Pod's dns subdomain cannot be resolved if headless service name not same as StatefulSet name

[dunjut]

What happened:

case A:
In namespace foo, create a StatefulSet named database and a headless service named database, I can successfully resolve database-0.database.foo.svc.cluster.local.

case B:
In namespace foo, create a StatefulSet named database and a headless service named db-svc, I failed to resolve database-0.db-svc.foo.svc.cluster.local.

What you expected to happen:

In case B, Pod's subdomain should be successfully resolved.

How to reproduce it (as minimally and precisely as possible):
Just create a StatefulSet and a headless service...

Anything else we need to know?:

Environment:

• Kubernetes version (use kubectl version):

Client Version: version.Info{Major:"1", Minor:"9", GitVersion:"v1.9.6", GitCommit:"9f8ebd171479bec0ada837d7ee641dec2f8c6dd1", GitTreeState:"clean", BuildDate:"2018-03-21T15:21:50Z", GoVersion:"go1.9.3", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"9", GitVersion:"v1.9.6", GitCommit:"9f8ebd171479bec0ada837d7ee641dec2f8c6dd1", GitTreeState:"clean", BuildDate:"2018-03-21T15:13:31Z", GoVersion:"go1.9.3", Compiler:"gc", Platform:"linux/amd64"}

• Cloud provider or hardware configuration:
• OS (e.g: cat /etc/os-release): Ubuntu 16.04.2 LTS
• Kernel (e.g. uname -a): Linux master 4.4.0-62-generic add travis integration #83-Ubuntu SMP Wed Jan 18 14:10:15 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
• Install tools: kubeadm
• Others: kubedns docker image version 1.14.7

[dunjut]

/sig network

[prameshj]

I just tried this using the yaml here - https://kubernetes.io/docs/tutorials/stateful-application/basic-stateful-set/#creating-a-statefulset

Verified the the service 'nginx` is indeed headless, returns the pod ips when resolving the service name - nginx.

I was able to resolve:
host web-0.nginx
web-0.nginx.default.svc.cluster.local has address 10.0.2.6

Can you share the yaml you were using?

[prameshj]

/assign @dunjut

[k8s-ci-robot]

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In response to this:

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[dunjut]

@prameshj Could you try to remove the spec.serviceName field in StatefulSet and resolve again?

Now I think this may be by design but I don't understand why serviceName is needed since we already have a name for Service...

[prameshj]

I can't remove the serviceName : 'missing required field "serviceName" in io.k8s.api.apps.v1.StatefulSetSpec'

if you don't specify serviceName, I don't see how the endpoints for the headless service can be updated to include the pods in the statefulSet. Can you share the yamls for how you created the service and statefulSets? It looks like this is working as intended.

[dunjut]

@prameshj Sorry I just noticed your reply.

This should be easy to reproduce, you can change serviceName to 'web' and keep the real headless service's name being 'nginx', then dns fails.

The headless service with a selector already includes the pods in StatefulSet, I just don't understand why serviceName is needed.

[prameshj]

I understood the issue now, maybe it should read "Pod's dns subdomain cannot be resolved if headless service name not same as serviceName in StatefulSet spec"? This is working as intended. From the api doc:

https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.10/#statefulset-v1-apps

"selector is a label query over pods that should match the replica count. It must match the pod template's labels."

"serviceName is the name of the service that governs this StatefulSet. This service must exist before the StatefulSet, and is responsible for the network identity of the set. Pods get DNS/hostnames that follow the pattern: pod-specific-string.serviceName.default.svc.cluster.local where "pod-specific-string" is managed by the StatefulSet controller.
templatePodTemplateSpec"

serviceName is used for dns records, selector is for determining count of replicas.

Closing issue since this is working as intended. Please reopen if needed.
/close

[k8s-ci-robot]

@prameshj: Closing this issue.

In response to this:

I understood the issue now, maybe it should read "Pod's dns subdomain cannot be resolved if headless service name not same as serviceName in StatefulSet spec"? This is working as intended. From the api doc:

https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.10/#statefulset-v1-apps

"selector is a label query over pods that should match the replica count. It must match the pod template's labels."

"serviceName is the name of the service that governs this StatefulSet. This service must exist before the StatefulSet, and is responsible for the network identity of the set. Pods get DNS/hostnames that follow the pattern: pod-specific-string.serviceName.default.svc.cluster.local where "pod-specific-string" is managed by the StatefulSet controller.
templatePodTemplateSpec"

serviceName is used for dns records, selector is for determining count of replicas.

Closing issue since this is working as intended. Please reopen if needed.
/close

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[vadasambar]

If anyone is wondering why this is said to be working as intended when the api reference does not mention anything about headless service, the docs talk about this under Limitations.

StatefulSets currently require a Headless Service to be responsible for the network identity of the Pods. You are responsible for creating this Service.