How to resolve pod hostnames from other pods?

Normally, only Services get DNS names, not Pods. So, by default, you can't refer to another Pod directly by a domain name, only by its IP address.

Pods get DNS names only under certain condidtions that include a headless Service, as explained in the documentation. In particular, the conditions are:

• The Pods have a hostname field
• The Pods have a subdomain field
• There is a headless Service (in the same namespace) that selects the Pods
• The name of the headless Service equals the subdomain field of the Pods

In this case, each Pod gets a fully-qualified domain name of the following form:

my-hostname.my-subdomain.default.svc.cluster.local

Where my-hostname is the hostname field of the Pod and my-subdomain is the subdomain field of the Pod.

Note: the DNS name is created for the "hostname" of the Pod and not the "name" of the Pod.

You can test this with the following setup:

apiVersion: v1kind: Servicemetadata:  name: my-subdomainspec:  selector:    name: my-test  clusterIP: None---apiVersion: v1kind: Podmetadata:  name: my-pod-1  labels:    name: my-testspec:  hostname: my-hostname-1  subdomain: my-subdomain  containers:  - image: weibeld/ubuntu-networking    command: [sleep, "3600"]    name: ubuntu-networking---apiVersion: v1kind: Podmetadata:  name: my-pod-2  labels:    name: my-testspec:  hostname: my-hostname-2  subdomain: my-subdomain  containers:  - image: weibeld/ubuntu-networking    command: [sleep, "3600"]    name: ubuntu-networking

After applying this, you can exec into one of the Pods:

kubectl exec -ti my-pod-1 bash

And you should be able to resolve the fully-qualifed domain names of the two Pods:

host my-hostname-1.my-subdomain.default.svc.cluster.localhost my-hostname-2.my-subdomain.default.svc.cluster.local

Since you're making the requests from the same namespace as the target Pods, you can abbreviate the domain name to:

host my-hostname-1.my-subdomainhost my-hostname-2.my-subdomain