS and firewall. The following information might help you get started. These settings used in this instruction are permissive, and you may want to use a different configuration that locks your system down more.
For UFW, set
DEFAULT_FORWARD_POLICY="ACCEPT"
in your configuration.For firewalld, add rules similar to the following to your policy. One for incoming requests, and one for outgoing requests.
<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>
Make sure that the interface names and chain names are correct.
Kernel 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.
If you don't need these capabilities, you can ignore the warning.
You can turn on 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 when Docker isn't 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 the GRUB boot loader.
$ sudo update-grub
An error occurs if your GRUB configuration file has incorrect syntax. In this case, repeat steps 2 and 3.
The changes take effect when you reboot the system.