STFC Docker Hub Mirror


The STFC cloud team provides an internal Docker Hub mirror at which is strongly recommended; users avoid rate limit issues, can pull large images faster over the local network and is free.

Docker Hub limit an IP to 100 pulls per 6 hours. Instances with floating IPs will have separate rate limits, whilst all other instances use a shared limit (as all pulls come from a single external IP).

Docker will automatically fall back to using Docker Hub directly if the mirror is ever unavailable, so there is minimal risk to applying these changes.

New Instances

By default all new STFC Cloud instances will use the mirror by default. To confirm the mirror is being used see Checking that the mirror is being used

Existing Instances

Existing instances will automatically update to pull in the required file described here Restarting Docker. A restart of the instance or docker service is required to start using the mirror.

Checking that the mirror is being used

Checking configuration exists

Users can quickly check if they have the mirror configuration present by running:

cat /etc/docker/daemon.json

An output similar to this indicates that the configuration is present:

    "registry-mirrors": [""]

To check whether the docker daemon is using this config you can run the following:

docker info

An output which includes the following near the bottom will indicate that the mirror is being used:

Registry Mirrors:

Monitoring Logs

For further peace of mind the logs can be checked. Docker will not produce any logs when pulling from the mirror successfully but will log an error if it fails to use direct connections:

  • If this an existing instance which has not been restarted recently see Restarting Docker.

  • In a terminal follow the Docker logs:

# Where -u means unit and -f means follow
journalctl -u docker -f
  • Select an image which has not been pulled on the System before, for example Ubuntu or Debian latest.

  • In a separate terminal run a docker pull of your image:

# Assuming ubuntu:latest has never been pulled on the machine
docker pull ubuntu:latest
  • If the mirror could be contacted there will be no output in the logs about it

  • If the mirror could not be used something similar to this will appear:

Jan 1 12:00:00 My-Instance-Name dockerd[900]: time="2021-01-01T12:00:00.000000000Z" level=info msg="Attempting next endpoint for pull after error: ...

For more support see Troubleshooting Mirror Connection.

Manually Configuring the Mirror

For most users this is not required; instances will automatically update the file as described in Checking that the mirror is being used.

If you have internal machines that are outside of Openstack or separately managed or you need to apply the changes proactively the following steps can be followed:

Docker Daemon

If you are using Docker (SL7 / Ubuntu / CoreOS / K8s < 1.20) the following steps can be performed using sudo or root. By default most distributions do not pre-create this file:

mkdir -p /etc/docker
<editor> /etc/docker/daemon.json # e.g. vi /etc/docker/daemon.json

Add or append the following JSON:

    "registry-mirrors": [""]

Then restart the service (see Restarting Docker).


As of Kubernetes 1.20, a future release (TBA) will use Containerd by default. The STFC Core OS image already contains the mirror information on users behalf at /etc/containers/registries.conf

Further documentation on manually on setting up containerd will be included when upstream Kubernetes uses containerd by default and further internal testing is completed.

Restarting Docker


Restarting the Docker Daemon will also pause and resume any running containers. This may result in service interruption or lost work.

The Docker Daemon can be restarted to apply the changes. Any running containers will be either paused and resumed, or restarted during this process.

Depending on the operating system the service is either called docker or dockerd, with the former being more common. To restart simply run:

systemctl restart docker  # or dockerd

To verify the service has resumed successfully:

systemctl status docker  # or dockerd

Troubleshooting Mirror Connection

The logs can be checked if you suspect the mirror is not being used (see Monitoring Logs).

If Docker is failing back some simple diagnostics steps can be performed and/or cloud support can be contacted. Including the output of these steps in your email will enable us to provide support faster:

  • Check that the VM has a connection to the internet with ping

  • Check config matches Manually Configuring the Mirror, paying attention to typos in the URL

  • Check that you can connect to the mirror using curl: curl you should get an empty response: {}

  • Check the systemd logs for additional errors related to Docker journalctl -u docker

  • Contact us for additional support