How can I debug "ImagePullBackOff"?

All of a sudden, I cannot deploy some images which could be deployed before. I got the following pod status:

[root@webdev2 origin]# oc get pods 
NAME                      READY     STATUS             RESTARTS   AGE 
arix-3-yjq9w              0/1       ImagePullBackOff   0          10m 
docker-registry-2-vqstm   1/1       Running            0          2d 
router-1-kvjxq            1/1       Running            0          2d

The application just won't start. The pod is not trying to run the container. From the Event page, I have got Back-off pulling image "172.30.84.25:5000/default/arix@sha256:d326. I have verified that I can pull the image with the tag with docker pull.

I have also checked the log of the last container. It was closed for some reason. I think the pod should at least try to restart it.

I have run out of ideas to debug the issues. What can I check more?

Solution

You can use the 'describe pod' syntax

For OpenShift use:

oc describe pod <pod-id>

For vanilla Kubernetes:

kubectl describe pod <pod-id>

Examine the events of the output. In my case it shows Back-off pulling image unreachableserver/nginx:1.14.22222

In this case the image unreachableserver/nginx:1.14.22222 can not be pulled from the Internet because there is no Docker registry unreachableserver and the image nginx:1.14.22222 does not exist.

NB: If you do not see any events of interest and the pod has been in the 'ImagePullBackOff' status for a while (seems like more than 60 minutes), you need to delete the pod and look at the events from the new pod.

For OpenShift use:

oc delete pod <pod-id>
oc get pods
oc get pod <new-pod-id>

For vanilla Kubernetes:

kubectl delete pod <pod-id>  
kubectl get pods
kubectl get pod <new-pod-id>

Sample output:

  Type     Reason     Age                From               Message
  ----     ------     ----               ----               -------
  Normal   Scheduled  32s                default-scheduler  Successfully assigned rk/nginx-deployment-6c879b5f64-2xrmt to aks-agentpool-x
  Normal   Pulling    17s (x2 over 30s)  kubelet            Pulling image "unreachableserver/nginx:1.14.22222"
  Warning  Failed     16s (x2 over 29s)  kubelet            Failed to pull image "unreachableserver/nginx:1.14.22222": rpc error: code = Unknown desc = Error response from daemon: pull access denied for unreachableserver/nginx, repository does not exist or may require 'docker login': denied: requested access to the resource is denied
  Warning  Failed     16s (x2 over 29s)  kubelet            Error: ErrImagePull
  Normal   BackOff    5s (x2 over 28s)   kubelet            Back-off pulling image "unreachableserver/nginx:1.14.22222"
  Warning  Failed     5s (x2 over 28s)   kubelet            Error: ImagePullBackOff

Additional debugging steps

try to pull the docker image and tag manually on your computer
Identify the node by doing a 'kubectl/oc get pods -o wide'
ssh into the node (if you can) that can not pull the docker image
check that the node can resolve the DNS of the docker registry by performing a ping.
try to pull the docker image manually on the node
If you are using a private registry, check that your secret exists and the secret is correct. Your secret should also be in the same namespace. Thanks swenzel
Some registries have firewalls that limit ip address access. The firewall may block the pull
Some CIs create deployments with temporary docker secrets. So the secret expires after a few days (You are asking for production failures...)