Obtain Pod DNS Name Programmatically

Question

I have a statefulset with 3 members. They are accessible from inside the cluster with something like:

podname-{0..n}.service.default.svc.cluster.local

I'm using the Kubernetes API from a Controller. I just created the Statefulset with:

import (
    "k8s.io/client-go/kubernetes"
    appsv1 "k8s.io/api/apps/v1"
)
...

s := appsv1.StatefulSet{ /* some data */ }
kubernetes.AppsV1().StatefulSets(nameSpace).Create(s)

Let's assume everything worked correctly, and after a few minutes I have 3 running Pods. How can I get the Pod DNS entries for the newly created Pods?

I know I can build the DNS with:

fmt.Sprintf("%s-%d.%s.%s.svc.cluster.local", s.Name, idx, service, S.Namespace)

But I'm finding some issues:

1. I will have to calculate the index depending on the amount of replicas
2. I need to know about the service name used for this StatefulSet
3. I'm assuming the cluster domain is cluster.local which is not necessarily true

What I think it should exists (but I'm not sure if it really exists), is an API, that given a StatefulSet will allow me to know the DNS names of the Replicas that were created. Does an API like this exists?

Answer 1

When you create the headless Service with the StatefulSet, it should create DNS SRV entries. You can query that to get a list of all host names of the pods corresponding to the headless Service without knowing the number. For example, if using Python you can do:

>>> import dns
>>> for item in dns.resolver.query('your-app-name-headless', 'SRV'): item
...
<DNS IN SRV rdata: 10 25 0 2555bb89.your-app-name-headless.your-project.svc.cluster.local.>
<DNS IN SRV rdata: 10 25 0 830bdb18.your-app-name-headless.your-project.svc.cluster.local.>
<DNS IN SRV rdata: 10 25 0 aeb532de.your-app-name-headless.your-project.svc.cluster.local.>
<DNS IN SRV rdata: 10 25 0 a432c21f.your-app-name-headless.your-project.svc.cluster.local.>

See:

• https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/#srv-records
• https://kubernetes.io/docs/concepts/services-networking/service/#headless-services